Solar Detoxification

Transcription

1 Electronic copy only Solar Detoxification by Julian Blanco Galvez, Head of Solar Chemistry and Sixto Malato Rodriguez, Researcher in the Solar Chemistry Area, Plataforma Solar de Almeria, Spain United Nations Educational, Scientific and Cultural Organization 2003

2 TABLE OF CONTENTS PART A. SOLAR DETOXIFICATION THEORY 1. Introduction Aims Objectives Notation and units 1.1 Solar Chemistry 1.2 Water contaminants 1.3 Photodegradation principles Definitions Heterogeneous photocatalysis Homogeneous photodegradation 1.4 Application to water treatment 1.5 Gas-phase detoxification Summary of the chapter Bibliography and references Self-assessment questions Answers 2. Solar irradiation Aims Objectives Notation and units 2.1 The power of light Ultraviolet light Visible light Infrared light 2.2 The solar spectrum 2.3 Solar ultraviolet irradiation 2.4 Atmospheric attenuation of solar radiation Annual available ultraviolet radiation 2.5 Solar radiation measurement Detectors Filters Input Optics Summary of the chapter Bibliography and references

3 Self-assessment questions Answers 3. Experimental systems Aims Objectives Notation and units 3.1 Laboratory systems 3.2 Solar detoxification pilot plants 3.3 Operation of pilot plant Once-through operation Batch operation Modelling once-through and batch operation 3.4 Evaluation of solar UV radiation inside photoreactors Radiometers calibration Correlation between radiometric and spectroradiometric data Collector efficiency Actinometric experiments 3.5 Simplified method for the evaluation of solar UV radiation inside photoreactors Summary of the chapter Bibliography and references Self-assessment questions Answers 4. Fundamental parameters in photocatalysis Aims Objectives Notation and units 4.1 Direct photolysis 4.2 Oxygen influence 4.3 ph influence 4.4 Catalyst concentration influence 4.5 Initial contaminant concentration influence 4.6 Radiant flux influence 4.7 Temperature influence 4.8 Quantum yield Summary of the chapter Bibliography and references Self-assessment questions Answers

4 5. Water decontamination by Solar Detoxification Aims Objectives Notation and units 5.1 Detoxification of pollutants Total mineralization Degradation pathways Toxicity reduction Detoxification of inorganic pollutants 5.2 Quantum yield improvement by additional oxidants Hydrogen peroxide Persulphate Other oxidants 5.3 Catalyst modification Metal semiconductor modification Composite semiconductors Surface sensitisation 5.4 Recommended analytical methods Original contaminants Mineralization measurements (TOC) Intermediate analysis (GC-MS/HPLC-MS) Extraction methods Toxicity analysis Summary of the chapter Bibliography and references Self-assessment questions Answers PART B. SOLAR DETOXIFICATION ENGINEERING 6. Solar Detoxification Technology Aims Objectives Notation and units 6.1 Solar collector technology generalities 6.2 Collectors for solar water detoxification. Peculiarities Peculiarities of solar UV light utilization Parabolic trough collectors Non-concentrating collectors Compound parabolic concentrator (CPC)

5 6.2.5 Holographic collectors 6.3 Concentrated versus non-concentrated sunlight 6.4 Technology issues Reflective surfaces Photocatalytic reactor 6.5 Catalyst issues Slurry versus supported catalyst Catalyst recuperation and re-use Summary of the chapter Bibliography and references Self-assessment questions Answers 7. Solar Detoxification Applications Aims Objectives Notation and units 7.1 Introduction 7.2 Industrial waste water treatment Phenols Agrochemical compounds Halogenated hydrocarbons Antibiotics, antineoplastics and other pharmaceutical biocide compounds Wood preserving waste Removal of hazardous metals ions from water Other applications 7.3 Maritime tank terminals 7.4 Groundwater decontamination 7.5 Contaminated landfill cleaning 7.6 Water disinfection 7.7 Gas-phase treatments Summary of the chapter Bibliography and references Self-assessment questions Answers 8. Economic Assessment Aims

6 Objectives Notation and units 8.1 Photochemical and biological reactors coupling 8.2 Cost calculations Example A: TiO 2 -based detoxification plant Example B: Photo-Fenton based detoxification plant 8.3 Solar or electric photons? 8.4 Solar resources assessment 8.5 Comparison with other technologies Thermal oxidation Catalytic oxidation Air stripping Adsorption Membrane technology Wet oxidation Ozone oxidation Advanced oxidation processes Summary of the chapter Bibliography and references Self-assessment questions Answers 9. Project engineering Aims Objectives Notation and units 9.1 Feasibility study Identification of target recalcitrant hazardous compounds Identification of possible pre-treatments Identification of most adequate photocatalytic process Determination of optimum process parameters Post-treatment process identification Determination of treatment factors 9.2 Feasibility study example Background Experimentation. TiO 2 -Persulphate tests Photo-Fenton tests Conclusions and Treatment Factors 9.3 Preliminary design

7 9.4 Preliminary design example 9.5 Final design and project implementation 9.6 Example of final design and project implementation Summary of the chapter Bibliography and references Self-assessment questions Answers 10. International collaboration Aims Objectives Notation and units 10.1 International Energy Agency: The SolarPACES Program 10.2 The European Union 10.3 The CYTED Program 10.4 Main research activities United Stated Spain 10.5 Guidelines to successful water treatment projects in developing countries Summary of the chapter Bibliography and references Self-assessment questions Answers

