9 UV/Titanium Dioxide 9.1 Introduction The photochemical reactions of titanium dioxide (TiO 2 ) have drawn much attention in the field of advanced oxidation processes for over 30 years. Research on photocatalysis using semiconductors began in the late 1940s with the work of Laidler et al., who investigated the chemical reactions of zinc oxide under illumination. At the same time, Mashio and Kato were studying the photocatalytic oxidant action of alcohols by TiO 2 . In the late 1960s, Honda and Fujishima made significant discoveries that led to the development of TiO 2 as a semiconductor photocatalyst (Fujishima and Honda, 1971). The irradiation of a TiO 2 electrode coupled with a platinum electrode was found to be able to split water into hydrogen and oxygen (Fujishima et al., 1969); the water was simultaneously oxidized and reduced. This discovery laid the foundation for further research on TiO 2 to generate hydrogen as a combustible fuel. It soon became apparent that redox reactions of organic and other inorganic compounds could be induced by band gap irradiation of a variety of semiconductor particles. The photocatalytic oxi- dation of simple organic compounds such as alcohols and carboxylic acids was further studied by Kraeutler and Bard (1998). Through the 1970s and by the end of the 1980s, photocatalytic reactions of TiO 2 were studied for almost all classes of organic compounds. In the 1990s, a UV/TiO 2 process for treating wastewaters containing organic pollutants was commercialized. Titanium dioxide is widely used in the production of plastics, enamels, artificial fibers, electronic materials, and rubber (Hadjiivanov and Klissur- ski, 1996). Its ability to photocatalyze the oxidation of organic materials has been known for years in the paint industry. For this reason, TiO 2 is used as a white paint pigment (Stafford et al., 1996). TiO 2 is also known as an excellent catalyst for semiconductor photocatalysis due to its nonselectivity for environmental engineering applications; it is nontoxic, insoluble, TX69272_C09.fm Page 321 Tuesday, November 11, 2003 12:14 PM © 2004 by CRC Press LLC 322 Physicochemical Treatment of Hazardous Wastes reusable, photostable, readily available, and inexpensive (Tang and Chen, 1996). UV/TiO 2 is an appealing advanced oxidation process (AOP) because it has been demonstrated that the process can mineralize a wide variety of organic pollutants. Titanium dioxide is the most widely used catalyst in photocatalytic deg- radation of organic pollutants due to its suitable band-gap energy of 3.05 eV over a wide range of pH (Tang and Huang, 1995). The chemical viability of UV/TiO 2 has been established, but the future of photocatalysis depends on new designs for photochemical reactors and fixation of TiO 2 onto supports to eliminate its separation from the treated effluent. For this purpose, catalyst supports, glass beads, glass plates, fiberglass mesh, and porous films on glass substrates have been studied (Peil and Hoffmann, 1996). 9.2 Fundamental Theory 9.2.1 Photoexcitation A semiconductor has a band structure that is characterized as the valance band (VB), which has a series of closely spaced energy levels associated with covalent bonding between atoms. The valence band is composed of a crys- tallite structure. The conduction band (CB) is a series of spatially diffuse, energetically similar levels at a higher energy. The magnitude of energy between the electron-rich valence band and the electron-deficient conduction band is responsible for the extent of thermal distribution of the conduction band and, ultimately, the electrical conductivity of the particle. This band gap also defines the sensitivity of the semiconductor to irradiation by pho- tons at different wavelengths. Photocatalytic oxidation by UV/TiO 2 involves the excitation of TiO 2 par- ticles by UV light from the valance band of the solid to the conduction band. TiO 2 + h n Æ e – CB + h + VB (9.1) The equation can also be illustrated in Figure 9.1. When a