32

CHAPTER 2

REVIEW OF LITERATURE

This review attempts to explain the diversity of the field, starting with

the nanotechnology and its applications, various methods of synthesis of

nanoparticles and the possible mechanistic aspects. The next section discusses

about the various dyes used in textiles and the methods adopted for

removal/biosorption of textile dyes. The last section highlights the recent

advances, possible applications of nanoparticles in photocatalytic studies with the

future perspectives. Though there are a few good reviews dealing with the

synthesis and applications of nanoparticles, there appears to be scarcity of

information regarding the possible mechanistic aspects of nanoparticle formation.

This review attempts to fill the void.

2.1 NANOTECHNOLOGY

The concept of nanotechnology though considered to be a modern

science has its history dating back from 9th century. Nanoparticles of gold and

silver were used by the artisans of Mesopotamia to generate a glittering effect to

pots. The first scientific description of the properties of nanoparticles was

relations of gold (and o 1857).

In 1959, Richard Feynman gave a talk describing molecular machines

built with atomic precision. This was considered the first talk on nanotechnology.

36

and the amount of biological material has come up to give hope in

implementation of these approaches in large scale and for commercial

applications. There are also the possibilities of producing genetically engineered

size and shape of biological nanoparticles. The combinatorial approach such as

photobiological methods as proved in the case of Fusarium oxysporum-mediated

silver nanoparticles production will help to increase the rate of production. While

exploring the natural secrets for the synthesis of nanoparticles by microbes, which

are regarded as potent ecofriendly green nanofactories, scientists have discovered

magnetite particles by magnetotactic bacteria (Lovley et al 1987; Dickson 1999),

siliceous materials by diatoms (Pum & Sleytr 1999), and gypsum and calcium

layers by S-layer bacteria (Milligan 2002). Interactions between metals and

microbes have been exploited f

bioremediation, biomineralization, bioleaching, and biocorrosion and the

research as nanobiotechnology inter connecting biotechnology and

nanotechnology.

2.1.2.1 Properties of nanoparticles

A number of physical phenomena become more pronounced as the size

of the system decreases. Certain phenomena may not come into play as the

system moves from macro to micro level but may be significant at the nano scale.

One example is the increase in surface area to volume ratio which alters the

mechanical, thermal and catalytic properties of the material. The increase in

surface area to volume ratio leads to increasing dominance of the behaviour of

atoms on the surface of the particle over that of those in the interior of the particle,

thus altering the properties. The electronic and optical properties and the chemical

reactivity of small clusters are completely different from the better known

property of each component in the bulk or at extended surfaces. Some of the size

37

dependant properties of nanoparticles are quantum confinement in

semiconductors, Surface Plasmon Resonance in some metallic nanoparticles and

paramagnetism in magnetic nanoparticles.

Surface plasmon resonance refers to the collective oscillations of the

conduction electrons in resonance with the light field. The surface plasmon mode

arises from the electron confinement in the nanoparticle. The surface plasmon

resonance frequency depends not only on the metal, but also on the shape and size

of the nanoparticle and the dielectric properties of the surrounding medium (Jain

et al 2007). For example, noble metals, especially gold and silver nanoparticles

exhibit unique and tunable optical properties on account of their Surface Plasmon

Resonance.

Super paramagnetism is a form of magnetism that is a special

characteristic of small ferromagnetic or ferromagnetic nanoparticles. In such

super paramagnetic nanoparticles, magnetization can randomly change direction

under the influence of temperature. Super paramagnetism occurs when a material

is composed of very small particles with a size range of 1- 10nm. In the presence

of an external magnetic field, the material behaves in a manner similar to

paramagnetism with an exception that the magnetic moment of the entire material

tends to align with the external magnetic field.

Quantum confinement occurs when one or more dimensions of the

nanoparticle is made very small so that it approaches the size of an exciton in the

bulk material called the Bohr exciton radius. The idea behind confinement is to

trap electrons and holes within a small area (which may be smaller than 30nm).

Quantum confinement is important as it leads to new electronic properties.

Scientists at the Washington University have studied the electronic and optical

changes in the material when it is 10nm or less and have related it to the property

of quantum confinement. Some of the examples of special properties that

nanoparticles exhibit when compared to the bulk are the lack of malleability and

38

ductility of copper nanoparticles lesser than 50nm. Zinc oxide nanoparticles are

known to have superior UV blocking properties compared to the bulk.

2.1.2.2 Kinds of Nanoparticles

Nanoparticles can be broadly grouped into two: namely organic and

inorganic nanoparticles. Organic nanoparticles may include carbon nanoparticles

(fullerenes) while some of the inorganic nanoparticles may include magnetic

nanoparticles, noble metal nanoparticles (like gold and silver) and semiconductor

nanoparticles (like titanium dioxide and zinc oxide). There is a growing interest

in inorganic nanoparticles as they provide superior material properties with

functional versatility.

Due to their size features and advantages over available chemical

imaging drugs agents and drugs, inorganic nanoparticles have been examined as

potential tools for medical imaging as well as for treating diseases. Inorganic

nanomaterials have been widely used for cellular delivery due to their versatile

features like wide availability, rich functionality, good biocompatibility, and

capability of targeted drug delivery and controlled release of drugs (Xu et al

2006). For example mesoporous silica when combined with molecular machines

prove to be excellent imaging and drug releasing systems.

Gold nanoparticles have been used extensively in imaging, as drug

carriers and in thermo therapy of biological targets. Inorganic nanoparticles (such

as metallic and semiconductor nanoparticles) exhibit intrinsic optical properties

which may enhance the transparency of polymer- particle composites. For such

reasons, inorganic nanoparticles have found special interest in studies devoted

to optical properties in composites. For instance, size dependant colour of gold

nanoparticles has been used to colour glass for centuries.

39

2.2 SILVER NANOPARTICLES

Silver nanoparticles are nanoparticles of silver, i.e. silver particles of

between 1 nm and 100 nm in size. While frequently described as being 'silver'

some are composed of a large percentage of silver oxide due to their large ratio of

surface-to-bulk silver atoms. Over the last decades silver nanoparticles have

found applications in catalysis, optics, electronics and other areas due to their

unique size-dependent optical, electrical and magnetic properties. Currently most

of the applications of silver nanoparticles are in antibacterial/antifungal agents in

biotechnology and bioengineering, textile engineering, water treatment, and

silver-based consumer products. Silver nanoparticles are commonly utilized

nanomaterials due to specific surface area is relevant for catalytic reactivity,

high electrical conductivity, and unique optical properties that could be used in

various applications (Sondia & Salopek- Sondi 2004). Metal nanoparticles are

synthesized and stabilized through chemical and mechanical methods

(Balantrapu & Goia 2009; Tripathi et al 2010), electrochemical techniques

(Patakfalvi & Dekany 2010), photochemical reactions in reverse micelles

(Rodriguez-Sanchez et al 2000) and nowadays via green chemistry method

(Taleb et al 1998).

