SHANGHAI UNIVERSITY SCHOOL OF ENVIRONMENTAL AND CHEMICAL ENGINEERING

上海大学环化学院

REVIEW AND RESEARCH PROPOSAL FOR MASTER’S THESIS

APPLICATION OF RADIATION TECHNOLOGY

FOR THE TREATMENT OF TOXIC

AND RECALCITRANT POLLUTANTS IN WATER.

辐射技术在处理水中的难降解有毒物污染物的应用

STUDENT (学生): ADELEKE Olukunle Francis 奥陆克 kadeleke@yahoo.com

02761281

MAJOR (专业): Environmental Engineering 环境工程

SUPERVISOR (导师): Zhou Ruimin (周瑞敏)

MAY 2003 (2003 年 5 月) Shanghai, People’s Republic of China (中国上海)

1. INTRODUCTION As a result of increasing industrialization throughout the world, especially in the last few decades, tens of thousands of new chemicals are being produced annually. These may be contained in the water discharges of manufacturing and industrial plants. In addition to this, increased use of various pesticides and chemical fertilizers in agriculture contributes to the contamination of groundwater. At the same time, the rapid growth of world population places a lot of pressure on the limited water resources. Over the past few decades, environmental regulatory requirements have become more stringent because of increased awareness of the human health and ecological risks associated with environmental contaminants. Some of these chemicals are toxic to aquatic life and may lead to reduction in the number of aquatic species or complete destruction of aquatic life in extreme cases. Some chemicals also bioaccumulate, i.e. they become more concentrated as they move up the food chain, posing toxicity problems even at very low concentrations. The effect of some of the chemicals on human health includes increased risk of cancer, neurological or reproductive effects, and even mortality. Conventional biological treatment methods are usually inadequate for the treatment of the toxic pollutants in water. Some of them are recalcitrant (non biodegradable) and others may be even toxic to the microorganisms responsible for the biodegradation. Thus other treatment methods including carbon adsorption, air stripping, chemical oxidation with ozone (O3) and hydrogen peroxide (H2O2) are often used to pretreat chemical wastes before biological treatment. However, these methods also have their limitations. For example, air stripping and carbon adsorption merely transfer contaminants from one medium to another, whereas biological treatment and conventional chemical oxidation have low removal rates for many environmental contaminants, including chlorinated organics.

Therefore, various alternative treatment technologies have been developed over the last 10 to 20 years in order to cost-effectively meet environmental regulatory requirements. One such group of technologies is commonly referred to as advanced oxidation processes (AOPs). Advanced oxidation processes generally involve generation and use of powerful but relatively nonselective transient oxidizing species, primarily the hydroxyl radical (*OH). The *OH has a very high oxidation potential compared to other oxidants including ozone and hydrogen peroxide. *OH can be generated by both photochemical processes (for example, ultraviolet [UV] radiation in combination with O3, H2O2, or a photosensitizer) and non-photochemical processes (for example, electron beam irradiation, O3, in combination with H2O2, or Fenton’s reagent).

Over the past years ionizing radiation processes have emerged as commercial technologies in many industrial fields; well known are their applications for medical supplies sterilization, foodstuffs treatment (decontamination, disinfestation, shelf-life

extension, etc.), crosslinking of wires, tires, plastics and foams, curing of video tapes, paper, wood panels, etc. However, environmental applications of irradiation technologies are comparatively recent. The earliest application is the disinfection of sewage sludge from municipal wastewater treatment plants, with the purpose of land application as a fertilizer and soil conditioner. The other possible environmental applications include the removal of SO2 and NOx from flue gases and the decomposition of toxic pollutants in water and wastewater.

The main focus of this paper is the use of ionizing radiation (mainly γ radiation and electron beam radiation) on the decomposition of toxic organic pollutants in water.

2. TOXIC POLLUTANTS IN WATER

Toxic pollutants, which are mainly from industrial wastewaters, are organic chemicals and metals, which are known environmental hazards and are also present in polluted waters. The primary concern is that even though this type of toxic pollutant may be discharged at levels that are non-toxic in the receiving waters they may accumulate to toxic levels in sediment or aquatic life through bioaccumulation. Some are also persistent in the environment. Toxic pollutants are primarily grouped into organics (including pesticides, solvents, polychlorinated biphenyls (PCBs), and dioxins) and metals (including lead, silver, mercury, copper, chromium, zinc, nickel, and cadmium). The majority of the toxic pollutants can be categorized into ten groups (Viessner and Hammer, 1993): Halogenated Aliphatics are used in fire extinguishers, refrigerants, propellants, pesticides and solvents. Health effects include damage to central nervous system and liver. Phenols are industrial compounds used primarily in production of synthetic polymers, pigments, and pesticides and occur naturally in fossil fuels. They impart objectionable taste and odor at very low concentrations, taint fish flesh, and vary in toxicity depending on chlorination of the phenolic molecule. Monocyclic aromatics (excluding phenols and phthalates) are used in the manufacture of chemicals, explosives, dyes, fungicides, and herbicides. These compounds are central nervous system depressants and can cause damage to liver and kidneys. Ethers are solvents for polymer plastics. They are suspected carcinogens and aquatic toxins. Nitrosamines, used in production of organic chemicals and rubber, are suspected carcinogens. Phthalate esters are used in production of polyvinyl chloride (PVC) and thermoplastics. They are aquatic toxin and can be biomagnified. Polycyclic aromatic hydrocarbons (PAHs) are in pesticides, herbicides, and petroleum products including a family of semi-volatile organic pollutants such as naphthalene,

anthracene, pyrene, and benzo(a)pyrene. Pesticides of concern are those that biomagnify in the food chain and are persistent in nature; common in this group of pesticides are chlorinated hydrocarbons including DDT, Aldrin, Chlordane, Endosulfan, Endrin, Heptachlor, and Diazinon. Poly chlorinated biphenyls (PCBs) are extremely persistent in the environment and has been proven to bioconcentrate in the food chain. PCBs were used in electric capacitors and transformers, paints, plastics, insecticides, and other industrial products. Heavy metals vary in toxicity, and some are subject to biomagnifications. They include arsenic, cadmium, chromium, lead mercury and zinc.

