Role of Reduction in the Photocatalytic Degradation of TNT

Jul 25, 1996 - The reduction of TNT by electrochemical and photoelectrochemical techniques has been investigated to obtain insight into the role of re...
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Role of Reduction in the Photocatalytic Degradation of TNT DANIEL C. SCHMELLING,† K I M B E R L Y A . G R A Y , * ,‡ A N D P R A S H A N T V . K A M A T * ,§ Department of Civil Engineering and Geological Sciences and Notre Dame Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556, and Department of Civil Engineering, 2145 Sheridan Road, Northwestern University, Evanston, Illinois 60208-3109

The reduction of TNT by electrochemical and photoelectrochemical techniques has been investigated to obtain insight into the role of reductive transformations in a photocatalytic degradation process. TNT was observed to be labile to reductive transformation by a platinum electrode at electrochemical potentials commensurate with the flat band potential of TiO2, and aminodinitrotoluene species were detected as early reduction products. Oxygen did not influence the rate of reductive TNT transformation, but byproducts were more stable in aerated than deaerated conditions. Photocatalytic reduction of TNT was analyzed using CdS as a chromophore coupled to TiO2 and visible light excitation so that direct photolysis of TNT was precluded. Under deaerated conditions, reduction of TNT occurred through both direct and sensitized mechanisms, predominantly forming aminodinitrotoluene compounds as relatively stable byproducts. Photocatalytic transformation and mineralization of TNT was also examined with particulate films of TiO2 immobilized on optically transparent electrodes. Varying levels of positive bias were applied to the photocatalyst in order to decrease the availability of photoexcited electrons. The rate of TNT degradation was found to decrease with increasing positive bias, indicating that conduction band electrons facilitate overall compound degradation. Results from studies with TiO2 thin films support the proposal that photocatalytic TNT destruction proceeds through oxidative pathways, where molecular oxygen accelerates byproduct degradation, and reductive pathways in which byproduct degradation is retarded by oxygen.

Introduction Residues of explosives are a widespread environmental contaminant and have been identified in soil and groundwater samples at hundreds of current and former U.S. Department of Defense sites (1). Extensive research has

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 1996 American Chemical Society

been conducted in search of effective remediation strategies for munitions-contaminated water and soil (2), and many of these investigations have focused on 2,4,6-trinitrotoluene (TNT) due to its prevalence at contaminated sites (3), its continued wide use as a secondary explosive (4), and its recalcitrance to complete degradation by traditional bioremediation techniques (5, 6). A number of research groups have lately examined the use of irradiated semiconductor particulate systems for the treatment of TNT-contaminated water (7-11). In a recent paper (8), we presented data on the photocatalytic degradation of dissolved TNT using aqueous TiO2 (P25) slurries. This study observed that when a 220 µM solution of TNT was exposed to the TiO2 photocatalyst in the presence of oxygen and near UV radiation (λ > 340 nm) more than 90% of the TNT was oxidized to CO2. Additionally, approximately 35% of the TNT nitrogen was recovered as ammonium ion, a reduced nitrogen species, while 55% was oxidized to nitrate ion. The formation of a substantial amount of ammonia during the photocatalytic mineralization of TNT is of interest because it suggests that the degradation pathway involved significant reductive transformations. Mahdavi et al. (12) observed that nitroaromatic compounds were reduced to the corresponding amine (e.g., nitrobenzene reduced to aniline) in high yield through interfacial electron transfer by excited TiO2 in solutions of deaerated ethanol. Our earlier findings (8) as well as work by Low et al. (13) and Pelizzetti et al. (14) indicate that this reaction can occur readily in aerated aqueous systems as well. Photocatalytic reactions transpire when an electron is promoted from the filled valence band to the empty conduction band of a semiconductor by excitation with ultra-bandgap radiation. The electron hole pair so formed then reacts either by recombination or through participation in interfacial redox reactions with reduction occurring by the photoexcited electron and oxidation taking place at the site of the positive hole. In aqueous solution, a preponderance of evidence, as summarized by Turchi and Ollis (15), indicates that oxidative degradation of aromatic compounds occurs primarily via either bound or free hydroxyl radicals formed through the oxidation of adsorbed H2O, hydroxide, or surface titanol groups by photogenerated holes trapped at the semiconductor surface (16). Direct oxidation by trapped holes has also been reported (17). The predominant initial reductive step in aerated systems is a transfer of the photoexcited electron to adsorbed molecular oxygen to create a superoxide radical anion that may then form other activated oxygen species such as HO2• and H2O2 (18-20). The superoxide radical has been proposed to enhance the oxidation of certain organic compounds (21). Preliminary results have indicated that nitroaromatic compounds effectively compete with O2 to * Address correspondence to either author. K.A.G. telephone: 847467-4252; fax: 847-491-4011; e-mail address: [email protected]. P.V.K. telephone: 219-631-5411; fax: 219-631-8068; e-mail address: Kamat@ marconi.rad.nd.edu. † University of Notre Dame. ‡ Northwestern University. § Notre Dame Radiation Laboratory.