2.2.1 Green Synthesis of Silver Nanoparticles

The worldwide demand for environmentally friendly and sustainable

methods to prepare nanomaterials requires the application of green chemistry

principles. Thus green nanoscience aims at using environmentally benign and

economically viable reagents or solvents, designing inherently safe

nanomaterials for reduced biological and ecological detriment, and enhancing

the material and energy efficiency of safe chemical processes (Dahl et al

2007). Nanotechnology is an upcoming field the benefits of which are

believed to revolutionize various other fields like computers, pharmaceuticals

40

etc. The synthesis of nanoparticles of different chemical composition, controlled

size is an important area of research in nanotechnology. Nanoparticles of metals

and semiconductors have immense use in various other branches of

sciences. There are various chemical methods (Murray et al 2002) and

physical methods (Ayyub et al 2001) to synthesize nanoparticles, but these routes

for synthesis of particles/crystallites require tedious and environmentally

challenging techniques.

The growing needs to develop clean, non-toxic and eco-friendly

procedures for synthesis of nanoparticles has resulted in researchers seriously

looking at biological systems for inspiration. Ever increasing pressure to

develop environmentally benign technique for nanoparticle synthesis has lead

to a renewed interest in biotransformation as a route to growth of nanoscale

structures. Biological systems have a unique ability to control the structure,

phase and nanostructural topography of the inorganic crystals (Cui & Gao

2003). It is well known that microbes such as bacteria (Brierley 1990), Yeast

(Huang et al 1990), fungi (Frilis & Myers-Keith 1986) and algae are able to

adsorb and accumulate metals and can be used in the reduction of

environmental pollution and also for the recovery of metals from waste.

Amongst these microorganisms, only a few groups have been confirmed to

selectively reduce certain metal ions (Klaus et al 1999, Mukherjee et al 2001;

Nair & Pradeep 2002; Oremland et al 2004). The potential of microbes to

Semiconductor bimetallic nanoparticles with immense use in the

semiconductor devices.

Biosynthetic methods can be employed either microorganism cells

or plant extract for nanoparticles production. Biosynthesis of nanoparticles is

an exciting recent addition to the large repertoire of nanoparticles synthesis

methods and now, nanoparticle has entered a commercial exploration period.

41

Gold and silver nanoparticles are presently under intensive study for applications

in optoelectronic devices, ultrasensitive chemical and biological sensors and as

catalysts. This chapter is devoted to biosynthesis and application of silver

nanoparticles.

Nanotechnology enables the development of nanoscale particles of

metals with novel and distinctive physic-chemical properties, and a broad range

of scientific and technological applications (Moore 2006). Another potential use

of silver nanoparticles in water filters in wastewater treatment plants. At

nanoscale silver exhibits remarkably unusual physical, chemical and biological

properties (Evan off & Chumanov 2005; Chen & Schluesener 2007). Recently it

was shown that silver ions may be reduced extracellularly using fungus

Phanerochaete chrysoporium (Vigneshwaran et al 2007) and Pleurotus sajor caju

(Nithya & Ragunathan 2009).

Silver nanoparticles are the promising products in the

nanotechnology industry. The development of consistent processes for the

synthesis of silver nanomaterials is an important aspect of current

nanotechnology research. One of such promising process is green synthesis.

Silver nanoparticles can be synthesized by several physical, chemical and

biological methods. However for the past few years, various rapid chemical

methods have been replaced by green synthesis because of avoiding toxicity of

the process and increased quality. Synthesis of nanoparticles to have a better

control over particles size, distribution, morphology, purity, quantity and

quality, by employing environment friendly economical processes has always

been a challenge for the researchers (Hahn 1997). The synthesis and assembly

of nanoparticles would benefit from the development of clean, nontoxic and

42

organisms ranging from bacteria to fungi and even plants (Sastry

et al 2004).

2.2.2 Significance of Silver Nanoparticles

The preparation of stable, uniform silver nanoparticles by reduction of

silver ions by ethanol is reported here. It is a simple process of recent interest for

obtaining silver nanoparticles. The samples have been characterized by X-Ray

diffraction (XRD) and Transmission Electron Microscopy (TEM), which reveal

the nano nature of the particles. These studies infer that the particles are mostly

spherical in shape and have an average size of 16 nm. The UV/Vis spectra show

that an absorption peak, occurring due to Surface Plasmon Resonance (SPR),

exists at 410 nm.

Uniform silver nano particles can be obtained through the reduction of

silver ions by ethanol at the temperature of 80°C to 100°C under atmospheric

conditions. In this synthesis process, 20 ml of aqueous solution containing silver

nitrate (0.5g of AgNO3), 1.5 g sodium linoleate (C18H32ONa), 8 ml ethanol and 2

ml linoleic acid (C18H32O2) are added in a capped tube under agitation. The

system is sealed and treated at the temperatures between 80°C to 100°C for 6

hours.

In the aqueous solution of silver ions, sodium linoleate and the mixture

of linoleic acid and ethanol are added in order. A solid phase of sodium linoleate,

a liquid phase of ethanol and linoleic acid, and water ethanol solution phase

containing silver ions formed in the system. Ethanol in the liquid and solution

phases reduced the silver ions into silver nanoparticles. Along with the reduction

process, linoleic acid is absorbed on the surface of the silver nanoparticles with

the alkyl chains on the outside which the produced silver nanoparticles of near

circular shape.

43

The product which collected at the bottom of vessel after cooling to

room temperature was dispersed in chloroform to form a homogenous colloidal

solution of silver nanoparticles. The colour of the sample (colloidal solution of

silver nanoparticle) becomes reddish brown. On changing the concentration of

electrolyte, it is found that the colour become reddish brown on adding linoleic

acid at the same proportions. This reddish brown colour of prepared nanoparticles

indicates nearly 100% conversions of silver ions into nanoparticles. The

preparation of silver nanoparticles with different electrolyte concentrations has

been tried, but neither the samples with concentration other than the present one is

found to be stable over 2 weeks nor of smaller size (more than 60 nm). Hence, we

have recorded the data of the particle of optimum size and of comparatively better

stability (over 4 months).

Silver nanoparticles are being used in numerous technologies and

incorporated into a wide array of consumer products that take advantage of their

desirable optical, conductive, and antibacterial properties. Silver nanoparticles

have advantages over other noble nanoparticles (e.g., gold and copper) because

the surface Plasmon resonance energy of silver is situated far from the

interband transition energy. Application of green chemistry to the synthesis of

nanomaterials has vital importance in medicinal and technological aspects

(Mondal et al 2011; Begum et al 2009). Biologically synthesized silver

nanoparticles (Ag-NPs) have wide range of applications because of their

remarkable physical and chemical properties. The literature on the extra cellular

biosynthesis of Ag-NPs using plants and pure compounds from plants are

insignificant (Song & Kim 2008; Gilaki 2010). Specifically, while there is

relatively little or no literature on the extra cellular synthesis of Ag-NPs by using

seaweeds (Govindaraju et al 2009). The following are some of the specific

applications of silver nanoparticles:

44

Diagnostic Applications: Silver nanoparticles are used in

biosensors and numerous assays where the silver nanoparticle

materials can be used as biological tags for quantitative detection.

Antibacterial Applications: Silver nanoparticles are

incorporated in apparel, footwear, paints, wound dressings,

appliances, cosmetics, and plastics for their antibacterial

properties.

Conductive Applications: Silver nanoparticles are used in

conductive inks and integrated into composites to enhance

thermal and electrical conductivity.

