Solar detoxification; 2003 - unesdoc

SolarDetoxification

by Julian Blanco Galvez, Head of Solar Chemistry and Sixto Malato Rodriguez, Researcher in the Solar Chemistry Area,

Plataforma Solar de Almeria, Spain

United Nations Educational, Scientific and Cultural Organization

2003

Electronic copy only

TABLE OF CONTENTS

PART A. SOLAR DETOXIFICATION THEORY

1. IntroductionAimsObjectivesNotation and units1.1 Solar Chemistry1.2 Water contaminants 1.3 Photodegradation principles 1.3.1 Definitions1.3.2 Heterogeneous photocatalysis1.3.3 Homogeneous photodegradation 1.4 Application to water treatment1.5 Gas-phase detoxification

Summary of the chapterBibliography and referencesSelf-assessment questionsAnswers

2. Solar irradiationAimsObjectivesNotation and units2.1 The power of light2.1.1 Ultraviolet light2.1.2 Visible light2.1.3 Infrared light2.2 The solar spectrum2.3 Solar ultraviolet irradiation 2.4 Atmospheric attenuation of solar radiation 2.4.1 Annual available ultraviolet radiation2.5 Solar radiation measurement2.5.1 Detectors2.5.2 Filters2.5.3 Input Optics

Summary of the chapterBibliography and references

Self-assessment questionsAnswers

3. Experimental systemsAimsObjectivesNotation and units3.1 Laboratory systems3.2 Solar detoxification pilot plants3.3 Operation of pilot plant3.3.1 Once-through operation3.3.2 Batch operation3.3.3 Modelling once-through and batch operation3.4 Evaluation of solar UV radiation inside photoreactors3.4.1 Radiometers calibration3.4.2 Correlation between radiometric and spectroradiometric data3.4.3 Collector efficiency3.4.4 Actinometric experiments3.5 Simplified method for the evaluation of solar UV radiation inside photoreactors


4. Fundamental parameters in photocatalysisAimsObjectivesNotation and units4.1 Direct photolysis 4.2 Oxygen influence4.3 pH influence4.4 Catalyst concentration influence4.5 Initial contaminant concentration influence4.6 Radiant flux influence4.7 Temperature influence4.8 Quantum yield


5. Water decontamination by Solar DetoxificationAimsObjectivesNotation and units5.1 Detoxification of pollutants 5.1.1 Total mineralization5.1.2 Degradation pathways5.1.3 Toxicity reduction5.1.4 Detoxification of inorganic pollutants5.2 Quantum yield improvement by additional oxidants5.2.1 Hydrogen peroxide5.2.2 Persulphate5.2.3 Other oxidants5.3 Catalyst modification 5.3.1 Metal semiconductor modification5.3.2 Composite semiconductors5.3.3 Surface sensitisation5.4 Recommended analytical methods5.4.1 Original contaminants5.4.2 Mineralization measurements (TOC)5.4.3 Intermediate analysis (GC-MS/HPLC-MS)5.4.4 Extraction methods5.4.5 Toxicity analysis


PART B. SOLAR DETOXIFICATION ENGINEERING

6. Solar Detoxification TechnologyAimsObjectivesNotation and units6.1 Solar collector technology generalities6.2 Collectors for solar water detoxification. Peculiarities6.2.1 Peculiarities of solar UV light utilization6.2.2 Parabolic trough collectors6.2.3 Non-concentrating collectors6.2.4 Compound parabolic concentrator (CPC)

6.2.5 Holographic collectors6.3 Concentrated versus non-concentrated sunlight6.4 Technology issues6.4.1 Reflective surfaces6.4.2 Photocatalytic reactor6.5 Catalyst issues6.5.1 Slurry versus supported catalyst6.5.2 Catalyst recuperation and re-use


7. Solar Detoxification ApplicationsAimsObjectivesNotation and units7.1 Introduction7.2 Industrial waste water treatment7.2.1 Phenols7.2.2 Agrochemical compounds7.2.3 Halogenated hydrocarbons7.2.4 Antibiotics, antineoplastics and other pharmaceutical biocide compounds7.2.5 Wood preserving waste7.2.6 Removal of hazardous metals ions from water7.2.7 Other applications7.3 Maritime tank terminals7.4 Groundwater decontamination7.5 Contaminated landfill cleaning7.6 Water disinfection 7.7 Gas-phase treatments


8. Economic AssessmentAims

ObjectivesNotation and units8.1 Photochemical and biological reactors coupling 8.2 Cost calculations8.2.1 Example A: TiO2-based detoxification plant 8.2.2 Example B: Photo-Fenton based detoxification plant8.3 Solar or electric photons?8.4 Solar resources assessment 8.5 Comparison with other technologies8.5.1 Thermal oxidation8.5.2 Catalytic oxidation8.5.3 Air stripping8.5.4 Adsorption8.5.5 Membrane technology8.5.6 Wet oxidation8.5.7 Ozone oxidation8.5.8 Advanced oxidation processes


9. Project engineeringAimsObjectivesNotation and units9.1 Feasibility study9.1.1 Identification of target recalcitrant hazardous compounds9.1.2 Identification of possible pre-treatments9.1.3 Identification of most adequate photocatalytic process9.1.4 Determination of optimum process parameters9.1.5 Post-treatment process identification9.1.6 Determination of treatment factors9.2 Feasibility study example9.2.1 Background 9.2.2 Experimentation. TiO2-Persulphate tests9.2.3 Photo-Fenton tests9.2.4 Conclusions and Treatment Factors9.3 Preliminary design

9.4 Preliminary design example9.5 Final design and project implementation9.6 Example of final design and project implementation


10. International collaborationAimsObjectivesNotation and units10.1 International Energy Agency: The SolarPACES Program10.2 The European Union10.3 The CYTED Program10.4 Main research activities10.4.1 United Stated10.4.2 Spain10.5 Guidelines to successful water treatment projects in developing countries


SOLAR DETOXIFICATION

1. INTRODUCTION

AIMSThis unit describes an alternative source of energy that combines sunlight and chemistry toproduce chemical reactions. It outlines the basic chemical and physical phenomena that arerelated with solar chemistry. This chapter will review approaches that have been taken,progress that has been made and give some projections for the near and longer term prospectsfor commercialisation of solar photochemistry. It also introduces the focus of this book: SolarDetoxification.

OBJECTIVESBy the end of this unit, you will understand the main factors causing the photochemicalreactions and you will be able to do five things:1. Distinguish perfectly between thermochemical and photochemical processes.2. Understand the impact of pollutants on the environment.3. Calculate the energy flux of a light source and its relationship with semiconductor

excitation.4. Understand the basic principles sustaining advanced oxidation processes.5. Describe the most important features of heterogeneous photocatalysis making it applicable

to the treatment of contaminated aqueous effluents.

NOTATION AND UNITSSymbol UnitsAλ Absorbance at wavelength λAOPs Advanced Oxidation Processesc Light speed nm/s, m/sci Concentration of component i molesEG Semiconductor band-gap energy eV, JEλ Spectral irradiance W m-2 nm-1

Eλo Spectral irradiances incident into the medium W m-2 nm-1

Eλl Spectral irradiances at a distance l W m-2 nm-1

EC50 Concentration that produce an effect in 50% of a population mg/L, mg/kgGAC Granulated activated carbonh Planck’s constant J sLC50 Concentration that produce death in 50% of a population mg/L, mg/kgNOEL No observed effect level mg/kg/daypi partial pressure of component i atmU energy of a photon eV, Jαλ Absorption coefficient cm-1 atm-1

ελ extinction coefficient mol-1 cm-1

φ quantum yieldλ. Wavelength nm, µm

1


1.1 SOLAR CHEMISTRYThe dramatic increases in the cost of oil beginning in 1974 focussed attention on the need todevelop alternative sources of energy. It has long been recognised that the sunlight falling onthe earth’s surface is more than adequate to supply all the energy that human activity requires.The challenge is to collect and convert this dilute and intermittent energy to forms that areconvenient and economical or to use solar photons in place of those from lamps. It must bekept in mind that today there is a clear world-wide consensus regarding the need for long-term replacement of fossil fuels, which were produced million of years ago and today aremerely consumed, by other inexhaustible or renewable energies. Under these circumstances,the growth and development of Solar Chemical Applications can be of special relevance.These technologies can be divided in two main groups:

1. Thermochemical processes: the solar radiation is converted into thermal energy thatcauses a chemical reaction. Such a chemical reaction is produced by thermal energyobtained from the sun for the general purpose of substituting fossil fuels.

2. Photochemical processes: solar photons are directly absorbed by reactants and/or acatalyst causing a reaction. This path leads to a chemical reaction produced by theenergy of the sun’s photons, for the general purpose of carrying out new processes.

It should be emphasized, as a general principle, that the first case is associated with processesthat are feasible with conventional sources of energy. The second is related only tocompletely new processes or reactions that are presently carried out with electric arc lamps,fluorescent lamps or lasers.

Heat Photons

Thermochemical ProcessSteam reforming of methane

CH4 + H2O → CO + 3H2 - 206 kJ/mol600º - 850ºC

Photochemical ProcessExcitation of a semiconductor

hν + SC →e- + p+

hν ≥ EG of SC

Increase of Temperature

Modification of chemical bonds

Figure 1.1 Schematic view of Solar Chemical Applications

From the outset, it was recognized that direct conversion of light to chemical energy heldpromise for the production of fuels, chemical feedstock, and the storage of solar energy.Production of chemicals by reactions that are thermodynamically ‘uphill’ can transform solarenergy and store it in forms that can be used in a variety of ways. Wide ranges of suchchemical transformations have been proposed. A few representative examples are given inTable 1.1 to illustrate the concept.

2


∆H (kJ/mol)

CO2(g) → CO(g) + 1/2O2 286

CO2(g) + 2H2O(g) → CH3OH (l) + 3/2O2 727

H2O(l) → H2(g) + 1/2O2 286

CO2(g) + 2H2O(l) → 1/6C6H12O6 (s) + O2 467

Table 1.1 Representative chemical reactions that can store solar energy (Thermochemicalprocesses)

These processes generally start with substances in low-energy, highly-oxidized forms. Theessential feature is that these reactions increase the energy content of the chemicals usingsolar energy. For such processes to be viable, they must fulfil the following requirements, asoutlined by NREL (1995) and slightly modified by the authors:• The thermochemical reaction must be endothermic.• The process must be cyclic and with no side reactions that could degrade the

photochemical reactants.• The reaction should use as much of the solar spectrum as possible.• The back reaction should be very slow to allow storage of the products, but rapid when

triggered to recover the energy content.• The products of the photochemical reaction should be easy to store and transport.

The other pathway for the use of sunlight in photochemistry is to use solar photons asreplacements for those from artificial sources. The goal in this case is to provide a cost-effective and energy-saving source of light to drive photochemical reactions with usefulproducts. Photochemical reactions can be used to carry out a wide range of chemicalsyntheses ranging from the simple to the complex. Processes of this type may start with morecomplex compounds than fuel-producing or energy-storage reactions and convert them tosubstances to which the photochemical step provides additional value or destroy harmfulproducts. The principles of photochemistry are well understood and examples of a wide rangeof types of synthetic transformations are known (Figure 1.2). Therefore, the problem becomesone of identifying applications in which the use of solar photons is possible and economicallyfeasible. The processes of interest here are photochemical, hence, some component of thereacting system must be capable of absorbing photons in the solar spectrum. Because photonscan be treated like any other chemical reagent in the process, their number is a critical elementin solar photochemistry (see Chapter 2).

OCHO

OCHO

O O

hυ<700nm

Methylene blue/O2

C6Cl5OH+9/2O2+ 2H2Ohυ<390nm

TiO2

6CO2 + 5HCl

Figure 1.2 Furfural photo-oxidation and pentachlorophenol mineralization (Photochemicalprocesses).

3


Because they are very technologically and environmentally attractive, solar chemicalprocesses have seen spectacular development in recent years. In the beginning, research insolar chemistry was centered only on converting the solar energy into chemical energy, whichcould then be stored and transferred over long distances. Together with this importantapplication, other environmental uses have been developed, so that today the entire range ofsolar chemical applications has a promising future. In principle, any reaction or processrequiring an energy source can be supplied by solar energy.

1.2 WATER CONTAMINANTSEnvironmental pollution is a pervasive problem with widespread ecological consequences.Recent decades have witnessed increased contamination of the Earth’s drinking waterreserves. The inventory of priority pollutants compiled by the U.S. Environmental ProtectionAgency provides a convenient frame of reference (in Table 1.2 only a partial list is shown) forunderstanding the importance of removing such contamination from the Earth.

1,1,2,2-Tetrachloroethane1,l -Dichloroethane1,2,4-Trimethylbenzene1,2-Dibromoethane1,2-Dichlorobenzene1,2-Dichloropropane1,2-Dinitrotoluene1,2-Diphenylhydrazine1,4-Dioxane2,2,4-Trimethylpentane2,4,6-Trichlorophenol2,4,6-Trinitrotoluene2,4 Diaminoanisole2,4-Dichlorophenol2,4-Dinitrophenol2,4-Dinitrotoluene2,4-Toluene diamine2-Chloroethyl vinyl ether2-Chlorophenol2-Nitropropane4,4’-Diaminodiphenyl ether4,4’-Methylenedianiline4-Aminoazobenzene4-Methylphenol5-Nitro-o-anisidineAcetaldehydeAcetamideAcetoneAcetonitrileAcetophenoneAcroleinAcrylamideAcrylic acidAcrylonitrileAldrinAnilineAnthraceneAtrazineBenzamideBenzene

BenzidineBenzo(a)pyreneBenzyl chlorideBenzenehexachloride)BiphenylBis(2-Chloroethoxy)methaneBromoethaneCaptanCarbarylCarbon disulfideCarbon tetrachlorideCatecholChlordaneChloroacetic acidChlorobenzeneChlorodibenzodioxins,variousChlorodibenzofuranso-,m-,p-CresolsCumeneCyclohexaneDiazomethaneDibenzofuranDichlorvosDicofolDiepoxybutaneDiethanolamineDimethyl phthalateDisulfotonEndosulfanEpichlorohydrinEthylbenzeneEthylene glycolEthylene thioureaFluometuronFormaldehydeHexachlorobenzeneHexachloroethaneHexane

HydroquinoneIsophoroneIsopropyl alcoholLindaneMalathionManebMechlorethamineMelamineMethanolMethoxychlorMethyl acrylateMethyl isocyanateMethyl tert-butyl etherMethylene bromideMethylhydrazineMirexMustard gasNitrilotriacetic acidNitrobenzeneNitrofenNitrogen mustardNitroglycerinNitrophenoln-Butyl alcoholn-Dioctyl phthalateN-NitrosodiethylamineN-NitrosopiperidineN-Nitroso-N-ethylureaOctachloronaphthaleneOctaneOxiraneo-Anisidinehydrochlorideo-Nitroanilineo-ToluidinehydrochlorideParathion (DNTP)PCBsPentachlorobenzenePentachlorophenol

PhenanthrenePhosgenePhthalic anhydridePolybrominatedbiphenylsBeta-PropoxurPyrenep-Chloro-m-cresolQuinoneQuintozeneSafroleSet-Butyl alcoholSevin (carbaryl)StyreneTerephthalic acidTert-Butyl alcoholTetrachlorvinphosTetrahydrofuranThioacetamideThioureaTolueneToluene diisocyanateTotal xylenesToxapheneTriaziquoneTrichlorfonTrifluralinUrethane (ethyl carbamate)Vinyl bromideVinyl chlorideVinylidene chlorideXylene (mixed isomers)Zineb

Table 1.2. Organic compounds that are included in various lists of hazardous substances identified bythe U.S. EPA

4


In any case, a consensus exists that the environmental impact of a given contaminant dependson the degree of exposure (its dispersion and the resulting concentration in the environment)and on its toxicological properties. The assessment of exposure involves comprehension ofthe dispersion of a chemical in the environment and estimation of the predicted concentrationto which organisms will be exposed. For example, the pesticide fenaminphos oxidizes veryquickly (half-life 10 days) into sulphoxide and sulphone, while its pesticidal properties remainunaffected. A half-life of 70 days has been found for degradation of fenaminphos and its twometabolites. Furthermore, the two metabolites are more mobile (soluble) than fenaminphos(Hayo and Werf, 1996). Assessment of the contaminant’s effect involves summarizing dataon the effects of the chemical on selected representative organisms and using these data topredict a no-effect concentration on a specific niche. Organisms may consume chemicalsthrough ingestion of food and water, respiration and through contact with skin. When achemical crosses the various barriers of the body, it reaches the metabolic tissue or a storagedepot. Toxicity of a chemical is usually expressed as the effective concentration or dose of thematerial that would produce a specific effect in 50% of a large population of test species(EC50 or ED50). If the effect recorded is lethal, the term LC50 (or LD50) is used. The ‘noobserved effect level’ (NOEL or NOEC) is the dose immediately below the lowest leveleliciting any type of toxicological response in the study. For example, the pesticidemethamidophos, which has been classified as a Restricted-Use Pesticide (RUP) by the U.S.EPA, is highly toxic for mammals (acute oral LD50 = 16 mg/kg in rats and 30-50 mg/kg inguinea pigs), birds (bobwhite quail 8-11 mg/kg) and bees. The 96-hour LC50 is 25-51 mg/L inrainbow trout, but concentrations as low as 0.22 ng/L are lethal to larval crustaceans in 96-hour toxicity tests. A 56-day rat feeding study resulted in a NOEL of 0.03mg/kg/day (Tomin,1994).

Decontamination of drinking water is mainly by procedures that combine flocculation,filtration, sterilization and conservation, to which a limited number of chemicals are added.Normal human sewage water can be efficiently treated in conventional biological processingplants. But very often, these methods are unable to reduce the power of the contaminant. Inthese cases, some form of advanced biological processing is usually preferred in the treatmentof effluents containing organic substances. Biological treatment techniques are wellestablished and relatively cheap. However, these methods are susceptible to toxic compoundsthat inactivate the waste degrading microorganisms. To solve this problem, apart fromreducing emissions, two main water treatment strategies are followed: (i) chemical treatmentof drinking water, contaminated surface and groundwater and (ii) chemical treatment of wastewaters containing biocides or non-biodegradable compounds.

Chemical treatment of polluted surface and groundwater or wastewater, is part of a long-termstrategy to improve the quality of water by eliminating toxic compounds of human originbefore returning the water to its natural cycles. This type of treatment is suitable when abiological processing plant cannot be adapted to certain types of pollutants that did not existwhen it was designed. In such cases, a potentially useful approach is to partially pre-treat thetoxic waste by oxidation technologies to produce intermediates that are more readilybiodegradable. Light can be used, under certain conditions, to encourage chemicals to breakdown the pollutants to harmless by-products. Light can have a dramatic effect on a moleculeor solid, because, when it absorbs light, its ability to lose or gain electrons is often altered.This electronically excited state is both a better oxidizing and a better reducing agent than itscounterpart in the ground. Electron transfer processes involving excited-state electrons andthe contact medium (for example water) can therefore generate highly reactive species like

5


hydroxide (•OH) and superoxide (O2•-) radicals (see Table 1.3). These can then be used to

chemically decompose a pollutant into harmless end-products. Alternatively, light can be useddirectly to break up pollutant molecule bonds photolytically. These processes are calledAdvanced Oxidation Processes (abbreviated as AOPs). Many oxidation processes, such asTiO2/UV, H2O2/UV, Photo-Fenton and ozone processes (O3, O3/UV, O3/H2O2) are currentlyemployed for this purpose.

Oxidizing reagent Oxidation Potential, V

Fluorine 3.06

Hydroxide radical (•OH) 2.80

Ozone 2.07

Hydrogen peroxide 1.77

Chlorine dioxide 1.57

Chlorine gas 1.36

Oxygen 1.23

Hypochlorite 0.94

Iodine 0.54

Superoxide radical (O2•-) -0.33

Table 1.3. Oxidation potentials of common substances and agents for pollution abatement.The more positive the potential, the better the species is an oxidizing agent

1.3 PHODEGRADATION PRINCIPLES1.3.1 DefinitionsFor the benefit of those who may have a limited background in photochemistry, a brief outlineof some basic concepts of photochemistry is presented here. In order for photochemistry totake place, photons of light must be absorbed. The energy of a photon is given by

λhc

U = (1.1)

where h is Planck’s constant (6.626 10-34 J s), c is the speed of light and λ is the wavelength.For a molecule’s bond to be broken, U must be greater than the energy of that bond.

When a given wavelength λ of light enters a medium, its spectral irradiance Eλ (W m-2 nm-1)is attenuated according to the Lambert-Beer law, which is expressed in two ways, one for gasphase and the other for liquid phase:

lpEEln ilo

λλλ α=)/( gas phase (1.2)

6


lcEElog ilo

λλλ ε=)/( liquid phase (1.3)

Eλo and Eλ

l are the incident spectral irradiances and at a distance l into the medium, αλ is theabsorption coefficient (cm-1 atm-1), pi is the partial pressure (atm) of component i, ελ is theextinction coefficient (M-1 cm-1), and ci is the concentration (M) of component i. Theabsorbence Aλ at wavelength λ is the product ελcil. The photochemical quantum yield (φ) isdefined as the number of molecules of target compound that reacts divided by the number ofphotons of light absorbed by the compound, as determined in a fixed period of time.Normally, the unit is the maximum quantum yield attainable.

The term photocatalysis implies the combination of photochemistry with catalysis. Both lightand catalyst are necessary to achieve or to accelerate a chemical reaction. Photocatalysis maybe defined as the “acceleration of a photoreaction by the presence of a catalyst”.Heterogeneous processes employ semiconductor slurries for catalysis, whereas homogeneousphotochemistry is used in a single-phase system. Any mechanistic description of aphotoreaction begins with the absorption of a photon, being sunlight the source of photons insolar photocatalysis. In the case of homogeneous photocatalytic processes, the interaction of aphoton-absorbing species (transition metal complexes, organic dyes or metalloporphyrines), asubstrate (e.g. the contaminant) and light can lead to a chemical modification of the substrate.The photon-absorbing species (C) is activated and accelerates the process by interactingthrough a state of excitation (C*). In the case of heterogeneous photocatalysis, the interactionof a photon produces the appearance of electron/hole (e- and h+) pairs, the catalyst being asemiconductor (e.g. TiO2, ZnO, etc). In this case, the excited electrons are transferred to thereducible specimen (Ox1) at the same time that the catalyst accepts electrons from theoxidizable specimen (Red2) which occupies the holes. In both directions, the net flow ofelectrons is null and the catalyst remains unaltered.

C C

C R R C

R P

C C e h

h Red Ox

e Ox Red

h

h

ν

ν

→

+ → +

→

→ +

+ →

+ →

− +

+

−

*

* * *

*

( )

2 2

1 1

(1.4)

(1.5)

(1.6)

(1.7)

(1.8)

(1.9)

1.3.2. Heterogeneous photocatalysisThe concept of heterogeneous photocatalytic degradation is simple: the use under irradiationof a stable solid semiconductor for stimulating a reaction at the solid/solution interface. Bydefinition, the solid can be recovered unchanged after many turnovers of the redox system.When a semiconductor is in contact with a liquid electrolyte solution containing a redoxcouple, charge transfer occurs across the interface to balance the potentials of the two phases.An electric field is formed at the surface of the semiconductor and the bands bend as the fieldforms from the bulk of the semiconductor towards the interface. During photoexcitation (aphoton with appropriate energy is absorbed), band bending provides the conditions for carrierseparation. In the case of semiconductor particles, there is no ohmic contact to extract themajority carriers and to transfer them by an external conductor to a second electrode. This

7


means that the two charge carriers should react at the semiconductor/electrolyte interface withthe species in solution. Under steady state conditions the amount of charge transferred to theelectrolyte must be equal and opposite for the two types of carriers. The semiconductor-mediated redox processes involve electron transfer across the interface. When electron/holepairs are generated in a semiconductor particle, the electron moves away from the surface tothe bulk of the semiconductor as the hole migrates towards the surface (see Figure 1.3). Ifthese charge carriers are separated fast enough they can be used for chemical reactions at thesurface of the photocatalyst, i.e., for the oxidation or reduction of pollutants.

Oxid1

Red1

Red2

Oxid2

hν

recombination

recombination

Figure 1.3. Fate of electrons and holes within a particle of illuminated semiconductor incontact with an electrolyte.

Metal oxides and sulphides represent a large class of semiconductor materials suitable forphotocatalytic purposes. Table 1.4 lists some selected semiconductor materials, which havebeen used for photocatalytic reactions, together with band gap energy required to activate thecatalyst. The final column in the table indicates the wavelength of radiation required toactivate the catalysts. According to Plank’s equation, the radiation able to produce this gapmust be of a wavelength (λ) equal or lower than that calculated by Eq. 1.10.

GE

hc=λ (1.10)

where EG is the semiconductor band-gap energy, h is Planck’s constant and c is the speed oflight.

Material Band gap (eV) Wavelength corresponding to band gap (nm)

BaTiO3 3.3 375

CdO 2.1 590

CdS 2.5 497

CdSe 1.7 730

Fe2O3 2.2 565

8


GaAs 1.4 887

GaP 2.3 540

SnO2 3.9 318

SrTiO3 3.4 365

TiO2 3.0 390

WO3 2.8 443

ZnO 3.2 390

ZnS 3.7 336

Table 1.4. Selected properties of several semiconductors

Summarizing, a semiconductor particle is an ideal photocatalyst for a specific reaction if:• The products formed are highly specific.• The catalyst remains unaltered during the process.• The formation of electron/hole pairs is required (generated by the absorption of photons

with energy greater than that necessary to move an electron from the valence band to theconduction band)

• Photon energy is not stored in the final products, being an exothermic reaction and onlykinetically retarded.

1.3.3. Homogeneous photodegradationThe use of homogeneous photodegradation (single-phase system) to treat contaminated watersdates back to the early 1970s. The first applications concerned the use of UV/ozone andUV/H2O2. The use of UV light for photodegradation of pollutants can be classified into twoprincipal areas:• Photooxidation. Light-driven oxidative processes principally initiated by hydroxyl

radicals.• Direct photodegradation. Light-driven processes where degradation proceeds following

direct excitation of the pollutant by UV light.

Photooxidation involves the use of UV light plus an oxidant to generate radicals. Thehydroxyl radicals then attack the organic pollutants to initiate oxidation. Three major oxidantsare used: hydrogen peroxide (H2O2), ozone and Photo-Fenton reaction. H2O2 absorbs fairlyweakly in the UV region with increasing absorption as the wavelength decreases. At 254 nm,ελ is 18 M-1 cm-1, whereas at 200 nm is 190 M-1 cm-1. The primary process for absorption oflight below 365 nm is dissociation to yield two hydroxyl radicals:

OHOH h •→ 222ν (1.11)

The use of hydrogen peroxide is now very common for the treatment of contaminated waterdue to several practical advantages: (i) the H2O2 is available as an easily handled solution thatcan be diluted in water to give a wide range of concentrations; (ii) there are no air emissions;(iii) a high-quantum yield of hydroxyl radicals is generated (0.5). The major drawback is thelow molar extinction coefficient, which means that in water with high UV absorption thefraction of light absorbed by H2O2 may be low unless very large concentrations are used.Furthermore, especially as concerns the focus of this text, H2O2 absorption is very low in theSolar UV range (up 300 nm).

9


Ozone is generated as a gas in air or oxygen in concentrations generally ranging from 1 to 8%(v/v). It has a strong absorption band centered at 260 nm with ελ = 3000 M-1 cm-1. Absorptionof light at this wavelength leads to formation of H2O2:

21

3 )( ODOO h +→ ν (1.12)

2221 )( OHOHDO →+ (1.13)

Hydroxyl radicals are then formed by reaction of ozone with the conjugate base of hydrogenperoxide:

+− +→+ OHHOOHOH 32222 (1.14)•−− +→+ 2332 HOOOHO (1.15)

−•− +→+ OHHOOHO 323 (1.16)

23 OOHHO +→ •• (1.17)Since the net result of ozone photolysis is the conversion of ozone into hydrogen peroxide,UV-ozone would appear to be only a rather expensive method of making hydrogen peroxide.However, there are other oxidation-related processes occurring in solution, such as the directreaction of ozone with a pollutant (see Table 1.3). Ozone may have advantages in water withhigh inherent UV absorbence, but it involves the same problem as hydrogen peroxide for usein solar energy processes.

The essential step of the Fenton reaction is the same as for all AOPs. Highly reactive radicals(like HO• and HO2

•) oxidize nearly all organic substances to yield CO2, water and inorganicsalts. In the case of Photo-Fenton, Fe2+ ions are oxidized by H2O2 while one •OH is produced(1.18), and the Fe3+ or complexes obtained then act as the light absorbing species that produceanother radical while the initial Fe2+ is recovered (1.19 and 1.20).

•−++ ++→+ OHOHFeOHFe 322

2 (1.18)

•+++ ++→++ OHHFehOHFe 22

3 ν (1.19)

•++ ++→+− RCOFehROOCFe 222)]([ ν (1.20)

Note that in equation (1.20) the ligand R-COO– can be replaced by other organic groups(ROH, RNH2 etc.). Compared to other homogeneous photooxidation processes, theadvantages of Photo-Fenton are the improved light sensitivity (up to a wavelength of 600 nm,corresponding to 35% of the solar radiation). On the other hand, disadvantages, such as thelow pH values required (usually below pH 4) and the necessity of removing iron after thereaction, remain.

Some pollutants are able to dissociate only in the presence of UV light. For this to happen, thepollutant must absorb light emitted by a lamp (or the sun) and have a reasonable quantumyield of photodissociation. Organic pollutants absorb light over a wide range of wavelengths,but generally absorb more strongly at lower wavelengths, especially below 250 nm (Figure1.4). In addition, the quantum yield of photodissociation tends to increase at lowerwavelengths, since the photon energy is increasing (eq. 1.1). The net chemical result ofphotodissociation is usually oxidation, since the free radicals generated can react withdissolved oxygen in the water. In practice, the range of waste waters that can be successfullytreated by UV alone is very limited. This defect is more relevant when solar energy is used(see Figure 1.4) because only photons up 300 nm are available.

10


Figure 1.4. UV spectra between 200 and 400 nm of Acrinathrin and sunlight.

1.4 APPLICATION TO WATER TREATMENTAs mentioned above, UV light can be used in several ways. But direct photolysis can occuronly when the contaminant to be destroyed absorbs incident light efficiently. In the case ofUV/ozone and UV/hydrogen peroxide this does not happen. But here too, absorption by somesensitizer must initiate the reaction, and limited absorption by the solute or the additiverestricts efficiency. Furthermore, these mixtures often still require large quantities of addedoxidant. By contrast, in heterogeneous photocatalysis, dispersed solid particles absorb largerfractions of the UV spectrum efficiently and generate chemical oxidants in situ fromdissolved oxygen or water (see Figure 1.5). These advantages make heterogeneousphotocatalysis a particularly attractive method for environmental detoxification. The mostimportant features of this process making it applicable to the treatment of contaminatedaqueous effluents are:• The process takes place at ambient temperature.• Oxidation of the substances into CO2 is complete.• The oxygen necessary for the reaction is obtained from the atmosphere.• The catalyst is cheap, innocuous and can be reused.• The catalyst can be attached to different types of inert matrices.

O2

•OH + H+

e-

O2-•

H2O

TiO2 Particle

WATER

hν ≥ 3.0eV

h+

Figure 1.5. Effect of UV radiation on a TiO2 particle dispersed in water

For all of the above reasons, from now on only this method is dealt with in this text.

Whenever different semiconductor materials have been tested under comparable conditionsfor the degradation of the same compounds, TiO2 has generally been demonstrated to be the

11


most active. Only ZnO is as active as TiO2. TiO2’s strong resistance to chemical andphotocorrosion, its safety and low cost, limits the choice of convenient alternatives (Pelizzetti,1995). Furthermore, TiO2 is of special interest since it can use natural (solar) UV. This isbecause it has an appropriate energetic separation between its valence and conduction bandswhich can be surpassed by the energy content of a solar photon (see Table 1.4). Othersemiconductor particles, e.g., CdS or GaP absorb larger fractions of the solar spectrum andcan form chemically activated surface-bond intermediates, but unfortunately, thesephotocatalysts are degraded during the repeated catalytic cycles involved in heterogeneousphotocatalysis. Therefore, degradation of the organic pollutants present in waste water usingirradiated TiO2 suspensions is the most promising process and R&D in this field has grownvery quickly during the last years.

•−−

+•−+−

+−

+−

→+

+−−−→+−−−

+→++

++→

)(2)(2

22

2

22

22

)()(

'/

adsBCads

IVIVBV

IVIV

h

OeO

HOHTiOTiOhOHTiOTiO

horandheatTiOTiOhe

TiOheTiO

ν

ν (1.21)

(1.22)

(1.23)

(1.24)

To date, evidence supports the idea that the hydroxyl radical (•OH) is the main oxidizingspecimen responsible for photooxidation of the majority of the organic compounds studied.The first effect, after absorption of near ultraviolet radiation, λ<390 nm, is the generation ofelectron/hole pairs, which are separated between the conduction and valence bands (Eq. 1.21).In order to avoid recombination of the pairs generated (Eq. 1.22), if the dissolvent isoxidoreductively active (water) it also acts as a donor and acceptor of electrons. Thus, on ahydrated and hydroxylated TiO2 surface, the holes trap •OH radicals linked to the surface (Eq.1.23). In any case, it should be emphasized that even trapped electrons and holes can rapidlyrecombine on the surface of a particle (Eq. 1.22). This can be partially avoided through thecapture of the electron by preadsorbed molecular oxygen, forming a superoxide radical (Eq.1.24).

Whatever the formation pathway, it is well known that O2 and water are essential forphotooxidation with TiO2. There is no degradation in the absence of either. Furthermore, theoxidative species formed (in particular the hydroxyl radicals) react with the majority oforganic substances. For example, in aromatic compounds, the aromatic part is hydroxylatedand successive steps in oxidation/addition lead to ring opening. The resulting aldehydes andcarboxylic acids are decarboxylated and finally produce CO2. However, the important issuegoverning the efficiency of photocatalytic oxidative degradation is minimizing electron-holerecombination by maximizing the rate of interfacial electron transfer to capture thephotogenerated electron and/or hole. This issue is discussed in more detail later.

Degradation by photocatalysis has been most investigated in monoaromatics andconsequently, these pollutants appear as model compounds in dozens of scientific papers.Some monoaromatics investigated have been benzene, dimethoxybenzenes, halobenzenes,nitrobenzene, chlorophenols, nitrophenols, benzamide, aniline, etc., most of which arerecognised as priority pollutants (see Table 1.2). In addition to these, several other types ofmolecules have also been investigated as substrates for photocatalytic degradation:• Haloaliphatics (trichloroethylen, tetrachloromethane, etc.). Important because so many of

these compounds have been released into the environment and contaminate waters. Somealso originate during water treatment by chlorinating.

12


• Water-miscible solvents (ethanol, alkoxyethanol, etc.). These compounds are verydifficult to detoxify since they are resistant to treatment and are poorly adsorbed on GAC.

• Pesticides. Contaminate waters where agricultural runoff is important. Among the recentlyinvestigated compounds are triazines, organophosphorous, carbamates, phenoxyacids,organochlorines, chloronicotinics, etc.

• Surfactants. Surface active agents enter domestic and industrial waste waters in increasingamounts. Because their biodegradability may be one of the more important constraints intheir use, photocatalytic degradation has received increasing attention.

• Dyes. Strongly colored compounds can be removed by adsorption but it is always better todestroy them by oxidation.

Three exhaustive reviews by Blake (1994, 1995, 1997) describe almost 1800 studies carriedout before 1996.

Despite encouraging laboratory-scale data and some industrial-scale tests, chemical oxidationdetoxification is still restricted to a few experimental plants. The broader application of thosetechnologies requires: i) reactor optimization and modeling and ii) assessment of theefficiency of oxidation technology to reduce the toxicity of effluents. The following chaptersof this book will attempt to highlight these matters.

1.5 GAS-PHASE DETOXIFICATIONAirborne pollutants (such as volatile compounds) can be treated during the gas phase with theUV/TiO2 process. Gas-phase treatment offers several advantages. In general, substrate mass-transport is an order of magnitude faster in the gas phase than in the liquid phase. This in turnleads to much faster reaction rates. Oxidant starvation (such as O2 supply) may be less of aproblem in the gas-phase than in water. There is also no interference on the photocatalyticsurface from other species that are invariably present in aqueous treatment media (forexample anions). In addition, photocatalysis separation after use is not a problem unlike withaqueous slurry suspensions. By using solar energy to drive the process, no fuel is required,gaseous affluent volume is reduced, no NOx is generated, no products of incompletecombustion are produced, CO2 emanating from fuel burning is avoided and substantial fuelsaving may be achieved. Since no burning takes place, oxygen is only necessary atstoichiometric ratio. Solar concentrators provide the opportunity for small-size solar furnacesand even mobile solar parabolic dishes for on-site destruction of low productions of highlytoxic compounds

On the other hand, there are indications that mineralization may not be complete with someorganic substrates in the gas-phase. The TiO2 photocatalyst loses its activity after prolongeduse and must be reactivated with moist air that presumably restores the original degree ofhydroxylation on the oxide surface. There are also indications that product (or intermediate)adsorption on the TiO2 surface may be problematic during the course of the reaction.

Pollutant substrates like trichloroethylene, acetone, formaldehyde, m-xylene and Nox havebeen treated with TiO2/UV in the gas-phase in bench-scale tests. Field tests have also beenconducted to treat effluent air emissions using this technique at different manufacturing plantsin the USA (Rajeshwar, 1996).

13


Class of Compound Chemicals Tested

Aromatics

Nitrogen-containing ring

Aldehydes

Ketones

Alcohols

Alkanes

Terpenes

Sulfur-containing Organics

Chlorinated E!hylenes

Acetyl Chlorides

Benzene, Toluene

Pyridine, Picoline, Nicotine

Acetaldehyde. Formaldehyde

Acetone

Methanol. Ethanol, Propanol

Ethylene. Propene, Tetramethyl Ethylene

α-Pinene

Methyl Thiophene

Dichloroethylene.Trichloroethylene.Tetrachloroetylene

Dichloroacetyl Chloride, Tetrachloroacetyl Chloride

Table 1.5. VOCs amenable to treatment via Photocatalytic Oxidation (Jakobi et al., 1996).

SUMMARY OF THE CHAPTERA description is given of how solar chemistry could become a significant segment of thechemical industry and how it can be used, under certain conditions, to provoke chemicalbreakdown of pollutants into harmless by-products. The behaviour of contaminants inenvironmental water is summarised. The basic concepts of photochemistry relating tophotolysis of chemical bonds, homogeneous photodegradation and heterogeneousphotocatalysis are reviewed. The use of semiconductors for wastewater treatment, withparticular reference to TiO2, has been discussed. Examples of the waste materials that havebeen treated successfully using TiO2, have been presented. Gas-phase photocatalysis has alsobeen introduced.

BIBLIOGRAPHY AND REFERENCESBlake, D.M.; Bibliography of Work on the Photocatalytic Removal of Hazardous Compounds

from Water and Air. National Technical Information Service, US Depart. ofCommerce, Springfield, VA22161, USA, May 1994. Update Number 1 To June 1995,October 1995. Update Number 2 To October 1996, January 1997.

Hayo, M.G. and van der Werf. Assessing the impact of pesticides on the environment. Agric.Ecosys. Environ., 60, 81-96, 1996.

Jacoby, W.A., Blake, D.M., Fennell, J.A., Boulter, J.E., Vargo, L.M., George, M.C. andDolberg, S. K. Heterogeneous Photocatalysis for Control of Volatile OrganicCompounds in Indoor Air. J. Air Waste Manage. Assoc. 46, no. 9, 891-8, 1996.

14


National Renewable Energy Laboratory, Solar Photochemistry-Twenty Years of Progress,What’s Been Accomplished, and Where Does It Lead? Report NREL/TP-433-7209,Golden, Colorado, USA, 1995.

Pelizzetti, E. Concluding Remarks on Heterogeneous Solar Photocatalysis. Solar En. Mat.Sol. Cells, 38, 453-457, 1995.

Rajeshwar, K. Photochemical Strategies for Abating Environmental Pollution. Chemistry &Industry, 17, 454-458, 1996.

Tomin, C. The Pesticide Manual, a World Compendium. 10th Edition. British Crop ProtectionCouncil. Croydon, UK, 1994.

SELF-ASSESSMENT QUESTIONS

PART A. True or False?

1. The solar energy is useful only to substitute fossil fuels converting it into thermal energythus provoking chemical reactions.

2. Toxicity of a chemical is the same for all the species.3. Biological treatment techniques are the cheapest wastewater treatment methods.4. The energy of a photon depends of the ambient temperature.5. Heterogeneous photocatalysis employs liquid catalysts.6. Light driven oxidative processes are initiated by excited electrons of the catalyst surface.7. Ozone can be produced from air.8. The most important characteristics of a photocatalysts are: stability to chemical and

photocorrosion, safety, cost and band-gap.9. The electron/hole recombination can be avoided increasing reaction temperature.10. Heterogeneous photocatalysis can be applied only to monoaromatics.

PART B.

1. Which is the most important difference between thermochemical and photochemical solarprocesses?

2. Which are the usual ways to express the toxicity of a chemical in the environment?3. Why biodegradation, which is a major mechanism in wastewater treatment, is quite

inefficient to treat certain types of wastewater?4. What is the percentage of absorbed photons in a solution with the following

characteristics: extinction coefficient = 1327 cm-1 M-1, concentration of substrate 0.01 M,illuminated pathlength = 5.6 cm? And if the extinction coefficient is 0.3?

5. What is the wavelength able to excite a semiconductor which band-gap is 4.0 eV?6. Name three important characteristics of heterogeneous photocatalysis to be used as water

treatment process.7. Why TiO2 is the most suitable photocatalyst for wastewater treatment?8. Which is the more important electron acceptor in water?9. Which is the most important product of photocatalytic degradation with organic

contaminants?10. Why hydroxyl radicals react with organic substances?

Answers

Part A

15


1. False; 2. False; 3. True; 4. False; 5. False; 6. False; 7. True; 8. True; 9. False; 10. False.

Part B

1. In thermochemical processes solar radiation is converted into thermal energy, inphotochemical processes the solar photons are absorbed directly by the reactants givingrise to the reaction.

2. Toxicity of a chemical is usually expressed as the effective concentration or dose of thematerial that would produce a specific effect in 50% of a large population of test species(EC50 or ED50).

3. Because when compounds are very toxic, the micro-organisms need an extended period ofadaptation, when they are not invaible.

4. 100% and 3.8 %.5. λ ≤ 310 nm6. The process takes place at ambient temperature, the oxygen necessary for the reaction is

obtained from the atmosphere, the catalyst is cheap, innocuous and can be reused.7. It has exhibited the highest activity. It is high stable to chemical and photocorrosion. It

can use natural UV.8. Dissolved oxygen.9. Carbon dioxide.10. Because of its very high oxidation potential.

16


2. SOLAR IRRADIATION

AIMSThis unit describes the power of light as a source of energy. It outlines the basic principlesthat are related to the light spectrum and specifically to the solar spectrum. This chapterdiscusses solar UV radiation and its photon flux in more detail, because this part of the solarspectrum is the most important for driving chemical processes. Moreover, the majoratmospheric variables determining the amount of UV solar radiation on the earth’s surface arediscussed. A method for calculating UV attenuation at a given site is presented. Finally, solarradiation measurement systems are described.

OBJECTIVESAt the end of this unit, you will understand the main factors affecting solar radiationbehaviour and you will be able to do six things:1. Discriminate between the different components of solar radiation and their principal

characteristics.2. Recognize typical solar spectra and understand the effect of sun position on the solar

power reaching the earth’s surface.3. Find the photon flux of a polychromatic source of energy with simple calculations.4. Describe the most important components of the earth’s atmosphere and the consequences

for power and spectral distribution of the solar radiation.5. Understand the procedures that permit solar power to be calculated from available

radiation at any given site.Comprehend the basic principles on which solar radiation measurement is based.

NOTATION AND UNITSSymbol UnitsAM Air mass ratiofn Clouds factorfλ Fraction of power associated with a wavelength nm-1

H Radiance exposure monthly average kJ m-2

TBDH TBDUV radiance exposure kJ m-2

I Photon flux density Einstein s-1 m-2

Na Quantity of photons absorbed by the system Photons s-1

N0 Avogadro’s number, 6.023 x 1023 Photons mol-1

Nλ Number of photons supplied by a source of light of wavelength λ Photons s-1

Qλ Energy of a monochromatic source of light of wavelength λ W m-2 µm-1

T TransmittanceTλ Transmittance of direct-bean solar radiation under cloudless skies

at a specific wavelengthTa,λ Transmittance related to absorption and dispersion by aerosolsTg,,λ Transmittance resulting from absorption of atmospheric gasesTo,λ Transmittance related to the effect of the ozone layerTR,λ Transmittance related to the molecules of airTv,,λ Transmittance resulting from absorption by steam.

17


TBDUV Typical “best day”. Completely clear sky during all the hours ofsunlight

Uλ Energy of one photon eV, JUVD Direct ultraviolet light W m-2

UVG Global ultraviolet light W m-2

UVλ Ultraviolet irradiance associated with a wavelength W m-2 nm-1

Φ Quantum yield No unitsλ. Wavelength nm, µm

2.1 THE POWER OF LIGHTLight is just one of various electromagnetic waves present in space. The electromagneticspectrum covers an extremely broad range, from radio wavelengths of a meter or more, downto x-rays with wavelengths of less than one billionth of a meter. Optical radiation lies betweenradio waves and x-rays on that spectrum and has a unique combination of ray, wave, andquantum properties. At x-ray and shorter wavelengths, electromagnetic radiation tends to bequite particle-like in its behaviour, whereas toward the long wavelength end of the spectrumbehaviour is mostly wavelike. The UV-visible portion occupies an intermediate position,having both wave and particle properties in varying degrees (See Figure 2.1a).

UV100-400 nm

Infrared770-106 nm

Visible

400-770nm

M icrowavesX-rays

W avelength λλλλ , nanometers

a)

b) λλλλ

100 1000 10000

Figure 2.1The optical portion of the electromagnetic spectrum (a)

and light wave front modelled as a straight-line (b).

Like all electromagnetic waves, light waves can interfere with each other, becomedirectionally polarised, and bend slightly when passing through an edge. These propertiesallow light to be filtered by wavelength or amplified coherently as in a laser. In radiometry,light’s propagating wave front is modelled as a ray travelling in a straight line (See Figure2.1b). Lenses and mirrors redirect these rays along predictable paths. Wave effects areinsignificant in a large-scale optical system, because the light waves are randomly distributedand there are plenty of photons.

2.1.1 Ultraviolet lightShort wavelength UV-light exhibits more quantum properties than its visible or infraredcounterparts. Ultraviolet light is arbitrarily broken down into three bands, according to itsanecdotal effects. UV-A (315-400 nm), which is the least harmful type of UV light, because ithas the least energy (recall Eq. 1.1), is often called black light, and is used for its relativeharmlessness and its ability to cause fluorescent materials to emit visible light – thusappearing to glow in the dark. UV-B (280-315 nm) is typically the most destructive form of

18


UV light, because it has enough energy to damage biological tissues, yet not quite enough tobe completely absorbed by the atmosphere. UV-B is known to cause skin cancer. Since theatmosphere blocks most of the extraterrestrial UV-B light, a small change in the ozone layercould dramatically increase the danger of skin cancer. UV-C (100-280 nm) is almostcompletely absorbed in air within a few hundred meters. When UV-C photons collide withoxygen atoms, the energy exchange causes the formation of ozone. UV-C is never observed innature, however, since it is absorbed so quickly. Germicidal UV-C lamps are often used topurify water because of their capability to kill bacteria.

2.1.2 Visible lightVisible light is concerned with the radiation perceived by the human eye. The lumen (lm) isthe photometric equivalent of the watt, weighted to match the eye response of the “standardobserver”. Yellowish-green light receives the greatest weight because it stimulates the eyemore than the blue or red light of equal radiometric power (1 W at 555 nm = 683.0 lumens).To put this into perspective: the human eye can detect a flux of about 10 photons per secondat 555 nm; this corresponds to a radiant power of 3.58 x 10-18 W (or J s-1). Similarly, the eyecan detect a minimum flux of 214 and 126 photons per second at 450 nm and 650 nm,respectively.

400 500 600 700 nm

Blue Green Yellow Red

Figure 2.2Visible light colours distribution

2.1.3. Infrared lightInfrared light contains the least amount of energy per photon of any other band and is uniquein that it has primarily wave properties. This can make it much more difficult to manipulatethan ultraviolet and visible light. Infrared is more difficult to focus with lenses, refract withlenses, diffracts more, and is difficult to diffuse. Since infrared light is a form of heat, farinfrared detectors are sensitive to environmental changes – such as a person moving in thefield of view. Night vision equipment takes advantage of this effect, amplifying infrared todistinguish people and machinery that are concealed in the darkness.

2.2 THE SOLAR SPECTRUMAll the energy coming from that huge reactor, the sun, from which the earth receives1.7x1014 kW, meaning 1.5x1018 kWh per year, or approximately 28000 times worldconsumption for one year (Figure 2.3a). Radiation beyond the atmosphere has a wavelengthof between 0.2µm and 50µm, which is reduced to between 0.3 µm and 3.0 µm when reachingthe surface due to the absorption of part of it by different atmospheric components (ozone,oxygen, carbon dioxide, aerosols, steam, clouds). The solar radiation that reaches the groundwithout being absorbed or scattered is called direct radiation; radiation that reaches theground but has been dispersed is called diffuse radiation, and the sum of both is called globalradiation. In other words, it is the direct radiation that produces shadow when an opaque

19


object blocks it; diffuse radiation does not. In general, the direct component of globalradiation on cloudy days is minimum and the diffuse component is maximum, and theopposite on clear days.

0.6 1.2 1.8 2.4 3.0 3.6 4.20

500

1000

1500

2000

0

500

1000

1500

2000

Extraterrestrial Direct Air Mass 1.5Global 37º Air Mass 1.5

Irra

dia

nce

, W m

-2µ µµµ m

-1

Wavelength, µµµµm

Figures 2.3a and 2.3b(a) World solar irradiance, MWh m-2 year-1)

(b) Spectral solar radiation plotted from 0.2 to 4.5 µm

Figure 2.3b shows the standard solar radiation spectra (Hulstrom et al., 1985) at ground levelon a clear day. The dotted line corresponds to the extraterrestrial radiation in the samewavelength interval. When this radiation enters the atmosphere, it is absorbed and scatteredby atmospheric components, such as air molecules, aerosols, water vapor, liquid waterdroplets and clouds. The spectral irradiance data are for the sun at a solar zenith angle of48.19º. This zenith angle corresponds to an air mass of 1.5, which is the ratio of the direct-beam solar-irradiance path length through the atmosphere at a solar zenith angle of 48.19º tothe path length when the sun is in a vertical position. AM =1 when the sun is directlyoverhead (zenith). As air mass increases, the direct beam traverses longer path lengths in theatmosphere, which results in more scattering and absorption of the direct beam and a lowerpercentage of direct-to-total radiation (for the same atmospheric conditions).

20


48.2º60º

SunriseSunset

Zenith

Diffuseradiation

Directradiation

G lobal radiation EA RTH

AM 2.0

AM

1.5 Atm osphere

Figure 2.4Air mass and solar components

The AM 1.5 global irradiance is shown for a flat surface facing the sun and tilted 37º from thehorizontal. The 37º tilt angle is used because it is the latitude of the Plataforma Solar deAlmería, where most of the research presented here was done. The scarce part of the solarspectrum that can be used in photocatalysis with TiO2 may be clearly seen (See Table 1.4)but, as the energy source is so cheap and abundant, even under these limitations its use is ofinterest.

2.3 SOLAR ULTRAVIOLET IRRADIATION

Solar ultraviolet radiation is, as explained above, only a very small part of the solar spectrum,between 3.5% and 8% of the total of the solar spectrum, as demonstrated by measurement,although this ratio may be different for a given location on cloudy and clear days. Thepercentage of global UV radiation (direct + diffuse) generally increases with regard to totalglobal when atmospheric transmissivity decreases mainly because of clouds, but also becauseof aerosols and dust. In fact, the average percentage ratio between UV and total radiation oncloudy days is up to two percentage points more than values on clear days.

The efficiency of a chemical reaction is calculated from the ratio between the products and thedeparting reactants. In photochemistry, it is very common to use the quantum yield concept,which is calculated from a known amount of photons absorbed in the reaction. Quantum yield(Φ) is defined as the ratio between the number of reacting molecules (∆n) and the quantity ofphotons absorbed by the system (Na):

Φ ∆ =

n

N a

(2.1)

Experimentally, the quantum yield is expressed as the number of moles of reactant in aninterval of time t, divided into the number of moles of photons absorbed during the sameperiod. Knowledge of the quantum yield is rather important for an understanding of themechanism of a photochemical reaction. If every absorbed photon produces a moleculartransformation, Φ = 1. If it is less than 1, it means that deactivation processes or otherreactions competing with the one studied exist. Over 1 indicates a series of reactions thepromoter of which has been excited by a photon. In the case of photocatalysis by UVradiation, the number of photons that reach the reacting mixture and are thereby susceptible to

21


being absorbed, will be in relation to the UV solar spectrum. For reactors using solarradiation, knowledge of the solar UV spectrum is important for the following reasons:• The radiation (sunlight) that reaches them is not constant. This prevents correct

comparison between experiments carried out at different times of the day or seasons of theyear or under different atmospheric conditions.

• The extensive bibliography on photocatalytic decomposition of organic compoundsindicates that the majority of the experiments in which the photon flux is known arecarried out in laboratory reactors illuminated by lamps. In order to compare these resultswith solar radiation or to use the information contained in those reports, it is necessary toknow the photon flux inside the solar reactor.

• The quantum yield of the reaction tested under a given experimental condition providesinformation on the optimum conditions for decomposition of the contaminant. Knowledgeof the photon flux in this situation is basic to the determination of the efficiency of thesolar reactor components (reflective surface, absorber tube, control system, concentrationfactor, etc.) and any possible modification, in each case, to improve photodegradationconditions.

• Any economic comparison between solar radiation and electric lamps as the UV photonsource requires knowledge of the photon flux incident on the solar reactor.

The two spectra shown in Figure 2.5 correspond to the same spectra shown in Figure 2.3 forthe solar UV spectrum range at ground level. The shorter of them (direct UV) reaches 22 Wm-2 between 300 and 400 nm, the longer (global UV) reaches 46 W m-2.

The number of photons, Nλ, supplied by a monochromatic source of light with wavelength λand energy Qλ is related to the energy of one photon, Uλ, by Planck’s equation (Eq.1.1):

λλ

λλλ

N = Q

W = Q

hc(2.2)

When a source of light is polychromatic as is solar radiation, the number of photons is givenby an integral covering the whole range of wavelengths of that source:

N = N )d = 1

hc Q( ) d

1

2

1

2(λ λ λ λ λλλ

λλ∫ ∫ (2.3)

0.30 0.32 0.34 0.36 0.38 0.400

5

10

15

20

0

5

10

15

20

UVD

UVG

Wavelength, µµµµm

Pho

ton

flux

, 10-2

0 ph

oton

s m

-2 s

-1µ µµµ m

-1

8.4 x 1019 photons m-2 s-1

3.6 x 1019 photons m-2 s-1

Figure 2.5Ultraviolet spectra on the earth surface (standard ASTM)

Equation 2.3 gives the ratio between photonic and radiometric quantities, defining from here

22


the photon flux density I [Einstein s-1 m-2] as the number of incident photons per unit ofsurface and time:

I = d N

N dt dA

2

0

(2.4)

where N0 is Avogadro’s number (6.023 x 1023). 1 Einstein = 1 mol of photons = 6.02 x 1023

photons.

Using the spectrum data and the above equations in congruent units [S.I], it is possible todetermine the photon flux density I (ID = 3.6 x 1019 photons m-2 s-1 = 6 x 10-5 Einstein m-2 s-1,IG = 8.4 x 1019 photons m-2 s-1 = 14 x 10-5 Einstein m-2 s-1). These two values give an idea ofthe energy coming from the sun and available for photocatalytic reactions with TiO2, whichonly uses the part of the UV spectrum up to 390 nm, as explained below. In any case, the UVradiation values described vary from one location to another, and obviously, at different hoursof the day and in different seasons, making it necessary to know these data for the particularlocation and in real time. This data will be very useful in those cases where this is notpossible.

2.4 ATMOSPHERIC ATTENUATION OF SOLAR RADIATIONA general expression for transmittance (T) of direct-bean solar radiation under cloudless skiesat a specific wavelength (λ) is (Iqbal, 1983):

T T T T T TR a o g vλ λ λ λ λ λ= , , , , , (2.5)

TR,λ is the spectral transmittance resulting from the dispersion produced by molecules of air(dimensions of many of which are ≈1Å, Raleigh dispersion). Ta,λ is the spectral transmittancerelated to absorption and dispersion by aerosols (solid or liquid particles suspended in the air).To,λ corresponds to the effect of the ozone layer. Tg,,λ is the transmittance resulting fromabsorption of atmospheric gases (such as carbon dioxide and oxygen). Tv,,λ corresponds to theabsorption by water vapour. The effect of each of these parameters within the range inquestion (300 nm-400 nm) would be the following (Riordan et al., 1990):

• TR,λ = exp (-0.008735 λ-4.08 M’), where M’ is the air mass corrected according to itsdensity, which depends on the pressure and, therefore, the altitude. In agreement withthis, this factor would be practically constant for a given site.

• Ta,λ = exp (-λ-α β M) where M is the mass of air, β is the coefficient of turbidity,which usually varies between 0 and 0.5 and is a reflection of the amount of aerosolsin the air and α is an index of the size of the aerosol molecules. Depending on theatmospheric contamination at the site, β varies differently. Where there is nocontamination it varies only slightly. The same reasoning is valid for α.

• To,λ is constant for a specific site since the ozone layer has a practically constantthickness (for now).

• Tg,λ only influences wavelengths over UV.• Tv,λ does not affect the UV spectrum.

Keeping in mind then, the different transmittances, it may be assumed that the solar UVspectrum does not vary substantially within a specific site throughout the year, unlessatmospheric conditions (except clouds) do so. The dominating attenuator of solar radiation isclouds. Under overcast skies there is no direct-beam radiation, and under partly cloudy skiesthere is intermittent direct-beam radiation when clouds are not obscuring the sun’s disk.Clouds are often assumed to have a wavelength-independent attenuation function in the UVrange; but in the near-infrared region (See Figure 2.6) they cause increased absorption due towater vapour and liquid water (Tv,λ).

23


400 600 800 1000 12000.0

0.2

0.4

0.6

0.8

1.0

1.2

Clear sky

Clouds

W m

-2 n

m-1

Wavelength, nm

300 325 350 375 4000.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Clear sky

Clouds

W m

-2 n

m-1

Wavelength, nm

Figure 2.6Solar spectra on the earth surface (Plataforma Solar de Almería) between 300 and 1100 nm.

Clouds modify the total UV energy reaching the earth’s surface, but the wavelengthdistribution is not affected. This cannot be guaranteed, however, if the data for all the spectrashown in Figure 2.6 are not represented in a standardized manner as in Figure 2.7. This can bedone for any wavelength interval by the following operation. Summations have been used totreat the discrete values nm to nm:

1fthereforeUV

UVf

nm

nmnm

nm

=,400

300400

300

∑∑

=

==

=

=λ

λλλ

λλ

λλ (2.6)

where fλ, is the fraction of power associated with wavelength λ and UVλ is the irradiance,W m-2 nm-1 corresponding to each wavelength and measured with a spectroradiometer. InFigure 2.7, the homogeneity of all the spectra recorded may be observed. If the spectrum ofUV radiation is assumed to have a fixed form, then standardized spectrum can be consideredas standard for each site. Therefore, the number of photons corresponding to this range ofwavelengths is only a function of the intensity (Measurable in real time with the radiometers,see the following section in this chapter).

300 325 350 375 4000.00

0.01

0.02

0.03

0.04

0.05

0.06

f λ λλλ,

nm-1

Wavelength, nm

Area below each curve = 1

1fthereforeUV

UVf

nm

nmnm

nm

=,400

300400

300

∑∑

=

==

=

=λ

λλλ

λλ

λλ

Figure 2.7Normalised solar UV spectra shown in Figure 2.6.

24


2.4.1 Annual available ultraviolet radiationA general index of atmospheric transmittance due to all the processes described above is theso-called “cloud factor” (fn). For the calculation of fn the annual average of ultravioletradiation must be found. Knowledge of this factor enables the amount of energy that reachesthe earth’s surface to be predicted for a given place at any time of the year. It should be notedthat the “cloud factor” for global radiation is always lower than the direct, as the diffusecomponent of solar radiation is maximum when direct radiation is absent (global = direct +diffuse). This factor is calculated (Eq. 2.7) from the ratio between average radiation (affectedby all atmospheric phenomena) and the highest attainable radiation at all times of the year.This is usually calculated for each month separately. To find out the highest UV radiationavailable each month, a typical “best day” (completely clear sky during all daylight hours) isselected for each month (TBDUV) from among all the days for which average radiation iscalculated. The parameters taken into account in selecting the TBDUV are absence of cloudsand proximity to the 15th day of each month (See Figure 2.8). Several days per month(corresponding to each year for period to be analyzed) must be chosen and compared to findthe day with the maximum TBDH .

−=

TBDn H

Hf 1 (2.7)

where TBDH (kJ m-2) is the TBDUV radiance exposure (direct or global). It has been obtainedintegrating the UV irradiance along the day. In this case, being discrete values, it has beencalculated integrating numerically the irradiance from sunrise to sunset. H (kJ m-2) is theaverage of the month. This was calculated by multiplying the monthly average irradiance bythe monthly average of hours of sunlight. The “cloud factor” for UV radiation has to becalculated from data collected with radiometers which are UV-specific. The UV-radiationdata base must be large enough to be considered statistically correct (at least 4-5 years).

SOLAR HOUR

UV

, W m

-2

6 8 10 12 14 16 18 200

10

20

30

40

50

October

June

April

January

Figure 2.8TBDUV of different periods of the year at Plataforma Solar de Almería (37º N)

2.5 SOLAR RADIATION MEASUREMENTDetectors translate light energy into an electrical current. Light striking a silicon photodiodecauses a charge to build up between internal “P” and “N” layers. When an external circuit isconnected to the cell, an electrical current is produced. This current is linear with regard to the

25


incident light over a dynamic 10-decade range. A wide dynamic range is a prerequisite formost applications. The radiometer should be able to cover the entire dynamic range of anydetector that will be plugged into it. This usually means that the instrument should be able tocover at least 7 decades of dynamic range with minimal linearity errors. The current orvoltage measurement device should be the least significant source of error in the system. Thesecond thing to consider when choosing a radiometer is the type of features offered. Ambientzeroing, integration ability, and a “hold” button should be standard. The ability to multiplexseveral detectors to a single radiometer or control the instrument remotely may also bedesired. Lastly, portability and battery life may be an issue for measurements made in thefield.