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scavenge conduction band electrons while also undergoing oxidative degradation (8, 13, 14). Reaction pathways proposing both reductive transformations by conduction band electrons and oxidation by hydroxyl radicals have been reported in previous work on the photocatalytic mineralization of aromatic compounds using TiO2 (22-25). For example, conduction band electrons have been proposed to be utilized in the reduction of organic radicals formed during photocatalysis of 4-nitrophenol (22) and in reduction of chloride radicals to chloride ions during photocatalytic transformation of 4-chlorophenol to hydroquinone (23, 24). Nevertheless, in the majority of published transformation pathways for the photooxidation of aromatic compounds by TiO2, the primary degradation step has been hydroxyl radical attack on the aromatic ring due to the lability of the parent compound to this reaction (15, 25, 26). In contrast, TNT is highly susceptible to reductive attack (27) while hydroxyl radical addition to the aromatic ring of TNT is hindered both by steric effects and the deactivating influence of the three nitro functional groups (28). Reduction by conduction band electrons may, therefore, play a more significant role in the photocatalytic degradation of TNT than is the case with less substituted nitroaromatic compounds. The objective of the research presented here is to detail the reductive interactions of TNT by conduction band electrons and to determine the importance of these reactions to overall compound degradation. Such an understanding should be important in efforts to make the application of photocatalysis to TNT remediation more efficient.

Experimental Methods Materials. The 2,4,6-trinitrotoluene used for experimental work was purchased from Chemservice (West Chester, PA). Cadmium sulfide and 3,5-dinitroaniline were purchased from Aldrich (Milwaukee, WI). Certified standards of 2,4,6trinitrotoluene, 1,3,5-trinitrobenzene, 2-amino-4,6-dinitrotoluene, 4-amino-2,6-dinitrotoluene, 2,4-dinitrotoluene, and 2,6-dinitrotoluene were purchased from AccuStandard Inc. (New Haven, CT) and were used for identification and quantification. All compounds were of the highest purity available from these vendors and were used without further purification. Optically transparent electrodes were cut (5 cm × 0.9 cm) from a conducting glass plate obtained from Donelley Corp. (Holland, MI). Titanium dioxide (P25) powder was acquired from DeGussa Corp. The water used in all experiments and analyses was ultrapure (18 MΩ‚cm) prepared using a Milli-Q purification system. Preparation and Characterization of Semiconductor Films. The TiO2 particulate film electrodes subjected to electrical bias were prepared by applying 400 µL of a TiO2 slurry to the surface of an optically transparent electrode using a hypodermic syringe and covering approximately 2 cm2. The TiO2 slurry concentration was varied from 0 to 2 g/L in order to achieve different TiO2 film masses. The electrode was then dried on a warm plate and baked at 400 °C for 1 h. For experiments in which CdS-TiO2-coupled catalysts were used, the catalyst was also immobilized on optically transparent electrodes. However, this was done only to affix the catalyst to an optically transparent surface, and no electrical bias was ever applied. CdS-TiO2 composite films were prepared by vigorously mixing a slurry of CdS (2 g/L) and varying amounts (0, 1.1, or 4.4 g/L) of TiO2. A 400-µL sample of the CdS-TiO2 slurry was applied to the