Optical Applications: Silver nanoparticles are used to efficiently

harvest light and for enhanced optical spectroscopies including

metal-enhanced fluorescence (MEF) and surface-enhanced

Raman scattering (SERS).

Environmental applications: Silver nanoparticles are used to

provide an efficient degradation of various pollutants from

industries.

2.3 SEAWEEDS AS A SOURCE OF SYNTHESIS OF SILVER

NANOPARTICLES

Earth is a biosphere sizzling with activities of all terms of life. It is

essentially a water planet, two thirds of which being covered with water

especially marine water. Seaweeds or marine microalgae are the renewable

living resources which are also used as food, feed and fertilizer in many parts of

the world. Seaweeds are of nutritional interest as they contain low calorie food,

but rich in vitamins, minerals and dietary fibres. In addition to vitamins and

minerals, seaweeds are also potentially good sources of proteins, polysaccharides

45

and fibres (Darcy-Vrillon1993). The lipids which are present in very small

amounts are unsaturated and afford protection against cardiovascular pathogens.

Seaweeds are considered as source of bioactive compounds and produce a great

variety of secondary metabolites characterized by a broad spectrum of biological

activities. Compounds with cytostatic, antiviral, antihelminthic, antifungal and

antibacterial activities have been detected in green, brown and red algae. Many

bioactive compounds can be extracted from seaweeds. Seaweeds have been

screened extensively to isolate life saving drugs or biologically active substances

all over the world. The present study was undertaken to investigate the

biosorption of textile dyes.

Seaweeds are rich and varied source of bioactive natural products and

have been studied as potential biocidal and pharmaceutical agents (Ara et al

2001). In recent years, there are numerous reports of macro algae derived

compounds that have a broad range of biological activities such as antibacterial,

antifungal, antiviral, antineoplastic, antifouling, anti inflammatory, antitumoric,

cytotoxic and anti-mitotic activities (Perry et al 1991). Presently seaweeds

constitute commercially important marine renewable resources which are

providing valuable ideas for the development of new drugs against cancer,

microbial infections and inflammations.

Seaweeds or benthic marine algae are the group of plants that live

either in marine or brackish water environment. The synthesis of nanoparticles

using algae as source has been unexplored and underexploited. More recently,

there are few, reported that algae being used as a biofactory for synthesis of

metallic nanoparticles.They are also rich in polysaccharides such as alginates,

fructans, and laminarians which have been reported to possess potential medicinal

46

a. Seaweeds are divided into three categories depending upon their

nutritional and chemical composition as brown, red and green algae.

Brown seaweeds are known to contain more bioactive components

than either green or red seaweeds. Marine algae in human consumption have been

documented since 600 BC. In China, Japan, France and Chile, seaweeds are

harvested to be included in a great variety of dishes, including sushi wrappings,

salads, soups, and condiments. The major seaweeds that are of dietary importance

in Asian countries belong to the genus Undari, Porphyra and Laminaria. Recently,

other countries, such as USA, South America, Ireland, Iceland, and France have

market in seaweeds

(McHugh 2003). The Atlantic coast of Ireland is one of the most productive

seaweed growing areas in the world. Its climate, over 5000 km of rocky habitat,

seaweed grow. More than 600 di erent species of seaweed have been

Irish waters. How

of this product from coastal regions and it may be collected from inshore marine

waters as well as from agricultural run-o which may perhaps be a source of

contamination. The retail of fresh seaweeds is limited and the harvest is processed

in order to ensure preservation by drying, freezing or fermenting. Not much

literature is available investigating the micro biology of this material.

Hence, the present study was performed to characterize the brown

seaweeds with respect to their biodiversity and the e ects of heat on its

microbiota.

2.3.1 Ulva lactuca

Ulva lactuca Linnaeus, a green alga in the division Chlorophyta, is the

type species of the genus Ulva, also known by the common name sea lettuce.

47

Ulva lactuca is a thin flat green algae growing from a discoid holdfast. The

margin is somewhat ruffled and often torn. It may reach 18 cm or more in length,

though generally much less, and up to 30 cm across. The membrane is two cells

thick, soft and translucent, and grows attached, without a stipe, to rock by a small

disc-shaped holdfast. Green to dark green in color, this species in the Chlorophyta

is formed of two layers of cells irregularly arranged, as seen in cross section. The

chloroplast is cup-shaped with 1 to 3 pyrenoids. There are other species of Ulva

which are similar and not always easy to differentiate.

2.4 TEXTILE INDUSTRY

Environmental pollution has increased with increasing industrial

developments (Noor-ul-Amin et al 2009). In recent years, globalization and

rapid industrialization have resulted in the establishment of variety of industrial

sectors. Among which, the invention of synthetic dyestuffs has enhanced the

growth rate of new textile sectors in the developing countries like India. Cheap

labor availability also has become a support to the growth of such textile sectors.

There are more than 1, 00,000 commercially available dye industries with over

approximately 7 × 105 tonnes of dyes utilized annually through out the world

(Nigam et al 1996). Thousands of small-scale dyeing units generate enormous

amount of wastewaters from dyeing and subsequent rinsing steps that form one of

the largest contributions to wastewater generation in the textile industry. Textile

dyeing and processing industries use variety of synthetic dyestuffs such as acid,

base, vat, sulphur, indigo, azo and reactive dyes depending on the nature of the

fabric and the intensity of color required thus leads to generation of various types

of wastewater differing in magnitude and quality. The main challenge for the

textile industry today is to modify production methods, so they are more

ecofriendly at a competitive price, by using safer dyes and chemicals and by

reducing cost of effluent treatment/disposal. Recycling has become a

48

necessary element, not because of the shortage of any item, but because of the

need to control pollution.

2.4.1 Wastewater from Textile Industry

Wastewater generated from different industries is posing a great

threat not only to mankind but also to the landmass fertility as well as natural

flora and fauna. In order to meet the stringent international standards,

treatment of industrial wastewater is mandatory. Different dyes and pigments

are extensively used in textiles, papers, plastics, cosmetics, pharmaceuticals

and food industries (Levin et al 2005). The total world colorants production is

estimated to be 8, 00,000 tons per year and at least 10 % of the used dyestuffs

enter the environment through textile and other mills effluents (Levin et al

2004; Palmieri et al 2005). The wastewater from textile plant is often rich in

color, containing residual dyes and chemicals. The presence of dyes in the effluent

can have acute and/or chronic effects on exposed organisms or living beings

depending on the exposure time and dye concentration. Increased color

concentrations not only make the water unfit for domestic or industrial uses, but

they also reduce light transmittance, thus limiting aquatic plant growth and self-

purification processes. In addition, dyes exert an adverse effect on fish life.

Currently the effluents are discharged into streams or canals after retention period

of few hours in a stabilization pond without any proper treatment. This effluent

has adverse impacts on under ground, surface water bodies and land in the

surrounding area. The ecological system of the neighborhood is badly affected

and hence proper treatment method is required before releasing into the

environment.