Light is all around us every day, yet it remains the most elusive form of energy to measureaccurately. A single photon of light travels in a straight line in one direction, at a givenwavelength. A light bundle consists of a jumbled mixture of billions and billions of photons atdifferent wavelengths, going in different directions, at different moments in time. The watt(W), the fundamental unit of optical power, is defined as a rate of energy of one joule (J) persecond. Optical power is a function of both the number of photons and the wavelength. Eachphoton carries an energy that is described by Planck’s equation (Eq. 1.1). All lightmeasurement units are spectral, spatial or temporal distributions of optical energy. Thebiggest hurdle in light measurement is the very spatial nature of light. Irradiance is a measureof the energy density received from a light source. Since light expands outward from a pointsource, the irradiance decreases with distance. The irradiance also decreases with incidentangle. Carefully designed input optics cannot prevent measurement errors caused by laxattention to the measurement geometry upon which the units systems are based. Spectralresponsivity and detectivity present a very different problem. Many properties of light aredependent on wavelength and the energy of one photon is inversely proportional to itswavelength. Since a detector measures only absorbed light, it cannot differentiate between 1photon (200 nm) and 10 photons (2000 nm). The light must be filtered by wavelength beforeit reaches the detector.

Sensitivity to the band of interest is a primary consideration when choosing a detector. Youcan control the peak responsivity and bandwidth through the use of filters, but you must havean adequate signal to start with. Filters can suppress out-of-band light but cannot boost signal.Another consideration is blindness to out-of-band radiation. If you are measuring solarultraviolet in the presence of massive amounts of visible and infrared light, for example, youwould select a detector that is insensitive to the long wavelength light that you intend to filterout. Lastly, linearity, stability and durability are considerations. Some detector types must becooled or modulated to remain stable. High voltages are required for other types. In addition,some can be burned out by excessive light, or have their windows permanently ruined by afingerprint.

2.5.1 DetectorsPlanar-diffusion-type silicon photodiodes are perhaps the most versatile and reliable sensorsavailable. The P-layer material at the light sensitive surface and the N material at the substratefrom a P-N junction which operates as a photoelectric converter, generating a current that isproportional to the incident light. Silicon cells operate linearly over a ten-decade dynamicrange, and remain true to their original calibration longer than any other type of sensor. Forthis reason, they are used as transfer standards at the NIST (National Institute of Standardsand Technology, USA).

26


The phototube is a light sensor that is based on the photoemissive effect. The phototube is abipolar tube which consists of a photoemissive cathode surface that emits electrons inproportion to incident light, and an anode which collects the electrons emitted. The anodemust be biased at high voltage (50 to 90 V) in order to attract electrons to jump through thevacuum of the tube. Some phototubes use a forward bias of less than 15 volts, however. Thecathode material determines the spectral sensitivity of the tube. Solar-blind vacuumphotodiodes use Cs-Ta cathodes to provide sensitivity only to ultraviolet light, providing asmuch as a million to one long wavelength rejections. A UV glass window is required forsensitivity in the UV down to 185 nm, with fused silica windows offering transmission downto 160 nm.

The thermopile is a heat sensitive device that measures radiated heat. Infrared light containsthe least amount of energy per photon of any other band. Because of this, an infrared photonoften lacks the energy required to pass the detection threshold of a quantum detector. Infraredis usually measured using a thermal detector such as a thermopile, which measurestemperature change due to absorbed energy. While these thermal detectors have a very flatspectral responsibility, they suffer from temperature sensitivity, and usually must beartificially cooled. The sensor is usually sealed in a vacuum to prevent heat transfer except byradiation. A thermopile consists of a number of thermocouple junctions in series, whichconvert energy into a voltage using the Peltier effect. Thermopiles are convenient sensors formeasuring the infrared, because they offer adequate sensitivity and a flat spectral response ina small package. More sophisticated bolometers and pyroelectric detectors need to be choppedand are generally used only in calibration labs.

The best method of operating a thermal detector is by chopping incident radiation, so that driftis zeroed out by the modulated reading. The quartz window in most thermopiles is adequatefor transmitting from 200 to 4200 nm, but for long wavelength sensitivity out to 40 microns,Potassium Bromide windows are used. Another strategy employed by thermal detectors is tomodulate incident light with a chopper. This allows the detector to measure differentiallybetween the dark (zero) and light states. Quantum-type detectors are often used in the nearinfrared, especially below 1100 nm. Specialized detectors such as InGaAs offer excellentresponsivity from 850 to 1700 nm. Typical silicon photodiodes are not sensitive above 1100nm. These types of detectors are typically employed to measure a known artificial near-IRsource without including long wavelength background ambient. Most radiometric IRmeasurements are made without lenses, filters, or diffusers, relying on just the bare detector tomeasure incident irradiance.

27


200 400 600 800 1000 12000

10

20

30

40

50

60

70

80

90

100

Sola

r-bl

ind

vacu

um p

hoto

diod

e

Thermopile Silicon photodiode

%

Wavelength, nm

220 240 260 280 300 3200.0

0.2

0.4

0.6

0.8

1.0

Combined responsivity

DetectorFilter

Wavelength, nm

Figure 2.9Responsivities of three detectors.

In the inset is shown a schematic of the effect of a filter on detector responsivity.

2.5.2 FiltersA detector’s overall spectral sensitivity is equal to the product of the responsivity of thesensor and the transmission of the filter. Given a desired overall sensitivity and a knowndetector responsivity, you can then solve a transmission curve for the ideal filter. Filterbandwidth decreases with thickness according to Lambert-Beer’s law (see Eqs. 1.2 and 1.3),so by varying filter thickness, you can selectively modify the spectral responsivity of a sensorto match a particular function. Multiple filters cemented in layers give a net transmissionequal to the product of the individual transmissions. Filters operate by absorption orinterference. Colored glass filters are doped with materials that selectively absorb light bywavelength, and obey Lambert-Beer’s law. The peak transmission is inherent to the additives,while bandwidth is dependent on thickness. Sharp-cut filters act as long pass filters, and areoften used to subtract out long wavelength radiation in a secondary measurement. Interferencefilters rely on thin layers of dielectric to cause interference between wave-fronts, providingvery narrow bandwidths. Any of these filter types can be combined to form a composite filterthat matches a particular photochemical process.

2.5.3 Input OpticsWhen selecting input optics for a measurement application, consider both the size of thesource and the viewing angle of the intended real-world receiver. Suppose, for example, thatyou were measuring the erythemal (sunburn) effect of the sun on human skin. While the sunmay be considered very much a point source, skylight, refracted and reflected by theatmosphere, contributes significantly to the overall amount of light reaching the earth’ssurface. Sunlight is a combination of a point source and a 2π steradian area source. The skin,since it is relatively flat and diffuse, is an effective cosine receiver. It absorbs radiation inproportion to the incident angle of the light. An appropriate measurement system should alsohave a cosine response. If you aimed the detector directly at the sun and tracked the sun’spath, you would be measuring the maximum irradiance. If, however, you wanted to measurethe effect on a person lying on the beach, you might want the detector to face straight up,regardless of the sun’s position. This example can be extended to solar collectors (see Chapter6).

28


Different measurement geometries necessitate specialised input optics. Radiance andluminance measurements require a narrow viewing angle (< 4º) in order to satisfy theconditions underlying the measurement units. Power measurements, on the other hand,require a uniform response to radiation regardless of input angle to capture all light. Theremay also be occasions when the need for additional signal or the desire to exclude off-anglelight affects the choice of input optics. A high-gain lens, for example, is often used to amplifya distant point source. A detector can be calibrated to use any input optics as long as theyreflect the overall goal of the measurement.

100%

100%

50%

50%

30º 30º

30º 30º

60º 60º

60º 60º±±±±1.5%

Figure 2.10Relative spatial response of an ideal cosine diffuser (up)

and a radiance lens barrel (down).

Cosine Diffusers. A bare silicon cell has a near perfect cosine response, as do all diffuseplanar surfaces. As soon as you place a filter in front of the detector, however, you change thespatial responsivity of the cell by restricting off-angle light. Fused silica or optical quartz witha ground (rough) internal hemisphere makes an excellent diffuser with adequate transmissionin the ultraviolet. Teflon is an excellent alternative for UV and visible applications, but is notan effective diffuser for infrared light.

Figure 2.11Solar Global UV detector (tilted 37º and facing south) with a cosine diffuser

Radiance Lens Barrels. Radiance and luminance optics frequently employ a dual lens systemthat provides an effective viewing angle of less than 4º. The trade-off of a restricted viewingangle is a reduction in signal. Radiance optics merely limit the viewing angle to less than theextent of a uniform area source. This input optic is used to measured direct sunlight, but

29


mounted on a mobile sun-tracking platform (one loop per day) to follow the sun from sunriseto sunset.

Figure 2.12Solar Direct UV detector installed on a solar tracking system

Fibre Optics. Fibre optics allow measurements in tight places or where irradiance levels andheat are very high. Fibre optics consist of a core fibre and a jacket with an index of refractionchosen to maximise total internal reflection. Glass fibres are suitable for use in the visible, butquartz or fused silica is required for transmission in the ultraviolet. Fibres are often used tocontinuously monitor UV curing ovens, due to the attenuation and heat protection theyprovide. Typical fibre optics restrict the field of view to about ±20º in the visible and ±10º inthe ultraviolet.

Integrating Spheres. An integrating sphere is a hollow sphere coated inside with BariumSulfate, a diffuse white reflectance coating that offers greater than 97% reflectance between450 and 900 nm. The sphere is baffled internally to block direct and first-bounce light.Integrating spheres are used as sources of uniform radiance and as input optics for measuringtotal power. Often, a lamp is place inside the sphere to capture light that is emitted in anydirection.

High Gain Lenses. In situations with low irradiance from a point source, high gain inputoptics can be used to amplify the light by as much as 50 times while ignoring off angleambient light. Flash sources such as tower beacons often employ fresnel lenses, making nearfield measurements difficult. With a high gain lens you can measure a flash source from adistance without compromising signal strength. High gain lenses restrict the field of view to±8º, so cannot be used in full immersion applications where a cosine response is required.

SUMMARY OF THE CHAPTERThe three principal components of light (ultraviolet, visible and infrared) and theirwavelength distribution have been described. Typical solar spectra and air mass effect (sunposition) have been shown. The calculation of photonic fluxes (Einstein) from radiometricmeasurement (W) and spectral data (nm-1) has been introduced. The attenuating componentsof the atmosphere and their effect on UV radiation have been discussed to achieve a finalconclusion: UV spectrum is constant at a definite emplacement under certain circumstances.This characteristic permits the standardisation of the solar-UV spectrum, which is veryhelpful for finding a standard photon flux. The “cloud factor” index has been described and itscalculation from an UV-radiation database has been explained. Finally, solar radiationmeasurement systems have been described, with special emphasis on their main components.Their correct combination will permit accurate analysis of solar radiation and correctevaluation of the quantum yield of photochemical reactions.

BIBLIOGRAPHY AND REFERENCES

30


Hulstrom, R.; Bird, R.; Riordan, C. Spectral Solar Irradiance Data Sets for SelectedTerrestial Conditions. Solar Cells, 15, 365-391, 1985.

Iqbal, M. An Introduction to Solar Radiation. Academic Press, Canada. 1983.Riordan, C.J.; Hulstrom, R.L.; Myers, D.R.. Influences of Atmosferic Conditions and Air

Mass on the Ratio of Ultraviolet to Total Solar Radiation. Solar Energy ResearchInstitute (SERI)/TP-215-3895. 1990.


PART A. True or False?1. The Visible portion of the light is more powerful than the UV portion.2. The most important portion of Solar UV light at earth’s surface is between 100 and 280

nm.3. The solar radiation that reaches the ground level without being absorbed or scattered, is

called direct radiation.4. Any economic comparison desired between solar radiation and electric lamps, as the UV

photon source, requires knowledge of the photon flux incident on the solar reactor.5. The UV photon flux density on earth’s surface is in the range of thousands of photons per

square meter.6. Clouds absorb UV light.7. Clouds scatter UV light.8. The “cloud factor” for UV light has to be determined using specific UV radiometers.9. The best detector to measure infrared light is a phototube.10. A detector’s overall spectral sensitivity is equal to the product of the transmission of the

sensor and the responsivity of the filter

PART B.1. Which is the most important difference between ultraviolet, visible and infrared light?2. Which are the usual units to express the solar spectrum power?3. Why the Air mass at Sun zenith is called AM 1?4. Cite at least two reasons to justify the importance of knowing the solar spectrum in

photochemistry?5. Which is the typical unit in photochemistry? Why?6. Why clouds do not absorb UV light?7. Which is the usefulness of Equation 2.6?8. Convert the following fraction of power associated with each wavelength (corresponding

to a standardised spectrum between 300 and 400 nm) in solar spectrum power knowingthat the total measured power is 40 W/m2.

λ, nm fλ300 0.0002302 0.0004304 0.0007306 0.0011308 0.0017310 0.0023312 0.0032

9. Why is it very difficult the measurement of infrared light measured with quantumradiometer?

31


10. How is it possible to vary the spectral responsivity of a sensor without changing its type offilter?

AnswersPart A1. False; 2. False; 3. True; 4. True; 5. False; 6. False; 7. True; 8. True; 9. False; 10. False.

Part B1. Wavelength, ultraviolet light between 100 and 400 nm, visible light between 400 and 770

nm and infrared light between 770 and 106 nm.2. W m-2 nm-1 or W m-2 µm-1

3. Because this is the direct beam solar irradiance minimum path length through theatmosphere.

4. (a) The radiation that reaches the solar reactors is not constant. (b) In order to comparelaboratory results with solar results or to use the information obtained with lamps. (c) Thedetermination of the efficiency of the components of the solar reactor and the possiblemodifications to improve the conditions of photodegradation. (d) To perform economiccomparisons between solar radiation and electric lamps.

5. Einstein. Because it is 1 mol of photons and therefore, the quantum yield of aphotochemical reaction (rate 1 M/min) that absorbs 1 Einstein L-1 min-1 is 1.

6. Because pure water does not absorb UV light.7. To represent spectra in a standardised manner. This will permit the comparison between

spectra recorder at different sites, hour of the day and/or different seasons.8.

λ, nm UVλ, W m-2 nm-1

300 0.0080302 0.0160304 0.0280306 0.0440308 0.0680310 0.0920312 0.1280

9. Infrared light contains the least amount of energy per photon of any other band. Becauseof this, an infrared photon often lacks the energy required to pass the detection thresholdof a quantum detector.

10. Varying filter thickness.

32


3. EXPERIMENTAL SYSTEMS

AIMSThis unit describes the experimental systems necessary for performing pilot-plant-scale solarphotocatalytic experiments. Various laboratory set-ups are also shown in order to emphasisethe big differences between these small photoreactors and pre-industrial-scale pilot plants. Itoutlines the basic components of these pilot plants and the different possibilities for operatingthem. This chapter will review modelling of the experiments from a chemical engineeringpoint of view. Finally, an extensive overview of relationships between radiometermeasurements and the photons that actually reach the photoreactor is presented.

OBJECTIVESBy the end of this unit, you will know the most important features of large-scale outdoorphotocatalytic experiments and you will be able to do six things:1. Design a simple, versatile solar detoxification pilot system to fit the present and future

necessities of the research to be performed in it.2. Understand pilot plant operation and decide among different options.3. Calculate the kinetic constant using the appropriate method depending on pilot plant

characteristics and operation.4. Perform on-site calibration of solar-UV radiometers.5. Find a relationship between solar radiometer measurements and the photons that actually

reach the reaction.6. Employ chemical actinometers to validate all the calculations performed to obtain the

photon flux inside photoreactors.

NOTATION AND UNITSSymbol UnitsC or Ci Reactant concentration M or mg L-1

EUV Accumulated energy (per unit of volume) incident on thereactor.

kJ L-1

I Photon flux density Einstein s-1 m-2

IE Effective photon flux corresponding to the UV inside theabsorber of a photoreactor.

Einstein s-1 m-2

I*E Volumetric effective photon flux corresponding to the UV

inside the absorber of a photoreactor.Einstein L-1s-1

fλ Fraction of power associated with a wavelength nm-1

k First order kinetic constant min-1, h-1

N Number of incident photons Photons s-1 m-2

Q Flow rate L min-1, m3 h-1

r Reaction rate M min-1, mg L-1 min-1

Sp Surface area of the reflector of a photoreactor capturing theradiation.

m2

ST Surface area of the tube of a photoreactor. m2

texp Experimental time min, s, htR Residence time min, s, hUV Measurements provided by a radiometer W m-2

UVλ Ultraviolet irradiance associated with a wavelength W m-2 nm-1

UV*λ Spectral data calculated with the standard spectrum and

radiometric dataW m-2 nm-1

33


UVΣ Summation of the measurements provided by aspectroradiometer

W m-2

V Volume L, m3

VTOT Volume of the entire pilot plant. L, m3

x ConversionΦE Estimated quantum yield using moles of incident photons

instead of moles of photons absorbed by the catalyst.η Loss factors affecting the photon fluxλ Wavelength µm, nm,µλ Coefficient of absorptivity cm-1

τ One residence time min, s, h

3.1 LABORATORY SYSTEMSThe treatment of contaminated water necessarily includes the design of an efficientphotoreactor. Basic laboratory research on the process has mostly been performed withexperimental devices in which efficiency was not as important as obtaining appropriateconditions that would permit reproducibility of the results and exhaustive knowledge of theeffects of all the important parameters. This is correct when the goal is a fundamentalknowledge of the process, but not always sufficient to attempt a change of scale. Twodifferent concepts of laboratory photoreactors are presented here. One is a continuos stirredtank reactor (Figure 3.1) and the other a recirculating system (Figure 3.2).

Figure 3.1Laboratory typical stirred tank reactor.

Figure 3.1 shows a basic scheme of a microphotoreactor used in laboratory experiments. Thephotoreactor consists of a flask made of Pyrex with a flat bottom window constituted by anoptical filter generally made of quartz (fused silica) to let enter all the UV-visible radiationwith λ ≥ 220 nm. For an isothermal reaction, or when one wants to change the temperature ofthe medium for determining the activation energy of the reaction (see section 4.7), the systemis equipped with a jacketed envelope through with flows a temperature-constant fluid (e.g.water) delivered by a thermostat. There are several upper apertures to introduce the reactingmixture, the catalyst, a controlled gas atmosphere and also to withdraws samples, either fromthe slurry or from the gas phase through the ground top and the valve for analysis by GC (gaschromatography) or GC/MS (gas chromatography-mass spectrometry, see chapter 5). A goodstirring is obtained with a magnetic stirrer, curiously working perpendicularly to the rotatingmagnet stirrer, that it is at the right level. The determination of gaseous CO2 is made inoxygen confined atmosphere by sampling amounts of supernatant gas phase through theground top and the valve, linked to a GC by a stainless steel tubing.

34


The UV-light is provided by a xenon lamp introduced in a water-cooled envelope that isclosed by a silica optical disk and used for preventing the formation of ozone in the laboratoryatmosphere. All the IR beams, which could heat the slurry suspensions, are removed by awater cell, which contains an optical disk transparent to the wavelength domain desired. Theradiant flux (in W/m2) is measured for each experiment by a radiometer whose head is settledin the place of the photoreactor. The photograph represents a simple photoreactor, without acooled jacket and open to the air for a routine experiment of water detoxification byilluminated slurry of titania. A mere mixing is sufficient to provide gaseous oxygen to themedium (see section 4.2).

Figure 3.2Laboratory typical recirculating system.

Figure 3.2 shows a schematic and a photo of the recirculating system. Water, contaminantsand titanium dioxide are continuously recirculated through a 250 mL Pyrex reactor. Duringphotocatalytic degradation experiments some parameters as pH, CO2 and O2 can be monitoredin situ by specific electrodes (probes). Air is introduced inside the reactor flask through a flowcell and distributed via a sintered-glass tip to maintain a constant oxygen concentration in thesystem. Irradiation is carried out by a solar simulator equipped with a high-pressure xenon arclamp and a parabolic reflector. Since solar simulators contain a significant thermalcomponent, it is necessary to decrease the temperature using a small heat exchanger. Theentire reactor is fabricated with chemically resistant materials (Pyrex, Teflon and Viton).

3.2 SOLAR DETOXIFICATION PILOT PLANTSThe first outdoor engineering-scale reactor developed was a converted solar thermalparabolic-trough collector in which the absorber/glazing-tube combination had been replacedby a simple Pyrex glass tube through which contaminated water could flow (Goswami andBlake, 1996). Since that time, research all over the world has advanced a number of reactorconcepts and designs, including concentrating and non-concentrating reactors. The catalystcan be deployed in several ways, including as a fixed catalyst, slurry, or neutral-density large

35


particles. As mentioned above, a simple modification of the parabolic-trough solar thermalcollector was successfully developed and operated for experiments in which the catalyst wasdeployed in slurry. Parabolic concentrating-type reactors have been used for applications suchas groundwater remediation and removal of metals from water. A disadvantage ofconcentrating reactors is that they cannot concentrate diffuse solar radiation. This is not amajor problem for solar thermal applications, because diffuse radiation forms a small fractionof the total solar radiation. However, solar photocatalytic detoxification with TiO2 as acatalyst uses only the ultraviolet portion of solar radiation, and as much as 50 percent or moreof this may be present in diffuse form, especially at humid locations and during cloudy orpartly cloudy periods. Therefore, concentrating reactors would be more useful at dry, high,direct-insolation locations. Another disadvantage of concentrating reactors is that the quantumefficiency is low, due to a square root rather than linear dependence of rate on light flux (seeChapter 4). These disadvantages tend to favour the use of non-concentrating reactors. In anycase, these are aspects which will be discussed in detail in Chapter 6 and, whatever type ofcollector, solar detoxification pilot plants have several components in common and generaldesign characteristics which can be discussed here.

The design procedure for a pilot solar detoxification system requires the selection of a reactor,catalyst operating mode (slurry or fixed matrix), reactor-field configuration (series orparallel), treatment-system mode (once-through or batch), flow rate, pressure drop, pre-treatment, catalyst and oxidant loading method, pH control, etc., so a pilot plant has to be asversatile as possible to allow for these variables and, at the same time, provide sufficientconfidence in the experiments carried out in it. This is the crucial difference between pilot andprocess plants. A pilot plant must fulfil all the present and future requirements of the researchto be performed in it.

In Figure 3.3 a detailed drawing of a plant is given. Usually, a detoxification pilot plant isconstructed with several solar collectors. All the modules are connected in series, but withvalves that permit to bypass any number of them. Sampling valves are in the outlet of each ofthe modules. All the tubes and valves are black HDPE, material chosen because it is stronglyresistant to chemicals, weather-proof and opaque, in order to avoid any photochemical effectoutside of the collectors. There are storage-feeder tanks available, also made of HDPE andhaving different capacities, where the test mixtures are prepared. Four different operatingmodes are possible: recirculation, once-through, partial recirculation, and system cleaning.

O2

Pr

PIC

T

Pr O2

FI

FIC FCV

nmodules in series

T

Contaminant+ TiO2

Refrigeration system (optional)

GACfilter

Disposal

Clean water

SOLAR COLLECTOR

Closed valveOpened valveSensorsPump

Flow control

Batch tank

Holding tank

Discharge tank

Figure 3.3Photocatalytic Detoxification Pilot Plant (the batch mode is shown).

36


When concentrating solar collectors are used, the temperature of the water which flowsthrough them rises considerably. Obviously, the slower the flow rate used in once-throughexperiments and the longer recirculation experiments, the greater the increase of temperatureis. Therefore, to avoid evaporation and damage to plastics, cooling is necessary, and a closed-circuit water-cooling system has to be installed.

A centrifugal pump with an electric motor (calculated to provide sufficient flow when themaximum length of the system is used) has to be installed to move the treatment waterthrough the reactor. The flow rate (in batch mode) has to be such that it guarantees only asmall amount of reactant is converted each time through the reactor, and the concentrationthroughout the system remains relatively constant (this reasoning will become clear below).Either a flow-rate control loop made up of a flow meter connected to a controller, which inturn governs an automatic electric valve, or an electric pump with a speed controller has to beinstalled to regulate the flow to the rate desired. The most important sensors required for thesystem are temperature, pressure and dissolved oxygen (at least in the reactor outlet). Othersensors, such as pH, selective electrodes, etc., could be useful depending on the type ofexperiments to be carried out. As constant pure oxygen is required for the oxidation oforganics, an injection system at the reactor inlet allows oxygen to be added to the reactoreither at planned intervals by opening and closing in a predefined cycle, or continuously.Atmospheric oxygen can be also stirred into the reaction medium in the reservoir tank. In thiscase, the dissolved O2 is kept at a concentration of around 6-8 mg/L. An UV-radiation sensormust be placed in a position where the solar UV light reaching the photoreactor can bemeasured, permitting the evaluation of the incident radiation as a function of hour of the day,clouds, and atmospheric or other environmental variations. All these data have to be sent byan appropriate transmitter from the sensors to a computer, which stores the results for laterevaluation.

To clean the system, a drainage tube, with an active carbon filter to retain any organiccompound not decomposed during the experiments, must be hooked up to the sewagepipelines. Use of demineralised water (conductivity less than 10 µS, organic carbon content <0.5 mg/L) is recommended for cleaning as well as in the tests themselves.

3.3 OPERATION OF PILOT PLANTS

3.3.1 Once-through operationExperimental procedure begins when the pump is turned on and the system is filled with cleanwater. Those modules necessary are selected and the corresponding valves are set to bypassthe rest. Then the water is pumped through the circuit and the modules are covered.Obviously, the maximum pump flow rate is necessary for this procedure. The amountsrequired to obtain the initial concentrations of catalyst, contaminant and any other ingredientin the experiment are added to the holding tank. When the time needed for mixture to becompleted has expired, this is verified by taking samples at two different points in the reactorat the same time for analysis. A few minutes later two more samples are taken and, if the fourcoincide, the concentration of the reactives may be considered to be the same throughout thereactor. Simultaneously, the automatic control sets the flow rate (Q) which will then be keptconstant during the experiment, oxygen injection is activated and valves are adjusted so thatthe fluid goes to the discharge tank. After that the modules to be used are put into operation.This marks the beginning of the experiment.

37


The modules are kept illuminated a little longer (experimental time) than necessary to allowthe water in the holding tank to go through the reactor and approach the discharging tank.This time (texp) is:

Q

nVVt tube mod

exp

+= (3.1)

where Q is the flow rate, Vtube is the volume in the pipes between the modules and the tankand Vmod is the volume in each module, with n the number of modules in series. At this time,samples are taken at all the valves in the outlets of each of the modules in the experiment.This provides “n” number of samples with different residence or illumination times (tR,i) toenable determination of kinetics. Under these conditions, the reactor behaves according to theideal plug-flow model as explained later. The residence time corresponding to each samplecollected at the end of the experiment is calculated with the following equation:

Q

Vnt illui

iR =, (3.2)

where i is the number of modules through which the samples have passed before beingcollected and Villu is the volume in the illuminated section of each module. When the test isover, n samples have been obtained with a reactor residence time that is a function of the flowrate. Thus, if the procedure is repeated at a different flow rate, that group of samples has adifferent tR and the number of points (tR,i, Concentration) necessary to evaluate anyexperiment can be obtained.

3.3.2 Batch operationSolar detoxification pilot plants are frequently operated in a recirculating batch mode asdepicted in Figure 3.3. In this scheme, the fluid is continuously pumped between a reactorzone and a tank in which no reaction occurs, until the desired degradation is achieved. Thesystems are operated in a discontinuous manner by recirculating the slurry solution with anintermediate reservoir tank and centrifugal pump. This type of operation differs little from theprevious one.

When concentration of the reactives is the same throughout the reactor, oxygen injection (ifnecessary) is activated and the position of the valves is maintained so that the fluid begins andends up in the holding tank (now called the batch tank). The automatic control sets themaximum flow rate, which has to be such that it guarantees that only a small amount ofreactant is converted each time it goes through the reactor. Then the modules that are going tobe used are put into operation. This begins the experiment. Recirculation is continued and thetest lasts however long required, even up to several days. Samples may now be taken at any ofthe sampling ports, since as the system is in recirculation mode, tR is the same for samplestaken at any point in the system. The (t R,i, Concentration) pairs are thus obtained (Eq. 3.3).

iTOT

illuiR t

V

Vt exp,, = (3.3)

where VTOT is the volume of the entire pilot plant. Villu and VTOT are defined at the beginningof the experiment by the number of modules used and the level of water in the batch tank. Theexperimental time (texp) is the difference in time between the initial sample (initialconcentration of the pollutant, t = 0) and samples collected during the experiment (t > 0).

38


HoldingTank

DischargeTank

SOLAR REACTOR (n modules)

BatchTank

SOLAR REACTOR

Recirculation line

Villun1 niQ(L/min)

C0C1 Ci

C2(t)C1(t)

Figure 3.4Schematic of two pilot plant operation concepts: A once-through operation (top) and a batch

operation (bottom).

3.3.3 Modelling once-through and batch operationIn general, once-through operation can be modelled as plug-flow. The variation inconcentration (dC) over time (dt) for once-through experiments and for first order reactions(typical in photocatalysis) would be:

iRi tk

C

CisintegratedwhichkC

dt

dCr ,

0

ln:, −==−= (3.4)

where tR,i is calculated by Equation 3.2. For most laboratory-scale batch systems, the amountof conversion of reactant that passes through the reactor is small, and the concentrationthroughout the system is relatively constant. In that case, the entire system can easily bemodelled as a simple batch system in which only a fraction (i.e., the part in the reactor) of thetotal mixture is undergoing reaction. In this case, Equation (3.4) above can also be applied,but using Equation 3.3 to calculate tR,i.

In a large field system, the amount of conversion each time the mixture passes through thereactor is noticeable. As the relatively clean water in the reactor is mixed with the “dirty”water in the batch tank, the water sent from it to the reactor has a lower and lowerconcentration. Because the rate usually decreases with concentration, the overall rate in thereactor responds likewise. Thus, unless properly accounted for, the presence of the tank willalter the perceived performance of the photoreactor. Two solutions are available to solve thisproblem. The first, and also the easier of the two, is to use a very high flow rate to achievelow conversion each time through. This high flow rate (Eq. 3.5) must allow more than 1 %conversion per pass (C1(t) ≈ C2(t)) to be avoided. This 1% has been selected because is veryfar from the error associated to any chemical method applied for Ci analysis.

xC

rVQ TOT

0

≥ (3.5)

where r is the reaction rate, C0 the initial concentration of the reactant and x the conversion(0.01). For example, if the initial reactant concentration is 50 mg L-1, the reaction rate is 0.1mg L-1 min-1, and VTOT is 250 L, the flow rate must be over 50 L/min. When this is notpossible because the reaction rate is very high, initial concentration very low and/or the pumpis not strong enough, the mathematical formulation is the following:

In steady-state, the concentration in the reactor outlet at time t, C1(t) is determined from the

39


inlet concentration (which does not change with time) and the reaction kinetics. However,because the system is a transient process, the normal steady-state plug flow reactor equationcannot be used to model the photoreactor. Because the inlet concentration changes with time,C2(t) is defined by what went into the reactor one residence time prior to t, C2(t-τ), and thekinetics. The batch tank is still modelled as a well-mixed tank. Solving these equations ismore difficult than the low-conversion-per-pass case and a numerical routine is required to fitthe data from the batch test. This routine also takes into account the volume of the pipingbetween the reactor outlet and the batch tank.

The defining equations for the system components shown in Figure 3.4 are given by basicchemical engineering principles. For the plug flow reactor:

∫ ∫−=1

2

*

0

C

Cd

r

dC ττ (3.6)

In the batch process, both concentrations vary with time, that is, C1 = C1(tR) and C2 = C2(tR).A solution for C1(tR) and C2(tR) for all tR can be obtained if three different time regimes aredefined as follows:• Regime 1: tR = 0 (Reactor exposed to sunlight at t = 0). By definition we have C1=C2=C0

• Regime 2: 0 < tR <τ. Under these conditions, the fluid exiting the reactor has beenilluminated for less than one complete reactor residence time. In Eq. 3.6, the time spent inthe reactor is equal to tR, and the starting concentration is C0, thus:

∫ ∫−=1

0 0

C

C

tR

dr

dC τ (3.7)

• Regime 3: tR≥τ. Now the fluid exiting the reactor has been illuminated for one completeresidence time and τ*= τ. However, to solve for the reactor outlet concentration at time t,it must be known what went into the reactor at time tR-τ. This defines the lowest limit onthe rate integral:

∫ ∫−−=

)(

)( 0

1

2

R

R

tC

tCd

r

dCτ

ττ (3.8)

Having defined these three regimes, the balance of material in the well-mixed tank may nowbe written:

[ ])()()(

212

RRbatchR tCtCQV

dt

tdC−=− (3.9)

where Vbatch is the volume in the batch tank. Now, only linking Eq. 3.9 with the differentforms of Eq. 3.6 remains to be done. To do this, we must define an expression for the rate. Asimple first order rate expression (typical in photocatalysis) can be used as an example (seeEq. 3.4). The algorithm that solves Eq 3.6 for C1(t) as a function of C2, at the different timeregimes using first-order kinetics is the solution:• Regime 1: C1=C2=C0

• Regime 2: C1(t)= C0 exp(-kt)• Regime 3: C1(t)= C2(t-τ) exp(-kτ)

This expression for C1(t) is substituted into Eq 3.9 and the value for C2(t) predicted. Thisprediction is then compared to the experimental values for the concentration in the batch tank,with iteration until the k value which provides the best fit to the data is obtained. Thismethodology can be used to solve numerically for the concentration profile in any batchprocess. Although, in any case, this method is only really necessary when the recirculationflow is not high enough (See Eq. 3.5).

40


3.4 EVALUATION OF SOLAR UV RADIATION INSIDE PHOTOREACTORSSolar ultraviolet radiation is an essential parameter for the correct evaluation of data obtainedduring photocatalytic experiments in a solar water decontamination pilot plant. The kineticconstants of photocatalytic processes can be obtained by plotting substrate concentration as afunction of three different variables: time, incident radiation power inside the reactor andphotonic flux absorbed by the catalyst. Depending on the procedure, the complexity ofobtaining these constants, as well as their applicability, vary. When the photonic fluxabsorbed by the catalyst is used as an independent variable, extrapolation of the results isbetter. However, many parameters (incident photons passing through the reactor withoutinteracting with the catalyst, the directions in which light scatters, distribution of the sizes ofTiO2 particle suspended in the liquid, etc.) must be known for this, making it impractical in areactor used for pilot plant experiments.

Use of the experimental time as the calculation unit could give rise to misinterpretation ofresults, because the reactor consists of illuminated and non-illuminated elements. Largeexperimental reactors require much instrumentation and the reactor must also be as versatileas possible, substantially increasing the non-illuminated volume. With use of residence time,that is, the time the water has been exposed to the radiation, the conclusions would also beerroneous. This is because when time is the independent variable, the differences in theincident radiation in the reactor during an experiment are not taken into account. Furthermore,the more different the environmental conditions of the experiments to be compared (differentdays, different periods of the year or atmospheric variations), the more critical this becomes.The only way to avoid this problem is to use a relationship between experimental time, plantvolume, collector surface and the radiant power density measured by radiometers. Thispermits extrapolation of known data from one scale to another, as well as avoiding theproblem arising from a variable source of radiation (sunlight).

Recalling Eq. 2.4, the photon flux density I [Einstein s-1 m-2] is the number of incidentphotons per unit of surface and time:

dAdtN

Nd=I0

2

(3.10)

where N0 is Avogadro’s number (6.023 x 1023). Using Solar spectrum data (See Figures 2.6and 2.7) and the former equations in congruent units [S.I], it is possible to determine thephoton flux density. In any case, the UV radiation values vary from one location to another,and obviously, during the day and from season to season, so that it is necessary to know thesedata for a given location and in real time. The calculation of the photon flux in anyphotochemical reactor could be undertaken following the flow diagram shown in Figure 3.5.

41


EFFICIENCIESCALCULATIONNOT CORRECT

SPECTROPHOTOMETRIC MEASUREMENT OF UV

EQUATION TO OBTAIN PHOTON FLUX INSIDE THE PHOTOREACTORAS FUNCTION OF ON-LINE UV MEASUREMENTS.

ACTINOMETRIC EXPERIMENTS

EXPERIMENTAL ACTINOMETRIC QUANTUM YIELD =CALCULATED (BY PREVIOUS EQUATION) QUANTUM YIELD?

IS IT THE UV SPECTRUM CONSTANT?

DETERMINATION OF SPECTRAL COLLECTOREFFICIENCIES (REFLECTIVITY, TRASMISSIVITY, ETC..)

END

IT IS IMPOSSIBLE TOEVALUATE THE PHOTONFLUX WITHOUT ON-LINE

SPECTRUMMEASUREMENTS

NO

YES

NO

YES

RADIOMETERS CALIBRATION

IT IS POSSIBLE TO OBTAIN AN EQUATION WHICH CORRELATE THERADIOMETRIC AND SPECTROPHOTOMETRIC DATA

Figure 3.5Calculating procedure to find out the photon flux inside a solar reactor.

3.4.1 Radiometers calibrationSolar UV radiometers (See Chapter 2.5) provide data in terms of the incident W m-2 on eachof them, which gives an idea of the energy reaching any surface in the same position as theyare with regard to the sun. According to the manufacturer, UV instruments are sensitive toultraviolet radiation (See Figure 2.9). But in their technical description, the interval ofwavelengths covered in their calibration is not usually reported. Therefore, that specification,so important in the calculation of photonic flux, is not certifiable. In order to resolve thisuncertainty, on-site calibration is necessary. This is performed by comparing the dataprovided by the UV radiometers with those from a spectroradiometer, which gives data interms of W m-2 nm-1, in the same position as each of the radiometers. This gives:

∑=

=Σ =

n

nm

UVUVλ

λλ

300

(3.11)

where UVΣ (W m-2) is the summation of the radiation measurements (above 300 nm up to n in1 nm intervals) provided by the spectroradiometer. An example (with only a few measureddata) is shown in Table 3.1. The two ways of measuring UV at different times of day arecompared considering n = 400 nm, which is the ultraviolet-visible threshold, after which theradiometers should not measure.

Local time UV, W m-2 UVΣΣΣΣ, W m-2 UV-UVΣΣΣΣ, W m-2 100(UV-UVΣΣΣΣ)/UVΣΣΣΣ, %

10:31 13.69 11.68 2.01 +17.2

11:00 17.43 16.01 1.42 +8.9

12:59 26.96 25.42 1.54 +6.1

15:34 17.68 16.00 1.68 +10.5

Table 3.1Radiometric and spectroradiometric UV measurements at different times of day compared

considering n = 400 nm

42


In view of these results, the conclusion is that the UV radiometer measures beyond 400 nm. Inorder to find the real interval, the same procedure is carried out for n values over 400 nm at 1nm intervals. The results of the previous example are given in Table 3.2, together with the %of error at each interval.

UVΣΣΣΣ(1), W m-2

UV, W m-2 n=401 n=402 n=403 n=404 n=405

13.69 11.99(+14.2) 12.33(+11.0) 13.30(+2.9) 13.53(+1.2) 13.10(+4.5)

17.43 16.42(+6.2) 16.87(+3.3) 17.33(0.0) 17.79(-2.1) 18.25(-4.7)

26.96 26.01(+3.6) 25.4(+6.1) 27.32(-1.3) 27.0(0.0) 28.65(-6.3)

17.68 16.39(+7.6) 16.81(+5.2) 17.24(+2.5) 17.67(0.0) 18.51(-4.4)

Table 3.2Radiometric and spectroradiometric UV measurements at different times of day compared

considering n > 400 nm.It seems that, taking as the upper limit a wavelength of 403-404 nm, the radiation measuredby both instruments is in agreement. The radiometer measurement interval is fundamental forthe calculation of the photon flux that reaches the interior of the reactor. The alternative mightbe on-line measurement of the solar spectrum with a solar spectroradiometer, but this is veryoften not possible because of the price and/or the manpower required for a spectroradiometer.

3.4.2 Correlation between radiometric and spectroradiometric dataNow, recalling Eq. 2.6, it is possible to standardise the UV spectrum up to the radiometer cut-off wavelength. In this case, 404 nm is used. It is also used as an example in the abovecalculations to clarify the explanation, but similar reasoning could be applied to anywavelength interval.

1fthereforeUV

UVf

nm

nmnm

nm

=,404

300404

300

∑∑

=

==

=

=λ

λλλ

λλ

λλ (3.12)

If the UV radiation spectrum is assumed to have a fixed shape similar to that in Figure 2.6, astandardised spectrum is available in the wavelengths measured by the radiometer. Therefore,using the standardised spectrum and the irradiance data (W m-2) measured by the radiometer,the spectral distribution can be calculated for all of these data:

UVfUV λλ =* (3.13)

where UV*λ are the spectral data calculated with the standard spectrum (fλ) and the radiometer

data. Therefore, the number of photons is only a function of the intensity (measurable in realtime with the radiometers). Once the spectral distribution of the radiometer measurements isknown, the number of photons incident per unit of time and surface (N) corresponding tothose measurements can be found. Recalling Eq. 2.3, which relates the number of photonsfrom a given polychromatic source of light to the energy corresponding to each wavelength,this can be transformed for this case into the following (using summations of discrete values):

λλ

λλ∑

=

=− =

nm

nmnm UV

hcN

404

300

*404300

1(3.14)

43


where N300-404nm are incident photons. But, as the spectral distribution is assumed to beconstant, Eqs. 3.13 and 3.14 yield:

λλ

λλ∑

=

=− =

nm

nmnm f

hc

UVN

404

300404300 (3.15)

Therefore, the wavelength considered equivalent to the summation would be:

λλ

λλ∑

=

=

nm

nm

f404

300

= 368.79 nm

To calculate the values corresponding to other intervals, the same procedure is followed.Returning to Eq. 3.15, the number of photons corresponding to the average radiation at anygiven instant for the radiometer is:

UV101.856N 18nm ×=−404300 (3.16)

where N300-404nm is the incident photons, between 300 and 404 nm, per m2 and second, whenUV is measured in W m-2 and λ in nm, with speed of light c = 2.988 × 1017 nm s-1 and thePlanck’s constant h = 6.626 × 10-34 J s. If Einstein (moles of photons) is used as the unit, theresult is:

UVN

NI nm

nm6

0

404300404300 10083.3 −−

− ×== (3.17)

3.4.3 Collector efficiencyAll the factors contributing to collector efficiency are shown in Figure 3.6. With thecombination of these factors as defined below and Eq. 3.17, the photon flux may becalculated.• ηC includes all the errors produced while the collector was built up.• ηR,λ is the reflectivity of the parabolic mirror surface (usually aluminium, see Chapter 6).

Since the reflection spectrum does not vary over a wide range and that variation isuniformly distributed over the whole interval (300-400 nm), an average spectraldistribution may be considered. However, as the surface is outdoors and could get dirty ordamaged, it has to be measured periodically, in which case the value is ηR,i. With frequentcleaning (before every experiment), it may be considered constant and is then ηR.

• ηT,λ is the spectral transmissivity of the absorber tube.

11'

3 42

1+1’ I2 Reflected I 3+4 I inside photoreactor, IE

ηηηη Efficiency factors

ηηηη λλλλR,

ηηηη λλλλT ,

MIRROR

ABSORBER

Figure 3.6Drawing of the various loss factors (η) affecting the photon flux (I) inside a photoreactor.

The solar radiation that reaches ground level without being absorbed or scattered is calleddirect radiation, the radiation that has been dispersed but reaches the ground is called diffuseradiation and the sum of both is called global radiation (see Chapter 2). Global radiation iscollected directly by the transparent absorber tube without intervention of the collector. In

44


Figure 3.6 the path followed by I until it arrives inside the absorber tube is shown. It mustarrive at the surface and be reflected (part is lost due to ηR,i) in the right direction by the realmirror surface (ηC), before penetrating (ηT,λ) in the tube. Furthermore, the parabolicconcentration factor must also be considered (ratio of surface area of the parabola capturingthe radiation and surface area of the tube, Sp/ST) if it is a concentrating solar collector (seeChapter 6). Therefore, the effective photon flux corresponding to the UV inside the absorber(IE) is:

= ληηη ,, ,,,, TiRC

T

PE S

SIfI (3.18)

Only the transmissivity of the glass, ηT,λ, affects the radiation reaching the absorber withoutbeing reflected by the mirror (1’). In this case Eq. 3.18 is simplified:

),( ,ληTE IfI = (3.19)

Since ηT,λ depends on the wavelength, in order to evaluate IE, Eq. 3.15 must be recalculated asfollows:

ληηηλ

λλλ∑

=

=

××=

nm

nmTiRC

T

PE f

hc

UV

S

SI

404

300,, (3.20)

where, if UV is W m-2, the units of IE are Einstein m-2 s-1 incident in the inside of the tube.Sp/ST, as accurate as possible, is used to determine IE in each case. For this it is necessary tomake the corresponding trigonometric calculations based on the collector characteristics.Once this is known, the same ratio can be calculated for the reactor volume. Therefore, Sp/ST

in Eq. 3.20 is substituted by this ratio (collector area/collector volume). In this way, photonflux is obtained in units congruent with the reaction rate (M s-1) so that an estimated quantumyield (ΦE) similar to that in Eq. 2.1 can be obtained, but using moles of incident photonsinstead of moles of photons absorbed by the catalyst.

∗=ΦE

E I

ratereaction(3.21)

where I*E are Einstein L-1s-1 of UV irradiance incident inside the tube. To calculate the values

corresponding to other wavelength intervals, the same procedure is followed.

3.4.4 Actinometric experimentsIn a chemical actinometer, the photochemical conversion is directly related to the amount ofphotons absorbed. This method has been used since the 30’s but due to recent progress inradiation sensors, semiconductor and electronic equipment development, physical measuringdevices have become more popular for photochemistry. In the case of reactors with simplegeometries, they are preferable because they are very quick, simple and precise. In the case inhand, the chemical actinometer is needed for validation (see outline of work in Figure 3.5) ofall the calculations performed to obtain the equations detailed above. A good chemicalactinometer meets the following specifications:• The photochemical system should be simple and the reaction should be reproducible, under

well-defined and easily controlled conditions. The quantum yields should be well knownfor a wide range of wavelengths, if polychromatic wavelength radiation has to bemeasured.

• The quantum yield should be independent of the intensity of radiation, actinometerconcentration and temperature (large pilot plants cannot be thermostatised.)

• The reagents and products should be reasonably stable, so errors do not arise between thetime the sample is taken and the time it is analysed.

45


• The analytic methods should be simple and the reagents should be easily synthesised and,even better, commercially available. This is, if possible, much more important in the caseof pilot plants, because of the large volumes of actinometer that have to be prepared.

• The system should be sufficiently sensitive for low radiation intensities and the evaluationof photons absorbed should be simple.

For this case, a common uranyl-oxalate system is explained as an example. Very completeinformation on actinometric systems has been summarised by Kuhn et al. (1989):

( )( )

UOhv

UO

UO H C O UO CO CO H O

22

22

22

2 2 4 22

2 2

+ + ∗

+ +

→

+ → + + +*

(3.22)

Absorptivity of this solution may be estimated from:

( )Absorbed Photons

Incident Photonsb= − −1 2exp µλ (3.23)

where b is the inside reactor radius, 2b is the light path length and µλ is the coefficient ofabsorptivity. General characteristics of the actinometer and experimental details may be foundin Curcó, et al. (1996) and references therein.

Photon flux inside the reactor during the actinometric experiments may be calculated fromEq. 3.20. However, in this case the range of wavelengths is widened to 536 nm (actinometeractivity cut-off) and solar spectra up to 536 nm is used. If the photon flux inside the reactorand the characteristics of the actinometer are known, the oxalic acid degradation rate can becalculated from IE(300-536), µλ, oxalic acid quantum yield in the mixture and the area/volumeratios in each case. The comparison between this calculated acid decomposition rate and therate measured in the actinometric experiments gives an idea of the validity of the equations inpoint 3.4.3. The oxalic acid degradation rate calculated by the experiments or by Eq. 3.21 hasto be quite close. Therefore, all the equations developed are assumed to be valid. However,actinometry would not be useful for finding the photon flux, at any given time, inside aphotoreactor illuminated by solar radiation. The variations in solar intensity due to changes inweather, and the impossibility of using an actinometer inside the reactor while thephotocatalytic experiments are being performed, make it impossible.

3.5 SIMPLIFIED METHOD FOR THE EVALUATION OF SOLAR UV RADIATIONINSIDE PHOTOREACTORS

The above procedure is usually the most appropriate but, several parameters are very often notavailable: on-site solar spectrum, collector reflectivity, absorber transmissivity, etc. In thesecases, a shorter procedure can be used. Although not the best solution, it is frequently veryuseful. Nevertheless, an UV radiometer mounted at the same angle as the solar collector isalways necessary for data evaluation. This radiometer sends a signal to a computer in whichthe data (UV) are stored. As radiation data are collected continuously, it is very easy tocalculate the average incident radiation on the collector surface ( nUV ), for each period of t,and apply Eq. 3.24 to that average. Consequently, the amount of energy collected by thereactor (per unit of volume) from the start of the experiment until each sample is collectedmay be found by:

t-t=t

V

SUVt+E=E

1nnn

TOT

Pnn1-nUV,nUV,

−∆

∆(3.24)

46


where tn is the experimental time at which each sample was taken, SP is the collector surface,VTOT is the total plant volume and EUV,n is the accumulated energy (per unit of volume, kJ L-1)incident on the reactor for each sample taken during the experiment. This procedure is correctonly if the rate of the photoprocesses under examination is linear with the intensity ofradiation (see Chapter 4). For this purpose, several different experiments must be carried outin the photoreactor to determine this effect at very different solar radiation conditions(morning, noon, with and without clouds, summer, winter, etc) without changing the kineticorder of the reaction.

UV, W m-2

-r,µ µµµ

M m

in-1

Figure 3.7Typical photocatalytic degradation (-rDCA) in a solar pilot plant under different UV solar light

intensities.

The results of a typical test are shown in Figure 3.7. The reaction rate is calculated at thebeginning of the experiment (zero order) and UV corresponds to exactly the same periodused for the calculation of –r. Therefore, the rate is linear with regard to the radiation intensityunder the same experimental conditions and in the same reactor where Eq. 3.24 is going to beapplied. Figure 3.8 shows the improvement obtained using this equation to calculate thereaction rate in a two-day photocatalytic degradation experiment. Obviously, UV powerchanges during the day and clouds, on the first day, make this variation still more noticeable,but with Eq. 3.24, the data for both days can still be combined and compared with otherphotocatalytic experiments. Consequently, with EUV, the reaction rate (-r) is expressed interms of mass of reactant degraded per kJoule of UV incident on the collector surface. If r isexpressed in these units, collector efficiency is already included in it through the use ofincident surface radiation, since different r with the same substance and different solarcollectors, means collector efficiency is different.

47


t, min

EUV, kJ L-1

1st day2nd day

UV

, W m

-2

1st day 2nd day

C, m

g L

-1C

, mg

L-1

Figure 3.8Photocatalytic degradation in a solar pilot plant. Plots of concentration ( ) as a function of

experiment time (up) and accumulated energy (down). Solar UV power throughout theexperiment is also shown.

SUMMARY OF THE CHAPTERA pilot plant has to be as versatile as possible in order for any photocatalytic experiment to beperformed with sufficient confidence. The pilot plant must fulfil all the actual and futurenecessities of the research to be performed in it. They may be operated in once-through orrecirculation mode. The calculation of residence time in the reactor is different in each caseand thereby, the conversion. In any case, the use of residence time to calculate reaction rates(very common in chemical engineering) is not always recommendable because solar power isnot constant. The knowledge of the wavelength interval at which the UV radiometers used areactive is basic to data treatment. On-line measurement of UV power is essential and reactorefficiency must always be calculated considering, at least, the amount of energy incident on it.Therefore, it is necessary to find a relationship between the radiometer measurements and thephotons that actually reach the reaction. The estimation of quantum yields using Eq. 3.21enables results obtained in the photoreactor to be compared with others and thereby, make useof existing literature on the compounds that are going to be tested. This is very importantwhen working with a large reactor, where any test means a considerable outlay of time andexpense. So the more information available “a priori”, the fewer experiments are necessaryand the faster useful conclusions may be arrived at. Actinometric experiments have beenshown to be useful to contrast the validity of all those equations related to photon fluxcalculation.

BIBLIOGRAPHY AND REFERENCES.Curcó, D.; Malato, S.; Blanco, J. and Gimémez, J. Photocatalysis and Radiation Absorption

in a Solar Plant. Sol. En. Mat. Sol. Cells, 44, 199-217, 1996.Goswami D. Y. and Blake D. M. Cleaning up with Sunshine. Mechanical Engineering,

August, 56-59, 1996.Kunh, H.J., Braslavsky, S.E. and Schmidt, R. Chemical actinometry. Pure &Appl. Chem.,

Vol. 61, 2, 187-210, 1989.

48



PART A. True or False?1. The photocatalytic laboratory microreactors must be open to the atmosphere.2. The power of the pilot plant pump should be low for avoiding high cost.3. The residence time corresponding to each sample collected at the end of once-through

experiments is calculated with the following equation:

TOT

illuiiR V

Vnt =,

4. In the general case once-through operation system can be modelled as a plug flow reactor.5. The kinetic constants of photocatalytic processes can be only obtained by plotting

substrate concentration as a function of time.6. The interval of wavelengths covered in their calibration is not usually reported in UV

instruments technical description.7. The spectral transmissivity of the photoreactor absorber tube does not affect the

photocatalytic reaction rate.8. Quantum yield estimated using moles of incident photons is always less that quantum

yield calculated using of moles of photons absorbed by the catalyst.9. The quantum yield of a chemical actinometer should increase when the intensity of

radiation does.10. The amount of energy collected by the reactor (per unit of volume) from the start up of the

experiment until each sample is collected, calculated by the following equation, is only asimplified procedure.

t-t=t

V

SUVt+E=E

1nnn

TOT

Pnn1-nUV,nUV,

−∆

∆

PART B.1. Why is it necessary filtering IR in laboratory photoreactors?2. Why a pilot plant has to be as versatile as possible?3. Which are the main characteristics of the material to be used for connecting the pilot plant

photoreactors?4. Which are the most important sensors to be installed throughout and at a photocatalytic

pilot plant?5. Why is it very important the flow rate in batch experiments?6. Why residence time is not the best variable for obtaining kinetic constants in solar

photocatalysis?7. Which are the essential data, necessary for obtaining an equation similar to the following,

for correlating radiometric measurements and photonic flux inside a photocatalytic solarphotoreactor?

)(0

2121 UVf

N

NI == −

−λλ

λλ

8. Which is the wavelength considered equivalent to the following normalised spectrum?

λ, nm fλ300 0.0002310 0.0023320 0.0063

49


330 0.0109340 0.0118350 0.0124360 0.0127370 0.0159380 0.0156390 0.0160

9. If Einstein (moles of photons) is used as the unit, which is the number of photonscorresponding to the average radiation at any given instant in the previous example?

10. Which are the essential data for radiation evaluation in a solar photocatalytic reactor?

Answers

Part A1.False; 2. False; 3. False; 4. True; 5. False; 6. True; 7. False; 8. True; 9. False; 10. True.

Part B1. Because IR beams could heat the slurry suspensions very quick.2. Because the necessity of accomplishing all the experiments with enough confidence and

fulfilling all the actual and future necessities of the research to be performed in it.3. All the tubes and valves must be strongly resistant to chemicals, weather-proof and

opaque, in order to avoid any photochemical effect outside of the photoreactors.4. Temperature, pressure, dissolved oxygen and solar irradiation.5. Because it is directly related with the conversion per pass and, therefore, with the

modelling of the photoreactor.6. Because when time is the independent variable, the differences in the incident radiation in

the reactor during an experiment are not taken into account.7. Solar spectra.

8. 1390

300

=∑=

=

nm

nm

fλ

λλ ; nmf

nm

nm

6.373390

300

=∑=

=

λλ

λλ

9. 3.135 x 10-6 UV (Einstein s-1 m-2)10. Radiation data from a radiometer mounted on the same angle, as the solar collector.

50


4. FUNDAMENTAL PARAMETERS IN PHOTOCATALYSIS

AIMSThis unit describes the fundamental parameters related to heterogeneous photocatalysisreactions. Examples for better comprehension of oxygen, pH, catalyst concentration, initialsubstrate concentration, radiation intensity and effect of temperature are also shown. Itoutlines the basic tests for understanding experimental system behaviour when theseparameters change and why these changes affect the photocatalytic reaction rate. Finally, anoverview of the different ways of evaluating quantum yields is presented.

OBJECTIVESBy the end of this unit, you will know the main aspects of experimental photocatalysisvariables and you will be able to do 7 things:1. Understand why blank experiments are always necessary in photocatalysis.2. Design experiments that avoid the effects of oxygen and pH and, thereby, reduce the

number of variables affecting the tests.3. Comprehend the influence of catalyst concentration and how to find the optimum catalyst

mass in an experimental reactor.4. How a Langmuir Hinshelwood (L-H) model, which, although it does not explain the

photocatalytic mechanisms, may be used in heterogeneous photocatalysis to obtain thekinetic constants for reactor optimisation.

5. Arrange experiments to obtain L-H parameters.6. Describe the interdependence of the photocatalytic reaction rate and illumination intensity

and when this relationship is directly proportional.7. Determine useful parameters for describing photocatalysis efficiency without knowing the

amount of photons absorbed by the system and, why quantum yield is almost impossibleto be calculated in heterogeneous photocatalysis experimental systems.

NOTATION AND UNITSSymbol UnitsC or Ci Reactant concentration M or mg L-1

C0 Initial reactant concentration M or mg L-1

CS Solvent concentration M or mg L-1

E Activation energy kJ mol-1

I Photon flux corresponding to the UV inside a photoreactor. Einstein L-1 s-1

k’ Apparent reaction rate constant min-1

kr Reaction rate constant M min-1

K Reactant adsorption constant M-1

KS Solvent adsorption constant M-1

L-H Langmuir Hinshelwood modelNa Number of photons absorbed Photons s-1

pKa -log(acid ionisation constant)pO2 Oxygen partial pressure atmr Reaction rate M min-1

∆n Number of reacting molecules Molecules s-1

Φ Quantum yieldθx Fraction of surface covered by the substrateζr Relative photonic efficiency

51


4.1 DIRECT PHOTOLYSISAs mentioned in Chapter 1, some pollutants can only be dissociated in the presence of UVlight. For this, the pollutant must absorb the light of the lamp (or the sun) with a reasonablephotodissociation quantum yield. Although organic pollutants absorb light over a wide rangeof wavelengths, this is generally stronger at the lower wavelengths. However, such naturalphotodegradation is usually very slow. For example, at least 10 days under perfectly sunnyconditions is necessary to reduce 50 mg/L acrinathrin to half. One half of 100 mg/Lpentachlorophenol at pH 7.3 decomposes in 48 hours. So the photolytic reaction rate is usuallydifferent from one substance to another even in the same experimental device. The photolytichalf-lives of hundreds of substances has been summarised by Tomin (1994).

In any case, the focus here is on fundamental photocatalytic parameters and therefore thephotolytic effect will be discussed from this point of view. These tests have to be performed inorder to find out the decomposition rates without the semiconductor. As TiO2 readily sticks tothe glass in the photoreactors, it is necessary to carry out these tests at the beginning, beforethe catalyst comes into contact with the photoreactors. In pilot-plant-scale experiments,removal of the thin coating of catalyst on the tubes after TiO2 suspensions have circulatedthrough them is a very hard, complex and expensive task. In laboratory tests, TiO2 can only beremoved with an ultrasonic cleaner or by abrasion. This type of experiment will focus ondemonstrating the absence or evaluating the importance of the following effects:• The treatment is not feasible without a catalyst.• Increase in temperature (due to illumination) in the reactor does not cause product loss.• There is no adsorption of pollutant or its metabolites in the materials of the pilot plant.After these tests have been performed, the photocatalytic experiment results may beconsidered accurate and the kinetic parameters can be determined properly. Any side effect ofthe photocatalytic reaction rate can be quantified and subtracted from the global rate, resultingin the real photocatalytic reaction rate.

4.2 INFLUENCE OF OXYGENIn semiconductor photocatalysis for water purification, the pollutants are usually organic and,therefore, the overall process can be summarised by Eq.4.1. Given the reaction stoichiometryof this equation, there is no photomineralization unless O2 is present. The literature provides aconsensus regarding the influence of oxygen. It is necessary for complete mineralization anddoes not seem to be competitive with other reactives during the adsorption on TiO2 since theplaces where oxidation takes place are different from those of reduction (See Figure 1.5). TheO2 avoids the recombination of e-/h+ (Eq. 1.24) and, photoactivated oxygen (O2

•-) also reactsdirectly (Table 1.3).

acidsmineralOHCOOpollutantorganic torsemiconduc

energybandgapultra

+++ →−

222 (4.1)

The concentration of oxygen also affects the reaction rate, which is faster when the partialpressure of oxygen (pO2) in the atmosphere in contact with the water increases. In any case, itseems that the difference between using air (pO2 = 0.21 atm) or pure oxygen (pO2 = 1 atm) isnot drastic (See Figure 4.1). In an industrial plant it would be purely a matter of economy ofdesign. In Figure 4.1 it is clear that when all the oxygen contained in the water has beenconsumed, photodecomposition of TOC comes to a halt. At the moment injected oxygenreaches the reactor, photodecomposition continues. Therefore, injection of pure O2 becomesnecessary in once-through experiments (See Figure 3.3 and Figure 3.4) at low flow rates. At

52


high flow rates or with recirculation, the addition of oxygen is not always necessary since theillumination time per pass is short. The water again recovers the oxygen consumed when itreaches the tank (open to the atmosphere and stirred).

0 10 20 30 400.0

0.5

1.0

1.5

2.0

2.5

3.0

TOC

O2

C/C

0

Illumination time, min

0 20 40 60 80 100

air

reac

tion

rat

e

pO2

Figure 4.1Effect of the concentration of dissolved oxygen on photocatalytic mineralization. [O2]0 = 8.5

mg L-1. In the inset, the usual effect of partial pressure of oxygen on the photocatalyticreaction rate is shown.