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FIGURE 1. Scanning electron micrograph (90000× magnification) of a CdS-TiO2 composite film (1:1 molar ratio) on tin oxide-coated glass substrate. The cluster of larger particles (approximate diameter 300 nm) are CdS while the smaller particles (diameter 30 nm) are TiO2.

surface of a glass electrode using a hypodermic syringe, also covering approximately 2 cm2. The catalyst was then dried on a warm plate and baked at 200 °C for 1 h. A scanning electron micrograph of a composite CdS-TiO2 film with a CdS:TiO2 molar ratio of 1:1 is shown in Figure 1. The micrograph was recorded using a Hitachi S-4500 scanning electron microscope. It indicates that CdS colloids tended to coagulate in the composite film, resulting in the formation of larger CdS particles coated with a layer of TiO2. However, while TiO2 and CdS were not fully intermixed in the film, they were in sufficient contact with each other to induce charge separation. Efforts are underway in our laboratory to develop better techniques for preparation of composite catalyst films. Reaction Procedures. Unless otherwise noted, all electrochemical and photocatalytic experiments were carried out in a standard two-compartment cell in which the working and counter electrodes were separated by a fine glass frit. The volume of the working electrode chamber was 5 mL. A platinum wire electrode was always used as the counter electrode and a saturated calomel electrode was employed as the reference electrode. A Princeton Applied Research Model 173 potentiostat was utilized to apply the desired potential to the working electrode. Experimental solutions were sparged with either oxygen or nitrogen as indicated for 30 min before the experiment was initiated and were lightly sparged throughout the reaction. All experiments were conducted at room temperature. For the experiments that examined direct reduction of TNT by a platinum electrode, the working electrode chamber initially contained an aqueous solution of 200 µM TNT plus 100 mM KCl as an electrolyte. The counter electrode chamber held an identical solution plus 5% ethanol to serve as an electron donor. The working electrode was platinum mesh. A collimated light beam from a 250-W xenon lamp was used as a radiation source for all photocatalytic work. During experiments in which photocatalysis of TNT was studied using TiO2 electrodes under applied bias, the light was filtered using a 10-cm solution light filter consisting of 0.025 M CuSO4 and 0.025 g/L 2,7-dimethyl-3,6-diazacyclohepta1,6-diene perchlorate, which eliminated radiation with λ
340 nm.

FIGURE 6. Relationship between TiO2 film weight and the percent decrease in TNT concentration and absorbance at 254 nm after 60 min of irradiation using electrochemically assisted photocatalytic degradation with a TiO2 particulate film electrode. Initial TNT concentration ) 220 µM in oxygen-sparged aqueous solution. Excitation wavelengths >340 nm. During these experiments, the TiO2 electrode was maintained at an electrochemical bias of +0.75 V vs NHE.