Till recently, the discharging of waste into the environment was the

way to eliminate them. The permitted discharge levels have been vastly exceeded,

causing such environmental contamination that our natural resources cannot be

49

used for certain purposes and their characteristics have been altered. Dyes,

phenols, pesticides, fertilizers, detergents, and other chemical products are

disposed directly into the environment, without being treated, controlled or

uncontrolled and without an effective treatment strategy. Color

removal/adsorption from the textile wastewater has become an issue of interest

during the last few years because of the toxicity of the dyes and more often the

colored wastewater from the textile industries also decreases the visibility of the

receiving waters. Removal of dyes from textile waste effluents has been carried

out by physical, chemical and biological methods, such as flocculation,

membrane filtration, electrochemical techniques, ozonation, coagulation,

adsorption and fungal discoloration (Fu & Tiraraghavan 2004). Fungal

bioremediation is becoming an attractive option for removal of dyes from

environment. Without fungi and their decomposing activities, the world would

have become a heap of dead bodies of plants and animals. Removal of dyes from

industrial waste waters is of global concern because dyes cause many problems in

aqueous environments. Dyes may significantly affect photosynthetic activity in

aquatic life because of reduced light penetration and may also be toxic to some

aquatic life due to the presence of aromatics, metals, chlorides, etc. (Daneshvar

et al 2007). Dyes and pigments are widely applied in the textiles, paper,

plastics, leather, food and cosmetic industry to color products. Organic dyes

appear in many industrial effluents. Most commercial dyes due to their

chemical stability and difficulty in decomposition (Alzaydien 2009) cause

serious environmental and health problems such as mutagenic and

carcinogenic effects (Crini 2006). Such colored effluents reduce the sunlight

penetration to the aquatic environment and significantly hinder photosynthetic

processes. On the other hand, dyes by reducing oxygen levels in water lead to

suffocation of aquatic flora and fauna (Cheung et al 2009; Hu & Wu 2001).

50

2.4.2 Textile Dyes and Its Effects

Konstantinou & Albanis (2004) reported that textile dyes and other

industrial dyestuffs constitute one of the largest groups of organic compounds

that represent an increasing environmental danger. About 1 20% of the total

world production of dyes is lost during the dyeing process and is released in

the textile effluents (Zollenger 1991). The textile dyes and dye intermediates

with high aromaticity and low biodegradability have emerged as major

environmental pollutants (Arslan et al 2000; Sauer et al 2002) and nearly 10-15%

of the dye is lost in the dyeing process and is released in the wastewater which is

an important source of environmental contamination. Considerable amount of

water is used for dyeing and finishing of fabrics in the textile industries. The

wastewater from textile mills causes serious impact on natural water bodies and

land in the surrounding area. High values of COD and BOD, presence of

particulate matter and sediments, chemicals which are dark in color leading to

turbidity in the effluents causes depletion of dissolved oxygen, which has an

adverse effect on the marine ecological system. As dyes are designed to be

chemically and photolytically stable, they are highly persistent in natural

environments. The improper handling of hazardous chemicals in textile water also

has some serious impact on the health and safety of workers putting them into the

high-risk bracket for contracting skin diseases like chemical burns, irritation,

ulcers, etc. and respiratory problems.

Dye wastes represent one of the most problematic groups of pollutants

because they can be easily identified by the human eye and are not easily

biodegradable. This literature review paper highlights and provides an overview

of dye waste treatments performed over the three years period from 2008 2010.

Noteworthy processes for the treatment of dye waste include biological treatment,

catalytic oxidation, filtration and sorption process and combination treatments.

Some other methods have been developed for the treatment of textile dye

51

wastewater. These methods include membrane technology, activated carbon

adsorption, chemical precipitation and microbial treatment. However, these

methods have been ineffective because of the high salt contents resulting from

azo and direct dyes (Barclay & Buckley 2002). Dyes and dye intermediates

with high degree of aromaticity and low biodegradability are introduced into

the aquatic system resulting in increase of environmental risk. Dyepollutants

from the textile industry are an important source of environmental

contamination. Several studies of photocatalytic degradation of dyes have been

reported (Chatterjee & Mahata 2002; Herrera et al 2000; Poulios &Tsachpinis

1999; Yang et al 2001). Factors influencing the degradation rate of aqueous

systems have been studied such as effect of pH, dissolved oxygen contents and

the amount of photocatalyst added (Kiriakidou et al 1999; Schrank et al 2002).

2.4.3 Classification of Dyes

All aromatic compounds absorb electromagnetic energy but only those

that absorb light with wavelengths in the visible range (~350-700 nm) are colored.

Dyes contain chromophores, delocalized electron systems with conjugated

double bonds, and auxochromes, electron-withdrawing or electron-donating

substituents that cause or intensify the color of the chromophore by altering the

overall energy of the electron system. Usual chromophores are -C=C-, -C=N-, -

C=O, -N=N-, -NO2 and quinoid rings, while the auxochromes are -NH3, -COOH,

-SO3H and OH groups. Based on chemical structure or chromophore, 20-30

different groups of dyes can be discerned. Each different dye is given a C.I.

(Color Index) generic name determined by its application characteristics and its

color (Abrahart 1977).

The largest class of dyes in the Color index is Acid dyes. Acid dyes are

anionic compounds that find their main application in dyeing nitrogen-containing

fabrics like wool, polyamide, silk and modified acryl. They bind to the cationic

NH4+ ions of those fibres. Most acid dyes are azo, anthraquinone or

52

triarylmethane, azine, xanthene, nitro and nitroso compounds. Rather than the

presence of acid groups (sulphonate, carboxyl) in the molecular structure of these

Reactive dyes are dyes with reactive groups that are capable of forming

a covalent bond between a carbon atom of dye molecule and OH-, NH-, or SH-

groups in fibres (cotton, wool, silk, nylon). The reactive group is often a

heterocyclic aromatic ring substituted with a chloride or fluoride atom, e.g.

Dichlorotriazine. Another common reactive group is vinyl sulphone. Most

(~80%) reactive dyes are azo or metal complex azo compounds but also

anthraquinone and phthalocyanine reactive dyes are applied, especially for green

and blue color. In the Color Index, the reactive dyes form the second largest dye

class. During dyeing with reactive dyes, hydrolysis (i.e. inactivation) of the

reactive groups is an undesired side reaction that lowers the degree of fixation.

Salt and ureum (up to 60 and 200 g L-1respectively) is added during the dyeing

process to increase the degree of fixation.

The reactive dyes are commercially available important class of textile

dyes for which losses through processing operations are significant and the

treatment is problematic. It is estimated that 10 to 50% of the dye will not react

with the fabric and remain hydrolyzed or unfixed form in the water phase. The

problem of colored effluents is therefore mainly due to the use of reactive dyes.

Among acid and reactive dyes, many metal complex dyes can be found

(not listed as a separate category in the Color Index). These are strong complexes

of one metal atom (usually chromium, copper, cobalt or nickel) and one or two

dye molecules, respectively i.e. 1:1 and 1:2 metal complex dyes. Metal complex

dyes are usually azo compounds.

Direct dyes are relatively large molecules with high affinity for

cellulose fibres. Vander Waal forces make them bind to the fibre. Direct dyes are

53

mostly azo dyes with more than one azo bond or phthalocyanine, stilbene or

oxazine compounds. In the Color Index, the direct dyes form the second largest

dye class with respect to the amount of different dyes.

Basic dyes are cationic compounds that are used for dyeing acid-group

containing fibres, usually synthetic fibres like modified polyacryl. They bind to

the acid groups of the fibres. Most basic dyes are diarylmethane, triarylmethane,

anthraquinone or azo compounds.

