Photoelectrocatalytic Degradation of Triazine-Containing Azo Dyes at

Aug 29, 2007 - Triazine-containing azo dyes, anionic reactive brilliant red K-2G, reactive brilliant Red X-3B, and reactive yellow KD-3G were degraded...
0 downloads 0 Views 190KB Size
Download PDF
Environ. Sci. Technol. 2007, 41, 6802-6807

Photoelectrocatalytic Degradation of Triazine-Containing Azo Dyes at γ-Bi2MoO6 Film Electrode under Visible Light Irradiation (λ > 420 Nm) XU ZHAO, JIUHUI QU,* HUIJUAN LIU, AND CHUN HU State Key Laboratory of Environmental Aquatic Chemistry, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing, 100085, P.R. China

Triazine-containing azo dyes, anionic reactive brilliant red K-2G, reactive brilliant Red X-3B, and reactive yellow KD3G were degraded in the electrooxidation, photocatalysis and combined electrooxidation and phtocatalysis process at the γ-Bi2MoO6 film electrode under visible light irradiation. Furthermore, in the combined process, synergetic degradation of the dyes was observed by the analysis of degradation kinetics and total organic carbon variation. The synergetic mechanism was studied using X-ray photoelectron spectra and electrochemical impedance spectra as well as analysis of variation of current vs time in various processes. It is suggested that application of bias potential with a lower value than the redox potential of the targeted dyes efficiently increases the photocatalysis rate of the dyes by prohibiting recombination of electrons and holes. At the bias greater than the redox potential of the dyes, the degradation of the dyes was efficiently improved by the combined electrooxidation and photocatalysis process. The main active oxygen species involved in the dyes degradation was hydroxyl radicals as confirmed via DMPO spin-trapping electron spin resonance measurements and the effect of radical scavengers. Intermediates mainly including organic aromatic and aliphatic carboxylic acids were detected.

Introduction The use of semiconductor photocatalysis and electrochemical oxidation for destruction of organic contaminants in water has undergone rapid development (1, 2). Combined electrooxidation and photocatalysis using a semiconductor film electrode (PEC) has shown great potential compared with individual electrooxidation and photocatalysis (3-6). In the combined process, an applied bias potential not only leads to the electrochemical degradation of target contaminants but also promotes the photocatalysis by driving electrons and providing O2. Many active species generated on the electrode surface via photocatalysis will activate the electrode and promote the electrooxidation of target contaminants. Electrode materials including 70% TiO2/30% RuO2, Pt doped TiO2/Ti, and β-PbO2 electrodes modified by TiO2, and ZnWO4 films are all responsive to UV light (3-6). To obtain * Corresponding author phone: +86-10-62849151; fax: +86-1062923558; e-mail: [email protected]. 6802

9

ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 41, NO. 19, 2007

more efficient utilization of solar irradiation, development of a film electrode responsive to visible light is indispensable. It was reported that WO3 and porous Fe2O3 film electrodes showed good incident photo-to-current conversion efficiency (IPCE) for water decomposition under visible light (7, 8). Modification of TiO2 has also been intensively studied to shift its photocatalytic activity to the visible region (9-12). IPCE values for ZnChl-e6-Cro/TiO2 at 400 and 500 nm estimated to be ca. 23 and 8.9% have been reported (12). Considering the electrode materials for environmental purification, stability and electrocatalytic activities of these film materials still need to be improved. Recently, some novel complex semiconductor photocatalysts responsive to visible light have been developed and have shown great potential in applications such as splitting water, solar cells, and environmental purification (13, 14). In our previous work, the complex semiconductor ZnWO4 has exhibited high electrooxidation activity (5). γ-Bi2MoO6 consisting of a layered structure with corner-shared MoO6 units has arisen as a new visible light driven photocatalyst for O2 evolution from an aqueous silver nitrate solution (15). Its band gap is ca. 2.70 eV and can it can absorb visible light with an absorbance wavelength near to 460 nm. However, application of γ-Bi2MoO6 in photocatalysis and electrooxidation of organic contaminants has still not been reported. Herein, the photocatalytic activity of γ-Bi2MoO6 film toward the degradation of K-2G was exhibited. The results showed that γ-Bi2MoO6 film can efficiently degrade K-2G via anode oxidation. Moreover, a synergetic effect has been observed by the combined photocatalysis and electrooxidation. KD-3G and X-3B can also be efficiently degraded. The efficiency and mechanism of γ-Bi2MoO6 film electrode in degrading K-2G were investigated. Azo dyes constitute a significant portion of contaminants and probably have the least desirable consequences in terms of the surrounding ecosystem (16). Three triazine-containing group azo dyes of K-2G, X-3B, and KD-3G were selected in the present study. The degradation of these dyes have been studied in detailed, as reported previously (17). Thus, it is convenient to study the degradation process of these dyes.