8 1. INTRODUCTION AIMS This unit describes an alternative source of energy that combines sunlight and chemistry to produce chemical reactions. It outlines the basic chemical and physical phenomena that are related with solar chemistry. This chapter will review approaches that have been taken, progress that has been made and give some projections for the near and longer term prospects for commercialisation of solar photochemistry. It also introduces the focus of this book: Solar Detoxification. OBJECTIVES By the end of this unit, you will understand the main factors causing the photochemical reactions and you will be able to do five things: 1. Distinguish perfectly between thermochemical and photochemical processes. 2. Understand the impact of pollutants on the environment. 3. Calculate the energy flux of a light source and its relationship with semiconductor excitation. 4. Understand the basic principles sustaining advanced oxidation processes. 5. Describe the most important features of heterogeneous photocatalysis making it applicable to the treatment of contaminated aqueous effluents. NOTATION AND UNITS Symbol Units A λ Absorbance at wavelength λ AOPs Advanced Oxidation Processes c Light speed nm/s, m/s c i Concentration of component i moles E G Semiconductor band-gap energy ev, J E λ Spectral irradiance W m -2 nm -1 o E λ Spectral irradiances incident into the medium W m -2 nm -1 l E λ Spectral irradiances at a distance l W m -2 nm -1 EC 50 Concentration that produce an effect in 50% of a population mg/l, mg/kg GAC Granulated activated carbon h Planck s constant J s LC 50 Concentration that produce death in 50% of a population mg/l, mg/kg NOEL No observed effect level mg/kg/day p i partial pressure of component i atm U energy of a photon ev, J α λ Absorption coefficient cm -1 atm -1 ε λ extinction coefficient mol -1 cm -1 φ quantum yield λ. Wavelength nm, µm 1

9 1.1 SOLAR CHEMISTRY The dramatic increases in the cost of oil beginning in 1974 focussed attention on the need to develop alternative sources of energy. It has long been recognised that the sunlight falling on the earth s surface is more than adequate to supply all the energy that human activity requires. The challenge is to collect and convert this dilute and intermittent energy to forms that are convenient and economical or to use solar photons in place of those from lamps. It must be kept in mind that today there is a clear world-wide consensus regarding the need for longterm replacement of fossil fuels, which were produced million of years ago and today are merely consumed, by other inexhaustible or renewable energies. Under these circumstances, the growth and development of Solar Chemical Applications can be of special relevance. These technologies can be divided in two main groups: 1. Thermochemical processes: the solar radiation is converted into thermal energy that causes a chemical reaction. Such a chemical reaction is produced by thermal energy obtained from the sun for the general purpose of substituting fossil fuels. 2. Photochemical processes: solar photons are directly absorbed by reactants and/or a catalyst causing a reaction. This path leads to a chemical reaction produced by the energy of the sun s photons, for the general purpose of carrying out new processes. It should be emphasized, as a general principle, that the first case is associated with processes that are feasible with conventional sources of energy. The second is related only to completely new processes or reactions that are presently carried out with electric arc lamps, fluorescent lamps or lasers. Increase of Temperature Heat Thermochemical Process Steam reforming of methane CH 4 + H 2 O CO + 3H kj/mol 600º - 850ºC Photons Modification of chemical bonds Photochemical Process Excitation of a semiconductor hν + SC e - + p + hν E G of SC Figure 1.1 Schematic view of Solar Chemical Applications From the outset, it was recognized that direct conversion of light to chemical energy held promise for the production of fuels, chemical feedstock, and the storage of solar energy. Production of chemicals by reactions that are thermodynamically uphill can transform solar energy and store it in forms that can be used in a variety of ways. Wide ranges of such chemical transformations have been proposed. A few representative examples are given in Table 1.1 to illustrate the concept. 2

10 CO 2 (g) CO(g) + 1/2O CO 2 (g) + 2H 2 O(g) CH 3 OH (l) + 3/2O H 2 O(l) H 2 (g) + 1/2O CO 2 (g) + 2H 2 O(l) 1/6C 6 H 12 O 6 (s) + O H (kj/mol) Table 1.1 Representative chemical reactions that can store solar energy (Thermochemical processes) These processes generally start with substances in low-energy, highly-oxidized forms. The essential feature is that these reactions increase the energy content of the chemicals using solar energy. For such processes to be viable, they must fulfil the following requirements, as outlined by NREL (1995) and slightly modified by the authors: The thermochemical reaction must be endothermic. The process must be cyclic and with no side reactions that could degrade the photochemical reactants. The reaction should use as much of the solar spectrum as possible. The back reaction should be very slow to allow storage of the products, but rapid when triggered to recover the energy content. The products of the photochemical reaction should be easy to store and transport. The other pathway for the use of sunlight in photochemistry is to use solar photons as replacements for those from artificial sources. The goal in this case is to provide a costeffective and energy-saving source of light to drive photochemical reactions with useful products. Photochemical reactions can be used to carry out a wide range of chemical syntheses ranging from the simple to the complex. Processes of this type may start with more complex compounds than fuel-producing or energy-storage reactions and convert them to substances to which the photochemical step provides additional value or destroy harmful products. The principles of photochemistry are well understood and examples of a wide range of types of synthetic transformations are known (Figure 1.2). Therefore, the problem becomes one of identifying applications in which the use of solar photons is possible and economically feasible. The processes of interest here are photochemical, hence, some component of the reacting system must be capable of absorbing photons in the solar spectrum. Because photons can be treated like any other chemical reagent in the process, their number is a critical element in solar photochemistry (see Chapter 2). O CHO hυ<700nm Methylene blue/o 2 O O O CHO hυ<390nm C 6 Cl 5 OH+9/2O 2 +2H 2 O 6CO 2 + 5HCl TiO 2 Figure 1.2 Furfural photo-oxidation and pentachlorophenol mineralization (Photochemical processes). 3