4.3 pH INFLUENCEThe pH of the aqueous solution significantly affects TiO2, including the charge of the particleand the size of the aggregates it forms. The pH at which the surface of an oxide is unchargedis defined as the Zero Point Charge (pHzpc), which for TiO2 is around 7. Above and below thisvalue, the catalyst is negatively or positively charged according to:

++ +↔− HTiOHTiOH 2 (4.2)+− +−↔− HTiOTiOH (4.3)

The equilibrium constants of these reactions (Kormann et al. 1991) are pKTiOH2+ = 2.4 andpKTiOH = 8.0, the abundance of all the species as a function of pH: -TiOH ≥ 80% when3<pH<10; -TiO- ≥ 20% if pH>10; -TiOH2

+ ≥ 20% when pH<3. Under these conditions, thephotocatalytic degradation of the ionisable organic compounds is affected by the pH. At firstsight, and for pollutants for which pKa is outside the range of 1-13, a very acidic solutionappears to be detrimental and a very basic solution to be favourable, since the variations aremodest or non-existent around neutrality. Because even at extreme pHs the change in thephotocatalytic rate is generally less than one order of magnitude, the TiO2 water treatmentdefinitively possesses an advantage over other processes. In many cases, a very importantfeature of photocatalysis is not taken into account when it is to be used for decontamination ofwater, is that during the reaction, a multitude of intermediate products are produced that maybehave differently depending on the pH of the solution. To use only the rate of decompositionof the original substrate could yield an erroneous pH as the best for contaminant degradation.

53


Therefore, a detailed analysis of the best pH conditions should include not only the initialsubstrate, but also the rest of the compounds produced during the process.

Mean particle-size measurements (presented in Figure 4.2) have been found to be constant atpH far from pH ≈ 7. 300 nm sizes increase to 2-4 µm when dispersion reaches pHzpc. The zerosurface charge yields zero electrostatic surface potential that cannot produce the interactiverejection necessary to separate the particles within the liquid. This induces a phenomenon ofaggregation and TiO2 clusters become larger. The large mean size in suspension at pH≈7becomes much smaller at pH far from 7. This effect is clearly related to the capability of thesuspension for transmitting and/or absorbing light. Furthermore, larger clusters sediment morequickly than small particles, thus the agitation necessary to maintain perfect homogeneitymust be more vigorous. In contrast, these variations in particle size could be an advantage forseparating the catalyst from water (by sedimentation and/or filtration) at completion ofphotocatalytic treatments.

2 4 6 8 10 120

500

1000

1500

2000

2500

3000

pHZPC

= 6.9

Mea

n si

ze, n

m

pH

Figure 4.2Mean particle size of TiO2 (P-25) suspended in water versus pH. [TiO2]=0.2 g/L.

To unambiguously exclude the pH effect, the reagents used to change the pH must containcounterions that have no effect on the rate of water treatment. Sodium hydroxide andhydrogen chloride or sulphuric acid have generally been chosen to produce basic or acid pH,respectively. Organic buffers must be avoided because they are potential consumers of •OH,and toxic inorganic acids or bases the same, for evident reasons.

4.4 INFLUENCE OF CATALYST CONCENTRATIONWhether in static, slurry or dynamic flow photoreactors, the initial reaction rates were foundto be directly proportional to catalyst mass. This indicates a truly heterogeneous catalyticregime. However, above a certain value, the reaction rate levels off and becomes independentof catalyst mass. This limit depends on the geometry and working conditions of thephotoreactor and is for a definite amount of TiO2 in which all the particles, i.e. the entiresurface exposed, are totally illuminated. When catalyst concentration is very high, aftertravelling a certain distance on an optical path, turbidity impedes further penetration of light inthe reactor. In any given application, this optimum catalyst mass ([TiO2]OPT) has to be foundin order to avoid excess catalyst and ensure total absorption of efficient photons.

54


UV

UVUV

UVUV UV

UVUV

REACTION CHAMBER

UV path length

LAMPPROTECTION

CHAMBER

UV LAMP

Coolingwater

a) b)

UV LAMP

UV

UV

c)

Figure 4.3Different laboratory photoreactor designs and zones of radiation penetration when

illuminated in different ways.

There are a number of studies in the literature on the influence of catalyst concentration onprocess efficiency. The results are very different, but from all of them it may be deduced thatradiation incident on the reactor and path length inside the reactor are fundamental indetermining the optimum catalyst concentration:

• If the lamp is inside of the reactor and coaxial with it (see Figure 4.3a), [TiO2]OPT is veryhigh (on the order of several grams per litre) if the path length is short (several mm). Onthe other hand, [TiO2]OPT is low (hundreds of mg per litre) if several centimetres arecrossed (see Figure 4.4 solid circles).

• If the lamp is outside (see Figure 4.3c), but the path length is short (1-2 cm max.), themaximum rate is obtained with 1-2 g L-1 of TiO2 (see Figure 4.4 solid squares).

• If the lamp is outside, but the path length is several centimetres long, similar to a reactorilluminated by solar radiation (see Figure 4.3b and 3.6), the appropriate catalystconcentration is several hundreds of milligrams per litre. An example of this is shown inFigure 4.4, where open circles corresponds to a photoreactor with a large diameter andsolid triangles to one with a smaller diameter, but both several centimetres. In this case, itis very clear than the optimum rate is attained at lower catalyst concentrations when thephotoreactor diameter is wider.

In all the cases described above, a “screening” effect is produced when the TiO2 concentrationis very high. The reaction rate diminishes due to the excessive opacity of the solution, whichprevents the catalyst farthest in from being illuminated. Besides, the larger the size (see Figure4.2), the less the opacity of the suspension. When the radiation comes from a parabolic troughcollector, something similar to what is shown in Figure 4.3b occurs. In any case, these areonly approximations based on the results obtained by different authors and, because of all ofthe above, it is always necessary to find out, experimentally, the optimum catalystconcentration for the plant studied. That is, the minimum concentration at which themaximum reaction rate is obtained. But it does not seem to be necessary to test a very widerange of concentrations. This effect is shown in Figure 4.4. Usually, the reaction rate increasesvery quickly with TiO2 concentration, but only at low catalyst concentrations (usually below100 mg/L). After that, the reaction rate stabilises and at very high catalyst concentrations the

55


reaction rate decreases. When this point is found, it is not really necessary to continuechecking any higher because no more useful information is going to be obtained.

0.0 0.4 0.8 1.2 1.6 2.00.0

0.2

0.4

0.6

0.8

1.0

1.2

rela

tive

rat

e

TiO2, g L-1

0.0 0.2 0.4 0.6 0.80.0

0.5

1.0TOC

rela

tive

rat

e

TiO2, g L-1

Figure 4.4Influence of catalyst concentration on the rate of photocatalysis (normalised rates have beenused to make it more easily understood) in different reactors (see text). The continuous line isonly to clarify the tendency. In the inset, relative reaction rates of degradation (two differentphotoreactors) and mineralization (TOC) of the same contaminant are shown. See text for

comments.

As shown in Figure 4.4, initial substrate behaviour is not always the same as that of TOC.This is due to the influence of intermediate products generated during the photocatalyticreaction. That is, the screening effect influences the decomposition of the original product in adifferent way than the other organic species present in the reaction and, in the examplepresented, to mineralise the contaminant it is unnecessary to use more than 0.2 g L-1 of TiO2.

4.5 INITIAL CONTAMINANT CONCENTRATION INFLUENCE

As oxidation proceeds, less and less of the surface of the TiO2 particles is covered as thecontaminant is decomposed. Evidently, at total decomposition, the rate of degradation is zeroand a decreased photocatalytic rate is to be expected with increasing illumination time (or theaccumulated energy, see Eq. 3.24). Most authors agree that, with minor variations, theexpression for the rate of photomineralization of organic substrates with irradiated TiO2

follows the Langmuir Hinshelwood (L-H) law for the same saturation-type kinetic behaviourin any of four possible situations: (i) the reaction takes place between two adsorbedsubstances; (ii) the reaction occurs between a radical in the solution and the adsorbedsubstrate; (iii) the reaction takes place between the radical linked to the surface and thesubstrate in the solution; and (iv) the reaction occurs with both species in solution. In allcases, the expression of the equation rate is similar to the L-H model. From kinetic studiesonly, it is therefore not possible to find out whether the process takes place on surface or insolution. Although the L-H isotherm has been rather useful in modelling the process, it isgenerally agreed that both rate constants and orders are only "apparent". They serve todescribe the rate of degradation, and may be used for reactor optimization, but they have nophysical meaning, and may not be used to identify surface processes. Thus, while not a useful

56


tool for describing the active species involved in oxidation, engineers and solar designersseem to have a common understanding on the usefulness of the unmodified L-H model.

0 5 10 15 20 250.0

0.2

0.4

0.6

0.8

1.0

Nor

mal

ised

Con

cent

rati

on

EUV, kJ/L or time, min

(C0 - C) = krt

0 5 10 150.0

0.5

1.0

1.5

2.0

2.5

ln (

C0/C

)

EUV

, kJ/L or time, min

ln (C0/C) = k’t

Figure 4.5Typical photocatalytic degradation. The insert shows data adjusted to Eq. 4.6.

Due to the above, for L-H standard data treatment, it is assumed that the reaction occurs onthe surface, which is also the assumption most widely accepted as possible. Under theseconditions, two extreme situations are defined to illustrate the adsorption on the catalystsurface: (I) substrate and water compete for the active catalyst sites and (II) the reactant andthe solvent are adsorbed on the surface without competing for the same active catalyst sites.According to the L-H model, the reaction rate (r) is proportional to the fraction of surfacecovered by the substrate (θx). In each case the following expression can be obtained:

CK+KC+1

KCk=k=dt

dC-=r

ss

rxrθ (4.4a)

KC+1

KCk=k=dt

dC-=r r

xrθ (4.4b)

where kr is the reaction rate constant, K is the reactant adsorption constant, C is heconcentration at any time, KS is the solvent adsorption constant and CS is its concentration (inwater CS ≈ 55.5 M). As CS >>C and, CS remains practically constant, the part of the catalystcovered by water is unalterable over the whole range of C and the previous equations can beintegrated:

tCK+1

Kk=C)-C(CK+1

K+

CC

ss

r0

ss

0ln (4.5a)

Ktk=C)-CK(+CC

r00ln (4.5b)

When C0 is very small, both equations can be reduced to an order one-reaction rate equation:

tk=CC0 ′ln (4.6)

So, if ln (C0/C) is represented versus t (or the accumulated energy, see Eq. 3.24), a line, theslope of which is the apparent reaction rate constant k’, should be obtained (see Figure 4.5).Likewise, at higher concentrations, both equations can be simplified by adjusting them to zeroorder, (C0 - C) = krt, as might be the case at the beginning of the experiment represented inFigure 4.5.

57


Using an L-H model, graphics similar to those depicted in Figure 4.6 may be obtained fromthe experimental data and from the linearisation of the previous equations. The effect of theinitial concentration on the degradation rate is shown in Figure 4.6a, where, due to thesaturation produced on the semiconductor surface as the concentration of the reactantincreases, it reaches a point at which the rate becomes steady. Figure 4.6b shows alinearisation of Eqs. 4.4, where the slope of that straight line is (1+KSCS)/krK, but could alsobe 1/krK. Finally, Eqs. 4.7 are obtained from Eqs. 4.5 when the concentration is half of theinitial (C/C0 = 0.5):

k

C0.5+

Kk

CK0.693=t

r

0

r

SS1/2

)1( +(4.7a)

k

C0.5+

Kk

0.693=t

r

0

r1/2 (4.7b)

r 0

C0

r 0-1

C0

-1

t 1/2

C0

a) c)b)Figure 4.6

Graphics related to the adjustment of data to a L-H type kinetic model

It should be emphasised that photodecomposition gives rise to intermediates, which could alsobe adsorbed competitively on the surface of the catalyst. The concentration of theseintermediates varies throughout the reaction up to their mineralization and thus, Eq. 4.4 mayalso take the following form:

( )∑=

=++=

n

iii

r

niCKKC

KCkr

1

,11(4.8)

where i is the number of intermediates formed during degradation (the solvent is also includedin the summation).

An understanding of the reaction rates and how the reaction rate is influenced by differentparameters is important for the design and optimisation of an industrial system. The L-Hreaction rate constants are useful for comparing the reaction rate under different experimentalconditions. Once the reaction constants kr and K have been evaluated, the disappearance ofreactant can be estimated if all other factors are held constant. Due to this, a series of tests atdifferent initial substrate concentrations has to be performed to demonstrate whether theexperimental results could be adjusted with this model. The concentration range has to bewide enough to allow correct fit of the L-H linearisation. This means, from the lowestconcentration at which the initial rate could be determined until the limit where therelationship between initial reaction and initial concentration remains constant (see Figure4.6). The results shown in Figure 4.5 correspond to an example of each of the experiments tobe carried out for this purpose. From the slope of the line corresponding to the starting pointsof each of the experiments, the initial degradation rate can been calculated. Figure 4.7 shows

58


the initial rate calculated. From 0.2-0.4 mM of substrate the initial rate is steady. At thisconcentration, catalyst saturation occurs and the reaction rate becomes constant.

0 25 50 750

1

2

3

1/r 0

1/C0

(krK)-1 = 0.0381

(kr)-1 = 0.638

0.0 0.2 0.4 0.6 0.8 1.00.0

0.5

1.0

1.5

2.0

0.0

0.5

1.0

1.5

2.0

r 0, m

M h

-1

C0, mM

Figure 4.7Initial degradation rate as function of the initial substrate concentration. The insert shows thelinear transformation of Eq. 4.4 from which the rate constant and the adsorption coefficient

can be estimated from the intercept and the slope, respectively.

The constants can be calculated from the graphic inserted in Figure 4.7 using the L-H model.

C

1

Kk

1+

k

1=

r

1

KC+1KCk=r

0rr00

0r0 → (4.9)

kr = 1.57 mM h-1

K = 16.75 mM-1

With known kr and K the time required to degrade a definite substrate concentration (C0)down to a certain level (C) may be found from Eq. 4.5. With this, definition of reactor volumeand surface (see Eq. 3.2 or 3.3) is possible. If accumulated energy is used instead of time,them Eq. 3.24 should be used for obtaining the kinetic constants and, afterwards, for reactordesign.

4.6 RADIANT FLUX INFLUENCE

Since 1990, the kind of solar technology, which should be involved in detoxification, has beenclarified. Initial experiments with parabolic troughs for water and dishes or furnaces for gas-phase treatments have evolved to lower flux systems. The reason for using one-sun systemsfor water treatment is firmly based on two factors, first the high percentage of UV photons inthe diffuse component of solar radiation and second the low order dependence of rates on lightintensity. It has been demonstrated by experiment that above a certain UV photon flux,reaction rate dependency on intensity goes down from one to a half order (Ollis, 1991;Herrmann, 1995). This does not seem to occur at a particular radiation intensity, as differentresearchers obtain different results, but presumably is significantly affected by experimentalconditions. Some authors impute the transition of r = f (I1.0) to r = f (I0.5), to the excess of

59


photogenerated species (e-, h+ and •OH). This can be demonstrated as follows. According tosection 1.4, the five basic simplified equations are:

+− +→+ hehTiO ν2 (4.10a)

energyNhe +→+ +− (4.10b)−− →+ AeA (4.10c)++ →+ DhD (4.10d)

ProductstesIntermediaDA →→+ +− (4.10e)and the rate-limiting step is the reaction in the adsorbed phase (Eq. 4.10e). Therefore:

[ ][ ]+−== DAkrr ee (4.11)

In an n-type semiconductor such as titania, the photo-induced holes are much less numerousthan electrons (photo-induced electrons plus n-electrons): [p+]<<[e-]. Therefore holes are thelimiting active species. Thence:

[ ][ ]+=== pDkrrr dde (4.12)

At any instant, one has:[ ] [ ][ ] [ ][ ]++−

+

−−==−−= pDkpekkrrrdt

pddbadba 0 (4.13)

Thence:

[ ] [ ] [ ]Dkek

Ikp

db

a

+= −

+ (4.14)

and:[ ]

[ ] [ ]Dkek

IDkkr

db

da

+= − (4.15)

From the above equation, it can be seen that the reaction rate is directly proportional to lightflux. In the case of high fluxes, the instantaneous concentrations [e-] and [p+] become muchlarger than kd[D] and [e-] ≈ [p+]. Therefore Eq 4.14 becomes:

[ ] [ ] [ ]b

a

b

a

k

IkpTherefore

ek

Ikp ≈≈ +

−+ 2

; (4.16)

The reaction rate becomes:

[ ]2/1

===

b

adde k

IkDkrrr (4.17)

which means that r is proportional to I0.5 and that the rate of electron-hole formation is greaterthan the rate of photocatalysis, which favours electron-hole recombination. Thereforeoptimum use of light power corresponds to the region where r is proportional to I.

RE

AC

TIO

N R

AT

E

FOTONIC FLUX

r = k I

r = kr = k I½

Figure 4.8

60


Relation between the photocatalytic reaction rate and the intensity of the radiation received.

At higher radiation intensities, another transition from r = f (I0.5) to r = f(I0) is produced. Atthis moment, the photocatalytic reaction is no longer dependent on the radiation received,depending only on mass transfer within the reaction. So, the rate is constant although theradiation increases. This effect may be due to different causes, such as a lack of electronscavengers (i.e. O2), or organic molecules in the proximity of the TiO2 surface and/or excessof products occupying active centres of the catalyst, etc. Actually, these phenomena appearmore frequently when working with a supported catalyst, and/or at slow agitation speeds,which implies less catalyst surface in contact with the liquid and less turbulence. It does notfavour reactant contact with the catalyst or dispersion of products from the proximity of thecatalyst to the liquid.

Many articles on this aspect of photocatalysis provide information on the light intensity atwhich the change of order is produced. The values found are very dissimilar. It may only beintuited that at an intensity of several suns (1 sunUV = 22 WUV m-2), quantum yield diminishes.This effect should be measured experimentally in each device, but this limit (several suns) isusually accepted as a general rule.

4.7 TEMPERATURE EFFECT

Because of photonic activation, photocatalytic systems do not require heating and operate atroom temperature. The true activation energy Et is nil, whereas the apparent activation energyEa is often very low (a few kJ/mol) in the medium temperature range (20ºC-80°C ). However,at very low temperatures (-40°C-0°C), activity decreases and activation energy Ea becomespositive. By contrast, at "high" temperatures (>70-80°C) for various types of photocatalyticreactions, the activity decreases and the apparent activation energy becomes negative. Thisbehaviour can be easily explained within the frame of the Langmuir-Hinshelwood mechanismdescribed above. The decrease in temperature favours adsorption, which is a spontaneousexothermic phenomenon. In Eqs. 4.4 θ tends toward unity, whereas KC becomes >>1. Inaddition, the lowering temperature also favours adsorption of the final reaction products,desorption of which tends to be the rate-limiting step. To the contrary, when temperatureincreases above 80°C, nearing the boiling point of water, the exothermic adsorption ofreactants is disfavoured and this tends to become the rate-limiting step.

In addition to these mechanical effects, other consequences of plant engineering must beconsidered. If temperature is high, the materials (see point 3.2) used for the plant should betemperature-resistant (more expensive), and oxygen concentration in water decreases.Consequently, the optimum temperature is generally between 20 and 80°C. This absence ofneed for heating is attractive for photocatalytic reactions carried out in aqueous media and inparticular for environmental purposes (photocatalytic water purification). There is no need towaste energy heating water that already possesses a high thermal capacity.

4.8 QUANTUM YIELD

In photochemistry, a concept called quantum yield is used to evaluate the results obtained andcompare different experimental conditions. Recalling Eq. 2.1 (Φ = ∆n Na

-1), the quantumyield of a photochemical reaction is defined with regard to the number of reacting moleculesand the number of photons absorbed.

61


In this book, a heterogeneous system made up of a suspended solid (TiO2), a gas (O2) inbubbles and/or dissolved aqueous solution of a multitude of compounds (initial substrate,intermediates, H+, anions…) is described. Finding out the amount of photons absorbed by thecatalyst, from the behaviour of the radiation incident on a suspension such as this, is verydifficult. In order to calculate this, if so desired, one would have to: a) evaluate the lightabsorption of a very complex reactive mixture, which, moreover, changes its compositionthroughout the reaction, b) from this basis, determine the photon flux that arrives at eachparticle of the catalyst to photoactivate it, and c) estimate the photons absorbed and dispersed.Furthermore, it seems that this, for the moment, is a difficult undertaking (Serpone et al.,1996). Remember that, in heterogeneous catalysis, the reaction rate is usually expressed as afunction of the grams of catalyst. In photocatalysis, it should include the number of activecentres, as well as the surface area of catalyst. But as a consequence of the above comments,the number of active centres is unknown and the surface of catalyst exposed to light isundetermined.

This is therefore simplified by considering only the radiation of a certain wavelength (Eq.3.21) incident on the inside of the reactor for calculation of Na. The value obtained from thisis called the estimated quantum yield: ΦE. No distinction is made between the photonscorresponding to each wavelength, assuming that all of them have the same effect on thesurface of the catalyst. In all cases, this simplification is accepted as valid by a multitude ofauthors and widely used in the bibliography. Consequently, the reported “quantum yields”have sometimes been reported as lower limits not allowing for scattered light. A simple meansof assessing process efficiencies for equal absorption of photons is therefore desirable inheterogeneous photocatalysis (Eq. 4.18). The initial photoconversion of phenol has beenchosen as the standard process and Degussa P-25 titania as the standard photocatalyst. Thiscompromise has been adopted by a group of scientist (belonging to different research groups)considered among the most important in the world (Serpone et al., 1996). The choice ofphenol was dictated by the recognition that the molecular structure of phenol is present inmany organic pollutants and, like many of theses, is essentially degraded by oxidation ratherthan reduction.

phenolofncedisappearaofrate

substrateofncedisappearaofrater =ζ (4.18)

where ζr is called relative photonic efficiency. When the reaction rate for the test substancesand phenol (secondary actinometer) are obtained under identical experimental conditions thereis no need to measure the photon flux. The use of relative photonic efficiency renderscomparison of process efficiencies between studies carried out in different laboratories orpilot plants possible because ζr is basically independent of the fundamental photocatalysisparameters (light intensity, reactor geometry and TiO2 concentration for a given catalyst).However, it depends on the initial concentration of substrate and on temperature. In any case,based on initial rates of degradation, ζr illustrates only one aspect of photodegradation and isalso useful to compare different photocatalyst materials for water treatment purposes.

An example of an application of ζr to degradation of pesticides (lufenuron and propamocarb)under sunlight is shown in the following paragraphs. Phenol (20 mg/L) has been used tocalculate ζr following the method proposed above. In Figure 4.9 the experiments carried outwith phenol are shown.

62


0 5 10 15 20 250.0

0.2

0.4

0.6

0.8

1.0

0.0

0.2

0.4

0.6

0.8

1.0

C/C0

Average TOC/TOC

0

Average

Nor

mal

ized

Con

cent

rati

on

EUV, kJ/L

Figure 4.9Plots of the normalised concentration as a function of the accumulated energy for the

photodegradation and mineralization of phenol. C0 = 20 mg/L , TiO2 = 200 mg/L, pH0 = 5.

The mineralization rate (measured by TOC analysis) is also included because efficienciesbased on the disappearance of organic carbon (ζr ,TOC, Eq. 4.19) provide more practicalinformation.

phenolfromTOCofncedisappearaofrate

TOCsubstrateofncedisappearaofrateTOCr =,ζ (4.19)

Phenol and pesticide experiments have been performed under the exact same conditions. Withsunlight, it is not possible to work under constant illumination conditions. Therefore, Eq. 3.24is used to avoid this uncertainty and reaction rates used to determine ζr and ζr ,TOC arecalculated using EUV instead of time. Relative photonic efficiencies of the two pesticidestested are reported in Table 4.1. All the efficiencies are lower than unity, indicating that theinitial photocatalytic oxidative degradations of the test substances, at the selected initialconcentration, are less efficient than for phenol.

Substrate Rate µµµµmol/kJ ξξξξr ξξξξr,TOC

Propamocarb 5.8 0.21 0.48

Lufenuron 8.8 0.32 0.73

Table 4.1 belongs here.Relative photonic efficiencies for the two pesticides treated with phenol as the standard

reference (C0 = 20 mg/L).

SUMMARY OF THE CHAPTER

Photolysis tests have to be performed always before photocatalysis tests in order to find outdecomposition rates without the semiconductor. The water to be treated must contain enoughdissolved oxygen. To unambiguously exclude the effect of pH, the reagents used to modify thepH must contain counterions that have no effect on the rate of water treatment. The optimumcatalyst concentration always depends on the experimental device used and, therefore, must be

63


always tested first. The direct application of the Langmuir-Hinshelwood model produces anempirical equation, which fits the degradation experimental data accordingly. This equation isuseful in a wide range of initial concentrations and is necessary for engineering plant design.Experimentation at pilot plant level is essential to obtain these equations. Above a certain fluxof UV photons, reaction rate changes depending on intensity. It may only be intuited that atintensities of several suns, quantum yield diminishes. This effect should be measuredexperimentally in each device, but this limit is generally accepted. The optimum temperatureis generally between 20 and 80°C. The use of relative photonic efficiencies renderscomparison of process effectiveness between studies carried out in different reactors possible.


Herrmann, J.M. Heterogeneous Photocatalysis: an Emerging Discipline Involving MultiphaseSystem. Catalysis Today, 24, 157-164, 1995.

Ollis, D.F. Solar-Assisted Photocatalysis for Water Purification: Issues, Data, Questions.Photochemical Conversion and Storage of Solar Energy, 593-622, Kluwer AcademicPublishers, 1991.

Serpone, N., Sauvé, G., Koch, R., Tahiri, S., Pichat, P., Piccinni, P., Pelizzetti, E., Hidaka, H.Standardisation protocol of process efficiencies and activation parameters inheterogeneous photocatalysis: relative photonic efficiencies ζr J. Photochem.Photobiol. A: Chem., 95, 191-203, 1996.

Tomin C. The Pesticide Manual, a World Compendium. 10th ed. British Crop ProtectionCouncil and Royal Society of Chemistry. Croydon, UK, 1994.

Kormann, C., Bahnemann, D.W. and Hoffmann M. R. Photolysis of Chloroform and otherOrganic Molecules in Aqueous TiO2 Suspensions. Environ. Sci. Technol., 25, 494-500, 1991.


PART A. True or False?1. Oxygen avoids recombination of e-/h+ and promotes •OH formation.2. The reagents used to change the pH must contain organic buffers.3. The optimum concentration of the catalyst is always around 1 g/L.4. Initial substrate behaviour is always the only fundamental parameter for choosing

[TiO2]OPT.5. By applying the Langmuir Hinshelwood (L-H) rate law it is possible to find out whether

the photocatalytic process is on the surface or in solution.6. When C0 is very small, L-H isotherm equations can be reduced to an order one-reaction

rate equation.7. It has been demonstrated that above a certain UV-photon flux, reaction rate changes from

one to half-order dependence to the intensity.8. Temperature does not affect photocatalytic reaction rate.9. The amount of photons absorbed by the catalyst must be known to compare photocatalytic

reactors.10. Simplifications must be employed for calculating photocatalytic efficiencies.

PART B.1. Why is it necessary to carry out a blank test before putting TiO2 in a photoreactor?2. What are blank photocatalysis experiments mainly for?

64


3. What is the main difference between bubbling oxygen and air inside a photoreactor?4. To unambiguously exclude the pH effect, what are the reagents most often used to change

the pH?5. Why are L-H constants called “apparent” in photocatalysis?6. Calculate the L-H constant corresponding to the following experimental data:

Table 4.2 belongs here.Experimental data from a series of photocatalytic experiments at different initial

concentration.7. What is the most accepted reason for photocatalytic reaction rate dependence on change of

order of intensity?8. What is the usual UV power of light intensity where the change of order is produced?9. What is the optimum temperature range for heterogeneous photocatalytic reactions?10. Why is use of a secondary actinometer necessary?

Answers

Part A1.True; 2. False; 3. False; 4. False; 5. False; 6. True; 7. True; 8. False; 9. False; 10. True.

Part B1. As TiO2 readily sticks to the materials of the photoreactors.2. This type of experiment evaluates the importance of the treatment without catalyst, the

increase in temperature in the reactor and the adsorption of pollutant or its metabolites inthe materials of the pilot plant.

3. The concentration of O2 inside the reactor. By bubbling oxygen, the [O2] is almost fivetimes greater than by bubbling air.

4. Sodium hydroxide and hydrogen chloride or sulphuric acid.5. Because with kinetic studies only, it is impossible to find out whether the process is on the

surface or in solution and therefore, the constants obtained have no physical meaning, andmay not be used to identify surface processes.

6. kr = 2.19 mg L-1 min-1; K = 0.0192 (mg/L)-1

7. The transition of r = f(I1.0) to r = f (I 0.5) is usually attributed to the excess ofphotogenerated species (e-, h+ and •OH).

8. Above 50-75 WUV per square meter of photoreactor surface, the quantum yielddiminishes.

9. Between ambient temperature and 60-70 ºC.10. Because ζr is basically independent of photocatalysis fundamental parameters (light

intensity, reactor geometry and TiO2 concentration for a given catalyst).

65


5. WATER DECONTAMINATION BY SOLARDETOXIFICATION

AIMSThis unit describes the most important applications of photocatalysis for waterdecontamination. It outlines the decomposition of organic and inorganic contaminants and theusefulness of biological testing in addition to chemical analyses. Examples are also given tofacilitate comprehension of the degradation pathways involved in the decomposition of theinitial compounds. The use of modified catalysts and additional oxidants to increase processefficiency by trapping the photogenerated electrons and/or by producing extra oxidisingspecies is discussed. Finally, an overview of the most common sophisticated analyticaltechniques is presented.

OBJECTIVESBy the end of this unit, you will know why photocatalysis for the treatment of contaminatedwater is important and the best analytical techniques for determining the degree ofdecontamination during treatment. You will:1. Be able to describe the stoichiometric parameters of the reactions involved in the

photocatalytic process.2. Understand how it is possible to combine photocatalytic and biological processes.3. See how photocatalysis can treat not only organic but also inorganic contaminants.4. Find out different ways to improve process efficiency by modifying the catalyst and/or

adding other extra-electron acceptors.5. Review the most common analytical tools used to determine the degradation rate of the

original contaminants.6. Learn the most sophisticated analytical techniques used to describe the complicated

mechanisms involved in the photocatalytic degradation of organic substances.7. Understand how to determine the toxicity of water and why this is so important for water

treatment.

NOTATION AND UNITSSymbol UnitsAPI Atmospheric pressure ionisationBOD Biological Oxygen Demand mg O2/LCI Chemical ionisationDPs Degradation productsEo Oxidation potential VECD Electron capture detectorEI Electron impactFID Flame ionisation detectorGC Gas chromatographyHPLC High pressure liquid chromatographyLC50 Concentration of a chemical that is lethal to 50% of the exposed

populationmg/L

LLE Liquid-liquid extractionPoct Water/octanol partition coefficientMS Mass spectrometryNMR Nuclear magnetic resonance

66


NPD Nitrogen phosphorus detectorSPE Solid phase extractionTOC Total organic carbon mg/L

5.1 DETOXIFICATION OF POLLUTANTSChemists play a significant role in devising new methods to solve environmental problems.Existing methods deal with concentrated contaminants; what is still needed is an effectivemethod for handling toxic materials widely dispersed in the environment. Irradiated TiO2

efficiently degrades nearly every significant functional group, including the mostenvironmentally recalcitrant materials. The primary processes associated with semiconductor-sensitised photodegradation of organic substances are illustrated in Figure 5.1. As shown byEqs. 1.21-1.24, the hydroxyl radical (•OH.) is the major intermediate reactive responsible fororganic substrate oxidation. The free radical HO2

• and its conjugate O2•- are also involved in

degradation processes, but those radicals are much less reactive than free hydroxyl radicals.These radicals react strongly with most organic substances by hydrogen abstraction orelectrophilic addition to double bonds. Free radicals further react with molecular oxygen togive a peroxy radical, initiating a sequence of oxidative degradation reactions that may lead tocomplete mineralization of the contaminant. In addition, hydroxyl radicals may attackaromatic rings at positions occupied by a halogen, generating a phenol homologue. Theelectrons of the conduction band can also degrade organic compounds by reductive pathwaysas is shown in Figure 5.1 in the case of tetrachloromethane.

Figure 5.1 belongs hereMajor general processes for the photo-oxidative or photo-reduction degradation of organiccompounds in aqueous solution sensitised by semiconductor particles. Examples of photo-

oxidation (PCP) and photo-reduction (CCl4) are shown

In general, the types of compounds that have been degraded include alkanes, haloalkanes,aliphatic alcohols, carboxylic acids, alkenes, aromatics, haloaromatics, polymers, surfactants,herbicides, pesticides and dyes. A partial tabulation of organic compounds successfullydegraded by photocatalysis is provided in Table 5.1. Eq. 5.1 generally holds true for anorganic compound of general formula CnHmOp.

OHm

nCOOnpm

OHC pmn 222 24)2( +→

+−+ (5.1)

In the case of organic compounds containing halogens, Eq. 5.2 shows how the correspondinghalide is formed.

qHXOHqm

nCOOnpm

XOHC qpmn +−+→

+−+ 222 24

)2((5.2)

Under photocatalytic oxidative conditions, sulphur is recovered as sulphate in sulphurcontaining compounds according to Eq. 5.3

42222 SOzHOyHnCOxOSOHC rpmn ++→+ (5.3)

Table 5.1 belongs here.Some examples of TiO2-sensitised photodegradation of organic substrates.

5.1.1 Total mineralizationIn photodegradation, transformation of the parent organic compound is desirable in order toeliminate its toxicity and persistence, but the principal objective is to mineralise all pollutants.

67

Figure 5.1

+

-hνCO2

Inorganic acidsWater

TiIVOH

TiIIIOH A

A-

TiIVOH

TiIIIOH+••••

Oxidisedintermediate

Organicpollutant

OHCl

ClCl

Cl

ClO

Cl

ClCl

Cl

ClO

Cl

ClO

Cl

ClO

Cl

ClCl

Cl

ClOH.

OCl

ClO

Cl

Cl

OH.

OH

-H2O

. OH OH. .-HCl

CCl4 + ecb-→CCl3

•+Cl-

CCl3•+ ecb

-→:CCl2 + Cl-

If A = O2

O2-•••• HO2

•, HO2-, H2O2, •OH H2OO2

Oxidisedintermediate

Organicpollutant

68


The effectiveness of degradation is not demonstrated only because the entire initial compoundis decomposed. As observed in the HPLC chromatograms (Figure 5.2), oxamyl(chromatogram A, retention time 4.45 min) disappears nearly completely (chromatogram B).However, many new organic compounds appear.

Figure 5.2 belongs hereHPLC-UV chromatograms of photodegradation of oxamyl before photocatalytic treatment (A)

and when the oxamyl has completely disappeared (B).

Furthermore, the stoichiometry proposed for the general reactions (Eqs. 5.1-5.3) has to bedemonstrated in each case by a correct mass balance. Reactives and products might be lostand this will originate not confident results. Mineralization rate is determined by monitoringinorganic compounds, such as CO2, Cl-, SO4

2-, NO3-, PO4

3-, etc. When organics decompose, astoichiometric increase in the concentration of inorganic anions is produced in the watertreated and, likewise, very often an increase in the concentration of hydrogen ions (decrease inpH). For this reason, the analysis of these two products of the reaction is of interest for thefinal mass balance. However, the decrease in pH is not a very reliable parameter of thisbalance, save in some cases, because it is influenced by other processes which take place inthe medium: the effect of the TiO2 suspension, the formation of CO2 and intermediates, etc.

In order to demonstrate that there are no product losses, the molar ratio must be in accordancewith the organic substrate structure. For example, the pentachlorophenol decompositionreaction to which mass balances should be adjusted is shown in Eq. 5.4. Therefore, [Cl-] = 5[PCP]0, but this only occurs at the end of the experiment, when the TOC is almost 0. Duringthe degradation, the formation of intermediates impedes this, since these intermediates containdiffering amounts of chlorides

HClCOTiOOHOOHClC

h

56229

222562

+++ →υ

(5.4)

In Figure 5.3, a complete pentachlorophenol degradation test is shown. The products obtainedare practically stoichiometrically correct. The slight difference may be attributed to the factsthat all the TOC is not completely decomposed and that results obtained with four differentanalytical techniques were compared.

Figure 5.3 belongs hereEvolution of H+ and Cl- during pentachlorophenol degradation. To more clearly demonstrate

that reaction 5.4 is completed, the concentration of TOC in mM is calculated considering1 mMol TOC = 6 mMol of C = 72 mg of C.

The oxidation of carbon atoms into CO2 is relatively easy. It is, however, in general, markedlyslower than the dearomatization of the molecule. Until now, the absence of totalmineralization has been observed only in the case of s-triazine herbicides, for which the finalproduct obtained was essentially 1,2,5-triazine-2, 4,6, trihydroxy (cyanuric acid), which is,fortunately, not toxic. This is due to the strong stability of the triazine nucleus, which resistsmost methods of oxidation. For chlorinated molecules, Cl- ions are easily released into thesolution. Nitrogen-containing molecules are mineralised into NH4+ and mostly NO3

-.Ammonium ions are relatively stable and the proportion depends mainly on the initialoxidation degree of nitrogen and on the irradiation time. The pollutants containing sulphuratoms are mineralised into sulphate ions. Organophosphorous pesticides produce phosphate

69

A

B

OXAMYL

min.

Figure 5.2

70

Figure 5.3

H+

Cl-

TOC

PCP

0 5 10 15 20 25 30 350.00

0.02

0.04

0.06

0.08

0.10

0.12

0.0

0.1

0.2

0.3

0.4

0.5

0.6

[PC

P],

TO

C],

mM

tR, min

71


ions. However, phosphate ions in the pH range used remained adsorbed on TiO2. This strongadsorption partially inhibits the reaction rate that, however, remains acceptable. Until now, theanalyses of aliphatic fragments resulting from the degradation of the aromatic ring have onlyrevealed formate and acetate ions. Other aliphatics (presumably acids, diacids, andhydroxylated compounds) are very difficult to separate from water and to analyse. Formateand acetate ions are rather stable, which in part explains why total mineralization takes muchlonger than dearomatization.

5.1.2 Degradation pathwaysAs pointed out above, a variety of degradation products (DPs) are formed in photocatalyticprocesses. Nevertheless, in most cases no attention is paid to the possible formation of theseDPs which, on the other hand, allow the degradation processes to be better understood andevaluated and thereby comparison among the degradation pathways of different types ofcompounds, functional groups, chemical oxidation parameters, etc. Cost-effective treatment tocomplete compound mineralization is usually not feasible and the generation of by-productsappears to be unavoidable with photocatalytic degradation. Identification of those by-productsis the key to maximising overall process efficiency. Since hydroxyl radicals react non-selectively, numerous by-products are formed at low concentrations. On the other hand someof the degradation products obtained are of interest, because they may be more toxic andpersistent than the parent compound. In some cases of photocatalytic TiO2 treatments, onlytraces of these metabolites are detectable because they are degraded faster than the parentcompounds and mineralization is almost total in a short time. But in other cases, such as s-triazine herbicides mentioned above, overall conversion to the final degradation productstakes longer. For example, primary intermediates of the photocatalytic degradation of variousaromatic pollutants detected and identified correspond to hydroxylation of the benzene ring(see Figure 5.1). Maximum transient concentrations are much lower for these intermediatesthan the initial pollutants, since CO2, acetate and formate are formed in the initial stages ofdegradation. The orientation of the hydroxylation of the aromatic ring depends on the natureof the substituents. For instance, for chlorophenols and dimethoxybenzenes, the para andortho positions (with respect to OH for the chlorophenols) are favoured as is expected. Bycontrast, for benzamide and nitrobenzene, the hydroxylation occurs at all free sites, whereas ameta orientation is expected for electron-withdrawing substituents. A list of compoundsproduced during photodegradation of a pesticide (pyrimethanil) with TiO2 is shown in Figure5.4. It will give an idea of the complexity of the chemical reactions involved in photocatalyticmethods.

Figure 5.4 belongs hereChemical structures of pyrimethanil and its degradation products obtained during a

photocatalytic treatment with TiO2.

5.1.3 Toxicity reductionThe success of photocatalytic degradation in wastewater treatment depends on how much thecost of illumination, i.e., energy, can be brought down. This can be achieved by improving theefficiency of photocatalytic degradation by modifying the catalyst, the photoreactor design, byusing additional oxidants, etc. It is also possible to envisage incomplete photocatalyticdegradation to detoxify the wastewater. The products of incomplete degradation and theirconcentrations may be sufficiently innocuous as to be permissible for discharge directly intothe environment or for further biological treatment. Biological treatment of biodegradableresidual waters is presently the most compatible with the environment and the least expensive.

72

NH2

OH

OH

OH

OH

N

O

C NH2

N

O

C CO

CH3NH

NH

O

C CH3

O

NHCH

NH

O

C CH3

OH

PHENYL DERIVATIVES

NH

N

N CH3

CH3

PYRIMETHANIL

HIDROXY DERIVATIVES

CH3

CH3

N

NN

OH

CH3

CH3

N

NNH

OH

CH3

CH3

N

NNHOH

CH3

CH3

N

NN

OH

OH

CH3

CH3

N

NNHOH

OH

PYRIMIDIL DERIVATIVES

CH3

CH3

N

NNH2

ALIPHATIC DERIVATIVES

CCH3

ONH

OC

NH2

ONH2

CH3

C

O NH2CH

Figure 5.473


However, it is very difficult to identify all the intermediary compounds en-route to completemineralization. Toxicity testing of the photocatalytically treated wastewater is thereforenecessary, particularly when incomplete degradation is planned. Any number of whole-organism bioassays could be used for the assessment of water quality before disposal. Since itis not feasible to determine the specific toxicity of every toxic intermediary compound, whole-effluent toxicity testing using aquatic organisms is a direct, cost-effective and relevant meansof determining effluent toxicity. Therefore, bioassays can provide a more direct andappropriate measure of mixtures of toxins than chemical analyses alone, which are notsufficient to assess potential effects on aquatic biota (Tapp et al., 1996). A review of thesetests is shown at the end of this chapter.

Another useful technique for evaluating the feasibility of disposing photocatalytically treatedwaters before total mineralization is determining the biodegradability (BOD) in sludge frombiological activated sludge plants. This sludge is inoculated with the treated water and BODtests are carried out. In this case, the results are obtained in terms of “relativebiodegradability”. This means that once biodegradability is achieved, it can only be assured ifthe same biological activated sludge used for the BOD tests is used for disposing the treatedwater. This test produces non-universal results, but could be very useful if the biologicaltreatment plant that will receive the effluent from the photocatalytic treatment is known priorto design of the photocatalytic plant. This permits disposal of the pre-treated water into abiological (low cost) treatment.

5.1.4. Detoxification of inorganic pollutantsPhotocatalytic processes may also transform inorganic substances from the environment.Specifically, H2S and CN- can be converted into less toxic materials and various strategicand/or toxic metals can be removed from waste effluents. The toxicity of H2S is comparableto that of HCN. In the presence of light and a semiconductor catalyst, the products ofoxidation are molecular hydrogen and sulphur. Free cyanides are generated in large quantitiesin heat-treating operations and in metal-finishing industries. The greatest amounts of cyanide-containing wastes are produced by precious metals milling operations and coal gasificationprocesses. CNO- is the first product of photocatalytic oxidation of cyanides in the presence ofpolycrystalline TiO2 in an aqueous medium. The proposed mechanism implies the oxidationof cyanide by the photogenerated holes and the reduction of oxygen by electrons according toEqs. 5.5. The reacting mixture must be at pH 10 to avoid the formation of volatile HCN.

OHCNOOHhCN 222 +→++ −+− (5.5)Cyanate ions are photo-oxidised to nitrates and a satisfactory nitrogen balance is achievedaccording to the overall reaction shown in Eq. 5.6. At strong oxidant conditions volatilenitrogen-containing species are not produced or are quickly photooxidised to NO2

- and NO3-.

2232322 4324 OHNOCOOHOHOCNO ++→+++ −−−− (5.6)

It has now been discovered that ozonation (production of drinking water) generates low levelsof bromate ions, recognised as a suspect agent of cancer. Drinking water treated with ozonewill typically have in the range of tens of µg/L of bromate and TiO2 is able to sensitise thephotodecomposition of bromate (Eq. 5.7).

2/

3 322 2 OBrBrO TiOh + → −− ν (5.7)

Heavy metals are generally toxic and can be removed from industrial waste effluents as smallcrystallites deposited on the photocatalyst as demonstrated by Eq. 5.8:

20/

2 422 O

nnHMOH

nM TiOhn ++ →+ ++ ν (5.8)

74


Under identical conditions, the following reactivity pattern has been found: Ag > Pd > Au > Pt>> Rh >> Ir >> Cu = Ni = Fe = 0 for AgNO3, PdCl2, AuCl3, H2PtCl6 or Na2PtCl6, RhCl3,H2IrCl6, Cu(NO3)2, Ni(NO3)2 and Fe(NO3)3. As the photodeposition conversion increases, themetal particles form agglomerates, reaching several hundreds of nm. Silver photodepositionhas been applied in the recovery of Ag from used photographic baths in which the silver-thiosulphate complex is decomposed, Ag+ being reduced to Ag°. Because of their favourableredox potentials, only noble metals can be photodeposited. This property has been used toselectively recover noble heavy metals. For instance, silver has been separated from copper insolutions simulating industrial electrolytic baths. Other toxic, heavy non-noble metals couldbe removed from water. Mercury, because of its favourable redox potential, was photoreducedas a zero-valent metal. The cations Pb2+ and Tl3+ have been deposited on UV-irradiated TiO2

powder as PbO2 and Tl2O3. Similarly, uranium has been photodeposited on TiO2 as U3O8

from uranyl solutions. From an application point of view, the recovery of silver fromphotographic baths seems the most promising issue, provided legislation on discharge watercontaining Ag becomes stricter.

5.2 QUANTUM YIELD IMPROVEMENT BY ADDITIONAL OXIDANTS

One practical problem in using TiO2 as a photocatalyst is electron/hole recombination (reverseof Eq. 1.21), which, in the absence of proper electron acceptors, is extremely efficient andthus represents a major energy-wasting step and limitant to achieving a high quantum yield.Oxygen has been chosen in most of the applications for this purpose, although its role is notonly related to electron scavenging (see Chapter 4). But with only dissolved oxygen as anoxidant, low mineralization photo-efficiencies (production of CO2) are obtained (in the rangeof 1-5%).

One strategy for inhibiting e-/h+ recombination is to add other (irreversible) electron acceptorsto the reaction. Outstanding enhancement of the rate of degradation of various organiccontaminants through the use of inorganic peroxides has been demonstrated (Pelizzetti et al.1991, Malato et al. 1998). The addition of other oxidising species could have several differenteffects:• Increase the number of trapped e- in the e-/h+ pairs and, consequently, avoid

recombination.• Generate more •OH and other oxidising species.• Increase the oxidation rate of intermediate compounds.• Avoid problems caused by a low O2 concentration.

It must be mentioned here that in highly toxic wastewater where degradation of organicpollutants is the major concern, the addition of an inorganic anion to enhance the organicdegradation rate may often be justified. For better results, these additives should fulfil thefollowing criteria: dissociate into harmless by-products and lead to the formation of •OH orother oxidising agents. There is another advantage related to the use of this type of oxidantwhen solar energy is the photon source. Although scientific research on photocatalyticdetoxification has been conducted for at least the last three decades, industrial/commercialapplications, engineering systems and engineering design methodologies have only beendeveloped recently. In this type of installation, the photoreactor is by far the most expensivecomponent and a barrier to commercialisation. The increase of the photocatalytic reaction ratewith these additives would decrease photoreactor dimensions proportionally and dramaticallydecrease overall costs.

75


5.2.1. Hydrogen PeroxideHydrogen peroxide is the obvious candidate. It can increase the efficiency of the process athigh irradiance (see Section 4.6) and it has been tested with a large number of compounds.Also, it is a very commonly used chemical and, therefore, very cheap. Being an electronacceptor, hydrogen peroxide reacts with conduction band electrons (Eq. 5.9) to generatehydroxyl radicals, which are required for the photomineralization of organic pollutants.

−•− +→+ OHOHeOH 22 (5.9)

The following reactions (Eq. 5.10 and 5.11) can also produce •OH (reaction 5.11, whichrequires λ < 300 nm, does not take place with solar radiation).

2222 OOHOHOOH ++→+ −•−• (5.10)

OHhOH •→+ 222 ν (5.11)The effect of this electron acceptor deserves some further comment. In some cases, theaddition was found to be beneficial, increasing the degradation rate. The effect depends on theH2O2 concentration, generally showing an optimum range of concentration. At higherconcentrations the improvement starts to lessen. Whereas this beneficial effect can easily beexplained in terms of prevention of electron/hole recombination and additional •OHproduction through reactions 5.9, 5.10 and 5.11, inhibition could be explained in terms ofTiO2 surface modification by H2O2 adsorption, scavenging of photoproduced holes (Eq. 5.12)and reaction with hydroxyl radicals (Eq. 5.13).

++ +→+ HOhOH 22 222 (5.12)•• +→+ 2222 HOOHOHOH (5.13)

The inhibition of adsorption not only depends on the characteristics of the pollutant, but alsoon the hydrogen peroxide/organic concentration ratio. This may be explained in terms ofLangmuir-Hinshelwood kinetics, rC = krKC/(1+KC) and competitive adsorption (see Chapter4). If pollutant concentration (C) is too low and H2O2 concentration too high, organicadsorption decreases because of adsorption of hydrogen peroxide and, therefore, the additionalhydroxyl radicals generated by H2O2 do not react efficiently. When C is higher, the radicalsreact more easily and rC increases and, when C is still higher, the reaction rate is not asaffected by adsorption (1+ KC ≈ KC; rC = kr). In the latter situation the reaction rate is onlydependent on •OH concentration related to favourable (Eqs. 5.9-5.11) and unfavourablereactions (Eqs. 5.12-5.13). There is an optimum ratio of H2O2/C under these circumstanceswhereby, organic material is sufficient to consume generated hydroxyls and, to avoiddetrimental reactions, if peroxide is not too high. At high molar ratios an inhibition effectwould be expected because the unfavourable reactions become more and more important. Allthis may be summarised: (i) if pollutant concentration is low, the hydrogen peroxide easilyinhibits the degradation rate; (ii) if the molar ratio between H2O2 and pollutant is too high, thesame is true.

5.2.2. PersulphateIn homogeneous reactions, the persulphate ion accepts an electron and dissociates (Eq. 5.14).The sulphate radical anion is also generated adequate thermal and photolytic (wavelength <270 nm) conditions. This radical goes through the reactions explained below (Eq 5.16 and5.17). Persulphate can therefore be a beneficial oxidising agent in photocatalyticaldetoxification because SO4

•- is formed from the oxidant compound by reaction with thephotogenerated semiconductor electrons (e-

CB, Eq. 5.15). In addition, it can trap thephotogenerated electrons and/or generate hydroxyl radicals. The sulphate radical anion (SO4

•-)

76


is a very strong oxidant (Eo = 2.6 V) and engages in at least three reaction modes with organiccompounds: by abstracting a hydrogen atom from saturated carbon, by adding to unsaturatedor aromatic carbon and by removing one electron from carboxylate anions and from certainneutral molecules.

−•−−− +→+ 244

282 SOSOeOS aq (5.14)

−•−−− +→+ 244

282 SOSOeOS CB (5.15)

−−•− →+ 244 SOeSO CB (5.16)

+−••− ++→+ HSOOHOHSO 2424 (5.17)

5.2.3. Other oxidantsOther compounds could also potentially increase the reaction rate because they are alsoelectron scavengers. The most frequently tested so far in heterogeneous photocatalysis aredescribed in Eqs. 5.18-5.21.

−−− +→++

IOHeIO H2

84 48 (5.18)

−−− +→++

ClOHeClO H2

63 36 (5.19)

−−− +→++

BrOHeBrO H2

63 36 (5.20)

•−−−−−•−− +→++→+ 45245 SOOHeHSOorSOOHeHSO (5.21)

Chlorate has been proven insufficient to improve effectiveness. However, both IO4- and

bromate increase the mineralisation rate in all cases tested. Nevertheless, these additives arevery expensive compared to hydrogen peroxide and peroxydisulphate, and their applicationwould dramatically increase treatment cost. Even more importantly, they do not dissociateinto harmless products (Br- and I-), because hundreds of mg/L of these anions are undesirablein water. Potassium peroxymonosulfate (commercially called oxone) was also examined as anirreversible electron acceptor. The formula of this salt is 2KHSO5.KHSO4.K2SO4, written inaqueous solution as HSO5

-. Upon accepting an electron from the conduction band, HSO5-

would dissociate into two different pathways (Eq. 5.21). The disadvantage is its highmolecular weight. Many grams of product are necessary for 1 active mol (HSO5

-).

5.3 CATALYST MODIFICATION

The TiO2 band-gap represents only 5% of the solar spectrum (see Chapter 2). From thestandpoint of solar collecting technology, it is therefore a rather inefficient process even for ahigh added-value application. In contrast to other Advanced Oxidation Technologies,photocatalysis has the advantage of being solarizable and of being an environmentally friendlytechnology. TiO2 is a cheap photostable catalyst, and the process may be run at ambienttemperature and pressure conditions. Furthermore, the oxidant, molecular oxygen (O2), is themildest one. Therefore, basically, TiO2 is a mild catalyst that works at mild conditions withmild oxidants. However, as concentration and number of contaminants increase, the processbecomes more complicated and challenging problems, such as catalyst deactivation, slowkinetics, low photoefficiencies and unpredictable mechanisms need to be solved. It is clearthat naked TiO2 needs extra help to undertake practical applications of industrial andenvironmental interest and this could lead to the loss of some of the charm of its mildoperation. Moreover, even reactor set-ups using artificial light, and the cost of running thelamps involved in them, will be much cheaper if visible radiant flux can be employed.

77


The redox process is based on the migration of electrons and holes to the semiconductorsurface and two further oxidation and reduction steps (see Figure 5.1). Two basic lines ofR&D attempt to balance both half-reaction rates, one by adding electron acceptors (additionaloxidants, already commented on above) and the other by modifying catalyst structure andcomposition. Both try to promote competition for electrons and avoid recombination of e-/h+

pairs. A third approach has focused not only on increasing quantum yield but finding newcatalysts with band-gaps that match the solar spectrum better. Unfortunately, the choice ofconvenient alternatives for substituting titanium dioxide in photocatalytic detoxificationsystems is limited. The appropriate semiconducting material should be:• Non toxic.• Stable in aqueous solutions containing highly reactive and/or toxic chemicals.• Not photo-corrodible under band-gap illumination.• Economical, that is, an increase in photocatalytic reaction rates must be always be

accompanied by a non-proportional increase in overall process costs.

Generally, metal oxides fulfil these criteria but most metal oxides are wide band-gapsemiconductors or insulators. Although iron (III) oxide is one of the few exceptions (see Table1.4), it has demonstrated satisfactory activity in a limited number of cases. Other non-oxidesemiconductors (e. g. CdS) are usually unstable and photodegrade in time. A few exampleswill be given in the following sections to illustrate the large body of work conducted in thearea of photocatalyst modification.

5.3.1. Metal semiconductor modificationThe addition of noble metals to a semiconductor could modify the photocatalytic process bychanging the semiconductor surface properties. Figure 5.5 is an illustration of the capacity of ametal in contact with a semiconductor surface to capture electrons. After excitation, theelectron migrates to the metal where it becomes trapped and e-/h+ recombination is avoided.The hole is then free to migrate to the surface where oxidation of the organics can occur (seeFigure 5.1). The Pt/TiO2 system (Pt deposited on titania by the impregnation and reductionmethod) is the metal-semiconductor system most commonly studied (see Figure 5.6). Loadingof Pt is optimum to achieve the maximum photocatalytic rate, affecting the distribution ofelectrons in the system. Above the optimum metal content, the efficiency decreases becauseonce negatively charged, Pt particles become attractive for holes, which recombine withelectrons into inefficient thermal energy. Care must therefore be taken in studies conductedwith modified metal semiconductors to use the optimum quantity of metal.

Figure 5.5 belongs hereElectrons capture by a metal in contact with a semiconductor surface.

Figure 5.6 belongs hereConcentrating solar reactor with platinum/titanium dioxide catalyst on ceramic saddles.Tested on air contaminated by spray paint at Fort Carson Army Base in Colorado (USA).

Courtesy of National Renewable Energy Laboratory (USA).

5.3.2. Composite semiconductorsCoupled semiconductors provide a way to increase the separation between charges and reducethe energy (increasing wavelength) necessary to excite the system. Figure 5.7 shows CdS-TiO2 as an example. The energy from light (λ<497 nm) is large enough to cause an electron toleave the CdS valence band and go over to the conduction band. The hole remains in the CdS

78

Figure 5.5

hνννν

h+

e-

Metal

MetalMetal

79

Figure 5.6

80


while the electron is transferred to the TiO2 conduction band. The separate charges are thenfree to undergo electron transfer with the species adsorbed on the surface.

Figure 5.7 belongs hereThe excitation process in a semiconductor-semiconductor photocatalyst.

5.3.3. Surface sensitisationSurface sensitisation of a wide band-gap semiconductor via chemisorbed or physisorbed dyescan increase the efficiency of the excitation process. The wavelength range can be expandedby charge transfer from the sensitiser to the semiconductor. Figure 5.8 illustrates theexcitation, charge transfer and regeneration steps. If the oxidative energy level of the excitedstate of the dye is more negative than the semiconductor conduction band, then the dye cantransfer the electron to the conduction band of the semiconductor. The electron in turn can betransferred to reduce an organic acceptor adsorbed on the surface. Without the presence of aredox couple, the dye-semiconductor system can also be used in oxidative degradation of thedye itself. This is important due the large number of dye substances found in industrial textilewastewater.

Figure 5.8 belongs hereSteps of excitation with a sensitizer in the presence of an adsorbed organic electron acceptor

(A).

5.4 RECOMMENDED ANALYTICAL METHODS

5.4.1 Original contaminantsThe analyses performed on samples collected during photocatalytic treatment have to be ascomplete as possible in order to adjust the mass balance for the photocatalytic decompositionof the contaminants and assure that the analytical data are reliable and the organic compoundshave not disappeared in some other way (evaporation, adsorption in the reactor, adsorption inthe catalyst, etc.) besides photocatalysis. Liquid chromatography (HPLC) with UV detection isthe method of choice for analysis of samples from photocatalytic treatment since directinjection of the aqueous sample into the analytical column is allowed, avoiding the necessityof previous extraction. Samples require only filtering to remove TiO2 or any other particlesthat could damage the chromatographic column before they are injected into the equipment. Adevice made up of a syringe and 0.22 µm membrane filters is used for this purpose. Gaschromatography is only suggested if HPLC is not viable or when a pre-concentration isnecessary due to the very low concentration of the parent compounds.

5.4.2 Mineralization measurements (TOC)Total organic carbon analyses of samples taken during photocatalysis degradation experimentsare vital for the following reasons:• Since identification of all the intermediates generated during photodecompositon is not

possible, pinpointing the moment at which only CO2 remains and water is consideredcompletely decontaminated is crucial.

• Determination of the CO2 produced might be reasonable, since this must be stoichiometricwith the organic carbon present at the beginning in the organic molecule. However, sincethe reactors are usually large and not airtight, loss into atmosphere of the carbon dioxideproduced prevents this. For the same reasons, the samples might become contaminated byatmospheric CO2 and falsify results.

81

Figure 5.7

h+

e-

EG= 2.5 eV

hνννν

EG= 3.2 eV

e-

TiO2

CdS

82

Figure 5.8

Dye

Dye*hνννν

CB

VB

e-

ACB

VB

ACB

VB

A-

83


• It is a reliable, simple and rapid way to close the mass balance at any moment to get anidea of the remaining amount of intermediates and check the extraction methods andanalysis of intermediates.

The basic techniques for the determination of TOC in water have remained relativelyunchanged for 20 years. Organic compounds are converted to CO2 using a combination thatmay include chemical oxidizing agents, ultraviolet radiation or high-temperature combustion(Wangersky, 1993). The CO2 is then measured using non-dispersive infrared absorption,micro-coulometry or conductimetric techniques. Since many water samples contain inorganicforms of carbon (HCO3

- and CO32-), it is usually necessary to remove theses species, typically

using a gas stripping technique prior to measurement of TOC. Some part of the organic carbon(VOCs) may also very often be removed by this procedure. In the case of heterogeneousphotocatalysis, it is necessary to use a TOC analyser able to manage non-filtered samples.When TiO2 is filtered from the sample, there is an important loss of organic compoundsbecause they are adsorbed onto the solid retained in the filter.

5.4.3 Intermediates analysis (GC-MS/HPLC-MS)In order to analyse a reaction mixture containing pesticides and their degradation products(DPs), it is necessary to have analytical screening methods available that permit separationand identification of compounds with very different hydrophilic-hydrophobic characteristicsand spanning very different concentration ranges. The use of gas chromatography withclassical detectors, like ECD, NPD or FID is obviously insufficient. But gas or liquidchromatographic systems coupled to mass spectrometry (GC/MS and LC/MS) represent agood fast alternative for achieving very useful structural information on the compoundsgenerated in such processes (Agüera and Fernández-Alba, 1998).

Important advantages of the GC/MS-based methods are: a) the large amount of structuralinformation they yield and the spectra libraries available either from data bases or fromresearch papers which make DP identification feasible; b) the durability and reliability of theGC-MS interface; and c) the highly efficient sensitivity and separation that avoids overlappingcompounds with similar structures. However, GC-MS based methods have importantdrawbacks because of their low capacity to analyse very polar, low volatile and thermallyunstable compounds. Identification of DPs is usually based on their EI (Electron Impact) massspectra, mainly by comparing the unknown compound spectrum with published spectra. Adisadvantage of EI is that it does not usually provide molecular weight information.Additional and very useful structural information on DPs can be obtained by chemicalionisation (CI). From the molecular weight and interpretation of fragmentation patterns, it ispossible to hypothesise a molecular structure. However, even an EI mass spectrum does notprovide enough information about the location of the functional groups (e.g., position of ahydroxyl group on a benzyl ring). A comparison with a standard, when commerciallyavailable, is required for unequivocal confirmation. When by-product standards are notcommercially available, the laboratory synthesis of DPs could be a solution.

Because GC methods require compounds with high vapour pressures. Derivation, of at leastthe acid fraction (BF3/MeOH or diazomethane procedure), has become a typical procedure foridentifying the less volatile and polar compounds. Nevertheless, derivation is not easy and it isalso time-consuming. Because of this, LC-MS techniques are gaining in importance. LCtechniques present several advantages over GC: (i) little or no cleaning of the sample isrequired, (ii) high polar, low volatile and thermally labile compounds are more easily

84


analysed, (iii) direct analysis of the samples is possible, avoiding polar DPs to escape fromextraction procedures. The major role of LC-MS in the degradation processes studied is (i) tocheck DP molecular weight and (ii) to detect DPs which are not directly amenable with GC-MS techniques.