Experimental Section Fabrication and Characterization of γ-Bi2MoO6 Film. Indium-tin oxide (ITO) glass (thickness, 1.1 mm; a sheet resistance, 15 Omega/square) was purchased from China Southern Glass Co. Ltd. All other chemicals are of analytical grade reagents and used without further purification. Deionized water was used throughout the experiment. Porous γ-Bi2MoO6 films with a thickness of ca. 100 nm were deposited onto the ITO glass from an amorphous heteronuclear complex via a dip-coating method as described previously (18). X-ray photoelectron spectra (XPS) were obtained using a PHI Quantera SXM (PHI-5300/ESCA, ULVAC-PHI, INC). Prior to the measurement, the electrode was washed with water to remove electrolyte. X-ray diffraction (XRD) of the film was recorded on a Scintag-XDS-2000 diffractometer with Cu KR radiation. UV-visible absorption spectra of the samples were recorded on a UV-vis spectrophotometer (Hitachi UV-3100) with an integrating sphere attachment. The morphology of the γ-Bi2MoO6 film was characterized using a JSM 6301 scanning electron microscope (SEM). The zeta potential of catalysts in KNO3 (10-3 M) solution was measured with a Zetasizer 2000 (Malvern Co., UK). Every reading of the instrument was recorded after three consistent readings were attained. Incident photo-to-current conversion 10.1021/es070598b CCC: $37.00

 2007 American Chemical Society Published on Web 08/29/2007

FIGURE 1. X-ray diffraction analysis of the γ-Bi2MoO6 film and the ITO substrate; inset, UV-vis diffusion reflectance spectra of the γ-Bi2MoO6 and TiO2 powders. efficiency (IPCE) was determined as described previously (19). Degradation Experiment. Degradation experiments were performed in a rectangular (50 × 50 × 100 mm) reactor made from quartz glass. The reactor, which contained 100 mL sample solution allowing 3.5 cm of the supported film electrode to be immersed into the solution, was placed 3 cm in front of a 150 W Xe lamp purchased from the German Osram. A UV cutoff filter (λ > 420 nm) filter purchased from Shanghai Seagull Colored Optical Glass Co., Ltd. was used to remove light with wavelengths below 420 nm. The intensity of light, as measured by a visible-irradiance meter (Instruments of Beijing Normal University) was c.a. 28 mW/cm2 at 4 cm into the reactor, the position where the film electrode was placed. The PEC reaction employed a basic electrochemical system (Princeton Applied Research) connected with a counter-electrode (Pt wire, 70 mm in length with a 0.4 mm diameter), a working electrode (Bi2MoO6 film, active area of 12 cm2), and a reference electrode (a saturated calomel electrode (SCE)). 0.1 mol/L (M) Na2SO4 solution was used as electrolyte solution. The initial concentration of the dye solution was 30 mg/L, and its concentration variation was examined using a U-3010 spectrophotometer. A total organic carbon (TOC) analyzer (multi N/C 3000) was employed for mineralization degree analysis of the dye solutions. Prior to injection into the TOC analyzer, the samples were filtrated with a 0.45 µm Millipore filter. All experiments were carried out at least in duplicate. The reported values were within the experimental error range of (3%. Electrochemical impedance spectra (EIS) were recorded in the potentiostatic mode. The amplitude of the sinusoidal wave was 10 mV, and the frequency range of the sinusoidal was 100 kHz to 0.01 Hz. The electron spin resonance (ESR) signals of radicals spintrapped by spin-trap reagent 5,5′-dimethyl-1-pirroline-Noxide (DMPO) (purchased from Sigma Chemical Co.) were detected on a Bruker model ESP 300E spectrometer. A 350-W Xe arc lamp with a UV cutoff filter (γ > 420 nm) served as the irradiation light source. The •OH radical spin adduct of DMPO was prepared by instantaneously sampling the solution during PEC reaction with a syringe containing a constant volume of 30 mM DMPO. The formation of H2O2 in the electrooxidation and PEC process was determined by a photometric method as described in the literature (20).