Priority pollutants are a subset of the toxic pollutants defined in the Clean Water Act (CWA) of the United States Environmental Protection Agency (USEPA). These 126 pollutants were assigned a high priority for development of water quality criteria and effluent limitation guidelines because they are frequently found in wastewater. Priority pollutants are hazardous or radioactive organic and inorganic chemicals present in an environmental setting, such as air, water, or vegetation. The Integrated Water Discharge Standard of China (GB 8978-1996) also gave a maximum discharge concentration of some priority organic pollutants. These include AOX, BTEX, chlorinated benzenes, phenols, chlorophenols, phtalate esthers, trichloroethylene, tetrachloroethylene, etc.

3. WASTEWATER TREATMENT METHODS Wastewater treatment methods can be generally divided into preliminary (or pretreatment), primary, secondary and tertiary. Conventional water and wastewater treatment processes have been long established in removing many chemical and microbial contaminants of concern to public health and environment. A conventional municipal wastewater treatment consists of preliminary processes (pumping, screening and grit removal), primary settling and secondary biological aeration to metabolize and flocculate colloidal and dissolved organics. Waste sludge drawn from these unit operations is thickened and processed for ultimate disposal. However in recent years, the use of conventional water and wastewater treatment processes becomes increasingly challenged with the identification of more and more contaminants, rapid growth of population and industrial activities, and diminishing availability of water resources (Zhou and Smith, 2002).

Treatment of industrial wastewater is achieved by physical, chemical and biological means. Often, a combination of these processes is employed for treatment in order to take advantage of the properties of each technique. Each wastewater is unique in its combination of characteristics of composition and flow. These characteristics determine the combination of treatment processes required to achieve satisfactory treatment. This

integrated system normally comprises preliminary and primary treatment by physical processes followed by secondary biological treatment similar to those used in a conventional sewage treatment works. Additional pre-treatment and tertiary treatment processes are used according to the characteristics of the particular industrial effluent.

The main methods of treating industrial wastewater containing toxic pollutants are: (1) they may be treated separately in an industrial waste treatment plant prior to discharge to watercourse, (2) raw wastewaters may be discharged directly to municipal sewers for complete treatment at municipal treatment plants (referred to as publicly-owned treatment works, or POTW), or (3) industrial waste can be pretreated at the industrial site to reduce the amount of pollutant present and then discharged into the municipal sewer for final treatment at the POTW. 3.1 BIOLOGICAL TREATMENT Primary and secondary treatment processes in municipal treatment plants handle most of the nontoxic wastewaters; other wastes have to be pretreated before being added to municipal sewers. Primary treatment prepares the wastewater for biological treatment. Wastewater treatment usually begins with preliminary treatment by physical means in order to remove large or dense solids and floating material, which may damage mechanical equipment or cause blockages. Large solids are removed by screening, and grit, if present is allowed to settle out. Equalization, in a mixing basin, levels out the hour-to-hour variations in flows and concentrations. Neutralization, when required, follows equalization because streams of different pH partly neutralize each mixed. Primary sedimentation in a sedimentation tank removes both settleable and floatable solids.

Secondary treatment is the biological degradation of soluble organic compounds in solution or suspension by microorganisms until the BOD of the water has been reduced to acceptable levels. This is usually done aerobically, in the presence of added oxygen in open vessel or lagoon, but wastewaters may be pretreated anaerobically in a pond or closed vessel. After biotreatment, the microorganisms and other carried-over solids are allowed to settle in a secondary sedimentation tank. A fraction of this sludge is recycled in certain processes, but ultimately the excess sludge, along with sedimented solids, has to be disposed of. The most commonly used aerobic biological processes are the activated sludge process, (2) aerated lagoons, (3) trickling filters, (4) rotating biological contactors, and (5) stabilization ponds. Anaerobic decomposition involves the breakdown of organic wastes to gas (methane and carbon dioxide) in the absence of oxygen. The common types of anaerobic processes are anaerobic filter reactor, anaerobic contact process, fluidized-bed reactor (FBR), upflow anaerobic sludge blanket (UASB) process and the ADI-BVF process.

Applications of BOD and COD Testing BOD is the most common parameter for defining the strengths of untreated and treated municipal and biodegradable industrial wastes. The oxygen requirement and tank sizing for aerobic treatment processes are based on BOD loadings. It is also used in quantifying the quality of an effluent from a treatment plant to determine if the treated water meets discharge standards. COD is commonly used to define the strength of industrial wastewaters that are either not readily biodegradable or contain certain compounds that inhibit biological activities. Unlike the BOD test, which requires 5 days, the COD test has the advantage of rapid analysis and reproducible results. The relationship between BOD and COD concentrations must be defined for each individual wastewater. The ratio BOD/COD gives an indication of the fraction of pollutants in the wastewater that is biodegradable. 3.2 PHYSICAL AND CHEMICAL TREATMENT Physical and chemical processes are often used for the pretreatment of wastewaters containing toxic chemicals and also for tertiary treatment of treated wastewater containing non-biodegradable chemicals after biological treatment. In-plant pretreatment is necessary for streams rich in heavy metals, pesticides, and other substances that would pass through primary treatment and inhibit biological treatment. It is also better for low-volume streams rich in nondegradable materials, because it is easier and much less costly to remove a specific pollutant from a small, concentrated stream than from a large dilute one. Processes for in-plant treatment include precipitation, activated carbon adsorption, chemical oxidation, air or steam stripping, ion exchange, reverse osmosis, electrodialysis, and wet air oxidation. Tertiary or advanced treatment processes are added on after biological treatment in order to remove specific types of residuals. Filtration removes suspended or colloidal solids; adsorption by granulated activated carbon (GAC) removes organics; and chemical oxidation also removes organics. The treatment methods that are used for removal of toxic or recalcitrant organic pollutants are discussed below. Air Stripping Volatile organic compounds (VOCs) can be removed by desorption into air (or stripping). This is a physical process, which relies on the VOC having a low solubility in water. The wastewater is aerated to give a large contact area between water and liquid. This can be accomplished by injection of air into water by diffused or mechanical aeration systems in aeration basins or in specially designed packed towers, where a counter current of air passes over the water as it falls through a packed tower. Because of their volatility, VOCs are rarely found in surface waters (Viessman and Hammer, 1993). They might however be found in contaminated ground waters. The compounds include vinyl chloride,

1,2-dichloroethane, benzene, carbon tetrachloride, trichloroethylene, etc.

