TiO2 Photoelectrocatalytic Oxidation

Development of an E-H2O2/TiO2. Photoelectrocatalytic Oxidation. System for Water and Wastewater. Treatment. X. Z. LI* AND H. S. LIU. Department of Civ...
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Environ. Sci. Technol. 2005, 39, 4614-4620

Development of an E-H2O2/TiO2 Photoelectrocatalytic Oxidation System for Water and Wastewater Treatment X. Z. LI* AND H. S. LIU Department of Civil and Structural Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China

In this study, an innovative E-H2O2/TiO2 (E-H2O2 ) electrogenerated hydrogen peroxide) photoelectrocatalytic (PEC) oxidation system was successfully developed for water and wastewater treatment. A TiO2/Ti mesh electrode was applied in this photoreactor as the anode to conduct PEC oxidation, and a reticulated vitreous carbon (RVC) electrode was used as the cathode to electrogenerate hydrogen peroxide simultaneously. The TiO2/Ti mesh electrode was prepared with a modified anodic oxidation process in a quadrielectrolyte (H2SO4-H3PO4-H2O2-HF) solution. The crystal structure, surface morphology, and film thickness of the TiO2/Ti mesh electrode were characterized by X-ray diffraction and scanning electron microscopy. The analytical results showed that a honeycomb-type anatase film with a thickness of 5 µm was formed. Photocatalytic oxidation (PC) and PEC oxidation of 2,4,6-trichlorophenol (TCP) in an aqueous solution were performed under various experimental conditions. Experimental results showed that the TiO2/Ti electrode, anodized in the H2SO4-H3PO4-H2O2HF solution, had higher photocatalytic activity than the TiO2/ Ti electrode anodized in the H2SO4 solution. It was found that the maximum applied potential would be around 2.5 V, corresponding to an optimum applied current density of 50 µA cm-2 under UV-A illumination. The experiments confirmed that the E-H2O2 on the RVC electrode can significantly enhance the PEC oxidation of TCP in aqueous solution. The rate of TCP degradation in such an E-H2O2assisted TiO2 PEC reaction was 5.0 times that of the TiO2 PC reaction and 2.3 times that of the TiO2 PEC reaction. The variation of pH during the E-H2O2-assisted TiO2 PEC reaction, affected by individual reactions, was also investigated. It was found that pH was well maintained during the TCP degradation in such an E-H2O2/TiO2 reaction system. This is beneficial to TCP degradation in an aqueous solution.

Introduction TiO2-based photocatalysis has been extensively studied for water and wastewater treatment because of its nontoxicity, photochemical stability, and low costs (1, 2). In this technique, photocatalytic (PC) oxidation of organic substances can be normally achieved either with TiO2 powder in aqueous suspension or on the surface of a TiO2 film in aqueous solution under UV-A illumination. Recently, photoelectrocatalytic * Corresponding author phone: (852) 2766 6016; fax: (852) 2334 6389; e-mail: [email protected]. 4614

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(PEC) oxidation has proven to be more efficient than PC oxidation by driving the photogenerated electrons to a counter electrode via an external circuit (3-7). In most PEC oxidation systems, a particulate TiO2 film electrode is usually used as a photoanode by coating TiO2 on supporting media such as a conducting glass or a metallic material, while a counter electrode such as a Pt electrode is used as a cathode. Unfortunately, the role of the cathode beyond a counter electrode is usually disregarded in this kind of PEC system. However, some further advantage of utilizing the cathode for generating useful oxidant species such as hydrogen peroxide (H2O2) within the photoreactor could be realized, if the cathode were to be used as a counter electrode. In fact, the addition of the H2O2 chemical as a sacrificial oxidant to scavenge the conduction band electrons has been proven beneficial for improving the efficiency of PC oxidation in a TiO2 suspension system. In this case, H2O2 takes effect by capturing the photoinduced conduction band electrons to form the hydroxyl radical (•OH) as indicated by eqs 1 and 2 (8, 9). However, these studies also indicated that the H2O2 dosing method plays a very important role. Any overdosing of H2O2 may inhibit the photocatalytic oxidation of organics, and a batch chemical dosing mode is not as good as a continuous dosing mode. Alternatively, recent developments of electrochemistry (10, 11) have demonstrated that H2O2 could be electrochemically generated on a carbon electrode, such as graphite or reticulated vitreous carbon (RVC), via eq 3.

