Photocatalytic Oxidation of Arsenite on TiO2: Understanding the

Response to Comment on “Photocatalytic Oxidation Mechanism of As(III) on TiO2: Unique Role of As(III) as a Charge Recombinant Species”. Damián ...
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Environ. Sci. Technol. 2006, 40, 7034-7039

Photocatalytic Oxidation of Arsenite on TiO2: Understanding the Controversial Oxidation Mechanism Involving Superoxides and the Effect of Alternative Electron Acceptors JUNGHO RYU AND WONYONG CHOI* School of Environmental Science and Engineering, Pohang University of Science and Technology, Pohang 790-784, Korea

Previously we have reported that superoxide plays the primary role as oxidant of As(III) in the UV/TiO2 system, however, since then there has been a controversy over the true identity of the major As(III) oxidant. This study aims to establish a comprehensive understanding of the oxidative mechanism which satisfactorily explains all of the observed results during the photocatalytic oxidation (PCO) of As(III). The key step that has masked the true oxidative mechanism is related to the fact that the adsorbed As(III) on TiO2 serves as an external charge-recombination center where the reaction of As(III) with an OH radical (or hole) is immediately followed by an electron transfer to make a null cycle. This was confirmed by the observation that the photoanodic current obtained with a TiO2 electrode immediately decreased upon spiking with As(III), portraying the superoxide-mediated PCO as the dominant pathway. The degradation of competitive substrates (benzoic acid and formic acid) was delayed until As(III) was fully converted into As(V) since the normal PCO mechanism that is based on the action of adsorbed OH radicals (or holes) is not working as long as As(III) is present on the TiO2 surface. However, the As(III) PCO mechanism is entirely altered when alternative electron acceptors (Ag+, Cu2+, polyoxometalate) are present. When these alternative electron acceptors are more efficient than O2 they are able to intercept the CB electron, impeding the recombination pathway and enabling an anoxic oxidation mechanism in which OH radicals and holes play the role of main As(III) oxidant. In the presence of polyoxometalate or Cu2+, the above-mentioned photoanodic current immediately increases upon spiking As(III), indicating that the PCO mechanism has changed in the presence of more efficient electron acceptors. Comprehensive mechanisms of As(III) PCO and experimental factors that alter the mechanism are discussed.

Introduction Arsenic contamination in groundwater has been recognized as a great threat to the public health worldwide (1-3). Since WHO and EPA recently lowered the allowed arsenic levels in drinking water to 10 µg/L, extensive efforts have been made to develop effective methods to remove arsenic from * Corresponding author e-mail: [email protected]; phone: +82-54-279-2283; fax: +82-54-279-8299. 7034

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drinking water (2-4). Arsenic in groundwater occurs in its common oxidation states of As(III) and As(V) (5). Since As(III) is more toxic and mobile than As(V), the preoxidation of As(III) to As(V) is a desirable process in water treatment. Various remediation techniques which oxidize As(III) to As(V) have been reported in the literature (6-12). TiO2 photocatalysis has been demonstrated to be a promising technology to remediate polluted air and water (13-19). On illuminated TiO2, valence band (VB) hole, OH radical, or superoxide may initiate the oxidation of As(III) (reactions 1-3) (20, 21).

As(III) + OH• f As(IV) + OH(k ) 9 × 109 M-1 s-1) (20) (1) As(III) + hVB+ (or surface-bound OH•) f As(IV) (2) As(III) + O2•- + H+ f As(IV) + HO2-

(k ) 3.6 × 106 M-1 s-1) (22) (3)

As(IV) + O2 f As(V) + O2•-/HO2•

(k ) 1.1 × 109 M-1 s-1) (21) (4)

Note that the rate constant of reaction 1 is about 3 orders of magnitude higher than that of reaction 3. In our previous studies that investigated TiO2 photocatalytic oxidation (PCO) of As(III), we proposed that superoxide is the primary oxidant of As(III) to As(VI) (23, 24). The claim was rather unusual because it is well accepted that OH radicals are main oxidants in most TiO2-mediated PCO processes. The proposition of the superoxide-mediated PCO mechanism of As(III) is based mainly on the following observations: (1) the addition of excess t-BuOH (OH radical scavenger) did not reduce the rate of As(III) oxidation at all and hence the role of OH radicals should be insignificant; (2) surface-fluorinated TiO2 that had a markedly reduced adsorptive capacity for As(III) did not show a reduced PCO rate, which indicates that the direct hole transfer path is not important, either; (3) As(III) oxidation on dye-sensitized TiO2 under visible light (where superoxides should be dominant as a main oxidant) occurred with a rate similar to that of the case of TiO2/UV; and (4) the As(III) PCO rate was significantly reduced (by about half) in the presence of superoxide dismutase. However, it is not clearly understood how such a weak oxidant like superoxide can be more important than OH radicals or VB holes. Several studies were followed to address the issue of PCO mechanism of As(III) to confirm or to dispute our claim. Ferguson et al. (25) reinvestigated the PCO of As(III) in TiO2 suspension in an environmentally relevant concentration range of As(III) and supported our claim that superoxide plays a major role in the oxidation of As(III) to As(V). On the contrary, Dutta et al. (26) investigated the effect of benzoic acid (an OH radical scavenger) on the PCO mechanism and argued that OH radicals are main oxidants. Xu et al. (22) also asserted that the adsorbed OH radicals play the central role in As(III) PCO and the contribution of superoxides in the PCO mechanism is insignificant. Yoon and Lee (27) recently proposed that the direct electron transfer between As(III) and TiO2 VB holes is the most rational oxidation mechanism on the basis of their study that investigated the effects of various hole scavengers and electron acceptors. The issue on the As(III) PCO mechanism seems to be highly controversial and there is no consensus about what plays the role of the main oxidant of As(III) in TiO2-mediated PCO. 10.1021/es0612403 CCC: $33.50