Carbon Adsorption Adsorption is used for the treatment of toxic or recalcitrant wastewaters or for tertiary treatment following biological oxidation. Adsorption consists of transferring the contaminant from the aqueous waste phase to the surfaces of an adsorbent material. The most commonly used adsorbent is activated carbon because of its large surface area, ability to adsorb a wide variety of compounds and its low cost compared to other comparable adsorbent. Activated carbons are made from a variety of materials including wood, lignin, bituminous coal, lignite, and petroleum residues (Eckenfelder, 2000). In most cases, it is used in granular form (granulated activated carbon, GAC) and is enclosed in a packed bed or fluidized bed reactor. In a few cases, it is used in powdered form, mixed with wastewater and then removed by filtration (Bishop, 2000). Powdered activated carbon (PAC) can also be combined with biological treatment. A variety of organic compounds adsorb to the surface of GAC including compounds, which cause taste and odor problems, industrial organic compounds, pesticides and herbicides. Adsorption is not a steady-state process. As solutes accumulate, the amount of media available for adsorbance diminishes until there is no removal of solutes or the effluent concentration increases to unacceptable values. Thus there is need for regeneration of the media before this deterioration in treatment. Chemical Oxidation Chemical oxidation is the process in which one or more electrons are transferred from the chemical being oxidized to the chemical initiating the transfer (the oxidizing agent). Oxidation of a substance results in loss of electron from the substance whereas reduction of a substance results in gain of electron in the substance. Pollutants are often oxidized or reduced to products that are less toxic, more readily biodegradable, or readily removed by adsorption. The common oxidizing agents are ozone, hydrogen peroxide, chlorine and potassium permanganate. They are currently used to oxidize cyanides, sulfides, phenols, pesticides and some other organic compounds (Bishop, 2000). However, recent concerns regarding the formation of toxic chlorinated by-products, such as chloroform has greatly limited the use of chlorine in the oxidation of organics for wastewater treatment. Hydrothermal Processes These refer to aqueous treatment of wastes at elevated temperatures and pressures. Wet air oxidation (WAO) This is a commonly used process for toxic and/or biologically refractory concentrated wastes. The process uses an oxidant, primarily O2 from air, to partially oxidize organics, yielding a variety of low-molecular weight organic acids that are readily biodegradable. Usual temperatures range from 150 to 320oC and pressures from 1.0 to 20.7 Mpa (Eckenfelder, 2000).

Supercritical Water oxidation (SCWO) Oxygenation of wastewater and increasing the temperature and pressure to above the critical point of water (374oC and 218atm) allows complete oxidation of organics to quickly occur, producing inorganic salts. The salts are nearly insoluble in supercritical water and precipitate out. The heat produced by the oxidation helps to maintain the reactor temperature. Typical operating conditions are 400 to 650oC and 24.1 to 34.5 Mpa (Eckenfelder, 2000). Upon depressurization, cooling, and vapor-liquid separation, the water should be essentially free of contaminants. Pilot-scale studies have indicated destruction efficiencies greater than 99.99 percent but the process is not yet in commercial operation for waste treatment (Bishop, 2000). Advanced Oxidation Processes (AOPS) Advanced oxidation processes generally involve generation and use of powerful but relatively nonselective transient oxidizing species, primarily the hydroxyl radical (.OH). Table 1 shows the oxidation potential of different oxidants commonly used for the treatment of pollutants in water. Table 1. Oxidation Potential of several oxidants in water. Oxidant Oxidation Potential (Eo) (V) Hydroxyl radical (.OH) O3 H2O2 Perhydroxy radical Permanganate ion Chlorine dioxide Chlorine Oxygen

2.80 2.07 1. 77 1.70 1. 67 1. 60 1. 38 1.23

Source : USEPA From the table, it can be seen that .OH has the highest thermodynamic oxidation potential. In addition, most environmental contaminants react 1 million to 1 billion times faster with .OH than with O3, a conventional oxidant (USEPA, 1998). .OH can be generated by both photochemical processes (for example, ultraviolet [UV] radiation in combination with O3, H2O2, or a photosensitizer); and nonphotochemical processes (for example, electron beam irradiation, O3, in combination with H2O2, or Fenton’s reagent). From laboratory testing and very limited industrial applications (Zhou and Smith), it is believed that AOPs offer several distinct advantages over conventional treatment processes because (i) they are very effective at removing resistant organic compounds, (ii) they are capable of complete mineralization of organic contaminant into carbon dioxide if desired, (iii) they are less susceptible to the presence of toxic chemicals, and (iv) they produce less harmful by-products. Because hydroxyl radicals are very unstable in water,

the use of AOPs can lower the effective disinfectant concentration. Thus, AOPs should offer few benefits for microbial disinfection. In addition, there is little evidence that the complete mineralization of organic compounds is either necessary or economically practical. Nevertheless, they could still be very useful by integrating with other treatment processes. Along with biological oxidation, for example, AOPs can be used as a pretreatment process for the partial oxidation of organic compounds that are either too toxic or refractory to biodegradation. The common AOPs currently used in water and wastewater treatment include O3–H2O2 process, O3–UV process, H2O2–UV process, and photo-Fenton reaction. They are mainly used for the oxidation of refractory contaminants in water and disinfection purposes.

The use of ionizing radiations (especially electron beam irradiation) for generation of highly reactive radicals including .OH is also generally regarded as an AOP process. However, the practical application of these processes is still limited at this stage. Compared with reactions initiated by ultraviolet light, ionizing radiation offers the advantage of greater penetrating power so that more uniform reactions takes place at larger volume of reactant, avoiding the build-up on the walls of the reaction vessel. The use of metal rather than glass vessel is possible and reaction can be initiated in media that are not transparent to UV light. Selective decomposition of products at particular wavelength is also excluded. However, capital and operation costs of ionizing radiation are higher and the gamma-radiation sources must be replaced at intervals to maintain uniform source strength. Ionizing radiation is also less specific in its action and if prolonged, may bring about secondary changes in product. It is also dose-rate dependent, especially for chain reactions and this may make it difficult to increase output by simply increasing the radiation dosage rate (Woods and Pikaev, 1993).