The ready availability of atmospheric pressure ionisation (API), electrospray (ESP)/ionspray(ISP) and atmospheric pressure chemical ionisation (APCI) LC-MS interfaces that providegeneral structural information and sensitivity, has expanded the applicability of LC-MS,allowing identification of a large range of polar transformation products. LC-MS showsimportant weaknesses since the information usually achieved lacks detailed structural data andis less sensitive and discriminating than GC-MS. These disadvantages can frequently preventDP identification as a consequence of the low detection threshold or overlapping peaks. Sincethe DPs obtained are very complex (see Figure 5.9), mass spectrometry-based techniquestherefore do not provide enough structural information for the unequivocal structuralelucidation of the DPs in many cases and it must be done by more complex time-consumingtechniques such as LC/ MS/MS or NMR. But the combination of both GC/MS and LC/MS isvery useful and, in many cases, enough structural information to evaluate the degradationprocess may be found fast. In addition, other analytical measures such as Total OrganicCarbon (TOC) or Ion Chromatography (IC) can be of great help to assess the success of thedegradation process by evaluating the mineralization rate achieved or by establishing the massbalance of the whole process.

Figure 5.9 belongs hereDegradation pathway proposed for pirimiphos-methyl dissolved in water when illuminated in

the presence of TiO2.

5.4.4 Extraction methodsCommon handling procedures for analysis of samples of chemically treated water involve theuse of extraction methods because identification of degradation products has to be carried outat sub-mg/L level. Trace organic compounds in wastewater are still typically enriched byliquid-liquid extraction (LLE) using an appropriate solvent; however, solid-phase extraction(SPE) is gaining in acceptance (Chiron et al., 1997), mainly because SPE generates lessmatrix interference and a wide range of new adsorbents (able to trap DPs having a wide rangeof polarities) are commercially available, including: alkylsilanes modified silica, e.g. C-18 andend-capped C-18; porous polymers, e.g. poly (styrene-divinylbenzene) PRP-1 or PLRP-S; andcarbon modified materials, e.g. porous graphitic carbon (PGC). SPE materials are expected toshow different behaviours with respect to capacity and breakthrough volumes of both analyteand matrix interferences. Because the retention of a compound is higher in its neutral formthan in its ionic form, phenol and benzoic acid extractions are better under acidic conditionswhile amines are best recovered at alkaline conditions. Problems may arise withmultifunctional compounds. The retention of hydrophilic compounds (Log Poct <0) such asaminophenols or cyanuric acid is poor but can be high with porous graphitic carbon. On theother hand, short chain aliphatics such as oxalic or formic acids (usually the final organicphotooxidation degradation products) may not be recovered at all. In order to identify as manychemicals as possible, a sequential extraction scheme, involving different SPE adsorbents,could be proposed. A C-18 phase is used initially to select all neutral hydrophobic compoundsat pH7. The C-18 filtrates are then passed through a polymeric adsorbent also at pH7, wheremedium polar compounds are retained. Subsequently filtrates are acidified to pH4.5 andpH2.5, respectively, to extract the majority of acidic compounds with a polymeric or carbon-

85

Pyrimiphos Methyl

R2=+

N

NCH3

OH

NCH2 CH3

CH2 CH3PSCH3

OCH3

OCH3

O

P

OCH3

OCH3

O

OCH3 +

N

NCH3 NCH2 CH3

CH2 CH3

O PH3CO

H3COO

R2 NCH2CHO

CH2CH3

R2 NCH2COOH

CH2CH3

CH2CH3R2 N

CH3

CH2CH3R2 N

CHO

CH2CH3

R2 NH

N

NCH3

O P

S

H3CO

H3CO

NCH2 CH3

CH2 CH3

R1=

R1 NCH2CH2OH

CH2CH3R1 N

CHOHCH3

CH2CH3+

R2 NCH2CHO

CH3

R2 NCH3

CH3

R2 NCH2COOH

CH2CHO

R2 NH

CHO

N

N

OH

HOH2C NH2

OH

R1 NCH2CH3

CH2COH

R1 NCH2CH3

CH2COOH

CH2CH3R1 N

CH3

R1 NCH2CH3

H

R1 NCH2CH3

H

N

N

OCH3

OCH3

S

PO

O

O

HOC

Figure 5.9

86


type adsorbent. Polymeric adsorbents can be used over pH2-13 without decomposition of theadsorbent material.

5.4.5 Toxicity analysisThe test organisms in these assays include representatives from four groups: micro-organisms,plants, invertebrates and fish. The Organisation for Economic Cooperation and Development(OECD) has suggested a set of minimum data to assess the effects of chemicals on theenvironment. The OECD lists mortality in fish, impaired reproduction in crustaceans andinhibition of growth in algae as examples of ecotoxicity tests needed to predict the impact ofreleased chemicals upon ecosystems. The two main invertebrate toxicity tests routinely usedare the 21-day Dapinia and the 7-day Ceriodaphinia survival and reproduction tests. Tests areconducted by exposing the organisms to the toxins under control conditions and after therequired incubation period, live organisms are counted. Fish bioassays have been conductedfor decades. The distinct physiological and behavioural responses of fish to low levels ofpollutants has been employed in the development of fish monitors that act as indicators ofwater quality (Tothill et al., 1996). The tests are usually based on larval growth and survival,where newly hatched fish are exposed to a range of effluents for 1-2 days or up to 7 days. Theacute lethality test with fish measures the concentration of a chemical that is lethal to 50% ofthe exposed population after 96 h (LC50). Species such as rainbow trout (Oncorhynchusmykiss) and fathead minnow (Pimephales promelas) are commonly used. During recent years,research has been carried out to reduce or replace acute fish tests with in vitro assays, usingcultured fish cell lines. The use of algae in bioassays has proved useful in detecting metals,herbicides, pesticides and crude oil compounds. However, the culturing and preparation of thealgae suspension is a lengthy process and, further, it is difficult to maintain an identicalculture of algae each time the bioassay is conducted.

Many of the above tests require specialised equipment and operator skills and are timeconsuming. The use of higher organisms such as fish may also be ethically undesirable. In thelast few years, there has been increased interest in bacterial screening to assess toxicity.Studies of effects on microbial function or activity constitute a more direct, rapid and sensitiveapproach to measure chemical stress. These can be classified by the type of measurementused:• Monitoring transformation of carbon, sulphur or nitrogen.• Determination of the activity of microbial enzymes such as dehydrogenases, adenosine

triphosphatases and other enzymes.• Measurement of growth, mortality and photosynthesis.• Determination of glucose uptake activity using radioisotopes.• Measurement of oxygen consumption using a dissolved oxygen electrode or respirometer.• Measurement of luminescence using a photometer.

The development and applications of biological toxicity testing are rapidly increasing.Numerous bioassay procedures are now available, however, it is difficult to state thesensitivity of these tests, and therefore, a universal-monitoring device for toxicity testing isunlikely to be available. In any case, most of the recent studies have dealt with the use of thebacterial luminescence assay for toxicity screening. The use of toxicity tests for evaluation ofphotocatalytic treatments is not very common for the moment, but several papers have alreadybeen published (Jardim et al., 1997; Herrmann et al. 1999).

87


SUMMARY OF THE CHAPTERPhotocatalytic degradation of organic and inorganic compounds follows a definitestoichiometry that has to be determined by appropriate analytical techniques duringphotocatalytic experiments in order to close the mass balance between reactives and products.Furthermore, degradation pathways and by-products formed are so complex that analysis ofwater toxicity during the treatment is recommended to assure that it decreases. Thephotocatalytic degradation of contaminants by photocatalysis can be enhanced by the use ofadditional oxidants like hydrogen peroxide and persulphate and/or the modification of thecatalyst using deposited metals, composite semiconductors or dye sensitisers.

The degradation of the original contaminants is monitored preferably by HPLC, and the finalmineralization by TOC and Ionic Chromatography, but identification of intermediate productshas to be by MS techniques (GC-MS and HPLC-MS). A combination of several MStechniques with several extraction methods is better in order to assure the identification andquantification of a significant number of DPs. As the detection of all DPs is almostimpossible, the use of different organism for determining toxicity is very important in order toguarantee that the treatment is correct.


Agüera, A., Fernández-Alba, A. R., GC-MS and LC-MS Evaluation of Pesticide DegradationProducts Generated from Advanced Oxidation Processes in Waters: An overview.Analusis, 26, 123-130, 1998.

Chiron, S., Fernandez-Alba, A.R. and Rodriguez, A., Pesticide Chemical OxidationProcesses: An Analytical Approach. Trends Anal. Chem., 16, 518-526, 1997.

Herrmann, J.M., Guillard, Ch., Argüello, M., Agüera, A., Tejedor, A., Piedra, L. andFernández-Alba, A. Photocatalytic Degradation of Pesticide Pyrimiphos-Methyl.Determination of the Reaction Pathway and Identification of Intermediate Products byVarious Analytical Methods. Catalysis Today, 1999, in press.

Herrmann, J.M. Heterogeneous Photocatalysis: an Emerging Discipling Involving MultiphaseSystem. Catalysis Today, 24, 157-164, 1995.

Jardim, W.F., Moraes, S.G. and Takiyama, M.M.K. Photocatalytic Degradation of AromaticChlorinated Compouns using TiO2: Toxicity of Intermediates. Wat. Res., 31, 1728-1732, 1997.

Malato, S., Blanco, J., Richter, C, Braun, B. and M. I. Maldonado. Enhancement of the Rate ofSolar Photocatalytic Mineralization of Organic Pollutants by Inorganic OxidisingSpecies. Appl. Catal. B: Environ., 17, 347-360, 1998.

Pelizzetti, E., Carlin, V., Minero C. and M. Grätzel. Enhancement of the Rate ofPhotocatalytic Degradation on TiO2 of 2-Chlorophenol, 2,7-Dichlorodibenzodioxinand Atrazine by Inorganic Oxidizing Species. New J. Chem., 15, 351-359, 1991.

Tapp, J.F., Wharfe, J.R and Hunt, S.M. Toxic Impacts of Wastes on the Aquatic Environment,Royal Soc. Chem., 1996.

Tothill, I.E. and Turner, A.P.F. Developments in Bioassay Methods for Toxicity Testing inWater Treatment. Trends in Anal. Chem., 15, 178-187, 1996.

Wangersky, P.J. Dissolved Organic Carbon Methods: a Critical Review. Marine Chem., 41,61-74, 1993.

88


SELF-ASSESSMENT QUESTIONS PART A. True or False?1. Electrons in the conduction band cannot degrade organic compounds.2. When organics decompose, an increase in the concentration of hydrogen ions is produced

in the water.3. The final objective of photocatalytic treatment is always the total mineralization of the

organic contaminants.4. Inorganic compounds are degraded through oxidative and/or reductive photocatalytic

pathways.5. The addition of other oxidants in the photocatalytic process only produces additional

oxidant species.6. Hydrogen peroxide always enhances the decomposition of organics.7. Liquid chromatography (HPLC) with UV detection is the method of choice for analysing

samples from photocatalytic treatment because it is the easiest.8. TOC analysis is a method for measuring CO2 production during experiments.9. GC/MS and LC/MS are complementary methods.10. There is a universal-monitoring device for toxicity testing.

PART B.

1. Which is the general stoichiometry for the total mineralization of methamidophos(C2H8NO2PS)?

2. Why is the analysis of the initial compound not enough to determine the efficiency ofcontaminant degradation?

3. Why is toxicity determination during a photocatalytic experiment very important?4. What are the final products in photocatalytic treatment of CN-?5. Which are the main advantages of an additional oxidant?6. What are the most common oxidants used for photocatalytic degradation of contaminated

water?7. What metals are used for doping TiO2 and how do they increase photoefficiency?8. What are the commonest analytical techniques for determining the disappearance of the

initial compound during photocatalysis in water? Why?9. What is the principal advantage of applying MS-based analytical methods to the

photocatalytic degradation of organic compounds?10. What are the most common toxicity tests in use today?

Answers

Part A

1.False; 2. True; 3. False; 4. True ; 5. False; 6. False; 7. True; 8. False; 9. True; 10. False.

Part B

1. C H NO PS 7O 2CO H PO H SO HNO H O2 8 2 2TiO h

2 3 4 2 4 3 22+ → + + + +/ υ

2. Because reactives and products could be lost causing results not to be reliable.3. Because the products of incomplete degradation and their concentrations may be

sufficiently innocuous for discharge directly into the environment or for further biologicaltreatment.

89


4. NO3- and CO2.

5. Increasing the number of trapped e- of the e-/h+ pairs, generating more •OH as well asother oxidising species, increasing the oxidation rate of intermediate compounds andavoiding problems caused by a low concentration of O2.

6. Hydrogen peroxide and persulphate.7. Noble metals. After excitation, the electron migrates to the metal where it becomes

trapped and e-/h+ recombination is avoided.8. Liquid chromatography (HPLC). This method permits direct injection of the aqueous

sample into the analytical column avoiding the necessity of extraction procedures.9. The identification of unknown compounds produced during photocatalytic treatment of

contaminated water.10. Bacterial screening tests.

90


FIGURE CAPTIONS

Figure 5.1. Major general processes for the photo-oxidative or photo-reduction degradation oforganic compounds in aqueous solution sensitised by semiconductor particles. Examples ofphoto-oxidation (PCP) and photo-reduction (CCl4) are shown.

Figure 5.2. HPLC-UV chromatograms of photodegradation of oxamyl before photocatalytictreatment (A) and when the oxamyl has completely disappeared (B).

Figure 5.3. Evolution of H+ and Cl- during pentachlorophenol degradation. To more clearlydemonstrate that reaction 5.4 is completed, the concentration of TOC in mM is calculatedconsidering 1 mMol TOC = 6 mMol of C = 72 mg of C.

Figure 5.4. Chemical structures of pyrimethanil and its degradation products obtained during aphotocatalytic treatment with TiO2.

Figure 5.5. Electrons capture by a metal in contact with a semiconductor surface.

Figure 5.6. Concentrating solar reactor with platinum/titanium dioxide catalyst on ceramicsaddles. Tested on air contaminated by spray paint at Fort Carson Army Base in Colorado(USA). Courtesy of National Renewable Energy Laboratory (USA).

Figure 5.7. The excitation process in a semiconductor-semiconductor photocatalyst.

Figure 5.8. Steps of excitation with a sensitizer in the presence of an adsorbed organicelectron acceptor (A).

Figure 5.9. Degradation pathway proposed for pirimiphos-methyl dissolved in water whenilluminated in the presence of TiO2.

91


6. SOLAR DETOXIFICATION TECHNOLOGY

AIMSThis unit discusses the basic factors related to solar photocatalytic process technology andapplications. Main key issues related to the use of solar ultraviolet radiation and theirimplications for appropriate materials and reactors for efficient light collection and use arediscussed here.

OBJECTIVESWhen you have completed this unit, you will have a basic knowledge and understanding ofthe following areas:1. The key issues in collector technology related to solar water detoxification.2. The specific peculiarities of the use of solar ultraviolet light.3. Main characteristics of different types of collectors for solar water detoxification

applications.4. Advantages and disadvantages of using non-concentrated sunlight.5. Factors concerned with main technological components: reactor, reflectors and catalyst.

NOTATION AND UNITSSymbol Description UnitsCPC Compound Parabolic ConcentratorCPVC Chlorinated polyvinyl chlorideCR Concentration Ratio (solar collector)C Geometric concentration ratioCε Effective concentration ratiod Reactor tube diameter mmD Aperture width (parabolic collector) mmf Focal length (parabolic trough collector) mmECTFE Ethylenechloride tetrafluroethyleneETFE EthylenetetrafluoroethyleneFEP Fluorinated ethylenepropyleneIEP Isoelectric PointIFSH Institut für Solarenergieforschung GmbH (Hannover, Germany)IR Infrared light (solar radiation)NREL National Renewable Energy LaboratoriesPSA Plataforma Solar de AlmeríaPTFE PolytetrafluoroethylenePTC Parabolic Trough CollectorPVDF Polyvinylidene fluoridePZC Point of Zero ChargeRe Reynold numberUV Ultraviolet light (solar radiation)TFE Tetrafluoroethyleneθa Semi-acceptance angle (parabolic collectors) degreesρs Specular reflectanceσ Optical error (reflective surfaces) mrad

6.1 SOLAR COLLECTOR TECHNOLOGY GENERALITIESTraditionally, different solar collector systems have been classified depending on the level ofconcentration attained by them. The concentration ratio (CR) can be defined as the ratio of

92


the collector aperture area to the absorber or reactor area. The aperture area is the areaintercepting radiation and the absorber area is the area of the component (either fullyilluminated or not) receiving concentrated solar radiation. This CR is directly related to theworking system temperature and, according to this criterion, there are three types ofcollectors:

• Non concentrating or low-temperature, up to 150º C• Medium concentrating or medium temperature, from 150º C to 400º C• High concentrating or high temperature, over 400º C.

This traditional classification considers only the thermal efficiency of the solar collectors.However, in photocatalytic applications, the thermal factor is irrelevant (as already explainedin Chapter 4) whereas the amount of useful radiation collected (in the case of the TiO2

catalyst, with a wavelength shorter then 385 nm) is very important.

Non-concentrating solar collectors (Figure 6.1) are static and non-solar-tracking. Usually,they are flat plates, often aimed at the sun at a specific tilt, depending on the geographiclocation. Their main advantage is their simplicity and low cost. An example is domestic hot-water technology.

Figure 6.1 Non-concentrating solar collectors for domestic heat water application

Medium concentrating solar collectors concentrate sunlight between 5 and 50 times, socontinuous tracking of the sun is required. Parabolic Trough Collectors (PTC) andholographic collectors (Fresnel lenses) are in this group. The first have a parabolic reflectingsurface (Figure 6.2) which concentrates the radiation on a tubular receiver located in the focusof the parabola. They may be one-axis tracking, either azimuth (east-west movement around anorth-south-oriented axis) or elevation (north-south movement around an east-west-orientedaxis), or two-axis tracking (azimuth + elevation). Fresnel lens collectors consist of refractingsurfaces (similar to convex lenses) which deviate the radiation at the same time theyconcentrate it onto a focus.

Figure 6.2 Medium concentrating solar collector. Recirculating parabolic trough reactor for water purificationusing titanium dioxide slurry at NREL. Courtesy of National Renewable Energy Laboratory (USA)

High concentrating collectors have a focal point instead of a linear focus and are based on a

93


paraboloid with solar tracking. Typical concentration ratios are in the range of 100 to 10000and precision optical elements are required. They include parabolic dishes and solar furnaces.

Figure 6.3 High concentration solar collector. Fix Focus solar reactor (PSA, Spain)

Up to now, the solar collectors used for photocatalysis have been in the two first categories. Inorder to illustrate the variation in performance between different orientations, Figures 6.4 and6.5 show, respectively, a comparative analysis of medium concentrating and nonconcentrating solar collector efficiency with regard to direct incident radiation. Directradiation is the radiation which has no interference from the atmosphere and, consequently, aknown direction, and can therefore be concentrated. Global radiation is composed of directand diffuse radiation. The data represented in Figure 6.4, correspond to direct radiation in anideal cloudless year (based on average meteorological data on sunny days at the PlataformaSolar de Almería) and show the energy available from direct radiation on the aperture plane ofa one-axis parabolic-trough collector with different orientations: elevation tracking, azimuthtracking and azimuth tracking slightly tilted 8º with regard to the horizontal.

Figure 6.4 belongs hereYearly efficiency of solar collectors: PTC-one axis with different orientations

The calculations performed are geometric and based on the cosine of the incident angle, thisangle being the one formed by the solar ray with the line normal to the aperture plane of thecollector. They allow to know the amount of direct radiation available at any given time foreach collector configuration. In Figure 6.4, it may be observed that the annual efficiency ofazimuth tracking (east-west movement around a north-south-oriented-axis) is about 10%better than elevation tracking (north-south movement around an east-west-oriented axis). Inthe first case, this efficiency increases notably in the summer and decreases in the winter(identical in the Northern and Southern Hemispheres) whereas it is almost constant around theyear in the second case. A slight 8º tilt to the south in the northern hemisphere and theopposite in the south increases yearly efficiency about 5% in the azimuth-trackingconfiguration due to the reduction of the cosine factor over the year.

Figure 6.5 belongs hereYearly efficiency of solar collectors: flat plate with different inclinations

In the case of non-concentrating flat-plate collectors, it may be observed that efficiencies arelower than one-axis PTCs, attaining maximum efficiency with an inclination (to the south inthe Northern Hemisphere and to the north in the Southern Hemisphere) from the horizontalequal to the local latitude. This configuration, that is, angle of tilt set at the angle of latitude ofthe site, maximizes the annual energy collection in a flat-plate collector. Although thecalculations made here are for a specific location and latitude, the comparisons of solarradiation collection and conclusions obtained are qualitatively valid for any other location.

94


6.2 COLLECTORS FOR SOLAR WATER DETOXIFICATION. FEATURES6.2.1.- Specific features of solar UV light utilization.The specific hardware needed for solar photocatalytic applications have much in commonwith those used for thermal applications. As a result, both photocatalytic systems and reactorshave followed conventional solar thermal collector designs, such as parabolic troughs andnon-concentrating collectors. At this point, their designs begin to diverge, since:

- the fluid must be exposed to ultraviolet solar radiation, and, therefore, the absorbermust be UV-transparent, and

- temperature does not play a significant role in the photocatalytic process, so noinsulation is required.

The first engineering-scale outdoor reactor for solar detoxification was developed by SandiaNational Laboratories (USA) at the end of the eighties (see Figure 6.6) where a parabolic-trough solar thermal collector was modified simply by replacing the absorber/glazing-tubecombination with a Pyrex tube through which contaminated water could flow. Since then,many different concepts with a wide variety of designs have been proposed and developed allover the world, in a continuous effort to improve performance and reduce the cost of solardetoxification systems. Among these different concepts, several of the most important withregard to the definition of the overall system are those related to whether or not radiation mustbe concentrated, the type of reflective surface to be used, the way the water circulates throughthe reactor (tube, falling film or stirred vessel) and the way in which the catalyst is employed.

Figure 6.6 First engineering scale outdoor solar detoxification reactor using one-axis parabolic troughcollector. Part of the 465 m2 parabolic trough system at Sandia National Laboratory. Courtesy

of National Renewable Energy Laboratory (USA)

One of the most important reactor design issues is the decision between concentrating or non-concentrating collector system. Concentrating systems have the advantage of a much smallerreactor-tube area, which could mean a shorter circuit in which to confine, control and handlethe contaminated water to be treated. The alternative of using high-quality ultraviolet-light-transmitting reactors and supported-catalyst devices also seems more logical, botheconomically and from an engineering point of view, if concentrating collector systems are tobe used.

Nevertheless, concentrating reactors have two important disadvantages compared to non-concentrating ones. The first is that they cannot concentrate (i.e., use) diffuse solar radiation,which is unimportant for solar thermal applications, because diffuse radiation is a smallfraction of the total solar radiation. However, solar photocatalytic detoxification with TiO2 asa catalyst uses only the UV fraction of the solar spectrum and, since this radiation is not

95


absorbed by water vapour, as much as 50 percent of this, or more in very humid locations orduring cloudy or partly cloudy periods, can be diffuse. As non-concentrating solar collectorscan make use of both direct and diffuse UV radiation, their efficiency can be noticeablyhigher. The second disadvantage of concentrating collectors is their complexity, cost andmaintenance requirements. The consequence of these disadvantages is that present state-of-the-art favours the use of non-concentrating reactors for solar photocatalytic applications. Anadditional disadvantage of concentrating reactors is that the quantum efficiency is low, due toa square root rather than linear dependence of rate on light flux, as already explained inchapter 4.

For many of the solar detoxification system components, the equipment is identical to thatused for other types of water treatment and construction materials are commercially available.Most piping may be made of polyvinylidene fluoride (PVDF), chlorinated polyvinyl chloride(CPVC), or simply polyethylene. In any case, piping, as well as the rest of the materials, mustbe resistant to corrosion by the original contaminants and their possible by-products in thedestruction process. Neither must materials be reactive, interfering with the photocatalyticprocess. All materials used must be inert to degradation by UV solar light in order to becompatible with the minimum required lifetime of the system (10 years).

Optical material requirements are similar to other closed solar systems, but photocatalyticreactors must transmit UV light efficiently because of the process requirements. In somecases, when a steam pressure of contaminants in water is sufficiently low, a closed systemcould not be required and then a transmissive UV containment material could be avoided.

All pipes, reactor and connection devices must be strong enough to withstand the necessarywater-flow pressure. Typical parameters are 2 to 4 bar for nominal system pressure drop and amaximum of 5 to 7 bar. Concentrating system materials must also be able to withstandpossible high temperatures that could result from absorption of concentrated visible andinfrared light in the reactor.

With regard to the reflecting/concentrating materials, aluminium is the best option due to itslow cost and high reflectivity in the solar terrestrial UV spectrum. Commercially availablefilm products incorporate a thin aluminium foil with an acrylic coating. The last peculiarity ofsolar photocatalytic systems is the requirement of a catalyst; in the case of TiO2 it can bedeployed in several ways, such as a slurry or as a fixed catalyst (like a fiberglass matrixinserted in the reactor tube).

6.2.2 Parabolic Trough CollectorsSolar photoreactors for water detoxification were originally designed for use in line-focusparabolic-trough concentrators. This was in part because of the historical emphasis on troughunits for solar thermal applications. Furthermore, PTC technology was relatively mature andexisting hardware could be easily modified for solar photocatalytic processes. PTCs,considered medium concentrating collectors, are of two types:

a) One-axis parabolic troughb) Two-axis parabolic trough

As explained previously, the first engineering-scale facility was developed from one-axisPTCs (Sandia National Labs, USA, 1989) and the second from two-axis PTCs (PlataformaSolar de Almería, Spain, 1990, Figure 6.7). Both facilities are considerably large pilot plants

96


(hundreds of square meters of collecting surface) and can be considered the first steps inindustrialisation of the photocatalytic process.

Figure 6.7 CIEMAT 384 m2 solar detoxification facility using two-axis parabolic troughcollectors at Plataforma Solar de Almería (PSA, Spain)

Although one-axis tracking has been demonstrated to be the most economically suitable forsolar thermal applications, certain particularities of photocatalytic research make two-axis-tracking PTCs efficient for finding out exactly how much radiation reaches the photoreactorat any given time and also, therefore, permitting accurate evaluation of all the otherparameters related to the photocatalytic process. This accuracy allows comparison ofexperiments carried out in such large photoreactors with lab-scale photoreactors, where thecalculation of incident radiation is much easier. This also makes it possible to reduce thenumber of variables during testing, using the knowledge acquired by other authors.

The equation of the parabola is:

f4

xy

2

= (6.1)

where f is the focal length. If D is the aperture width and d, the reactor tube diameter, thegeometric concentration of the collector C is:

dð

DC = (6.2)

The basic components of a parabolic-trough collector for photocatalytic applications are: thereflecting concentrator, the absorber tube (photoreactor), the drive-tracking system and theoverall structure. Of these, the last two do not differ in photocatalysis from the applicationsfor which they were originally designed and are identical to those existing for thermalapplications. Reflective surfaces and photoreactor technology are specifically discussed underpoint 6.4.1, as it can be considered independently of the solar collector used.

The collector structure supports the reflecting concentrator system, which reflects directinsolation onto the receiver tubes. Two-axis PTCs consist of a turret on which there is aplatform supporting several parallel parabolic trough collectors with the absorber in the focus.The platform has two motors controlled by a two-axis (azimuth and elevation) tracking system.Thus the collector aperture plane is always perpendicular to the solar rays, which are reflected bythe parabola onto the reactor tube at the focus through which the contaminated water to bedetoxified circulates. One-axis PTCs have only one motor and a one-axis solar-tracking system;the reactor tube (linear focus of the parabola) is then positioned in the same plane containing the

97


normal vector of the collector aperture plane and the solar vector (See Figure 6.8). The angleformed by these two vectors is called the incident angle of solar radiation.

Figure 6.8 belongs hereSolar ray reflection on a one-axis parabolic trough collector

After all optical losses have been considered, the effective concentrating ratio of PTCs isusually between 5 and 20. Typical overall optical efficiencies in a PTC are in the range of 50to 75 percent, with the following breakdown:

− Tracking system: 90%-95%− Reflector/Concentrator (reflectivity): 80%-90%− Absorber/Reactor (transmittance): 80%-90%− Mechanical collector errors: 90%-95%

Parabolic-trough collectors make efficient use of direct solar radiation and, as an additionaladvantage, the thermal energy collected from the concentrated radiation could be used inparallel for other applications. The size and length of the reactor is smaller, receiving a largeamount of energy per unit of volume, so handling and control of the liquid to be treated issimpler and cheaper, and the risk of leaks, which in many cases can be dangerous, is lower. Ingeneral, this can also be translated into a reactor able to withstand higher pressures and able toemploy potentially costly supported-catalyst configurations.

6.2.3 One-Sun (Non-Concentrating) CollectorsOne-sun non-concentrating collectors (CR = 1) are, in principle, cheaper than PTCs as theyhave no moving parts or solar tracking devices. They do not concentrate radiation, so theefficiency is not reduced by factors associated with reflection, concentration and solartracking. Manufacturing costs are cheaper because their components are simpler, which alsomeans easy and low-cost maintenance. Also, the non-concentrating collector supportstructures are easier and cheaper to install and the surface required for their installation issmaller, because since they are static they do not project shadows on the others.

Based on extensive effort in the designing of small non-tracking collectors, a wide number ofnon-concentrating solar reactors have been developed for solar photocatalytic applications,which can be classified as follows:- Trickle-down flat plate, based on a tilted plate facing the sun over which the water to be

treated falls slowly; the catalyst is fixed on plate surface.- Free-falling film, similar to the trickle-down flat plate, but with a higher flow rate and

normally with a catalyst attached to the surface on which the liquid to be treatedcirculates. It is usually open to the atmosphere, so it can be used only when volatilecompounds are not present.

- Pressurized flat plate, consisting of two plates between which water circulates using aseparating wall which can be filled in with fibre to which the catalyst is attached.

- Tubular: this kind of collector usually consists of many small tubes connected in parallelto make the flow circulate faster than a flat plate, but functioning basically the same.

- Shallow solar ponds. This is a very interesting variety, as pond reactors are easily built on-site, especially for industrial wastewater treatment. Since manufacturing industries alreadyuse ponds for microbiological treatment of wastewater, shallow solar ponds can be usedfor the front or back end of a combined solar/microbiological treatment scheme (see alsochapter 8.1).

98


Figure 6.9 Experimental set up of a thin film fixed bed reactor tested by ISFH at PSAinstallations. The housing is made of plexiglas and the catalyst is fixed on a flatglass plate. Courtesy of Institut für Solarenergieforschung GmbH (Hannover,

Germany)

Any falling film or flat reactor must be covered to avoid direct contact with the atmosphere.The use of an uncovered reactor is not recommended due to many factors: loss of volatilecontaminants, dust and dirt inside the reaction mixture, etc.

Although one-sun designs possess important advantages, the design of a robust one-sunphotoreactor is not trivial, due to the need for a large area of weather-resistant, and chemicallyinert ultraviolet-transmitting reactors. The amount of materials required makes it necessaryfor them to be relatively inexpensive. Those that best meet these requirements are certaintypes of plastics, e.g., fluoropolymer films, but these, although highly versatile, possess lowertensile strength than the rigid glass pipe and reduces the pressure capacity of the photoreactorsystem. This combination of low pressure capacity and large volume, coupled with the needto either keep a catalyst slurry suspended or ensure good mass transfer to a supported catalyst,requires carefully designed fluid control..

Non-concentrating systems require significantly more photoreactor area than concentratingphotoreactors and, as a consequence, full-scale detoxification systems (hundred of squaremeters of collectors) must be designed to withstand the operating pressures anticipated forfluid circulation through a large field. As a consequence, the use of tubular photoreactors hasa decided advantage because of the inherent structural efficiency of tubing; tubing is alsoavailable in a large variety of materials and sizes and is a natural choice for a pressurized fluidsystem. Finally, its construction must be economical and should to be efficient with lowpressure drop.

If a supported catalyst is used, the photoreactor has to be much larger than in a concentratingsystem. If the catalyst is circulated in a slurry, the design would have to avoid low-flowregions where the catalyst could settle out of suspension, which means that turbulent flowmust be assured throughout the hydraulic circuit. Containment of volatile organiccontaminants to prevent their escape into the atmosphere is also of concern in a large reactor.

99


Figure 6.10 (a) and 6.10 (b)One-sun water treatment reactors with PTFE tubes at NREL: early reactor (a) and reactor

with titanium dioxide immobilized on glass fiber bundles (b).Courtesy of National Renewable Energy Laboratory (USA)

6.2.4 Compound Parabolic Concentrator (CPC)CPC collectors are a very interesting cross between trough concentrators and one-sun systemsand are one of the best options for solar photocatalytic applications. CPCs have been found toprovide the best optics for low concentration systems and these non-imaging concentratorswere extensively employed for evacuated tubes. CPCs are static collectors with a reflectivesurface following an involute around a cylindrical reactor tube; it can be designed with aCR=1 (or near one), then having the advantages of both PTCs and one sun collectors.

Figure 6.11 belongs hereSolar reflection on a CPC collector

Thanks to the reflector design, almost all the UV radiation arriving at the CPC aperture area(not only direct, but also diffuse) can be collected and is available for the process in thereactor. The UV light reflected by the CPC is more or less distributed around the back of thetubular photoreactor and as a result most of the reactor tube circumference is illuminated, butdue to the ratio of CPC aperture to tube diameter, no one point on the tube receives muchmore than one sun of UV light. As a result, the UV light incident on the reactor is very similarto that of a one-sun photoreactor and, as in the case of flat-plate collectors, maximum yearlyefficiency is obtained at the same collector angle inclination as the local latitude.

Performance is very close to that of the simple tubular photoreactor, but only about l/3 of thereactor tube material is required. As in a parabolic trough, the water is more easily piped anddistributed than in many one-sun designs. All these factors contribute to excellent CPCcollector performance in solar photocatalytic applications.

100


The explicit equation for a CPC reflector with a tubular reactor can be obtained from Figure6.11; a generic reflector point S can be described in terms of two parameters, angle θ ,subtended by lines originating at O (centre of the reactor tube) to A and R, and distance ρ ,given by segment RS:

OROA=θ (6.3)

RS=ρ (6.4)

RS being tangent to the reactor tube at R. One important parameter for CPC definition is theangle of acceptance a2θ , which is the angular range over which all or almost all rays areaccepted (i.e., reflected into the reactor tube) without moving the collector.

Figure 6.12 belongs hereObtention of CPC involute

The solution is given in two separate portions, an ordinary involute for A to B and an outerportion from B to C:

2forr a πθθθρ +≤= part AB of the curve (6.5)

( )( ) aa

a

aa

2

3

2for

sin1

cos2r θπθπθ

θθθθπθθ

ρ −≤≤+−+

−−++= part BC of the curve (6.6)

The CPC concentration ratio (CR) is given by:

asin

1C

θ= (6.7)

In the special case of aθ =90º, CR=1 and every CPC curve is an ordinary involute (points B

and C are coincident). So, optimum CPC acceptance half-angles ( aθ ) for photocatalyticapplications are obtained from 60 to 90 degrees either side of the normal. This wideacceptance angle allows the reflector to direct both direct-normal and diffuse sunlight onto thereactor, as UV light collection is not highly sensitive to these acceptance angles. Anadditional advantage is that these wide acceptance reflectors forgive the reflector-tubealignment errors, which is an important virtue for a low-cost photoreactor array.

101

θa

θa

θ

A

S

O

B

x

y

R C

r

Fig. 6.12

102


Figures 6.13(a) and 6.13(b) View of CPC shape (a) and CPC photoreactor array (b). PSA(Spain)

CPC reflectors are usually made of polished aluminium and the structure can be a simplephotoreactor support frame with connecting tubing. Since this type of reflector is considerablyless expensive than tubing, their use is very cost-effective compared to deploying non-concentrating tubular photoreactors without use of any reflectors, but preserving theadvantages of using tubing for the active photoreactor area.

6.2.5 Holographic CollectorsAnother innovative idea is the holographic concentrator. This concept has been extensivelyexplored with regard to solar thermal applications as well as in the area of concentrators forphotovoltaic systems. Holographic surfaces are highly wavelength-selective and theirdevelopment for solar thermal applications, which require the full solar spectrum, has provedto be a very difficult task. However, the holographic technologies could be very appropriatefor narrow-wavelength-band processes such as photovoltaics and photochemistry.

Holography is basically a diffractive technology. It records the interference pattern between areference beam of highly coherent monochromatic light and an object beam using the samelight source. In the case of solar holographic concentrators, the object beam is the one in thefocal region (point-focus or line-focus concentrator) and the reference beam is the virtualimage of the sun as a source. Once created, sunlight incident on the holographic optic elementwill focus back to the focal region by either transmission or reflection depending on whetherthe reference beam used to create the hologram strikes the diffractive material from the sameor opposite direction as the object beam. As a hologram is a passive optical device, it is notpossible to track the sun without moving.

Figure 6.14 belongs hereHolographic concentration of solar light

Normally, holographic elements are made with highly coherent monochromatic laser light inorder to obtain the most efficient hologram at that wavelength. Efforts carried out with the sunas the source of light have resulted in a maximum usable bandwidth of about 100 nm,obviously insufficient for thermal applications. However, as the photocatalytic process withTiO2 uses 300 to 385 nm photons, the holographic concentrator could very well be a goodway not only to supply these photons while filtering out those that are unnecessary, but alsominimising thermal heating of the photoreactor.

103

Focus

Holographic Optic Element

Focus

Holographic Optic ElementA B

Fig. 6.14

104


Furthermore, solar holographic concentrators can theoretically avoid the technologicalcomplexities associated with the curved support of conventional reflecting concentrators.However, many significant issues remain unresolved. One of the most important is whetherthe devices developed in research programs can be feasibly translated to realistic cost-effective sizes for the collection of solar energy. Also, there are not many holographicmaterials in the UV wavelength region and their ability to survive in an outdoor environmentis unknown.

6.3 CONCENTRATED VERSUS NON-CONCENTRATED SUNLIGHT.The use of concentrating or non-concentrating collectors is based on laboratory andengineering-scale experiments carried out by many different research groups. As alreadymentioned, during these experiments it was found that non-concentrating collectors have theadvantage of collecting diffuse light. UV light is more susceptible to Rayleigh scattering byatmospheric gases, mainly water vapour, than visible light. The same mechanism scatters bluelight more than red light, which is what causes the sky to appear blue. Because of thisscattering, as much as half of the UV radiation arrives at the earth’s surface as diffuse light,even on a clear day. Near-UV wavelengths (from 285 to 385 nm) comprise only 2-3% of theenergy in direct sunlight, but they make up 4-6% of combined diffuse and direct sunlight.

Concentrating collectors focus only the direct sunlight and cannot collect the diffuse light.Thin clouds, dust, and haze reduce the direct-beam component of sunlight more than thediffuse component. As a result, non-concentrating collectors can use a resource that is notonly larger but also less variable than that available to concentrating collectors, permitting, inmany locations, continual operation of the non-concentrating detoxification system. Undercloudy conditions, non-concentrating devices can continue operating (although at lowerrates), while a trough unit would have to shut down. This fact has been successfullydemonstrated even in northern European locations with small solar detoxification pilot plants(Figure 6.15).

Figure 6.15 Solar detoxification pilot plant in Koln (Germany)Courtesy of Deutsche Forschungsanstalt für Luft- und Raumfahrt (DLR)

An additional benefit of a non-concentrating design is that the efficiency of the photocatalyticprocess frequently decreases as light intensity increases (as explained in Chapter 4). Thisbehaviour means that catalysts are unable to process all of the UV energy available to thedesired pollutant destruction reactions when subjected to high UV fluxes. Lower lightintensities also slow down the rate of recombination, thus yielding higher efficiency (seefigure 4.8). Nevertheless, different researchers have obtained different results when testingwith low radiation intensities, so presumably, they are significantly affected by experimentalconditions.

105


Some authors attribute the transition of ( ) ( )5.00.1 IfrIfr =→= , to the excess ofphotogenerated species (e-, h+ and •OH). At higher radiation intensities, another transitionfrom ( ) ( )05.0 IfrIfr =→= is produced. At this moment, the photocatalytic reaction departsfrom its dependence on radiation received, depending only on mass transfer within thereaction, so the rate remains constant although radiation increases. This effect may be due toseveral causes, such as the lack of electron scavengers (i.e. O2), or organic molecules in theproximity of TiO2 surface and/or excess of products occupying active catalyst centres, etc.Actually, this phenomenon appears to be more frequent with supported catalysts and/or atslow mixing speeds. This implies small catalyst surface in contact with the liquid and lessturbulence, which does not favour contact of reactants with the catalyst and dispersion ofproducts.

Finally, one-sun photoreactors have additional advantages, such as decreasing optical lossesfrom reduced or non-existing reflective surface. On the other hand, among possibleadvantages of concentrating systems is a much smaller reactor-tube area requirement (smallercircuit, better control and handling of contaminated water to be treated). Nevertheless, state-of-the-art technology cannot match the higher efficiency of non-concentrating systems. Thishas been demonstrated by many comparative efficiency studies, such as the one performed byNREL (National Renewable Energy Laboratories, USA) with seven different small collectors(from 18 to 157 litres total capacity and from 0.4 m2 to 53 m2 reflecting surface). Resultsshowed that one-sun collectors were significantly more efficient than concentrating collectors.

Due to all the above, and, furthermore, although it was first thought that PTCs were the idealtechnological alternative, their high cost and the fact that they can only be operated withdirect solar radiation (implying location only in highly insolated areas), have decided thequestion in favour of the static non-concentrating collector alternative. Their intrinsicsimplicity, low maintenance and operating cost and potential for reducing the manufacturingcost, make one-sun systems the natural selection for solar water detoxification.

Among the different technologies previously described, CPCs seem to be one of the bestdeveloped options for system design and implementation. In this specific case, and moreover,since the absorber is illuminated differently during the morning and the afternoon, if theradiation distribution is integrated over the solar day, an almost regular distribution of light isobtained along the reactor perimeter (Figure 6.16). This agrees with the optical characteristicof low-concentrating CPC-type collectors, which can collect, within their acceptance angle,the radiation coming from the hemisphere and place it on the absorber perimeter.

Figure 6.16 belongs hereFlux distribution on absorber along the solar day (6:00 to 18:00). Simulation of CPC

behaviour with the following data: collector orientation: East-West; semi-acceptance angle:60º; truncation angle: 80º; absorber radius: 13.6 mm; optical gap: 2 mm; concentration

ratio: 1.17. Courtesy of Renewable Energy Dept. INETI (Portugal)

6.4 TECHNICAL ISSUES

In addition to the solar collector type, the most important technical issues related with solardetoxification hardware are the reflective surface and the reactor tube, which are addressedhere in more detail.

6.4.1 Reflective Surfaces

106

Fig. 6.16

107


The optical quality requirements of reflective surfaces for solar applications are usuallyrelated to the concentration required by the particular application under consideration. Thehigher the concentration desired, the stricter the requirements for quality of parameters. Lightreflected off a polished or mirrored surface obeys the law of reflection: the angle between theincident ray and the normal to the surface is equal to the angle between the reflected ray andthe normal. When light reflects off a rear surface mirror, the light first passes through theglass substrate, resulting in reflection losses, secondary reflections, refraction, absorption, andscattering of light passing through the transparent substrate (second-surface mirrors).Precision optical systems use first-surface mirrors that are aluminized on the outer surface toavoid these phenomena.

When light obeys the law of reflection, it is termed a specular reflection. Most hard polished(shiny) surfaces are primarily specular in nature. Even transparent glass specularly reflects aportion of incoming light. Diffuse reflection is typical of particulate substances like powders.If you shine a light on baking flour, for example, you will not see a directionally shinycomponent. The powder will appear uniformly bright from every direction. Many reflectionsare a combination of both diffuse and specular components. One manifestation of this is aspread reflection, which has a dominant directional component that is partially diffused bysurface irregularities (Figure 6.17).

Figure 6.17 belongs hereSpecular, diffuse and spread reflection from a surface

In the case of solar detoxification applications, the strictest requirements are those of PTCs,for example, UV-mirror materials need to have a specular reflectance between 300-400 nm inorder to achieve concentration ratios of from 1 to 20. For this configuration, the effectiveconcentration ratio (Cε) can be related to the optical performance parameters as:

=

σσ

ρεsum

sCC (6.8)

Where:C = concentration in the absence of surface and tracking errors, as defined in eq. 6.2

sρ = specular reflectance

sumσ = half angular extent of sun (Gaussian distribution) = 2.73 mrad

σ = total optical error, which is function of the slope, specularity and tracking errors

The greater the errors are, and particularly the reflective surface errors, the lower the effectiveconcentration ratio is. So, the reverse is also true: the lower the effective concentration ratiois, the higher the optical errors may be and therefore, the lower the quality of reflectivesurface required. This is an important additional factor in favour of low or non-concentratingsystems, since these lower quality requirements (lower specular reflectance) are directlytranslated into lower manufacturing cost, since the reflector element can represent aconsiderable fraction of collector cost.

Another important factor is the reflective base material. For solar photocatalytic applications,the reflective surface must clearly be made of a highly reflective material for ultravioletradiation. The reflectivity between 300 and 400 nm of traditional silver-coated mirrors is verylow (reflected radiation/incident radiation) and aluminium-coated mirrors is the best option inthis case (Figure 6.18).

108

Specular Diffuse Spread

Fig. 6.17

109


Figure 6.18 belongs hereReflectivity of fresh metal coatings for mirrors

Aluminium is the only metal surface that is highly reflective throughout the ultravioletspectrum. Reflectivities range from 92.3 percent at 280 nm. to 92.5 percent at 385 nm.Comparable values for silver are 25.2 percent and 92.8 percent, respectively. A new depositedaluminium surface is fragile and needs to be protected from weathering and abrasion, but theconventional glass cover used for silver-backed mirrors has the drawback of significantlyfiltering UV light (an effect that is duplicated due to the light path through the glass). The thinoxide layer that forms naturally on aluminium is not sufficient to protect it in outdoorenvironments. Under such exposure conditions, the oxide layer continues to grow and UVreflectance drops off dramatically.

Various optical technologies require UV-reflective elements, such as UV mirrors for medicalimaging, astronomical telescopes, microscopy, UV curing, indoor lighting, microlithography,industrial micro machining and UV laser reflection. Nevertheless the specific requirements ofthese applications are very different from those of solar, mainly with regard to outdoordurability. The ideal reflective surface for solar photocatalytic applications must be:- highly reflective in the UV range,- acceptable durability under outdoor conditions for extended service lifetimes and- reasonable price to permit the technology to be competitive against alternative

technologies

The surfaces currently available that best fit these requirements are:− electropolished anodized aluminium (electrolytically formed aluminium oxide outer layer)− organic plastic films with an aluminium coating (three-part “sandwich”-type plastic-

aluminium-plastic composition)

Anodized coatings with tin oxide can provide good protection against some chemicals andgood resistance to abrasion. Typically, thin (2-3 µm) oxide layers are used to provide somemeasure of resistance to abrasion but little protection against moisture or pollutants isprovided. Thicker oxide layers (up to 50 µm) are usually specified when anodized aluminiumis intended for engineering/marine applications but, unfortunately, such a coating results inconsiderably lower reflectance in the UV range.

An interesting alternative approach is to cover the aluminium with a protective acryliclacquer. Acrylic lacquering provides impressive outdoor resistance (more than 1000 hours insalt-spray fog chamber without significant degradation), but also reduces UV reflectance. Acompromise between outdoor resistance and UV reflectance could be an optimum solution.

Another possible solution is an aluminium-coated plastic film. Several commercial coatedplastic film products have been used successfully in parabolic trough applications.− ECP-244 (3M film no longer manufactured) consisting of a 10-µm-thick aluminium

surface covered by a 76-µm-thick acrylic surface. Average reflectivity of new filmbetween 280 and 385 nm thick is about 63 percent.

− SA-85P (manufactured also by 3M); this film has a 50-µm-thick polyester backing, an 10-µm-thick aluminium surface, and a very thin 2.5-µm-thick acrylic covering. Averagereflectivity in the same range is 87 percent for new films. The superior reflectivity is dueto the much thinner protective acrylic coating.

110


− ECP-305 (3M); a silver reflective film similar in composition to SA-85.− Other companies and even some research institutions have developed similar coated films,

usually a combination of over-100-µm-thick polyester backing on 10-µm-aluminium foiland a thin outer film of acrylic or some copolymer highly resistant to outdoor exposurewith good UV transmittance.

Normally, due to their lack of rigidity, these films must be bonded over a stiff substrate andabout two percent specular reflectivity is lost in this process. Also, the reflectivity of eachfilm at the end of its lifetime (from 5 to 10 years) would be only 88 percent of the new bondedvalue.

Figure 6.19 belongs hereReflectivity of different aluminium and plastic film surfaces.

Curve (d) by courtesy of Alanold Aluminium-Veredlung GmbH & Co (Germany)

6.4.2 Photocatalytic ReactorThe requirements for the photocatalytic reactor are similar to other advanced water or airoxidation processes, with the additional necessity of an illuminated photocatalyst. Thephotocatalytic reactor must contain the catalyst and be transparent to UV radiation providinggood mass transfer of the contaminant from the fluid stream to an illuminated photocatalystsurface with minimal pressure drop across the system.

As mentioned before, the square-root dependence on light intensity provides better photo-efficiencies for one-sun designs, which leads to a flat-plate geometry. This geometry is widelyused for solar-powered domestic hot water heater systems in large part because of its simpledesign. Nevertheless, for water treatment, the reactor must be hard enough to work underusable water pressure and tube configurations clearly seem the most appropriate for fluidcontainment and pumping. Adequate flow distribution inside the reactor must be assured, asnon-uniform distribution leads to non-uniform residence times inside the reactor, resulting indecreased performance compared to an ideal-flow situation. If the catalyst is used insuspension (slurry in the case of TiO2), the Reynold number (Re) must always be over 4000in order to guarantee turbulent flow. This is critical in avoiding catalyst settlement. Anotherimportant design issue is that internal reactor materials must not react with either the catalystor the pollutants to be treated or their by-products.

The choice of materials that are both transmissive to UV light and resistant to its destructiveeffects is limited. Also, temperatures inside a one-sun solar photocatalytic reactor can easilyexceed 40°C due to the absorption of the visible portion of the solar spectrum. Therefore, aone-sun reactor must be able to withstand summer temperatures of around 60 to 70°C in orderto insure that there will be no damage which could reduce the flow. Finally, low pH resistanceis needed since the production of inorganic acids as reaction by-products is quite normal (i.e.the destruction of chlorinated hydrocarbons leads to the production of HCl).

Common materials that meet these requirements are fluoropolymers, acrylic polymers andseveral types of glass. Quartz has excellent UV transmission and temperature and chemicalresistance, but the slight advantage in transmission in the terrestrial solar spectrum over othermaterials does not justify its high cost, which makes it completely unfeasible forphotocatalytic applications.

111


Figure 6.20 belongs hereTransmittance of different materials suitable for the manufacture of photoreactor tubes

Plastics work well as long as they fulfil transmittance, pressure and thermal resistancespecifications as well as maintaining their properties during outdoor operation.Fluoropolymers are a good choice of plastic for photoreactors due to their good UVtransmittance, excellent ultraviolet stability and chemical inertness. Several different types,such as ETFE (ethylenetetrafluoroethylene), PTFE (polytetrafluoroethylene), ECTFE(ethylenechloridetetrafluroethylene), PVDF (polyvinylidene fluoride), FEP (fluorinatedethylenepropylene), PFA and TFE (tetrafluoroethylene), can be extruded into tubing and usedas a photoreactor. Tubular fluoropolymers are very strong, possess excellent tear resistance,and are flexible and lighter than glass. One of their greatest disadvantages is that, in order toachieve a desired minimum pressure rating, the wall thickness of a fluoropolymer tube mayhave to be increased, which in turn will lower its UV transmittance. In addition, due to thelack of rigidity, tube connections can withstand much lower pressures than glass tubes. ETFEand FEP are among the best candidates; ETFP has higher tensile strength (extrude-like) thanFEP; this could mean thinner-walled tubes resulting in cost savings (since less material isused) and higher UV transmittance and therefore higher photoreactor performance. Theproblem is that ETFE tubing is not as readily available as FEP tubing. 50-mm-outer-diameter,0.6-mm-wall FEP tubing has a UV hemispherical transmittance (300 to 400 nm), of 61.6%.This light is transmitted as diffuse, as fluoropolymer materials are poor IR-diffusers, but makean excellent visible / UV diffusers (diffusion usually varies with wavelength).

Acrylics could also potentially be used to enclose the photoreactor. However, acrylics arevery brittle and would have to be employed in sheets, which increases their cost. On thepositive side, acrylic polymer sheets can be shaped with channels and flow patterns that couldthemselves be used as solar reactors. Other lower-cost polymers are available in tube form,but none possess the necessary UV and chemical stability for detoxification of water that maybe contaminated with a variety of solvents or other pollutants. Also, low cost polymericmaterials are significantly more susceptible to attack by the pollutant molecules and thedissolution of organic contaminants in polymer materials could be a way of avoiding thedegradation process.

Glass is another alternative for photoreactors. Standard glass, used as protective surface, is notsatisfactory because it absorbs part of the UV radiation that reaches it, due to its iron content.Borosilicate glass has good transmissive properties in the solar range with a cut-off of about285 nm. Therefore, such a low-iron-content glass would seem to be the most adequate.

Figure 6.21 Glass tubes manufacturing. Different compositions mean that the glass can beused for a wide variety of applications. Courtesy of Schott-Rohrglas GmbH

112


Two undesirable effects reduce the performance of a glass reactor for solar detoxification:increased absorption in the solar UV-range between 300 and 400 nm and a further decrease ofUV-transmittance during operation due to the damaging impact of solar radiation in the samewavelength region (UV-solarisation). Both effects are caused to a large extent by polyvalentions that change charge. The effect of the Fe-ions in the glass, which change charge from Fe2+

to Fe3+ due to photo-oxidation by photons having a wavelength below 400 nm, is especiallyharmful. Furthermore, the oxidised Fe3+ ion absorbs in the UV. As a result, enhancement oftransmittance in the 300-400 nm region could only be accomplished by strong reduction iniron content down to 50 ppm (Figure 6.22), but penalised by a corresponding increase in cost.

Figure 6.22 belongs hereInfluence of Iron on transmittance of Borosilicate Glass Light Transmission (oxidativeconditions). Samples: flat glass; 3 mm thickness. Courtesy of Schott-Rohrglas GmbH

Therefore, as both fluoropolymers and glass are valid photoreactor materials, cost becomes animportant issue. In large volumes, glass piping could be more expensive than fluoropolymertubing, but from the perspective of performance, the choice is the material that has the bestcombination of tensile strength and UV transmittance. On this basis, if a large field is beingdesigned, large collector area means also a considerable number of reactors and, asconsequence, high system pressure rating. Thus, fluoropolymer tubes are not the best choiceof material since high-pressure is linearly related to thickness and could result in higher cost.A detailed analysis is recommended for any specific design.

One of the most important parameters in a tubular photoreactor design is the diameter, as inboth homogeneous or heterogeneous photocatalysis it must be guaranteed that all arrivinguseful photons are kept inside the reactor and do not go through it without intercepting acatalyst particle. The intensity of illumination affects the relationship between reaction rateand catalyst concentration. The dispersion and absorption of light causes photon density todiminish almost exponentially over the length of the optical path within a catalyst suspension.At higher light intensity, the catalyst concentration can be higher.

In the case of TiO2 heterogeneous photocatalysis, when catalyst concentration is very high, a“screening” effect produces excessive opacity of the solution, preventing the catalyst particlesfarthest in from being illuminated and reducing system efficiency. The lower the catalystconcentration, the less opaque the suspension. 1 g L-1 of TiO2 catalyst reduces transmittanceto zero in a 10-mm-inner-diameter cylinder with concentrated light in a parabolic troughcollector (Figure 6.23). Therefore, in a wider diameter tube, only an outer layer is illuminated.This means that larger inner reactor diameter permits use of lower optimum catalystconcentrations. Practical inner diameters for tubular photoreactor would be in the range of 25to 50 mm. Diameters that are very small do not make sense because of the associated highpressure-drop and very large diameters imply a considerable dark volume, thus reducingoverall system efficiency.

Figure 6.23 belongs hereZone of tubular reactor where light penetrates if the catalyst concentration is 1 g L-1

(TiO2 heterogeneous photocatalysis)

6.5 CATALYST ISSUESThe catalyst plays a major role, not only because of its importance to the process, but alsofrom a technological point of view. This is especially relevant in heterogeneous

113

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

250 275 300 325 350 375 400

W avelength [nm ]

Tra

nsm

itanc

e

A: 0ppm Fe, 91% (300- 400 nm )B: 50ppm Fe, 88%C : 100ppm Fe, 84.5%D : 150ppm Fe, 83.5%E: 200ppm Fe, 81%F: 250ppm Fe, 80%

A

D

C

B

E

F

Fig. 6.22

114

Illuminated area[TiO2] = 1 g L-1

R = 29 mm

ID = 58 mm

1 cm

Fig. 6.23

115


photocatalysis, where Degussa P-25 titanium dioxide is often the standard particulate materialagainst which other catalysts have been and continue to be measured. TiO2 in aqueous phaseapplications can be used in suspensions (slurry) or supported. In the first case, the catalystmust be recovered. The technological implications of this are discussed below.

6.5.1 Slurry versus Supported CatalystOne of the major solar photoreactor system design issues is whether to use a suspended orsupported catalyst. The majority of experiments to date have used small TiO2 particlessuspended in the contaminated water, which makes it necessary to recover them aftertreatment. This process is addressed under the next point.

Supported catalyst configurations eliminate the need for catalyst filtration, but with the mainobjection of an important reduction in system efficiency. The idea is to attach the catalyst to asupport inside the reactor as is done for gas-phase stream treatment, which requires thecatalyst to be anchored onto some type of inert support. Desirable characteristics of such asystem would include being very active (comparable to slurries), have a low pressure-drop,long lifetime, and reasonable cost but, in the case of water treatments, to present this has notbeen possible.

Fixed-catalyst designs must solve several problems. As the catalyst must be exposed tosunlight and in contact with the pollutant, the support must be configured to efficiently routethe pollutant to the illuminated zone and, at the same time, maintain a high flow rate in thewater to ensure good mixing without significantly increasing system pressure, which meansmore power for pumping, and thereby higher operating costs. Also, the same criteriadiscussed for photoreactor materials must be kept in mind and applied when choosing asupport. Supports tested so far have included fibreglass beads, metal fibres, steel mesh,aluminium, many types of plastic (Figure 6.24) and ceramics such as alumina, silicon carbideand silica, in the most diverse shapes. TiO2 coatings on tiny hollow glass beads calledmicroballoons for catalyst removal, by screening rather than filtering, have also been tested.

Figure 6.24(a) and 6.24(b) (a) Experimental concentrating solar reactor using titaniumdioxide immobilised on glass wool for treating contaminated air streams. (b) Parabolic trough

reactor for water purification with immobilised titanium dioxide. Courtesy of NationalRenewable Energy Laboratory (USA)

Support of TiO2 on a stiff surface by adherence can be done using several differenttechniques, such as dip-coating with solvents, deposits from precursors, vapour deposition,

116


and sol-gel formation. Several important performance requirements are directly related to theprocess used for catalyst fixation, such as the durability of the coating, catalyst activity,lifetime, etc. Surface area of the catalyst coating must also be considered because goodcontact of photons and target molecules with the catalyst is required for efficient performance.

Studies performed to date have not yet identified a fixed-catalyst system that performs asefficiently as slurry systems. Several exceptions, such as a test performed by NREL (USA)using silica beads as the catalyst support, have approached the efficiency of the slurry, but thehigh pressure-drop across the bed made the system impractical. In aqueous systems,compared to an unsupported catalyst, immobilisation of TiO2 results in a reduction inperformance around 60 to 70 percent. An important direct consequence of this fact is thenecessity of multiply by a factor of about 3 the size of necessary solar collector field if similarefficiencies want to be obtained, making the overall system clearly less cost efficient andcompetitive than slurry systems. In addition to the above, a key question is how long asupported catalysts will last in a real stream of water; a short period of activity would meanfrequent replacement and, consequently, an important rise in the overall system cost.

To the contrary of fixed catalyst configurations, slurry configurations have the advantage ofhigher throughputs (in the range of 200 to 400 percent) a low pressure-drop through thereactor and excellent fluid-to-catalyst mass transfer.

6.5.2 Catalyst Recuperation and reuseThe need to remove the catalyst from the clean water after treatment was initially considered amajor disadvantage of a slurry system because ultrafiltration, which was believed difficult andcostly, would be required for efficient separation of the submicron-sized TiO2 particles fromthe slurry. Although it is true that titanium dioxide powder particles are about 0.03 µm (30nm), or even smaller in some cases (specially manufactured TiO2 can be up to 0.01 µm), oncein water, the particles always agglomerate into larger ones (from 0.3 to 0.6 µm), facilitatingthe problem considerably, since microfiltration, which is much less expensive thanultrafiltration, can be used. 0.2-µm carbon-graphite membranes can effectively concentratetitanium dioxide slurry from less than 1 g L-1 to up to 100 g L-1 with about 2 to 3 bar trans-membrane pressure and permeate flow rates of about 2500 l h-1 per square meter ofmembrane. Recovery rates of over 99% have been obtained with 0.5 µm membranes and notitanium dioxide has been detected in water filtered with 0.2 µm membranes.

However, best performance can be obtained when microfiltration is combined with previoustitanium dioxide sedimentation. About 90% of the catalyst can easily be recovered bysedimentation and the rest by microfiltration. This means a significant reduction in time andelectricity in the typical recovery process for TiO2 concentrations of about 200 mg L-1. Thelifetime of membranes and the time between cleanings is also increased considerably. Thiscould be particularly important with high volumes of water.

Titanium dioxide sedimentation is closely related to colloidal stability and TiO2 aggregationconditions. The suspension can easily be destabilized by adding an electrolyte (such as NaCl)and/or adjusting the pH to point zero charge (PZC) and the isoelectric point (IEP) on thesurface of the catalyst particles, as both factors modify the surface charge. Progressive particleagglomeration (sizes from 1 to 10 µm) and settlement is then obtained. In the case of TiO2

(Degussa P-25), at concentrations of 200 mg L-1, the PZC is obtained at about pH 7 (6.8±0.2,in experiments at the Plataforma Solar de Almería) when NaCl concentration is about 10-6

molar.

117


Therefore, more than 200 hours are needed for 75% of titanium dioxide to settle at pH 4.5, butat pH 7, only 5 hours of storage are needed to recover 90 to 95%. This can be directlyrecovered in a conic-bottom tank and 5 to 10% of the remaining catalyst can be recovered bymicrofiltration (see also chapter 9).