TiO2 + hv f h+ + e-CB

(1)

H2O2 + e-CB f •OH + OH-

(2)

O2 + 2H+ + 2e- f H2O2

(3)

Because of the advantage that the electrochemical reaction is easily controlled by current or potential, the appropriate and continuous supply of H2O2 can be achieved by an electrochemical method instead of the chemical dosing method. Therefore, it is possible to utilize an electrical current to generate H2O2 continuously on the surface of the cathode during the photoreaction in a PEC reaction system to further improve the efficiency of the TiO2-based PEC oxidation. However, the photoinduced current of the particulate TiO2 film electrode is usually limited by the impedance of electron mass transfer between the film and substrate (12). A TiO2/Ti mesh electrode with a microporous structure, by forming TiO2 directly on the surface of titanium mesh through anodic oxidation in the H2SO4 electrolyte solution, was introduced into water and wastewater treatment in our previous studies (13-15). It was confirmed that this TiO2/Ti mesh electrode makes good electric contact with less impedance between the TiO2 film and supporting medium than other materials such as indium-tin oxide conducting glass. However, the photooxidation efficiency of this TiO2/Ti mesh electrode is limited by the low-valent titanium oxides and a limited film thickness, because most of the anatase phase changes to a less photoactive rutile phase with the growth of film thickness. Fortunately, Iwasaki et al. (16) reported recently that lowvalent titanium oxides can be dissolved in dilute fluoride solution and a thick TiO2 film with a microporous structure can be formed in a trinary (H2SO4-H3PO4-H2O2) electrolyte by anodic oxidation. Beranek et al. (17) found that the appropriate hydrofluoric acid (HF) facilitates the formation 10.1021/es048276k CCC: $30.25

 2005 American Chemical Society Published on Web 05/07/2005

of ordered micropores on the anodized TiO2 film. It was indicated that the anodized TiO2/Ti electrode synthesized by a modified art has hopes of gaining higher efficiency of photocatalytic activity. In this work, a TiO2/Ti mesh electrode was first prepared using a modified anodic oxidation process in a quadrielectrolyte (H2SO4-H3PO4-H2O2-HF) solution instead of a H2SO4 single-electrolyte solution. An innovative E-H2O2/TiO2 (E-H2O2 ) electrogenerated H2O2) PEC reactor system was then designed using the anodized TiO2/Ti mesh as a photoanode and a RVC electrode as a cathode. In this system, while organics in aqueous solution are photocatalytically oxidized on the surface of the TiO2/Ti electrode (anode), E-H2O2 can be continuously generated on the surface of the RVC electrode (cathode). It is expected that such an E-H2O2assisted TiO2 PEC oxidation would achieve more efficient organic degradation in aqueous solution than normal TiO2 PEC oxidation.

Experimental Section Materials. 2,4,6-Trichlorophenol (TCP) with a purity of 99.0% was purchased from Aldrich Chemical Co. and used as a model pollutant in the experiments. Titanium mesh (purity >99.6%, nominal aperture 0.19 mm, and wire diameter 0.23 mm) was obtained from Goodfellow Co., U.K., and used as a precursor material to make an anodized TiO2/Ti electrode. RVC material (100 psi) was obtained from the ERG Materials and Aerospace Corp. and was used to prepare an RVC electrode. Preparation of Anodized TiO2/Ti Mesh. The anodized TiO2/Ti electrode was prepared by an optimized procedure based on our previous work (13-15) and the reports by Iwasaki et al. and Beranek et al. (16, 17). A piece of Ti mesh (50 mm × 10 mm × 0.5 mm) was cleaned with alcohol and then placed into 500 mL of 1.5 M H2SO4-0.3 M H3PO4-0.3 M H2O2-0.03 M HF solution accompanied by Pt foil with the same size as a counter electrode. An electrical current was applied between the two electrodes by an electrophoresis power supply (EPS 600, Pharmacia Biotech). The anodic oxidation process was conducted in two stages, in which the galvanostatic anodic oxidation with a constant current density of 50 mA cm-2 first took place until the voltage reached 200 V. The voltage was then maintained at 200 V until the end of the anodic oxidation process, while the current density was decreasing gradually without control. The whole anodic oxidation process lasted for 30 min. The freshly anodized TiO2/Ti mesh was washed by distilled water and dried at 105 °C for 2 h. Characterization of Anodized TiO2/Ti Mesh. X-ray diffraction (XRD) was first carried out to determine the crystal structure of the anodized TiO2/Ti mesh using a Phillips analytical X-ray BV diffractometer with Cu KR radiation (λ ) 0.15418 nm). An accelerating voltage of 35 kV and emission current of 30 mA were used. Data were collected using a step scanning mode with a step size of 0.01° and a counting time of 1 s per step in the range of 10-80°. The surface morphology of the anodized TiO2 film was observed using scanning electron microscopy (SEM; Leica Stereoscan 440). The film thickness was determined by SEM on a cross section of resin blocks, in which a piece of anodized TiO2/Ti mesh was solidified. The resin blocks were cut with a diamond powder disk, mechanically polished to mirror brightness with silica gel (in a sequence down to a particle size of 0.04 mm), and cleaned with distilled water and alcohol. Electrochemical and Photoelectrochemical Measurements. Both electrochemical and photoelectrochemical measurements were carried out with a standard threeelectrode assembly, which consists of an anodized TiO2/Ti mesh as a working electrode, a Pt wire as a counter electrode, and a saturated calomel electrode (SCE) as a reference