A detailed study compared three advanced oxidation technologies: UV/H2O2, UV/TiO2, and e-beam, for their cost-effectiveness in two benchmark tasks: bleaching of methylene blue and the decay of phenol. The e-beam technology was dramatically superior, because of the poor quantum yield (~5%) for UV, and because of the necessity in the other technologies to continuously modulate H2O2 concentration with temperature and contaminant load and to regenerate the TiO2. 4. ENVIRONMENTAL APPLICATIONS OF RADIATION TECHNOLOGY The main environmental applications of radiation technology in waste management include flue gas irradiation, disinfection of wastewater and sewage sludge, and decontamination of toxic chemicals in water and wastewaters (Woods and Pikaev, 1993). Sources of ionizing radiation The most widely used commercial sources of ionizing radiation at the present time are cobalt-60 for gamma (γ) irradiation, and electron accelerators for electron-beam (EB)

irradiation. Theγsources require substantial radiation shielding and are generally housed in concrete cells with walls several feet thick; they are used mainly to sterilize medical products and for food irradiation, applications where the greater penetration ofγradiation is an advantage. Electron accelerators are generally the preferred radiation source for environmental applications (Woods, 1998). Electron accelerators are designed in a variety of forms and can be designed to produce electron beams with electron energies ranging from 0.15 to 10 MeV for commercial applications. Beam power (the product of electron energy and beam current range from 5 to 300kW. Electrons are less penetrating thanγ-radiation, but they can be more easily tailored to the application by controlling energy of the electrons, and the radiation source can be easily turned off by simply turning off the electric power. The lower-energy EB generators may also be self-shielding. Useful Definitions and Cost The absorbed dose is the energy transferred from the incident radiation to the material being irradiated. It is responsible for producing ions and excited species in the irradiated material which than induce radiation-induced chemical and biological changes in the material. The SI unit is joule per kilogram (J/kg) called gray (symbol Gy). Another commonly used unit is rad (1 rad = 0.01 Gy) and sometimes the electronvolt per gram (eV/g). The absorbed dose rate is the absorbed dose per unit time, and the unit is Gy/s. The radiation chemical yield describes the yield of chemical products produced on irradiation. It is expressed as the quantity of product divided by the absorbed dose. It is reported in term of G values, where G(X) and G(-Y) are the number of molecules of product X formed, or of starting material Y changed respectively, per 100 eV of energy absorbed. The SI unit is in μmol/J (1μmol/J = 9.649 molecules/100 eV).

Only the absorbed radiation can initiate physical, chemical or biological effects. The energy absorption in a medium e.g. water takes place in 10-15 s, in the course of which the dose distribution is not uniform, because of electron scattering and “build-up” effects occurring during interaction between radiation and matter. As radiation passes through a material, energy is lost at a rate depending on the stopping power of the material being irradiated and the energy of the radiation source. The dose distribution gradients, illustrated by depth-dose curves can be used to determine the dose distribution within the irradiated material. The shape of the depth-dose curve in a material is a function of electron energy, absorber density and incidence angle of radiation. γrays have more deeper penetration in water than electrons (Getoff, 1996). Radiation processing cost and annual output of an irradiation facility are related to the adsorbed dose required, D by the equations (Woods, 1998)

Processing cost = 2.78 x 10-7 SD

$/kg (1) f

Where S $/kWh is the cost of the radiation energy in dollars per kilowatt-hour and f is the

fraction of the available radiation energy absorbed by the material being irradiated. The annual output in tons (t) material processed per year is given by

Annual output = 3.16 x 107 Tf

t/yr (2) D

Where T kW is the beam power. Flue gas irradiation Irradiation of flue gases is done to reduce or eliminate the emission of sulfur and nitrogen oxides by coal-fired power stations and industrial plants. When electron-beam process is used to clean the flue gas, the gas passes through an evaporative spray cooler where the gas temperature is decreased, as the humidity is increased. The gas then passes to a process vessel, where it is irradiated by a beam of high-energy electrons in the presence of ammonia. Under irradiation, SO2 and NOx gases are oxidized to form H2SO4 and HNO3 respectively. These acids subsequently react with the ammonia to form ammonium sulfate and ammonium nitrate, which are recovered as dry powder using conventional particle collector. The collected powder can be used as agricultural fertilizer (Salimov et. al, 1998). The procedure with the addition of ammonia is known as the “Ebara process” because of the pilot plant constructed by the Ebara Co. in Japan (1974-1977) to assess the technical feasibility of the process by the pioneering Japanese scientists and engineers. Large industrial demonstration plants utilizing the Ebara process were installed in 1985 in Indianapolis, USA and Karlsruhe, Germany. Both plants operated successfully and were then decommissioned (Woods, 1998).

Also in China, the main environmental application of ionizing radiation is the electron-beam flue gas treatment (EFGT). The emission of coal-fired boilers containing large amounts of SO2 and NOx gases is a serious air pollution problem in China. Experiments to remove SO2 and NOx by EFGT method were started in 1984 at the Shanghai Institute of Nuclear Research (SINR). Recently some universities and power plants have been involved in the development of this technology. A model plant based on the Ebara process was set up for the removal of the SO2 and NOx gases generated by the Chengdu Electric Power Plant. A pilot plant was also set up in Sichuan in the same year at a coal-fired electrical power station. Tests results from the plants and research institutes have given satisfactory results (Lin, 2001). Sludge Disinfection. Municipal sewage sludge typically contains potential harmful infectious organisms (viruses, bacteria, parasites), heavy metals and chemicals. It also contains nitrogen, phosphorus and other nutrients beneficial to plant growth. Gamma irradiator from cobalt- 60 is typically used for irradiation disinfection of sludge. The gamma rays pass through the sludge killing microorganisms and parasites leaving no residue in the sludge. The

irradiation process will not change moisture content, or the levels of nutrients or heavy metals; it is solely used to eliminate the pathogens. Disinfected sludge can be safely recycled for use as a fertilizer or soil conditioner. Because it is organically based, sludge products offer long-term soil improvements compared to chemical fertilizers. The use of irradiation systems to disinfect liquid sludges prior to application on farmland has been successfully adopted in Germany, India, Italy and Canada (Swinwood et. al., 1994). At the India’s Sludge Research Irradiator (SHRI), the digested/undigested sludge is first passed into a silo and a measured volume is fed into the irradiation vessel containing cobalt 60 gamma irradiator. A pump then circulates the sludge for a predetermined duration to prevent settling and impart the desired dose. After irradiation, the sludge is drained into a storage tank from where it is pumped to drying beds. The disinfected and dried bed has been used as fertilizer at the SHRI facility’s garden (Swinwood et. al., 1994).