Mordant dyes are fixed to the fabric by the addition of a mordant, a

chemical that combines with the dye and the fibre. Though mordant dyeing is

probably one of the oldest ways of dyeing, the use of mordant dyes is gradually

decreasing. They are used with wool, leather, silk, paper and modified cellulose

fibres. Most mordant dyes are azo, oxazine or triarylmethane compounds. The

mordants are usually dichromates or chromium complexes.

Disperse dyes are scarcely soluble dyes that penetrate synthetic fibres

(cellulose acetate, polyester, polyamide, acryl, etc.). This diffusion requires

swelling of the fibre, either due to high temperatures (>120 °C) or with the help of

chemical softeners. Dyeing takes place in dyebaths with fine disperse solutions of

these dyes. Disperse dyes form the third largest group of dyes in the Color Index.

They are usually small azo or nitro compounds (yellow to red), anthraquinones

(blue and green) or metal complex azo compounds (all colors).

Pigment dyes (i.e. organic pigments) represent a small fraction of

widely applied group of colorants. These insoluble, non-ionic compounds or

insoluble salts retain their crystalline or particulate structure throughout their

application. Pigment dyeing is achieved from a dispersed aqueous solution and

therefore requires the use of dispersing agents. Pigments are usually used together

with thickeners in print pastes for printing diverse fabrics. Most pigment dyes are

54

azo compounds (yellow, orange, and red) or metal complex phthalocyanines (blue

and green). Also anthraquinone and quinacridone pigment dyes are applied.

Vat dyes are water-insoluble dyes that are particularly and widely used

for dyeing cellulose fibres. The dyeing method is based on the solubility of vat

dyes in their reduced (leuco) form. Reduced with sodium dithionite, the soluble

leuco vat dyes impregnate the fabric. Next, oxidation is applied to bring back the

dye in its insoluble form. Almost all vat dyes are anthraquinones or indigoids.

Anionic dyes and Ingrain dyes (naphthol dyes) are the insoluble

products of a reaction between a coupling component usually naphthols, phenols

or acetoacetylamides and a diazotized aromatic amine. This reaction is carried out

on the fibre. All naphthol dyes are azo compounds.

Sulphur dyes are complex polymeric aromatics with heterocyclic S-

containing rings. Though representing about 15% of the global dye production,

sulphur dyes are not so much used in Western Europe. Dyeing with sulphur dyes

involves reduction and oxidation, comparable to vat dyeing. They are mainly used

for dyeing cellulose fibres.

Solvent dyes (lysochromes) are non-ionic dyes that are used for dyeing

substrates in which they can dissolve, e.g. plastics, varnish, ink, waxes and fats.

They are not often used for textile-processing but their use is increasing. Most

solvent dyes are diazo compounds that underwent some molecular

rearrangement. Also triarylmethane, anthraquinone and phthalocyanine solvent

dyes are applied.

Apart from the dye classes mentioned above, dye intermediates

are highly aromatic compounds with low biodegradability which are formed

as a result of fragmentation of large dye moieties during the process of

degradation. Such organic compounds being toxic when introduced into the

aquatic system cause various health hazards thereby increasing the environmental

55

risks. Various dyes intermediates are found to be chloro or bromo derivatives of

naphthalene or sulphonated, hydroxy, amino or nitro aromatic compounds.

2.4.4 Dyes Used in this Study

Congo red (CR) is a secondary diazo dye that has complex chemical

structure and high solubility in aqueous solution. CR is metabolized to

benzidine, a known human carcinogen and exposure to this dye can cause

some allergic responses. CR mainly occurs in the effluents discharged from

textile, paper, printing, leather industries, etc (Han et al 2008). There are many

processes to remove CR molecules from colored effluents and the treatment

methods can be divided into three categories: (1) physical methods such as

adsorption (Namasivayam & Kavitha, 2002; Mall et al 2005; Chatterjee et al

2009); (2) chemical methods such as ozonation (Gharbani

et al 2008; Khadhraoui et al 2009), photo degradation (Wahi et al 2005) and

electrochemical process (Elahmadi et al 2009); and (3) biodegradation

(Gopinath et al 2009). Adsorption techniques have potential for removing

organics from water due to their high efficiency and ability to separate a wide

range of chemical compounds (Slejko 1985; Suffet & McGurie 1985; Jain &

Sikarwar 2008).

various planar, highly conjugated molecular structures that also contain one or

-

soluble, anionic compounds (Types of Dyes). Their flat shape and their length

enable them to lay along-side cellulose fibers and maximize the Vander Waals,

dipole and hydrogen bonds. Direct dyes are commonly used in the textile

industry. They are advantageous since they are available in powered form,

making them much easier to handle and measure. In addition, they can be

applied directly to textiles without needing a separate adhesion mechanism. A

disadvantage to direct dyes, however, is that it is common for a textile dyed

56

with a direc

treatments that can be used to help keep the direct dyed textiles from losing

their color.

2.4.5 Conventional Methods for Treatment of Textile Dyes

Degradation of dyes in industrial wastewaters has therefore received

increasing attention and some methods of remediation have been preffered.

The conventional methods for removal of color from wastewater are chemical

coagulation, flocculation, chemical oxidation and adsorption. Due to the stability

in waste water they are not totally degraded by conventional treatments.

Moreover textile waste water quality is variable with time and may include

many types of dyes, detergents, sulphide compounds, solvents, heavy metals

and inorganic salts,theamount depends on the kind of process that generates

effluent. The decolorization is a challenge for textile industry as well as for

waste water system. However, these techniques are non-destructive process

which the toxic substances are only removed and transferred. So, the new type

of pollution is rise and further treatment has also provided (Arslan et al

2000; Chaudhuri & Sur 2000; Stock et al 2000).

Adsorption is the most common technique among the conventional

methods used for removal of color from wastewater. The major limitation of this

technique is the disposal of the adsorbent after the adsorption process becomes

difficult as the dyes without any chemical transformations have been transferred

as such from the effluent on to the adsorbent. Hence the adsorbent when disposed

affects the environment. The other problems are that it is an inherently slow

process and its performance is limited by the equilibrium. Chemical separation

occurring either by sorption or bonding mechanism has been shown to be

effective in decolorizing both soluble and insoluble dyes. But the main

disadvantage of this method is the disposal of the sludge generated through the

57

flocculation of the reagent and the dye molecules. The conventional method of

incineration of the sludge results in environmental pollution. The process

though cationic dyes do not coagulate at all, acid, direct, vat, mordant and reactive

dyes do coagulate, but the resulting sludge is of poor quality and does not settle

well, resulting in poor results.

The photo chemical method which uses UV light that produces high

concentrations of hydroxyl radicals and degrades dye molecules to CO2 and H2O

(Yang et al 1998). In this method the rate of dye removal is influenced by the

intensity of the UV radiation, pH, dye structure and the dye bath composition.