semiconductor such as TiO 2 absorbs photons, the valence band electrons are excited to the conduction band. For this to occur, the energy of a photon must match or exceed the band-gap energy of the semiconductor. This excitation results in the formation of an electronic vacancy or positive hole at the valence band edge. A positive hole is a highly localized electron vacancy in the lattice of the irradiated TiO 2 particle. This hole can initiate further interfacial electron transfer with the surface bound anions. The primary steps in photoelectrochemical mechanism are as follows: (1) formation of charge carriers by a photon; (2) charge-carrier recombination to liberate heat; (3) initiation of an oxidative pathway by a valence-band hole; (4) initiation of a reductive pathway by a conduction-band electron; (5) further TX69272_C09.fm Page 322 Tuesday, November 11, 2003 12:14 PM © 2004 by CRC Press LLC UV/Titanium Dioxide 323 thermal and photocatalytic reactions to yield mineralization products; (6) trapping of a conduction band electron in a dangling surficial bond to yield Ti(III); and (7) trapping of a valence-band hole at a surficial titanol group. The photogenerated electrons and holes of a semiconductor lose their energy by interactions with oxidants and reductants, respectively, in an aqueous solution. They are excited from the bottom of the valence band to the upper edge of the conduction band. Photoexcitation of TiO 2 and the generation of an electron/hole pair creates the potential not only for oxida- tion but also for reduction. The energy level of the bottom of the conduction band can be considered to be a measure of the reduction strength of the photoexcited electrons. The energy level of the upper level of the valance band is a measure of the oxidation strength of the holes. The positions of the valence and conduction bands of TiO 2 at pH = 1, relative to standard carbon electrode (SCE), are –0.1 and +3.1 V, respectively (O’Shea and Car- dona, 1994). Conduction band electrons generated within TiO 2 molecules have chemical reactivity patterns that can be monitored. The potential of semiconductors for oxidation and reduction can be classified into four groups according to the water-splitting reaction shown in Table 9.1. This classification based on water splitting is important to understanding the redox potential of a given semiconductor. Although this classification is simple, it is convenient in selecting a semiconductor that is appropriate for a desired reaction. For a more detailed reactor design, factors such as the lifetimes of carriers; energy levels of surface states; adsorption and desorp- tion of molecules on the surface; kinetic nature of the surface; and electron kinetics must be considered (Serpone and Pelizzetti, 1989). 9.2.2 Hydroxyl Radical Formation The production and reactivity of the radical intermediates are the main factors limiting the entire oxidation reaction rate. The formation of the FIGURE 9.1 Excitation of electron from valance band to conduction band; semiconductor potential of TiO 2 . O 2 O 2 - CO 2 Organics H 2 O H 2 - H 2 O OH - ⋅ OH 3eV O 2 hv O 2 TX69272_C09.fm Page 323 Tuesday, November 11, 2003 12:14 PM © 2004 by CRC Press LLC 324 Physicochemical Treatment of Hazardous Wastes hydroxyl radicals on irradiated TiO 2 in an aqueous system has been probed by pulse radiolysis. The detection of singly oxidized transients suggests that surface-bound hydroxyl radicals initiate the oxidation of the surface-bound substrate rather than diffuse into the bulk solution. Several other studies on this topic have also reached the same conclusion (Fox and Dulay, 1993). Superoxide and perhydroxyl radicals are other radicals formed from the reactions of electrons with adsorbed oxygen. The generated radicals can then oxidize organic pollutants at the solid–liquid interface. Direct oxidation of organic pollutants may also be possible by photogenerated holes and may proceed in competition with hydroxyl radical oxidation, as proposed for benzene