Sludge irradiation using high-energy electron beams has also been demonstrated to be a practical and economic method of disinfecting liquid municipal wastes and sludges. Research has shown that sludges can be successfully disinfected by EB irradiation, which also often accelerates sedimentation and filtration, facilitating dewatering (Woods, 1998). Research using high-energy EB has been carried out in Boston, USA, and Takasaki, Japan. Pilot plants have also been established at Boston and New Mexico (USA) and a commercial plant in Munich (Germany). Economic feasibility studies indicate that radiation treatment of sewage sludges is generally less costly than conventional methods when the irradiation facilities have high output (Woods, 1998). Natural and Polluted Drinking Water Domestic water is treated before use to remove microorganisms that are harmful to health, and organic compounds that give rise to objectionable color and odor. Treatment typically involves the addition of a coagulant, pH adjustment (with the addition of lime), settling, filtration and disinfection with chlorine or ozone. Investigation has shown that the application of ionizing radiation at low doses can bring about decolorization, deodorization and disinfection of natural waters (Woods and Pikaev, 1993). For color removal in water, the radiation effect is enhanced if air is bubbled through the water during irradiation. Ionizing radiation at low doses thus provides an opportunity for reagent-free process for the purification and disinfection of natural water. Radiation also can be used to decompose organic pollutants and carcinogenic chlorinated organic compounds (formed by the treatment of water containing humic substances with chlorine). Radiation-purification has not yet, however been applied on a large scale. Wastewater There is a greater variety of pollutants present and at higher concentration in domestic and industrial wastewaters as compared to polluted drinking water. Industrial wastewaters often contain substances that are toxic or difficult to destroy. Because of high-absorbed

dose necessary to treat wastewater by irradiation alone, combined process is being developed in which radiation treatment is used in combination with conventional chemical or biological processes. Radiation can be used as a tertiary treatment following chemical or biological treatment for disinfection, color removal or toxic pollutant degradation. As a preliminary treatment process, irradiation can be used for decomposing toxic or recalcitrant pollutants in industrial wastewater before treatment by biological processes. The higher molecular weight pollutants can be degraded to lower molecular weight aldehydes and acids that are more biodegradable. 5. RADIATION-INDUCED DEGRADATION OF WATER POLLUTANTS The radiation-induced degradation of pollutants in water is a very powerful technique. Based on the very high radiation power of the modern EA-machines, this method is marked out by a great output, high efficiency and economics in comparison to other technologies (Getoff, 1996). Some applications are already potential candidates for wider commercialization; others are at demonstration stage with operating pilot-scale facilities. Some others have given promising laboratory scale results, and are under study to evaluate the actual technical-economic feasibility of potential industrial processes. Radiation of Water and Aqueous Systems The interaction between ionizing radiations and water produces electronically excited and ionized molecules. Subsequently, this leads to the production of several very reactive species (eaq

-, OH, H and HO2 or O2-) and molecular products (H2 and H2O2). Major

reactions in the radiolysis of liquid water are (Woods and Pikaev, 1993).

H2O rad. H2O+, e-, H2O* (3) H2O+ + H2O → H3O+ + OH (4) H2O* → H2O (5) e- + nH2O → e-

aq (6) e-

aq + H2O → H + OH- (7) 2 e-

aq →H2O H2 + 2OH- (8) e-

aq + H →H2O H2 + OH- (9) e-

aq + OH →OH- (10) e-

aq + H3O+ → H + H2O (11) 2H → H2 (12) H + OH →H2O (13) 2OH →H2O2 (14) OH + H2O2 →H2O + HO2 (15) 2HO2 → H2O2 + O2 (16)

In the presence of oxygen: H + O2 → HO2 (17) e-

aq + O2 → O2- (18)

The three species of greatest interest in the destruction of toxic organic compounds are the hydrated electrons (e-

aq), hydrogen atom (H·) and hydroxyl radical (OH·). Hydrated electrons (e-

aq) are the dominant reducing species in the radiolysis of deaerated water, then followed by H· atoms. Hydroxyl radical, OH· is the main oxidizing specie. The decay of the e-

aq is accelerated by agents, which are expected to react with electrons including H+, O2, and N2O. e-

aq is the strongest known reducing species with a reduction potential Eo' = - 2.9 V at pH 7. In oxygenated systems, e-

aq and H-atom are converted to perhydroxyde radical anion (O2

- ) and perhydroxyl radicals (HO2) respectively, which are strong oxidizing agents, which together with OH-radicals can initiate degradation of water pollutants (Photobiology). The reducing radicals (e-

aq and H·) and the oxidizing radical (OH·) dominate for γ and EB radiations with a low molecular yield of H2 and H2O2. The relative yields of the water radiolysis products by ionizing radiation depend on the pH.

The yields, G-values (number of changed molecules per 100eV) of the product of water radiolysis at pH 7 are e-

aq (2.7), H (0.6), OH (2.8), H2 (0.45), H2O2 (0.7), H+aq (3.2),

OH-aq (0.5) (Getoff, 1996).

Numerous studies have shown that ionizing radiation can be effective for the removal of several classes of hazardous organic compounds, such as halogenated alkyl hydrocarbons, aromatic hydrocarbons and chlorobenzenes. The efficiency of removal of organic chemicals from contaminated water depends on radiation dose, initial concentration of contaminant, pH and turbidity. A descriptive, empirical model was developed for estimating the removal of selected organic compounds as a function of these factors and also for estimating the dose required to meet specific treatment objectives (Kurutz and others, 1995). The developed model, however, can overstate the removal of compounds sensitive to particular radicals in waters with higher concentrations of scavengers such as nitrates, carbonates and alcohols. Examples of radiation-induced pollutant degradation TCE and PCE Chlorinated ethylenes such as trichloroethylene (TCE) and perchloroethylene (PCE) have been widely used in industrial and commercial applications including metal degreasing, cleaning of electronic components and dry-cleaning. As a result, they found their ways to natural and groundwater sources in addition to industrial wastewaters. The radiolysis in the presence of air results in the formation of aldehydes and simple carboxylic acids. A complete degradation to carbon dioxide and water can be achieved at higher dose. Getoff (1996) reported a complete degradation of 10-6 mol dm-3 TCE solution at a dose of 150Gy.