 2006 American Chemical Society Published on Web 10/17/2006

This study aims to provide a unanimous and consistent explanation for what has been observed for As(III) PCO in different studies. By carrying out photoelectrochemical measurements and investigating the effects of competitive substrates and alternative electron scavengers on As(III) PCO, we reconfirmed that superoxides are the main oxidant in aqueous TiO2 suspensions with dissolved O2 as a sole electron acceptor. The PCO mechanisms, however, could be changed in the presence of interfering substances (e.g., alternative electron acceptors) that were used as mechanistic probes, which explains why the suggested mechanisms disagree among different research groups. More elaborate and comprehensive mechanisms of As(III) PCO which can not only explain the results from this group but also from others are discussed.

Experimental Section Materials and Chemicals. NaAsO2 (As(III), Aldrich) and Na2HAsO4‚7H2O (As(V), Kanto) were used as the arsenic sources. Other chemicals used included LiClO4 (Aldrich), benzoic acid (Aldrich), formate (HCOONa, Acros), AgNO3 (Aldrich), 4-chlorophenol (4-CP, Sigma), KO2 (Aldrich), KH2PO4 (Kanto), tertbutyl alcohol (t-BuOH, Shinyo), and H4O40SiW12 (POM, Fluka), all of which were of reagent grade and used as received. Degussa P25 with an average surface area of 50 m2/g was selected as a photocatalyst. Deionized water used was ultrapure (18 MΩ‚cm) and prepared by a Barnstead purification system. Photoelectrochemical Experiments. Photocurrents were obtained with a TiO2/ITO electrode immersed in aqueous electrolyte solution (10 mM LiClO4). For preparing the TiO2/ ITO electrode, an ITO plate was coated with TiO2 film using Carbowax as a binder (28). The TiO2-coated ITO plate was then dried for 20 min in air and calcined at 450 °C for 30 min to burn off the organic binder. The TiO2/ITO electrode, a saturated calomel electrode (SCE), and a graphite rod were immersed in a reactor as working (collector), reference, and counter electrodes, respectively. Photocurrents were monitored as a function of time in aqueous electrolyte solution before and after spiking an aliquot of As(III) or As(V) solution with an applied potential (from -0.8 to +0.3 eV vs SCE) using a potentiostat (EG&G 263A2) connected to a computer. Photolyses and Analyses. The concentrations of TiO2 and As(III) were fixed at 0.5 g/L and 500 µM, respectively, in most experiments. The pH of the suspension was adjusted with HCl or HClO4 standard solution and the suspension was stirred for 20 min to allow equilibrium adsorption of As(III) on TiO2. For the PCO experiments in the absence of dissolved oxygen, the suspension was continuously sparged with nitrogen gas during the irradiation. A 300-W Xe arc lamp (Oriel) combined with a 10-cm IR water filter and a UV cutoff filter (λ > 300 nm) was used as a light source. The 90-mL Pyrex reactor was open to the ambient air (air-equilibrated conditions) or closed with a rubber septum under continuous purging of nitrogen gas (N2-saturated conditions), and stirred magnetically during irradiation. Sample aliquots were withdrawn by a 1-mL syringe intermittently during the photoreaction and filtered through a 0.45-µm PTFE filter (Millipore) to remove TiO2 particles. Quantitative analysis of As(V) and formate was performed using an ion chromatograph (IC, Dionex DX-120), which was equipped with a Dionex IonPac AS 14 (4 mm × 250 mm) column and a conductivity detector. The eluent solution was 3.5 mM Na2CO3/1 mM NaHCO3. The concentration of benzoic acid was monitored using a UV/vis spectrophotometer (Agilent).