Results and Discussion Characterization of the γ-Bi2MoO6 Film. As shown in Figure 1, the film samples can be identified as γ-Bi2MoO6 film (JCPDS. 21-102), and the films were composed of small particles with a size of ca. 100 nm; a porous structure was also observed (Supporting Information Figure S1). UV-vis diffusion reflectance spectra of γ-Bi2MoO6 film and TiO2 are shown in the inset of Figure 1. Clearly, γ-Bi2MoO6 film absorbs

FIGURE 2. (A) Degradation of K-2G with reaction time in the photocatalysis, electrooxidation, photoelectrocatalysis processes; (B) Temporal variation of UV-vis in the photoelectrocatalysis of K-2G. (30 mg/L K-2G, 0.1 M Na2SO4, 3.0 V potential, visible light irradiation). a larger portion of the visible light than TiO2. Based on the absorption curve, the band gap can be estimated to be 2.67 eV. Thus, it is concluded that γ-Bi2MoO6 film can be excited by the irradiation light with wavelength less than 464 nm (21). The anodic photocurrent was observed under visible light irradiation of the γ-Bi2MoO6 film electrode, and the photocurrent increased steadily with applied positive potential (Supporting Information Figure S2). The IPCE decreased with longer wavelength, and the shape of IPCE action spectrum was similar to that of the absorption spectrum (shown in Figure. 2). Degradation of the Dyes. As shown in Figure 2(A), K-2G can be photocatalytically degraded using the γ-Bi2MoO6 film electrode; it can also be degraded via the electrooxidation process at the bias potential of 3.0 V. Clearly, the degradation rate of K-2G was the highest under the PEC process with the same bias potential. Pseudo-first-order kinetics was confirmed in the electrochemical, photocatalytic, and PEC processes by the linear transforms ln (K-2G0/ K-2Gt) ) Kt (K is the kinetic constant) (22). The pseudo-first kinetic constant of K-2G for the PEC process is 0.588 h-1 which is larger than the sum of the electrochemical 0.166 h-1 or photocatalytic process 0.042 h-1 individually. Thus, it can be concluded that a sort of synergetic effect occurs during the PEC process in the degradation of K-2G. Additionally, no concentration variation was observed in the presence of the film electrode without visible light irradiation. Thus, it was concluded that the adsorption effect can be ignored within the reaction process. As shown in Figure 2(B), the peak at 510 nm of K-2G decreased quickly with reaction time in the PEC process, which will be discussed subsequently. The variation of current with the reaction time in the dye electrooxidation and PEC degradation process are shown in Figure 3(A). It can be seen that the current decrease with the prolonged reaction time in the electrooxidation process. In contrast, it increases with the reaction time in the PEC process. It is suggested that a certain kind of polymer substance could be formed on the film surface. Polymeric products tend to have low solubility in aqueous media and are adsorbed on electrode surfaces, preventing further VOL. 41, NO. 19, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

9

6803

FIGURE 3. Temporal variation of current (A) and conductivity (diluted 20 times) (B) in the photoelectrocatalysis and electrooxidation of K-2G (30 mg/L K-2G; 3.0 V applied potential bias; 0.1 M Na2SO4 aqueous solution). reaction (5). Thus, the current decrease in the electrooxidation process. In the PEC process, the light intensity at the electrode surface may have increased as the reaction proceeded because the dye was decolorized and, as the PEC reaction proceeded, more and more acid intermediates such as SO42were produced with the PEC reaction, which could increase the electronic conductivity of the reaction solution, as shown in Figure 3(B). The photogenerated active species could keep the surface of the electrode fresh and promote the PEC degradation of the target compounds. The XPS analysis as shown in Figure 4 indicated that the intensity of Bi and Mo for the film sample used in the PEC process (3.0 V) for 5 h decreased slightly. However, their intensity greatly decreased for the film sample used in the electrochemical process under the same conditions. The binding energy of the O changed greatly in comparison with that during the PEC process. Moreover, a certain amount of N, Cl, and S were clearly observed on the film electrode surface. It is suggested that a certain kind of polymer substance could be formed on the film surface in the electrooxidation process (5). In contrast, a similar reaction did not occur in the PEC process. These results explain the synergetic effect. As shown in Figure 5, TOC can be reduced in the electrooxidation and PEC processes with the reaction time. The degradation rate in the PEC process is faster than that in the electrooxidation process. Additionally, it should be noted that the TOC content of the solution could not decrease much further even with prolonged electrooxidation reaction. However, it could be reduced in the PEC process efficiently within 5 h. Furthermore, as shown in Supporting Information Figure S3, the KD-3G and X-3B dyes were degraded efficiently over γ-Bi2MoO6 film electrodes in the electrooxidation and PEC degradation under visible light irradiation. These results further confirm the PEC activities of γ-Bi2MoO6 film toward degrading azo dyes. Effect of Applied Bias Potential. The redox potential of K-2G (EK-2G) at the γ-Bi2MoO6 film electrode was determined as ca. 1.3 V as shown in Supporting Information Figure S4. It is known that limited electrochemical degradation of K-2G could take place when the applied potential is well below the 6804