electrode. All the potentials in this study are referred to SCE. The experimental setup was composed of a ZF-9 potentiostat/ galvanostat, a ZF-4 signal generator, a ZF-10B data collector (Shanghai, Zhenfang Electronic Ltd.), and a personal computer to control and record potential and current signals. To analyze the response of the anodized TiO2/Ti electrode to potential and UV-A illumination, linear sweep voltammetry (LSV) was employed with a scanning rate of 20 mV s-1 in the range of 0-4 V. The photocurrents at different applied potentials were recorded to determine an optimal potential, and a corresponding current was applied for the PEC oxidation using the anodized TiO2/Ti electrode. The potential vs time curves were also recorded to determine the stability of the electrode under the optimal applied currents. Experimental Setup and Procedure. The TCP degradation in aqueous solution was studied in the E-H2O2/TiO2 PEC oxidation reactor using the TiO2/Ti mesh electrode as the anode, the RVC electrode as the cathode, and a saturated calomel electrode (SCE) as the reference electrode. A cylindrical quartz cell with an effective volume of 100 mL was used as the photoreactor equipped with an 8 W UV-A lamp which has a main emission at 365 nm. Four sets of experiments were carried out with different objectives. The first set of experiments was performed using different TiO2/ Ti meshes prepared in a single-electrolyte solution (0.5 M H2SO4) and a quadrielectrolyte solution (1.5 M H2SO4-0.3 M H3PO4-0.3 M H2O2-0.03 M HF) under UV-A illumination to evaluate the effect of state-of-the-art synthesis on the efficiency of PC activity. The second set of experiments was aimed at determining the effects of applied potential in the range of 0-2.5 V on the PEC oxidation and electrochemical oxidation of TCP using the TiO2/Ti electrode. The third set of experiments used the TiO2/Ti electrode as the anode and the RVC electrode as the cathode to study the efficiency of the E-H2O2/TiO2 PEC system. In the meantime, the variation of E-H2O2 concentration during individual reactions was also investigated. The fourth set of experiments was accomplished to determine the PEC oxidation of TCP in aqueous solution with a pH buffer at pH 5.5 ( 0.2. In the above experiments, TCP solution with an initial concentration of 10 mg L-1 was used and 0.01 M Na2SO4 solution was added to simulate the salinity of Hong Kong’s sewage. The initial pH of this mixture solution was around 5.5. Oxygen gas was purged into the solution at a flow rate of 30 mL min-1, accompanied by continuous magnetic stirring. In addition, the Pt wire was used as the cathode in the TiO2 PEC system and as the anode in the E-H2O2 system, while the RVC electrode was used as the cathode in the E-H2O2/TiO2 PEC system. To avoid the effects of electrochemical reactions on the Pt electrode, it was placed in another compartment with the same solution, which connected the reaction solution with a KCl electrolyte bridge. Before these experiments started, the systems were kept in the dark for 15 min to establish an adsorption/ desorption equilibrium of TCP between the solution and electrodes. Chemical Analyses. The TCP concentration in aqueous solution was analyzed using high-performance liquid chromatography (HPLC; Waters 486) equipped with a reversedphase column (Waters, XTerra MS C-18, 5 µm) and a UV detector. The mobile phase consisted of acetonitrile and water (65:35) with 0.1% acetic acid. The detection wavelength was set at 296 nm. The retention time of the TCP peak under this condition was recorded at 6.20 min. The H2O2 concentration was determined by detecting the absorption intensity of the mixture of H2O2 and K2Ti(C2O4)3 in 2 M H2SO4 solution at 400 nm using a UV-vis spectrophotometer (Spectronic, GENISIS2) as reported by Seller (18). The pH value was measured by a pH/ISE meter (Orion, EA 940) equipped with an Orioncomposed electrode. VOL. 39, NO. 12, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. XRD patterns of anodized TiO2/Ti mesh prepared in (a) a quadrielectrolyte (H2SO4-H3PO4-H2O2-HF) and (b) a single electrolyte (H2SO4). The markers at the bottom show the sites of diffraction peaks of titanium, rutile, and anatase.