Figures 6.25 belongs hereSedimentation experiments at different pH. [TiO2]=0,2 g/L; [NaCl]=0 M. The Y axis showsthe absorbance of the solution at 800 nm; no absorbance means absence of TiO2 (see also

Figure 4.2)

Catalyst recovered, usually in highly concentrated slurry, can be reused but not indefinitely.Slurry lifetime has been tested with satisfactory results under laboratory conditions (i.e.,deionized water and only one contaminant), but with real water treatments catalyst lifetimewould be diminished due to poisoning by contaminants. However, in specific applications,tests have demonstrated that the catalyst can be reused up to 10 times, or even more, withoutany problem.

If part of the catalyst is lost in the drainage water, the percentage of catalyst that remainsuseful after each run is a relevant parameter. In field tests conducted by NREL in Florida, itwas reported that approximately 10% of the catalyst was washed away in the discharge water.In tests performed at the Plataforma Solar de Almería, 2.5% of catalyst was lost. From thispoint of view, about a 5 to 10% addition of catalyst could be an interesting option tocompensate possible loss and periodic replacement in case of catalyst poisoning.

SUMMARY OF THE CHAPTERSolar collectors are traditionally divided in three categories: non-concentrating (or lowtemperature, up to 150º C), medium concentrating (or medium temperature, from 150º C to400º C) and high concentrating (or high temperature, over 400º C). Concentrating solarsystems make use of direct radiation and need solar tracking mechanisms. Non-concentratingsystems are much simpler as they do not need solar tracking and can collect direct and diffusesolar radiation with slightly lower yearly efficiencies. The specific hardware needed for solarphotocatalytic applications is very similar to that used for conventional thermal applicationswith the following main differences: the fluid must be exposed to the ultraviolet solarradiation, so the absorber must be transparent to this radiation and no thermal insulation isrequired as the temperature does not play a significant role in the photocatalytic process.Besides, solar photoreactors for water detoxification were originally designed to use line-focus parabolic trough concentrators, non-concentrating collectors are the choice for solarphotocatalytic applications. They are more efficient than concentrator-based systems due tothe use of both direct and diffuse UV light, the square root dependence between reaction rateand light intensity and their intrinsic simplicity. The CPC (static collectors with a reflectionsurface following an involute around a cylindrical reactor tube) are a very interesting crossbetween trough concentrators and one-sun systems and have been found to provide the bestoptics for low concentration systems. CPC’s designed with a CR=1, or near one, are one ofthe best options for solar photocatalytic applications. Aluminium is the only metal surface thatoffers high reflectivity values in the UV spectrum. Electropolished anodized aluminium andorganic plastic films with an aluminium coating film are the most appropriate reflectivesurfaces to be used for solar detoxification applications. Photocatalytic reactors must be bothtransmissive and resistant to UV light. Common materials that meet these requirements arefluoropolymers, acrylic polymers and borosilicate glass and tubular photoreactors designs are

118


the best option. In TiO2 heterogeneous photocatalysis, suspended catalyst systems givesefficiencies higher than supported catalysts. After their use, TiO2 can be agglomerated andsedimented. Best recovery performances are obtained with a two step process: sedimentationand microfiltration.

BIBLIOGRAPHY AND REFERENCES1. Rabl, A. “Active Solar Collectors and Their Applications”. Oxford University Press.

1985.2. Blake, D. M.; Link, H. F.; Eber, K. “Solar Photocatalytic Detoxification of Water”.

Advances in Solar Energy, ed. Karl W. Boer, 167-210, 7. Boulder, CO: American SolarEnergy Society, Inc., 1992.

3. Blake, D. M.; Magrini, K.; Wolfrum, E.; May E.K. “Material Issues in SolarDetoxification of Air and Water.” Optical Materials Technology for Energy Efficiencyand Solar Energy Conversion XV, eds. Carl M. Lampert, Claus G. Granqvist, MichaelGratzel, and Satyen K. Deb, 154-62, Proceedings of SPIE, Bellingham, WA: SPIE–TheInternational Society for Optical Engineering, 1997.

4. Blanco, J.; Malato, S. et al. “Final Configuration of PSA Solar Detoxification Loop”.Plataforma Solar de Almería. Technical Report: TR 06/91. 1991.

5. Fernández, P.; De las Nieves, F.J.; Malato, S. “TiO2 Sedimentation Procedure”. Proc. 2ndUsers European Workshop Training and Mobility of Researchers Programme atPlataforma Solar de Almería, Serie Ponencias, CIEMAT (Ed.), Madrid, 1999.

6. Jorgensen, G.; Rangaprasad, G. Ultraviolet Reflector Materials for Solar Detoxification ofHazardous Waste, SERI/TP-257-4418. Solar Energy Research Institute, Golden, CO,1991. DE91002196.

7. Kreider, J. F. “Medium and High Temperature Solar Processes”. Academic Press. 1979.8. May, E. K.; Gee, R.; Wickham, D.T.; Lafloon, L.A.; Wright, J.D. Design and Fabrication

of Prototype Solar Receiver/Reactors for the Solar Detoxification of Contaminated Water,Industrial Solar Technology Corporation, Golden, Colorado, 1991. Final report for anNREL subcontract.

9. Pacheco, K.; Watt, A.S.; Turchi, C.S. “Solar Detoxification of Water: Outdoor Testing ofPrototype Photoreactors.” ASME/ASES Joint Solar Energy Conference, eds. AllanKirkpatrick, and William Worek, 43-49, New York, NY: American Society of MechanicalEngineers, 1993.

10. Wendelin, T. “A Survey of Potential Low-Cost Concentrator Concepts for Use in Low-Temperature Water Detoxification.” ASME International Solar Energy Conference, eds.William B. Stine, Jan Kreider, and Koichi Watanabe, 15-20, New York, NY: AmericanSociety of Mechanical Engineers, 1992.


PART A. True or False?1. The best yearly efficiency of a static non-concentrating solar collector is obtained when

the inclination from the horizontal is equal to the local latitude.2. Higher inclination than the local latitude gives higher efficiencies in winter than in

summer (non-concentrating solar collector).3. Temperature plays an important role in solar photocatalytic degradation of water

contaminants.4. Diffuse UV solar light is a small portion of total UV radiation, especially in the case of

clouds.5. Non-concentrating solar collectors can use both direct and diffuse solar light.

119


6. Concentrating solar collectors obtain better efficiencies than non-concentrating ones insolar water detoxification applications.

7. Silver is not used to produce UV solar reflector due to its price.8. Quartz is the best material for photocatalytic reactors.9. Supported catalyst systems have similar efficiencies when compared with slurry systems.10. Besides the particles of titanium dioxide are very small, once in water, they agglomerate

into bigger ones, 10 to 20 times bigger.

PART B.1. Why are concentrating solar systems more efficient than non-concentrating ones, when

thermal applications are considered?2. Why are non-concentrating solar systems more efficient than concentrating ones, when

UV photocatalytic applications are considered?3. Which must be the diameter of a PTC reactor tube if the aperture width is 2.5 m and a

geometric concentration ratio of 10 is desired to be achieved?4. What is the geometric concentration ration of a CPC if their semi-aceptance angle is 70º?5. A PTC with a geometric concentration ratio of 6 has an effective concentration ratio of 4.

What is the total optical error of the reflective surface if their specular reflectance is 85%?6. What is the main factor that limits the UV transmittance of standard glass?7. Why reactor tubes of 200 mm inner diameter are not practical at TiO2 slurry systems?8. Cite at least three of the main problems of supported catalyst systems.9. How can titanium dioxide slurry particles be aggregated and sedimented?10. Why the Reynold number must be higher than 4000, at TiO2 slurry systems?

ANSWERS

PART A1. True; 2. True; 3. False; 4. False; 5. True; 6. False; 7. False; 8. True; 9. False; 10. True

PART B1. Because they need a solar tracking system and the collectors are always looking at the

sun.2. Because, besides non-concentrating systems have not solar tracking device, they can

collect the UV solar light, which is an important contribution to the yearly efficiency.3. Using equation (6.2); d = 2500 / (10 x 3.1416) = 79.6 mm.4. Using equation (6.7); C = 1 / sin (70) = 1.06.5. Using equation (6.8); σ = 6 x 0.85 x 2.73 / 4 = 3.48 mrad.6. The presence of Fe ions, specially Fe+3, which absorbs the UV light.7. Because of most of the inner volume of the reactor would be dark due to the opacity of the

catalyst suspension.8. Less efficiency, much higher pressure-drop and the necessity of periodic replacement.9. TiO2 slurry suspensions can be destabilized by adding a small quantity of electrolyte, such

as NaCl, and/or adjusting the pH to 7 in order to get the point of zero charge and theisoelectric point of the colloidal solution.

10. Because turbulent flow regime must be assured to guaranty adequate fluid mixture and toavoid possible catalyst settlement.

120


7 SOLAR DETOXIFICATION APPLICATIONS

AIMSThis unit describes main proven and potential applications of solar detoxification processeswith demonstrated technical feasibility. Advantages and limitations of these applications arealso discussed.

OBJECTIVESWhen you have completed this unit, you will have an appreciation of the following subjects:1. Limiting factors and necessary conditions to feasible solar detoxification applications.2. Main applications related with contaminated water.3. Technical considerations about gas phase treatment processes.

NOTATION AND UNITSSymbol Description UnitsAOT Advanced Oxidation TechnologiesBTEX Bencene, Toluene, Ethyl benzene, and XilenesBWTP Biological Wastewater Treatment PlantCVOC Chlorinated Volatile Organic CompoundsEDTA Ethylene Diamine Tetraacetic AcidEPA Environmental Protection Agency (USA)HMX 1,3,5,7-tetranitro-1,3,5,7-tetrazacycloctaneIR Infrared lightLLNL Lawrence Livermore National Laboratory (USA)NREL National Renewable Energy Laboratory (USA)PAH Polynuclear Aromatic HydrocarbonsPCP PentachlorophenolPCO Photocatalytic Oxidation ProcessesPSA Plataforma Solar de Almería (Spain)PTFE PolytetrafluoroethylenePVC Polyvinyl ChlorideRDX Hexahydro-1,3,5-trinitro-1,3,5-triazineSNL Sandia National Laboratories (USA)TCE TrichloroethyleneTNT TrinitrotolueneVOC Volatile Organic Compounds

7.1 INTRODUCTIONDetoxification is today the most successful photochemical application of solar photons, withseveral relevant installations and projects already in operation. This is due not only to the factthat solar detoxification is an outstanding demonstration of how well suited solar energy is toenvironmental conservation, but also because, contrary to most photochemical processes, it isnon-selective and can be employed with complex mixtures of contaminants. During the lastdecade, the number of references and related patents on heterogeneous photocatalytic removalof toxic and hazardous compounds from water and air can be counted by thousands and thenumber of applications and target compounds are numerous.

However, as indicated by Dr. M. Romero (CIEMAT, Spain), an analysis of the historicalevolution of solar photocatalysis clearly identifies three different stages of development.

121


Initially, the efforts of solar-conscious researchers focused on transferring laboratory researchcarried out by photochemical groups to solar-engineered testing with existing technology.These first results produced excitement in the photochemical research community. Theirextrapolation to practical situations assumed an ability to degrade almost any organiccontaminant as well as some metals. Later, more appropriate collectors and designs weredeveloped, but the need to know the fundamentals of certain aspects of the reaction led to anincreasing number of studies on kinetics, mechanisms, performance of mixtures and operatingparameters, with dissimilar results. It is a period of both promising and discouraging results.At present, a third scenario seems to be underway, in which the boundary conditions ofapplications are determined, and the technology is focusing on a few specific initialapplications, with the peculiarity that early development and still unsolved questions coexistwith near-commercial and industrial applications of the technology. As a result, theenvironmental market, although very receptive to clean energy sources, is still reluctant to"risk assuming" initiatives with regard to solar detoxification processes.

From the point of view of practical applications, heterogeneous photocatalysis with asemiconductor (TiO2) is the process for which the solar technologies are most extensivelyemployed today, although in some specific applications, a well-known homogeneous phasereaction, Photo-Fenton, shows higher water-phase degradation rates. In other solardetoxification options, such as sensitized-photochemical oxidation by singlet oxygen, there isstill very limited experience to date.

Photocatalytic oxidation processes (PCO) currently under development are included in thesame group of Advanced Oxidation Technologies (AOT) with other radical-promotingprocesses like plasma, electron-beam, etc. The main advantage of PCO over other AOTs is itspotential for incorporating solar energy in the form of solar photons, whereby the degradationprocess acquires significant additional environmental value. If solar photons can be collectedand used efficiently at low cost, the number of opportunities for PCO may increasedramatically. Although solar driven PCO was at first considered a universal method ofdegrading organic pollutants, a profusion of contradictory results during recent years (positiveresults in almost real problems together with other experimental results pointing outuncertainties and negative performance) has lead to confused public perception. Solar PCOtechnology is currently viewed as very sensitive to many things and scientific opinion isevolving toward a more conservative period of specific applications.

Within this context, treatment of industrial wastewater, though difficult to develop, seems oneof the most promising fields of application of solar detoxification. The only really general ruleis that there is no general rule at all, each real case being completely different from any other.In some cases, the Photo-Fenton process has demonstrated higher degradation efficienciesthan heterogeneous TiO2 photocatalysis, but in others, the Fe cycle is affected by thecontaminants and Photo-Fenton does not work at all. As consequence, preliminary research isalways required for assessing potential pollutant treatment and optimising the best option forany specific problem, nearly on a case-by-case basis.

To find out if a specific water-contamination problem can be treated with solar detoxificationtechnology is not always easy, since low efficiencies produce hydroxyl radicals and slowkinetics may limit economic feasibility. As mentioned above, preliminary tests are normallyneeded for assessment of process viability. In attempt to provide some guidelines for thereader, the following general affirmations may be made:

122


- Maximum organic concentration of several hundred mg L-1. Photodegradation processeswork well in low or medium concentrations up to a few hundred ppm of organics. Thelimit always depends on the nature of the contaminants, but concentrations over 1 gr L-1

normally are not suitable for solar photocatalytic processes unless previously diluted.- Non-biodegradable contaminants. When possible, biological treatments are always the

most cost-effective processes. Only when the contaminants are persistent (non-biodegradable) do photocatalytic processes make sense.

- Hazardous contaminants present within complex mixtures of organics. One of the mainadvantages of solar photocatalysis is that it is non-selective, so non-biodegradablecontaminants can be treated within a complex mixture of other organic compounds. Solardetoxification also works very well with individual contaminants, but a mixture could bean indication of its utility. Hazardous contaminants also usually appear in concentrationssusceptible to photocatalytic treatment.

- Contaminants with no easy conventional treatment. An additional hint of whether solarphotocatalysis will be useful can be provided by the fact that contaminants are present inconcentrations that make conventional treatment difficult.

The above recommendations provide an indication of the type of industrial wastewater forwhich solar detoxification can potentially be employed. Nevertheless, several additionalconditions are needed before a complete solar detoxification feasibility study can beundertaken.

- Throughputs should be reasonable. Spatial velocities or surface throughputs in the solarcollector and solar-photon consumption related to the total volume to be treated should beacceptable. The treatment capacity must be high enough to make photocatalyticdegradation practical. Many aqueous organic oxidation processes are too slow to beeconomically viable.

- Solar photons must be used efficiently. The technology to be applied must optimise thecollection of solar photons to be used. The overall energy needed per molecule destroyedmust also be low enough to make the process feasible and the use of external oxidants,such as electron scavengers, like S2O8

=, H2O2 or O3, must be possible to increase thequantum yield.

- The photocatalytic process should be reliable (no catalyst deactivation). The degradationprocess must work continuously without problems, such as catalyst deactivation.Collectors' components, catalyst and overall system must also be durable, guaranteeinglong periods of operation without incident.

- Operation and maintenance processes must be simple. The implementation of any realsolar detoxification technology application requires minimum operation and systemsupervision. As treatment processes are non-productive, personnel cost associated must bereduced to a minimum. These considerations also apply to maintenance.

- Batch system treatment. It is clear that water treatment with solar detoxification should berun in recirculation mode with batch loads of contaminants to guarantee completedestruction. This means that the treatment must be independent of the process generatingthe wastewater and that on-line treatments normally are not feasible.

Applications that fulfil both groups of requirements may be considered serious candidates forsolar detoxification and a detailed feasibility test study would be worth to be considered.

123


7.2 INDUSTRIAL WASTE WATER TREATMENTAs indicated above, solar detoxification technology may be considered feasible for industrialeffluents containing highly toxic compounds for which biological waste treatment plants areunfeasible at medium or low pollutant concentrations. “Solardetox” is a bidimensionaltechnology that is linearly dependent on the energy flux. It is therefore also linearly dependenton the collector surface, and investment in turn depends on the collector surface. Landlimitations must also be considered. Normally, several hundred or even 1000 to 2000 m2 arenot a problem for a medium-sized factory. As a conclusion, reasonable order of magnitude ofinflows for a Solar Detoxification plant, with the actual state-of-the-art of the technology,would be in the range from several dozens up to a few hundreds of m3 per day.

At the moment, and from the experience accumulated by many scientists and researchers inthe last 10 years, solar detoxification seems to be a good solution for destroying, amongothers, the following contaminants found in industrial waste-water:

− Phenols− Agrochemical waste− Halogenated hydrocarbons− Antibiotics, antineoplastics and other chemical biocide compounds from the

pharmaceutical industry− Wood preservative waste (PCP, fungicides)− Hazardous metal ions

7.2.1 PhenolsThese substances include phenol, cresol and other phenol substitutes. All the phenols are verytoxic. Their maximum concentration in a biological wastewater treatment plant inlet (BWTP)is 1-2 mg/L. Even very low concentrations of phenols (1-10 mg/L) in fresh water produceunpleasant odour/taste during the chlorinating process, so any discharge of phenols must beavoided. The solar detoxification technology would therefore be useful for treatment of watercontaining this type of contaminants.

Phenols can easily be degraded by solar photocatalysis. A few hundred square meters of solarCPC collectors could crack or completely destroy the phenols contained in small volumes ofindustrial effluents prior to discharging to a BWTP. Figure 7.1 shows two degradation testscarried out on chemical industry waste water containing a large number of compounds,mainly phenols, and including formol, phthalic acid, fumaric acid, maleic acid, glycolcompounds, xylene, toluene, methanol, butanol and phenylethylene, among others.

Figure 7.1 belongs hereComplete mineralization of a complex mixture of organic contaminants containing phenols

using persulphate as electron scavenger

As observed in Figure 7.1, complete degradation of all initial substances and theirintermediates is possible at reasonable degradation rates. Table 7.1 shows examples ofindustrial processes that generate wastewater-containing phenols that could be good targetsfor solar photocatalytic processes.

124


Chemicals Process Waste description Quantity1.Phenols &formaldehyde

Phenol-formaldehyderesins

Condensation water containing lessthan 2000 mg/L of phenols andformaldehyde

500 L/ton of phenolicresin

2.Phenols Storage tanks, ships,lorries, piping

Rinsing water containing less than15000 mg/L

0.1% of the totalproduction diluted in 60volumes of water

3.Phenols Phenols production Spills, cleaning water 0.1% of the totalproduction diluted in 60volumes of water

Table 7.1 Examples of processes producing phenol residues in waste water

One such application is the treatment of condensation water from the manufacture of phenol-formaldehyde, one of the oldest synthetic resins used in industry, for which phenols are theraw materials. Phenolic resins are obtained from the reaction of phenol or a phenol substitutewith formaldehyde. The reaction may be portrayed as indicated in Figure 7.2.

Figure 7.2 belongs hereFormation of phenolic resins. Courtesy of Ecosystem S.A. (Spain)

The ingredients are boiled in a reflux condenser (Figure 7.3) and condensation water isusually removed by vacuum distillation. This condensation water contains several mg/L ofreagents, being of which are very toxic.

Figure 7.3 belongs hereReactor condensation waste-water from manufacture of phenolic resins.

Courtesy of Ecosystem S.A.(Spain)

Phenol-formaldehyde resins are usually prepared with 40% formaldehyde by volume, then,approximately 750 L of water is removed for each ton of final product. When solidformaldehyde is used, only 160 L of condensation water are removed from the final mixture.These phenolic residues contain between 600–2000 mg/L of phenol and between 500-1300 mg/L of formaldehyde.

7.2.2 Agrochemical compoundsThis family includes a broad range of chemicals, extensively used in agriculture. Some ofthese compounds are soluble in water, others are used as suspensions, some are oil-basedpesticides and some are used as dusts. However, the majority of them are dissolved,suspended or emulsified in water prior to spraying and the amount of wastewater generatedcould vary greatly depending on handling. Water-borne contamination originates in manydifferent processes, such as cleaning and rinsing of spraying equipment, dumping of leftoverspray solutions, plastic bottle recycling, etc.

Pesticides are one of the best fields of application of solar detoxification technology for whichsolar detoxification appears to be an ‘omnivorous’ technique. This is because they areemployed in very diluted solutions or suspensions of mullet-component formulas, in smallvolumes and in batches. Very good results have been obtained with organohalogenated andorganophosphorous pesticides, carbamats, thiocarbamats, 2,4-D (2,4 dichlorophenoxyaceticacid), triazines, etc. Besides the large amount of pesticide waste generated in agriculture,there is also a huge amount of industrial waste from factories producing the active ingredients

125

Fig. 7.2

126


and, especially, in factories where the active components and other additives are stored,weighed according to the product formula, mixed and packed.

Chemicals Process Waste description QuantityPesticides Packaging Wet scrubbing alkaline solution (NaOH)

containing dissolved, suspended andpartially degraded pesticides (the wholefamily at high/medium concentration)

500 m3/year for a medium sizecompany

Pesticides Manufacturing /Packaging

Floor cleaning water, mixer & reactorcleaning and rinsing water

2000 m3/year for a medium sizecompany, with 0.05% of theactive components dissolved orsuspended in 100 parts of water

Pesticides Land application Cleaning and rinsing water of applicationequipment. It is formed by diluted spraysolution (< 100 mg/L of the whole family)

------

Pesticides Land application Spraying solution prepared in excess ------Pesticides Plastic container

recyclingCleaning and rinsing water of plasticbottles prior to crushing and pelletisation.(< 100 mg/L of the whole family)

0.2 to 0.5 g of pesticide per eachempty plastic bottle of 2Laverage volume, dissolved in 500volumes of water can beestimated

Table 7.2 Examples of processes potentially producer of wastewatercontaining agrochemical residues

A complete description of an example of use of the photocatalytic degradation technology forthese compounds may be found in chapter 9. Another potential example is the treatment ofrinse water from cleaning sprayers in agrochemical applications. Correct management of thisrinse water containing pesticides is necessary to avoid contaminating the soil, groundwaterand surface water that can occur when this material is improperly discharged. In many areas,farmers prepare the spraying solution at the spring used by the whole town. Such places aresuitable for the installation of a small-medium sized solar treatment plant. It could also be ofinterest to large farm owners or pesticide application companies.

7.2.3 Halogenated hydrocarbonsSolar detoxification has also been demonstrated to be efficient in the degradation ofhalogenated solvents. Halogenated compounds are found in much of pharmaceutical industrywaste and increasing concern about volatile organic compound (VOC) emissions andenvironmental regulations and directives are pushing industrial managers to control VOCemissions. One of the VOC emission control methods is wet scrubbing, and the contaminatedwater from the scrubbers can be treated by solar detoxification. Other sources of halogenatedwastes are factories of halocompounds manufacturing. The waste can be estimated as a lowpercentage of the total production, dissolved at 100 – 200 mg/L. One example could be thefactories of PVC production, where each ton of produced PVC gives 2.5 m3 of wastewatereffluent contaminated with short-chain polymers or monomer of PVC.

Figure 7.4 belongs herePhotocatalytic degradation of dichloromethane, chloroform, trichloroethylene and

tetrachloroethylene using a TiO2 catalyst manufactured by CISE. Courtesy of University ofTorino (Italy)

Example: estimation of the required field. A feasibility study has determined that a 6 m2

photoreactor can completely mineralize 250 L of wastewater containing 100 mg/L of

127


chloroform in 8 hours of sunlight and 25000 L are to be treated. As all the parameters arelinear, assuming unchanged weather and waste-water conditions, the direct result is 600 m2 ofcollector field in 8 hours, or 300 m2 in 16 hours to treat the 25000 L, while for a 100-m2 field48 sunny hours would be necessary to complete the treatment.

Another potential halocompound application of solar detoxification is treatment of theeffluents from the manufacture of PVC (polyvinyl chloride), which is produced, in hugequantities worldwide. The volume of these effluents is 2.5 times greater than the finalproduction of PVC (1 ton of PVC produces approximately 2.5 m3 of effluent) and may becontaminated with short-chain polymers or vinyl chloride, the PVC monomer.

7.2.4 Antibiotics, antineoplastics and other pharmaceutical biocide compoundsThere are two main types of pharmaceutical industries: factories that produce fine chemicalsand those that produce and mix raw materials and components to obtain a final product. Sinceantibiotics are intrinsically biocides, they cannot be treated in BWTP. Antineoplastics arecitotoxic (some of them inhibit cellular division) and are commonly used in chemotherapy ofleukemia and tumors. Both of them are ecotoxic and should be degraded before dumping intostreams or sewage systems. Manufacturing equipment (hoppers, conveyors/ belt feeders,weighing and mixing equipment, pill conforming or vial filling machines) is normally cleanedthoroughly with a huge amount of water which can be stored and treated in batches, havingthen the ideal conditions for solar detoxification treatment.

Chemicals Process Waste description Quantity1.Antibiotics Pill conforming & raw

material mixing process atmanufacturing ofpharmaceutical specialities

Cleaning water coming frompill and vials filling equipment,containing less than 1000 mg/Lof organic compounds

3-5000 L/d at a mediumsized factory

2. Antibiotics Antibiotic production. Fermentators and liophilisatorscleaning

0.05% of total productiondissolved in 500 volumesof water

3. Antineoplastics Idem 14. Antineoplastics Idem 2

Table 7.3 Examples of processes potentially producer of antibiotic and antineoplastic waste

The chemical industry also produces a great variety of biocides used as preservatives,especially in paint. Common examples of these compounds are: phenyl mercuric acetate,dithiocarbamates and other sulfuric compounds, halogenated phenols, halogenated phenol-formaldehyde condensates, quaternary ammonium salts, etc. A medium sized chemicalcompany could typically produce 500 t/yr of this type of product. As equipment must becleaned between each batch, approximately 500–1000 m3 of rinse water could be generatedper year, with a normal concentration of 200–500 mg/L of the biocide, for which solartreatment is feasible.

7.2.5 Wood preservative wasteThe most toxic and persistent of the wood preservatives is pentachlorophenol (PCP). Thisinsecticide is very well know to solar detoxification researchers because it was one of the firstcompounds studied exhaustively in photocatalytic degradation. Although it is now banned,PCP is still produced and used in wood protecting varnishes because of its effectivenessagainst woodworms and termites.

128


Other compounds in this group are creosote and organic insecticides and fungicides. Althoughprecise information on the amount and chemical composition of these residues is scant, thegeneral impression is that they produce a huge amount of waste. In one Spanish provincealone there are about 100 wood-preserving firms and the wood (timber and sawed pieces) aretreated in concentrated pesticide baths. When the baths become weakened, their content isreplaced with fresh product. A conservative assumption is that 5% of the chemicals in thebath are discharged as waste.

Figure 7.5: Degradation of PCP at PSA Solar Detoxification Facility(Helioman’s collectors loop) See also Figure 5.3 (solar photocatalytic mineralization of PCP)

7.2.6 Removal of hazardous metal ions from waterOne additional advantage of the heterogeneous photocatalytic process is the presence of areductive chemistry pathway that can be exploited to remove reducible species, such ashazardous heavy metal ions. It is important to remember that both oxidation and reductionmust occur simultaneously to maintain process activity, so it is possible to use the presence oforganics as reductants for the reduction of metals and, at the same time, the metals as oxidantsfor the oxidation of organics. In general, the higher the concentrations of organics, the fasterthe metal reduction rates. Similarly, an increase in the concentration of metals increases theorganics oxidation rate.

Oxygen is usually the oxidant when there are no other oxidants or metals present and wateracts as the reductant element. Dissolved oxygen can compete with dissolved metals forconduction-band electrons and therefore can inhibit the rate of metal removal. However,dissolved oxygen can be helpful for the degradation of the organics if the concentration ofmetals is insufficient. The ability to remove metals depends on the standard reductionpotential for the reduction reaction. For example, Ag2+, Cr6+, Hg2+, and Pt2+ can be treated byphotocatalytic reduction, but Cd2+, Cu+2, and Ni+2 cannot.

Figure 7.6 Cr+6 to Cr+3 solar photocatalytic reduction at PSA Solar Detoxification Facility(CPC’s collectors loop). See also Figure 7.7

129


Specific applications described in the literature are elimination of specific organometalliccompounds (organic agents forming complexes with metal ions) like phenylmercury or theremoval of silver from black-and-white photoprocessing waste. The presence oforganometallic compounds usually complicates wastewater treatment, as the metal becomesmore inert and the effectiveness of traditional processes for aqueous metal removal, such asalkaline precipitation and ion exchange, is reduced. Although the rate of metal removal isgenerally slow, it remains attractive to solar detoxification technology, as these applicationsare normally widely distributed and can be processed in batches. When there are only metals,inexpensive and non-hazardous sacrificial reductants (such as EDTA or citric acid) can beadded to enable photocatalytic treatment.

Figure 7.7 belongs hereSimultaneous oxidation of phenol and reduction of Cr+6 to Cr+3 using solar detoxification

technology (PSA, Spain)

Another application of interest is the reduction of Cr+6 to Cr+3. Photocatalytic reduction ofCr(VI) is very sensitive to pH, and is more efficient below pH 2 and leaves the reductionproduct, Cr(III), in solution in this range of pH. Around pH 5, Cr(III) forms a stableprecipitate. The Cr(VI) reduction rate is very sensitive to the nature of the organic that issimultaneously destroyed. Generally, the more easily oxidized the organic compounds, thehigher the photocatalytic reduction rate; this means that different waste streams may exhibitdramatically different treatment rates depending on their specific chemical composition.

7.2.7 Other applicationsMany other potential applications of solar photocatalysis may be found in the literature.Among them, the destruction of organics in recyclable water from the microelectronicsindustry, the removal of explosives (such as TNT, RDX and HMX) from aqueous munitionswaste and the oxidative degradation of cyanides.

Because they are highly toxic, photocatalytic degradation of cyanides is another potentialindustrial application having the advantage that it neither produces highly toxic substancessuch as cyanogen chloride nor sludges and avoiding the use of additional chemical reactantssuch as chlorine.

OHOCNhOHCN VB 222 +→++ −+−− (7.1)

Furthermore, photocatalytic oxidation can transform −CN into less toxic substances, such as−OCN , and by carefully selecting reaction conditions, complete oxidation to CO2 and N2 can

be obtained:

+−+− ++→++ HNCOhOHOCN VB 432 2212

32 (7.2)

In all the applications mentioned in this chapter, the presence of inorganic ions (such aschloride, phosphate, nitrate, sulphate, etc) in water can have a negative effect on thedestruction rates of the target compounds. High concentrations of inorganic ions have beenfound to reduce the performance of the titania catalyst and, as a consequence, reducesignificantly the feasibility of the overall treatment process.

130


7.3 SEAPORT TANK TERMINALSA large percentage of the international chemical trade is shipped by sea and there are manytank terminals in harbours all over the world for the main purpose of receiving, storing anddistributing raw chemicals and fuels to industry. Final distribution of the stored product is inbulk by tank trucks, except for a small amount in drums in the same terminal enclosure.

Seaport tank terminals must carry out cleaning operations and also have to control emissionsfrom the stored products. These cleaning operations usually produce huge amounts of watercontaminated with low concentrations of the chemical from tanks, pipelines, jetty pipelinesand hoses. Among the compounds normally transported and distributed this way, manychemicals, e.g., phenol, metham sodium, perchloroethylene, trichloroetylene, methylenchloride, sodium dichromate, etc., and other compounds, such as styrene, toluene, nonilphenol, benzene chloride, etc., may be successfully treated by solar detoxification.

Example: Cleaning of phenol tanks. At the end of a contract for the rental of a 600-m3 tankthat had contained phenol, terminal personnel proceed to clean it for refill with styrene. Thisis done in a two-step operation. First saturated steam is injected to dissolve any phenolcrystals remaining in the top and valves. 7400 L of condensed water containing 6.4% phenolis then discharged into a storage waste tank for later recovery of the phenol by distillation orits destruction by thermal treatment. Then the inside of the tank is rinsed out using a hose witha special spray nozzle. This rinse water contains 450 mg/L of phenol in a total volume of16000 L. The conditions (volume, concentration, transparency) of the waste generated in thissecond step are ideally suited to solar detoxification.

Figure 7.8 shows an example of another potential application: treatment of wastewater fromcleaning a tank that had contained metham sodium. Metham sodium or Vapam is adithiocarbamate used as soil fumigant to control weeds, nematodes, fungi and insects (soilsteriliser). This compound is a direct competitor of methyl bromide, another soil disinfectantand one of the ozone-depleting chemicals.

Figure 7.8 belongs hereDegradation of metham sodium wastewater from tank cleaning. A stop in the process is

observed after partial degradation of initial TOC content (PSA, Spain)

In a medium-sized seaport tank terminal (typically about 70 tanks with 28000 m3 averagestorage capacity) at least 15 cleaning operations are completed each year, with huge amountsof residues generated annually in the pipeline connection pit (snake pit), hoses and cleaning ofships. There is also a potential application in any wet scrubber these plants may use for VOCemission-control equipment. All these residues can easily be managed by solar detoxificationbecause they can be stored individually in small tanks or containers until the treatmentprocess can be run.

7.4 GROUNDWATER DECONTAMINATIONRemediation of contaminated groundwater is a problem that can be found in many parts of theworld and especially in developed countries. Typical substances found in contaminatedgroundwater are chlorinated hydrocarbons (including PCB’s), aromatics from a variety ofsources including fuels (e.g., benzene and toluene), and a wide variety of other chemicals thatinclude pesticides, solvents, phenols (PCP, creosote), TNT & DNT, dyes, polynucleararomatic hydrocarbons (PAHs) and even dioxins.

131


Solar detoxification can be considered a good solution for ‘in-situ’ treatment anddecontamination of groundwater containing substances where conventional biologicaltreatment is difficult due to dilution of pollutants. Also, groundwater is clear and transparentand there is always good storage capacity (the aquifer is in itself a holding ‘tank’ forcontaminated water). Tests have shown that the majority of pollutants found in contaminatedgroundwater are easily destroyed by photocatalysis. Groundwater remediation also has theadvantage of a processing time-scale of months or years.

The difficulty is that standard discharge must normally be drinking-water quality, which isdifficult and costly (for any treatment technology to be used). Groundwater also tends to havea high mineral and salt content, resulting in the need for pre- and post-treatment systems.

Typically contaminated sites are old chemical plants, oil recycling plants (engine lubricantoil), refineries, chemical weapon and explosive factories, pesticide plants, coke and gasfactories, airforce bases, harbours, railway stations, power plants and substations. Thecontaminated groundwater must be pumped out from a series of extraction wells locateddownstream of the contamination plume. After treatment, the effluent is injected again intothe water table through upstream injection wells.

Figure 7.9 belongs hereGeneric concept of contaminated groundwater treatment

Courtesy of ECOSYSTEM S.A. (Spain)

The first known treatment of contaminated groundwater by solar photocatalysis was carriedout on the grounds of the Lawrence Livermore National Laboratory (LLNL) in Livermore,California. This was also the first on-site application of solar photocatalysis technology.During World War II, a part of the grounds now occupied by LLNL was a Naval Air Stationtraining and maintenance facility. Trichloroethylene (TCE) and other toxic chemicals wereused extensively in normal operations to clean engine parts and other machinery. Over theyears, unconfined TCE and other volatile organic compounds entered local ground water,where they are now slowly migrating off site. Today, TCE is present in the groundwater atconcentrations ranging up to 500 ppb, which means 100 times the acceptable EPA limit fordrinking water. The field experiment, developed by three US government laboratories(NREL, SNL and LLNL), was conducted at LLNL in 1991 using available trough technologyand demonstrated the technical feasibility of this application.

The system consisted of two solar troughs, each 36.5 m long, with effective concentrationratio of approximately 20 and total solar collector area of 158 m2 using a TiO2 slurry andyielded outlet concentrations below 5 ppb, which meets the limits for drinking-water processfeasibility demonstration. One of the main problems reported was the required acidpretreatment which had an important negative effect on the effectiveness of the process.

132


Figure 7.10 Part of the 156 m2 parabolic trough water treatment system tested oncontaminated ground water at a site at LLNL (USA) in 1992. Courtesy of National

Renewable Energy Laboratory (USA)

Another application in the literature is the one-sun solar detoxification facility installed in1993 on a groundwater site contaminated by a former jet-fuel handling area at Tyndall AirForce Base in Florida (USA). Contaminants of interest that were present in the groundwaterincluded benzene, toluene, ethyl benzene, and xylenes (BTEX). 30 one-sun collector modules(1.22 m x 2.4 m) connected in series were used, each collector module made up of 66 UVtransparent parallel tubes with a 0.64-cm inner diameter and 2.4-m length (Figure 7.11).Treatment of initial concentrations of 1 to 2 mg/L of BTEX was performed in a batch systemobtaining typical destruction levels of 50% to 75% in 3 hours. The total volume treated duringeach run was 530 L.

Figure 7.11 One-sun reactor built by American Energy Technology, Inc. for treatingcontaminated groundwater in Florida (USA) in 1992. Courtesy of National Renewable Energy

Laboratory (USA)

One last example is the photocatalytic cleaning of groundwater contaminated by gasolinefrom underground storage tanks (Figures 7.12 a & b). In this case the groundwater iscontaminated with BTEX (benzene, toluene, ethyl benzene and xylenes) at a concentration ofabout 1 ppm. The test site is a gasoline station and the system consists of six non-concentrating solar reactors of 3 m2 each using TiO2 slurry and a 1900 litre storage tank.

133


Figures 7.12(a) and 7.12(b)Non-concentrating solar detoxification system for BTEX-contaminated groundwater at acommercial site in Gainesville, Florida (USA), 1996. Courtesy of the Solar Energy and

Energy Conversion Laboratory, University of Florida

7.5 CONTAMINATED LANDFILL CLEANINGSoil decontamination is another potential application of solar detoxification. There arecontaminated landfill sites all over the world for which, depending on the nature of thecontaminants, water or gas-phase photocatalytic treatments can be used. If the contaminantsare soluble in water, water can be used to extract them, and this water can then be treated laterby solar detoxification. If the contaminants are volatile organic compounds, they can bedesorbed from the solid as a gaseous product using either vacuum extraction or heat andpumped into a gas-phase solar detoxification system (See also chapter 7.7).

One example of this application is the mineralization of lindane suspended in landfillleachate. The reaction between benzene and chlorine under UV light produces 6 stableisomers of hexachlorocyclohexane which are carcinogens. Only one of them, Lindane, whichmakes up 14% of the final mixture, is actually an insecticide. The other isomers, very toxicand also very stable in the environment, accumulate in factories, are dumped in very badlymanaged landfills or have been spread throughout the environment for decades. Lindane isone of the oldest chlorinated insecticides, which, due to its stability in the environment, isbanned for agrochemical and veterinary use in the majority of countries. However, it is stillused to eliminate lice in children (ectoparasiticide) and other pharmaceutical and medicinaluses with the approval of sanitary authorities.

Figure 7.13 belongs hereSimulation of contaminated landfill treatment using solar detoxification. Mineralization of

Lindane (PSA Solar Detox Facility, Spain)

Many sites are contaminated by lindane, some of them very well known in Europe. Figure7.13 shows the photocatalytic degradation of Lindane (technical grade) suspension in water,simulating the final step of contaminated landfill treatment and demonstrating the technicalfeasibility of the solar treatment process (maximum solubility of lindane: 10 mg/L).

7.6 WATER DISINFECTIONChlorine is the most commonly used chemical for water disinfection because of its ability toinactivate bacteria and viruses. Nevertheless, the presence of organic impurities in the watercan generate unwanted by-products of disinfection, such as trihalomethanes and othercarcinogens. As a result, other water disinfection technologies are becoming increasingly

134


important. Among them, ultraviolet irradiation with lamps is widely used to destroy biologicalcontaminants, primarily at a 254-nanometer wavelength. Solar ultraviolet, which is primarilyat 290 to 400 nm wavelengths, is much less active as a germicide (see Figure 7.14).

Figure 7.14 belongs hereCommon ultraviolet band designations,based on biological effects.

Courtesy of International Light, Inc.(USA)

Despite the broad spectrum of research, the potential use of solar detoxification technologyfor water disinfection is still essentially unexplored. However, the antibacterial effect of TiO2

has been demonstrated on several microorganisms, including Escherichia Coli, Lactobacillus,Streptococcus, and others. In all cases, the photocatalytic oxidation effect of TiO2 particleswas able to effectively sensitize bacteria to photo-killing by solar exposure. This photo-killingaction is associated with the disruption of the cell wall and membrane throughphotocatalytically induced surface oxidation resulting in the disintegration of the cell.Disinfection of viruses, such as Phage MS2 and poliovirus 1, can also be found in theliterature.

Homogeneous methylene-blue-sensitized photochemical disinfection has also been found tobe highly effective with E. Faecalis bacillus spores through singlet oxygen photooxigenation.Singlet oxygen is generated by absorption of a photon by a photosensitizer (dye) which isexcited and transfers its energy to a dissolved oxygen molecule. Photo-killing occurs whenthis singlet oxygen penetrates into the microorganism resulting in oxidation and inactivationof cellular components. Successful pilot plant experiments using this technique have beencarried out in Israel and Germany.

Two major disadvantages of sensitized photodisinfection in comparison with conventionaltechniques are the generally slow kinetics and the lack of residual disinfection capacity (afterexposure to light). On the positive side, the presence of biological and organic contaminationin surface waters is usually highest in summer, when the greatest amount of solar radiation isavailable for the process. One possible realistic approach might be preliminary solardetoxification to partially disinfect water and reduce the level of organic contaminants,followed by limited chlorination to maintain disinfection in distribution pipelines and avoidformation of undesired by-products.

7.7 GAS-PHASE TREATMENTSGas-phase photocatalytic oxidation is one of the Advanced Oxidation Technologies (AOTs)with promising applications for end-of-pipe treatment of gaseous emissions and airpurification. The first pilot experiments have only been carried out in the last few years. In theUSA, development of the photocatalytic oxidation technology was motivated by theambitious DOE Solar Industrial Program. Some examples of applications are cleaning ofexhaust streams from the microelectronics industry and treatment of off-gases from extractionof contaminated soil vapour. Indoor air quality in houses, vehicles and spacecraft has alsobeen a matter of study both in the USA and Japan. In Europe several initiatives also addressthe development of gas-phase solar PCO as an end-of-pipe technology for the treatment ofVOC emissions but, unlike water detoxification, all the above mentioned experiments havebeen conducted with lamps. No solar installations can be found in the literature and therecontinues to be a serious lack of solar-driven gas-phase PCO.

135


The fundamental background of radical attack on organic contaminants photocatalyzed withthe assistance of smog have been addressed by several tropospheric chemistry specialists but,unfortunately, some crucial questions related to the basic fundamentals of gas-phase PCO stillpersist. A critical screening of the related literature reveals conflicting results among theauthors regarding the influence of different process parameters. The reaction mechanism ingas-phase, based on hydroxyl radicals, has also recently been revised by other possibleoptions, resulting in a presently confusing stage for gas-phase photocatalytic treatmentprocesses.

Figure 7.15 belongs hereScheme of TCE gas-phase mineralization with PCO and a monolithic catalyst based on

sepiolite/TiO2/Pt

Figure 7.15 shows the possible trichloroethylene (TCE) PCO degradation pathways. Someintermediates detected are formed in the presence of chloride, so it has been proposed that °Clcould be the driving force instead of °OH. Other authors have proposed that both mechanisticapproaches via °Cl and °OH are essentially correct and the eventual pathway depends on thecharacteristics of the catalyst (e.g., internal surface or porosity). TCE is one of the moststudied compounds because it is a major gas-phase air pollutant. Their photocatalyticoxidation with titanium dioxide has received good reports in the literature.

Figure 7.16 belongs hereDestruction efficiencies of TCE 400 ppm. Thermocatalytic and photocatalytic processes at

different temperatures by using a MgSiO4/TiO2/Pt monolithic catalyst (CIEMAT, Spain)

Figure 7.16 shows a lab experiment performed on TCE using a monolithic TiO2-basedcatalyst at different temperatures. Photoeffect was clearly differentiated by using appropriatefilters. As it can be observed, temperature played a negative role due to adsorption-desorptionphenomena. Nevertheless, apart from high photonic efficiencies (up to 95%) reported withTCE, it is not easy to find applications for pure photocatalytic gas-phase treatment processes,as not many compounds have been found to be significantly affected by photocatalysis, beingnecessary in most cases a combination of photocatalytic degradation and destructivethermocatalysis.

In addition to that, a major problem for solar gas-phase PCO is the difficulty, or evenimpossibility, of working in batch systems, implying an important additional difficulty forsolar-driven processes due to the natural uncontrollability of the sun as the energy source. Asa result, it is not trivial to find a niche for solar driven PCO applied to gas-phase and airpurification. Hybrid solar-electric or pure electric devices using lamps are therefore envisagedfor initial practical applications.

136


Compound Rate Compound Rateacetaldehydeacetonitrileacetonebenzenebenzaldehydebutyl acetatebutyl aminedecanedichloroacetyl chloridedichlorobenzenediethyl aminediisopropyl amineethaneethanolethyl acetateethylbenzeneethylenefuel oil

msssmmmmmsmmsmmmmm

methanolmethylene chloridemethylethyl ketonemethylisobutyl ketoneperchloroethylenepropyl aminepropylnitriletetraethylethylenetoluenetrichloroethylenexylenessec-butyl aminet-butyl amine1-butanol1-propanol1,1-dichloroethylene2-propanol1,2-dichloroethylene

msmmfmsmsfmmsmmfmf

Table 7.4 Degradation rates at different compounds screened for photocatalytic activity: slow(s), medium (m) and fast (f). Courtesy of National Renewable Energy Laboratory (USA)

Table 7.4 summarises NREL experience with gas-phase photocatalitic degradation ofdifferent contaminants. Aromatic compounds like BTEX have slow or medium reaction ratesand TCE is revealed as the fastest. BTEX tests demonstrate deactivation of the TiO2 catalystand the need to add ozone to enhance and make PCO degradation possible.

This and other possible technology combinations could be another way of promoting gas-phase applications. Possible targets are the majority of hazardous air pollutants requiringabatement technologies: halogenated, aliphatic and aromatic hydrocarbons, alcohols, glycols,ethers, epoxides and phenols normally present in air streams at concentrations of less than5000 ppm. Some applications for air purification or degradation of emissions in thesemiconductor industry are also under development.

Figures 7.17(a) and 7.17(b)Flat plate reactor for treating contaminated air exiting from air stripping units. McClelland Air

Force Base. California, 1997. Courtesy of National Renewable Energy Laboratory (USA)

137


Figure 7.17 shows a recent one-sun reactor with PTFE window material and titanium dioxideimmobilized on polypropylene supports for gas-phase solar detoxification successfully testedby NREL (USA). For additional information on gas-phase experimental treatment systems,see also Figure 6.24 and Figure 5.6.

Contrary to water phase, in which catalyst may be used in the more efficient slurryconfigurations, in gas phase this is not possible and all experimental systems developed todate use an immobilised catalyst. This is apparent in the figures above. As discussed inSection 6.5, the catalyst must be anchored onto some type of inert support inside the gas-phase stream treatment reactor. Characteristics of such a system must be being very active(high degradation efficiency), have a low pressure-drop, long lifetime, and reasonable cost.Very diverse types of catalysts have been used in gas-phase systems, from MgSiO4/TiO2/Ptmonoliths to TiO2 immobilised on glass fibers, ceramic substrates, inert plastic materials, etc.As in water phase, residence times must be as long as possible to increase degradationefficiency.

One interesting potential application of gas-phase solar detoxification technology is the gastreatment from contaminated landfill recovering (see chapter 7.5). Chlorinated volatileorganic compounds (CVOC) are among the most pervasive subsurface ground contaminantsand the remediation technologies most used for CVOC ultimately produce a gas-phase streamcontaining CVOC, either from soil venting from the unsaturated zone or ground waterrecovery from the saturated zone and subsequent air stripping. In the past, direct discharge ofgaseous CVOC to the atmosphere was normal, but early in the 90s, many regulations began torequire the abatement of these emissions using the best available technologies.

Other possible applications are the small air pollution sources. These applications are notuncommon and some examples are dry cleaners, auto repair shops, bakeries, or coffeeroasters, etc. Additional potential solar applications include remote sites, storage tank vents,or potentially explosive waste streams. Finally, the regeneration of a carbon bed or otheradsorbent materials could be another promising application for a solar system. In thisapplication, the adsorbent can perform 24-hour VOC removal, and the solar system can beused to purge and destroy the contaminants during daylight hours.

With regard to air disinfection, most of the work carried out has been focused on the use ofTiO2 particles as sensitizers for the destruction of bacteria and viruses in air. In this field, avery interesting application is the fabrication of self-disinfecting surfaces. Several studieshave reported the antibacterial effect of titanium dioxide on indoor air in combination withUV fluorescent lamps. The use of thin TiO2 films as a method of keeping surfaces free ofbiological material would be of particular value where sterile surfaces are essential, such asoperating rooms in hospitals. Although very few demonstrations have been reported to date, itis expected that more will take place in the future.

138


Finally, solar photocatalysis is the process that makes self-cleaning glasses work efficiently.Fouling of glasses is mainly due to dust and/or atmospheric particles stuck on the surface ongreasy stains, mainly of fatty acids. Self-cleaning glasses are coated with an invisible thinlayer of titania, which under the simultaneous presence of oxygen (air), atmospheric watervapour and solar UV-light, is able to decompose fatty acids by successivephotodecarboxylation (photo-Kolbe) reactions and allow coated glasses to recover their initialclearness (Figure 7.18).

Figure 7.18 Self-cleaning windows. Evolution of photocatalytic treatment process on glassescoated with titanium dioxide. Courtesy of Laboratoire de Photocatalyse, Catalyse et

Environnement (E.R. au CNRS), Ecole Central de Lyon (France).

SUMMARY OF THE CHAPTERSolar detoxification technologies can provide the environmental waste management industrywith a powerful new tool to destroy waste with clean energy from the sun. Nevertheless, inspite of the large number of patents and publications, industry is still reluctant to adopt theprocess due to some discouraging and contradictory results that have led research to focus onspecific potential applications. Solar detoxification is a useful technology for addressing non-biodegradable hazardous contaminants where no easy treatment by conventional technologiesis available at concentrations up to several hundred mg L-1. Phenols, agrochemical wastes,halogenated hydrocarbons, biocide compounds from the pharmaceutical industry, woodpreserving waste, hazardous metal ions, cyanides, aqueous munitions waste, etc, are amongthe industrial waste-water applications. Other interesting applications are the treatment ofgroundwater contamination, seaport tank terminals, cleaning of contaminated landfills andwater disinfection. Gas-phase applications are in an even more confusing situation as somecrucial questions related to the basic fundamentals still persist. In gas gas-phase PCO, catalystmust be used immobilised, being used diverse types supporting substrates (glass fibers,ceramic substrates, inert plastic materials, monoliths, etc). Some potential gas-phaseapplications are gas treatment from contaminated landfill recovering, degradation ofemissions in the semiconductor industry, air purification, air disinfection, self-cleaningwindows, etc.

BIBLIOGRAPHY AND REFERENCES1. Acher, A.; Fischer, E.; Zellingher, R.; Manor, Y. “Photochemical disinfection of effluents.

Pilot plant studies”. Wat. Res. Vol. 24, No. 7, pp. 837-843. 1990.2. Anon. “Development of a Homogeneous Aqueous Phase Photocatalyst for the Solar

Detoxification of Water”. Final Report to NREL. Solarchem Environmental Systems,Markham, Ontario, 1995.

3. Goswami, D. Yogi, J. Klausner, G. D. Mathur, A. Martin, K. Schanze, P. Wyness, CraigT. Turchi, and E. Marchand. “Solar Photocatalytic Treatment of Groundwater at Tyndall

139


AFB: Field Test Results.” Solar 1993. Proceedings of the American Solar Energy SocietyAnnual Conference; pp. 235-239. ASES, 1993.

4. Malato, S. “Solar photocatalytic decomposition of pentachlorophenol dissolved in water”.Doctoral thesis. Editorial Ciemat. 1999.

5. Mehos, M.S.; Turchi, C.S. “Field Testing Solar Photocatalytic Detoxification on TCE-Contaminated Groundwater”. Environmental Prog. 12(3); pp. 194-199. 1993.

6. Nimlos, R; Wolfrum, E.J.; Gratson, D.A.; Watt, A.S.; Jacoby, W.A.; Turchi, C. “Reviewof Research Results for the Photocatalytic Oxidation of Hazardous Wastes in Air”.NREL/TP-433-7043, 1995.

7. Prairie, M. ; Evans, L.R.; Martinez, S.L. “Destruction of Organics and Removal of HeavyMetals in Water Via TiO2 Photocatalysis”. Chemical Oxidation: Technology for theNineties, Second International Symposium. Lancaster, PA: Technonomic PublishingCompany, 1994.

8. Vincent, M. “Solar Detox Market”. Training and Mobility of Researchers SummerSchool: Industrial Applications of Solar Chemistry. September 1998. Editorial Ciemat.

9. Watts, R.J.; Kong, S.; Orr, M.P.; Miller, G.C; Henry, B.E. “Photocatalytic inactivation ofcoliform bacteria and viruses in secondary wastewater effluent”. Wat. Res. Vol. 29, No. 1pp. 95-100, 1995.

10. Yves, P.; Blake, D.; Magrini-Bair, K.; Lyons, C.; Turchi, C.; Watt, A.; Wolfrum, E.;Prairie. M. “Solar Photocatalytic Processes for the Purification of Water: State ofDevelopment and Barriers to Commercialization.” Solar Energy 56, No. 5, pp. 429-437,1996.


PART A. True or False?1. Preliminary research is always required for assessing potential treatment of pollutants in

any specific problem on a case-by-case basis.2. Solar Photocatalytic processes work well with organic concentrations of over 1 gr/L.3. Non biodegradable organic contaminants are the most logical target for solar

photocatalytic processes.4. There is no sense in addressing complex mixtures of hazardous contaminants with solar

photocatalysis.5. A hint of solar photocatalytic technology being of possible application could be lack of an

easy conventional treatment.6. External oxidants must always be used as electron scavengers, when feasible, to increase

the quantum yield of degradation processes.7. There is usually no difference between treatment of contaminated water in a batch or on-

line process.8. Typical solar detoxification plants process from several dozen up to a few hundred m3 per

day.9. The presence of high concentrations of inorganic ions negatively affects the destruction

rate and the overall feasibility of solar detoxification processes.10. It is difficult to find applications for pure photocatalytic gas-phase treatment processes as

not many compounds have been found to be affected by a significant photocatalytic effect.

PART B.1. Why is the environmental market still reluctant to accept solar detoxification processes?2. Indicate the three existing solar photocatalytic processes.

140


3. What is the main advantage of Photocatalytic Oxidation Processes over the rest of theAdvanced Oxidation Technologies?

4. In addition to the use of solar energy, indicate one main advantage of solar photocatalysis.5. Indicate at least three conditions indicative of a viable solar photocatalytic process.6. Indicate why water treatment with solar photocatalysis must normally be in a batch

system.7. As solar photocalysis is an oxidation-reduction process, what is the normal reductant

agent when only organic compounds are present?; and when metals are present?8. From testing it has been found that a 4-m2 collector system can treat a 50-L sample of

wastewater containing 200 mg/L of TOC from organic compounds in 4 hours of sunlight.How long would it take to treat 100 m3 of the same wastewater with a 500-m2 treatmentsystem?

9. What are the proposed mechanisms for gas-phase photocatalytic degradation ofchlorinated organic compounds?

10. What is the worst situation for water treatment process based on TiO2 photocatalysis?

ANSWERS

PART A1. True; 2. False; 3. True; 4. False; 5. True; 6. True; 7. False; 8. True; 9. True; 10. True.

PART B1. Because initially successful experimental results presumed a wide range of practical

applications, but later confusing and discouraging results led to current concentration onapplications with specific required boundary conditions.

2. Heterogeneous TiO2 photocatalysis, homogeneous photo-Fenton and homogeneoussensitized-photochemical oxidation by singlet oxygen.

3. Their potential solarization, introducing an important additional environmental value inthe pollutant degradation process.

4. The fact that it is a non-selective process, making possible the treatment of complexmixtures of non-biodegradable organic contaminants.

5. Reasonable throughputs, efficient use of solar photons and reliable photocatalytic processwith no catalyst deactivation.

6. Because the degradation process is driven by solar photons, a source which can not becontrolled, making it necessary to separate the treatment from the wastewater generatingprocess.

7. When only organic compounds are present, oxygen is the normal oxidant and water actsas the reductant element. When metals are present, oxygen is also the oxidant and themetals themselves are the reductant elements.

8. sunlight.ofhours64

450

500100000

4R =

=

9. Via °Cl, °OH or °Cl + °OH radicals attack, depending the eventual pathway on thecatalyst characteristics.

10. The presence of high concentration of organic contaminants and inorganic ionssimultaneously.

141


8 ECONOMIC ASSESSMENT

AIMSThis unit discusses the main factors entering into the cost of solar detoxification systems anddescribes how to go about the economic assessment of specific solar photocatalyticapplications. They are also qualitatively compared with other treatment technologies.

OBJECTIVESAfter completing this unit, you will be able to:

1. Determine the feasibility of a two-step photocatalytic biological treatment process.2. Estimate the cost of solar photocatalytic treatment.3. Estimate the feasibility of solar detoxification as a function of the local solar resource.4. Compare possible alternative or complementary treatment technologies.

NOTATIONS AND UNITSSymbol Description UnitsAOPs Advanced Oxidation ProcessesASTM American Standards for Testing and Materialscλ Atmosphere attenuation coefficient at λ. dimensionlessCOD Chemical Oxygen Demand moles of O2 L

-1

cos ϕ Cosine of the incident angle of solar radiation to collectorsurface

CPC Compound parabolic concentratorE(λ) Extraterrestrial irradiation W m-2 µm-1

EPFL Ecole Politechnique Federale de Laussanne (Switzerland)f Cloud Factor %FCR Fixed Charge Rate %GAC Granular-activated carbonhs Solar time angle degreesH Henry´s Law constant dimensionlessHL Total yearly hours of operation of a lamp based system hoursHS Total yearly hours of operation of a solar detoxification system hoursIb,λ Intensity of solar irradiation over terrestrial surface at

specific wavelength, λ.W m-2

Io,λ Intensity of extraterrestrial solar irradiation at the samewavelength, λ.

W m-2

Isc,λ Solar Constant associated with a spectral interval (λ1,λ2) W m-2

UVgI Yearly average global UV irradiation W m-2

L Local latitude degreesm Air mass ratio dimensionlessmo Air mass ratio at sea level dimensionlessM Molar Moles L-1

mM Mili Molar 10-3 moles L-1

MW Molecular Weight gr mol-1

N Daynumber dimensionlessNREL National Renewable Energy Laboratory (USA)NL Number of electric lampsNUV Total yearly collected UV photons (energy higher than 3.2 eV) photons

142


NΦ (E>3.2 eV) Number of photons with energy higher than 3.2 eV per unit oftime and surface

photons m-2 h-1

PCP PentachlorophenolRDX Hexahydro-1,3,5-trinitro-1,3,5-triazineS Surface of solar collector field m2

TCE TrichloroethyleneTNT TrinitrotolueneTOC Total Organic Carbon mg L-1 or

moles of C L-1

Tf Treatment FactorTfm Mass Treatment Factor g h-1 m-2

Tfv Volumetric Treatment Factor L h-1 m-2

UV-A Ultraviolet Radiation Az Elevation over sea level mα Solar altitude degreesδs Solar declination degreesφPH Nominal average photonic flux electric lamp photons s-1

ΨAOS Average Oxidation State dimensionless

8.1 PHOTOCHEMICAL AND BIOLOGICAL REACTORS COUPLINGThe Mass Treatment Factor (Tfm) of a specific solar detoxification system can be defined asthe amount of organic substances the system is able to treat per unit of time and solarcollector surface:

∆=

2

)(

mh

g

St

TOCmT fm (8.1)

The Volumetric Treatment Factor (Tfv) of a specific solar detoxification system can be definedas the volume of contaminated water the system is able to treat per unit of time and solarcollector surface:

∆=

2mh

L

St

VT fv (8.2)

Tfm and Tfv obviously depend on the specific solar system and the waste water to be treated.The same system yields different treatment factors with different contaminants and it is alsodifferent depending on the solar irradiation, higher on sunny days than cloudy days. The mostpractical units are those indicated in Equations 8.1 and 8.2: the grams of TOC degraded perhour and square meter of solar collector field, in the case of Tfm, and the litres of water treatedper hour and square meter of solar collector field in the case of Tfv.

Example: calculation of treatment factors. If a specific solar detoxification plant having300 m2 of collectors can treat 5 m3 of waste water containing 50 mg L-1 of organiccompounds in 90 minutes, the treatment factors will be:

22

1

mh

g0.55

mg

g10

h1.5m300

L5000Lmg50T 3

fm == −−

xx

x (8.3)

143


22 mh

L11.11

h1.5m300

L5000T fv ==

x (8.4)

Treatment Factors are a powerful tool for comparison of different detoxification systems andchecking treatment feasibility for different waste-water problems. The efficiency of solardetoxification systems is usually around 0.05 to 1 g of organic compounds degraded per hourand square meter. This is a function of several parameters, such as system geometry andmaterials, the contaminants and their concentration, UV-A irradiation, catalyst concentration,etc. Nevertheless, this parameter does not necessarily imply better or worse performance of aspecific system, because this depends on the discharge objectives. Contaminant treatment, inits strictest meaning, is the complete mineralization (TOC = 0) of the contaminants, but, asindicated in Chapter 7, normally photocatalytic processes only make sense for hazardous non-biodegradable pollutants (toxic organic pollutants not treatable in conventional biologicaltreatment plants). When feasible, biological treatment of biodegradable residual waters are thecheapest treatment technologies and also the most compatible with the environment.Therefore, biologically recalcitrant compounds could be treated with photocatalytictechnologies until biodegradability is achieved, later transferring the water to a conventionalbiological plant. Such a combination of photocatalytic and biological treatments, reducestreatment time and optimises the overall economics, since the solar detoxification system canbe significantly smaller. Due to the kinetic mechanism explained in chapter 4, the first part ofthe photocatalytic process is the quickest, producing the main part of TOC degradation, andthe last phase is the longest, with a minimum contribution to complete degradation.