oxidation, although Okamoto et al. (1985) did not report the exist- ence of this pathway for phenol. Oxidation by holes was suggested for some acids, such as trichloroacetic acid, which are formed as intermediates from the oxidation of chlorinated ethanes by hydroxyl radicals. Photogeneration of radical species can be represented by the following reactions: h + VB + OH – (surface) Æ OH • (9.2) h + VB + H 2 O (adsorbed) Æ OH • + H + (9.3) e – CB + O 2 (adsorbed) Æ O 2 • (9.4) e – CB + h + VB Æ heat (9.5) where h n represents UV radiation, represents valence-band holes, represents conduction-band electrons, and represents a superoxide ion. H 2 O 2 , which can also be generated from the superoxide ion by various mechanisms, can also be a source for hydroxyl radicals by further reacting with electrons or superoxide ions. It may be decomposed to hydroxyl radi- cals by photolytic disassociation. Kormann et al. (1991) determined that the TABLE 9.1 Classification of Semiconductors Based on Water Splitting Reaction Classification Reaction OR type The oxidation and reduction power is strong enough to promote hydrogen and oxygen production. Examples include TiO 2 , SrTiO 3 , and CdS. OR indicates a strong ability for both oxidation and reduction. R type Only the reduction power is strong enough to reduce water; the oxidation power is too weak. Examples include CdTe, CdSe, and Si. O type The valence band is located deeper than the O 2 /H 2 O level so the oxidation power is strong enough to oxidize water, but the reduction power is not strong enough to reduce water. Examples include Fe 2 O 3 , MoS 2 , and Bi 2 O 3 . X type The conduction and valence bands are located between the H + /H 2 and O 2 H 2 O levels; therefore, both the oxidation and reduction powers are so weak that neither oxygen nor hydrogen can be produced. h VB + e CB – O 2 • TX69272_C09.fm Page 324 Tuesday, November 11, 2003 12:14 PM © 2004 by CRC Press LLC UV/Titanium Dioxide 325 reaction of surface-bound hydroxyl radicals with the adsorbed organic com- pound was the rate-determining step. The recombination of electrons and holes was reportedly the main factor in limiting oxidation rates of organic substrates. The holes and the electrons at the surface of the TiO 2 molecule can then form hydroxyl radicals from the oxidation of oxygen, water, or hydroxide ions (Venkatadri and Peters, 1993); however, the oxidative degradation rates of organic pollutants are based upon the energy needed to cleave a given chemical bond and the concentration of dissolved molecular oxygen. The formation of hydroxyl radicals has been observed when TiO 2 is irra- diated with ultraviolet light. Hydroxyl radical is the most powerful oxidizing species after fluorine. In the UV/TiO 2 process, if organic compounds are rich in p electrons, hydroxylation will proceed as previously described in Equa- tion (4.1). Hydrogen abstraction usually occurs in reaction with unsaturated organic compounds. Peroxyl radicals are produced by the reaction between the organic radicals and the molecular oxygen: R • + O 2 Æ RO 2 • (9.6) 9.2.3 The Role of Adsorption in the UV/TiO 2 Process Photocatalytic oxidation is a surface-catalyzed reaction; therefore, a chemical must first be adsorbed onto the TiO 2 surface before it can undergo photo- catalytic oxidation. One of the requirements of the Langmuir–Hinshelwood model is the adsorption of the compound has to occur before oxidation takes place. For UV/TiO 2 systems, Fox et al. (1990) proposed that strong adsorp- tion of substrates on the TiO 2 surface is required due to the very fast recom- bination of electron/hole pairs. Photocatalytic degradation has been demonstrated to follow the Langmuir–Hinshelwood kinetic model (Mat- thews, 1991; Davis and Huang, 1990). It provides strong evidence of substrate preadsorption onto the TiO 2 surface. Langmuir–Hinshelwood (LH) kinetics are widely used to quantitatively delineate substrate preadsorption in both solid–gas and solid–liquid reac- tions. The model