9

ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 41, NO. 19, 2007

FIGURE 4. XPS of γ-Bi2MoO6 film before (a) and after PEC reaction (3.0 V) (b) and electrooxidation (3.0 V) (c) in 0.1 M NaSO4 solution with 30 mg/L dyes (5 h reaction).

FIGURE 5. Temporal variation of TOC in the photoelectrocatalysis and electrooxidation of K-2G (30 mg/L K-2G, 3.0 V applied bias potential, 0.1 M Na2SO4 aqueous solution).

FIGURE 6. Effects of applied bias potentials on K-2G degradation in the electrooxidation and photoelectrocatalysis process (30 mg/L K-2G, 3.0 V applied bias potential, 0.1 M Na2SO4). redox of K-2G. When the bias potential is higher than EK-2G, an electrooxidation reaction may take place. As shown in Figure 6, with the increase of external potential, the degradation rate of K-2G increases gradually. The application of potentials greater than the γ-Bi2MoO6 flat band potential across a photoelectrode will remove the

excited electrons to external circuit and increase the concentration of photogenerated active species on the surface by decreasing the rate of recombination of photogenerated holes and electrons. As a result, as the potential increased, the rate of K-2G degradation increased until most of the photogenerated electrons are removed either by the electric field or by reaction with dissolved oxygen. Further increase of the applied potential beyond the redox potential of K-2G greatly improved the degradation. In this case, the degradation of K-2G was carried out simultaneously by the combined electrooxidation and photocatalysis simultaneously. In the electrochemical degradation process of dye pollutants, the reactions at the anode depended on the nature of the dye, the type and composition of the organic and inorganic ingredients, and the nature of the electrode materials used. Of the two possible ways of anodic reactions, the direct oxidation of the dye molecules depends on the potentials of the dye and the organic ingredients. When the potentials are greater than the decomposition of water, the direct oxidation of the dye is extremely difficult unless electrocatalytic electrodes such as platinum are employed, or it is done by indirect ways. In the indirect methods, electroactive species already present or externally added to the dye solution itself to its nearest high valent state at the electrode surface can immediately convert back to its original state in the subsequent chemical reactions with the dye molecules. However, at the bias potential greater than 3.0 V, the synergetic effect decreases gradually. A similar phenomenon was observed in our previous work (5), and the explanation has been presented in detail. The size of the arc radius on the EIS Nyquist plot is reduced because of the photoirradiation; the sizes of the arc radius are reduced by applying the applied potential of 0.3, 0.6, and 0.9 V (Supporting Information Figure S5(A)). The size of the arc radius on the EIS Nyquist plot reflects the rate of electrode reaction (23). These results confirmed that the applied bias increased the degradation rate of dyes. As shown in Supporting Information Figure S5(B), the sizes of the arc radius are largely reduced under application of 1.6, 2.0, and, 3.0 V bias potential or combination of a photoirradiatin and applied bias potential of 1.6, 2.0, and, 3.0 V. It is the smallest at the 3.0 V applied potential combined with photoirradiation, which means an effective separation of photogenerated electron-hole pair and a fast interfacial charge transfer to the electron donor/electron acceptor as suggested by Leng et al. (23). Therefore, the dye can be effectively degraded. Involved Active Species in PEC Degradation of K-2G. Four characteristic peaks of DMPO-•OH were observed in the PEC process; no such signals were detected in the dark (Figure 7(A)). DMPO-•O2-• species were not detected in methanolic media during the PEC process under otherwise identical conditions. These results mean that •OH radicals are produced on the surface of the film electrode in the PEC process. To further examine whether the •OH is generated in the PEC process, the formation of H2O2, whose presence is widely accepted as evidence for the production of •OH, was measured (24). The possible formation of H2O2 as a cathodic reaction was ruled out due to the use of platinum cathode where H2O2 formation is known to be unfavorable. As shown in Figure 7(B), the amount of H2O2 increased with increasing photocatalysis, electrooxidation, and PEC reaction time at the applied bias potential of 3.0 V. Moreover, the amount of H2O2 in the PEC process is much greater than the sum of the individual photocatalysis and electrooxidation processes, which further confirmed the synergetic effect of the combined photocatalysis and electrooxidation in the dyes degradation. In the combined PEC process, adding excess tert-butylalcohol (t-BuTH) (0.04 M) as an •OH scavenger markedly inhibited the formation of H2O2, thus confirming the production of •OH during the PEC process. The role of