Higher yield of Cl- ions (taken as an indicator for the decomposition) are however reported for air-free solutions. It is believed that e-

aq is responsible for the attack on the Cl-atom of the TCE and thus oxygen, if present competes with the TCE for e-

aq and thus reduces the reaction rate. The OH radicals add to the double bond of the TCE molecule (Cl2C==CHCl) forming OH-adducts, which are unstable and decompose. The combination of ozone (O3) for the degradation of TCE in water had a synergistic effect and very efficient degradation. Chlorinated aromatic compounds Phenols and chlorophenols are widely distributed in natural waters. Chlorophenols are also common in pesticides and other industrial chemicals. The results obtained for irradiation of aqueous solutions of various chlorophenols without scavengers demonstrate efficient removal of all examined species by irradiation doses from 1 to 2 kGy (Trojanowicz, 2000). The most difficult to decompose is simple phenol, which is also a product of radiolysis of all chlorophenols except pentachlorophenol, occurring largest concentrations (up to several ppm). Doses up to 2.0 kGy have not decomposed it completely. Degradation of chlorophenols in synthetic aqueous solutions takes place almost completely at 0.2 kGy dose, however, for river water matrix containing scavengers such as carbonates or oxygen it requires a larger dose. For the same dose used for degradation of higher chlorophenols in river water, smaller amounts of difficult- to- decompose phenols are produced (Trojanowicz, 2000). For the pesticide 2,4-dichlorophenol (2,4-DCP), at doses not exceeding 1 kGy, the yield of decomposition essentially depends on initial concentration of 2,4-DCP. For 50 ppm 2,4-DCP, only 40% has been decomposed, and a dose 10 kGy is needed for complete decomposition. The efficiency of radiolytic degradation is additionally decreased in the presence of scavengers such as nitrate. Dehalogenation of chlorophenols leads to formation of di- and trihydroxybenzenes, and then to opening of aromatic ring and formation of various carboxylic acids. Formic and acetic acids are the main products in irradiated samples. Benzene, Toluene and Phenol An investigation was conducted by Zele and others (1998) for the degradation of Benzene, Phenol and Toluene using potable water with a DO of 3.5 mg/L with EB radiation. The byproducts in the irradiation of benzene is phenol and dihydroxy compounds (mainly catechol and hydroquinone. The phenol formed also reacts with hyroxyl radicals to form dihydroxy phenols similar to phenol irradiation. The formed phenol and the dihydroxy compounds consume OH. At high rate and compete with benzene removal. Initial benzene concentrations of 13.2μM and 85μM require absorbed doses of 50 and 100 krad for complete and 85% benzene removal respectively. Toluene on degradation reacts with hydroxyl radicals to form o-cresol, which further consumes OH. at high rates and competes with toluene removal. Toluene with initial concentration of about 7μM requires about 50 krad for complete removal, while an initial

concentration of about 53μM needed about 100 krad-absorbed dose for complete removal. The most important byproducts of irradiation of phenol are catechol and hydroquinone. 50 krad of absorbed dose was sufficient for complete removal of an initial 14μM solution. At higher doses of 135μM and 750μM, absorbed doses of 100 krad and 300 krad led to about 85% removal in both cases respectively. It can be seen that the required dose required for phenol is less than that of benzene since phenol is one of the byproducts of benzene irradiation. Phenol There are 11 phenols on the list of priority pollutants of the US EPA. Phenols are also one of the most frequently found hazardous compounds at Superfund sites. Although phenol is biodegradable at low concentration, conventional biological treatment methods are unsuitable at high concentration. Numerous studies have been conducted on the degradation of phenol in water by ionizing radiation. Phenol is often chosen because it is water-soluble and the most simple aromatic substance. It is also a by-product of radiation-induced degradation of other aromatic compounds.

Studies using high-energy EB have been conducted on the destruction of phenol in potable water in a large-scale flow-through system by Lin et. al. (1995). The tests were carried out at several solute concentrations, in the presence and absence of clay, and at different pHs and alkalinity concentrations. In the experiments, greatest percentage removal was observed at the lowest initial solute concentration (10.6 μmol/L) and in every case increased with increasing dose. The percentage removal decreased with increasing pH from 5 and 7 to 9, and appears unaffected by the presence of 3% solids as clay. At any given solute concentration and pH, the removal efficiency decreased with increasing dose. The O2 concentration in the effluent sample after irradiation was lower with respect to the influent concentration. The apparent reduced phenol removal with increased pH was found to be affected by increasing carbonate alkalinity rather than pH itself. The main by products of phenol reaction are di-hydroxy derivates, mainly hydroquinone and catechol with trace amounts of resorcinol at low absorbed doses. Carbonyl compounds (formaldehyde, acetaldehyde and glyoxal) and formic acid were also found to be other byproducts of the irradiation of phenol. It is believed that some mineralization must have taken place. It is suggested that at high doses, tri-hydroxy phenol derivatives, such as pyrogallol, hydroxyhydroquinone and phloroglucinol, might also be formed and that the decomposition of phenol may result in polymerization and formation of humic substances. For the recirculation experiment, a >95% removal of phenol at pH 7.75 from an initial concentration of 950μmol/L was observed but only 2 % decrease in TOC occurred. The results suggest that phenol was not mineralized at the high initial concentration, but the removal pathway may be through polymerization.