11 Because they are very technologically and environmentally attractive, solar chemical processes have seen spectacular development in recent years. In the beginning, research in solar chemistry was centered only on converting the solar energy into chemical energy, which could then be stored and transferred over long distances. Together with this important application, other environmental uses have been developed, so that today the entire range of solar chemical applications has a promising future. In principle, any reaction or process requiring an energy source can be supplied by solar energy. 1.2 WATER CONTAMINANTS Environmental pollution is a pervasive problem with widespread ecological consequences. Recent decades have witnessed increased contamination of the Earth s drinking water reserves. The inventory of priority pollutants compiled by the U.S. Environmental Protection Agency provides a convenient frame of reference (in Table 1.2 only a partial list is shown) for understanding the importance of removing such contamination from the Earth. 1,1,2,2-Tetrachloroethane 1,l -Dichloroethane 1,2,4-Trimethylbenzene 1,2-Dibromoethane 1,2-Dichlorobenzene 1,2-Dichloropropane 1,2-Dinitrotoluene 1,2-Diphenylhydrazine 1,4-Dioxane 2,2,4-Trimethylpentane 2,4,6-Trichlorophenol 2,4,6-Trinitrotoluene 2,4 Diaminoanisole 2,4-Dichlorophenol 2,4-Dinitrophenol 2,4-Dinitrotoluene 2,4-Toluene diamine 2-Chloroethyl vinyl ether 2-Chlorophenol 2-Nitropropane 4,4 -Diaminodiphenyl ether 4,4 -Methylenedianiline 4-Aminoazobenzene 4-Methylphenol 5-Nitro-o-anisidine Acetaldehyde Acetamide Acetone Acetonitrile Acetophenone Acrolein Acrylamide Acrylic acid Acrylonitrile Aldrin Aniline Anthracene Atrazine Benzamide Benzene Benzidine Benzo(a)pyrene Benzyl chloride Benzenehexachloride) Biphenyl Bis(2- Chloroethoxy)methane Bromoethane Captan Carbaryl Carbon disulfide Carbon tetrachloride Catechol Chlordane Chloroacetic acid Chlorobenzene Chlorodibenzodioxins, various Chlorodibenzofurans o-,m-,p-cresols Cumene Cyclohexane Diazomethane Dibenzofuran Dichlorvos Dicofol Diepoxybutane Diethanolamine Dimethyl phthalate Disulfoton Endosulfan Epichlorohydrin Ethylbenzene Ethylene glycol Ethylene thiourea Fluometuron Formaldehyde Hexachlorobenzene Hexachloroethane Hexane Hydroquinone Isophorone Isopropyl alcohol Lindane Malathion Maneb Mechlorethamine Melamine Methanol Methoxychlor Methyl acrylate Methyl isocyanate Methyl tert-butyl ether Methylene bromide Methylhydrazine Mirex Mustard gas Nitrilotriacetic acid Nitrobenzene Nitrofen Nitrogen mustard Nitroglycerin Nitrophenol n-butyl alcohol n-dioctyl phthalate N-Nitrosodiethylamine N-Nitrosopiperidine N-Nitroso-N-ethylurea Octachloronaphthalene Octane Oxirane o-anisidine hydrochloride o-nitroaniline o-toluidine hydrochloride Parathion (DNTP) PCBs Pentachlorobenzene Pentachlorophenol Phenanthrene Phosgene Phthalic anhydride Polybrominatedbiphenyls Beta-Propoxur Pyrene p-chloro-m-cresol Quinone Quintozene Safrole Set-Butyl alcohol Sevin (carbaryl) Styrene Terephthalic acid Tert-Butyl alcohol Tetrachlorvinphos Tetrahydrofuran Thioacetamide Thiourea Toluene Toluene diisocyanate Total xylenes Toxaphene Triaziquone Trichlorfon Trifluralin Urethane (ethyl carbamate) Vinyl bromide Vinyl chloride Vinylidene chloride Xylene (mixed isomers) Zineb Table 1.2. Organic compounds that are included in various lists of hazardous substances identified by the U.S. EPA 4

12 In any case, a consensus exists that the environmental impact of a given contaminant depends on the degree of exposure (its dispersion and the resulting concentration in the environment) and on its toxicological properties. The assessment of exposure involves comprehension of the dispersion of a chemical in the environment and estimation of the predicted concentration to which organisms will be exposed. For example, the pesticide fenaminphos oxidizes very quickly (half-life 10 days) into sulphoxide and sulphone, while its pesticidal properties remain unaffected. A half-life of 70 days has been found for degradation of fenaminphos and its two metabolites. Furthermore, the two metabolites are more mobile (soluble) than fenaminphos (Hayo and Werf, 1996). Assessment of the contaminant s effect involves summarizing data on the effects of the chemical on selected representative organisms and using these data to predict a no-effect concentration on a specific niche. Organisms may consume chemicals through ingestion of food and water, respiration and through contact with skin. When a chemical crosses the various barriers of the body, it reaches the metabolic tissue or a storage depot. Toxicity of a chemical is usually expressed as the effective concentration or dose of the material that would produce a specific effect in 50% of a large population of test species (EC 50 or ED 50 ). If the effect recorded is lethal, the term LC 50 (or LD 50 ) is used. The no observed effect level (NOEL or NOEC) is the dose immediately below the lowest level eliciting any type of toxicological response in the study. For example, the pesticide methamidophos, which has been classified as a Restricted-Use Pesticide (RUP) by the U.S. EPA, is highly toxic for mammals (acute oral LD 50 = 16 mg/kg in rats and mg/kg in guinea pigs), birds (bobwhite quail 8-11 mg/kg) and bees. The 96-hour LC 50 is mg/l in rainbow trout, but concentrations as low as 0.22 ng/l are lethal to larval crustaceans in 96- hour toxicity tests. A 56-day rat feeding study resulted in a NOEL of 0.03mg/kg/day (Tomin, 1994). Decontamination of drinking water is mainly by procedures that combine flocculation, filtration, sterilization and conservation, to which a limited number of chemicals are added. Normal human sewage water can be efficiently treated in conventional biological processing plants. But very often, these methods are unable to reduce the power of the contaminant. In these cases, some form of advanced biological processing is usually preferred in the treatment of effluents containing organic substances. Biological treatment techniques are well established and relatively cheap. However, these methods are susceptible to toxic compounds that inactivate the waste degrading microorganisms. To solve this problem, apart from reducing emissions, two main water treatment strategies are followed: (i) chemical treatment of drinking water, contaminated surface and groundwater and (ii) chemical treatment of waste waters containing biocides or non-biodegradable compounds. Chemical treatment of polluted surface and groundwater or wastewater, is part of a long-term strategy to improve the quality of water by eliminating toxic compounds of human origin before returning the water to its natural cycles. This type of treatment is suitable when a biological processing plant cannot be adapted to certain types of pollutants that did not exist when it was designed. In such cases, a potentially useful approach is to partially pre-treat the toxic waste by oxidation technologies to produce intermediates that are more readily biodegradable. Light can be used, under certain conditions, to encourage chemicals to break down the pollutants to harmless by-products. Light can have a dramatic effect on a molecule or solid, because, when it absorbs light, its ability to lose or gain electrons is often altered. This electronically excited state is both a better oxidizing and a better reducing agent than its counterpart in the ground. Electron transfer processes involving excited-state electrons and the contact medium (for example water) can therefore generate highly reactive species like 5