FIGURE 7. (A) DMPO spin-trapping ESR spectra recorded at ambient temperature under PEC process (the applied potential bias is 3.0 V, visible light irradiation); (B) Formation of H2O2 in the combined process (a); electrooxidation (b); photocatalysis (c); combined process + t-BuTH (d); inset, concentration variation of K-2G in the PEC process in the presence and absence of excess t-BuOH (the applied potential bias is 3.0 V).

FIGURE 8. Effect of solution pH values on PEC degradation of K-2G (applied bias potential 3.0 V, 0.1 M Na2SO4, 30 mg/L K-2G). •OH

in dye degradation was investigated by adding excess t-BuOH. The presence of 0.04 M t-BuOH halted dye degradation (inset), implying that electrogenerated •OH is a major species responsible for the dye degradation. To assess the possible role of valence band (VB) holes in PEC dye degradation, the PEC experiment was carried out in the presence of excess oxalates as an efficient holes scavenger. As shown in Supporting Information Figure S6, although most of the VB holes have been scavenged by oxalates, the PEC degradation rate of the dye was only moderately retarded, which indicates the minor role of VB holes in PEC dye degradation (25). Effect of pH. The pH of the dye solution was adjusted to pH 3.4 and 9.3 from 5.4 by employing minimum quantities of H2SO4 or NaOH solutions. It can be seen from Figure 8 that the degradation rate is higher at the pH values of 3.4 and 9.3 than at 5.4. The pH of the isoelectric point for γ-Bi2MoO6 particles obtained at 500 °C for 4 h, with the calcination conditions the same as that for the γ-Bi2MoO6 film was found to be 9.3 in 0.01 mol L-1 KNO3 solution (Supporting Information Figure S7). At pH values higher than the isoelectric point, negative ions are repelled from the γ-Bi2VOL. 41, NO. 19, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

9

6805

TABLE 1. Main Intermediates of the Photoelectracalysis and Electrooxidation of K-2G Detected by GC-MS

further confirms the destruction of K-2G into small molecular acids. These results suggested that K-2G degradation mainly proceeds by cleavage of the azo bond leading to decolorization, followed by opening of the phenyl and naphthalene ring to form small molecular organic acids. In the electrooxidation process, more intermediates were detected and some phenol compounds or benzene were also detected. The benzene molecules may be released from the breaking of the C-N single bond between the azo group and benzene ring (27). In the electrooxidation of phenol or chlorinated phenols process, phenoxy radical compounds are first formed, and can be further oxidized to form soluble products or can undergo radical-radical or radicalpolymeric products. These polymeric compounds tend to have low solubility in aqueous media and to be adsorbed on electrode surfaces, passivating electrode from further reaction (6). As shown in Supporting Information Figure S8, the pH in the reaction solution decrease with the electrooxidation and PEC reaction proceeds, which indicates that more acid compounds were produced in the PEC process than that in the electrooxidation process. Similar intermediates of KD3G in the PEC process were also identified ( Supporting Information Table S1), which indicated that the degradation pathway of KD-3G in the PEC process may follow a pathway similar to that of K-2G. There is no noticeable change in the XPS (Figure 4) of the γ-Bi2MoO6 film after 5 h PEC reaction at 3.0 V bias potential. PEC degradation of K-2G was performed five times at the bias potential of 3.0 V for 5 h; the results indicated that the degradation efficiencies were rather stable with a relative standard deviation of 3.1% (data not shown), which further confirms the stability of the γ-Bi2MoO6 film electrodes.