assumptions are stated in Table 9.2. Under these TABLE 9.2 Langmuir–Hinshelwood Model Assumptions The number of surface adsorption sites is fixed at equilibrium. Only one substrate may bind at each surface site. The heat of adsorption by the substrate is identical for each site and is independent of surface coverage. No interaction occurs between adjacent adsorbed molecules. The rate of surface adsorption of the substrate is greater than the rate of any subsequent chemical reaction. No irreversible blocking of active sites by binding to product occurs. TX69272_C09.fm Page 325 Tuesday, November 11, 2003 12:14 PM © 2004 by CRC Press LLC 326 Physicochemical Treatment of Hazardous Wastes assumptions, the relationship between surface coverage, q; initial substrate concentration, C ; and adsorption equilibrium constant, K , can be expressed by the following equation: q = KC /(1 + KC ) (9.7) The initial degradation rate of a substrate can be described by its initial concentration and adsorption characteristics: r LH = –d C /d t = kKC /(1 + KC ) (9.8) where k is the rate constant, and t is the reaction time. For two or more species competing for one adsorption site, the following expression was suggested by Davis and Huang (1990): r LH = kKC /(1 + KC + S i K i C i ) (9.9) where i represents a competitively adsorbed species. Good linearity of a plot 1/ r LH vs. 1/ C reflects the validity of the LH model, which demonstrates the preadsorption of a target compound during photocatalytic degradation. Oxi- dation rate constants and adsorption equilibrium constants can be obtained from the slope (1/ kK ) and intercept (1/ k ) of the straight line. 9.2.4 Characteristics of TiO 2 Surface The characteristics of the TiO 2 surface determine its adsorptive capacity and photocatalytic activity. TiO 2 exists as anatase, rutile, and brookite; however, the catalytic activities of each are significantly different. The rutile form has the most practical applications, as it is the most stable of the three. The effectiveness of TiO 2 as a catalyst is determined by its surface properties. The TiO 2 surface has many active sites that contribute to its high catalytic activity. The anatase surface is highly heterogeneous and has three kinds of Lewis acid sites, due to the differently coordinated Ti 4+ ions. The surface also has at least two kinds of hydroxy groups present on the surface (Hadijiivanov and Klissurski, 1996). Rutile and anatase have similar crystal structures, both tetragonal (Stafford et al., 1996). The difference in the catalytic activities of the anatase and rutile form is due to differences in lattice structure. It has been reported that the reducing properties of conduction-band electrons are dependent on lattice structures (Stafford et al., 1996). Anatase has the highest energy for the lowest unoccu- pied molecular orbital (LUMO), making it the least reactive of the three forms of TiO 2 (Gratzel and Rotzinger, 1985). Titanium dioxide is synthesized by various methods according to the structure that is desired; therefore, the surface properties depend on prepa- TX69272_C09.fm Page 326 Tuesday, November 11, 2003 12:14 PM © 2004 by CRC Press LLC UV/Titanium Dioxide 327 ration methods. Two methods that are used to produce TiO 2 are known as the sulfate method and vapor-phase oxidation of TiCl 4 . Vapor-phase oxida- tion is the most widely used preparation technique, and the product formed is anatase. The most popular commercial TiO 2 is Degussa P25, which is a fumed TiO 2 that consists of both anatase and rutile in proportions of 80:20. The product is mostly anatase with a coating of rutile on the surface. Degussa TiO 2 usually has a surface area of 50 m 2 /g and an average particle size of 30 nm. The preparation of TiO 2 has been studied to enhance its photocatalytic properties (Scalfani et al., 1990). For example, it has been found that the fastest photocatalytic degradation rates exist for anatase samples that have been prepared by precipitation of titanium isoperoxide at 