13 hydroxide ( OH) and superoxide (O 2 - ) radicals (see Table 1.3). These can then be used to chemically decompose a pollutant into harmless end-products. Alternatively, light can be used directly to break up pollutant molecule bonds photolytically. These processes are called Advanced Oxidation Processes (abbreviated as AOPs). Many oxidation processes, such as TiO 2 /UV, H 2 O 2 /UV, Photo-Fenton and ozone processes (O 3, O 3 /UV, O 3 /H 2 O 2 ) are currently employed for this purpose. Oxidizing reagent Oxidation Potential, V Fluorine 3.06 Hydroxide radical ( OH) 2.80 Ozone 2.07 Hydrogen peroxide 1.77 Chlorine dioxide 1.57 Chlorine gas 1.36 Oxygen 1.23 Hypochlorite 0.94 Iodine 0.54 Superoxide radical (O - 2 ) Table 1.3. Oxidation potentials of common substances and agents for pollution abatement. The more positive the potential, the better the species is an oxidizing agent 1.3 PHODEGRADATION PRINCIPLES Definitions For the benefit of those who may have a limited background in photochemistry, a brief outline of some basic concepts of photochemistry is presented here. In order for photochemistry to take place, photons of light must be absorbed. The energy of a photon is given by hc U = (1.1) λ where h is Planck s constant ( J s), c is the speed of light and λ is the wavelength. For a molecule s bond to be broken, U must be greater than the energy of that bond. When a given wavelength λ of light enters a medium, its spectral irradiance E λ (W m -2 nm -1 ) is attenuated according to the Lambert-Beer law, which is expressed in two ways, one for gas phase and the other for liquid phase: o l ln( E / E ) = α p l gas phase (1.2) λ λ λ i 6

14 o l log( E / E ) = ε c l liquid phase (1.3) λ E o λ and E l λ are the incident spectral irradiances and at a distance l into the medium, α λ is the absorption coefficient (cm -1 atm -1 ), p i is the partial pressure (atm) of component i, ε λ is the extinction coefficient (M -1 cm -1 ), and c i is the concentration (M) of component i. The absorbence A λ at wavelength λ is the product ε λ c i l. The photochemical quantum yield (φ) is defined as the number of molecules of target compound that reacts divided by the number of photons of light absorbed by the compound, as determined in a fixed period of time. Normally, the unit is the maximum quantum yield attainable. The term photocatalysis implies the combination of photochemistry with catalysis. Both light and catalyst are necessary to achieve or to accelerate a chemical reaction. Photocatalysis may be defined as the acceleration of a photoreaction by the presence of a catalyst. Heterogeneous processes employ semiconductor slurries for catalysis, whereas homogeneous photochemistry is used in a single-phase system. Any mechanistic description of a photoreaction begins with the absorption of a photon, being sunlight the source of photons in solar photocatalysis. In the case of homogeneous photocatalytic processes, the interaction of a photon-absorbing species (transition metal complexes, organic dyes or metalloporphyrines), a substrate (e.g. the contaminant) and light can lead to a chemical modification of the substrate. The photon-absorbing species (C) is activated and accelerates the process by interacting through a state of excitation (C*). In the case of heterogeneous photocatalysis, the interaction of a photon produces the appearance of electron/hole (e - and h + ) pairs, the catalyst being a semiconductor (e.g. TiO 2, ZnO, etc). In this case, the excited electrons are transferred to the reducible specimen (Ox 1 ) at the same time that the catalyst accepts electrons from the oxidizable specimen (Red 2 ) which occupies the holes. In both directions, the net flow of electrons is null and the catalyst remains unaltered. λ λ i C C C + R R + C R hν * * * * * P (1.4) (1.5) (1.6) + hν C C( e + h ) h + Red Ox 2 2 e + Ox Red (1.7) (1.8) (1.9) Heterogeneous photocatalysis The concept of heterogeneous photocatalytic degradation is simple: the use under irradiation of a stable solid semiconductor for stimulating a reaction at the solid/solution interface. By definition, the solid can be recovered unchanged after many turnovers of the redox system. When a semiconductor is in contact with a liquid electrolyte solution containing a redox couple, charge transfer occurs across the interface to balance the potentials of the two phases. An electric field is formed at the surface of the semiconductor and the bands bend as the field forms from the bulk of the semiconductor towards the interface. During photoexcitation (a photon with appropriate energy is absorbed), band bending provides the conditions for carrier separation. In the case of semiconductor particles, there is no ohmic contact to extract the majority carriers and to transfer them by an external conductor to a second electrode. This 7