Figure 8.1 Conceptual scheme of photocatalytic + biological technologiescoupling Courtesy of EPFL (Switzerland)

An example might be agrochemical compounds, which are normally quickly cracked byphotocatalytic degradation processes. The active component of the pesticide disappears afterseveral minutes of irradiation, but TOC becomes negligible only after two or three hours ofirradiation. Design of a solar detoxification system to reduce the TOC (i.e., disappearance of aspecific compound or compounds) only until toxicity is low enough to be sent on to abiological treatment plant might therefore be of interest.

144


Figure 8.2 (a) and 8.2 (b)Elimination of tensioactive compounds from agrochemical industrial wastewater after a few

minutes of photocatalytic treatment. (PSA, Spain)

In the above example (equation 8.2), “treat” could mean totally degrade organic contaminantsor only to biodegradability. Therefore, the Treatment Factor may also depend on the systemobjective and a very low Treatment Factor for complete mineralization could be transformedinto a feasible one for integration with biodegradation.

The feasibility of combining photocatalytic and biological treatments to achieve highlyefficient abatement of pollutants that cannot be treated by either biological or photocatalyticmethods alone has already been demonstrated at EFPL (Pulgarin et al., 1999). This strategy isbased on the Average Oxidation State (ΨAOS), which can be defined from the followingformula:

−

=Ψ=TOC

CODTOC4StateOxidationAverage AOS x (8.5)

Where TOC (Total Organic Carbon) and COD (Chemical Oxygen Demand) are expressed inmoles of C L-1 and moles of O2 L

-1, respectively. Average Oxidation State can be between +4for CO2, the most oxidised state of carbon, and -4 for CH4, the most reduced state of carbon.The latter is given by the reaction:

OH2COO2CH 2224 +→+ (8.6)

Where, TOC = 1 (moles of C L-1) and COD = 2 (moles O2 L-1) so, according to equation 8.5,

ΨAOS = –4. With CO2, as oxidation is impossible, COD = 0 and ΨAOS = +4. An example isoxidation of formic acid:

OH2COOCOOHH 222 +→+− (8.7)

In this case, TOC = 1 (moles of C L-1) and COD = 1 (moles O2 L-1), so ΨAOS = 0. Whenphotodegradation is applied, the Average Oxidation State of intermediates is a function ofphoto-treatment time and could be a good indication when to switch from the AdvancedOxidation Process to Biological Treatment.

145


Figure 8.3 belongs herePhotodegradation Process: usual relationship between the Average Oxidation State and

Time. Points A and B are also related to figure 8.4. Courtesy of EPFL (Switzerland)

Oxidised substances are typical of biological processes. Usually treatment performance is bestfor low-molecular-weight organic compounds. Photodegradation treatment performance isalso better at the beginning, when organic molecules are broken up into simpler ones. As aconsequence, it makes sense to always check the feasibility of a two step process in which thephotocatalytic treatment is employed first to increase biodegradability and, afterward,biological treatment. Figure 8.3 shows the typical evolution of ΨAOS when recalcitrant toxiccompounds are treated. Normally, a plateau is observed after a time, suggesting that thechemical nature of intermediates formed will no longer vary significantly. As Figure 8.4shows, there are two possible extreme situations (A and B) with regard to the evolution oftoxicity.

Figure 8.4 belongs herePhotodegradation Process: normal relationships between Toxicity and Time. Points A and B

are also related with figure 8.3. Courtesy of EPFL (Switzerland)

Sometimes, there is a direct correlation between ΨAOS and toxicity. This is the best situation,as reflected in Curve A (Figures 8.3 and 8.4), because it means that the overall chemicalstatus of the degradation process does not progress, ΨAOS remains constant, and toxicity isdecreasing, thereby increasing biodegradability. Nevertheless, even at low concentrations,toxic substances could be formed during the degradation process (Curve B, Situation B)increasing overall toxicity. This does not necessarily impede connection to biologicaltreatment, but it does force the point of connection to be moved to the right (Point B).

As previously indicated, the feasibility of combining Photo-Fenton (see section 1.3.3), and abiological process to treat certain non-biodegradable toxic compounds, has been demonstratedat EPFL (Switzerland). A TiO2 photocatalytic process followed by aerobic biotreatment hasalso been successfully employed by NREL (USA) for the treatment of waste water frommunitions manufacturing plants (pinkwater) containing low concentrations of high explosives,such as RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine), TNT (trinitrotoluene), HMX andPETN.

It must be pointed out that photocatalytic degradation of recalcitrant organic compounds quiteoften initially increases the toxicity of the wastewater. The reason is the proliferation at thebeginning of the process of many reaction by-products as well as with the initial contaminantsthemselves. Therefore, it is important that the photocatalytic treatment be long enough toeliminate this increased toxicity. Also, with regard to the parameters of toxicity, it is veryimportant that no intermediates formed be more toxic than the initial compounds, because thiscould disable the overall treatment process.

The feasibility of such a photocatalytic-biological process combination must always beassessed, because it could provide an important cost reduction in the treatment of hazardouscompounds in wastewater, by reducing the size of the necessary solar collector field. It mustbe taken into account that, as with most solar systems, economics of the water detoxificationsystems are dominated by their capital cost. Thus, by reducing the capital cost of the system,processing cost is decreased.

146


8.2 COST CALCULATIONSCost is always a key topic when innovative technologies are considered and standardcommercial procedures indicate that any new technology should provide important reductionsin processing costs over competing technologies or significant new technical contributions forsuccessful marketing. In the case of Solar Detoxification, its main contribution is theenvironmental added value of using solar energy to solve contamination problems.

This section addresses the main factors influencing final installed cost, operating cost andtreatment cost of a solar detoxification system. The installed cost includes all costs associatedwith the design, construction, and start-up of the facility, but it is assumed that normalservices and facilities such as water, electricity power, etc, are available on site and that landis also available at no cost. The four main components of installed cost are:

− Facility Cost. The total field cost includes the direct cost of equipment and systemcomponents, plus the direct cost of installation labour. The major components are:concrete foundations, civil engineering, connection to power and water supply, solarcollectors, tanks, manifolds, piping, pumps and valves, structures and supports, electricalequipment, control system, instrumentation, catalyst separation unit, collector, and pipingand connection assembly, etc.

− Project Contingency. Since the general conceptual design cannot include all the details ofthe specific final design, at that point, total field costs are still necessarily incomplete, andto properly account for any unidentified costs, a contingency item must be added to thecost. This could be from 10% to 20% of the Facility Cost, depending on the degree towhich the design definition. The sum of Facility Cost and Contingencies is the TotalFacility Cost.

− System Engineering and Assembly. This includes system design and integration intoexisting installations, specifications, procurement of system components, scheduling,project management, system assembly, training of operators and engineering support.Engineering costs are estimated to be 50% of the Total Facility Cost.

− Spare Parts. 0.5% of the total facility cost is normally included as a spare parts allowance.

The sum of the facility cost, contingencies, engineering and spare parts is the Total InstalledCost. It is important to notice that the cost of a previous feasibility study is not consideredhere, because the extension and depth of this study depends on many factors, specific to eachsituation, which are difficult to estimate. Table 8.1 shows the cost for different typical facilitysizes considering a CPC solar field. These costs are based on real bids and plants which havebeen built.

147


Collector Area (m2) 100 200 300 500 1,000 10,000

Control, pumps and instrumentation 20,221 23,228 25,190 27,900 32,048 50,793

Piping, valves, manifolds, tanks,structures and supports

14,647 22,201 28,315 38,471 58,311 232,139

Catalyst separation and recovery 2,314 3,507 4,473 6,078 9,212 36,673

Collectors and reactor tubes 21,220 42,440 63,660 106,100 168,556 1,338,891

Civil engineering 6,025 12,050 18,075 30,125 60,250 602,500

Equipment transport 601 911 1,162 1,579 2,393 9,525

TOTAL INVESTMENT 65,028 104,337 140,875 210,251 330,770 2,270,522

Cost / m2 650 522 470 420 331 227

Table 8.1 Estimated cost of typical sizes of a Solar Detoxification Facility. Costare indicated in 1999 Euros

In addition to the total installed cost, the second determining factor in the cost of treatment isthe operating cost. The Operating Cost must include all personnel and materials costs requiredto operate and maintain the facility (supervision, procurement, maintenance, etc), chemicalsupplies and electricity costs.

− Personnel. Personnel is linearly dependant on facility size. Although solar detoxificationfacilities normally operate 7 days per week and 52 weeks per year, operating personnelrequirements for a 500-m2 solar plant are estimated to be only 0.1 man-year, since theplant is completely automated. Also due to the simplicity of one-sun collectors, the totalannual maintenance requirement is estimated to be 0.25 man-year, mainly for periodiccleaning.

− Cost of Maintenance Materials. Annual cost of maintenance materials are mainly due tomechanical, electrical and instrumentation equipment which could amount to about 2% ofthe Total Facility Cost yearly. It is assumed that reactor tubes and solar collectors last forthe facility lifetime.

− Electricity. The facility consumes electricity for pumps, control system, instrumentationand lighting, the vast majority (around 90%) by the main system pump(s). The averageannual consumption of this auxiliary power consumption is difficult to estimate per m3 oftreated water as the ratio between total treated volume and plant size is directly dependenton the nature of the contaminants. It is more easily estimated as a function of the solarfield size, as this determines total pressure drop and, consequently, pump specifications.In view of this, yearly power consumption can be estimated as between 40 to 80 kWh/m2

of installed solar field.− Chemical supplies. Although one of the main factors in the yearly system operating cost is

the chemicals purchased, an estimation cannot be given here because it strongly dependson each specific treatment process. The chemicals used by Photo-Fenton processes arecompletely different from TiO2 processes. They also depend on the nature andcomposition of the wastewater, for which some pre or post-treatment might be needed,with specific chemicals in each case. A case-by-case cost study must therefore beperformed. Two examples can be found on sections 8.2.1 and 8.2.2.

The sum of personnel, maintenance materials, electricity and chemical supplies, is the TotalOperating Cost of the solar detoxification system. To obtain the Annual Treatment Cost, perm3 of treated wastewater, total installed costs are converted to an annual levelized costconsidering a fixed-charge rate (FCR), which is obtained by calculating all the fixed costs

148


(excluding operation) for the life of the plant. The annual levelized cost and treatment costcan be calculated as:

CostOperatingFCRCostInstalledTotalCostLevelizedAnnual += x (8.8)

CapacityTreatmentAnnual

CostAnnualmEurosCostTreatment 3 =)/( (8.9)

The Fixed Charge Rate represents the equivalent revenue that must be generated annually tomeet all the charges on each Euro of facility investment. FCR is normally equal to the sum ofthe return on debt, taxes, depreciation, insurance, etc. For solar detoxification treatmentplants, a plant life of 12 years with a depreciation period of 10 years may normally beassumed. With this data, and considering, for example, that the plant is financed by a debtwith a 5% interest rate and that the total annual expenses due to taxes, insurance, etc, are 2%of the total installed cost, an FCR of 17% is obtained.

8.2.1 Example A: TiO2-based Solar Detoxification PlantA 300-m2 solar detoxification plant has been designed (see Chapter 9 on this point) for theyearly treatment of 6,000 m3 of wastewater contaminated by pesticide, using TiO2-Persulfate(see Section 5.2) photocatalytic system:

)(2 nm390hehTiO VBCB <+→+ +− λν (8.10)+•+ +→+ HOHOHhVB 2 (8.11)

−−•−− +→+ 244

282 SOSOeOS CB (8.12)

+−•−• ++→+ HSOOHOHSO 2424 (8.13)

It is estimated that the plant would operate about 3,000 hours per year. An estimation of theyearly treatment cost, in 1999 Euros, can be performed as follows:

149

A Facility Cost Data from table 8.1 (300 m2 of solar collector systemaperture area)

140,875 Euros

B Project Contingency 15% of the Facility Cost is estimated 21,131 Euros

C Engineering and Setup 50% of A+B (Total Facility Cost) 81,003 Euros

D Spare Parts 0.5% of A+B (Total Facility Cost) 8,100 Euros

E Total Installed Cost A+B+C+D 251,109 Euros

F Personnel cost 0.25 man-year is considered. A 20,000 Euros/man-year ofcost is considered

5,000 Euros

G Maintenance material cost 2% of A+B (Total Facility Cost) is estimated 3,240 Euros

H Electricity 5 kW of average electricity consumption so the 3,000yearly operation hours would consume 15,000 kWh ofelectricity. A cost of 0.1 Euro/kWh is considered

1,500 Euros

I Chemical supplies TiO2 catalyst used at 200 mg L-1 of concentrationand reusing up to 10 times. This means 120 kgper year.

S2O8Na2 (MW = 238) used at concentrations from5 to 10 mM with an overall consumption perbatch cycle of 15 mM. This means 21,420 kg peryear.Sodium hydroxide (NaOH; 50% solution) for pHadjustment and neutralization of the treated water. 4,800 kgof yearly estimated consumption

1,080 Euros

53,550 Euros

1,344 Euros

J Total Operating Cost F+G+H+I 65,714 Euros

Table 8.2150




5,000 Euros


H Electricity 5 kW of average electricity consumption so the 3,000yearly operation hours would consume 15,000 kWh ofelectricity. A cost of 0.1 Euro/kWh is considered

1,500 Euros

I Chemical supplies TiO2 catalyst used at 200 mg L-1 of concentration andreusing up to 10 times. This means 120 kg per year.

S2O8Na2 (MW = 238) used at concentrations from 5 to 10mM with an overall consumption per batch cycle of 15mM. This means 21,420 kg per year.

Sodium hydroxide (NaOH; 50% solution) for pHadjustment and neutralization of the treated water. 4,800 kgof yearly estimated consumption

1,080 Euros

53,550 Euros

1,344 Euros


Table 8.2 Estimated operating cost of a TiO2-Persulfatea Solar Detoxification plant in 1999 Euros

To obtain the treatment cost, an FCR of 17%, with 10-year plant depreciation, is estimated.

K Annual Levelized Cost E x FCR + J (formula 8.8) 108,402 Euros

L Annual Treatment Cost K is divided by 6,000 (yearly treated volume) 18.0 Euros/m3

Table 8.3 Estimated annual treatment cost of a TiO2-Persulfate a SolarDetoxification plant in 1999 Euros

It should be noted that a 4-percent reduction in the FCR (from 0.17 to 0.13) has a greatereffect on the Annual Treatment Cost than a 50% reduction in the Solar Collector Field. Also,about 50% of the yearly operating cost are the cost of the persulfate itself, so a reduction in itsprice very significantly affects the treatment cost. In this way, it must be taken into accountthat addition of consumable reagents (such as persulfate) may not necessarily be required byspecific TiO2 photocatalytic applications.

8.2.2 Example B: Photo-Fenton based Solar Detoxification PlantSimilar to the previous case, a 200-m2 solar detoxification plant has been designed for theyearly treatment of 6,000 m3 of wastewater contaminated by pesticides, using Fenton’sreagent (H2O2 and Fe2+) irradiated in the UV-Vis range (Photo-Fenton method):

OHOHFeOHFe •−++ ++→+ 322

2 (8.14)

)(22

3 nm580OHHFehOHFe <++→++ •+++ λν (8.15)

As in the previous case, 3000 operating hours per year are estimated. The yearly treatmentcost would then be:

151


A Facility Cost Data from table 8.1 (200 m2 of solar collector systemaperture area)

104,337 Euros

B Project Contingency 12% of the Facility Cost is estimated 12,520 Euros

C Engineering and Setup 50% of A+B (Total Facility Cost) 58,429 Euros

D Spare Parts 0.5% of A+B (Total Facility Cost) 5,843 Euros



4,000 Euros


H Electricity 4 kW of average electricity consumption so the 3000 yearlyoperation hours would consume 12,000 kWh of electricity.A cost of 0.1 Euro/kWh is considered

1,200 Euros

I Chemical supplies FeSO4 7 H2O (MW = 278) catalyst at 1mM concentration,which means 1,668 kg per year.

Hydrogen peroxide (H2O2; 30% solution) with an overallconsumption per treatment cycle of 33.75 mM. As MW =34, the total yearly needed amount is 22,950 kg.

Sulphuric acid (SO4H2; 98%) at 1 mM concentration foriron sedimentation by pH adjustment. As MW = 98, theyearly consumption is 580 kg.

Sodium hydroxide (NaOH; 50% solution) for batch modeneutralization of the treated water. MW = 40; 3,036 kg ofyearly estimated consumption.

549 Euros

17,442 Euros

180 Euros

849 Euros


Table 8.4 Estimated operating cost of a Photo-Fenton Solar Detoxification plant in 1999 Euros

Also as in the previous example, an FCR of 17% with a 10-year plant depreciation period isestimated.

K Annual Levelized Cost E x FCR + J (formula 8.8) 57,349 Euros

L Annual Treatment Cost K is divided by 6,000 (yearly treated volume) 9.5 Euros/m3

Table 8.5 Estimated annual treatment cost of a Photo-Fenton Solar Detoxificationplant in 1999 Euros

Two additional conclusions from this example may be added:− For a depreciation period of 20 years (longer facility lifetime), the FCR would be 12% and

this would mean a very significant reduction in treatment cost.− Operation is a significant portion of the cost of treated water. Reduction is likely to be

difficult due to the low capacity factor of the facilities and the costs of consumablesupplies, which are proportional to the volume of treated water.

Two additional examples of solar photocatalytic treatment cost found in the literature are thefollowing:− Groundwater remediation at Livermore, California, by NREL (USA). Average treatment

capacity of 4.4 L/s of water, with a peak flow of 30 L/s, inlet concentration of 400 ppbTCE and maximum outlet concentration of 5 ppb. Total treatment cost was reported as$4.07/m3.

152


− Study for a site at the Rocky Flats Plant near Boulder, Colorado (Bechtel Corp., 1991) totreat an annual volume of 2246 m3 with a peak water flow of 6.3 L/s. The levelized cost oftreated water was estimated at $10.57/m3. This cost was heavily dominated by theexpensive system required to treat inorganic components of the water at Rocky Flats.

8.3 SOLAR OR ELECTRIC PHOTONS?Another important aspect to be in mind in an economic assessment is the threshold at whichthe collection of solar photons required to drive the photochemical reactions becomes cheaperthan photons generated by electricity. Obviously, when only a small amount of solarirradiation is available, solar-driven processes make no sense, so the measurement of the solarresource available at a specific location might permit a reasonable initial estimation of theadvisability of a more in-depth study on solar photocatalytic processes.

Electric ultraviolet lamps are currently available on the market for a variety of applications,such as lighting, food processing, medical treatment, tanning, lacquer drying, photochemicalsynthesis, photopolymerisation processes, attracting insect, etc. They can also be used forphotocatalytic degradation processes.

The most suitable of the different types of electric lamps available for UV-photon productionare the low-pressure mercury fluorescent lamps (the same common fluorescent light normallyused for illumination), which are based on the generation of an electric arc through a sealedchamber containing mercury vapour. The result is 245-nm radiation which is absorbed by aphosphorous wall coating, producing a fluorescent radiation which, depending on the coating,can be adjusted to any spectrum. As for any arc lamp, ballast is required to provide theappropriate current and voltage to start. Several studies have demonstrated that these lampsare the easiest, simplest and cheapest way to produce UV photons and, in fact, are already beingused in commercially available TiO2 photocatalytic lamp systems.

Another type is the germicidal lamp, which is basically the same, but the 254-nm light passesdirectly through transparent glass. As their peak light emission is centred at the mentionedwavelength, they produce fewer valid photons and are less efficient for photocatalyticapplications. Medium and high-pressure mercury lamps are also similar to the low-pressurelamps, but the higher pressure generates higher intensity radiation and also tends to shift thespectrum towards the visible light. Finally, metal halide lamps, high-pressure sodium lampsand incandescent lamps cannot be used for UV light production.

Figure 8.5 shows two typical low-pressure 40-W mercury fluorescent lamps with dominantemission spectrum centred at 313 and 340 nm. According to manufacturer data, the initialstandard efficiency of these lamps is between 25% and 30% of UV-photon production and anaverage efficiency (lifetime) of about 20%. Average lamp life is around 20,000 hours.

Figure 8.5 belongs hereUltraviolet spectra: solar (standard and PSA measured spectrum) and two low-pressure 40 W

mercury lamps (QUV fluorescent lamps: UVB-313 and UVA-340)

With the above data, it is easy to obtain the equivalence between UV-lamp and solar systems. Asan example, we can calculate how many standard 40-W UV-fluorescent lamps are equivalent toa 500-m2 TiO2-based solar plant with an average yearly global UV irradiation of 20 W m-2. Theparameter used to compare them is the number of useful UV photons generated or collected. In

153

Fig. 8.5

154


the case of the solar plant, this can be obtained directly from the standard ASTM solar spectrum(assuming constant spectral distribution) using the following equation (see also Section 8.4):

=>Φ

hm

photonsI105.8N

2UVg21

eVE )2.3( (8.16)

Where:NΦ (E>3.2 eV) = Number of photons with energy over 3.2 eV (wavelength up to 387 nm) per unit oftime and surface.

UVgI = Yearly average global UV irradiation (W m-2).

The total amount of UV photons collected yearly by the solar plant would be:

2921SeVEUV 101.74500300020105.8SHNN xxxxx)2.3( === >Φ (8.17)

Where:NUV = Total UV photons (with energy over 3.2 eV) collected yearly.HS = Total yearly hours of operation of a solar detoxification system. This value depends on thegeographic location, but it may be estimated at 3500 (near the equator to parallel 20) to 2500(40th to 50th parallels). In the example, 3000 is used.S = Surface area of solar collector field (500 m2 in the example).

UVgI = Yearly average global UV irradiation during the 3000 estimated hours of operation. 20 W

m-2 in this example (see also Chapter 8.4).

So if the lamps must supply the same amount of UV energy:

L22

LLPH29

UV N8760105.04NH101.74N xxxx =Φ== (8.18)

Where:φPH = Nominal average photonic flux of the lamp. Manufacturer’s information usually places thisvalue between 1.3 to 1.4 x 1019 photons per second (standard mercury UV lamps with emissionspectrum centred at 360 nm). In the example, this has been translated to photons per hour.HL = Total yearly hours of operation of a lamp-based detoxification system = the full year (8760hours).NL = Number of lamps of the system.

So, according to equation 8.18, in the example considered, the equivalent number of lampswould be 394.

From this particular example, the following general equation for estimating the equivalentnumber of any type of electric lamps can be obtained:

SH

HI105.8N

L

S

PH

UVg21L

Φ

= x (8.19)

155


Based on these equations, the cost of collecting UV photons with solar technology may becompared to the generation of the same amount of UV photons using electric lamps. This is doneconsidering the following amounts of photons to drive the photochemical process:- 1028 (1.E+28)- 5 x 1028 (5.E+28)- 1029 (1.E+29)- 5 x 1029 (5.E+29)- 1030 (1.E+30)

Using equations 8.16 and 8.17, the solar field necessary (with different possible yearly averageUV global irradiation) can be calculated to collect the targeted amounts of photons. Once thesolar fields have been calculated, their cost can be estimated from the data in Table 8.1 (using apolynomial fitting). Finally, with the estimated cost of the solar field, the annual cost can becalculated using equation 8.8, considering an FCR of 17% as in the previous examples. Nooperating costs are considered. It should be noticed that the only cost included here is the cost ofthe solar collector, including the reactor tubes, as it is the only hardware related to UV-photoncollection. The results are shown in Tables 8.6 to 8.10.

UVgI

(WUV/m2)

Hs(hours)

S(m2)

Investment Cost(1999 Euros)

Yearly Cost(FRC = 17%)

Photon Cost(Euros/1.E+25 photons)

40 3500 12 13345 2269 2.2735 3500 14 13638 2319 2.3230 3500 16 14029 2385 2.3925 3000 23 15124 2571 2.5720 3000 29 16082 2734 2.7315 2500 46 18953 3222 3.2210 2500 69 22779 3872 3.875 1500 230 49457 8408 8.41

Table 8.6 Estimated Yearly Cost of collecting 1.E+28 solar UV-photons at different yearlyaverage UV global irradiation. Costs are indicated in 1999 Euros.

UVgI

(WUV/m2)

Hs(hours)

S(m2)




40 3500 62 21550 3663 0.7335 3500 70 23013 3912 0.7830 3500 82 24963 4244 0.8525 3000 115 30419 5171 1.0320 3000 144 35187 5982 1.2015 2500 230 49457 8408 1.6810 2500 345 68404 11629 2.335 1500 1.149 198524 33749 6.75

Table 8.7 Estimated Yearly Cost of collecting 5.E+28 solar UV photons at different yearlyaverage UV global irradiation. Costs are indicated in 1999 Euros.

UVgI Hs S Investment Cost Yearly Cost Photon Cost

156


(WUV/m2) (hours) (m2) (1999 Euros) (FRC = 17%) (Euros/1.E+25 photons)40 3500 123 31782 5403 0.5435 3500 141 34701 5899 0.5930 3500 164 38589 6560 0.6625 3000 230 49457 8408 0.8420 3000 287 58942 10020 1.0015 2500 460 87262 14835 1.4810 2500 690 124709 21201 2.125 1500 2.299 376772 64051 6.41


UVgI

(WUV/m2)

Hs(hours)

S(m2)




40 3500 616 112712 19161 0.3835 3500 704 126990 21588 0.4330 3500 821 145945 24811 0.5025 3000 1.149 198524 33749 0.6720 3000 1.437 243928 41468 0.8315 2500 2.299 376772 64051 1.2810 2500 3.448 546036 92826 1.865 1500 11.494 1479332 251486 5.03


UVgI

(WUV/m2)

Hs(hours)

S(m2)




40 3500 1.232 211554 35964 0.3635 3500 1.407 239321 40684 0.4130 3500 1.642 276016 46923 0.4725 3000 2.299 376772 64051 0.6420 3000 2.874 462527 78630 0.7915 2500 4.598 706316 120074 1.2010 2500 6.897 999924 169987 1.705 1500 22.989 2048971 348325 3.48

Table 8.10 Estimated Yearly Cost of collecting 1.E+30 solar UV photons atdifferent yearly average UV global irradiation.

Costs are indicated in 1999 Euros.

The comparison of electric and solar UV photon collection is only affected by the collectorand reactor system, since the rest of the system components are quite similar in design andcost. The only exception is the electrical installation, which would logically be more

157


expensive in the case of lamps, depending on the total installed power. Using equation 8.18the equivalent number of lamps (NL) to generate the same amount of UV photons can beobtained directly:- 1.E+28 photons ⇒ 23 lamps (40 W low pressure mercury fluorescent tubes)- 5.E+28 photons ⇒ 113 lamps (40 W low pressure mercury fluorescent tubes)- 1.E+29 photons ⇒ 226 lamps (40 W low pressure mercury fluorescent tubes)- 5.E+29 photons ⇒ 1132 lamps (40 W low pressure mercury fluorescent tubes)- 1.E+30 photons ⇒ 2265 lamps (40 W low pressure mercury fluorescent tubes)

With these data, Table 8.11 shows the estimated cost of UV photon generation with electriclamps. The main advantage of an electric system is their total availability (24 hours a day),which is an average of 3 times higher than solar systems. Total yearly cost is obtained usingequation 8.8 with an FCR of 17% as calculated above for the solar system. The operating costincludes only electricity, replacement of lamp and the cost of labour involved in replacement. Toobtain these values, a 20,000-hour lifetime has been considered for the lamps. The cost of thelamp system may be considered linearly dependent on the number of lamps.

Number of photons 1.00E+28 5.00E+28 1.00E+29 5.00E+29 1.00E+30Number of lamps Unit 23 113 226 1132 2265Lamp, ballast and accessories 6.80 154 770 1540 7701 15402Reactor cost 100.00 2265 11325 22650 113249 226499A) INVESTMENT COST 106.80 2419 12095 24190 120950 241900Electricity cost (0.15 E/kWh) 52.56 1190 5952 11905 59524 119048Lamp replacement 2.98 67 337 675 3373 6746Labour cost to replacement 3.15 71 357 714 3571 7143B) OPERATION COST 58.69 1329 6647 13294 66468 132937TOTAL COST: A x FCR + B 76.85 1741 8703 17406 87030 174060

Cost per 1E+25 UV photons 1.74 1.74 1.74 1.74 1.74

Number of photons 1.00E+28 5.00E+28 1.00E+29 5.00E+29 1.00E+30Number of lamps Unit 23 113 226 1132 2265Lamp, ballast and accessories 6.80 154 770 1540 7701 15402Reactor cost 200.00 4530 22650 45300 226499 452997A) INVESTMENT COST 206.80 4684 23420 46840 234199 468399Electricity cost (0.15 E/kWh) 52.56 1190 5952 11905 59524 119048Lamp replacement 2.98 67 337 675 3373 6746Labour cost to replacement 3.15 71 357 714 3571 7143B) OPERATION COST 58.69 1329 6647 13294 66468 132937TOTAL COST 93.85 2126 10628 21256 106282 212564

Cost per 1E+25 UV photons 2.13 2.13 2.13 2.13 2.13

Table 8.11 Estimated Yearly Cost of UV photon generation with electric lamps consideringtwo different reactor costs: 100 and 200 Euros/lamp. Electricity cost = 0.15 Euros/kWh. FCR

= 17%. Costs are indicated in 1999 Euros.

158


The main uncertainty in Table 8.11 is the cost of the reactor in the lamp-based photocatalyticsystem. It is clear that this item would necessarily be an expensive component of the electricsystem as it must include piping, external UV-reflectors to avoid loss of UV photons,supports, wiring, and even, depending on how compactly the system assembly is, a coolingsystem. As this cannot be estimated with any precision, Table 8.7 attempts to cover acomplete range of possible costs by presenting two conservative estimates, one in which theper-lamp reactor cost is estimated low (100 Euros) and another in which it is estimated high(200 Euros).

The main conclusions arrived at from a comparison of Table 8.11 and Tables 8.6 to 8.10, arethe following:- The available solar UV-irradiation for a specific location is the main factor in determining

the cost of a solar-based system.- The cost of collecting photons with solar technology decreases proportionally as the

yearly total increases because of the reduction in collector production cost for largernumbers. This is not the estimated case of lamp-based systems as they are much morecompact.

- When fewer photons are needed (1.E+28), lamp-based systems are cheaper than solar.- Solar-based systems with collector apertures of over 100 m2 are clearly cheaper than

lamp-based systems when the average yearly global UV irradiation is over 15 W/m2.- The threshold UV radiation (below which lamp-based systems are cheaper than solar)

decreases as the yearly amount of photons increases.

All data included in Tables 8.6 to 8.11 are summarised in Figure 8.6, which shows thecomparative cost of solar and electric technologies for UV-photon collection and generation,respectively. Three different amounts of photons are plotted (1.E+29, 5.E+29, 1.E+30) againstthe average global solar UV-irradiation by adjusting the data in Tables 8.8, 8.9 and 8.10(yearly cost).

Figure 8.6 belongs hereComparative cost of UV photon collection/generation with solar technology (CPCs) and

electric lamps (electricity cost=0.15 Euros/kWh), respectively. Data from tables 8.6 to 8.11.Costs are indicated in 1999 Euros.

Figure 8.6 enables easy calculation of the solar threshold which determines the advantage ofsolar over lamp-based technology, as the yearly UV-photon requirement of the majority ofpossible applications will be in the range of 1.E+29 to 1.E+30 This threshold is normallybetween 10 and 12 WUV/m2, which corresponds to 0.015 Euros/kWh for electricity. If a differentcost is considered, Table 8.11 must be updated. This is the case of Figure 8.7, which correspondsto a cost of 0.05 Euros/kWh. In this case, the threshold solar advantage over lamp-based systemsis logically higher than before.

Figure 8.7 belongs hereComparative cost of UV photon collection/generation with solar technology (CPCs) and

electric lamps (electricity cost=0.05 Euros/kWh), respectively. Data from tables 8.6 to 8.11.Costs are indicated in 1999 Euros.

8.4 SOLAR RESOURCES ASSESSMENTBecause the size of a solar photocatalytic system is directly related to the amount of UV lightavailable, accurate assessment of the solar resource is important. Solar UV-light (300 to 400

159


nm) comprises roughly 2-3% of the direct beam energy and 4-6% of the combined directbeam and diffuse irradiation (global). Non-concentrating systems have an advantage withregard to resource assessment as there are extensive data on the total energy (direct anddiffuse) available from sunlight and the 4-6% conversion factor makes resource assessment astraightforward process for non-concentrating systems.

The cost of a specific solar detoxification treatment plant may be easily estimated from theindications in Sections 8.1, 8.2 and 8.3, once the degradation process has been assessed andtreatment factors have been and identified. Nevertheless, the yearly average UV irradiation atthe specific location of potential solar detoxification plants must be known. In the so-called“solar belt”, this is usually between 15 and 30 W m-2. It would be best if the location had adatabase of historical global UV irradiation available, such as the case in Figure 8.8.

Figure 8.8 belongs hereAverage direct and global UV irradiance (sunrise to sunset) at PSA (Almería, Spain).

Meteorological data records from 1991 to 1995

According to Section 2.4, for reliable estimation of the annual average UV radiation, the so-called “cloud factor” (f) must be found. This is the average percentage of global UV radiationthat is expected to be lost due to clouds throughout the year. The cloud factor is also intendedto be a measurement of atmospheric transparency and is affected by all the atmosphericcomponents, which can absorb or scatter solar radiation. Figure 8.9 shows the results obtainedat PSA, following the procedure indicated in Section 2.4.

Figure 8.9 belongs hereAverage “cloud factor” for global UV irradiance (sunrise to sunset) at PSA (Almería, Spain).

Meteorological data records from 1991 to 1995

Another interesting alternative for estimating the annual UV radiation availability is throughthe Lambert-Beer Law (also called the Bourguer Law), which indicates that light attenuationthrough a continuous medium is proportional to the flux radiation and the distance covered:

mcob eII λλλ

−= ,, (8.20)

where:Ib,λ = Intensity of solar irradiation on the earth’s surface at a specific wavelength, λ.Io,λ = Intensity of extraterrestrial solar irradiation at the same wavelength, λ.cλ = Atmosphere attenuation coefficient at λ.m = Air mass ratio (see Chapter 2), defined as the oblique optical path described by a photonin the atmosphere relative to the minimum vertical path to arrive at the same terrestriallocation. If the elevation of the sun is 90º then m = 1.

The air mass ratio can be obtained for any moment by the following equations:

Zo emm 0001184.0−= (8.21)

( ) αα sinsinmo 6146141229 2 −+= (8.22)

where:mo = Air mass ratio at sea levelz = elevation over sea level in meters

160


α = Solar altitude, defined as the angle formed by the solar vector to the horizontal surface ofthe earth.

Solar altitude can be obtained as follows:

( ) ( ) ( ) ( ) ( )SSS hcoscosLcossinLsinsin δδα += (8.23)

( ) ( )

+

=25,365

5,102cos45,23

NsinS

πδ (8.24)

Where:L = Local latitudeδs = Solar declinationhs = Solar time angle, which is zero at noon, positive before and negative after noon. It can becalculated at any time by adding 15º of each hour of difference with respect to the noon(Example: at 15’00 hours of solar time, hs = -45º).N = Day number; it is 1 at January 1st and 365 (or 366) at December 31st.23.45 is the angle formed by the earth’s axis and the plane formed by the sun and the ellipticaltrajectory of the earth.

Equations 8.21 to 8.24 allow solar irradiation at any specific location on earth to be detailed;Ib,λ must be measured at the specific location and Io,λ can easily be calculated. Due to theearth’s slightly eccentric elliptical orbit, solar extraterrestrial radiation varies ±3.4% along theyear depending on the Day number:

( )

+==

365

2cos034.01,,,

NINII SCoo

πλλλ (8.25)

( )∫=2

1

,

λ

λλ λλ dEI SC (8.26)

Where:Isc,λ = Solar Constant associated with the spectral interval (λ1,λ2) to which is intended toobtain the atmospheric attenuation coefficient (cλ). This interval must also be coherent withthe measure of Ib,λ (i.e., if we are using a solar radiometer which measures within the interval295 to 400 nm, Isc,λ must be calculated to the same interval).E(λ) = Extraterrestrial irradiation, which can be found in Table 8.12.

161


λ(µm)

Eλ

(W m-2 µm-1)λ

(µm)Eλ

(W m-2 µm-1)λ

(µm)Eλ

(W m-2 µm-1)

0,115 0,007 0,43 1639 0,90 8910,14 0,03 0,44 1810 1,00 7480,16 0,23 0,45 2006 1,2 4850,18 1,25 0,46 2066 1,4 3370,20 10,7 0,47 2033 1,6 2450,22 57,5 0,48 2074 1,8 1590,23 66,7 0,49 1950 2,0 1030,24 63,0 0,50 1942 2,2 790,25 70,9 0,51 1882 2,4 620,26 130 0,52 1833 2,6 480,27 232 0,53 1842 2,8 390,28 222 0,54 1783 3,0 310,29 482 0,55 1725 3,2 22,60,30 514 0,56 1695 3,4 16,60,31 689 0,57 1712 3,6 13,50,32 830 0,58 1715 3,8 11,10,33 1059 0,59 1700 4,0 9,50,34 1074 0,60 1666 4,3 5,90,35 1093 0,62 1602 5,0 3,80,36 1068 0,64 1544 6,0 1,80,37 1181 0,66 1486 7,0 1,00,38 1120 0,68 1427 8,0 0,590,39 1098 0,70 1369 10,0 0,240,40 1429 0,72 1314 15,0 0,00480,41 1751 0,75 1235 20,0 0,00150,42 1747 0,80 1109 50,0 0,0004

Table 8.12 Extraterrestrial solar irradiation

The Solar Constant for the whole solar spectrum (total solar radiation) is 1353 W m-2. In thecase of the UV spectrum (295 to 400 nm), using Table 8.12, it is:

( )∫ −=−==400

295

2, mW104.413.6118.1dEI UVSC λλ (8.27)

This value of 104.4 W m-2 represents 7.72% of total extraterrestrial irradiation. As thepercentage on the earth’s surface is lower (normally from 4 to 6 percent), this means that theatmosphere filters UV radiation relatively more than the overall spectrum.

With this background, by measuring Ib,λ, the atmospheric attenuation coefficient (cλ) can becalculated. This parameter include all the factors relevant to solar irradiation, such as Raleighdispersion, ozone absorption, atmospheric turbidity and spray, absorption by water molecules,clouds, etc. The knowledge of cλ at a specific location makes it possible to forecast solarirradiation during the year. Variations in cλ over the year also enable conclusions to be madeabout changes in atmosphere performance (with regard to solar light) at the specific location.

8.5 COMPARISON WITH OTHER TECHNOLOGIESA general qualitative approach to the range of application of different treatment technologies(concentration of contaminant versus flow rate to be treated) is provided in Figure 8.10.

162


Figure 8.10 belongs hereTechnology fit map. Range of application of different water treatment technologies

The following technologies may be considered the main competitors of aqueous-phase solardetoxification for the treatment of hazardous water contaminants. As only non-biodegradablecontaminants are considered here, biological treatment technologies are not discussed.

8.5.1 Thermal OxidationMost of the thermal oxidation technologies (also known as thermal incineration) are used todestroy hazardous wastes at high temperatures (normally higher than 650ºC). At suchtemperatures, chemical bonds are effectively broken and transformed into other more benignsubstances. There are different technologies (rotary kilns, infrared furnaces, conventionalincinerators, etc), but almost all of them are variations of the same process. Incineration istypically a two-step process in which a primary combustion chamber burns the waste materialat temperatures of about 1000ºC and the exhaust gases containing desorbed oxidized orpartially oxidized organic contaminants are transferred to the secondary combustion chamber(afterburner) where these gases are burned to carbon dioxide, carbon monoxide, inorganicacids and water. Afterburners generally operate at about 1200ºC and residence times in bothchambers depend on the contaminants to be destroyed. Natural gas or propane is the usualenergy source. Electricity can also be used in the primary chamber, as in infrared furnaces.Incineration is the most frequently used treatment for highly concentrated wastes orcontaminated soil. The standard performance requirement is 99.9999% (required by law inmany countries).

Incineration is a source of debate with opposition to the construction of new incinerators fromthe local populations. This is because of the increasing perception of incineration as a cause ofhealth and safety problems. Incineration is also an energy-intensive process subject to streamsize and concentration constraints with significant capital equipment and operating costs. As aconsequence, it is expensive compared to other treatment processes. Typical operating costsare also high, in the range of 200 to 1000 Euros/ton depending on the nature of thecontaminants and the facility utilisation factor. Fuel is the primary factor affecting operatingcost.

The advantages of incineration are that the volume of waste is greatly reduced, often by asmuch as 90%. Furthermore, the resulting ash is generally more stable and less likely to leachinto groundwater than the parent compounds. On the other hand, poor combustion duringincineration often produces by-products that are at least as toxic as the parent material. Metalssuch as lead, mercury, and chromium could be released into the air. The burning of someplastics can produce hydrogen chloride, which becomes hydrochloric acid rain if combinedwith the moisture in air. Benzene, chloroform, and TCE have frequently been found in stackemissions. Finally, generation of organic compounds and dioxins due to incompletedestruction has always been a concern.

8.5.2 Catalytic OxidationCatalytic oxidation (also known as catalytic incineration) is very similar to thermal oxidation.Contaminants are preheated, mixed, and combusted at high temperatures to form carbondioxide and water. The main difference is the presence of a catalyst inside the combustionunit that reduces the activating energy necessary for combustion. Therefore, combustionoccurs at a lower temperature than for thermal oxidation and, as a result, fuel costs forcatalytic oxidation are usually lower than for an equally applicable thermal oxidation system.

163

0 10 20 30 40 50 60 70 801

10

100

1000

TO

C (

mg

L-1)

2 000

Biological O

3

H2O

2

Biological

Redox Processes Fenton Biological

WAO & Biological

Wet Advanced Oxidation (WAO)

WAO & Incineration RecoveryIncineration

Flow Rate (m3 h

-1)

Fig. 8.10

164


Catalyst materials, such as platinum, palladium, and metal oxides such as chrome-alumina,cobalt oxide, etc, are introduced into the combustion unit in either a monolithic or beadedconfiguration. The average catalyst lifetime is normally from two to five years, after whichdeactivation by inhibitors, blinding by external particles and thermal ageing, render itineffective.

The most relevant characteristics of catalytic in comparison to thermal oxidation systems arethe following:- Not every organic contaminant may be treated by catalytic oxidation systems, which are

not effective for streams containing lead, arsenic, phosphorus, bismuth, antimony,mercury, iron oxide, tin, zinc or other catalyst deactivators.

- Catalytic oxidation systems are usually applied to low concentration streams, since highconcentrations may be associated with high heat content which can generate enough heatfrom combustion to deactivate the catalyst.

- Associated with the above mentioned characteristics, air is usually not required for thecombustion process.

- Temperature and pressure through the catalyst bed should be monitored to preservecatalyst activity. Catalytic oxidation systems normally operate at temperatures between250ºC and 500ºC, since excessive heat can deactivate most catalysts.

- Catalyst poisoning from metals or halogens and / or binding from particulate matter overtime can decrease destruction efficiency. Periodic catalyst replacement or reactivationwould then be required.

Catalytic incinerators have a higher initial cost compared with thermal incinerators, but itslower fuel costs and thereby lower operating cost offset this.

8.5.3 Air StrippingAir-stripping is a liquid-to-gas transfer process whereby polluted water is sprayed into the air,allowing volatile organics to escape. It is also widely used for contaminated soil containingVOC’s. The basic principle of air-stripping technology is the process of mass transfer ofdiffusion and volatilisation. The natural tendency of any organic compound to volatilize fromthe aqueous to gaseous phase is given by the Henry´s Law, which states the partial pressure pA

produced by a compound of concentration cA dissolved in a liquid solvent.

AA cHp = (8.28)

The Henry´s Law Constant H can then be expressed as a dimensionless ratio of a compound’sconcentration in air to its concentration in water. The application of Henry´s Law Constantmakes it possible to assess the maximum efficiency of an air-stripping system theoreticallypossible under ideal conditions. A compound with high H values is more likely to be removedfrom water and soil by air-stripping and, normally, compounds with an H over 0.001 can beremoved effectively.

H is over 0.01 for many organic compounds, which is the reason the air-stripping process isso widely used. Conventional air-stripping treatments of hazardous organic compounds inwater are based on packed or tray columns in which water and air usually flow downward andupward respectively, with a very high contact area between the two phases (spray chambers,venturi scrubbers, plate or tray towers, packed towers, etc). The technology has been appliedto a wide range of organic compounds with varying degrees of effectiveness at contaminant

165


concentrations between 250 and 10,000 mg L-1. The initial investment in air stripping may beconsidered medium and operating costs are low.

The main environmental objection to air stripping is the undeniable fact that this technologydoes not really destroy hazardous compounds, but simply transfers them to another medium,as the organic compounds stripped from water are released directly into the atmosphere.Tighter restrictions on emissions from air strippers are limiting their applicability andincreasing their cost, as secondary air-phase treatment systems (i.e., real treatment processes)are required afterward.

8.5.4 AdsorptionAdsorption of organic compounds on a solid adsorbent is widely used for treating hazardousorganic compounds in water. Contaminated water is forced through tanks containing activatedcarbon and contaminants are retained by weak intermolecular forces. Many organic and someinorganic compounds are efficiently removed, including chlorinated hydrocarbons, organicphosphorus, carbonate-based pesticides, polychlorinated biphenyls (PCB), and trace metals.Carbon, derived from wood, coal, or other carbonaceous raw materials, is the most commonlyused adsorbent. Other adsorbents include silica gel, alumina, and zeolite. For carbonadsorption, the organic compounds are transferred to a carbon canister that must betransported for disposal or regeneration.

Three types of carbon adsorbents are common: activated granules, activated powders andfibres. Granular activated carbon (GAC) is currently the most common type of carbonadsorbent because of the significant surface area provided by the granules. Powderedactivated adsorbents are generally cheaper. They are of lower quality than granular activatedcarbon and they cannot be regenerated. They are used in packed columns with high-pressuredrops. Powder coatings are used exclusively in batch operations. Carbon fibres are used in ahoneycomb structure to maximise surface area with adsorption directly on the fibre’s surface.

Carbon regeneration and recovery and disposal of contaminants are the primary factors in theoperating cost. Periodical regeneration in which the carbon is cleaned and reactivated,restoring its capacity for adsorption and further use, is necessary. A thermal regenerationprocess fueled by electricity, natural gas, or oil is the most commonly used. The adsorbedmaterials are pyrolyzed, forced off, and finally oxidized at over 900ºC.

As with air stripping, the main objection of adsorption by GACs is that they only displacecontamination to a large sorbent phase, which remains a regeneration or disposal problem, thecontaminants not really having been treated. The use of active carbon adsorption is alsofeasible only as long as current legislation continues to allow storage of this type of waste. Inthe European Union, regulations on waste generation are becoming stricter. The commonpractice of using carbon only once is expensive and polluting because the used carbon isusually destroyed by open burning. Carbon regeneration by indirect firing is feasible only forabout four cycles, after which the carbon is destroyed by burning. Furthermore, each time it isregenerated, only about 50% of the carbon is reused. The remainder is too fine and restrictsthe flow in filtration units. Often, the regenerated carbon also loses some of its activity and isnot as efficient as new. Finally, increasing transportation and carbon regeneration costs arereducing its cost-effectiveness. Consequently, GAC processes are expensive to operate andgenerate undesirable secondary waste gases and changing environmental regulations indicatethat open burning of these materials may be prohibited within a few years.

166


The initial investment is small and operating costs are medium. GAC is a mature technologyand significant cost reduction is not anticipated. Sensitivity studies have shown how costs arelikely to be affected by different contaminants, concentration and plant size. These studiesindicate that cost does not increase linearly with concentration, but much slower, because asconcentration increases, contaminant loads of carbon also increase. Therefore, GACprocessing costs are lower in large plants due to the economies of scale.

8.5.5 Membrane TechnologyMembrane technology refers to the use of a semi-permeable membrane to separatecontaminants from a wastewater stream; this separation technique is often used to purifymaterials or to pre-concentrate waste streams prior to treatment, so it is not a real treatmenttechnology. A waste stream (typically aqueous) is passed across a porous membrane, wherediffusion of specific species takes place, separating and concentrating species of interest. Themembrane is a composite of a polyamide barrier on a polysulfone support, which normallyrejects organic molecules with a high molecular weight.

The main membrane processes are reverse osmosis, ultrafiltration and microfiltration, all ofwhich are pressurised. Ionic exchange by resins can also be considered a membrane process.Nevertheless, the most common process is reverse osmosis, which has been used for years totreat drinking water. Polluted water is forced through semipermeable membranes (about 150micrometers thick) under pressures as high as 100 bar, separating contaminants from the cleanwater. Membranes (normally ceramic or polymeric) are manufactured in differentconfigurations such as plate-and-frame, spiral-wound, tubular, capillary tube, hollow fiber,etc.

Many types of organic compounds can be separated by membranes, including salts andchlorinated and organophosphoric pesticides. Reverse osmosis requires periodic cleaning andrejuvenation of filters. Residues, which contain high concentrations of salts, heavy metals,and toxins, must be disposed of either in landfills or by incineration. Additionally, reverseosmosis is an especially intensive energy consumer, because of the high hydraulic pressuresneeded to offset osmotic pressure, which require substantial pumping power. Depending onmembrane porosity and the nature of water contaminants, 2.5 to 5 kWh, are needed per cubicmeter of water treated. As a result, the major drawbacks of the membrane technology are itsrelatively high cost for large-scale applications and the fact, again, that it is not a realtreatment process but only a separation technology.

8.5.6 Wet OxidationWet oxidation, by which organic and inorganic compounds present in water (or any otherliquid) are oxidized with oxygen or air in the presence of water, has been commerciallyavailable since the seventies (Zimpro Passavant Environmental Systems, Inc.). This process isnormally used with water containing 1% to 30% hazardous organic matter (10 to 300 g L-1)and treatment conditions vary from 180ºC at 20 bar to 400ºC and 280 bar. During the process,water favours the dissolution of oxygen and heat transfer to the compounds to be treated.

When treatment conditions are below the critical point of water, 374ºC and 225 bar, reactionstake place in the water and the process is denominated subcritical wet oxidation or lowpressure wet oxidation; otherwise, it is called supercritical wet oxidation. Low-pressure wetoxidation achieves 99% efficiency in the destruction of contaminants with residence times ofaround one hour; supercritical wet oxidation can achieve 100% efficiency with a residence

167


time of 5 minutes, but higher temperatures and pressures are required. The main disadvantageof wet oxidation systems is their very high installation and operating costs.

8.5.7 Ozone oxidationOzone (O3), used as an oxidizing agent since the beginning of the 20th century, is consideredan effective treatment process. Except for some controversial reports, ozone is not known toproduce toxic or mutagenic substances. Ozone can be generated by a high voltage dischargein the presence of oxygen. After generation it must immediately be diluted in the water to betreated, as it is a very unstable gas. This means that ozone must be generated at the point oftreatment.

Ozone treatment has become very common in recent years thanks to improvement in ozonegenerators and the technology is quickly replacing many traditional oxidation processes basedon chlorine, hydrogen peroxide, permanganate, etc. Ozone is used to oxidise lowconcentrations of organic contaminants and also inorganic compounds and water-solublemetals converting them to their insoluble form to permit separation of e.g. Fe++ and Mn++.Another important application of ozone is disinfection. Investment costs of ozone technologyare medium as are its operating cost of from 5 to 15 Euros/m3.

Ozone treatment is reported to be more effective for hazardous organic contaminants whenused in combination with UV oxidation, similar to the synergistic effect that can be observedbetween UV radiation and an oxidiser such as hydrogen peroxide in wastewater treatment.This treatment process can be used selectively and reportedly acts on chlorinatedhydrocarbons faster than on other organic compounds. Halogenated organic compounds areoxidised to simpler organic forms and, in some cases, are oxidised to carbon dioxide, waterand innocuous salts. The combined UV-ozone system has also been observed to precipitateheavy metals, such as oxides or metals, although UV oxidation is not usually used to removemetals. The concentration of inorganic chemicals in wastewater must be low so as not toabsorb or shield the UV rays.

8.5.8 Advanced Oxidation processesIn addition to all the conventional technologies indicated above, a new type of treatmentprocess, commonly referred as Advanced Oxidation Processes (AOPs), is emerging. AOPsare increasingly being considered as alternatives to more conventional technologies becausethey destroy hazardous organic compounds rather than transferring them to other media, havea potentially lower cost and greater effectiveness. AOPs are generally characterized by theirability to generate hydroxyl radicals, some examples being UV/hydrogen peroxide, UV/ozoneand UV photocatalytic oxidation (Solar Detoxification).

Solar Detoxification has some unique advantages over other AOPs, such as:- The use of sunlight as the photon source, which means it is a “green” technology.- The process can be either heterogeneous (TiO2) or homogeneous (Photo-Fenton), with the

possibility of providing chemical pathways and surface interactions not available in othertreatment systems.

- A reductive chemical pathway is used to remove reducible species, such as heavy metalions and some organic compounds.

- It can be operated in the liquid or gas phase in contrast to processes using ozone orhydrogen peroxide, which are generally applied only in the liquid phase.

168


Solar Detoxification is a modular technology offering the advantage of system flexibility,which is important when treating low to moderate flow rates. However, this same attributedecreases the possibilities for the economies of scale that other pollution control technologiesbenefit from. Thus, while the cost of Solar Detoxification remains relatively constant as flowrate increases, the normalised costs for other competing technologies drop.

From the economic perspective, to date, Solar Detoxification cost estimates have been basedon limited field experience. However, this experience and projections of capital and operatingcosts show that solar photocatalytic oxidation of water costs from a few Euros up to 20-30Euros/m3. This cost is higher than for the air stripping, adsorption or membrane technologies,but with the important advantage that while solar detoxification is a real treatment process,the others are only contaminant separation technologies. There is also a possibility ofoptimising the cost of a specific treatment process by combining two (or more) differenttechnologies; Figure 8.11 shows the combination of solar detoxification and GAC for removalof pentachlorophenol (PCP).

Figure 8.11 belongs hereOptimisation of Solar Detoxification and GAC technologies to PCP degradation

The investment and operating costs of solar detoxification are lower than for technologiessuch as incineration or wet oxidation while other AOP technologies have similar investmentand operating costs.

It is unclear at present how much of the hazardous-waste treatment market could be capturedby Solar Detoxification technology, but potentially it is very large. Environmentalremediation field studies have demonstrated that technology selection is clearly not based oncost alone. Factors such as complete on-site treatment (which limits owner liability),community acceptance and absence of undesirable by-products are commonly consideredcarefully in addition to cost. It is expected that, because of it is so attractive in these areas, theSolar Detoxification system should be able to build a significant market share in the nearfuture.

SUMMARY OF THE CHAPTERAs photocatalytic processes only make sense for hazardous non-biodegradable pollutants and,when feasible, biological treatment of residual water is always the cheapest, it makeseconomic sense to combine the two processes. Biologically recalcitrant compounds can betreated with photocatalytic technologies until biodegradability is achieved, transferring thewater to a conventional biological plant later. Such a combination, based on the AverageOxidation State parameter, reduces treatment time and optimizes the overall economics.Operation and investment costs of Solar Detoxification plants are estimated to be lower thanalternative technologies such as incineration or wet oxidation, in the same range of ozone andhigher than air stripping, adsorption or membrane separation technologies. But the latter arenot real treatment technologies. Solar collecting of UV photons is logical where yearlyaverage solar UV irradiation is higher than 15 W m-2, which means almost the entire “sun-belt”.

BIBLIOGRAPHY AND REFERENCES1. Bechtel Corporation. “Conceptual Design of a Photocatalytic Wastewater Treatment

Plant”. Sandia National Laboratory Report. SAND91-7005. 1991.

169

% Eliminated PCP

Cos

ts

GAC

Photocatalysis

Photocatalysis + GAC

Fig. 8.11

170


2. Blanco, J.; Malato, S. “Photocatalytyc Treatment of Hazardous waste Water; costcomparison between solar and electrical technologies”. Int. Conf. on ComparativeAssessments of Solar Power Technologies. A. Roy, W. Grasse Eds. pp. 217-230.Jerusalem, Israel, 1994.

3. Blanco, J.; Malato, S. “Tecnología de Fotocatálisis Solar”. Instituto de EstudiosAlmerienses and CIEMAT Eds. ISBN 84-8108-106-X. Almeria, (1996).

4. Kenneth, M.E.; Gee, R.; Wickham, D.T.; Lafloon, L.A.; Wright, J.D. “Design andFabrication of Prototype Solar Receiver/Reactors for the Solar Detoxification ofContaminated Water”. Industrial Solar Technology Corporation. NREL report. 1991.

5. Malato, S.; Blanco, J.; Richter, C.; Braun, B.; Maldonado, M.I. "Enhancement of the rate ofsolar photocatalytic mineralization of organic pollutant by inorganic oxidizing species“.Applied Catalysis B: Environ. Vol. 17, pp. 347-356, 1998.

6. Mehos, M., Williams, T.; Turchi, C.S. “Overview of Solar Detoxification Activities in theUnited States”, NREL/TP-471-7262. National Renewable Energy Laboratory, Golden,CO, 1994. DE95000264.

7. NEPCCO Environmental Systems. “Gas Phase Photocatalytic Oxidation Remediation andProcess Pollution Market Study”. Final Report for NREL. 1997.

8. O’Brien & Gere Engineers, Inc. “Innovative Engineering Technologies for HazardousWaste Remediation”. International Thomson Publishing Inc. 1995.

9. Pulgarin, C.; Invernizzi, M.; Parra, S.; Sarria, V.; Polania, R.; Péringer, P. “Strategy forthe Coupling of Photochemical and Biological Flow Reactors useful in Mineralization ofBiorecalcitrant Industrial Pollutants”. Catalysis Today, 1999, in press.

10. Ruppert, G.; Bauer, R. “UV-O3, UV-H2O2, UV-TiO2 and the Photo-Fenton reaction-comparison of adv. oxidation processes for wastewater treatment“. Chemosphere, Vol.28,No 8, pp.1447-1454, 1994.

11. Schertz, P.; Kelly, D.; Lammert, L. “Analysis of Cost of Generating or CapturingUltraviolet Light for Photocatalytic Water Detoxification Systems”. Solar Kinetics, Inc.Final Report for NREL. 1992.



1. The Average Oxidation State of any organic compound is a number between 0 and +4.2. When contaminated water is treated by oxidation, its Average Oxidation State increases.3. If toxicity increases with oxidation, biodegradability will not be achieved.4. If toxicity decreases during photocatalytic treatment, biodegradability increases.5. The solar UV-irradiation available for a specific location is the main factor in determining

the cost of a solar system.6. The threshold below which electrical systems are cheaper than solar decreases when the

total yearly amount of photons needed increases.7. The air mass ratio is equal to 1 at solar noon at any latitude.8. Standard required performance for thermal oxidation technologies is 99.9999%.9. Catalytic oxidation processes usually require higher temperatures than non-catalytic

oxidation processes.10. Ozone oxidation can normally be applied to high concentrations of organic contaminants.

PART B.

171


1. Operating 2800 hours per year, a 300-m2 solar detoxification plant treats 5,000 m3 ofwater containing 250 mg L-1 of hazardous contaminants yearly. If, once 50% of TOCdegradation has been attained, the wastewater is biodegradable and it is transferred to abiological treatment plant, what would be the mass and volumetric treatment factors of thephotocatalytic facility?

2. Why may toxicity of wastewater increase at the beginning of photocatalytic treatment?3. What are the main factors in the operating cost of a solar detoxification treatment plant?4. What is the Fixed Charge Rate? How can it be calculated?5. What is the standard average efficiency of UV photon production of mercury fluorescent

lamps?6. A solar TiO2 detoxification facility can treat 3,500 m3 of contaminated water yearly with

an average global solar irradiation of 20 W m2. How much water could be treated at adifferent location with an average solar irradiation of 25 W m2 ?

7. How many mercury fluorescent lamps (40W tube) would be equivalent to a 350-m2 solarcollector field working 3500 hours per year where average solar UV irradiation is 26 Wm2 ?

8. What are the main differences between catalytic and non-catalytic thermal oxidation ?9. What is the main environmental objection to air-stripping, absorption and membrane

treatment technologies of hazardous wastewater?10. When is an oxidation process defined as supercritical?

ANSWERS

172


Part A

1. False; 2. True; 3. False; 4. True; 5. True; 6. True; 7. False; 8. True; 9. False; 10. False

Part B

1. Using equations 8.1 and 8.2:

2fmmh

g0.74

30028002

5000250

St

TOCmT ==

∆=

x

x)(

2fvmh

L5.9

3002800

5000000

St

VT ==

∆=

xThe total mass of organic substances is divided in two as only 50% is really degraded, butnot the total volume which may be considered completely treated.

2. Due to the presence of not only the initial toxic compounds, but also the many by-products of the degradation reaction at the beginning of the process.

3. Personnel, maintenance materials, electricity and chemicals cost.4. FCR is the factor converts total installed cost (or total plant investment) into annual

treatment cost, considering the volume of wastewater to be treated. FCR is obtained bycalculating all the fixed costs (except operation) for the life of the plant.

5. 20%.6. As solar irradiation is proportional to the amount of useful photons and this is linearly

dependent on the reaction rate:

3m437520

253500V ==

7. Using equation 8.19:

lamps4343508760

3500

3600101.35

26105.8S

H

HI105.8N

19

21

L

S

PH

UVg21L ==

Φ

=xx

xx

8. Catalytic oxidation a) does not treat every organic contaminant; b) is typically applied tolow concentrations; c) does not usually require air for combustion; d) requires monitoringof temperature and pressure to preserve catalyst activity; e) requires periodic replacementor reactivation of the catalyst.

9. The air-stripping, absorption and membrane technologies only transfer the contaminantsto a different, more easily managed, medium, but they can not be considered realhazardous-wastewater treatment technologies.

10. When it takes place at temperatures and pressures higher than the critical point of water(225 bars and 374ºC). If a fluid other than water is used, these conditions are defined bythe specific critical point of the working fluid.

173


9 PROJECT ENGINEERING

AIMSThis unit describes the systematic process of feasibility study, preliminary design, final designand construction of a solar detoxification plant, using the knowledge explained in the previouschapters. Some important aspects of project management are also indicated.

OBJECTIVESAfter completing this unit, you will have a basic knowledge of the following subjects:1. Feasibility study for Solar Detoxification applications.2. Initial pre-design of an engineering system.3. Implementation and management of Solar Detoxification projects.