oxygen species resulting from reductive transformations of molecular oxygen. 3,5-dinitroaniline is a reductive transformation product of 1,3,5-trinitrobenzene. Neither 1,3,5-trinitrobenzene nor 3,5-dinitroaniline were observed in the nitrogen-sparged reactor. Dinitrotoluene was not detected in either the oxygen- or the nitrogen-sparged reactors. The use of cyclic voltammetry to examine oxidative transformations of TNT was also attempted. However, the oxidation peak of TNT could not be resolved before it was obscured by oxidation of the supporting electrolyte, which occurred at approximately +1.4 V vs NHE. Electrochemically Biased Photocatalysis Using TiO2 Films. The photocatalytic degradation of TNT was examined using particulate films of TiO2 immobilized on optically transparent electrodes. Thin semiconductor films generated from colloidal suspensions have been shown to retain the photophysical and photochemical properties of individual semiconductor particles (39). While photoexcited semiconductor colloids in a suspension behave as shortcircuited microelectrodes, promoting oxidation and reduction on the same particle, immobilization of the semiconductor onto an electrode makes it possible to drive the photoexcited electrons from the semiconductor particle into an external circuit through application of a positive bias. TiO2 electrodes were employed in this work to examine photocatalysis of TNT in the presence and absence of conduction band electrons and, thereby, to gain insight into their importance to the degradation process. Additionally, comparison of aerated and deaerated experiments allowed us to elucidate the role of oxygen in the photocatalytic degradation of TNT. A schematic diagram of the reactor used in this work is illustrated in an earlier paper (40). (a) Effect of Film Thickness. Initially a series of experiments was run to examine the influence of the mass of TiO2 in the thin film electrode on the TNT photocatalysis rate. The results of these experiments are shown in Figure 6, which displays the percent decrease in both TNT concentration and absorbance at 254 nm for oxygen-

sparged 220 µM TNT solutions after 60 min of irradiation using near UV (λ > 340 nm) light. During these experiments, the electrode was maintained at a positive bias of +0.75 V vs NHE. In the absence of all TiO2 there was approximately a 23% decrease in TNT concentration and a 11% decrease in 254-nm absorbance due to direct photolysis of TNT. The increase in the decay rate of both TNT concentration and absorbance at 254 nm was roughly linear with increasing TiO2 film mass until a value of 200 µg/cm2 was reached. At this point, 60% of the TNT was transformed, and absorbance at 254 nm decreased by 50% after the 60-min irradiation period. Film masses above 200 µg/cm2 did not provide an additional increase in degradation rates, possibly due to inadequate light penetration into the TiO2 film and increased rates of charge recombination. These results illustrate that the immobilized TiO2 used in these experiments was photocatalytically active and substantially increased TNT degradation rates. They also provide an optimal film mass for use in later work under these same experimental conditions. (b) Effect of Applied Bias. The photocatalytic degradation of TNT was next examined using a constant TiO2 film mass (200 µg/cm2) and varying levels of positive bias. As stated earlier, placing the photocatalyst under a positive bias drives photoexcited electrons into a counter electrode which, in this work, was a platinum electrode in a separate compartment. In cases where the photocatalytic degradation of a compound is initiated oxidatively, application of a positive bias to the photocatalyst has been shown to increase transformation rates by lowering the degree of charge recombination (40-42). However, if the photocatalytic degradation of a compound were initiated reductively or if the path to mineralization involved conduction band electrons, then a decrease in degradation rates might be observed upon subjecting the TiO2 film to a positive bias. The objective of this group of experiments was to assess the relative importance of conduction band electrons in TNT photocatalysis by maintaining the TiO2 film at different electrochemical bias. Results of these experiments are compiled in Table 1, which presents the pseudo-first-order rate constants for both TNT transformation and decrease in absorbance at 254 nm for each level of applied bias. Three levels of applied bias (+1.1, +0.6, and +0.1 V vs NHE) as well as no applied bias were considered. The initial TNT concentration was 220 µM in oxygen-sparged aqueous solution, and the light

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FIGURE 7. Percentage of the initial dissolved organic carbon (DOC) remaining after 6 h of irradiation for different levels of applied bias during electrochemically assisted photocatalytic degradation of TNT with a TiO2 particulate film electrode. (V vs NHE). Initial TNT concentration ) 220 µM in oxygen-sparged aqueous solution. Excitation wavelengths >340 nm.