15 means that the two charge carriers should react at the semiconductor/electrolyte interface with the species in solution. Under steady state conditions the amount of charge transferred to the electrolyte must be equal and opposite for the two types of carriers. The semiconductormediated redox processes involve electron transfer across the interface. When electron/hole pairs are generated in a semiconductor particle, the electron moves away from the surface to the bulk of the semiconductor as the hole migrates towards the surface (see Figure 1.3). If these charge carriers are separated fast enough they can be used for chemical reactions at the surface of the photocatalyst, i.e., for the oxidation or reduction of pollutants. hν recombination Red 1 Oxid 2 Oxid 1 Red 2 recombination Figure 1.3. Fate of electrons and holes within a particle of illuminated semiconductor in contact with an electrolyte. Metal oxides and sulphides represent a large class of semiconductor materials suitable for photocatalytic purposes. Table 1.4 lists some selected semiconductor materials, which have been used for photocatalytic reactions, together with band gap energy required to activate the catalyst. The final column in the table indicates the wavelength of radiation required to activate the catalysts. According to Plank s equation, the radiation able to produce this gap must be of a wavelength (λ) equal or lower than that calculated by Eq hc λ = (1.10) E G where E G is the semiconductor band-gap energy, h is Planck s constant and c is the speed of light. Material Band gap (ev) Wavelength corresponding to band gap (nm) BaTiO CdO CdS CdSe Fe 2 O

16 GaAs GaP SnO SrTiO TiO WO ZnO ZnS Table 1.4. Selected properties of several semiconductors Summarizing, a semiconductor particle is an ideal photocatalyst for a specific reaction if: The products formed are highly specific. The catalyst remains unaltered during the process. The formation of electron/hole pairs is required (generated by the absorption of photons with energy greater than that necessary to move an electron from the valence band to the conduction band) Photon energy is not stored in the final products, being an exothermic reaction and only kinetically retarded Homogeneous photodegradation The use of homogeneous photodegradation (single-phase system) to treat contaminated waters dates back to the early 1970s. The first applications concerned the use of UV/ozone and UV/H 2 O 2. The use of UV light for photodegradation of pollutants can be classified into two principal areas: Photooxidation. Light-driven oxidative processes principally initiated by hydroxyl radicals. Direct photodegradation. Light-driven processes where degradation proceeds following direct excitation of the pollutant by UV light. Photooxidation involves the use of UV light plus an oxidant to generate radicals. The hydroxyl radicals then attack the organic pollutants to initiate oxidation. Three major oxidants are used: hydrogen peroxide (H 2 O 2 ), ozone and Photo-Fenton reaction. H 2 O 2 absorbs fairly weakly in the UV region with increasing absorption as the wavelength decreases. At 254 nm, ε λ is 18 M -1 cm -1, whereas at 200 nm is 190 M -1 cm -1. The primary process for absorption of light below 365 nm is dissociation to yield two hydroxyl radicals: h ν H 2O2 2 OH (1.11) The use of hydrogen peroxide is now very common for the treatment of contaminated water due to several practical advantages: (i) the H 2 O 2 is available as an easily handled solution that can be diluted in water to give a wide range of concentrations; (ii) there are no air emissions; (iii) a high-quantum yield of hydroxyl radicals is generated (0.5). The major drawback is the low molar extinction coefficient, which means that in water with high UV absorption the fraction of light absorbed by H 2 O 2 may be low unless very large concentrations are used. Furthermore, especially as concerns the focus of this text, H 2 O 2 absorption is very low in the Solar UV range (up 300 nm). 9

17 Ozone is generated as a gas in air or oxygen in concentrations generally ranging from 1 to 8% (v/v). It has a strong absorption band centered at 260 nm with ε λ = 3000 M -1 cm -1. Absorption of light at this wavelength leads to formation of H 2 O 2 : h 1 O3 ν O( D) + O2 (1.12) 1 O( D) + H 2O H 2O2 (1.13) Hydroxyl radicals are then formed by reaction of ozone with the conjugate base of hydrogen peroxide: + H 2 O2 + H 2O HO2 + H 3O (1.14) HO 2 + O3 O3 + HO2 (1.15) O3 + H 2O HO3 + OH (1.16) HO 3 OH + O 2 (1.17) Since the net result of ozone photolysis is the conversion of ozone into hydrogen peroxide, UV-ozone would appear to be only a rather expensive method of making hydrogen peroxide. However, there are other oxidation-related processes occurring in solution, such as the direct reaction of ozone with a pollutant (see Table 1.3). Ozone may have advantages in water with high inherent UV absorbence, but it involves the same problem as hydrogen peroxide for use in solar energy processes. The essential step of the Fenton reaction is the same as for all AOPs. Highly reactive radicals (like HO and HO 2 ) oxidize nearly all organic substances to yield CO 2, water and inorganic salts. In the case of Photo-Fenton, Fe 2+ ions are oxidized by H 2 O 2 while one OH is produced (1.18), and the Fe 3+ or complexes obtained then act as the light absorbing species that produce another radical while the initial Fe 2+ is recovered (1.19 and 1.20) Fe + H O Fe + OH + OH (1.18) 2 2 Fe 3+ ν (1.19) H 2O + h Fe + H + OH Fe( OOC R)] + h Fe + CO2 R [ ν + (1.20) Note that in equation (1.20) the ligand R-COO can be replaced by other organic groups (ROH, RNH 2 etc.). Compared to other homogeneous photooxidation processes, the advantages of Photo-Fenton are the improved light sensitivity (up to a wavelength of 600 nm, corresponding to 35% of the solar radiation). On the other hand, disadvantages, such as the low ph values required (usually below ph 4) and the necessity of removing iron after the reaction, remain. Some pollutants are able to dissociate only in the presence of UV light. For this to happen, the pollutant must absorb light emitted by a lamp (or the sun) and have a reasonable quantum yield of photodissociation. Organic pollutants absorb light over a wide range of wavelengths, but generally absorb more strongly at lower wavelengths, especially below 250 nm (Figure 1.4). In addition, the quantum yield of photodissociation tends to increase at lower wavelengths, since the photon energy is increasing (eq. 1.1). The net chemical result of photodissociation is usually oxidation, since the free radicals generated can react with dissolved oxygen in the water. In practice, the range of waste waters that can be successfully treated by UV alone is very limited. This defect is more relevant when solar energy is used (see Figure 1.4) because only photons up 300 nm are available. 10