NOTATIONS AND UNITSSymbol Description UnitsCIEMAT Centro de Investigaciones Energéticas, Medioambientales y

Tecnológicas (Spain)COD Chemical Oxygen Demand mg L-1

CPC Compound Parabolic CollectorDG-XII Directorate General XII (of European Commission)EC European CommissionEUV,n Accumulated UV energy incident on the reactor, since the

start of the degradation process up to tn, per unit of volumekJ L-1

GAC Granulated Active CarbonHS Total yearly hours of operation of a solar detoxification system hLLNL Lawrence Livermore National Laboratory (USA)mM Mili Molar 10-3 moles L-1

PLC Programmable Logic Controllerppm Parts per million (equivalent to mg L-1)PSA Plataforma Solar de AlmeríaRe Reynolds number DimensionlessS Solar collector field area m2

T Experimentation time minTfm Mass Treatment Factor g h-1 m-2

Tfv Volumetric Treatment Factor L h-1 m-2

TOC Total Organic Carbon mg L-1

tR Residence Time min / htT Total elapsed time (since the beginning of photocatalytic

process)min

UVG Global ultraviolet solar irradiation WUV m-2

UVG,n Average incident radiation on the collector surface for each ∆tinterval (∆tn = tn – tn-1)

WUV m-2

VR Illuminated reactor volume LVT Total volume of hydraulic circuit LVTOT Total solar detoxification plant volume L

9.1 FEASIBILITY STUDYSolar Detoxification projects, just as any other engineering project, must follow the logicalsequence of pre-design, preliminary design and final design with the final objective of

174


designing and building facilities of feasible construction and satisfactory operation, for whichcontractors can confidently make bids. Without competent design, implementation andoperation, no engineering project can succeed.

The pre-design phase must define the scientific and engineering data required before the finaldesign may proceed. In Solar Detoxification projects the pre-design phase might be moreproperly called a Feasibility Study, as any new potential application of solar detoxificationmust be tested before going ahead with the design and implementation of the treatment plant.When complex mixtures of hazardous contaminants are present, degradation pathways andreaction kinetics could be very different even from wastewater of a similar original. Thismeans that the design of a specific solar detoxification system is a complex procedurerequiring a prior feasibility study following the method defined below:

- Identify targeted recalcitrant hazardous compounds.- Identify any pre-treatment that could enhance photocatalytic efficiency.- Identify the best photocatalytic process (TiO2 / TiO2 + persulfate / TiO2 + other oxidant /

Photo-Fenton).- Determine optimum process parameters (catalyst concentration, additional oxygen, etc).- Assess toxicity and biodegradability to determine the best moment to stop the

photocatalytic process.- Identify possible post-treatment processes.- Determine treatment factors.

All this can be done in the laboratory using indoor reactors with simulated solar radiation, asdescribed in Section 3.1. Nevertheless, it is always much better to carry out the feasibilitystudy using experimental solar devices, as the results obtained will necessarily be morereliable when extrapolated to the engineering level. Section 3.2 describes several experimentalsolar systems.

Figure 9.1 belongs hereExperimental mobile pilot plant for solar water detoxification. CIEMAT (Spain), 1996.

9.1.1 Identification of target recalcitrant hazardous compoundsThe first step must be a complete analysis of the wastewater to be treated in order to identifythe target chemicals and their initial and desired final concentrations. This is very important toavoid inadequate data in the following phases of the project. Also, depending on the specificapplication, the nature of the contaminants and/or their concentration may change before thesystem is actually erected, as the time between the study and its implementation could takemonths or even years. As some differences normally arise when laboratory experiments arecompared to engineering scale pilot plant tests, at a later stage of the project, it is important toknow whether the differences originated in reactor and system configurations or fromvariations in the wastewater.

9.1.2 Identification of possible pre-treatmentsThe objective of pre-treatment, if necessary, is to condition contaminated waters for organicdestruction. This can be achieved by addressing any constituents of the contaminated waterthat interfere with the process. The presence of initial impurities in the wastewater to betreated could produce a significant reduction in process efficiency because of:- Unexpected chemical reactions due to the presence of specific compounds.- Incomplete destruction of one or several components.

175


- Presence of substances that inactivate the catalyst.- Presence of substances that reduce the oxidation kinetics.- Presence of substances that block UV solar radiation.

These possible impurities, mainly inorganic ions, could even lead to the no viability of thedegradation process and the only way to avoid it is by designing some form of pre-treatment,to adjust the chemistry of the water to conditions suitable for the detoxification process. Itshould be noticed that such treatment increases both capital and system operating costs andmay also affect the control strategy of the treatment plant.

Examples of pre-treatment are the following:- Filtering, in the case of turbidity or cloudy water that could block incoming solar UV-

irradiation.- Acidification, to eliminate substances such as bicarbonate.- Adjustment of pH to reduce the concentration of total dissolved solids by precipitation.- Addition of hydrogen peroxide to precipitate substances such as dissolved iron ions,

which react forming a ferric oxide precipitate, which may then be removed by filtering.- Cation bed exchange, in the case of impurities such as calcium ions that would adversely

affect the process.

In the end, the specific impurities for each case must be identified and, depending on theireffect on the overall photocatalytic process, design or not a particular pre-treatment.

Other pre-treatments are independent of the presence of impurities. This is the case of oxygen,hydrogen peroxide, persulfate or any other oxidant injected into the process stream before thestart of the photocatalytic process to act as whole receptors on the titanium dioxide catalyst. Inmany cases reduction of initial pH to between 5 and 6 is also recommended to enhance theinitial degradation rate.

9.1.3 Identification of most adequate photocatalytic processAs already indicated with regard to solar photocatalysis, when complex mixtures of organiccontaminants are present, no general indication can be given at all and each case is completelydifferent from every other. It is therefore recommended that different photocatalytic processesbe tested in the beginning to identify the one that is the most adequate. Until bothheterogeneous TiO2 photocatalysis and homogeneous Photo-Fenton processes have beenextensively analysed, no advantage can be previously attributed to any of them for anyparticular application.

A good example is shown in Figure 9.2, where the Photo-Fenton photocatalytic process isseen to be highly efficient in degrading about 6000 mg L-1 of initial TOC, compared to therelative inefficiency of TiO2 heterogeneous photocatalysis. The reason for this is the wastewater, taken from a cataphoretic painting process at an automobile assembly plant, whichcame from an ultrafiltration process, leaving no inorganic impurities at all present, and thehomogeneous process works quite well. By contrast, heterogeneous degradation does notwork due to the high presence of organic matter.

Figure 9.2 belongs hereTreatment wastewater from painting section at car assembly factory(ultrafiltration from cataphoresis process). CIEMAT (Spain), 1999

176


In other cases, the Photo-Fenton Fe cycle may be affected by the presence of impurities thatproduce just the opposite result. In the case of Photo-Fenton, pH must also be around 2.5 toavoid the formation of non-soluble iron hydroxides. It is thus clear that this first research stepis absolutely necessary in order to decide on the most appropriate process for further in depthstudy.

9.1.4 Determination of the optimum process parametersThis phase seeks the definition of the essential scientific and engineering data required beforeproceeding with system design. This can be done by defining, implementing and conductingan experimental plant to determine the following parameters:- catalyst load,- pH required,- oxygen- additional oxidant load, if any,- contaminant concentration, if this can be varied

and assessment of the photocatalytic degradation process over time:- Total Organic Carbon of the waste water,- individual concentration of key hazardous compounds identified during the first step

(Section 9.1.1),- toxicity,- Average Oxidation State

Figure 9.3 Catalyst mixing system for the LLNL water treatment system. Courtesy ofNational Renewable Energy Laboratory (USA)

It might also be very advisable to individually test the photocatalytic degradation of the keyrecalcitrant contaminants identified in order to check whether there are significant differencesfrom those obtained with the real wastewater problem. Reaction rates are normally higherwhen individual compounds are tested, but if there are significant differences, it might beworthwhile repeating the pre-treatment process again to try and identify possible additionalpre-treatments, which could speed up the detoxification process. Obviously, potential gains inoverall reaction rate must always be weighed against the cost of adding additional processesand reagents.

The disappearance of hazardous contaminants normally must be parallel to the toxicityreduction and the biodegradability increasing. These facts and the evolution of the AverageOxidation State would determine the optimum point to stop the oxidative process and transferthe wastewater to a conventional biological treatment plant (see section 8.1).

177


9.1.5 Post-treatment process identificationPost-treatment processes adjust the water chemistry to conditions suitable for discharge. Inthe case of industrial wastewater for later biological treatment, the post-treatment may only bereduced to:- pH adjustment and- catalyst separation/precipitation and recovery

Depending on the nature of the waste water treated and its later use, and whether solardetoxification is used for water purification, such as in ground water decontamination, thepost-treatment system could then require additional processes, such as:- carbon dioxide removal,- elimination of possible residual hydrogen peroxide (if used in the photocatalytic process),- cation-exchange bed to remove specific existing inorganic compounds,- GAC filters to remove trace quantities of organic materials.

And any other possible treatment process necessary to meet the specific local regulatoryrequirements before discharging water.

Another possible factor that could require post-treatment is the water temperature.Temperature has a negligible influence on the photocatalytic process and the use of non-concentrating solar technology considerably reduces the increase in water temperature duringthe treatment process. However, the usual recirculation or batch system design may cause thewater to become too hot for the process in summer, volatilising compounds with low boilingpoints, or for the system, because, e.g., a cation exchanger at the inlet of a post-treatmentsystem limits the inlet temperature. If so, a water-to-air heat exchanger must be installed at the inlet of the post-treatment system, or inserted in thewater circuit in the solar collector field.

9.1.6 Determination of treatment factorsAfter all the above mentioned factors have been analysed and the steps indicated have beenfollowed systematically, a feasibility study to calculate the treatment factors for the bestsystem configuration proposed may be carried out. These treatment factors [Mass TreatmentFactor (Tfm) and Volumetric Treatment Factor (Tfv)], were already defined in Section 8.1(equations 8.1 and 8.2) and must be associated with the average solar UV irradiation recordedduring the tests performed. These values will permit the necessary solar field to be sized.

All the information assembled during the feasibility study must be documented in a report,which may also include any additional relevant data collected and recommendations for futureactivities.

9.2 FEASIBILITY STUDY EXAMPLEOne example of feasibility study could be the treatment of pesticide contaminated water fromthe recycling of agrochemical plastic containers.

9.2.1 BackgroundIn the Mediterranean, intensive agriculture in greenhouses has become a very important partof the economy, especially during recent years. In southeastern Spain, the province ofAlmería alone has more than 40,000 hectares of such greenhouses, and this is now the mostimportant economic activity in the area. However, important environmental problems

178


associated with this activity have also arisen, such as the extensive and intensive use ofpesticides (requirements are about 200 times greater than for conventional agriculturalmethods).

One particular problem is caused by the enormous number of empty pesticide bottles disposedof each year. According to 1995 data, 5,200 tons of pesticides were consumed in the region,producing around 1.5 million empty bottles and containers, 99% of that are plastic, and havingan average volume of 1.9 litres. A small amount of pesticide residue always remains in the usedbottles and, therefore, they are hazardous waste, which cannot be handled in the same way asconventional garbage. To date, there has been no way to dispose of these bottles and recoverthem for reuse, and most of them are thrown away with the rest of the regular agriculture wasteor just dumped anywhere. In order to solve this environmental problem, a process was designedto recycle the high-quality plastic in these bottles into a valuable raw material. The recyclingprocess shreds the plastic, which is then washed, leaving a relatively small amount of watercontaminated by a few hundred mg L-1 total organic carbon content of persistent toxiccompounds. As the water must be reused, those contaminants have to be treated and eliminated.Solar Photocatalytic Detoxification was proposed to treat the wastewater and a feasibilitystudy was carried out.

Figure 9.4 belongs hereTons of pesticides used in Almería region (1995 data). Source: AEPLA (Spain)

The experimental research was performed at the PSA Solar Detoxification Facility. Definition ofthe work necessary started with research (Fig. 9.4) on the pesticide market in Almería thatprovided the qualitative and quantitative distribution of pesticides consumed in the region. Fromthese, 10 pesticides, with all the main chemical families present, were selected as representativefrom among those most used by the greenhouses, to carry out a complete, previously defined,solar degradation test program. Table 9.1 shows the list of pesticides used.

Table 9.1 Selected pesticides for the feasibility study assessment. CIEMAT (Spain), 1996

9.2.2 Experimentation. TiO2-Persulphate testsThe Test Program included individual and combinations of tests of selected compounds, underdifferent conditions. In order to simplify the study, and the number of tests to be carried out, thefollowing hypothesis were assumed:

Commercial name Producer ActiveIngredient

Formula

Rufast Rhône-Poulenc Acrinatrin C26H21F6NO5

Vertimec Merck Abamectin C48H72O14

Thiodan AgrEvo Endosulfan α-β C9H6Cl6O3SDicorzol AgrEvo Formetanate C11H16ClN3O2

Confidor Bayer Imidacloprid C9H10ClN5O2

Match Ciba-Geigy Lufenuron C17H8Cl2F8N2O3

Tamaron 50 Bayer Methamidofos C2H8NO2PSVydate DuPont Oxamyl C7H13N3O3SScala AgrEvo Pyrimethanil C12H13N3

Previcur AgrEvo Propamocarb C9H20N2O2

179

Nematocides (2314)

Molluskicides (23)

Insecticides (827)

Acaricides (52)

Fungicides (1,115) Others (55)

Phytoregulators (698)

Herbicides (134)

Fig. 9.4

180


- although pesticides, which are sprayed in concentrations between 200 ppm and 3000 ppmdepending on the product, are diluted in water before their application, pesticides in waterfrom the washing process are always more diluted than in normal use.

- the 10 pesticides selected are considered representative of the entire market (more than 300).- the same number of empty plastic containers are generated by each of the 10 selected

products.- the same amount of residue remains in all the containers.

Under these hypotheses, a mixture of the ten selected pesticides, each at the same concentration,with a total TOC of 100 mg L-1, was considered representative of the water to be found afterwashing, very similar what may be expected and was used in solar degradation experiments inboth Helioman (Fig. 7.5) and CPC (Fig. 7.6) systems, were performed.

Figures 9.5(a) and (b) belongs here(a) Solar mineralization of TOC from the insecticide abamectin; test performed in a parabolic

trough system; average direct UV light: 38.1 watts m-2. (b) Samples of photocatalyticdegradation in CPC system; average global UV solar radiation were: 26.2 watts m-2

(acrinatrin test); 33.6 watts m-2 (methamidophos) and 33.7 watts m-2 (lufenuron test). TiO2

(Degussa P25):200 mg L-1. CIEMAT (Spain), 1996

Figure 9.5 (a) shows mineralization of total organic carbon from the insecticide abamectin inthe parabolic trough system. The TOC is observed to practically disappear after one hour ofexposition to sunlight (good weather conditions). Photocatalytic degradation tests performedin a CPC system show a very similar pattern. Figure 9.5 (b) shows degradation of TOC fromthe insecticides, acrinathrin, methamidophos and lufenuron (three different experiments onsunny days). Each one (listed in Table 9.1) was individually tested and similar degradationwas observed: degradation of about 100 ppm of TOC in a residence time between 1 and 2hours, with good solar irradiation. Residence times (tR) were calculated as:

TT

RR t

V

Vt = (9.1)

where tT is the total time elapsed since the beginning of the experiment, VR the (illuminated)reactor volume and VT the total volume of the hydraulic loop.

However, the experiments of most practical interest are those in which the mixture of all 10 ofthe selected compounds was used. These were performed in the CPC system as well as inparabolic troughs assuming the four hypotheses considered. For these experiments 10 ppm ofeach pesticide were added to 250 L of distilled water, and homogenised for 30 minutes.Samples were taken periodically and the total organic carbon (TOC) in the suspension wasanalysed and pesticide concentration monitored by liquid chromatography. Figure 9.6 showssome of the results obtained, which included toxicity measurements throughout theexperiment. As the complete mineralization of all organics present in the water was notintended, it should be observed that no highly toxic compound is generated. Microtox(widely accepted toxicity measurement system), is usually expressed in terms of the EC50,which is the effective concentration causing a 50% reduction in light from a luminescentmarine bacterium (photobacterium phosporeum), indicating that 50% of the bacteria presenthave been killed.

181

Fig. 9.5 (a)

182

Fig. 9.5 (b)

183


Figure 9.6 belongs hereTOC mineralization of a mixture of 10 selected pesticides (parabolic troughs); direct UV

light: 36.3 watts/m2; TiO2 (Degussa P25) concentration: 200 mg L-1; persulfate addition: 0.01molar; treated volume: 250 l. EC50 toxicity measured by Microtox. CIEMAT (Spain), 1996

In the case displayed in the figure 9.6, a TOC reduction of 90% is followed by an importanttoxicity reduction (to obtain the EC50, a 40% of concentration is just needed from initialsample, while for the final one the needed concentration is 135%).

Figure 9.7 belongs hereTOC mineralization of a mixture of 10 selected pesticides (CPCs). TiO2 (Degussa P25)

concentration: 200 mg L-1, slurry. CIEMAT (Spain), 1998

All titanium dioxide tests were conducted in 200-mg L-1 slurry concentrations, with 5 to 10mM persulfate, which has been found to be the optimum concentration in PSA experiments.Na2S2O8 was added at the beginning of the tests and at regular intervals to assure continuouspresence throughout tests by measuring the S2O8

2- consumed. This addition of persulfate doesnot imply any environmental problem since it only produces small concentrations of sulphateswhich increase the salinity of the water treated (maximum sulphates permissible in drinkingwater is 250 mg/L; there is no limit for waste water). In the experiments indicated in Figure 9.7,80% of the TOC was removed in from 2 to 4 hours and 90% in 2,5 to 5 hours.

9.2.3 Photo-Fenton testsPhoto-Fenton experiments were conducted to determine the effect of the initial TOC and ironconcentration on the degradation rate. Initial concentrations were 100 to 500 ppm of TOC andFe of 0,25 mM to 2 mM. After homogenisation during 30 minutes, the pH was adjusted to 2.5or 2.8 by addition of concentrated sulphuric acid. Ferrous sulphate was added immediatelyafterwards and H2O2 was added in portions of 10 to 20 percent of the stoichiometry (fromprevious COD measurements) until the experiment was completed. More iron increases thedegradation until a maximum rate is reached. This occurred when iron compounds absorbedall the irradiated photons. Figure 9.8 shows the degradation curves for different experimentswith approximately 100 ppm of pesticides.

Figure 9.8 belongs herePesticide degradation by Photo-Fenton process. Comparison of different iron concentrations

for 100 ppm of pesticides in wastewater. CIEMAT (Spain), 1998

An increase in the concentration of iron did not improve the degradation rate as much as hadbeen expected from previous laboratory experiments. The poor performance of the 0.5-mMiron test may have been caused by the high initial TOC and a slight deviation in pHadjustment. Nevertheless, 80% of the initial TOC was removed in less than 3 hours. With anexcess of H2O2, degradation can be improved up to 90%.

One of the difficulties in both the Photo-Fenton and TiO2-Persulfate experiments giving riseto possible error, was measurement of the initial amount of pesticide, since the highly viscousliquids strongly adhered to the glass-measuring cylinder. Furthermore, the amount of distilledwater to be added could not be calculated accurately. Both of these factors caused fluctuationin the concentration of total TOC. Since not all the pesticide could be dissolved at once in thedistilled water, TOC increased up to 60 minutes reaction time.

184

Fig. 9.6

185


9.2.4 Conclusions and Treatment FactorsTotal volume treated was the same for all the experiments (250 L), and the CPC collectorfield (2 x 60° semi-aperture angle, 1 sun concentration and total irradiated volume of 110 L)was formed by 3 stationary modules with an overall aperture area of 9 m2. Collectors wereorientated to the south and tilted 37º, which is similar to the local latitude, to obtain themaximum yearly efficiency. With these data and the results in Figures 9.7 and 9.8, the MassTreatment Factor, Tfm (eq. 8.1), and Volumetric Treatment Factor, Tfv (eq. 8.2) for both theTiO2-persulfate and Photo-Fenton processes can be obtained. Table 9.2 showed the obtainedresults.

TiO2-Persulfate(90% degradation)

Photo-Fenton(80% degradation)

Tfm Tfv Tfm Tfv

Test 1 0.81 6.44 2.28 18.52Test 2 0.59 4.69 3.88 31.45Test 3 0.86 7.41 3.11 26.88Test 4 0.89 7.65 2.43 22.83Test 5 0.60 6.06 3.07 28.74Test 6 1.18 13.89 -- --Test 7 0.93 11.26 -- --Average 0.84 8.20 2.95 25.68

Table 9.2 Degradation of pesticide mixture. Mass Treatment Factor (Tfm) and VolumetricTreatment Factor (Tfv) obtained for TiO2-persulfate and Photo-Fenton processes. CIEMAT

(Spain), 1998

Treatment factors are observed to be significantly higher with Photo-Fenton, and the last 10%of degradation cannot be achieved with this process. This fact is an important issue in plantdesign philosophy at the preliminary design stage.

9.3 PRELIMINARY DESIGNThe goal of the preliminary design is to develop specific parameters for the sitting, layout,and size of the solar detoxification facility, based on the prior feasibility study. Thepreliminary design is complete when provides all necessary basic elements for the finaldesign and construction and usually this is often considered equivalent to 30 to 40 percent ofthe complete final design. No additional data should have to be developed after thecompletion of the preliminary design because this should include sufficient engineeringdetails to proceed rapidly to the final design. This step typically formulates a schematicprocess diagram and a site layout plan.

The final figures for volume of water to be treated, hazardous contaminant and itsconcentration, necessary collector surface, pre and post-treatment processes, etc, must beassessed at this point. A typical list of the data to required for the preliminary design might bethe following:

− Amounts and characteristics of waste water.− Photocatalytic process.− Collector field surface and residence time.− Throughput capacity.− Flow rates (maximum, minimum and mean).

186


− Loading rates.− Cleanup targets.− Discharge rates.− Chemicals and dosages.− Horsepower.− Utility requirements.− Site security measures.− Spill-containment and leak-detection provisions.− Construction materials.− Instrumentation and control.− Monitoring and alarms.− Operating requirements.− Process diagram.− Plant layout.

These data are basically no different from any other hazardous-waste remediation engineeringprocess. The main difference here is the need to calculate the size of the collector field.Determination of treatment factors gives a good idea of how the photocatalytic process workswith a specific waste water and there are valid figures for a feasibility study estimation, butthis cannot be used as the reference parameter for calculating the collector field area becausethe solar radiation, which obviously is an essential parameter, is not included in thecalculation of the treatment factors, as Table 9.2 (obtained from Figures 9.7 and 9.8 in theprevious example) shows.

Treatment factors, Tfm and Tfv, are based on the use of residence time, tR , or the time the waterhas been exposed to radiation, as the unit of calculation for analysis, and this could lead toerroneous conclusions when there are important differences in the radiation incident in thereactor due to clouds or time of day. One way to avoid this problem is to use a relationshipbetween experimental time, plant volume, collector surface and the radiant power density,UVG, as measured by an UV solar radiometer. The amount of energy collected by the reactor(per unit of volume) from the start of the experiment until each sample is collected may thenbe found by equation 9.2:

t-t=t;V

SUVt+E=E 1nnn

TOT

CPCnG,n-1nUV,nUV, − (9.2)

Where:

UVG,n is the average UV radiation incident on the collector surface for each ∆t interval;tn is the experimental time for each sample taken to monitor the degradation process;SCPC is the solar collector surface (CPC or any other type of collector);VTOT is the total plant volume; EUV,n is the accumulated energy (per unit of volume, kJ L-1) incident on the reactor for eachsample taken during the degradation process.

Equation 9.2 permits any specific photocatalytic degradation experiment to refer to the usefulenergy available to the process instead of the residence time, making direct comparisonbetween different experiments performed on different days with different weather conditionspossible, as well as calculation of the size of the solar field specific to the solar conditions atthe site. It must be remembered that only solar irradiation up to 390 nm is useful for the TiO2

process, but for Photo-Fenton, it is useful up to 580 nm.

187


Equation 9.3, for calculation of the total solar collector field area, can be obtained fromequation 9.2. HS is the yearly total of hours of solar detoxification system operation. 20 to 25percent larger collector area (than the theoretical figure) is always recommended.

GS

TOTUV

UVH

VES = (9.3)

The feasibility study and preliminary design are basic components of the final design, as theymust supply all the data essential for the engineers and scientists to proceed confidently withthe final design. In addition, these steps must demonstrate that the final solar detoxificationplant will accomplish the treatment objectives before significant expenditures are made indetailed design or implementation.

9.4 PRELIMINARY DESIGN EXAMPLEAs with the example of treatment of water contaminated by pesticides from the recycling ofagrochemical plastic containers (Section 9.2) the main item in the preliminary design is thecalculation of the collector field surface necessary, for which the following data have eitherbeen previously collected or assumed:− The yearly amount of empty pesticide bottles generated throughout the province of Almería

is 1.5 million.− The plant is to be designed to treat an initial 50% of this yearly total of bottles (realistic

taking into account their geographical distribution): 750,000 plastic bottles.− The CPC collector technology has been selected for the plant.

And the following hypotheses have previously been formulated:− Average residual TOC in an empty pesticide bottle (from prior analysis) is about 0.7 g; if the

bottle has previously been rinsed, the quantity is 0.1 g.− 0.5 g average residue content per bottle is used as a conservative estimate. This would mean

a total pesticide weight of 375 kg (750000 x 0.5 mg) to be treated yearly by the solardetoxification facility.

− Average volume of plastic bottles is 1.9 L.− Yearly total of operating hours in Almería for a solar detoxification facility is about 3000.

With equation 9.2 (ACPC = 9 m2; VTOT = 250 L), Figure 9.7 can be transformed into Figure9.9, where TOC degradation paths are plotted against the useful UV energy collected, insteadof the residence time (see Section 9.3).

Figure 9.9 belongs hereTOC degradation of a mixture of 10 selected pesticides by TiO2 (Degussa P25)- persulfateprocess. Degradation in function of UV collected energy (300-400 nm). CIEMAT (Spain),

1998

With the same procedure and the data from Figure 9.8, Figure 9.10 is obtained. In order tomake direct comparison of the Photo-Fenton and TiO2 processes possible, the amount of UVlight collected in Figure 9.10 was calculated the same way as for TiO2 (using data from thesame radiation sensor having a measuring range of 300-400 nm), even though ironcompounds absorb light up to wavelengths of 580 nm. From PSA measurements (Licor-1800Spectroradiometer), an average of 7.11 times more solar energy up to 580 nm is availablethan from the solar spectrum up to 390 nm.

188


Figure 9.10 belongs hereTOC degradation of a mixture of 10 selected pesticides by Photo-Fenton process and with

different iron concentrations. Degradation in function of UV collected energy (300-400 nm).CIEMAT (Spain), 1998

From the different various tests carried out, the best conditions found for the titanium dioxideprocess are a TiO2 catalyst concentration of 200 mg L-1 with 10 mM of persulfate addition. In thecase of the Photo-Fenton process, iron concentration is 1 mM. When Figures 9.9 and 9.10 arecompared, it seems clear that the Photo-Fenton process, although unable to achieve fullmineralization of the contaminants in the water, in this case, is more energy-efficient. The TiO2-persulfate process requires 27 kJUV L-1 for 80 percent of TOC degradation or 30 kJ L-1 for the 90percent mineralization. The Photo-Fenton process needs only the equivalent of 11.5 kJUV L-1 for80 percent disappearance of TOC.

The most appropriate conceptual design is, therefore, the one shown in Figure 9.11, where thewater contaminated with the pesticide is treated in a batch process until 80 percent mineralizationof TOC is achieved. At this point, water is transferred to the post-treatment process (ironprecipitation, sedimentation and recuperation), and either reused for bottle washing or dischargedthrough an activated carbon filter to guaranty discharge quality. The water to be reused ispumped back to wash the shredded plastic until contamination reaches a TOC of 100 ppm. Inthis closed cycle, water may be reused about 5 to 10 times before final discharge. With thisdesign, about 95 percent of the contaminants are mineralized by solar photocatalysis and theremaining 5 percent would be removed with a GAC filter.

Figure 9.11 belongs hereConceptual design of solar detoxification plant for pesticide bottles treatment and recycling.

CIEMAT (Spain)

The size of the solar field can be calculated with the following design parameters:− Photo-Fenton is the photochemical degradation process selected.− The initial TOC of water entering the solar detoxification facility will be 100 mg/L, which

includes not only the active ingredient, but also the rest of the components in thecommercial formulation.

− A TOC of 100 ppm is considered equivalent to about 200 mg L-1 of contaminantconcentration (from the ratio of average carbon weight against the average molecularweight of the selected pesticides).

− The final TOC signifying water removal from the solar plant is 20 mg/L.− The plant is designed to treat 375 kg of pesticides from the plastic-bottle washing process

yearly.− The total volume of water to be treated yearly is 1,875 m3 (375,000 mg / 200 mg L-1).− 3,000 hours of operation yearly.− The average local global UV irradiation is 18.6 W m-2.− The average solar energy necessary to degrade the contaminants is 12 kJUV L-1 (from

Figure 9.10).

So, using equation 9.3, the collector field area will be:

189

Batch process

Pump

Filter

Mixer

Tank

Contaminatedwater

Oxygen(Air)

Sun

Solar UVlight

Chemical reactor(CPC solar

collector field)

Chemicaloxidant

Catalyst

Pre-treatment(pH adjustment,

filtering, etc)

TOC > 100 ppm

Post-treatment(catalyst recovering,pH adjustment, etc.)

TOC < 20 ppmReutilization ?GAC filter

YES

NO

Discharge(irrigation

water)

WASHING CYCLE

Pump

Industrialbottles

washingprocess

DETOXIFICATION CYCLE

Fig. 9.11

190

Recirculation

Pump

Filter

Mixer

Tank

Contaminatedwater

Oxygen(Air)

Sun

UV light

Chemical reactor(solar collectors field)

Chemicaloxidant

TiO2

WASHING CYCLE

Industrial bottles

washing process

Pump

SOLAR DETOXIFICATION CYCLE

Catalyst separation

Pretreatments (filtration, pH

adjustment, etc)

TOC < 10 ppm

TOC > 100 ppm

OR

191


2

2

133

m112mWs

LLJ

18.636003000

1018751011.5

UVH

VES

GS

TOTUV =

==

−

−

xxxxx

(9.4)

And the proposed size of the solar collector field (CPC) would be 140 m2 (including a 25percent margin). With this and the data above, the cost of the treatment facility may beestimated following the procedure indicated in chapter 8. Figure 9.12 shows a proposedlayout for the complete facility.

Figure 9.12 belongs hereLayout design of solar detoxification plant for pesticide bottles treatment and recycling.

CIEMAT (Spain)

Two important items in the layout in Figure 9.12 with regard to the pre and post-treatmentprocesses are sludge removal before treatment of the contaminated water in the solar field andone-step iron sedimentation by neutralisation (in batch-mode), after the photocatalytictreatment.

9.5 FINAL DESIGN AND PROJECT IMPLEMENTATION.The fundamentals of the final engineering design process are no different from any othertreatment facility project. The objective is to develop the documentation necessary forselected contractors and vendors to proceed with construction of the facility. This phase of theproject is usually associated with the construction of the treatment plant through a contractbetween the owner or responsible party and the engineering firm responsible for the project.Final design and construction are normally on a turnkey basis.

After the feasibility study and the preliminary design phase, the final design process might beaffected by different factors, such as:− Uncertainty of key assumptions.− Availability of design data.− Availability of competent contractors.− Project cost or size.− Possible restrictions imposed by local regulations.− Owner’s acceptance of risk, due to the implementation of a new technology based on

energy from the sun, input which obviously cannot be controlled.

The last factor, project risk, must not be dismissed, as it could be one of the key factors inimplementing the solar detoxification technology. The possibility of clouds blocking sunlightfor a period of several days (or even weeks) could force a similar period of plant inactivity,which could well be incompatible with 24-hour-a-day operation. To avoid this risk, anadequate buffer system must be designed and, even so, there is always a certain risk for theowner.

One way to mitigate this problem, should it arise, might be to design a hybrid systemcombining a solar collector and electric UV lamp systems. Such a concept would benefit fromcontinuous operation, even during prolonged periods of bad weather, because the lamp systemcan be turned on when sunlight is insufficient. It can also increase the treatment rate to handleoccasional peak demand. However, it requires the additional installation of the UV-lampsystem and the energy to operate the lamps when sunlight is unavailable. Depending on the

192

SS

Solar CPC field (about300 m2 of collectors)

Office

Lab

Chemicaltreatment

unit

Micro-filtration

unit

15 m3

15 m3

StoreArea

Hot air dried &bag packed area

Washingarea

Shreddingarea

Inspectionarea

Lighterage quay(empty bottles)

Loading berth(final product)

Total plant required area:2000 to 3000 square meters

Fig. 6.12

193


local solar resources (see Section 8.3 for a comparison between solar and electric photoncosts), solar-electric hybrid systems may be more expensive than solar-only systems, but theycould reduce the above-mentioned risk of solar detoxification installations at specificlocations.

Another possible problem could be the uncertainty of important assumptions used during thefeasibility study or the preliminary design phase. This could be the case of the example ofpesticide treatment shown in Sections 9.2 and 9.4, where the real wastewater to be treatedmay contain more than 300 possible different compounds. As it is impossible to carry out afeasibility study with all these contaminants (it was performed with 10), a real uncertaintyexists with regard to the final design of the plant.

A possible solution for this problem is to approach the final design and construction of thefacility in a two-step process, designing and installing a pilot plant first, with a reducednumber of solar collectors to check and validate the figures obtained in the feasibility andpreliminary phases. After some tests have been performed in this pilot plant the necessaryarea of solar collectors is confirmed or modified and the plant is enlarged to its finaldimensions. Pilot-plant construction must be based on the confidence that all installedsystems are valid, with or without slight modifications to the plans.

When the final design is complete, a detailed project cost estimation must be prepared. This isusually based on normally accepted sources such as bids from other recently builtdetoxification plants, quotes from vendors and equipment suppliers, construction industry costestimation guides, etc.

All these possible alternatives, when appropriate, in detail, must be reflected in the contractbetween the engineering firm and the owner or responsible party. Typically, the main contractdocuments are the following:− Bidding documents− General and supplementary terms and conditions− Technical specifications− Contract drawings

The bidding documents describe the different items of the plant and are the basis for payment.These documents should include additional information such as the proposed schedule andmilestones, possible alternatives to the price, etc., and an agreement among the parties on thetotal price of the plant.

The general and supplementary terms and conditions must contain the standard and specificclauses of the construction contract. Issues such as responsibility sharing among the parties,payment procedures, settlement of disputes, insurance, guaranty, etc must also be included. Itis important to specify such issues as training of personnel, documentation to be provided,technology confidentiality, initial plant performance check period and any other servicesrequired by the contractor or engineering firm.

The technical specifications must contain the performance requirements and the criteria forfacility acceptance. Specific details of the equipment and materials to be used in the treatmentplant depending on the characteristics of the wastewater to be treated should also be included.

194


The contract drawings should identify the conditions of the plant location and the possiblerequirements and/or restrictions for construction. As appropriate, they may include plans,sections and general details relevant to plant location.

The complete design and construction schedule for a solar detoxification facility could bevary considerably, depending mainly on the requirements of the feasibility study and anypossible delays in obtaining the necessary permits. A sample schedule might be the following:− Feasibility study: 2 to 5 months.− Preliminary design: 1 to 2 months.− Licensing: variable, depending on local regulations.− Final design and contracts: 2 to 3 months.− Vendor selection. Equipment and solar collector procurement: 4 to 6 months.− Facility erection and installation: about 2 months.− Start up, checkout and training: 1 to 2 months.

Finally, the documentation necessary for proper operation and maintenance of the facilitymust be supplied.

9.6 EXAMPLE OF FINAL DESIGN AND PROJECT IMPLEMENTATIONThis section describes the design and construction of the solar detoxification plant within theproject entitled “Solar detoxification technology in the treatment of persistent non-biodegradablechlorinated water contaminants”. The project, partially financed by the DG-XII of EC, wasaimed at the treatment of wastewater containing C1 and C2 chlorinated hydrocarbons such asmethylene chloride, trichloroethylene, tetrachloroethylene, chloroform, methyl chloroform, etc.

For this project, a specific TiO2 catalyst was developed in the ENEL laboratories (Italy) bysynthesising titania powders with an innovative process in which a suitable reactant vapourinduced by a CO2 laser beam is pyrolysed. In the preliminary laboratory experiments, animpressive 10 percent photon efficiency was obtained with this catalyst, when selectedchlorinated solvents at concentrations close to solubility were treated (Calza, Minero andPelizzetti, 1997). Real contaminated water generally contains chlorinated solvents at similarconcentrations. Figure 9.13 shows the manufacturing process for this catalyst.

195


Figure 9.13 Laser device for TiO2 catalyst powders manufacturing. Courtesy of ENEL SpA(Italy), 1999

This 10-percent photon efficiency represents a significant improvement in catalyst efficiencyover Degussa P-25 titania, attributable to both the higher specific surface area of the laserpowders and to better crystallisation due to the thermal treatment.

The design solar-collector was a 100-m2 CPC, having a flexible modular structure adjustableto different angles of inclination (Figure 9.14) for easy on-site assembly and installation. Thecollectors were erected based on the following design data:− Acceptance angle: 90º− Truncation angle: 90º− Internal absorber radius: 14.6 mm− External absorber radius: 16.0 mm− Optical gap: 1.4 mm− Sunlight concentration ratio: 1.0

Figure 9.14 belongs hereModular collector structure to CPC easy assembly. Courtesy of AO SOL ENERGIAS

RENOVÁVEIS Lda. (Portugal), 1999

The CPC collectors were fabricated from a galvanised sheet frame containing 16 parallelhighly reflective anodised-aluminium CPC reflectors with 1.5-m-long reactor tubes. Eachtube had a connector at the ends to join it to the previous and following adjacent collectortube forming a complete module of collectors connected in a row. Figure 9.15 shows theCPC-trough manufacturing process. The reflector on which the glass reactor tube isassembled is later mounted in a box frame. The overall unit is installed on the supportingstructure on-site (see also Figure 9.19).

196

Fig. 9.14

197


Figures 9.15 (a), (b) and (c) Manufacturing of CPC shape reflector.Courtesy of AO SOL ENERGIAS RENOVÁVEIS Lda. (Portugal), 1999

The detoxification plant was installed at the facilities of a waste management and treatmentcompany in Madrid (Spain). Design batch-system treatment efficiency was 2 m3 of watercontaminated with non-biodegradable chlorinated solvents, in approximately two to threehours (depending on the solar irradiation). One of the first problems was the location withinthe factory for aesthetic reasons, because the solar facility had to be oriented to the south andbecause of the orientation of existing buildings. This is a common problem in any solarinstallation in existing buildings since they are normally built without considering theirorientation. One possible solution to this problem is the installation of the solar facility on theroof, however, in this case, it was not possible and the plant was installed on the ground asshown in the layout in Figure 9.16. Obviously, an important factor that must always beavoided is shadowing of the solar collector field by other buildings or constructions during theyear.

Figure 9.16 belongs hereSolar Detoxification plant layout. Courtesy of ECOSYSTEM S.A. (Spain), 1999

The main plant components designed were the civil engineering, i.e., foundations and pits,pumps, piping and fittings, hydraulic system, tanks, automation and control, electrical andmechanical installation and TiO2 recovery system. The main plant parameters are:− 2 Modules (21 collectors each) in parallel rows,− total collector aperture area: 100 m2,− total circuit volume: 800 L,− total plant volume: 2000 L,− catalyst configuration: slurry,− completely airtight with air injection (oxygen supply).

The plant consisted of two parallel rows of 21 collectors and 31 m length each. East-Westorientation was chosen with a small structural tilt (1%) in the same orientation as a way todry-out and to avoid the accumulation of rain water on the CPC troughs (Figure 9.17).

Figure 9.17 belongs hereSolar Detoxification plant design. Front and lateral view. Courtesy of ECOSYSTEM S.A.

(Spain), 1999

Civil engineering must consider the possibility of an accident (e.g., broken glass reactor) andspillage of hazardous water during treatment. For this possibility, a small sidewall and a sumpfor containment and collection of possible leaks, were designed and constructed (Figure 9.18).

198

Fig. 9.16

199

Fig. 9.17

200


Figures 9.18 (a) and (b) Solar Detoxification plant construction. Lateral wall and pit forpossible leaks containment and collection. Courtesy of HIDROCEN S.L. (Spain), 1999

Once the frame support has been prepared, the next step is the installation of the solarcollector (Figure 19). Collector inclination is equal to local latitude (40 degrees North) andthe distance between rows was calculated to minimize shadowing of collectors. To this end,the angle of sunlight at noon on December 21st (lowest maximum sun elevation) was used as adesign parameter to define row separation.

Figures 9.19 (a) and (b) Supporting structure and CPC units installation. Courtesy of AO SOLENERGIAS RENOVÁVEIS Lda. (Portugal), 1999

The final system design was modular with glass collector reactor tubes connected in series byHDPE quick-connections. Water flows simultaneously through all the parallel tubes and thereis no limit to the number of collector components in the modules. Pipes are made of PVC-Cand tanks are made of polyester-resin-reinforced glass. The hydraulic circuit was carefullydesigned to obtain the highest volumetric efficiency with minimum “dark zones”. Thedifferent sections of the pipes must be carefully calculated to guarantee similar flow rates inall the reactor tubes. Nominal flow was turbulent (Re between 10000 and 20000) to avoidcatalyst settlement. Water input and output manifolds at the ends of the modules connect thephotoreactor array to the main feed pipe (Figure 9.20). All materials in contact with the waterto be treated must be carefully selected according to the nature of the contaminants and therequired pH.

201


Figure 9.20 Photoreactor array input water manifold system. Source: CIEMAT (Spain), 1999

The Solar Detoxification facility was designed to be operated in batch mode. The water to betreated is initially stored in the 2-m3 storage tank, from which it flows into the buffer tank andsolar collector loop, completely filling them by force of gravity, and then recirculatescontinuously through the reactors until desired contaminant destruction is achieved. The TiO2

catalyst and chemical additives are prepared separately in small tanks and are fed into thetreatment loop in the equivalent of two recirculation cycles to guarantee completehomogenisation. The total treatment-loop volume is about 800 L, 600 L being continuouslyexposed to the solar radiation in the reactors. Once the desired destruction is obtained, thewater is transferred to the catalyst separation tank, the treatment circuit is filled again withnew wastewater to be treated (Figures 9.21) and the process is restarted.

Figures 9.21 (a), (b) and (c) (a) Installation of main tanks of Solar Detoxification facility:buffer tank (smaller in right place), storage tank (left) and catalyst separation tank (conic tank

on the back). (b) Installation of tank level sensors. (c) Installation of catalyst separationsystem. Source: CIEMAT (Spain), 1999

The plant was designed with full automatic control systems and minimum operation andmaintenance requirements. Achieved level of water treatment is indirectly measured bymeasuring sunlight availability. In this way, a solar UV-A sensor is incorporated within the

202


electronic control devices, with the function of solar UV integration from the beginning of thetreatment process (Figure 9.22). This sensor is connected to a Programmable Logic Controller(PLC) and, once the level of energy to fulfil the treatment has been achieved (previouslydetermined from preliminary test for plant design, according to each specific contaminatedwastewater to be treated), the PLC stops the main pump, transfer the water to the catalystseparation tank and advise the operator that the treatment has been completed.

Figures 9.22 (a) and (b) (a) Installation and testing of the UV-A sensor device (front left). (b)Testing the PLC and electronic equipment. Courtesy of ECOSYSTEM S.A. (Spain), 1999

The PLC also receives other data signals (flow-rate, tank levels, temperatures, etc) for systempump and valve control. Specifically developed software controls all normal operatingprocedures and sequences, so very little direct human intervention is needed. Orders areintroduced through a keyboard and a printer indicates alarms and main system events.

Figures 9.23 (a) and (b) Two views of the completed Solar Detoxification treatment plant.Source: CIEMAT (Spain), 1999

CHAPTER SUMMARYImplementation of any Solar Detoxification project must follow a three-step sequence:Feasibility Study, Preliminary Design and Final Engineering Design. The objective of theFeasibility Study is the assessment of the practicability of the solar photocatalytic technologyfor the treatment of the wastewater problem by preliminary testing. In addition, the feasibilitystudy must identify the recalcitrant hazardous compounds, possible pre- and post-treatment

203


processes, the most appropriate photocatalytic process, the optimum process parameters andthe treatment factors. The objective of the preliminary design is to develop specificparameters for the positioning, layout, and size of the solar detoxification facility from thefeasibility study previously performed. This phase should include sufficient engineeringdetails to proceed rapidly to the final design, including a schematic process diagram and a sitelayout. The final figures for the volume of water to be treated, presence of hazardouscontaminants and their concentration, necessary collector surface, pre and post-treatmentprocesses, etc, must also be defined at this preliminary design level. The objective of the finalengineering design is to develop the necessary documentation to proceed with theconstruction of the facility through selected contractors and vendors. This phase is usuallycarried out for construction of the treatment plant by contract between the owner orresponsible party and the engineering firm responsible of the project.

BIBLIOGRAPHY AND REFERENCES1. Bechtel Corporation. “Conceptual Design of a Photocatalytic Wastewater Treatment

Plant”. Sandia National Laboratory Report. SAND91-7005. 1991.2. Blanco, J. et. al. "Solar detoxification plant for a hazardous plastic bottle recycling plant in

El Ejido: feasability study". 8th Symp. on Solar Thermal Conc. SolarPACES. 1996.3. Blanco, J. et. al. “Compound Parabolic Concentrator Technology Development to

Commercial Solar Detoxification Applications”. International Solar Energy SocietySymposium. Jerusalem, Israel. 1999.

4. Calza P., Minero C. and Pelizzetti E. “Photocatalytic Transformations of ChlorinatedMethanes in the Presence of Electron and Hole Scavengers”. J. Chem. Soc. FaradayTrans., 93, 3765-3771. 1997.

5. Goswami, D.; “Engineering of Solar Photocatalytic Detoxification and DisinfectionProcesses”. Advances in Solar Energy. Vol. 10, pp. 165-209. (1995).

6. O’Brien & Gere Engineers, Inc. “Innovative Engineering Technologies for HazardousWaste Remediation”. International Thomson Publishing Inc. 1995.

7. Radian Corporation. “Conceptual Design Report for the Mobile Solar Detoxification Unit–Draft Report”. Solar Energy Research Institute. Denver, CO, 1991.



1. The reason for identifying possible pre-treatment processes is to make possible or enhancephotocatalytic wastewater degradation.

2. It may be possible to avoid pre and post-treatment processes, depending on the specificconditions of the photocatalytic wastewater degradation process.

3. When TiO2-persulfate degradation is employed, specific post-treatment is required toremove the excess sulphates in the discharge water.

4. Once the preliminary design is completed, no additional data should initially be requiredto proceed with the detailed engineering development (final design).

5. The technical and economic feasibility of the solar detoxification process must bedemonstrated by the feasibility study and the preliminary design, respectively.

6. As Photo-Fenton uses more photons than the TiO2 process (photons up to 580 nmcompared to 390 nm of the solar spectrum, respectively), it is always the best option.

7. Final project design is usually associated with the construction of the treatment plant, butnot necessarily.

8. A technology confidentiality agreement among the parties should be included, ifappropriate, in the general conditions of the contract.

204


9. Any specific documentation to be provided with the treatment facility, in addition to theoperation and maintenance manuals, should be defined in the Technical Specifications ofthe contract.

10. When wastewater is treated by batch processing, the flow rate must be as low as possibleto save pump energy.

PART B.

1. Is the implementation of a specific pre-treatment process always recommended to increasethe efficiency of the photocatalytic treatment?

2. In which cases could water temperature play a significant role in the solar detoxificationprocess?

3. Why shouldn’t treatment factors be used to calculate the solar collector area for a specificcase of wastewater treatment?

4. An experimental solar detoxification system has 375 L of total volume and 250 L ofreactor volume (irradiated volume). What factor is used to transform the experiment timeto residence time?

5. Why is an increase in TOC concentration observed in many photocatalytic degradationexperiments?

6. One experimental system, made up of 25 m2 of solar collectors, can degrade 75% of theTOC in 150 L of a specific wastewater in 150 minutes, with an average solar UVirradiation of 33 W/m2. Another experimental system with 15 m2 of solar collectors canalso degrade 75% of the TOC in 100 L of the same wastewater in 120 minutes with anaverage solar UV irradiation of 38 W/m2. Which of the two systems is more efficient?

7. The following results were obtained for a specific waste water in several photocatalytictests with TiO2-persulfate:

Test Treatment factor (L h-1 m-2) Average solar UV irradiation (W m-2)1 11 28.52 13 33.43 10 31.84 9 26.95 12 33.2

Using these data, estimate the size of a solar detoxification plant necessary to treat 10,000m3 of the same wastewater yearly operating 3000 h per year at a location with 24.5WUV/m2 average solar irradiation.

8. Why is it always advisable to check the feasibility and preliminary design data beforebeginning the final project design?

9. How can the risk of long cloudy periods (which could reduce the possibilities ofinstallation of a solar detoxification plant) be mitigated?

10. What are the main issues to be considered when designing the hydraulic loop for a solardetoxification facility?

ANSWERS

Part A

1. True; 2. False; 3. False; 4. True; 5. True; 6. False; 7. True; 8. True; 9. False; 10. False.

205


Part B

1. Not always, because potential gains in overall reaction rate must always be weighedagainst additional associated costs.

2. Because when hazardous compounds with low boiling points, such as volatile organiccompounds (VOCs), are to be treated, possible transfer of the contaminants from liquid togas phase must be taken into account.

3. Because the solar radiation is not considered. Treatment factors are a good tool forcomparing degradation performance in experimental systems, but as local available solarirradiation is an essential design factor, they cannot be used to calculate the required solarfield.

4. Using equation 9.1, the factor is 0.746:

TTR t0.746t375

280t ==

5. The initial presence of non-dissolved compounds in the wastewater.6. The problem can be solved by using equation 9.3, modified as follows:

TOT

GUV V

UVTSE = (9.4)

( ) 11

−== LJk49.5150

10336015025E

-3

UV

xx

( ) 12

−== LJk41.04100

10386012015E

-3

UV

xx

The second system may be considered more efficient than the first one, as it needs lessspecific energy to achieve the same degradation.

7. From the five tests performed, the average treatment factor is 11 L h-1 m-2, and the averagesolar irradiation, 30.76 W/m2. As there is a linear relationship with solar irradiation, if theyearly average is 24.5, the equivalent treatment factor is:

2x −= mhL8.7630.76

24.511 -1

And the estimated solar field would be:

22-1

3

m380h3000mhL8.76

L1010000 =−

The final estimate, with a 25% increase, would be 475 m2.8. Because final design and project implementation due not normally follow immediately

after the feasibility study and preliminary design phases, but some time later.Furthermore, sometimes-important specific assumptions and design data must bevalidated in a pilot plant previous to the final plant design.

206


9. A first approach is the design of an adequate buffer system. A second and more radicalapproach is the design of hybrid systems, combining solar collectors and electric UVlamps.

10. To guarantee the same flow rate in all the reactor tubes, to minimise the “dark volume”(volume not exposed to solar radiation) and to avoid catalyst settlement when aheterogeneous photocatalytic process is implemented.

207


10 INTERNATIONAL COLLABORATION

AIMSThis unit describes some of the programmes and initiatives promoting national orinternational collaboration in research, development and implementation of innovativesustainable technologies, that include Solar Detoxification projects within their scope.

OBJECTIVESAt the end of this unit, you will appreciate the scale, nature and content of these programmes,and some of the national and regional initiatives in progress around the world. You willacquire basic information concerning the mechanisms and possibilities for proposing andestablishing international collaboration in the field of solar detoxification and you will alsoknow how to receive updated information on those possibilities.

NOTATION AND UNITSSymbol Description UnitsCIEMAT Centro de Investigaciones Energéticas, Medioambientales y

Tecnológicas (Spain)DG-XII Directorate General XII (of European Commission): Science,

Research and DevelopmentDG-XVII Directorate General XVII (of European Commission): EnergyDOE Department of Energy (USA)DOD Department of Defense (USA)EC European CommissionEPA Environmental Protection Agency (USA)EU European UnionExCo Executive Committee (SolarPACES)FP5 Fifth Framework Programme (EU)IEA International Energy AgencyNREL National Renewable Energy Laboratory (USA)OEDC Organization for Economic Co-operation and DevelopmentPCO Photocatalytic OxidationPSA Plataforma Solar de AlmeríaPSI Paul Scherrer Institute (Switzerland)SANDIA Sandia National Laboratories (USA)SolarPACES Solar Power And Chemical Energy SystemsRTD Research and technological developmentsSERI Solar Energy Research Institute (USA)SMEs Small and medium-sized enterprisesSNL Sandia National Laboratories (USA)SSPS Small Solar Power SystemsWHO World Health Organisation

10.1 INTERNATIONAL ENERGY AGENCY: THE SolarPACES PROGRAMThe IEA (International Energy Agency), founded in 1974, is the energy forum forindustrialised countries. Based in Paris, the IEA is an autonomous agency within theframework of the Organisation for Economic Co-operation and Development (OEDC). Animportant function of the IEA is the promotion of enhanced international collaboration onenergy research and the development and application of new and efficient energytechnologies. The IEA has set up more than 60 “Implementing Agreements” linking member

208


Countries in R&D, technology demonstration and information initiatives. One of theseImplementing Agreements is called SolarPACES (Solar Power and Chemical EnergySystems).

SolarPACES is one of the international co-operative programs managed under the umbrella ofthe IEA to help find solutions to worldwide energy and environmental problems, bringingtogether teams of national experts from around the world to focus on the development andmarketing of systems based on solar technologies.

The SolarPACES program was initiated in 1977 under its former name of SSPS (Small SolarPower Systems). Two dissimilar solar facilities were designed in the project’s Stage 1 by tenContracting Parties from Austria, Belgium, Germany, Greece, Italy, Spain, United Kingdomand the United States. All the countries, with the exception of the UK, continued the projectthrough Stage 2 (Building, testing and Evaluation) which was completed in Almería, insouthern Spain, at the end of 1984. In the course of the subsequent Stage 3, eight countries(all but Greece) proceeded with solar-related research and development in various forms,especially in advanced solar thermal and solar chemical applications. The two SSPS facilitieswere transformed into what has since become the world’s most versatile solar test centre, thePlataforma Solar de Almería (PSA), which continues to serve as the site of multiple co-operative international testing and development efforts.

In 1991, Germany, Spain, Switzerland and the USA decided to go on to a Stage 4 and soughtincreased participation from both member and non-member countries. As of 1999, there arefourteen members of SolarPACES: Australia, Brazil, Egypt, the European Commission (DGXII and DG XVII), France, Germany, Israel, Mexico, Russia, South Africa, Spain,Switzerland, the United Kingdom and the United States. In 1998 alone, contacts weremaintained with representatives of Azerbaijan, Chile, Ghana, India, Italy, Japan, Jordan,South Africa, Turkey, Uzbekistan and Zimbabwe (1998 SolarPACES Annual Report).Membership is open to all countries, subject to Executive Committee approval, and involves agovernment (or its nominated contracting party) becoming a signatory to the program’s“Implementing Agreement”, which defined the SolarPACES charter and conditions ofmembership. The current Implementing Agreement, valid from 1996 untilDecember 31, 2001, is an amendment of the original one signed on September 23, 1977. TheImplementing Agreement may be extended by agreement of two or more participants, thenbeing applied only to those participants.

All SolarPACES activities are overseen by an Executive Committee (ExCo) composed ofindividuals nominated from each member country. The ExCo meets twice yearly to formulatestrategic objectives, direct the program of work, review results and accomplishments, andreport to the IEA. An elected Chairperson presides over the ExCo meetings, and throughoutthe year, an Executive Secretary deals with day-to-day program management.

The ongoing work and activities are co-ordinated through specific “Tasks” or areas of work,defined within the Implementing Agreement. SolarPACES currently has three such on-goingtasks:− Task I; Concentrating Solar Energy Power Systems.− Task II; Solar Chemistry Research, where solar detoxification is included.− Task III; Solar Technology and Applications.

209


An Operating Agent, nominated by the ExCo, is responsible for overseeing the work of eachTask and each member country nominates a National Co-ordinator within each of the threeTasks. Each task maintains a detailed program of work that defines all task activities,including their objectives, participants, plans and budgets. In addition to technical reports ofthe activities and their participants, accomplishments and progress are summarised in theSolarPACES annual report. Many SolarPACES activities involve close co-operation amongmembers countries (either through sharing of task activities or, occasionally, cost-sharing),although some co-operation is limited to sharing of information and results with otherparticipants.

The activities formally identified within Task II (Solar Chemistry Research) are related withthe development of technologies and systems in the field of solar-driven thermochemical,photochemical and electrochemical processes for the production of energy carriers, chemicalcommodities and for the detoxification and recycling of waste materials. As indicated in thecurrent Implementing Agreement, Task II activities are divided into three sectors, Sector II.3being completely devoted to solar detoxification activities and research.

(a) Sector II.1: Solar production of Energy Carriers.The objectives of this Sector are to:− Explore new ideas and concepts for he thermochemical, photochemical and

electrochemical production of chemical fuels and chemical heat pipes for storageand transportation of solar energy;

− Develop and test the required solar process technology;− Assess their technical and economic feasibility and implementation;− Set priorities of research and development needs;

(b) Sector II.2: Solar Production of Chemical Commodities.The objectives of this Sector are to:− Identify chemical processes for the solar production of fine and bulk chemical

commodities;− Develop and test the required solar process technologies;− Assess their technical and economic feasibility and implementation;

(c) Sector II.3: Solar Detoxification and Recycling.The objectives of this Sector are to:− Test and evaluate solar detoxification processes;− Further develop and demonstrate solar detoxification systems up to commercial

level.

The core of the work of SolarPACES is development of new and advanced concentratingsolar technologies and solving the wide range of technical problems associated with theircommercialisation. This means that, from advanced solar concentrating technologies ingeneral to solar detoxification applications in particular, industrial participation plays acritical role. Many of the Task’s international activities and teams involve industrial co-operation. In fact, in some countries (e.g., the UK and Australia), the SolarPACES contractingparty is an industrial company.

SolarPACES attempts to give added value to national work already funded by its membergovernments. It is, therefore, not in itself a “big-budget” operation and normally does notprovide funding for work to be carried out in member countries. The small annual fee paid by

210


member countries is used to support a limited range of co-operative activities approved by theExCo, such as publication and distribution of documents, scholarships and activitiespromoting international awareness.

The Task II Operating Agent is the Paul Scherrer Institute (PSI) of Switzerland, which co-ordinates the activities in close co-operation with the National Co-ordinators. OperatingAgent and National Co-ordinators normally meet once a year to review the progress of Taskactivities, discuss technical issues and prepare future Task development.

Full additional information on activities, conferences, reports, newsletters and contactaddressed can be found at the following web site:

http://www.demon.co.uk/tfc/SolarPACES.html

10.2 THE EUROPEAN UNIONIn recent years, the European Commission (EC) has become one of the most activeinstitutions in the financing and promoting of both solar detoxification research andinternational collaboration. A considerably high number of projects related to photocatalyticresearch has been approved and financed during the 90s (3rd and 4th Framework Programmes).A large part of the information contained in this book was obtained from projects, networksand activities promoted and partially financed by the European Commission, through theseFramework Programmes.

The Fifth Framework Programme, adopted in December 1998, defines the Communityactivities in the field of research, technological development and demonstration for 1998-2002. Its differs notably from its predecessors in that it focuses on a limited number ofobjectives and areas combining technological, industrial, economic, social and culturalaspects. Environmental Protection is one of these priority areas, water treatment being one ofits specific objectives, thereby providing a good scenario for co-operative research,development and demonstration initiatives related with solar detoxification of water.

The Fifth Framework Programme consists of seven Specific Programmes, of which four areThematic Programmes and three are Horizontal Programmes. The Thematic Programmes are:− Quality of life and management of living resources− User-friendly information society− Competitive and sustainable growth− Energy, environment and sustainable development

The Horizontal Programmes, complementing these Thematic Programmes, are:− Confirming the international role of Community research− Promotion of innovation and encouragement of participation of small and medium-sized

enterprises (SMEs)− Improving the human research potential and socio-economic knowledge base

Exhaustive documentation related to all these Programmes is provided by the EC at thefollowing web sites:− http://europa.eu.int/comm/dg12/index.html (European Commission DGXII)− http://www.cordis.lu/fp5/home.html (Fifth Framework Work programmes)− http://www.cordis.lu/home.html (CORDIS, big European database)

211


http://europa.eu.int/comm/dg12/index.html

http://www.cordis.lu/fp5/home.html

http://www.cordis.lu/home.html


The specific topic of water treatment is in the “Energy, environment and sustainabledevelopment” programme. The strategic goal of this programme is to promote environmentalscience and technology to improve quality of life and boost growth, competitiveness andemployment, while meeting the need for sustainable management of resources and protectionof the environment. Within this programme, research and technology development (RTD)will concentrate on six key actions (two for the “energy” area, four to the “environment andsustainable development” area). The first of the four key actions in the “environment” sectionis “Sustainable management and quality of water”, specifically addressed to water treatmentand purification technologies, with the objectives, among others, of:− developing improved waste-water treatment techniques and technologies,− developing technologies for rational water reuse,− developing technologies for water purification,− enhancing waste-water treatments,− minimising environmental impacts from waste water treatment.

Priority attention will be given to research initiatives addressed to waste water treatment andre-use, water pollution abatement from contaminated land, landfills and sediments and groundand surface waters diffuse pollution (persistent organic chemicals) abatement. The existingbudget for RTD initiatives related to the key action “Sustainable management and quality ofwater”, for the period 1998-2002, is about 450 million Euro.

It may be observed that all these objectives and research initiatives are perfectly coherent withthe processes and technologies indicated in this book, providing an adequate framework forinternational co-operation on solar detoxification applications and further research initiatives.

The “key action” concept is an important characteristic of the Fifth Framework Programme.Its objective is to address the many and varied aspects of the economic and social issues to betargeted, by integrating the entire spectrum of activities and disciplines needed to achieve thespecified objectives, using a problem-solving approach.

An important aspect of the overall European research strategy, in addition to the FifthFramework Programme’s basic support of European research, is international co-operation.Entities of non-EU countries and international organisations may participate in allProgrammes, as well as in the Horizontal Programme “Confirming the international role ofCommunity research”. Conditions for participation of third countries in FP5 may differ fromone Programme to another depending on the status of the country, with regard to theparticipation in EC research activities. Specific rules apply for the Programme “Confirmingthe international role of Community research”.

In addition to EU member countries, institutions and entities from other states have a specialstatus when participating in EC research activities. Countries that have signed AssociationAgreements may participate under the same conditions as EU member countries. Iceland,Liechtenstein, Norway, Israel and candidates for EU-membership (currently Bulgaria,Republic of Cyprus, Czech Republic, Estonia, Hungary, Latvia, Lithuania, Poland, Romania,Slovakia and Slovenia) have Association Agreements either in force or expected to enter intoforce during 1999. Switzerland has also concluded the Association Agreement negotiations.Other countries, such as Argentina, Australia, Canada, China, Russia, South Africa and USAhave signed Co-operation Agreements with the EU for participation and collaboration inresearch activities. In addition, some specific regions also have special relationship with theEU, such as other European countries (Albania, Bosnia-Herzegovina, Former Yugoslav

212


Republic of Macedonia Malta, Turkey and European Microstates and Territories), the so-called Mediterranean Partnership (Algeria, Republic of Cyprus, Egypt, Jordan, Lebanon,Malta, Morocco, Palestine Authority, Syria, Tunisia and Turkey) or the European NIS(Armenia, Azerbaijan, Belarus, Georgia, Moldova, Russia and Ukraine).