was filtered to block λ < 340 nm. The pH of the solution varied between 6 and 5 during the course of the reactions, which means that the conduction band energy of TiO2 varied between -0.45 and -0.40 V vs NHE. All applied potentials were therefore positive with respect to the conduction band energy of TiO2. Control experiments run in the dark indicated that no reaction occurred at any level of bias in the absence of irradiation. Little difference in the TNT transformation rate was observed as the applied bias was increased, although the reaction rate when the TiO2 was under no applied bias was approximately 10% greater. The same is true of the rate of decrease in 254-nm absorbance. The percent of the initial dissolved organic carbon (DOC) remaining after 6 h of irradiation is shown in Figure 7. These data indicate that the percent DOC removed in the reaction decreased slightly as the level of applied bias was increased. Approximately 27% of the initial DOC remained when the photocatalysis experiments were performed at +1.1 V bias and +0.6 V bias. However, when no bias was applied to the photocatalyst, only 14% of the DOC was left after 6 h. The margin of error for the indicated values is 2%. These data show that applying a positive bias to TiO2 slightly decreases the rate of TNT transformation as well as the rate of mineralization. Earlier work has demonstrated that if the degradation were influenced solely by oxidative steps, the reaction rate should increase upon application of a positive bias to the semiconductor due to a decreased rate of charge recombination (40-42). These results are therefore consistent with the assertion that TNT is photocatalytically transformed through both oxidative and reductive steps. With the increasing positive bias, the relative rate of oxidative reactions was increased while the rate of reductive transformations dropped, resulting in little net change in overall reaction rate. Reactions with conduction band electrons do, therefore, facilitate the photocatalytic transformation and mineralization of TNT. (c) Role of Oxygen. It is also of interest to consider the importance of oxygen in the photocatalytic degradation of TNT. During TiO2 photocatalysis in aerated systems, photoexcited electrons are typically considered to react

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primarily with adsorbed molecular oxygen (19). This scavenging of conduction band electrons is crucial to prevent charge recombination and loss of reaction efficiency. Other research has indicated that the presence of molecular oxygen further influences photocatalytic oxidation of certain organic compounds through additional interactions (21, 25, 26) including (i) reaction of molecular oxygen with organic radicals, generated upon the hole or •OH radical reaction with the reactant, to produce organoperoxy radicals; (ii) reaction of superoxide, produced by reduction of molecular oxygen by conduction band electrons, with organoperoxy radicals to form an unstable tetraoxide (21); (iii) production of additional hydroxyl radicals through a three-electron reduction of molecular oxygen by conduction band electrons; (iv) strong adsorption to the TiO2 surface thereby displacing poorly adsorbed species (40). When a positive bias is applied to the photocatalyst film, scavenging of conduction band electrons is no longer a limiting factor because photoexcited electrons are driven away to an external circuit. Consequently, comparison of reaction rates in aerated and deaerated conditions when the catalyst is under a positive bias should provide insight into the role of oxygen in the photocatalytic degradation of a specific compound. This type of experiment was carried out by both Vinodgopal et al. (40) and Kim and Anderson (41), who examined the photocatalytic oxidation of 4-chlorophenol and formic acid, respectively, on TiO2 thin films placed under a positive bias. Both groups observed an increase in degradation rates in the presence of oxygen. However, the transformations that were monitored in these cases were exclusively oxidative. In contrast, TNT appears to degrade through both oxidative and reductive reaction steps, and the reductive degradation of TNT byproducts appears to be inhibited by oxygen as evidenced by the electrochemical experiments discussed earlier. It was therefore of value to investigate the extent to which oxygen might accelerate the oxidative degradation of TNT in the oxidizing environment of a TiO2 film under a positive bias. The photocatalysis of TNT using TiO2 thin film electrodes was repeated at a positive bias of +1.1 V vs NHE in nitrogensparged conditions. The results of this experiment are compared to the data obtained at a bias of +1.1 V in oxygensparged conditions in Figure 8, which presents both TNT transformation and the percent decrease in absorbance at 254 nm. The rate of TNT transformation in each experiment was approximately equal, indicating that molecular oxygen was not important in the initial degradation step. However, substantial differences were observed in the change in absorbance at 254 nm with a 93% loss in the oxygen-sparged system as compared to only a 60% decrease in the nitrogensparged system after 4 h of irradiation. Using 254-nm absorbance as a general measure of aromaticity and overall degree of compound degradation, these data suggest that molecular oxygen does play a crucial role in reactions leading to the photocatalytic mineralization of TNT and is not limited to simply scavenging electrons from the photocatalyst surface. This result supports the proposal that TNT degradation proceeds both through oxidative pathways, where molecular oxygen accelerates byproduct degradation, and reductive pathways in which byproduct degradation is retarded by oxygen. Photocatalytic Reduction of TNT Using Coupled CdSTiO2. Although the photocatalytic role of TiO2 was established in the previous section, it is not clearly evident to