18 Figure 1.4. UV spectra between 200 and 400 nm of Acrinathrin and sunlight. 1.4 APPLICATION TO WATER TREATMENT As mentioned above, UV light can be used in several ways. But direct photolysis can occur only when the contaminant to be destroyed absorbs incident light efficiently. In the case of UV/ozone and UV/hydrogen peroxide this does not happen. But here too, absorption by some sensitizer must initiate the reaction, and limited absorption by the solute or the additive restricts efficiency. Furthermore, these mixtures often still require large quantities of added oxidant. By contrast, in heterogeneous photocatalysis, dispersed solid particles absorb larger fractions of the UV spectrum efficiently and generate chemical oxidants in situ from dissolved oxygen or water (see Figure 1.5). These advantages make heterogeneous photocatalysis a particularly attractive method for environmental detoxification. The most important features of this process making it applicable to the treatment of contaminated aqueous effluents are: The process takes place at ambient temperature. Oxidation of the substances into CO 2 is complete. The oxygen necessary for the reaction is obtained from the atmosphere. The catalyst is cheap, innocuous and can be reused. The catalyst can be attached to different types of inert matrices. O 2 - hν 3.0eV O 2 e- TiO 2 Particle h+ H 2 O WATER OH + H + Figure 1.5. Effect of UV radiation on a TiO 2 particle dispersed in water For all of the above reasons, from now on only this method is dealt with in this text. Whenever different semiconductor materials have been tested under comparable conditions for the degradation of the same compounds, TiO 2 has generally been demonstrated to be the 11

19 most active. Only ZnO is as active as TiO 2. TiO 2 s strong resistance to chemical and photocorrosion, its safety and low cost, limits the choice of convenient alternatives (Pelizzetti, 1995). Furthermore, TiO 2 is of special interest since it can use natural (solar) UV. This is because it has an appropriate energetic separation between its valence and conduction bands which can be surpassed by the energy content of a solar photon (see Table 1.4). Other semiconductor particles, e.g., CdS or GaP absorb larger fractions of the solar spectrum and can form chemically activated surface-bond intermediates, but unfortunately, these photocatalysts are degraded during the repeated catalytic cycles involved in heterogeneous photocatalysis. Therefore, degradation of the organic pollutants present in waste water using irradiated TiO 2 suspensions is the most promising process and R&D in this field has grown very quickly during the last years. ( TiO IV O 2 Ti e IV + h + ) OH O 2( ads) + TiO 2 TiO + h + e BV BC hν + e + h + TiO (1.21) 2 TiO 2( ads) + heat and / or 2 IV ( TiO O O 2 Ti IV hν ' ) OH + H + (1.22) (1.23) (1.24) To date, evidence supports the idea that the hydroxyl radical ( OH) is the main oxidizing specimen responsible for photooxidation of the majority of the organic compounds studied. The first effect, after absorption of near ultraviolet radiation, λ<390 nm, is the generation of electron/hole pairs, which are separated between the conduction and valence bands (Eq. 1.21). In order to avoid recombination of the pairs generated (Eq. 1.22), if the dissolvent is oxidoreductively active (water) it also acts as a donor and acceptor of electrons. Thus, on a hydrated and hydroxylated TiO 2 surface, the holes trap OH radicals linked to the surface (Eq. 1.23). In any case, it should be emphasized that even trapped electrons and holes can rapidly recombine on the surface of a particle (Eq. 1.22). This can be partially avoided through the capture of the electron by preadsorbed molecular oxygen, forming a superoxide radical (Eq. 1.24). Whatever the formation pathway, it is well known that O 2 and water are essential for photooxidation with TiO 2. There is no degradation in the absence of either. Furthermore, the oxidative species formed (in particular the hydroxyl radicals) react with the majority of organic substances. For example, in aromatic compounds, the aromatic part is hydroxylated and successive steps in oxidation/addition lead to ring opening. The resulting aldehydes and carboxylic acids are decarboxylated and finally produce CO 2. However, the important issue governing the efficiency of photocatalytic oxidative degradation is minimizing electron-hole recombination by maximizing the rate of interfacial electron transfer to capture the photogenerated electron and/or hole. This issue is discussed in more detail later. Degradation by photocatalysis has been most investigated in monoaromatics and consequently, these pollutants appear as model compounds in dozens of scientific papers. Some monoaromatics investigated have been benzene, dimethoxybenzenes, halobenzenes, nitrobenzene, chlorophenols, nitrophenols, benzamide, aniline, etc., most of which are recognised as priority pollutants (see Table 1.2). In addition to these, several other types of molecules have also been investigated as substrates for photocatalytic degradation: Haloaliphatics (trichloroethylen, tetrachloromethane, etc.). Important because so many of these compounds have been released into the environment and contaminate waters. Some also originate during water treatment by chlorinating. 12