350°C. Anatase samples that have been heated to 800°C will form rutile TiO 2 . Titanium dioxide particles that are colloidal in size have been proposed for improving the efficiency of photocatalysis. Decreasing the particle size is known to cause a widening of the bandgap (Brus, 1990). This quantum size effect may also contribute to the increased reactivity of surface holes and electrons (Stafford et al., 1996). The particle size of TiO 2 has also been correlated to its charge distribution. In photocatalytic experiments with UV light, illumination of TiO 2 has shown the zeta potential to become more positively charged, thus yielding better adsorption and degradation rates of organic anions (Kim and Anderson, 1996). Titanium dioxide can be used in the stationary phase attached to a support medium such as silica gel or fiberglass, to glass beads, or in ceramic mem- branes. Use of TiO 2 in the stationary phase avoids the need for separation. TiO 2 can also be suspended in a heterogeneous system, which requires sep- aration by microfiltration or centrifugation. TiO 2 has also been used as a coating on glass tubes as a catalyst support. In reactions using TiO 2 suspen- sions, degradation rates are two to six times faster than when the TiO 2 is fixed on a support. Titanium dioxide photodegradation rates can be significantly enhanced with H 2 O 2 . With the addition of H 2 O 2 , degradation times of trichloroethylene (TCE) dropped from 75 to 20 min in a study by Tanaka et al. (1989). This enhancement was most likely due to an increase of hydroxyl radicals. The half-lives of pesticides DDVP and DEP were demonstrated to be shortened with the addition of H 2 O 2 (Harada et al., 1990). Similar enhancements were shown for the photodegradation of chloral hydrate, phenol, and chlorophe- nols (Venkatadri and Peters, 1993). The catalytic activity of TiO 2 can be increased with the loading of metals such as silver or platinum. The loading of silver onto the surface of TiO 2 has been shown to increase the removal of chloroform from 35 to 45% and the removal of urea from 16 to 83% (Kondo and Jardim, 1991). The draw- back of this treatment is the dissolution of silver into solution at a level of 0.5 ppm, which is 10 times the regulatory limit (Venkatadri and Peters, 1993). TX69272_C09.fm Page 327 Tuesday, November 11, 2003 12:14 PM © 2004 by CRC Press LLC 328 Physicochemical Treatment of Hazardous Wastes 9.2.5 Adsorption of Organic Compounds on TiO 2 In the past, the degradation of most classes of organic compounds has been studied using an UV/TiO 2 system. Detailed mechanisms and kinetic data are presented in several recent literature reviews by Legrini et al. (1993), Mills et al. (1993), and Hoffmann et al. (1995). Adsorption has great effects on photocatalytic oxidation kinetics. For example, a strong correlation between the adsorptive capacity of thiocarbamates and the extent of photo- catalytic oxidation has been demonstrated (Sturini et al., 1996). The reaction rate of thiocarbamate has been found to be governed by its adsorption kinetics. Thiocarbamates are not soluble in water and were observed to be adsorbed onto the TiO 2 surface. It was found that only substrates adsorbed onto the surface of the TiO 2 molecule are photodegraded (Sturini et al., 1996). Tunesi and Anderson (1991) studied the influence of chemisorption on the photodecomposition of salicylic acid and several other compounds. The role of adsorption may be even more important for compounds that exhibit pH- dependent adsorption behavior in aqueous solutions. The presence of water in TiO 2 suspensions has been shown to strongly affect the bonding behavior of compounds to be oxidized. It has been observed that water and hydroxide groups readily adsorb onto the TiO 2 surface. In chemisorption, these ligands are exchanged with absorbing solutes, while in physisorption no interactions between the TiO 2 surface and the compound of interest are observed (Tunesi and Anderson, 1991). The degradation kinetics of