Depending on initial components and the extent of the decolorization treatment,

additional by-products, such as, halides, metals, inorganic acids, organic

aldehydes and organic acids, may be produced (Yang et al 1998) which have

adverse effects on the environment. Electrochemical destruction developed in the

mid 1990s is a relatively new technique. It is an effective method for dye removal

as there is little consumption of chemicals and no sludge builds up. The

metabolites produced are generally not hazardous and hence it is safe for treated

wastewaters to be released back into waterways. It is an efficient and economic

method as it results in high efficiency of color removal and degradation of

recalcitrant pollutants (Ogutveren & Kaparal 1994). One of the major drawbacks

of the process is that it requires more electric power.

Membrane filtration method has the ability to produce clarified liquid

and concentrated residue by the separation of dye continuously from effluent. It

has added advantages such as resistant to temperature, an adverse chemical

environment, and microbial attack. A few disadvantages of this method are high

investment and the possibility of clogging, and membrane replacement and the

disposal of the concentrated residue left after separation. Also this method is

suitable for water recycling within a textile dye plant if the effluent contains low

58

concentration of dyes, but it is unable to reduce the dissolved solid content. Ion

exchange has not been widely used for the treatment of dye-containing effluents;

mainly due to the fact that ion exchangers cannot accommodate a wide range of

dyes. Solvents used in ion exchange process are expensive. Now-a-days, the

remediation of several pollution problems is a target of the scientific

researchers. Technical ways of solving environmental concerns and menaces

are available a long time ago, but making them cheaper and sustainable is still

a challenge, especially when searching for natural raw materials as a possible

source that could provide a successful low cost solution (Demirbas 2008;

Pavan et al 2008; Alenor et al 2009; Sanchez-Martin et al 2010).

Apart from this, the ion exchange process is not very effective for dis-

perse and reactive dyes (Mishra & Tripathy 1993). Biological treatment process,

called as biodegradation has overcome the difficulties of physico-chemical

treatment processes. The process makes use of the microorganisms such as

bacteria and fungi, resulting in the break down of complex dye compounds into

simpler molecules such as CO2, H2O, etc and hence does not result in the

production of secondary pollutants. Due to the large degree of organics present

in these molecules and stability of modern textile dyes, conventional biological

treatment methods are ineffective for their decolourisation and degradation

(Stock et al 2000). This led to the study of other effective methods. Recent

studies have demonstrated that photocatalysis can be used to destroy dye

compounds using semiconductor photocatalysts under light irradiation

(Neppolian et al 1999). This also (Sleiman et al 2007; Chakrabarti &

Dutta2004; Silva et al 2006; Sun et al 2006) have been devoted to the use of

photocatalysis in the removal of dyes from wastewaters, particularly, because

of the ability of this method to completely mineralize the target pollutants

(Madhavan et al 2008). Heterogeneous photocatalysis is a process of great

potential for pollutant abatement and waste treatment. In order to improve the

overall performance of the photo process, heterogeneous photocatalysis is

59

being combined with physical or chemical operations, which affect the

chemical kinetics and/or the overall efficiency. This review addresses the

various possibilities to couple heterogeneous photocatalysis with other

technologies to photodegrade organic and inorganic pollutants dissolved in

actual or synthetic aqueous effluents. A survey of literature was carried out to

know the latest advancements in the field of heterogeneous photocatalysis for

the treatment of dyes and textile wastewater.

2.5 SILVER NANOPARTICLES IN REMOVAL OR

BIOSORPTION OF TEXTILE DYES

Amongst the numerous techniques of pollutant removal, adsorption

is an effective and useful process. Adsorption is considered to be the superior

method in comparison with the other techniques of wastewater treatment in

terms of low cost, easy availability, simplicity of sign, high efficiency, simple

operation, biodegradability and the ability to treat dyes in more concentrated

form (Saad et al 2007). Adsorption techniques have gained momentum due to

its efficiency in the removal pollutants. Adsorption produces a high quality

product which is economically feasible. Adsorption technique is popular one

and a generally accepted wastewater treatment method due to advantages such

as simplicity as well as the availability of wide range of adsorbents (Aksu

2005). A successful adsorption process depends on the adsorption performance

of the adsorbents and the constant supply of the materials for the process.

Hazrat Ali et al (2009) have reported 96.91% of decolorization of Malachite

Green by Aspergillus flavus. In this study, when compared with nanoparticle, the

plain culture could degrade only 76% of the dye and this might be due to the

complex nature of the dye. A slower rate of decolorization was attributed to

higher molecular weight, structural complexity and the presence of inhibitory

groups like NO2 and SO3Na in the dyes (Hu & Wu 2001). After decoloration,

the absorbance at wavelength 498 nm decreased is noted in the study. A general

60

decrease of this kind is usually attributed to dye degradation rather than

adsorption (Wesenberg et al 2002). These data show that decolorization was

caused by degradation of dye. Sadowski et al (2008) and Kalishwaralal et al

(2008) reported the nanoparticles synthesized in the size range of 100nm and

50nm and herein we obtained particles in the size range of 200nm. This

preliminary study shows silver nanoparticle can be effectively used for the

decolorization processes. We have made an attempt to decolorize the dyes by

silver nanoparticle synthesized by using seaweed (Ulva lactuca) and its

comparison with its plain seaweeds.

The adsorption technique, which is based on the transfer of pollutants

al

wastewater treatment method (Ghaedi et al 2011; Kismir & Araguz 2011; Ghaedi

2012). The cost effectiveness, availability and adsorptive properties are the main

criteria in selection of an adsorbent to remove organic compounds from

wastewater (Demirbas et al 2008). Performance is dependent on the type of

activated carbon used and the characteristics of the wastewater. Removal rates

can be improved by using massive doses of carbon. Activated carbon, like

many other dye-removal treatments, is well suited for one particular waste

system and ineffective in another. Metallic nanoparticles tend to either react

with surrounding media or agglomerate to result in significant loss of

reactivity (Chen et al 2011). Out of economic consideration, it is not used to

treat a large quantity of effluents by a great scale of adsorbent such as

activated carbon.

Activated carbon has been quite successful for removal of impurities

from exhaust gas and waste water streams. The highly porous nature of the

carbon provides a large surface area for contaminants to get deposited. The

adsorption takes place because of the attractive force between the molecules.

There is wide variety of activated carbons which exhibit different

61

characteristics depending upon the raw materials and the activation techniques

used in their synthesis. Agricultural waste material can effectively be

converted in to activated carbon using straw, leaves, stem, seeds & husk.

Researchers and scientists have been working on synthesis of activated carbon

from agricultural waste materials for removal of dyes from the solution. A

number of research papers have been reported in the literature.

The interest in having more economical adsorbents, which are

relatively abundant and efficient in the removal of specific solutes, has

encouraged the search of low-cost materials. Among those grape bagasse, rice

bran and peat can be emphasized as the natural origin. Bioadsorbents have an

additional advantage which is that the organic load is adsorbed. The activated

carbon associated with the nanoparticles have interesting physicochemical

properties, such as ordered structure with high aspect ratio, ultra-light weight,

high mechanical strength, high electrical conductivity, high thermal

conductivity, metallic or semi-metallic behaviour and high surface area. It is a

common behaviour that nanoparticles easily aggregate due to their magnetic

behaviour and so the particles exist in chain-like manner (Arami

et al 2005; Ardejani et al 2007). Commercial activated carbons are very

expensive; therefore various low cost new adsorbents are being studied by

researchers which are suitable for water pollution control (Selvi et al 2001).