20 Water-miscible solvents (ethanol, alkoxyethanol, etc.). These compounds are very difficult to detoxify since they are resistant to treatment and are poorly adsorbed on GAC. Pesticides. Contaminate waters where agricultural runoff is important. Among the recently investigated compounds are triazines, organophosphorous, carbamates, phenoxyacids, organochlorines, chloronicotinics, etc. Surfactants. Surface active agents enter domestic and industrial waste waters in increasing amounts. Because their biodegradability may be one of the more important constraints in their use, photocatalytic degradation has received increasing attention. Dyes. Strongly colored compounds can be removed by adsorption but it is always better to destroy them by oxidation. Three exhaustive reviews by Blake (1994, 1995, 1997) describe almost 1800 studies carried out before Despite encouraging laboratory-scale data and some industrial-scale tests, chemical oxidation detoxification is still restricted to a few experimental plants. The broader application of those technologies requires: i) reactor optimization and modeling and ii) assessment of the efficiency of oxidation technology to reduce the toxicity of effluents. The following chapters of this book will attempt to highlight these matters. 1.5 GAS-PHASE DETOXIFICATION Airborne pollutants (such as volatile compounds) can be treated during the gas phase with the UV/TiO 2 process. Gas-phase treatment offers several advantages. In general, substrate masstransport is an order of magnitude faster in the gas phase than in the liquid phase. This in turn leads to much faster reaction rates. Oxidant starvation (such as O 2 supply) may be less of a problem in the gas-phase than in water. There is also no interference on the photocatalytic surface from other species that are invariably present in aqueous treatment media (for example anions). In addition, photocatalysis separation after use is not a problem unlike with aqueous slurry suspensions. By using solar energy to drive the process, no fuel is required, gaseous affluent volume is reduced, no NO x is generated, no products of incomplete combustion are produced, CO 2 emanating from fuel burning is avoided and substantial fuel saving may be achieved. Since no burning takes place, oxygen is only necessary at stoichiometric ratio. Solar concentrators provide the opportunity for small-size solar furnaces and even mobile solar parabolic dishes for on-site destruction of low productions of highly toxic compounds On the other hand, there are indications that mineralization may not be complete with some organic substrates in the gas-phase. The TiO 2 photocatalyst loses its activity after prolonged use and must be reactivated with moist air that presumably restores the original degree of hydroxylation on the oxide surface. There are also indications that product (or intermediate) adsorption on the TiO 2 surface may be problematic during the course of the reaction. Pollutant substrates like trichloroethylene, acetone, formaldehyde, m-xylene and N ox have been treated with TiO 2 /UV in the gas-phase in bench-scale tests. Field tests have also been conducted to treat effluent air emissions using this technique at different manufacturing plants in the USA (Rajeshwar, 1996). 13

21 Class of Compound Aromatics Nitrogen-containing ring Aldehydes Ketones Alcohols Alkanes Terpenes Sulfur-containing Organics Chlorinated E!hylenes Acetyl Chlorides Chemicals Tested Benzene, Toluene Pyridine, Picoline, Nicotine Acetaldehyde. Formaldehyde Acetone Methanol. Ethanol, Propanol Ethylene. Propene, Tetramethyl Ethylene α-pinene Methyl Thiophene Dichloroethylene.Trichloroethylene.Tetrachloroetylene Dichloroacetyl Chloride, Tetrachloroacetyl Chloride Table 1.5. VOCs amenable to treatment via Photocatalytic Oxidation (Jakobi et al., 1996). SUMMARY OF THE CHAPTER A description is given of how solar chemistry could become a significant segment of the chemical industry and how it can be used, under certain conditions, to provoke chemical breakdown of pollutants into harmless by-products. The behaviour of contaminants in environmental water is summarised. The basic concepts of photochemistry relating to photolysis of chemical bonds, homogeneous photodegradation and heterogeneous photocatalysis are reviewed. The use of semiconductors for wastewater treatment, with particular reference to TiO 2, has been discussed. Examples of the waste materials that have been treated successfully using TiO 2, have been presented. Gas-phase photocatalysis has also been introduced. BIBLIOGRAPHY AND REFERENCES Blake, D.M.; Bibliography of Work on the Photocatalytic Removal of Hazardous Compounds from Water and Air. National Technical Information Service, US Depart. of Commerce, Springfield, VA22161, USA, May Update Number 1 To June 1995, October Update Number 2 To October 1996, January Hayo, M.G. and van der Werf. Assessing the impact of pesticides on the environment. Agric. Ecosys. Environ., 60, 81-96, Jacoby, W.A., Blake, D.M., Fennell, J.A., Boulter, J.E., Vargo, L.M., George, M.C. and Dolberg, S. K. Heterogeneous Photocatalysis for Control of Volatile Organic Compounds in Indoor Air. J. Air Waste Manage. Assoc. 46, no. 9, 891-8,

22 National Renewable Energy Laboratory, Solar Photochemistry-Twenty Years of Progress, What s Been Accomplished, and Where Does It Lead? Report NREL/TP , Golden, Colorado, USA, Pelizzetti, E. Concluding Remarks on Heterogeneous Solar Photocatalysis. Solar En. Mat. Sol. Cells, 38, , Rajeshwar, K. Photochemical Strategies for Abating Environmental Pollution. Chemistry & Industry, 17, , Tomin, C. The Pesticide Manual, a World Compendium. 10 th Edition. British Crop Protection Council. Croydon, UK, SELF-ASSESSMENT QUESTIONS PART A. True or False? 1. The solar energy is useful only to substitute fossil fuels converting it into thermal energy thus provoking chemical reactions. 2. Toxicity of a chemical is the same for all the species. 3. Biological treatment techniques are the cheapest wastewater treatment methods. 4. The energy of a photon depends of the ambient temperature. 5. Heterogeneous photocatalysis employs liquid catalysts. 6. Light driven oxidative processes are initiated by excited electrons of the catalyst surface. 7. Ozone can be produced from air. 8. The most important characteristics of a photocatalysts are: stability to chemical and photocorrosion, safety, cost and band-gap. 9. The electron/hole recombination can be avoided increasing reaction temperature. 10. Heterogeneous photocatalysis can be applied only to monoaromatics. PART B. 1. Which is the most important difference between thermochemical and photochemical solar processes? 2. Which are the usual ways to express the toxicity of a chemical in the environment? 3. Why biodegradation, which is a major mechanism in wastewater treatment, is quite inefficient to treat certain types of wastewater? 4. What is the percentage of absorbed photons in a solution with the following characteristics: extinction coefficient = 1327 cm -1 M -1, concentration of substrate 0.01 M, illuminated pathlength = 5.6 cm? And if the extinction coefficient is 0.3? 5. What is the wavelength able to excite a semiconductor which band-gap is 4.0 ev? 6. Name three important characteristics of heterogeneous photocatalysis to be used as water treatment process. 7. Why TiO 2 is the most suitable photocatalyst for wastewater treatment? 8. Which is the more important electron acceptor in water? 9. Which is the most important product of photocatalytic degradation with organic contaminants? 10. Why hydroxyl radicals react with organic substances? Answers Part A 15