FIGURE 8. Comparison of oxygen-sparged and nitrogen-sparged results during electrochemically assisted photocatalytic degradation of TNT with a TiO2 particulate film electrode. The TiO2 electrode was biased at +1.1 V vs NHE. Initial TNT concentration ) 220 µM in oxygen-sparged aqueous solution. Excitation wavelengths >340 nm.

FIGURE 9. Schematic diagram illustrating charge injection from excited CdS into TiO2. CB and VB refer to the energy levels of the conduction and valence bands, respectively, of the semiconductor particle.

what extent the photochemistry of TNT contributed to the overall degradation. This is because TNT is very labile to direct photolysis and readily transforms in the presence of radiation with λ < 400 nm (8). Titanium dioxide has a bandgap energy of approximately 3.2 eV and consequently requires radiation with a wavelength e385 nm to achieve charge separation. It is, therefore, impossible to degrade TNT using TiO2 as a photocatalyst while avoiding direct photolysis of TNT. In order to clarify the role of a semiconductor photocatalyst, we chose a short bandgap semiconductor, CdS, with a bandgap of 2.4 eV, which corresponds to a wavelength of 516 nm. Charge separation and subsequent photocatalytic reactions may thus be promoted selectively with cadmium sulfide using visible light (λ > 400) that is not sufficiently energetic to photolyze TNT. CdS thin films cast on a glass plate were used in this work both alone and as a sensitizer coupled to TiO2. Earlier work has shown that in a coupled CdS-TiO2 system a photoexcited electron may transfer from CdS to TiO2 due to the higher reduction potential of the TiO2 conduction band (43-47). In colloidal systems, electron transfer from CdS to TiO2 was observed to occur with a time constant of about 2 ps (47). The principle of charge separation in a TiO2-CdS system is illustrated in Figure 9. Such coupled systems are primarily investigated for their ability to

FIGURE 10. Results from photocatalysis of TNT using coupled CdSTiO2 photocatalysts. CdS:TiO2 in legend indicates the molar ratio of cadmium sulfide to titanium dioxide in the composite film. Initial TNT concentration ) 120 µM in aqueous nitrogen-sparged solution with 30% (v/v) methanol. Excitation wavelengths >400 nm.