Acknowledgments This work was supported by the Funds for Creative Research Groups of P.R. China (No. 50621804) and the National Natural Science Foundation of China (No. 50538090). We gratefully acknowledge the three anonymous reviewers whose comments helped to improve the manuscript.

Supporting Information Available a

Electrooxidation of 7 h. b photoelectrocatalysis of 4 h. c photoelectrocatalysis of 7 h. (30 mg/L K-2G; 3.0 V applied potential bias; 0.1 M Na2SO4 aqueous solution).

MoO6 surface. As a consequence, the PEC degradation of K-2G was favorable in the solution with a lower pH value. Moreover, the pH of the dye solution greatly influences the degradation process as it affects the protonation/deprotonation of some of the basic sites present in the dye or the formation and stability of active intermediates responsible for decomposition. The promotion in the desired reaction at higher pH may be attributed to the production of more •OH in basic solution since •OH is responsible for the decomposition of dye molecules (26). Analysis of PEC Degradation Intermediates. As shown in Figure 2(B) and Supporting Information Figure S3, the peak at 510 nm of K-2G, KD-3G, and X-3B decreased quickly with reaction time in the PEC process. These results indicated that the doubly bonded nitrogen in the dyes was destroyed efficiently. The degradation intermediates of K-2G in the electrooxidation for 7 h and PEC oxidation for 4 and 7 h were analyzed by a GC/MS, respectively. The major intermediate products detected are listed in Table 1. In the PEC process, many organic acids, such as formic acid, acetic acid, oxalic acid, succinic acid, and phthalic acid were detected. Moreover, the intermediates such as phenol that appeared at 4 h were not detected at 7 h; they were degraded as the PEC reaction proceeded. The decrease of TOC in the PEC process 6806

9

ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 41, NO. 19, 2007

Additional details are found in eight figures and one table. This material is available free of charge via the Internet at http://pubs.acs.org.

Literature Cited (1) Hoffmann, M. R.; Martin, S. T.; Choi, W. Y.; Bahnemann, D. W. Environmental applications of semiconductor photocatalyst. Chem. Rev. 1995, 95, 69. (2) Chen, G. H. Electrochemical technologies in wastewater treatment. Sep. Purif. Technol. 2004, 38, 11-41. (3) Pelegrini, R. T.; Freire, R. S.; Duran, N.; Bertazzoli, R. Photoassisted electrochemical degradation of organic pollutants on a DSA type oxide electrode: process test for a phenol synthetic solution and its application for E1 bleach kraft mill effluent. Environ. Sci. Technol. 2001, 35, 2842-2853. (4) Quan, X.; Chen, S.; Su, J.; Chen, J. W.; Chen, G. H. Synergetic degradation of 2,4-D by integrated photo- and electrochemical catalysis on a Pt doped TiO2/Ti electrode. Sep. Purif. Technol. 2004, 34, 73-79. (5) Zhao, X.; Zhu, Y. F. Synergetic degradation of rhodamine B at a porous ZnWO4 film electrode by combined electro-oxidation and photocatalysis. Environ. Sci. Technol. 2006, 40, 3367-3372. (6) Li, G. T.; Qu, J. H.; Zhang, X. W.; Ge, J. T. Electrochemically assisted photocatalytic degradation of acid orange 7 with β-PbO2 electrodes modified by TiO2. Water Res. 2006, 40, 213-220. (7) Santato, C.; Odziemkowski, M.; Ulmann, M.; Augustynski, J. Crystal oriented mesoporous WO3 films: synthesis, characterization, and applications. J. Am. Chem. Soc. 2001, 123, 10639. (8) Sartoretti, C.; Alexander, B.; Solarska, R.; Rutkowska, W.; Augustynski, J.; Cerny, R. Photoelectrochemical oxidation of water at transparent ferric oxide film. J. Phys. Chem. B. 2005, 109, 13685.