Pesticides and Herbicides Some early laboratory studies have been carried out by gamma irradiation at 3 kGy on active substances used in commercial pesticides, diluted at 10 ppm concentration, observing the following removal efficiencies: 100% for Paraquat and Coumachlor, 99% for Atrazine, 97% for Methyl Parathion, 95% for Dimethoate and 90% for Malathion. Some other active agents such as Parathion, Diazinone, Stimazine, Lindane and Heptachlor have shown to be more resistant: under the same experimental conditions their removal efficiencies were respectively 74%, 80%, 52%, 65% and 31%. Some tests have also been run on Monuron, widely used as herbicide; 0.4 ppm and 4.2 ppm solutions were irradiated at doses up to 0.3 kGy. In 0.4 ppm samples the active agent was totally destroyed at 0.25 kGy, while in 4.2 ppm solutions 97% removal was observed at 0.3 kGy. Dieldrin has been irradiated at 0.1 ppm concentration; total removal was achieved at doses greater than some tens of kGy (Tata et.al., 1995). Nitrophenols Nitrophenols are frequently used as intermediates in the production of explosives, pharmaceuticals, pesticides, pigments, dyes, wood preservatives and rubber chemicals. A study on the degradation of 2-nitrophenol, 3-nitrophenol, 4-nitrophenol and 2,4-nitrophenol was carried out by Weihua et.al (2002). Aqueous solutions, 8x10-4 M, 2-, 3-, 4- nitrophenol and 5.4x10-4 M 2,4-dinitrophenol were prepared in distilled water in the pH 4-6. An absorbed dose of 8 kGy was sufficient to remove the solutes in all the nitrophenols. In all the cases, a reduction of DOC of about 10% was observed which shows that there is only a little mineralization and that the main by-products are organic compounds. Actual wastewater and polluted groundwater A pilot plant study was carried out on real industrial effluent from chemical and pharmaceutical industries in Brazil (Sampa, Rela and Duarte, 1998). The industrial wastes were irradiated with and without air mixing. The parameters analyzed and their initial concentrations are COD (952 mg/L), phenol (2.6 ppm), chloroform (0.83 ppm), PCE (0.87ppm), TCE (3.57ppm), CCl4 (9.74 ppm), dichloroethane (87.93ppm), benzene (10 ppm), toluene (7 ppm), and xylene (9 ppm). For all the pollutants apart from phenol, there was more than 80% degradation at an applied dose of 2 kGy with air mixing and above 90% in samples without air mixing. For phenol, only a little degradation was observed in the presence of air, but 50% degradation was achieved without air addition. A decrease of about 30% in COD was observed in samples with air mixing at a dose of 2 kGy but the COD started rising at higher doses up to 15 kGy, and then fell again at 20 kGy. For the sample without air, there was no significant change in COD until 15 kGy, and at 20 kGy, the COD value decreased by 30%. Color decrease was more significant in the samples exposed to high doses (>10 kGy) and without air mixing. Thus, the irradiation was efficient for degrading toxic pollutants in a complex water system like a

real industrial effluent. A field treatability study (by Nickelsen and others, 1998) was carried out in Germany to determine the effectiveness of the EB process in destroying hazardous organic contaminants from various sources, and to make the treated effluent more amenable to biological remediation using a mobile electron-beam system. At a contaminated groundwater site in Berlin, the initial contaminant concentrations are; total chlorobenzene (277μg/L), total hexachlorocyclohexane (3.24 μg/L), total chlorinated ethane(s) and ethene(s) (86,884μg/L), total BTEX (230μg/L), and total PAH concentration (24.4μg/L). A dose of 10 Mrad was sufficient to achieve greater than 95% removal efficiency for all the pollutants. For a chemical process wastewater from a company producing phenolic compound, a COD (initial concentration = 749 mg/L) and phenol(s) (initial concentration = 46 mg/L), removal efficiency of 61.5 and 99.6% respectively were achieved at a dose of 9 Mrad and the overall BOD remains almost unchanged. For a refinery process wastewater contaminated with mainly BTEX (7125μg/L) and phenols (2230μg/L), a dose of 2.8 Mrad achieved overall removal efficiencies of > 99% for the BTEX, phenols and PAHs. The results demonstrate that the high-energy electron beam accelerators can efficiently and effectively treat complex mixtures of hazardous compounds in aqueous solutions. Effect of O3, H2O2 and TiO2 in combination with ionizing radiation Zele S. R and others (1998) reported that the removal efficiency of organic solutes could be improved by the addition of hydrogen peroxide at low doses (40 – 150 krad). For a benzene concentration of 80μM irradiated with 50krad, the removal was highest (85%) when 50mg/l of hydrogen peroxide was added compared to (70%) without any hydrogen peroxide addition. However with higher H2O2 concentration, decreased removal efficiency is predicted. This is because at higher concentration, the H2O2 scavenges OH. which is the most important radical causing destruction of benzene. Thus, there exist an optimum quantity of hydrogen peroxide that can cause sufficient radical enhancement, resulting in best removal efficiency. In an experiment to examine the effect of the photocatalyst TiO2 on aqueous phenol solutions by irradiation from UV, gamma and electron beam sources, the degradation by gamma and EB irradiation was much higher than that from UV radiation in the presence of TiO2. However, there was no significant difference in the degradation obtained for the ionizing radiations in the presence and absence of TiO2. Thus, the TiO2 has no effect on the phenol degradation by gamma and EB radiation, but shows a drastic TOC removal to below detectable level, which increases with increasing TiO2 concentration. The results suggest that TiO2 may have effect on some of the byproducts of phenol degradation to carbon dioxide (Chitose and others, 2003). The combination of ozone with ionizing radiation has proven very effective in the degradation of some pollutants. The addition of ozone to a phenol solution before irradiation led to a 20-fold reduction in applied dose (Getoff, 1996). Gehringer and