enhance charge separation and thereby improve the quantum yield of photocatalytic reactions (48). For example, Spanhel et al. (46) demonstrated that the efficiency of photocatalytic methyl viologen reduction could be increased by a factor of 10 through coupling TiO2 to CdS in comparison to CdS alone. In the work presented here, coupled CdS-TiO2 systems were used so that photogenerated electrons could be accumulated in TiO2 using visible light via charge transfer from excited CdS, hence allowing the interfacial reduction of TNT at a TiO2 surface to be studied without directly photoexciting TNT. This is of interest because substantial direct photoreduction of TNT in aqueous solution to form aminodinitroaromatic compounds has been reported (50, 51). The presence of the reduced species, ammonium, observed in the photocatalytic degradation of TNT (8) is not, therefore, conclusive evidence that nitro groups are involved in interfacial reduction reactions on TiO2. However, by using CdS as a chromophore coupled to TiO2 and visible light excitation, these experiments permit analysis of photocatalytic reduction of TNT by TiO2 in the absence of direct photolysis. This work also demonstrates the possibility of sensitized photocatalytic reduction of TNT by visible light. A series of experiments was run in which the photocatalytic degradation of TNT was examined using visible light (λ > 400 nm) excitation and different relative ratios of CdS:TiO2 in the composite film. The CdS:TiO2 molar ratios employed were 1:0, 1:1, 1:4, and TiO2 alone was used as a control. The initial TNT concentration was 120 µM in deaerated aqueous solution with 30% methanol v/v as a hole scavenger. Figure 10 presents the decrease in TNT concentration resulting in these experiments. With TiO2 alone as a photocatalyst there was no reaction since the radiation was not sufficiently energetic either to photolyze TNT or to promote charge separation in TiO2. The slight increase in TNT concentration seen in this experiment can be attributed to solvent evaporation. When the film of pure cadmium sulfide was used, TNT was transformed with a pseudo-first-order rate constant of 0.23 h-1. In the composite film of TiO2 and CdS at a ratio of 1:1, the decay rate constant increased by 75% to 0.4 h-1. Such an increase in rate confirms enhanced charge separation in the pho-

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Sciences of the Department of Energy. This is Contribution No. 3893 from the Notre Dame Radiation Laboratory.

Literature Cited

FIGURE 11. Evolution of 2-amino-4,6-dinitrotoluene and 4-amino2,6-dinitrotoluene during photocatalysis of TNT using coupled CdSTiO2 photocatalysts. The concentration shown is the molar sum of the two species. CdS:TiO2 in legend indicates the molar ratio of cadmium sulfide to titanium dioxide in catalyst. Initial TNT concentration ) 120 µM in aqueous nitrogen-sparged solution with 30% (v/v) methanol. Excitation wavelengths >400 nm.

tocatalytic system caused by electron injection from CdS into TiO2. No additional rate enhancement was observed in further increasing the TiO2 concentration in the CdSTiO2 film. Figure 11 displays the evolution of the predominant byproducts formed in these reactions: 2-amino-4,6-dinitrotoluene and 4-amino-2,6-dinitrotoluene. The concentration given in Figure 11 is the molar sum of the two species. The formation of these compounds clearly shows that TNT was transformed reductively as expected. When TiO2 was coupled to CdS, the rate of formation of these byproducts increased in proportion to the increased rate of TNT transformation. This suggests that the transformations of TNT on the CdS and the TiO2 surfaces were similar. These data, therefore, show that TNT can be reductively transformed on a TiO2 surface in aqueous solution and that the predominant byproducts are the aminodinitrotoluene species. These byproducts were also observed when TNT was reductively degraded by a platinum electrode maintained at -0.4 VNHE. However, while these compounds were rapidly degraded by the platinum electrode in deaerated conditions, they were relatively stable to photocatalytic degradation by CdS-TiO2. This suggests that photocatalytic reduction alone is not an effective method of completely degrading TNT. Additionally, the data demonstrate that photocatalytic reduction can occur through sensitized photocatalysis using visible light. This may be of importance if photocatalysis is used to treat nitroaromatic compounds in natural waters since natural organic acids have been shown to be capable of sensitizing TiO2 (52).

Acknowledgments D.C.S. and K.A.G. would like to acknowledge the support of the National Science Foundation (Grant BCS91-57948) and the Department of Education’s Graduate Assistance in Areas of National Need Program. The authors would also like to thank the Center for Bioengineering and Pollution Control for the use of their facilities and Richard Frankovic for providing scanning electron micrographs. P.V.K. acknowledges the support of the Office of Basic Energy

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Received for review November 29, 1995. Revised manuscript received April 4, 1996. Accepted April 5, 1996.X ES950896L X

Abstract published in Advance ACS Abstracts, June 1, 1996.

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