The agricultural waste products are much widely studied for their adsorption

efficiency. These products are readily available and low in cost also. Research

has been done on many materials like sugarcane bagasse, plant waste, leaves,

pomegranate peel, rice hull, saw dust, cotton seed, tea leaves etc. Coal and

straw are inexpensive but ineffective. Peat moss has been found effective in

adsorbing heavy metals. Coconut shell, crop milling waste and biomass also

gave good results. This work deals with the adsorption studies on the activated

carbons prepared from rice husk for the removal of textile dyes from aqueous

solutions.

62

2.6 PHOTOCATALYSIS

Photocatalysis is a rapidly expanding technology for wastewater

treatment. The photocatalytic detoxification has been discussed as an

alternative method for cleaning up of polluted water in the scientific literature

since 1976 (Carey et al 1976). In thisreview (Bhatkhande et al 2002), he

discussed the applications of photocatalytic degradation and chemical effects

of various variables on the rate of degradation of different pollutants. A

growing interest in the purification of water by semiconductor photocatalysis

especially in the removal of toxic organic pollutants is evident by the ever-

increasing number of studies in this area (Inel & Okte 1996; Mathews 1988;

Ollis et al 1989; 1991). A lot of studies have been reported on the

photocatalytic degradation (PCD) of refractory organics. Bhatkhande et al

(2001) reviewed recent works in this area and listed the compounds degraded

by photocatalysis by various researchers. Recent studies (Chakrabarti & Dutta

2004) have been devoted to the use of photocatalysis in the removal of dyes

from wastewaters, because of the ability of this method to completely

mineralize the target pollutants (Madhavan et al 2008). The use of

photochemical technologies has been shown to be a promising alternative for

the detoxification of industrial effluents (Legrini et al 1993), especially from

the environmental point of view (Munoz et al 2005). Herrmann (1999) has

described the basic fundamental principles as well as the influence of the main

parameters like mass of the catalyst, wavelength, initial concentration,

temperature and radiant flux governing the kinetics of heterogeneous

photocatalysis in wastewater treatment.

Heterogeneous photocatalytic treatment is a more attractive

alternative for the removal of soluble organic compounds. Paper, textile,

cosmetics, ink, ceramic, leather and food processing produce a large volume of

dyes in wastewater. These dyes create human health and environmental pollution

63

problems by releasing toxic substances into the aqueous phase, with carcinogenic

potential (Tezcanli-Guyer et al 2003). Photocatalysis has drawn a lot of attention

in environmental application in recent years (Jo 2013). Removal of pollution

from the environment using TiO2 as the photocatalyst is a research topic

focuses TiO2

availability, reusability, as well as the vast reactivity towards wide variety of

pollutants (Nakata & Fujishima 2012). The reaction mechanism is explained

by Kabra et al (2004), when absorbing photons holding energy equaling to or

more than the band gap of TiO2, the electron will transit from its valence band

to the conduction band, resulting in the separation of electrons and holes. The

photogenerated holes can either oxidize the organic pollutants directly, or react

with H2O molecules to form OH radicals, which show a strong oxidizability

towards organic chemicals almost unselectively (Colmenares et al 2009).

Most of the photocatalytic studies use either synthetic or

commercial TiO2 as the photocatalyst (Grzechulska & Morawski 2002).

Titanium dioxide is known to be the semiconductor with the highest

photocatalytic activity. It has the advantage of being non-toxic, relatively

inexpensive and stable in aqueous solution. Several reviews have been written

regarding the mechanistic and kinetic details as well as the influence of

experimental parameters (Rauf & Ashraf 2009; Akpan & Hameed 2009;

Wojciech Baran et al 2008) and it has also been demonstrated that degradation

by photocatalysis can be more efficient than by other wet-oxidation

techniques. Heterogeneous photocatalysis processes are the most efficient

methods for destroying organic pollutants in aqueous media (Andreozzi et al

1999). Recently, titanium dioxide is considered as an excellent inorganic

photocatalyst in the advanced oxidation process for degradation of a number of

different dyes by TiO2 under UV irradiation (Yamazaki et al 2001). In the present

study, the photocatalytic degradation of textile dyes by TiO2 photocatalytic system

64

was evaluated under the solar light by measuring the degradation rate of dye

aqueous solutions. Sunlight mediated photocatalysis of Reactive Blue 4 was

reported by Neppolian and his coworkers (Neppolian et al 2001) in a slurry

reactor. Titanium dioxide (TiO2) has been intensively investigated as a

photocatalyst since Fujishima and Honda discovered the photocatalytic

splitting of water on TiO2 electrodes in 1972. Recently, the application of TiO2

as a photocatalyst has mainly focused on the elimination of toxic and

hazardous organic substances and metal ions (Ohko et al 2001; Fox & Dulay

1993) in wastewater, drinking water, and air in view of environmental

protection (Hofmann et al 1995; Fujishima et al 2000; Legrini et al 1993).

There are certain distinct advantages such as chemical stability, low toxicity,

and low cost, when TiO2 is used as a photocatalyst.

Photocatalysis processes based on semiconductors are utilized for

purification and disinfection of air and water (Zhiyong et al 2008). This is a vital

subject studied in the recent years (Ji-Chuan Xu et al 2004). Metal oxide

semiconductors have been drawn much attention for a variety of practical

applications (Fuchs et al 2009). Among various semiconductors, titanium dioxide

technique has become a highly promising technology for the removal of the

pollutant from wastewater (Wan-Kuen Jo & Jong-Tae Kim 2009). Titanic is a

very useful photo catalyst because this has considerable properties such as: high

activity (Samari Jahromi et al 2009), chemical and biological internees (LiuWei et

al 2009), commercial availability etc (Xiao Q et al 2009). From the above said

properties, TiO2 is applicable in many area, for example solar cell (Xiwang et al

2009) biosensors, semiconductors electrode, UV radiation inhibitor (Shi et al

2008) etc. Photocatalytic activity is a remarkable function of titania (Kavitha et al

2007). Brookite, anatase and rutile are different structures of titanium dioxide

where reported in this photocatalytic processes (Yali li et al 2002).

65

TiO2 is generally used as a photocatalyst for the removal of organic

pollutants and dye pollutants (So et al 2002) due to its high photocatalytic activity

and stability, low cost, stability with respect to corrosion, and biological, nontoxic

and chemical inertness. The photocatalytic activity of TiO2 started with UV light

irradiation. The first step of this reaction is to begin by establishing the electron

hole pairs under appropriate illumination (Guosheng et al 2008). Generation of

electron hole pairs leads to hydroxyl (OH) and super oxide radicals on the surface

of TiO2 (Rengifo-Herrera et al 2008). Among the different structure of titanium

dioxide, anatase appears to be the most active form in photocatalytic process. The

band gap of anatase is 3.2 ev and for rutile is 3.1 ev. The efficiency from

photocatalytic activity of TiO2 depends on the crystalline structure originated

from synthetic process of TiO2 (Jing Yang et al 2008).