Results and Discussion Structure Characteristics of Anodized TiO2/Ti Mesh. Two TiO2/Ti meshes anodized in H2SO4 and H2SO4-H3PO4H2O2-HF solutions were examined by XRD analysis. The XRD patterns of both the TiO2/Ti meshes are shown in Figure 1. Compared to that of the TiO2/Ti mesh anodized in H2SO4 solution, the XRD pattern of the TiO2/Ti mesh, anodized in H2SO4-H3PO4-H2O2-HF solution, showed a higher peak intensity ratio of anatase to bulk titanium, which indicates the amount of TiO2 film is much increased. In addition, the noise baseline of the diffraction data decreased, implying that the mesh contained much less amorphous low-valent titanium oxides. Furthermore, no diffraction peak of the rutile phase occurred in the TiO2/Ti mesh anodized with H2SO4H3PO4-H2O2-HF, although the amount of TiO2 was increased significantly. These structure differences between two TiO2/ Ti meshes can be attributed to the different electrolytes used. It was indicated by some recent studies (16, 17) that the addition of H2O2 into H2SO4 electrolyte helps to increase the formation rate of the TiO2 film and addition of HF can reduce the amorphous TiO2 since it is easily dissolved in dilute HF solution prior to crystallization. The potential behavior of anodic oxidation of Ti in H3PO4 electrolyte is also more stable than that in H2SO4 solution (19), which means that less heat is produced and hence anatase is more difficult to transform to rutile. Morphology Characteristics of Anodized TiO2/Ti Mesh. As reported in our previous work (13), the TiO2/Ti mesh anodized in H2SO4 solution was light gray in color at a sparking breakdown voltage of 130 V, and became gold gray at 180 V due to the formation of the rutile phase. The maximum micropore diameter was found to be 200 nm, and the maximum film thickness was 1 µm only. In contrast, it was found that the TiO2/Ti mesh anodized in H2SO4-H3PO4H2O2-HF solution was ash gray in color even at a higher voltage up to 200 V. The SEM micrograph of the TiO2/Ti surface anodized in H2SO4-H3PO4-H2O2-HF (Figure 2A) showed that the film looks like an ordered honeycomb and the diameter of the micropores is enlarged to about 1 µm. The SEM micrograph of the cross section of this TiO2/Ti mesh (Figure 2B) shows that the thickness of the TiO2 film is approximately 5 µm. These results indicate that the addition of H3PO4, H2O2, and HF to H2SO4 electrolyte facilitates the formation of larger micropores and the growth of the oxide film efficiently. Apparently, these changes of morphology 4616

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FIGURE 2. SEM micrographs of the surface (A) and the cross section (B) of anodized TiO2/Ti mesh synthesized in quadrielectrolyte (H2SO4-H3PO4-H2O2-HF) solution. and structure are beneficial in improving the adsorption ability on the film surface and also UV light absorption. Photoelectrochemical Measurement of the Anodized TiO2/Ti Electrode. Although in laboratory studies an applied potential is generally controlled in such a PEC reaction system to drive the separation of photoinduced electrons and holes, the applied current mode is commonly used in industrial applications because it is technically easier to control a current than a potential. In fact, the effects of the two modes are theoretically equivalent. To study the electrochemical response to applied potential and also the photocurrent response to UV irradiation on the TiO2/Ti mesh electrode, three measurements of LSV were conducted in 0.01 M Na2SO4 solution, and the results of the V-i curves are presented in Figure 3. It can be seen that the curve for the Ti mesh electrode in the dark demonstrates a typical metal behavior, in which anodic current begins to increase slowly at 0.3 V and increases sharply over 1.0 V. This anodic current change is attributed to the electrochemical oxidation reaction of oxygen evolution, whereas the curve for the TiO2/Ti mesh electrode in the dark shows that the outburst potential of the current increased from 1.0 to 2.5 V as TiO2 was formed on Ti mesh. A typical characteristic of an N-type semiconductor, that a large anodic current only occurs over the breakdown potential, was thus demonstrated. Moreover, the anodic current of the TiO2/Ti mesh electrode should be derived from not only the reaction of oxygen evolution, but also anodic oxidation of the TiO2/Ti electrode itself. This is different from that of a Ti electrode (20). Relatively, the V-i curve of the TiO2/Ti mesh electrode under UV-A illumination shows that an obvious photocurrent appeared after the potentials greater than 1.0 V, and increased with the increase of potential until mixing occurred with other anodic currents. Apparently, the TiO2/Ti electrode is highly photoactive within a certain range of applied potentials. Considering the anodic breakdown of TiO2 and the anodic corrosion of the Ti electrode itself above 2.5 V, the maximum suitable applied potential seems to be around 2.5 V.