fenuron is affected by the pH solution (Rich- ard and Benagana, 1996). The products of the hydroxyl radical attack on fenuron are easily identified. It can also be concluded that the pH effect on the degradation of fenuron can be explained by differences in adsorption with changing pH. Because the point of zero charge of Degussa P-25 TiO 2 is equal to 6.3, the surface charge is neutral in neutral medium; however, at pH lower than 6.3, the surface is positively charged and molecules are attracted to the surface by their electronegative component (Richard and Benagana, 1996). As shown in Figure 9.2, the mechanism for the adsorption of fenuron onto TiO 2 can be explained in two different ways. In the first scheme, the positively charged TiO 2 surface repels the positively charged nitrogen atoms. This prevents the hydroxyl radicals attacking the methyl groups. In the second scheme, the carbon–nitrogen bonds are planar to the surface. This position seems more favorable because the lone electron pairs of the nitrogen and oxygen are close to the positive TiO 2 surface and the methyl groups can be oxidized. The degradation of fenuron in neutral medium is accomplished by oxida- tion of the methyl groups and the aromatic ring. In acidic TiO 2 suspensions, the degradation of fenuron is accomplished primarily by oxidation of the methyl groups and not oxidation of the aromatic ring. The oxidation of methyl groups can be explained by the position in which fenuron was adsorbed at the surface of TiO 2 (Richard and Benagana, 1996). TX69272_C09.fm Page 328 Tuesday, November 11, 2003 12:14 PM © 2004 by CRC Press LLC UV/Titanium Dioxide 329 Tunesi and Anderson (1991) reported that the electronic environment of the Ti 4+ cation is affected by the number of bonds with oxygen and by the coordination number of these oxygen ligands. In aqueous media, Ti 4+ cat- ions are capable of bonding to water molecules. For salicylic acid, benzoic acid, phenol, and 4-chlorophenol, the stereochemical configurations of the compounds are critical to adsorptive capacity. Salicylic acid has the stere- ochemical configuration that makes ring formation possible, while it is impossible for benzoic acid, phenol, and 4-chlorophenol to have this struc- ture. When this TiO 2 –salicylate chelate is formed, the salicylate bonds to the O surface orbitals of the TiO 2 molecule. Upon irradiation, the O orbitals become the source of holes and attract electrons from the ligand, leading to oxidation of the salicylate (Tunesi and Anderson, 1991). The photocata- lytic oxidation mechanisms of phenol and 4-chlorophenol were not observed to be affected by surface conditions. Phenol and 4-chlorophenol do not significantly adsorb on TiO 2 surfaces, so the degradation rates are independent of adsorption. The adsorption of organic ligands onto metal oxides and the parameters that have the greatest effect on adsorption were also studied (Stone et al., 1993). The extent of adsorption was measured by determining the loss of the compound of interest from solution. The physical and chemical forces that control adsorption into two general categories were classified as either specific or nonspecific adsorptions. Specific adsorption involves the phys- ical and chemical interaction of the adsorbent and adsorbate. Under spe- cific adsorption, the chemical nature of the sites influences the adsorptive capacity. Nonspecific adsorption does not depend on the chemical nature of the sites but on characteristics such as surface charge density (Stone et al., 1993). The interactions of specific adsorption can be explained in two ways. The first approach uses activity coefficients to relate the electro- chemical activity at the oxide/water interface to its electrochemical activ- ity in bulk solution (Stone et al., 1993). This approach is useful in situations FIGURE 9.2 Adsorption of fenuron onto TiO 2 . (From Richard, C. and Benagana, S., Chemosphere , 33, 635, 1996. With permission.) C NN O + + + N C O C N Scheme 1 Scheme 2 + + + TX69272_C09.fm