23 1. False; 2. False; 3. True; 4. False; 5. False; 6. False; 7. True; 8. True; 9. False; 10. False. Part B 1. In thermochemical processes solar radiation is converted into thermal energy, in photochemical processes the solar photons are absorbed directly by the reactants giving rise to the reaction. 2. Toxicity of a chemical is usually expressed as the effective concentration or dose of the material that would produce a specific effect in 50% of a large population of test species (EC50 or ED50). 3. Because when compounds are very toxic, the micro-organisms need an extended period of adaptation, when they are not invaible % and 3.8 %. 5. λ 310 nm 6. The process takes place at ambient temperature, the oxygen necessary for the reaction is obtained from the atmosphere, the catalyst is cheap, innocuous and can be reused. 7. It has exhibited the highest activity. It is high stable to chemical and photocorrosion. It can use natural UV. 8. Dissolved oxygen. 9. Carbon dioxide. 10. Because of its very high oxidation potential. 16

24 2. SOLAR IRRADIATION AIMS This unit describes the power of light as a source of energy. It outlines the basic principles that are related to the light spectrum and specifically to the solar spectrum. This chapter discusses solar UV radiation and its photon flux in more detail, because this part of the solar spectrum is the most important for driving chemical processes. Moreover, the major atmospheric variables determining the amount of UV solar radiation on the earth s surface are discussed. A method for calculating UV attenuation at a given site is presented. Finally, solar radiation measurement systems are described. OBJECTIVES At the end of this unit, you will understand the main factors affecting solar radiation behaviour and you will be able to do six things: 1. Discriminate between the different components of solar radiation and their principal characteristics. 2. Recognize typical solar spectra and understand the effect of sun position on the solar power reaching the earth s surface. 3. Find the photon flux of a polychromatic source of energy with simple calculations. 4. Describe the most important components of the earth s atmosphere and the consequences for power and spectral distribution of the solar radiation. 5. Understand the procedures that permit solar power to be calculated from available radiation at any given site. Comprehend the basic principles on which solar radiation measurement is based. NOTATION AND UNITS Symbol Units AM Air mass ratio f n Clouds factor f λ Fraction of power associated with a wavelength nm -1 H Radiance exposure monthly average kj m -2 H TBD TBD UV radiance exposure kj m -2 I Photon flux density Einstein s -1 m -2 N a Quantity of photons absorbed by the system Photons s -1 N 0 Avogadro s number, x Photons mol -1 N λ Number of photons supplied by a source of light of wavelength λ Photons s -1 Q λ Energy of a monochromatic source of light of wavelength λ W m -2 µm -1 T Transmittance T λ Transmittance of direct-bean solar radiation under cloudless skies at a specific wavelength T a,λ Transmittance related to absorption and dispersion by aerosols T g,,λ Transmittance resulting from absorption of atmospheric gases T o,λ Transmittance related to the effect of the ozone layer T R,λ Transmittance related to the molecules of air Transmittance resulting from absorption by steam. T v,,λ 17

25 TBD UV Typical best day. Completely clear sky during all the hours of sunlight U λ Energy of one photon ev, J UV D Direct ultraviolet light W m -2 UV G Global ultraviolet light W m -2 UV λ Ultraviolet irradiance associated with a wavelength W m -2 nm -1 Φ Quantum yield No units λ. Wavelength nm, µm 2.1 THE POWER OF LIGHT Light is just one of various electromagnetic waves present in space. The electromagnetic spectrum covers an extremely broad range, from radio wavelengths of a meter or more, down to x-rays with wavelengths of less than one billionth of a meter. Optical radiation lies between radio waves and x-rays on that spectrum and has a unique combination of ray, wave, and quantum properties. At x-ray and shorter wavelengths, electromagnetic radiation tends to be quite particle-like in its behaviour, whereas toward the long wavelength end of the spectrum behaviour is mostly wavelike. The UV-visible portion occupies an intermediate position, having both wave and particle properties in varying degrees (See Figure 2.1a). a) X-rays UV nm 40 Vis0- ibl 77 e 0 nm Infrared nm Wavelength λ, nanometers Microwaves b) λ Figure 2.1 The optical portion of the electromagnetic spectrum (a) and light wave front modelled as a straight-line (b). Like all electromagnetic waves, light waves can interfere with each other, become directionally polarised, and bend slightly when passing through an edge. These properties allow light to be filtered by wavelength or amplified coherently as in a laser. In radiometry, light s propagating wave front is modelled as a ray travelling in a straight line (See Figure 2.1b). Lenses and mirrors redirect these rays along predictable paths. Wave effects are insignificant in a large-scale optical system, because the light waves are randomly distributed and there are plenty of photons Ultraviolet light Short wavelength UV-light exhibits more quantum properties than its visible or infrared counterparts. Ultraviolet light is arbitrarily broken down into three bands, according to its anecdotal effects. UV-A ( nm), which is the least harmful type of UV light, because it has the least energy (recall Eq. 1.1), is often called black light, and is used for its relative harmlessness and its ability to cause fluorescent materials to emit visible light thus appearing to glow in the dark. UV-B ( nm) is typically the most destructive form of 18