FIGURE 3. Linear sweep voltammogram curves of (a) a Ti mesh electrode in the dark, (b) an anodized TiO2/Ti mesh electrode in the dark, and (c) an anodized TiO2/Ti mesh electrode under UV illumination. Electrolyte: 0.01 M Na2SO4 solution.

FIGURE 4. Photocurrent density (jph) of an anodized TiO2/Ti electrode following UV illumination in 10 mg L-1 TCP and 0.01 M Na2SO4 solution with an applied potential of 0.5, 1.5, 2.5, and 3.0 V vs SCE. The inset is a voltage vs time curve of an anodized TiO2/Ti electrode under UV illumination with an applied current density of 50 µA cm-2. To obtain an optimum applied potential for the anodized TiO2/Ti mesh electrode in such an E-H2O2/TiO2 PEC reaction system, the photocurrent response (jph) of the anodized TiO2/ Ti electrode was measured in an aqueous 0.01 M Na2SO4 solution containing 10 mg L-1 TCP with the applied potentials at 0.5, 1.5, 2.5, and 3.0 V, under the conditions of UV-A illumination on and off. The results shown in Figure 4 demonstrate that all curves of photocurrent density vs time behaved in a similar pattern. Before UV-A illumination, the current density was maintained at a low level. During UV-A illumination, the photocurrent density sharply increased to a higher level. After UV-A illumination, the current density dropped off rapidly to the original level. However, these changes of photocurrent were significantly affected by the electrical potential applied on the anodic TiO2/Ti electrode. The results show that, under UV-A illumination, the photocurrent density only increased to 2 µA cm-2 at 0.5 V and significantly increased to 12 µA cm-2 at 1.5 V. However, the photocurrent density remarkably increased to 50 µA cm-2 at 2.5 V. The results also demonstrated that, under the dark condition, the baseline of the current density was well maintained at a low level ( TiO2 PEC (Pt-TiO2/Ti-UV) > E-H2O2/UV (RVC-Pt-UV) > E-H2O2 (RVC-Pt). Compared to other processes, the E-H2O2/TiO2 PEC process achieved the highest k value of 0.0733 min-1, which is 5 times greater than that of the TiO2 PC process and 2.3 times that of the TiO2 PEC process. The above results indicate that the E-H2O2assisted TiO2 PEC reaction can significantly further improve

FIGURE 8. Accumulation of H2O2 concentration in TCP solution with a constant current density of 50 µA cm-2: (a) E-H2O2; (b) E-H2O2/ UV; (c) E-H2O2/TiO2; (d) E-H2O2/TiO2 PEC. the conventional TiO2 PEC reaction by replacing a common counter electrode such as a Pt electrode with an RVC electrode. In the E-H2O2-assisted TiO2 PEC reaction, E-H2O2 plays an important role as an electron acceptor to consume the photoinduced electrons generated from TiO2 under UV-A illumination. However, the E-H2O2 concentration during the reaction depends on its generation rate electrically and consumption rate in the reaction. In the third set of experiments, H2O2 concentration was also monitored, and the results are shown in Figure 8. It can be seen that the H2O2 concentration in the E-H2O2 reaction (RVC + Pt in the dark) increased gradually and continuously against time and reached its maximum value of 0.112 mM after 60 min. Under UV-A illumination, the H2O2 concentration demonstrated a slightly slower increasing rate compared to that in the dark condition. It is believed that the loss of H2O2 might result from the direct photolysis of H2O2 under UV-A illumination although insignificant, or a chemical reaction between H2O2 and the intermediates from the initial degradation of TCP. Once the Pt electrode as the anode was replaced by the anodized TiO2/Ti mesh as the E-H2O2/TiO2 reaction in the dark, the H2O2 concentration showed a slower rate of increase than those in the above two reactions and reached its maximum level of only 0.058 mM after 60 min. Loss of H2O2 in this system might be attributed to an easier reaction of H2O2 on the surface of the TiO2/Ti film than on the Pt electrode as shown by eq 4, where both the RVC and TiO2/Ti electrodes were placed in the same compartment.