Page 329 Tuesday, November 11, 2003 12:14 PM © 2004 by CRC Press LLC 330 Physicochemical Treatment of Hazardous Wastes where the making or breaking of covalent bonds is not occurring. Long- range electrostatic forces may be described using the Poisson–Boltzmann term: { I } x = { I } bulk exp (- zF Y / RT ) (9.10) where z is the ion charge, F is the Faraday constant, Y is the electrical potential, R is the universal gas constant, and T is the temperature of the reaction. The second approach postulates a new chemical species and an equilib- rium constant that relates the activity of the new species to the established species. The mechanism of nonspecific adsorption is believed to be due to long-range electrostatic forces on counterions near the charged TiO 2 surface. The extent of nonspecific adsorption can be calculated once the electrical potential on the TiO 2 surface is known (Stone et al., 1993). The extent of adsorption of organic ligands was further studied by Vasude- van and Stone (1996). Organic ligands have Lewis-base functional groups and are capable of forming bonds with protons, metal ions, and metal oxides. These ligands are polar and ionizable. Interaction between a ligand and the TiO 2 surface adds electrostatic forces to the interaction of a compound and the metal oxide surface. The extent of adsorption was affected by the sub- stituents on the aromatic ring (Vasudevan and Stone, 1996). TiO 2 has a high ionic contribution to bonding, and ligands with the greatest amount of ionic bonding capability are most readily adsorbed onto the TiO 2 surface. The interactions of ligands and metal oxides have been classified in two ways: those arising from long-range electrostatic forces and those that are difficult to quantify, which are referred to as near-range physical and chemical forces. According to the position of substituents on an aromatic ring, Lewis-base groups at the ortho position adsorb to a significant extent compared to com- pounds with groups at the meta and para positions (Vasudevan and Stone, 1996). This is due to the fact that Lewis-base groups at the ortho position can simultaneously coordinate a single metal ion, while the meta and para base groups cannot. The effect of mineral surfaces on the chemical transformation of organic chemicals was studied by Torrents and Stone (1991), who used the pesticide- like compound phenyl picolinate (PHP). Metal oxides, including TiO 2 , SiO 2 , Al 2 O 3 , Fe 2 O 3 , and FeOOH, were used as adsorbents. Effects of adsorbents on photocatalytic, redox, polymerization, and hydrolysis reactions have been widely studied, as it is necessary to understand the role that adsorption plays in these reactions. For these surface-catalyzed reactions to occur, the com- pound of interest must be adsorbed onto the surface of a metal oxide. If the adsorption is non-specific, the compound does not change in structure. If the adsorption is specific, the compound will change in structure and in chemical properties, making the compound more susceptible to reaction. Torrents and Stone (1991) concluded that mineral surfaces can catalyze reac- tions of carboxylic acids and esters in three ways: TX69272_C09.fm Page 330 Tuesday, November 11, 2003 12:14 PM © 2004 by CRC Press LLC [...]... the denitration of 4-NP (Dieckmann and Gray, 199 6) O e- O O OH H2O 4-NP.+ NO 2 NO2 4-NP NO2 + 4-NC 4-NP e-, -NO2, 4-NC.+ H2O O OH e - OH H2O HQ.+ OH HQ OH BT further oxidized products CO2 +H2O FIGURE 9. 11 Degradation pathways of 4-nitrophenol via sensitized photocatalysis (From Dieckmann, M and Gray, K., Water Res., 30(5) 11 69, 199 6 With permission.) © 2004 by CRC Press LLC TX 692 72_C 09. fm Page 354 Tuesday,... degradation of 4-nitrophenol was 50% lower for the deoxygenated system than for the oxygenated system The reaction mechanism for direct photocatalysis of 4-nitrophenol (4-NP) is summarized in Figure 9. 