(9) Sakthivel, S.; Kisch, H. Daylight photocatalysis by carbonmodified titanium dioxide. Angew. Chem., Int. Ed. 2003, 42, 4908-4911. (10) Sakthivel, S.; Kisch, H. Photocatalytic and photoelectrochemical properties of a nitrogen-doped titanium dioxide. ChemPhysChem. 2003, 4, 487-490. (11) Justicia, I.; Ordejon, P.; Canto, G.; Mozos, J. L.; Fraxedas, J.; Battiston, G. A.; Gerbasi, R.; Figueras, A. Designed self-doped titanium oxide thin films for efficient visible-light photocatalysis. Adv. Mater. 2002, 14, 1399-1402. (12) Tsurumoto, T.; Toyoda, M.; Amao, Y. Photovoltaic conversion system using carotenoid-chlorophyll assembled TiO2 film electrode. Colloids Surf., A 2006, 284-285, 623-626. (13) Zou, Z. G.; Ye, J. H.; Sayama, K.; Arakawa, H. Direct splitting of water under visible light irradiation with an oxide semiconductor photocatalyst. Nature. 2001, 414, 625. (14) Hwang, D. W.; Kim, H. G.; Lee, J. S.; Kim, J.; Li, W.; Oh, S. H. Photocatalytic hydrogen production from water over m-doped La2Ti2O7 (M ) Cr, Fe) under visible light irradiation (λ > 420 nm). J. Phys. Chem. B 2005, 109, 2093. (15) Shimodaira, Y.; Kato, H.; Kobayashi, H.; Kudo, A. Photophysical properties and photocatalytic activities of bismuth molybdates under visible light irradiation. J. Phys. Chem. B 2006, 110, 1779017797. (16) Pelizzetti, E.; Maurino, V.; Minero, C.; Carlin, V.; Pramauro, E.; Zerbinati, O. Photocatalytic degradation of atrazine and other s-triazine herbicides. Environ. Sci. Technol. 1990, 24, 15591565. (17) Hu, C.; Yu, J. C.; Hao, Z. P.; Wong, P. K. Photocatalytic degradation of triazine-containing azo dyes in aqueous TiO2 suspensions. Appl. Catal., B 2003, 42, 47-55. (18) Zhao, X.; Yao, W. Q.; Wu, Y.; Zhang, S. C.; Yang, H. P.; Zhu, Y. F. Fabrication and photoelectrochemical properties of porous ZnWO4 film. J. Solid State Chem. 2006, 179, 2529-2537. (19) Zhao, X.; Wu, Y.; Yao, W. Q.; Zhu, Y. F. Photoelectrochemical properties of thin Bi2WO6 film. Thin Solid Films. 2007, 515, 4753-4757.

(20) Bader, H.; Sturzenegger, V.; Hoigne, J. Photometric method for the determination of low concentrations of hydrogen peroxide by the peroxidase catalyzed oxidation of N,N-diethylp-phenylenediamine (DPD). Water Res. 1988, 22, 11091115. (21) Johnson, E. J. In Semiconductors and Semimetals; Willardson, R. K., Beer, A. C., Eds.; Academic Press, New York, 1967; Vol. 3. (22) Li, X. Z.; Liu, H. L.; Yue, P. T.; Sun, Y. P. Photoelectrocatalytic oxidation of rose Bengal in aqueous solution using a Ti/TiO2 mesh electrode. Environ. Sci. Technol. 2000, 34, 4401-4406. (23) Leng, W. H.; Zhang, Z.; Zhang, J. Q.; Cao, C. N. Investigation of the kinetics of a TiO2 photoelectrocatalytic reaction involving charge transfer and recombination through surface states by electrochemical impedance spectroscopy. J. Phys. Chem. B 2005, 109, 15008-15023. (24) Cho, M.; Lee, Y.; Chung, H.; Yoon, J. Inactivation of Escherichia coli by photochemical reaction of ferrioxalate at slightly acidic and near-neutral pHs. H2O2. Appl. Envrion. Microbiol. 2004, 70, 1129-1134. (25) Lee, J.; Choi, W.; Yoon, J. Photocatalytic degradation of N-Nitrosodimethylamine: mechanism, product distribution, and TiO2 surface modification. Environ. Sci. Technol. 2005, 39, 6800-6807. (26) Vaghela, S. S.; Jethva, A. D.; Mehta, B. B.; Dave, S. P.; Ramacandraiah, G. Laboratory Studies of electrochemical treatment of industrial azo dye effluent. Environ. Sci. Technol. 2005, 39, 2848-2855. (27) Spadaro, J.; Isabelle, L.; Renganathan, V. Hydroxyl radical mediated degradation of azo dyes: evidence for benzene generation. Environ. Sci. Technol. 1994, 28, 1389-1393.

Received for review March 9, 2007. Revised manuscript received June 13, 2007. Accepted July 18, 2007. ES070598B

VOL. 41, NO. 19, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

9

6807