Doping of TiO2 with metal or non metal decrease this recombination

recently. Resulting deposition element into titanic matrix increases whole

concentration, down bond gap energy to visible light and cause the suitable

photoactive system (Ying Yang et al 2004). Titanium dioxide is commonly

benefited in two forms: powder (suspension) and immobilize (film). Using of

suspension has several problems. Filtration and retrieval stage are time consuming

and costliness process for the suspension forms. While fixing and immobilizing

the TiO2 on various substrates solve these problems and optimize their catalytic

operation (Grieken et al 2002). Various techniques adopted for synthesizing

immobilizing titanic. Some of these are chemical vapor deposition (CVD)

(Bessergener et al 2002), sputtering, sol gel (Periro et al 2002) and

electrochemical process (Iwata et al 2003).

Electrochemical process is the best method because it possesses special

advantageous. The benefits are: Low cost, suitable process temperature, using low

energy, use of inexpensive solution, controls the condition of reaction, use of the

66

& Maria

Grzeszczuk 2009). Recently immobilizing TiO2 nanoparticles on metal Ti

substrates have done by electrochemical technique. Immobilizing reaction is

carried out in percents of DC current, electrolyte solution and excursuses voltage.

Changing the voltage can form different size of nanoparticles and resulting in

various colours on the substrates. Heterogeneous photocatalysis is a promising

method among advanced oxidation processes (AOPs), which can be used for

degradation of various organic pollutants in water and wastewater (Behnajady et

al 2006; Daneshvar et al 2004; Behnajady et al 2007a,b). In photocatalysis

system a combination of semiconductors (such as TiO2, ZnO, F2O3, CdS and

ZnS) and UV or visible light can be used. Upon irradiation, valence band

electrons are promoted to the conduction band leaving a hole behind. These

electron-hole pairs can either recombine or interact separately with other

molecules. The holes at the valence bond, having an oxidation potential of +2.6 V

versus normal hydrogen electrode (NHE) at pH=7, can oxidize water or

hydroxide to produce hydroxyl radicals (Behnajady et al 2006).The limitation of

the rate of photocatalytic degradation is attributed to the recombination of

photogenerated electron-hole (e--h+) pairs. Among the noble metals, silver ions

have attracted interests of several researchers, because of their novel effects on the

improvement of photoactivity of semiconductor photocatalysis (Sobana et al

2006; Mirkhani et al 2009). Silver can be doped with TiO2 using photodeposition

process chemical reduction of silver ion on TiO2 nanoparticles (Whang et al

2009), thermal deposition and pulsed laser deposition (Wang et al 2008).

Various attempts have been made to reduce e--h+ recombination in

photocatalytic processes. These studies include doping metal ions into the TiO2

lattice (Sakthivel et al 2004; Subba Rao et al 2003) and coupling semiconductors

(Wu 2004). Sakthivel et al (2004) have showed that the photonic efficiency of Pt

deposited on TiO2 is almost comparable with Au/TiO2 but higher than Pd/TiO2. In

67

this work, the highest photonic efficiency was observed with metal deposition

level of less then 1 wt%. Kondo and Jardim (1991) have showed the Ag-loaded

TiO2 is more effective than the pure TiO2 in the degradation of chloroform and

urea. Also, Sahoo et al (2005) found that Ag+ doped TiO2 is slightly more

efficient than the undoped TiO2 in the photocatalytic degradation of C.I. Basic

Violet 3. From the literature survey, the heterogenous photocatalysis with

titaniumdioxide as a semiconductor has proven to be an efficient process for

elimination of a large number of dyes from textile wastewater (Konstantinou

& Albanis 2004), which are difficult to degrade by conventional activated

sludge systems or standard chemical methods. Metal-doped titanium dioxide

has been used for the degradation of many organic pollutants in general (Gaya

& Abdullah 2008) and for phenols in particular (Wang et al 2000; Xu et al

2003). They are very toxic and poorly biodegradable compounds that are not

effectively degraded by direct biological methods (Xu et al 2003).

Consequently, their elimination in wastewaters and drinking water is of great

interest. Pandurangan et al (2001) carried out the PCD of basic yellow

Auramine O in a batch reactor, using ZnO exposed to solar radiation. The

process followed pseudo-first order kinetics. The rate constant decreased with

increase in initial dye concentration. A lower pH was more suitable. The

was indicated by a reduction of COD of the solution. Degradation of Acid

Green 16 was studied by Sakthivel et al (1999), using ZnO irradiated with

sunlight. Here also the photodegradation efficiency decreased with an increase

in initial dye concentration.

2.7 HPLC ANALYSIS

High Performance Liquid Chromatography (HPLC) is a powerful

chromatographic technique used to achieve separation easily. HPLC is a

68

chemistry based tool for quantifying and analyzing mixtures of chemical

compounds in solution. The vital part of HPLC includes the stationary phase,

mobile phase and a detector. A successful separation is achieved when a

suitable balance is established between the intermolecular forces involving the

sample, the mobile phase and the stationary phase (Wallace 2001). Some

researchers have succeeded in detecting the degradation products of textile

dyes by using HPLC.

Weber and Morris had presented intraparticle diffusion (IPD) model

in 1962. Of the total 57 articles surveyed, IPD model has been applied in three

different forms: (1) qt (the amount of adsorption at any time) is plotted against

t1/2 (the square root of time) to get a straight line that is forced to pass through

the origin (Zhang et al 2007; Wu et al 1998); (2) multi-linearity in qt vs t1/2

plot is considered (that is, two or three steps are involved to follow the whole

process). In this form, the external surface adsorption or instantaneous

adsorption occurs in the first step; the second step is the gradual adsorption

step, where intraparticle diffusion is controlled; and the third step is the final

equilibrium step, where the solute moves slowly from larger pores to

micropores causing a slow adsorption rate. The time required for the second

step usually depends on the variations of the system (including solute

concentration, temperature, and adsorbent particle size), which is difficult to

be predicted or controlled; qt is plotted against t1/2 to obtain a straight line but

does not necessarily pass through the origin; that is, there is an intercept.

Almost all the intercepts reported in the literature are positive, indicating that

rapid adsorption occurs within a shortperiod of time. To our knowledge, this

type of initial adsorption behavior has never been reported.

Photocatalysis process is a vital subject and more attention was paid

in the past few years. Metal oxide semiconductors have been said much

69

attention for a variety of practical applications. Among various

semiconductors, titanium dioxide has become a highly promising technology

for removal of the pollutant from water and environment. Titanic is a very

useful photocatalyst because this has considerable properties such as: high

activity, chemical and biological internees, commercial availability etc. From

this important property, TiO2 is applicable in many areas, for example solar

cell, biosensors, etc. In this thesis, an attempt has been made to develop a

simple but highly active and environment friendly process for the treatment of

industrial waste water which is expected to have a greater impact on the

economic development of the country.

In the present work, TiO2- doped with low concentration of Ag

using AgNO3 as a procedure and possesses higher activity under solar

irradiation. We will present the details of the TiO2 loading procedure. The use

of XRD gives detailed information on the surface of catalyst layer, there after

its activation is evaluated by the photodegradation of textile dyes as a model

for water pollution. The dependence of photocatalytic reaction was examined

on the following parameters: initial concentration of textile dyes, concentration

of Ag etc. It has been demonstrated that photocatalytic degradation rate

depends on photocatalyst pollutant molecules interactions and good

adsorption of pollutant molecule can improve the efficiency of photocatalytic

degradation (Tryba et al 2003; Zhang et al 2006; Zhao et al 2004).