10 © 2004 by CRC Press LLC TX 692 72_C 09. fm Page 352 Tuesday, November 11, 2003 12:14 PM 352 Physicochemical Treatment of Hazardous Wastes O- OH OH NO2 OH H abstraction -NO 2- OH 4-NP OH HQ - O O OH OH OH OH -NO2... capability of the –OH substituent at the 1-position OH OH OH ClO 3- lC OH 4-CD+ h+ products lC e- 4-CP OH Cl OH 4-CD O2 O e- O Cl OH 4-CDO OH + ClOH HQ FIGURE 9. 5 Reaction mechanism for the degradation of 4-chlorophenol by UV/TiO2 (From Martin et al., Environ Sci Technol., 30, 2535, 199 6 With permission.) © 2004 by CRC Press LLC TX 692 72_C 09. fm Page 344 Tuesday, November 11, 2003 12:14 PM 344 9. 3.6 Physicochemical... OH OH OH -NO2 OH - NO2 NO 2 OH 4-NP BT OH - O O -e NO 2 NO2 OH O2 -N OH OH further oxidized products OH 4-NP NO CO2 + H2O 4-Nitrophenol FIGURE 9. 10 Degradation pathways of 4-nitrophenol via direct photocatalysis (From Dieckmann, M and Gray, K., Water Res., 30(5) 11 69, 199 6 With permission.) In the oxygenated system, OH• abstracts hydrogen for solution phase nitrophenolate ion (4-NP-), leading to denitration... intermediates among the four compounds studied The major by-product was found to be 2,3,5-trichloro-1,4-hydroquinone After 60 min of reaction, the toxicity of the intermediates was less than that of 2,3,5-TCP The toxicity of 3,5-DCP and 2,4-DCP was shown to decrease with decreasing concentration of the parent compound After 90 min of irradiation, 2,4-DCP disappeared from solution without an increase in... aldehydes, ketones, and dibenzofurans The intermediates and by-products formed may be potentially more harmful that the parent compounds Six dichlorophenol isomers (2, 3-, 2, 4-, 2, 5-, 2, 6-, 3, 4-, and 3,5-DCP) were studied by Minero et al ( 199 5) to identify reaction by-products A 1500-W xenon lamp emitting light at >340 nm was used to irradiate 5-mL samples containing 20 mg/L of DCP and 250 mg/ L TiO2 for... permission.) © 2004 by CRC Press LLC TX 692 72_C 09. fm Page 342 Tuesday, November 11, 2003 12:14 PM 342 Physicochemical Treatment of Hazardous Wastes A dual hole-radical mechanism where H+ oxidation of 2,4-D (Equation 9. 14) exists in competition with H+ oxidation of surface hydroxyl groups (Equation 9. 15) can be described as below H+ + OHs– Æ O• (9. 14) H+ + OHs Æ R(oxidized) (9. 15) Holes carry out electron transfer... degraded in 100 min © 2004 by CRC Press LLC TX 692 72_C 09. fm Page 338 Tuesday, November 11, 2003 12:14 PM 338 9. 3.4 Physicochemical Treatment of Hazardous Wastes Anisoles (Methoxybenzenes) The photocatalytic oxidation of para- and meta-substituted anisoles with UV/ TiO2 was studied by Amalric et al ( 199 6) A 100-mL batch reactor containing aqueous solutions of the anisoles was irradiated with UV light at... oxidation of an alcohol and the reduction of the nitro compound are coupled (Mahdavi et al., 199 3) These findings are consistent with the generation of hydrogen gas Reduction of nitro compounds is highly dependent on the reduction potential of the compound being studied Nitro © 2004 by CRC Press LLC TX 692 72_C 09. fm Page 350 Tuesday, November 11, 2003 12:14 PM 350 Physicochemical Treatment of Hazardous Wastes. .. values on either side of pH 2 to 3 The optimum pH for photocatalytic oxidation of 2,4-D is pH 3 The observed products from the photocatalytic oxidation of 2,4-D are carboxyl CO2, formaldehyde, 2,4-DCP, and dichlorophenol formate The yields of the products suggest that the first step in the oxidation of 2,4-D is one-electron oxidation of the carboxyl group (Sun and Pignatello, 199 5); however, the reaction . major by-product was found to be 2,3,5-trichloro-1,4-hydro- quinone. After 60 min of reaction, the toxicity of the intermediates was less than that of 2,3,5-TCP. The toxicity of 3,5-DCP and 2,4-DCP. (2, 3-, 2, 4-, 2, 5-, 2, 6-, 3, 4-, and 3,5-DCP) were studied by Minero et al. ( 199 5) to identify reaction by-products. A 1500-W xenon lamp emitting light at >340 nm was used to irradiate 5-mL. the degradation of pentachlorophenol (PCP), 2,4-dichlo- rophenol (2,4-DCP), 3,5-dichlorophenol (3,5-DCP), and 2,3,5-trichlorophenol (2,3,5-TCP) as reported by Jardim et al. ( 199 7). Degradation