H2O2 + 2e- f 2OH-

(4)

In the E-H2O2/TiO2 PEC system under UV-A illumination, the H2O2 concentration initially increased and was then well maintained at a low level of 0.017 mM, which indicated that a high consumption rate of H2O2 was equivalent to the H2O2 generation rate. In other words, the photoinduced electrons from the conduction band of TiO2 under UV-A illumination were effectively eliminated by the H2O2 generated on the RVC electrode. Variation of pH in TCP Solution. In this E-H2O2/TiO2 PEC reaction system, several reactions would affect the pH of the TCP solution. From the chemical oxidation (eq 2), H2O2 reacts with e-CB to produce OH-, resulting in an increase of pH. From the electrogeneration of H2O2 (eq 3) on the RVC electrode, O2 reacts with H+, resulting in an increase of pH as well. On the other hand, the intermediate products of TCP degradation including some acidic compounds such as oxalic

FIGURE 9. Changes of pH value in different reaction systems under a constant current density of 50 µA cm-2: (a) E-H2O2; (b) E-H2O2/UV; (c) E-H2O2/TiO2 PEC; (d) TiO2PEC.

FIGURE 10. TCP degradation in the TiO2 PEC reaction with or without an HAc-NaOAc buffer solution. acid and formic acid (21) would result in a decrease of pH in TCP solution. To further study the pH variation effected by individual reactions, pH was monitored in the third set of experiments, and the results are shown in Figure 9. It can be seen that the E-H2O2 reaction (RVC + Pt) in the dark demonstrated an increase of pH in the TCP solution from 5.5 to 6.78, while the E-H2O2/UV reaction (RVC + Pt + UV) showed an increase of pH from 5.5 to 6.02. The pH increase resulted mainly from the electrogeneration of H2O2 with a side reaction between H2O2 and e- CB or e-. In contrast, the pH value during the TiO2 PEC reaction (TiO2 + Pt + UV) decreased from 5.5 to 4.1. This pH decrease resulted from the degradation of TCP. Furthermore, it was found that pH in the E-H2O2/TiO2 PEC reaction (RVC + TiO2 + UV) was kept almost stable with a minor decrease from 5.5 to 5.1. This phenomenon showed a net result of pH changes due to the individual reactions. It seems that the pH reduction resulting from TCP degradation can be satisfactorily neutralized by the generation of H2O2 in the same reaction system. To study the effect of pH on the TCP degradation reaction, the fourth set of two experiments was performed to carry out a TiO2 PEC reaction (TiO2 + Pt + UV) in a pH buffer solution at pH 5.5 ( 0.2 and also without pH buffer. The experimental results are shown in Figure 10 and Table 1. The results demonstrate that the rate of TCP degradation with a pH buffer was slightly faster than that without pH buffer. To compare the rates of TCP degradation in both the reactions, the pseudo-first-order kinetic constants (k) were calculated to VOL. 39, NO. 12, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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be 0.0321 min-1 for the reaction without pH buffer and 0.0401 min-1 for the reaction with pH buffer as listed in Table 1. The above experimental results indicate that the phenomenon of pH neutralization found in the TCP degradation reaction in the E-H2O2/TiO2 PEC reaction system might be a useful way to maintain pH values in practical water and wastewater treatment, when pH reduction results from degradation of organics. However, only limited data have been obtained so far, and further experiments on the degradation of various organics need to be conducted before any conclusions can be drawn.

Acknowledgments We thank the Research Grant Committee of the Hong Kong Government for financial support of this work (RGC Grant No. PolyU5148/03E). We also thank Mrs. Elaine Anson for English proofreading.

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Received for review November 5, 2004. Revised manuscript received March 15, 2005. Accepted April 6, 2005. ES048276K