Eshweiler (1998) reports that ozone addition during electron beam irradiation considerably reduces the radiation dose required for chlorinated ethylenes and thus improves the economy of the process or makes it economically feasible. An experiment was conducted by Gehringer and Eshweiler (2002) to determine the effect of ozone (O3), nitrous oxide (N2O), and H2O2 on the degradation of PCE in potable water by ionizing radiation. The three additives are all capable of producing OH free radical as a result of scavenging of solvated electrons. While the dose requirement for the EB radiation without any additive was 800 Gy, addition of N2O reduces the necessary dose to about 600 Gy. O3 addition however resulted in a dose requirement of just about 60 Gy, i.e. one order of magnitude less. H2O2 addition causes almost no improvement in the PCE decomposition, and at higher concentration (35mg/L), it actually caused abatement. Jung and others (2003) also found that while H2O2 has no effect on the decomposition of TCE and PCE irradiation by γ rays, theγ-ray/ozone combination increased TCE and PCE removal from 9% and 33% (without ozone) to 96% and 99% respectively. However, the limitations to the use of ozone are that it is unstable at higher concentrations and it cannot be stored, therefore it has to be produced on site. It also has low solubility in water, it is toxic and it is expensive to produce. Also, Gehringer and Eshweiler (1998) in their experiments on three chlorinated alkanes (CCl4, CHCl3 and CCl3-CH3), ozone addition did not improve their decomposition at all. Generally, ozone addition before or during irradiation can be effective only when oxidation is the main pathway for pollutant decomposition. For groundwater contaminated with trace amounts of chlorinated alkanes with high bicarbonate contents, ozone addition had no influence whatsoever on pollutant decomposition. This is because the pathway of decomposition is through reduction. Ozone addition also failed for the decomposition of 50 ppm PCE in deionized water. Under the condition, the solvated electrons decompose PCE even more efficiently than OH free radicals. Thus, ozone, which is supposed to convert solvated electron into OH free radical, did not prove helpful. Thus, ozone addition before or during irradiation will only improve pollutant decomposition in water and consequently the economy of the irradiation process if only the limitations of its use are carefully considered. (Gehringer and Eshweiler ,1998).

6. RESEARCH PROPOSAL Since the biological treatment methods are limited in their application to the treatment of wastewater containing recalcitrant or toxic organic pollutants, it is necessary to pretreat the wastewater before secondary biological treatment. Application of ionizing radiation is one of the methods by which the wastewater can be pretreated. This technology has been proved to be capable of rapid degradation and complete destruction of many toxic organic pollutants in water and wastewater. Other existing treatment methods e.g. carbon adsorption and air stripping merely transfers the pollutants from one phase to another without destruction of the pollutants. Chemical oxidants such as ozone (O3) and Hydrogen peroxide (H2O2) are also selective in their oxidation of different pollutants. The reaction rate of chemical oxidants can also be too slow for pollutant destruction, and may also require large quantities of oxidant. The biggest obstacle to the wide application of ionizing radiation (mainly electron beam irradiation) at the moment is the high cost of the process compared to other treatment method. The cost of the electron beam irradiation of pollutants is directly related to the absorbed dose for the destruction of the toxic pollutant, which determines the power consumption. Complete destruction of pollutant in wastewaters may usually require an absorbed dose whose cost may be prohibitive. Thus one of the methods proposed for cost reduction is the combination of radiation technology with biological treatment process in wastewater treatment. In this scheme, the polluted water is first irradiated to reduce the toxicity of the pollutants and make them more biodegradable by breaking down or altering the chemical structure and composition of the pollutants, but not complete mineralization to CO2 and water. The byproducts of the incomplete radiation may be more biodegradable, although they may also be more toxic in some cases. Then secondary biological treatment may follow. Also, the irradiation may serve as a tertiary treatment method for destruction of pollutants that may not be degraded in the biological treatment process.

Another scheme of reducing the absorbed dose is by the addition of catalysts or oxidants to the water or wastewater before irradiation. As at this moment, the only compound that has proved effective is ozone (O3). However ozone is a non-stable gas and therefore cannot be stored, it has to be generated onsite. The requirement for the installation of ozone generators will also lead to increased capital and operation cost.

Most of the existing research work on electron beam irradiation has focused on the absorbed dose required for the destruction of different pollutants at varying solute concentrations in water. Much research has also been done on the determination of the byproducts formed during the irradiation of the pollutants and their yield at different absorbed doses. However, for practical application purposes, it will be desirable to know the changes in the BOD and COD of the pollutant during the radiation process. This can be useful in two ways: 1. Since the BOD/COD ratio determines the biodegradability of raw wastewater, the

BOD/COD ratio can help determine the optimum absorbed dose required for the partial degradation of a pollutant before the biological treatment process. This will help in determine the minimum absorbed dose required for partial pollutant decomposition, and thus reduce the cost that would be required for complete pollutant destruction which may be unnecessary.

2. For treated wastewater, the irradiation process is a tertiary treatment method. Thus it is useful to know the impact of the byproducts of irradiated pollutant on the treated effluent. Since BOD and COD are among the parameters measured, the measurement will provide information on whether the irradiated by products may increase the effluent BOD and COD and thus may not be able to meet discharge standards.

3. In the case of contaminated drinking water sources (groundwater, surface water), it is also useful to know the additional BOD and COD that the irradiation process may add to the clean water, especially in the case of incomplete decomposition of the toxic pollutants in order to meet the standard maximum limit.

Gehringer and Eschweiler (1998) measured the changes in BOD and DOC of the wastewater of a molasses processing facility at different absorbed doses during irradiation; and Nickelsen and others (1998) also measured COD and BOD variation of the wastewater from a German chemical wastewater company at different doses during electron beam irradiation. However, at the moment, there is no published work on the determination of the changes in the BOD and COD solely due to individual pollutants in water at different absorbed doses during irradiation. I therefore propose to do the following research for my Masters Thesis: 1. The changes in the BOD, COD, and pH induced by priority organic pollutants

in water at different initial concentrations and varying absorbed doses during irradiation. The pollutants to be monitored are benzene, phenol, trichloroethylene (TCE), and 2,4-dinitrophenol. The pollutants were chosen because previous researchers have found that they can be decomposed by electron beam irradiation.

2. The changes in the COD and pH of the irradiated pollutant-containing water at different absorbed doses in the presence and absence of ozone. The ozone to be utilized is the one formed as a byproduct in the air layer between water surface and the electron beam accelerator-window. Thus, there is no need for additional ozone-generating equipment. At the moment, this ozone is usually wasted and discharged into the atmosphere and lead to atmospheric pollution. The addition of the ozone to the wastewater will thus have the potential of reducing irradiation treatment cost and save the environment.

The main equipments that will be required will be electron-beam accelerator, BOD, COD, alkalinity, DO and pH tests apparatus. The materials needed are distilled and potable water, the chemicals (benzene, phenol, TCE, and 2,4-dinitrophenol) and the test reagents.

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