In some Work programmes, the developing countries are grouped into the followinggeographic areas: African, Caribbean, Pacific (ACP) countries, Asian and Latin American(ALA) countries, and the Mediterranean countries (MC), so it is always recommended thatup-to-date information and conditions for participation be obtained for the specific call towhich it is intended to submit a proposal.

FP5 is implemented, as past Framework Programmes were, through specific WorkProgrammes, drawn up for each Programme and describing the specific activities and thevarious research areas. The Work Programme is, regularly revised to ensure its continuedrelevance in the light of evolving needs and developments, so potential proposers shouldensure that they are consulting the current version of the work programme when planningtheir proposal. The Work Programme appearing at the Specific Programme Website is alwaysthe current version. Work Programmes provide a means of focusing attention on areas or sub-areas, thereby optimising opportunities for launching collaborative projects and establishingtheme networks.

The EC partially finances RTD activities carried out under the Specific Programmesimplemented within its Framework Programmes. The types of activities normally aided are:

(a) Shared-cost activities− Research and technological development (R&D) projects: projects obtaining new

knowledge for product process or service development or improvement, and/or to meetthe needs of Community policies.

− Demonstration projects: projects designed to prove the viability of new technologiesoffering potential economic advantages, but which cannot be immediatelycommercialised.

− Combined R&D and demonstration projects: projects combining the above elements.− Support for access to research infrastructures: actions enhancing access to research

infrastructures for Community researchers.− “SME Co-operative” research projects: projects enabling at least three mutually

independent SMEs from at least two Member States or one Member State and oneAssociated State to jointly commission research carried out by a third party.

− “SME Exploratory” awards: support a project exploratory phase of up to 12 months (e.g.feasibility studies, validation, partner search, etc).

(b) Training fellowshipsThese may be either fellowships, whereby individual researchers apply directly to theCommission, or host fellowships, where institutions apply to host a number of researchers.There is also a bursary for young researchers from Developing Countries. When preparing ajoint research proposal or concerted action proposal for submission to any of the programmes,a consortium may include an application for an international co-operation-training bursary.These bursaries are intended to allow young researchers from Developing Countries,including Emerging Economies and Mediterranean Partner Countries to work for up to 6months in a European research institute participating in a FP5 project. These bursaryapplications must be submitted together with the proposal application and will be evaluated

213


together with it. The bursary applicant must not be more than 40 years of age, must be anational of one of the eligible countries and intending to return there at the end of the trainingperiod. Applications from female researchers are encouraged.

(c) Research training networks and thematic networksTraining networks for promoting training-through-research especially of researchers at pre-doctoral and at post-doctoral level and thematic networks for bringing together e.g.manufacturers, users, universities, research centres around a given objective.

(d) Concerted actionsActions co-ordinating RTD projects already in receipt of funding, for example to exchangeexperiences, to reach a critical mass, to disseminate results etc. These include co-ordinationnetworks between Community funded projects.

(e) Accompanying measuresActions contributing to the implementation of a Specific Programme or the preparation offuture activities of the programme. They will also seek to prepare for or to support otherindirect RTD actions.

As previously indicated, when planning an RTD proposal for submission to one of theprogrammes or to key actions, researchers should be aware of the conditions of participationby entities from non-EU countries and international organisations.

10.3 THE CYTED PROGRAMAnother possibility for international collaboration is the CYTED Program (ProgramaIberoamericano de Ciencia y Tecnología para el Desarrollo). CYTED is the Latin-AmericanScience and Technology for Development Program. It was created in 1984 by institutionalagreement between Argentina, Bolivia, Brazil, Chile, Colombia, Costa Rica, Cuba, Ecuador,El Salvador, Spain, Guatemala, Honduras, Mexico, Nicaragua, Panama, Paraguay, Peru,Portugal, Dominican Republic, Uruguay and Venezuela.

The CYTED Program objective is to promote co-operation in development through appliedresearch and technology for transferable results to the production systems of the participantcountries. The program is addressed to universities, research centres, institutions and privatecompanies for the use of scarce resources to better advantage to modernise productionsystems, improve the quality of life and enhance co-operative activities among LatinAmerican and European countries.

The Program is divided into 16 different subprograms, each directly managed by anInternational Co-ordinator appointed by the General International Secretariat, made up of therepresentatives of governmental research institutions in the participating countries and whichmanages the overall Program. Each subprogram also has national representatives. There arethree different ways to participate:- Field of Study Networks: to promote interaction, co-operation and transfer of knowledge

and technology among groups working on similar subjects.- Pre-competitive Research Projects: research projects performed through the creation of an

international complementary team. Immediate market application potential is notrequired.

- Innovative Projects: to promote technological development through co-operation betweenbusiness and research centres from different countries for industrial productivity and

214


competitiveness improvement. Innovation projects must be addressed to develop newtechnologies, products, processes and services near the market or with existing potentialmarket.

Networks, Pre-competitive Research Projects and Innovative Projects must be within thescope of the 16 sub-programs to be eligible. There are presently about 8600 Latin-Americanscientist participating in CYTED Program activities, with more than 1000 universities, R&Dinstitutions and companies involved. Solar detoxification activities can be promoted within atleast the following two sub-programs:- MATERIALS TECHNOLOGY. International co-ordinator: Miguel José Yacamán.

Consejo Nacional de Ciencia y Tecnología (CONACYT). Mexico.- CATALYSIS AND ADSORBENTS. International co-ordinator: Paulino Andreu.

Petróleos de Venezuela, S.A. Venezuela.

Some solar detoxification initiatives are already in progress under the CYTED umbrella.Among them are the “Latin-American Network of Semiconductor Oxides and MaterialsRelated to Optical Environmental Applications”, co-ordinated by Dr. Miguel Angel Blesa(Comisión Nacional de Energía Atómica, Argentina), with partner institutions fromArgentina, Brazil, Mexico, Cuba and Spain.

The program philosophy is to share existing national research resources to create a synergisticeffect, reinforcing and consolidating national research. There is an agreement to this endamong all the participating countries by which companies and research institutionsparticipating in approved (“certified”) projects are financed nationally. The type of supportand the financial mechanisms are those normally used within each country to promotedevelopment of scientific research and technology. Limited central financial support is onlyprovided some tasks for project co-ordination.

The normal procedure of a CYTED supported activity is the following:1. A project is initiated by Latin-American company or research institution.2. The National Co-ordinator of the appropriate CYTED sub-program is contacted.3. A preliminary project proposal is defined.4. An appropriate project partnership is sought. A collaboration agreement is reached.5. A project proposal is prepared by the project co-ordinator following a specific format.6. The CYTED National Co-ordinators involved confirms project eligibility.7. The project is certified and approved by the CYTED General Secretariat.8. National financing is requested through the National Co-ordinators.9. The project is carried out with follow-up, conclusion and reporting.

Complete updated information about the CYTED Program can be found at the following webaddress:

http://www.cicyt.es/ivpm/cyted.htm

10.4 MAIN RESEARCH ACTIVITIESAlthough many scientists in different countries are very well known for their continuouseffort in the development of photocatalytic techniques and technologies (theacknowledgement at the beginning of this book is just a small sample), only two countrieshave had government-financed solar detoxification technology research and development

215



programs with relevant industrial collaboration: The United Stated and Spain. Their researchprograms are briefly described below.

10.4.1 United StatesThe U.S. Department of Energy sponsored a 10-year effort to apply photochemicaltechnology to destruction of environmental contaminants. The Solar Detoxification Projectwas begun in the late 80s, initially funded by the Department of Energy (DOE) as part of itsSolar Thermal Program. In 1991 it was incorporated in a new Solar Industrial Program. Thegoal of the project was to develop marketable solar detoxification technology by the mid-1990s. Objectives of the project were to:− Advance the state of development of photocatalytic water treatment chemistry so that it

could be adopted by industry,− Bring the solar treatment of gas streams containing hazardous organic compounds to a

point where it could be transferred to industry, and− Develop remedial solar treatment technology for contaminated soil.

Work in these areas at the Solar Energy Research Institute (SERI), which became theNational Renewable Energy Laboratory (NREL), and at Sandia National Laboratories (SNL)had begun gradually in the mid-1980s. At the same time, there was a strong push, driven byregulatory pressure, to develop new technologies for correcting past environmentalcontamination of soil and of ground and surface water. The regulatory pressure created abusiness environment encouraging development of new, environmentally friendly processes.Many large and small companies studied a wide range of technologies. Solar technologieswere especially attractive because of the potential reduction in the cost of energy.

In order to reach the DOE goals, the following program elements were pursued:− Technology Research, to develop process chemistry− System Engineering, to develop reactors and solar concentrators and to create a solid

understanding of available solar resources applicable to the processes− System and Market Assessment, to evaluate process cost and application information that

could guide R&D for the project.

Subcontractors from industry and academia were heavily involved in all of these areas.Involving industry at an early phase was expected to smooth the path to commercialisation,and universities were expected to provide more basic understanding as a foundation for thetechnology. Initial research areas included aqueous phase applications, catalyst development,and concentrating solar reactor development.

The project was initially directed toward the development of processes that would useconcentrating solar hardware so the existing knowledge base in solar companies could betapped. This led to a field test at Lawrence Livermore National Laboratory using parabolic-trough reactors to treat contaminated ground water, as well as participation in the DOE, EPA,DOD Tri-Agency Project to treat contaminated soil. The latter project involved the U. S.Army, Environmental Protection Agency, and DOE. The NREL/SNL participation was toassist with bench-scale solar testing of the high-flux process. Science ApplicationsInternational Corporation (SAIC) was the prime contractor for the Army. That projectculminated in a test of a reactor on a solar dish by SAIC.

The Photochemistry research team at NREL/SNL conducted research and development workin all R&D areas: basic, applied, demonstration, and transfer to commercialization. Basic

216


research included core Photocatalytic Oxidation (PCO) R&D and catalysts developmentwork, as well as conducting research into new areas of photochemistry such as photo-inducedadsorption and high-temperature solar PCO. Applied research projects consisted ofremediation of chloroethylenes in the gas and water phase, gas and water phase solarphotoreactor development, and application research including indoor air quality, hybridbiological/PCO processes, and processes for treatment of munitions production wastewater.Many of the projects were co-funded by other agencies and programs, including the StrategicEnvironmental Research Defense Program, SEMATECH, and the U.S. Army.

A pilot-scale test conducted in 1991 at Lawrence Livermore National Laboratory treatedcontaminated groundwater containing 200 parts per billion (ppb) of TCE. PCO reduced TCElevels to below 5ppb, within the drinking water standard set by the Environmental ProtectionAgency (EPA). The results of this field pilot test suggested that the benefits of concentratedsunlight were insufficient to overcome the added costs of building concentrators. Efforts tofind a supported catalyst with activity close to that of slurried catalysts were also carried out.A second field test was conducted at Tyndall Air Force Base using a non-concentrating photoreactor system. The results of the Tyndall field test moved PCO research from the aqueousphase into gas-phase research, because the data gathered from this test suggested that airstripping the volatile organic pollutant compounds (VOCs), followed by PCO treatment,could be more cost-effective and more efficient when the water to be treated contained a highbackground level of substances which resulted in a reduction of catalyst activity.

In 1992, a co-operative research and development agreement was entered into with UnitedTechnologies to develop PCO for air treating applications. In 1993, another agreement wasentered into with International Technology Corporation (IT) for the purpose of conductingresearch and development to commercialise PCO remediation technologies coupled to airstripping systems. In 1994, a third co-operative research and development agreement wassigned with SEMATECH to apply PCO to semiconductor manufacturing. Work withSEMATECH resulted in a field demonstration at a semiconductor manufacturing plant-treating emissions from a semiconductor manufacturing operation in 1996. In 1995 and 1996,PCO applications were investigated for the Department of Defence for remediation oftrichloroethylene in water and air streams and for treating paint booth emissions. In 1997, twofield test demonstrations were completed for PCO/thermal treatment at a military paint boothinstallation and PCO remediation of TCE contaminated groundwater. Other industrialcollaboration partners have been the International Fabricare Institute and the American BakersAssociation.

In 1996, the DOE Office of Industrial Technology terminated dedicated funding to the SolarDetoxification Program. The Department of Defence project was successfully completed in1997 after four demonstrations had been completed. PCO work at NREL continues in appliedareas of gas-phase PCO for DOE Industries of the Future applications, indoor air quality andtandem processes such as PCO/biofiltration. This work is funded by a number of governmentand private sector organisations.

10.4.2 SpainThe CIEMAT’s Department of Renewable Energies, a public research institution devoted toenergy and the environment belonging to the Spanish Ministry of Industry and Energy, hasbeen working on solar chemistry processes since 1987. Research on Solar Detoxificationapplications also started at that time, mainly at the PSA, the CIEMAT’s solar research facility,which is located in southeastern Spain. In 1990, through the EU-DGXII “Access to Large

217


Installation Program”, the PSA designed and erected a large solar detoxification facility in co-operation with some relevant European photochemical research groups and the first solar testswere successfully carried out.

Due to reasonable initial expectations for the technical feasibility of solar detoxification, in1994, CIEMAT defined Solar Chemistry research as one of its areas of activity, focusingmainly on water and gas-phase solar detoxification processes and applications. This researchactivity was later transformed, in 1998, into a formal CIEMAT Research Project with themore specific scientific and technological objectives of developing and transferring a feasiblesolar detoxification technology to industry. The main project objective is the development ofa technology that would make the use of solar photons in environmental chemicalapplications in general, and solar detoxification processes in particular, technically andeconomically viable. This is to be done by assessing the scientific and technological basesthat make possible an engineering approach to photochemical processes using solar radiationat pre-industrial scale. Specific project objectives are the following:− Development and optimisation of solar water detoxification technology for the treatment

of hazardous non-biodegradable contaminants in industrial wastewater.− Development and optimisation of gas-phase solar detoxification technology (including

engineering, photoreactor and catalyst) to the photocatalytic detoxification of VOCs fromindustrial gaseous emissions.

− Assessment of technical and economic viability of other Solar Chemistry processes withpotential application in Spain and other countries with similar characteristics, such as hightemperature solar detoxification of hazardous wastes, solar reforming, solar gasification ofbiomass wastes, solar synthesis of fine chemicals, etc).

An important part of the contents of this book is the result of CIEMAT activities in solardetoxification of hazardous water contaminants during recent years.

During the entire period, a major source of scientific background has come from the EC“Access to Large-Scale Scientific Installations” (1990-1993), “Human Capital and Mobility”(1994-1995) and “Training and Mobility of Researchers” (1996-1998). These programs havefacilitated PSA Solar Detoxification Facility access for many relevant European universitiesand other research groups and have made possible a continuous exchange of ideas leading tophotochemical process improvement. In addition to this scientific database, a large number ofnational and international initiatives in solar detoxification, supported by Spanish andEuropean research programs, have provided CIEMAT an important complementarytechnological background. Some important initiatives are still underway.

Industrial participation is also a fundamental pillar of the overall CIEMAT activity in solardetoxification. Since 1993, it has collaborated with many Spanish companies in a largenumber of small projects that have made it possible to test the feasibility of photocatalyticdegradation of industrial waste water and identify the most suitable targets. Moreover, thesetests have been used to verify the continuous modification and improvement of the solartechnology. Industrial collaboration with the CIEMAT Solar Detoxification project hasincreased significantly since 1997, with the development of several important projects anddemonstration initiatives. Sections 9.2, 9.4 and 9.6 of this book are examples of thiscollaboration.

218


It may be affirmed that the result of this combined photochemical research from universityand industrial collaboration in hardware development and testing has lead to the current stateof the art of the solar detoxification technology, which this book has tried to reflect.

10.5 GUIDELINES TO SUCCESSFUL WATER TREATMENT PROJECTS INDEVELOPING COUNTRIESFinally, because of the high rate of failure of water system projects in developing countries,most of which are especially suitable for such applications since they are in the “sun belt”,some guidelines for water treatment projects to be carried out in those countries are includedhere as a complement to the previous sections.

Twenty guidelines indicating the requirements for water projects may be found in thedocument Lessons Learned in Water, Sanitation and Health (reference 6). As these guidelinesare basically addressed to water and sanitation projects funded by international aid, someimportant conclusions may be arrived at concerning solar detoxification water treatmentprojects. These and other guidelines were obtained as the result of many years of experienceand underline the necessity of promoting co-operation with local authorities and institutions,supporting local plans and requirements, rather than promoting the most convenient projectdictated by the developed country. Adequate information, training and local skill must also bebuilt up are to guarantee the long-term sustainability of any facility installed.

From such guidelines as these, lessons learned and general recommendations for addressingwater-related projects in developing countries, it may be affirmed that special emphasis mustbe placed on training local people and involving local institutions for collaborative projects onsolar detoxification. If local technical or managerial skills are inadequate, the success of anyrenewable energy project would be difficult, and this is more so in the case of solardetoxification technologies that require adequate previous training. Also, the collaboration oflocal institutions is very important in itself since any water treatment project is usually closelyrelated to sanitary and health issues.

A typical mistake is to focus the project on construction, forgetting the necessary previsionsand provisions to ensure the technical and financial sustainability of the renewable technologyinstallation. This issue must be foreseen from the beginning of the project. Therefore, it ishighly recommended that projects financed by foreign aid be designed in such a way that theiroperation and maintenance costs as well as part of the initial construction cost can be borne bythe user or users.

Another relevant recommendation is that projects should not promote dependency on foreignaid or foreign technical assistance. One possible way to avoid this when innovativetechnologies, such as solar detoxification, are implemented is to promote the development ofthe private sector on the area. Local companies, acting as interface between the users and theforeign engineering company, could provide the necessary technical assistance and alsopromote new projects and initiatives at local level.

As contaminated drinking water is a typical problem, water purification and disinfectionseems to be a solar detoxification technology application of interest in developing countries.Water contaminants, the main cause of diseases in developing countries, have been dividedinto five categories by the World Health Organisation (WHO): biological, inorganic, organic,aesthetic and radioactive contaminants. With regard to organic constituents of healthsignificance, main WHO recommendation is to encourage efforts to protect water sources

219


from organic contamination, as their treatment tends to be complicated. Whenever possible,these industrial and agricultural contaminants must be treated at their source. Solardetoxification is always an alternative to be considered for this.

Among the denominated Advanced Oxidation Processes (AOPs), based on catalytic andphotochemical techniques, Solar Detoxification has become especially attractive to thetreatment of water contaminants. This has been due to the synergistic combination of solvingdifficult environmental problems by using solar energy with the possibilities of creating newjobs and activities. Solar Detoxification could be considered, then, as a good example of theconcept of sustainable development which, as sooner the better, mankind must achieve.

SUMMARY OF THE CHAPTERAlong this chapter, a review of different existing mechanisms to promote internationalcollaboration in the subject of solar detoxification has been made: the International EnergyAgency SolarPACES project, the Research Programmes of the European Commission and theLatin American CYTED network. All three mechanisms are currently (1999) promotingand/or supporting activities and projects on solar detoxification topics with importantinternational collaboration. United States and Spain were the countries with more activityduring the nineties on solar detoxification technology development, resulting in the PresentState of the Art of the technology. A brief review of the governmental activities of bothcountries is also provided within the chapter. International collaboration with developingcountries must be addressed with special care due to the high rate of failure of water relatedprojects. Several general recommendations and lesson learned, from the wide experience ofmany international organisations, are particularised in the case of solar detoxificationinitiatives, when addressed to be implemented at developing countries.

BIBLIOGRAPHY AND REFERENCES1. Blake, D. M.; Carlson-Boyd, L. E.; Lee Recca, L.; Kissell, G. “Photochemical Pollution

Control: The Final Report of the Solar Industrial Program. Solar Detoxification Project”.U.S. Department of Energy. NREL/SANDIA read-only compact disk (CD-ROM). 1997.

2. IEA. “Implementing Agreement for the establishment of a project on Solar Power andChemical Energy Systems (SolarPACES)”. International Energy Agency. 1996 update of1977 document.

3. Niewoehner, J.; Larson, R.; Azrag, E.; Hailu, T.; Horner, J.; VanArsdale, P.“Opportunities for Renewable Energy Technologies in Water Supply in DevelopingCountry Villages”. National Renewable Energy Laboratory. NREL/SR-430-22359. 1997.

4. SolarPACES. “Towards the 21st Century, IEA/SolarPACES Strategic Plan”. SolarPACESBrochure, March 1996.

5. SolarPACES. “Solar Thermal Power and Solar Chemical Energy Systems, SolarPACESProgram of the International Energy Agency”. SolarPACES Brochure, Birmingham,United Kingdom, September 1994, and 1998 update.

6. WASH. “Lessons Learned in Water, Sanitation, and Health; Thirteen Years of Experiencein Developing Countries”. Water and Sanitation for Health Project (WASH). UpdatedEdition; Alexandria, VA. 1993.



1. IEA headquarters are located in London.

220


2. SolarPACES is one of the more than 60 Implementing Agreements of the InternationalEnergy Agency.

3. The PSA was built during the initial stages of the SolarPACES Project to test and evaluatethe different solar technologies to the electricity production existing at that moment.

4. The assistance to the SolarPACES periodic meetings is restricted to the ExecutiveCommittee members, Operating Agents and National Co-ordinators from the differentcountry members.

5. The Fifth Framework Programme of the European Union (1998-2002) consists of sevenSpecific Programmes, of which four are Thematic Programmes and three are HorizontalProgrammes.

6. “Sustainable management and quality of water” is one of the four key actions of theenvironment section of the EU-FP5 “Energy, environment and sustainable development”Work programme.

7. Participation from institutions, organisations and companies from non-EU countries ispossible in all EU-FP5 Programmes.

8. An application for an international Cupertino training bursary (EU-FP5 Programmes) canbe made to any of the programmes, independently of the submission of a joint researchproposal or concerted action initiative.

9. CYTED is a Latin American network of governmental research organisations to promote,support and reinforce national research initiatives.

10. When addressing international collaborative projects to be implemented at developingcountries it is highly recommended to get information from the experience of similar orrelated initiatives previously implemented in the same area.

PART B.

1. How would you define an Implementing Agreement of the International Energy Agency?2. What is SolarPACES?3. Indicate the initial members of the present stage of the SolarPACES project, initiated in

1991.4. How many countries have signed the SolarPACES Implementing Agreement in 1999?5. Indicate the different work areas defined at the SolarPACES Implementing Agreement.6. Which specific sector of SolarPACES Implementing Agreement includes the research

activities on Solar Detoxification?7. Indicate the name of the thematic programme, within the EU-FP5, more directly related

with environmental research.8. Indicate the name of the Horizontal Programme, within the EU-FP5, specifically

addressed to promote International Collaboration.9. Indicate the type of actions normally supported by the different EU-FP5 Programmes.10. Indicate the different shared-cost actions allowed within the different EU-FP5

Programmes.

ANSWERS

Part A

1. False; 2. True; 3. True; 4. False; 5. True; 6. True; 7. True; 8. False; 9. True; 10. True.

221


Part B

1. An “Implementing Agreement” is a specific contract between several IEA members tocollaborate in the research, development and application on a determined energy relatedissue.

2. SolarPACES (Solar Power And Chemical Energy Systems) is one of the IEAImplementing Agreements focused on the development and marketing of systems basedon solar technologies.

3. Germany, Spain, Switzerland and the United Stated.4. Twelve countries (Australia, Brazil, Egypt, France, Germany, Israel, Mexico, Russia,

Spain, Switzerland, United Kingdom and United States) plus the European Commission(DG XII and DG XVII).

5. Three on-going Tasks or thematic areas of work are defined within the SolarPACESImplementing Agreement:− Task I; Concentrating Solar Energy Power Systems.− Task II; Solar Chemistry Research.− Task III; Solar Technology and Applications.

6. The Sector II.37. Energy, environment and sustainable development.8. Confirming the international role of Community research.9. Shared-cost actions. Training fellowships. Research training networks and thematic

networks. Concerted actions. Accompanying measures.10. The different shared-cost actions allowed within the EU-FP5 Programmes are:

− Research and technological development (R&D) projects.− Demonstration projects.− Combined R&D and demonstration projects.− Support for access to research infrastructures.− “SME Co-operative” research projects.− “SME Exploratory” awards.

222


10 INTERNATIONAL COLLABORATION

AIMSThis unit describes some of the programmes and initiatives promoting national orinternational collaboration in research, development and implementation of innovativesustainable technologies, that include Solar Detoxification projects within their scope.

OBJECTIVESAt the end of this unit, you will appreciate the scale, nature and content of these programmes,and some of the national and regional initiatives in progress around the world. You willacquire basic information concerning the mechanisms and possibilities for proposing andestablishing international collaboration in the field of solar detoxification and you will alsoknow how to receive updated information on those possibilities.

NOTATION AND UNITSSymbol Description UnitsCIEMAT Centro de Investigaciones Energéticas, Medioambientales y

Tecnológicas (Spain)DG-XII Directorate General XII (of European Commission): Science,

Research and DevelopmentDG-XVII Directorate General XVII (of European Commission): EnergyDOE Department of Energy (USA)DOD Department of Defense (USA)EC European CommissionEPA Environmental Protection Agency (USA)EU European UnionExCo Executive Committee (SolarPACES)FP5 Fifth Framework Programme (EU)IEA International Energy AgencyNREL National Renewable Energy Laboratory (USA)OEDC Organization for Economic Co-operation and DevelopmentPCO Photocatalytic OxidationPSA Plataforma Solar de AlmeríaPSI Paul Scherrer Institute (Switzerland)SANDIA Sandia National Laboratories (USA)SolarPACES Solar Power And Chemical Energy SystemsRTD Research and technological developmentsSERI Solar Energy Research Institute (USA)SMEs Small and medium-sized enterprisesSNL Sandia National Laboratories (USA)SSPS Small Solar Power SystemsWHO World Health Organisation

10.1 INTERNATIONAL ENERGY AGENCY: THE SolarPACES PROGRAMThe IEA (International Energy Agency), founded in 1974, is the energy forum forindustrialised countries. Based in Paris, the IEA is an autonomous agency within theframework of the Organisation for Economic Co-operation and Development (OEDC). Animportant function of the IEA is the promotion of enhanced international collaboration onenergy research and the development and application of new and efficient energytechnologies. The IEA has set up more than 60 “Implementing Agreements” linking member

223


Countries in R&D, technology demonstration and information initiatives. One of theseImplementing Agreements is called SolarPACES (Solar Power and Chemical EnergySystems).

SolarPACES is one of the international co-operative programs managed under the umbrella ofthe IEA to help find solutions to worldwide energy and environmental problems, bringingtogether teams of national experts from around the world to focus on the development andmarketing of systems based on solar technologies.

The SolarPACES program was initiated in 1977 under its former name of SSPS (Small SolarPower Systems). Two dissimilar solar facilities were designed in the project’s Stage 1 by tenContracting Parties from Austria, Belgium, Germany, Greece, Italy, Spain, United Kingdomand the United States. All the countries, with the exception of the UK, continued the projectthrough Stage 2 (Building, testing and Evaluation) which was completed in Almería, insouthern Spain, at the end of 1984. In the course of the subsequent Stage 3, eight countries(all but Greece) proceeded with solar-related research and development in various forms,especially in advanced solar thermal and solar chemical applications. The two SSPS facilitieswere transformed into what has since become the world’s most versatile solar test centre, thePlataforma Solar de Almería (PSA), which continues to serve as the site of multiple co-operative international testing and development efforts.

In 1991, Germany, Spain, Switzerland and the USA decided to go on to a Stage 4 and soughtincreased participation from both member and non-member countries. As of 1999, there arefourteen members of SolarPACES: Australia, Brazil, Egypt, the European Commission (DGXII and DG XVII), France, Germany, Israel, Mexico, Russia, South Africa, Spain,Switzerland, the United Kingdom and the United States. In 1998 alone, contacts weremaintained with representatives of Azerbaijan, Chile, Ghana, India, Italy, Japan, Jordan,South Africa, Turkey, Uzbekistan and Zimbabwe (1998 SolarPACES Annual Report).Membership is open to all countries, subject to Executive Committee approval, and involves agovernment (or its nominated contracting party) becoming a signatory to the program’s“Implementing Agreement”, which defined the SolarPACES charter and conditions ofmembership. The current Implementing Agreement, valid from 1996 untilDecember 31, 2001, is an amendment of the original one signed on September 23, 1977. TheImplementing Agreement may be extended by agreement of two or more participants, thenbeing applied only to those participants.

All SolarPACES activities are overseen by an Executive Committee (ExCo) composed ofindividuals nominated from each member country. The ExCo meets twice yearly to formulatestrategic objectives, direct the program of work, review results and accomplishments, andreport to the IEA. An elected Chairperson presides over the ExCo meetings, and throughoutthe year, an Executive Secretary deals with day-to-day program management.

The ongoing work and activities are co-ordinated through specific “Tasks” or areas of work,defined within the Implementing Agreement. SolarPACES currently has three such on-goingtasks:− Task I; Concentrating Solar Energy Power Systems.− Task II; Solar Chemistry Research, where solar detoxification is included.− Task III; Solar Technology and Applications.

224


An Operating Agent, nominated by the ExCo, is responsible for overseeing the work of eachTask and each member country nominates a National Co-ordinator within each of the threeTasks. Each task maintains a detailed program of work that defines all task activities,including their objectives, participants, plans and budgets. In addition to technical reports ofthe activities and their participants, accomplishments and progress are summarised in theSolarPACES annual report. Many SolarPACES activities involve close co-operation amongmembers countries (either through sharing of task activities or, occasionally, cost-sharing),although some co-operation is limited to sharing of information and results with otherparticipants.

The activities formally identified within Task II (Solar Chemistry Research) are related withthe development of technologies and systems in the field of solar-driven thermochemical,photochemical and electrochemical processes for the production of energy carriers, chemicalcommodities and for the detoxification and recycling of waste materials. As indicated in thecurrent Implementing Agreement, Task II activities are divided into three sectors, Sector II.3being completely devoted to solar detoxification activities and research.

(a) Sector II.1: Solar production of Energy Carriers.The objectives of this Sector are to:− Explore new ideas and concepts for he thermochemical, photochemical and

electrochemical production of chemical fuels and chemical heat pipes for storageand transportation of solar energy;

− Develop and test the required solar process technology;− Assess their technical and economic feasibility and implementation;− Set priorities of research and development needs;

(b) Sector II.2: Solar Production of Chemical Commodities.The objectives of this Sector are to:− Identify chemical processes for the solar production of fine and bulk chemical

commodities;− Develop and test the required solar process technologies;− Assess their technical and economic feasibility and implementation;

(c) Sector II.3: Solar Detoxification and Recycling.The objectives of this Sector are to:− Test and evaluate solar detoxification processes;− Further develop and demonstrate solar detoxification systems up to commercial

level.

The core of the work of SolarPACES is development of new and advanced concentratingsolar technologies and solving the wide range of technical problems associated with theircommercialisation. This means that, from advanced solar concentrating technologies ingeneral to solar detoxification applications in particular, industrial participation plays acritical role. Many of the Task’s international activities and teams involve industrial co-operation. In fact, in some countries (e.g., the UK and Australia), the SolarPACES contractingparty is an industrial company.

SolarPACES attempts to give added value to national work already funded by its membergovernments. It is, therefore, not in itself a “big-budget” operation and normally does notprovide funding for work to be carried out in member countries. The small annual fee paid by

225


member countries is used to support a limited range of co-operative activities approved by theExCo, such as publication and distribution of documents, scholarships and activitiespromoting international awareness.

The Task II Operating Agent is the Paul Scherrer Institute (PSI) of Switzerland, which co-ordinates the activities in close co-operation with the National Co-ordinators. OperatingAgent and National Co-ordinators normally meet once a year to review the progress of Taskactivities, discuss technical issues and prepare future Task development.

Full additional information on activities, conferences, reports, newsletters and contactaddressed can be found at the following web site:


10.2 THE EUROPEAN UNIONIn recent years, the European Commission (EC) has become one of the most activeinstitutions in the financing and promoting of both solar detoxification research andinternational collaboration. A considerably high number of projects related to photocatalyticresearch has been approved and financed during the 90s (3rd and 4th Framework Programmes).A large part of the information contained in this book was obtained from projects, networksand activities promoted and partially financed by the European Commission, through theseFramework Programmes.

The Fifth Framework Programme, adopted in December 1998, defines the Communityactivities in the field of research, technological development and demonstration for 1998-2002. Its differs notably from its predecessors in that it focuses on a limited number ofobjectives and areas combining technological, industrial, economic, social and culturalaspects. Environmental Protection is one of these priority areas, water treatment being one ofits specific objectives, thereby providing a good scenario for co-operative research,development and demonstration initiatives related with solar detoxification of water.

The Fifth Framework Programme consists of seven Specific Programmes, of which four areThematic Programmes and three are Horizontal Programmes. The Thematic Programmes are:− Quality of life and management of living resources− User-friendly information society− Competitive and sustainable growth− Energy, environment and sustainable development

The Horizontal Programmes, complementing these Thematic Programmes, are:− Confirming the international role of Community research− Promotion of innovation and encouragement of participation of small and medium-sized

enterprises (SMEs)− Improving the human research potential and socio-economic knowledge base

Exhaustive documentation related to all these Programmes is provided by the EC at thefollowing web sites:− http://europa.eu.int/comm/dg12/index.html (European Commission DGXII)− http://www.cordis.lu/fp5/home.html (Fifth Framework Work programmes)− http://www.cordis.lu/home.html (CORDIS, big European database)

226


http://europa.eu.int/comm/dg12/index.html

http://www.cordis.lu/fp5/home.html

http://www.cordis.lu/home.html


The specific topic of water treatment is in the “Energy, environment and sustainabledevelopment” programme. The strategic goal of this programme is to promote environmentalscience and technology to improve quality of life and boost growth, competitiveness andemployment, while meeting the need for sustainable management of resources and protectionof the environment. Within this programme, research and technology development (RTD)will concentrate on six key actions (two for the “energy” area, four to the “environment andsustainable development” area). The first of the four key actions in the “environment” sectionis “Sustainable management and quality of water”, specifically addressed to water treatmentand purification technologies, with the objectives, among others, of:− developing improved waste-water treatment techniques and technologies,− developing technologies for rational water reuse,− developing technologies for water purification,− enhancing waste-water treatments,− minimising environmental impacts from waste water treatment.

Priority attention will be given to research initiatives addressed to waste water treatment andre-use, water pollution abatement from contaminated land, landfills and sediments and groundand surface waters diffuse pollution (persistent organic chemicals) abatement. The existingbudget for RTD initiatives related to the key action “Sustainable management and quality ofwater”, for the period 1998-2002, is about 450 million Euro.

It may be observed that all these objectives and research initiatives are perfectly coherent withthe processes and technologies indicated in this book, providing an adequate framework forinternational co-operation on solar detoxification applications and further research initiatives.

The “key action” concept is an important characteristic of the Fifth Framework Programme.Its objective is to address the many and varied aspects of the economic and social issues to betargeted, by integrating the entire spectrum of activities and disciplines needed to achieve thespecified objectives, using a problem-solving approach.

An important aspect of the overall European research strategy, in addition to the FifthFramework Programme’s basic support of European research, is international co-operation.Entities of non-EU countries and international organisations may participate in allProgrammes, as well as in the Horizontal Programme “Confirming the international role ofCommunity research”. Conditions for participation of third countries in FP5 may differ fromone Programme to another depending on the status of the country, with regard to theparticipation in EC research activities. Specific rules apply for the Programme “Confirmingthe international role of Community research”.

In addition to EU member countries, institutions and entities from other states have a specialstatus when participating in EC research activities. Countries that have signed AssociationAgreements may participate under the same conditions as EU member countries. Iceland,Liechtenstein, Norway, Israel and candidates for EU-membership (currently Bulgaria,Republic of Cyprus, Czech Republic, Estonia, Hungary, Latvia, Lithuania, Poland, Romania,Slovakia and Slovenia) have Association Agreements either in force or expected to enter intoforce during 1999. Switzerland has also concluded the Association Agreement negotiations.Other countries, such as Argentina, Australia, Canada, China, Russia, South Africa and USAhave signed Co-operation Agreements with the EU for participation and collaboration inresearch activities. In addition, some specific regions also have special relationship with theEU, such as other European countries (Albania, Bosnia-Herzegovina, Former Yugoslav

227


Republic of Macedonia Malta, Turkey and European Microstates and Territories), the so-called Mediterranean Partnership (Algeria, Republic of Cyprus, Egypt, Jordan, Lebanon,Malta, Morocco, Palestine Authority, Syria, Tunisia and Turkey) or the European NIS(Armenia, Azerbaijan, Belarus, Georgia, Moldova, Russia and Ukraine).

In some Work programmes, the developing countries are grouped into the followinggeographic areas: African, Caribbean, Pacific (ACP) countries, Asian and Latin American(ALA) countries, and the Mediterranean countries (MC), so it is always recommended thatup-to-date information and conditions for participation be obtained for the specific call towhich it is intended to submit a proposal.

FP5 is implemented, as past Framework Programmes were, through specific WorkProgrammes, drawn up for each Programme and describing the specific activities and thevarious research areas. The Work Programme is, regularly revised to ensure its continuedrelevance in the light of evolving needs and developments, so potential proposers shouldensure that they are consulting the current version of the work programme when planningtheir proposal. The Work Programme appearing at the Specific Programme Website is alwaysthe current version. Work Programmes provide a means of focusing attention on areas or sub-areas, thereby optimising opportunities for launching collaborative projects and establishingtheme networks.

The EC partially finances RTD activities carried out under the Specific Programmesimplemented within its Framework Programmes. The types of activities normally aided are:

(a) Shared-cost activities− Research and technological development (R&D) projects: projects obtaining new

knowledge for product process or service development or improvement, and/or to meetthe needs of Community policies.

− Demonstration projects: projects designed to prove the viability of new technologiesoffering potential economic advantages, but which cannot be immediatelycommercialised.

− Combined R&D and demonstration projects: projects combining the above elements.− Support for access to research infrastructures: actions enhancing access to research

infrastructures for Community researchers.− “SME Co-operative” research projects: projects enabling at least three mutually

independent SMEs from at least two Member States or one Member State and oneAssociated State to jointly commission research carried out by a third party.

− “SME Exploratory” awards: support a project exploratory phase of up to 12 months (e.g.feasibility studies, validation, partner search, etc).

(b) Training fellowshipsThese may be either fellowships, whereby individual researchers apply directly to theCommission, or host fellowships, where institutions apply to host a number of researchers.There is also a bursary for young researchers from Developing Countries. When preparing ajoint research proposal or concerted action proposal for submission to any of the programmes,a consortium may include an application for an international co-operation-training bursary.These bursaries are intended to allow young researchers from Developing Countries,including Emerging Economies and Mediterranean Partner Countries to work for up to 6months in a European research institute participating in a FP5 project. These bursaryapplications must be submitted together with the proposal application and will be evaluated

228


together with it. The bursary applicant must not be more than 40 years of age, must be anational of one of the eligible countries and intending to return there at the end of the trainingperiod. Applications from female researchers are encouraged.

(c) Research training networks and thematic networksTraining networks for promoting training-through-research especially of researchers at pre-doctoral and at post-doctoral level and thematic networks for bringing together e.g.manufacturers, users, universities, research centres around a given objective.

(d) Concerted actionsActions co-ordinating RTD projects already in receipt of funding, for example to exchangeexperiences, to reach a critical mass, to disseminate results etc. These include co-ordinationnetworks between Community funded projects.

(e) Accompanying measuresActions contributing to the implementation of a Specific Programme or the preparation offuture activities of the programme. They will also seek to prepare for or to support otherindirect RTD actions.

As previously indicated, when planning an RTD proposal for submission to one of theprogrammes or to key actions, researchers should be aware of the conditions of participationby entities from non-EU countries and international organisations.

10.3 THE CYTED PROGRAMAnother possibility for international collaboration is the CYTED Program (ProgramaIberoamericano de Ciencia y Tecnología para el Desarrollo). CYTED is the Latin-AmericanScience and Technology for Development Program. It was created in 1984 by institutionalagreement between Argentina, Bolivia, Brazil, Chile, Colombia, Costa Rica, Cuba, Ecuador,El Salvador, Spain, Guatemala, Honduras, Mexico, Nicaragua, Panama, Paraguay, Peru,Portugal, Dominican Republic, Uruguay and Venezuela.

The CYTED Program objective is to promote co-operation in development through appliedresearch and technology for transferable results to the production systems of the participantcountries. The program is addressed to universities, research centres, institutions and privatecompanies for the use of scarce resources to better advantage to modernise productionsystems, improve the quality of life and enhance co-operative activities among LatinAmerican and European countries.

The Program is divided into 16 different subprograms, each directly managed by anInternational Co-ordinator appointed by the General International Secretariat, made up of therepresentatives of governmental research institutions in the participating countries and whichmanages the overall Program. Each subprogram also has national representatives. There arethree different ways to participate:- Field of Study Networks: to promote interaction, co-operation and transfer of knowledge

and technology among groups working on similar subjects.- Pre-competitive Research Projects: research projects performed through the creation of an

international complementary team. Immediate market application potential is notrequired.

- Innovative Projects: to promote technological development through co-operation betweenbusiness and research centres from different countries for industrial productivity and

229


competitiveness improvement. Innovation projects must be addressed to develop newtechnologies, products, processes and services near the market or with existing potentialmarket.

Networks, Pre-competitive Research Projects and Innovative Projects must be within thescope of the 16 sub-programs to be eligible. There are presently about 8600 Latin-Americanscientist participating in CYTED Program activities, with more than 1000 universities, R&Dinstitutions and companies involved. Solar detoxification activities can be promoted within atleast the following two sub-programs:- MATERIALS TECHNOLOGY. International co-ordinator: Miguel José Yacamán.

Consejo Nacional de Ciencia y Tecnología (CONACYT). Mexico.- CATALYSIS AND ADSORBENTS. International co-ordinator: Paulino Andreu.

Petróleos de Venezuela, S.A. Venezuela.

Some solar detoxification initiatives are already in progress under the CYTED umbrella.Among them are the “Latin-American Network of Semiconductor Oxides and MaterialsRelated to Optical Environmental Applications”, co-ordinated by Dr. Miguel Angel Blesa(Comisión Nacional de Energía Atómica, Argentina), with partner institutions fromArgentina, Brazil, Mexico, Cuba and Spain.

The program philosophy is to share existing national research resources to create a synergisticeffect, reinforcing and consolidating national research. There is an agreement to this endamong all the participating countries by which companies and research institutionsparticipating in approved (“certified”) projects are financed nationally. The type of supportand the financial mechanisms are those normally used within each country to promotedevelopment of scientific research and technology. Limited central financial support is onlyprovided some tasks for project co-ordination.

The normal procedure of a CYTED supported activity is the following:1. A project is initiated by Latin-American company or research institution.2. The National Co-ordinator of the appropriate CYTED sub-program is contacted.3. A preliminary project proposal is defined.4. An appropriate project partnership is sought. A collaboration agreement is reached.5. A project proposal is prepared by the project co-ordinator following a specific format.6. The CYTED National Co-ordinators involved confirms project eligibility.7. The project is certified and approved by the CYTED General Secretariat.8. National financing is requested through the National Co-ordinators.9. The project is carried out with follow-up, conclusion and reporting.

Complete updated information about the CYTED Program can be found at the following webaddress:


10.4 MAIN RESEARCH ACTIVITIESAlthough many scientists in different countries are very well known for their continuouseffort in the development of photocatalytic techniques and technologies (theacknowledgement at the beginning of this book is just a small sample), only two countrieshave had government-financed solar detoxification technology research and development

230



programs with relevant industrial collaboration: The United Stated and Spain. Their researchprograms are briefly described below.

10.4.1 United StatesThe U.S. Department of Energy sponsored a 10-year effort to apply photochemicaltechnology to destruction of environmental contaminants. The Solar Detoxification Projectwas begun in the late 80s, initially funded by the Department of Energy (DOE) as part of itsSolar Thermal Program. In 1991 it was incorporated in a new Solar Industrial Program. Thegoal of the project was to develop marketable solar detoxification technology by the mid-1990s. Objectives of the project were to:− Advance the state of development of photocatalytic water treatment chemistry so that it

could be adopted by industry,− Bring the solar treatment of gas streams containing hazardous organic compounds to a

point where it could be transferred to industry, and− Develop remedial solar treatment technology for contaminated soil.

Work in these areas at the Solar Energy Research Institute (SERI), which became theNational Renewable Energy Laboratory (NREL), and at Sandia National Laboratories (SNL)had begun gradually in the mid-1980s. At the same time, there was a strong push, driven byregulatory pressure, to develop new technologies for correcting past environmentalcontamination of soil and of ground and surface water. The regulatory pressure created abusiness environment encouraging development of new, environmentally friendly processes.Many large and small companies studied a wide range of technologies. Solar technologieswere especially attractive because of the potential reduction in the cost of energy.

In order to reach the DOE goals, the following program elements were pursued:− Technology Research, to develop process chemistry− System Engineering, to develop reactors and solar concentrators and to create a solid

understanding of available solar resources applicable to the processes− System and Market Assessment, to evaluate process cost and application information that

could guide R&D for the project.

Subcontractors from industry and academia were heavily involved in all of these areas.Involving industry at an early phase was expected to smooth the path to commercialisation,and universities were expected to provide more basic understanding as a foundation for thetechnology. Initial research areas included aqueous phase applications, catalyst development,and concentrating solar reactor development.

The project was initially directed toward the development of processes that would useconcentrating solar hardware so the existing knowledge base in solar companies could betapped. This led to a field test at Lawrence Livermore National Laboratory using parabolic-trough reactors to treat contaminated ground water, as well as participation in the DOE, EPA,DOD Tri-Agency Project to treat contaminated soil. The latter project involved the U. S.Army, Environmental Protection Agency, and DOE. The NREL/SNL participation was toassist with bench-scale solar testing of the high-flux process. Science ApplicationsInternational Corporation (SAIC) was the prime contractor for the Army. That projectculminated in a test of a reactor on a solar dish by SAIC.

The Photochemistry research team at NREL/SNL conducted research and development workin all R&D areas: basic, applied, demonstration, and transfer to commercialization. Basic

231


research included core Photocatalytic Oxidation (PCO) R&D and catalysts developmentwork, as well as conducting research into new areas of photochemistry such as photo-inducedadsorption and high-temperature solar PCO. Applied research projects consisted ofremediation of chloroethylenes in the gas and water phase, gas and water phase solarphotoreactor development, and application research including indoor air quality, hybridbiological/PCO processes, and processes for treatment of munitions production wastewater.Many of the projects were co-funded by other agencies and programs, including the StrategicEnvironmental Research Defense Program, SEMATECH, and the U.S. Army.

A pilot-scale test conducted in 1991 at Lawrence Livermore National Laboratory treatedcontaminated groundwater containing 200 parts per billion (ppb) of TCE. PCO reduced TCElevels to below 5ppb, within the drinking water standard set by the Environmental ProtectionAgency (EPA). The results of this field pilot test suggested that the benefits of concentratedsunlight were insufficient to overcome the added costs of building concentrators. Efforts tofind a supported catalyst with activity close to that of slurried catalysts were also carried out.A second field test was conducted at Tyndall Air Force Base using a non-concentrating photoreactor system. The results of the Tyndall field test moved PCO research from the aqueousphase into gas-phase research, because the data gathered from this test suggested that airstripping the volatile organic pollutant compounds (VOCs), followed by PCO treatment,could be more cost-effective and more efficient when the water to be treated contained a highbackground level of substances which resulted in a reduction of catalyst activity.

In 1992, a co-operative research and development agreement was entered into with UnitedTechnologies to develop PCO for air treating applications. In 1993, another agreement wasentered into with International Technology Corporation (IT) for the purpose of conductingresearch and development to commercialise PCO remediation technologies coupled to airstripping systems. In 1994, a third co-operative research and development agreement wassigned with SEMATECH to apply PCO to semiconductor manufacturing. Work withSEMATECH resulted in a field demonstration at a semiconductor manufacturing plant-treating emissions from a semiconductor manufacturing operation in 1996. In 1995 and 1996,PCO applications were investigated for the Department of Defence for remediation oftrichloroethylene in water and air streams and for treating paint booth emissions. In 1997, twofield test demonstrations were completed for PCO/thermal treatment at a military paint boothinstallation and PCO remediation of TCE contaminated groundwater. Other industrialcollaboration partners have been the International Fabricare Institute and the American BakersAssociation.

In 1996, the DOE Office of Industrial Technology terminated dedicated funding to the SolarDetoxification Program. The Department of Defence project was successfully completed in1997 after four demonstrations had been completed. PCO work at NREL continues in appliedareas of gas-phase PCO for DOE Industries of the Future applications, indoor air quality andtandem processes such as PCO/biofiltration. This work is funded by a number of governmentand private sector organisations.

10.4.2 SpainThe CIEMAT’s Department of Renewable Energies, a public research institution devoted toenergy and the environment belonging to the Spanish Ministry of Industry and Energy, hasbeen working on solar chemistry processes since 1987. Research on Solar Detoxificationapplications also started at that time, mainly at the PSA, the CIEMAT’s solar research facility,which is located in southeastern Spain. In 1990, through the EU-DGXII “Access to Large

232


Installation Program”, the PSA designed and erected a large solar detoxification facility in co-operation with some relevant European photochemical research groups and the first solar testswere successfully carried out.

Due to reasonable initial expectations for the technical feasibility of solar detoxification, in1994, CIEMAT defined Solar Chemistry research as one of its areas of activity, focusingmainly on water and gas-phase solar detoxification processes and applications. This researchactivity was later transformed, in 1998, into a formal CIEMAT Research Project with themore specific scientific and technological objectives of developing and transferring a feasiblesolar detoxification technology to industry. The main project objective is the development ofa technology that would make the use of solar photons in environmental chemicalapplications in general, and solar detoxification processes in particular, technically andeconomically viable. This is to be done by assessing the scientific and technological basesthat make possible an engineering approach to photochemical processes using solar radiationat pre-industrial scale. Specific project objectives are the following:− Development and optimisation of solar water detoxification technology for the treatment

of hazardous non-biodegradable contaminants in industrial wastewater.− Development and optimisation of gas-phase solar detoxification technology (including

engineering, photoreactor and catalyst) to the photocatalytic detoxification of VOCs fromindustrial gaseous emissions.

− Assessment of technical and economic viability of other Solar Chemistry processes withpotential application in Spain and other countries with similar characteristics, such as hightemperature solar detoxification of hazardous wastes, solar reforming, solar gasification ofbiomass wastes, solar synthesis of fine chemicals, etc).

An important part of the contents of this book is the result of CIEMAT activities in solardetoxification of hazardous water contaminants during recent years.

During the entire period, a major source of scientific background has come from the EC“Access to Large-Scale Scientific Installations” (1990-1993), “Human Capital and Mobility”(1994-1995) and “Training and Mobility of Researchers” (1996-1998). These programs havefacilitated PSA Solar Detoxification Facility access for many relevant European universitiesand other research groups and have made possible a continuous exchange of ideas leading tophotochemical process improvement. In addition to this scientific database, a large number ofnational and international initiatives in solar detoxification, supported by Spanish andEuropean research programs, have provided CIEMAT an important complementarytechnological background. Some important initiatives are still underway.

Industrial participation is also a fundamental pillar of the overall CIEMAT activity in solardetoxification. Since 1993, it has collaborated with many Spanish companies in a largenumber of small projects that have made it possible to test the feasibility of photocatalyticdegradation of industrial waste water and identify the most suitable targets. Moreover, thesetests have been used to verify the continuous modification and improvement of the solartechnology. Industrial collaboration with the CIEMAT Solar Detoxification project hasincreased significantly since 1997, with the development of several important projects anddemonstration initiatives. Sections 9.2, 9.4 and 9.6 of this book are examples of thiscollaboration.

233


It may be affirmed that the result of this combined photochemical research from universityand industrial collaboration in hardware development and testing has lead to the current stateof the art of the solar detoxification technology, which this book has tried to reflect.

10.5 GUIDELINES TO SUCCESSFUL WATER TREATMENT PROJECTS INDEVELOPING COUNTRIESFinally, because of the high rate of failure of water system projects in developing countries,most of which are especially suitable for such applications since they are in the “sun belt”,some guidelines for water treatment projects to be carried out in those countries are includedhere as a complement to the previous sections.

Twenty guidelines indicating the requirements for water projects may be found in thedocument Lessons Learned in Water, Sanitation and Health (reference 6). As these guidelinesare basically addressed to water and sanitation projects funded by international aid, someimportant conclusions may be arrived at concerning solar detoxification water treatmentprojects. These and other guidelines were obtained as the result of many years of experienceand underline the necessity of promoting co-operation with local authorities and institutions,supporting local plans and requirements, rather than promoting the most convenient projectdictated by the developed country. Adequate information, training and local skill must also bebuilt up are to guarantee the long-term sustainability of any facility installed.

From such guidelines as these, lessons learned and general recommendations for addressingwater-related projects in developing countries, it may be affirmed that special emphasis mustbe placed on training local people and involving local institutions for collaborative projects onsolar detoxification. If local technical or managerial skills are inadequate, the success of anyrenewable energy project would be difficult, and this is more so in the case of solardetoxification technologies that require adequate previous training. Also, the collaboration oflocal institutions is very important in itself since any water treatment project is usually closelyrelated to sanitary and health issues.

A typical mistake is to focus the project on construction, forgetting the necessary previsionsand provisions to ensure the technical and financial sustainability of the renewable technologyinstallation. This issue must be foreseen from the beginning of the project. Therefore, it ishighly recommended that projects financed by foreign aid be designed in such a way that theiroperation and maintenance costs as well as part of the initial construction cost can be borne bythe user or users.

Another relevant recommendation is that projects should not promote dependency on foreignaid or foreign technical assistance. One possible way to avoid this when innovativetechnologies, such as solar detoxification, are implemented is to promote the development ofthe private sector on the area. Local companies, acting as interface between the users and theforeign engineering company, could provide the necessary technical assistance and alsopromote new projects and initiatives at local level.

As contaminated drinking water is a typical problem, water purification and disinfectionseems to be a solar detoxification technology application of interest in developing countries.Water contaminants, the main cause of diseases in developing countries, have been dividedinto five categories by the World Health Organisation (WHO): biological, inorganic, organic,aesthetic and radioactive contaminants. With regard to organic constituents of healthsignificance, main WHO recommendation is to encourage efforts to protect water sources

234


from organic contamination, as their treatment tends to be complicated. Whenever possible,these industrial and agricultural contaminants must be treated at their source. Solardetoxification is always an alternative to be considered for this.

Among the denominated Advanced Oxidation Processes (AOPs), based on catalytic andphotochemical techniques, Solar Detoxification has become especially attractive to thetreatment of water contaminants. This has been due to the synergistic combination of solvingdifficult environmental problems by using solar energy with the possibilities of creating newjobs and activities. Solar Detoxification could be considered, then, as a good example of theconcept of sustainable development which, as sooner the better, mankind must achieve.

SUMMARY OF THE CHAPTERAlong this chapter, a review of different existing mechanisms to promote internationalcollaboration in the subject of solar detoxification has been made: the International EnergyAgency SolarPACES project, the Research Programmes of the European Commission and theLatin American CYTED network. All three mechanisms are currently (1999) promotingand/or supporting activities and projects on solar detoxification topics with importantinternational collaboration. United States and Spain were the countries with more activityduring the nineties on solar detoxification technology development, resulting in the PresentState of the Art of the technology. A brief review of the governmental activities of bothcountries is also provided within the chapter. International collaboration with developingcountries must be addressed with special care due to the high rate of failure of water relatedprojects. Several general recommendations and lesson learned, from the wide experience ofmany international organisations, are particularised in the case of solar detoxificationinitiatives, when addressed to be implemented at developing countries.

BIBLIOGRAPHY AND REFERENCES1. Blake, D. M.; Carlson-Boyd, L. E.; Lee Recca, L.; Kissell, G. “Photochemical Pollution

Control: The Final Report of the Solar Industrial Program. Solar Detoxification Project”.U.S. Department of Energy. NREL/SANDIA read-only compact disk (CD-ROM). 1997.

2. IEA. “Implementing Agreement for the establishment of a project on Solar Power andChemical Energy Systems (SolarPACES)”. International Energy Agency. 1996 update of1977 document.

3. Niewoehner, J.; Larson, R.; Azrag, E.; Hailu, T.; Horner, J.; VanArsdale, P.“Opportunities for Renewable Energy Technologies in Water Supply in DevelopingCountry Villages”. National Renewable Energy Laboratory. NREL/SR-430-22359. 1997.

4. SolarPACES. “Towards the 21st Century, IEA/SolarPACES Strategic Plan”. SolarPACESBrochure, March 1996.

5. SolarPACES. “Solar Thermal Power and Solar Chemical Energy Systems, SolarPACESProgram of the International Energy Agency”. SolarPACES Brochure, Birmingham,United Kingdom, September 1994, and 1998 update.

6. WASH. “Lessons Learned in Water, Sanitation, and Health; Thirteen Years of Experiencein Developing Countries”. Water and Sanitation for Health Project (WASH). UpdatedEdition; Alexandria, VA. 1993.



1. IEA headquarters are located in London.

235


2. SolarPACES is one of the more than 60 Implementing Agreements of the InternationalEnergy Agency.

3. The PSA was built during the initial stages of the SolarPACES Project to test and evaluatethe different solar technologies to the electricity production existing at that moment.

4. The assistance to the SolarPACES periodic meetings is restricted to the ExecutiveCommittee members, Operating Agents and National Co-ordinators from the differentcountry members.

5. The Fifth Framework Programme of the European Union (1998-2002) consists of sevenSpecific Programmes, of which four are Thematic Programmes and three are HorizontalProgrammes.

6. “Sustainable management and quality of water” is one of the four key actions of theenvironment section of the EU-FP5 “Energy, environment and sustainable development”Work programme.

7. Participation from institutions, organisations and companies from non-EU countries ispossible in all EU-FP5 Programmes.

8. An application for an international Cupertino training bursary (EU-FP5 Programmes) canbe made to any of the programmes, independently of the submission of a joint researchproposal or concerted action initiative.

9. CYTED is a Latin American network of governmental research organisations to promote,support and reinforce national research initiatives.

10. When addressing international collaborative projects to be implemented at developingcountries it is highly recommended to get information from the experience of similar orrelated initiatives previously implemented in the same area.

PART B.

1. How would you define an Implementing Agreement of the International Energy Agency?2. What is SolarPACES?3. Indicate the initial members of the present stage of the SolarPACES project, initiated in

1991.4. How many countries have signed the SolarPACES Implementing Agreement in 1999?5. Indicate the different work areas defined at the SolarPACES Implementing Agreement.6. Which specific sector of SolarPACES Implementing Agreement includes the research

activities on Solar Detoxification?7. Indicate the name of the thematic programme, within the EU-FP5, more directly related

with environmental research.8. Indicate the name of the Horizontal Programme, within the EU-FP5, specifically

addressed to promote International Collaboration.9. Indicate the type of actions normally supported by the different EU-FP5 Programmes.10. Indicate the different shared-cost actions allowed within the different EU-FP5

Programmes.

ANSWERS

Part A

1. False; 2. True; 3. True; 4. False; 5. True; 6. True; 7. True; 8. False; 9. True; 10. True.

236


Part B

1. An “Implementing Agreement” is a specific contract between several IEA members tocollaborate in the research, development and application on a determined energy relatedissue.

2. SolarPACES (Solar Power And Chemical Energy Systems) is one of the IEAImplementing Agreements focused on the development and marketing of systems basedon solar technologies.

3. Germany, Spain, Switzerland and the United Stated.4. Twelve countries (Australia, Brazil, Egypt, France, Germany, Israel, Mexico, Russia,

Spain, Switzerland, United Kingdom and United States) plus the European Commission(DG XII and DG XVII).

5. Three on-going Tasks or thematic areas of work are defined within the SolarPACESImplementing Agreement:− Task I; Concentrating Solar Energy Power Systems.− Task II; Solar Chemistry Research.− Task III; Solar Technology and Applications.

6. The Sector II.37. Energy, environment and sustainable development.8. Confirming the international role of Community research.9. Shared-cost actions. Training fellowships. Research training networks and thematic

networks. Concerted actions. Accompanying measures.10. The different shared-cost actions allowed within the EU-FP5 Programmes are:

− Research and technological development (R&D) projects.− Demonstration projects.− Combined R&D and demonstration projects.− Support for access to research infrastructures.− “SME Co-operative” research projects.− “SME Exploratory” awards.

237

Solar Detoxificationby

Julian Blanco Galvez, Head of Solar Chemistryand

Sixto Malato Rodriguez, Researcher in the Solar Chemistry Area,Plataforma Solar de Almeria

Spain

SYNOPSIS

AIMS

The main object of the book is the translation of scientific, technological and engineering knowledge and experiences to make possible solar applications of water treatment within the "solar belt" of the world. As the book will define the necessary boundary conditions and the limits of the solar photocatalytic processes, it is the author’s main objective that, after its lecture, skilled people

could arrange the necessary infrastructure to carry out not only similar or related applications as the explained within the book, but also complete different ones.

SCOPE OF THE BOOK

The book is divided into two parts, with five chapters each. First one addresses the theory and fundamentals of the water decontamination by means of solar energy. The objective of this part is to provide enough background to the reader for the second part of the book, addresses the practical applications and systems engineering of the process.

LEVEL OF THE BOOK

The book is basically descriptive and available to a very wide range of students. Also, an important quantity of drawings, graphics and photos will make it more pleasant besides its technical and scientific rigor. Approximately 75% of the book could be considered descriptive with a 25% of a more deepen study of specific subjects.

PRE-REQUISITE KNOWLEDGE OF THE READER

The reader of the book must be a technician with university or bachelor degree, or a technical university student. Nevertheless, as the book will be written in a wide descriptive way since the beginning of the different subjects to be treated, no previous specific knowledge will be required (such as in solar or photocatalysis matters).

MOTIVATION FOR WRITING THE BOOK

Up to now, there not exist a single book compiling the extensive work performed on the engineering and applications of the solar detoxification process, besides the wide number of papers and articles published on the subject. Our main motivation is the compilation, in a comprehensive and extensive way, of all this work making it accessible not only to people interested in solar and photocatalytic applications, but also to all people interested into learning how environmental technology could help to solve environmental problems in general.

Solar detoxification; 2003 - unesdoc

Documents

Transcript of Solar detoxification; 2003 - unesdoc