Results and Discussion Photoelectrochemical Investigation of As(III) PCO. Let us start by looking into how the interfacial electron transfer

FIGURE 1. Time-dependent profiles of photocurrent generated with a TiO2/ITO electrode immersed in electrolyte solution. The change of photocurrent is shown upon spiking an aliquot of (a) As(III) and (b) As(V). Spiking the electrolyte solution alone did not influence the photocurrent at all. The experimental conditions were [LiClO4] ) 10 mM (electrolyte), [As(III)]spike ) [As(V)]spike ) 500 µM, applied potential ) +0.1 or -0.1 V (vs SCE), pHi ) 3, and air-equilibrated. between As(III) and TiO2 surface occurs under irradiation. This was done in this study by monitoring the change of photocurrent obtained with a TiO2/ITO electrode immersed in aqueous electrolyte upon spiking As(III). The photocurrent monitored under UV irradiation should indicate whether a substrate is oxidized (electron-donating) or reduced (electronaccepting). An electron-donating process occurring on the TiO2 electrode should increase the photoanodic current. Accordingly, if As(III) adsorbed on the surface of TiO2 is mainly oxidized by VB holes, the photoanodic current should be enhanced when As(III) is added into the illuminated electrochemical reactor. However, Figure 1a clearly shows that the photoanodic current immediately decreases as soon as 500 µM As(III) is added. We repeated the test of Figure 1a with varying concentrations of As(III) and the photocurrent decrease upon spiking As(III) was still observed with [As(III)]spike as low as 25 µM. The magnitude of the current drop was reduced by about half when decreasing the [As(III)]spike from 500 to 50 µM. The current drop was also observed when the test of Figure 1a was carried out under the N2-saturated condition, which rules out electron transfer to any oxygencontaining species as responsible for the current drop. The observed photocurrent drop indicates that As(III) oxidation cannot be induced by a direct hole transfer (reaction 2). The decline of the current should not be a sign of the reduction of As(III) since As(III) is oxidized on illuminated TiO2. On the other hand, the addition of As(V) hardly influenced the photoanodic current (Figure 1b). This is consistent with the observation that As(V) was neither further oxidized nor reduced back to As(III) on TiO2. The paradox is that As(III) is oxidized but its presence decreases the photoanodic current. This contradiction can VOL. 40, NO. 22, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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SCHEME 1. Schematic Illustration of As(III) PCO Mechanism (a) with Dissolved O2 as a Sole Electron Acceptor and (b) with Alternative Electron Acceptors That are More Efficient Than O2. The Band Edge Potentials of TiO2 are at pH 3. The Standard Reduction Potentials of Electron Acceptors are from Refs 30 and 34.

be rationalized when we make the hypothesis that the redox couple of As(III)/As(IV) (Eo ) 2.4 VNHE) (21) acts as an external charge recombination center at which reaction 1 or 2 is immediately followed by the reaction with a conduction band (CB) electron (reaction 5). Judging from the observations of Figure 1a, reaction 5 seems to occur much faster than reaction 4.

As(IV) + eCB- f As(III)

(5)

The related energy levels and electron transfers are illustrated in Scheme 1a. When this null cycling prevails, the addition of As(III) should decrease the photoanodic current (as observed) by providing an additional charge recombination path. If the reaction of As(III) with an OH radical or a VB hole (reactions 1 and 2) is immediately followed by reaction 5, it can explain why OH radicals or VB holes do not serve as a main oxidant of As(III) despite their strong oxidizing power. To put it another way, OH radicals and VB holes are immediately annihilated by CB electrons as long as As(III) is present on the TiO2 surface. Under this condition, the oxidation of As(III) might be carried out by an alternative oxidant such as superoxide (reaction 3). It is interesting to note that in their transient absorption study Xu et al. (22) showed that OH radicals adsorbed on TiO2 react with As(III) and concluded that adsorbed OH radicals are the primary As(III) oxidant. However, in their study the adsorbed OH radicals were generated through a pulse radiolysis in the presence of TiO2 colloid, not by photoexcitation of TiO2. Thus, the adsorbed OH radicals on TiO2were generated without generating CB electrons and therefore reaction 5 will be absent from their system. In this case the adsorbed OH radicals are expected to oxidize As(III) to As(IV) and in the absence of CB electrons the adsorbed As(IV) will eventually be converted to As(V) through reaction 4. Their observation is fully consistent with our hypothesis. On the other hand, superoxides are much longer-lived than OH radicals and will consequently diffuse farther into the bulk solution than OH radicals. It was observed that the superoxides generated in the UV-illuminated TiO2 suspension decayed very slowly and [O2•-] remained as high as 0.1 µM even 10 min after stopping the UV irradiation (29). Therefore, As(IV) produced via reaction 3 can be generated away from the interfacial region, 7036

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FIGURE 2. Effects of the presence of competitive substrates, (a) benzoic acid (BA) and (b) formate, on As(III) PCO. The experimental conditions were [TiO2] ) 0.5 g/L, [As(III)]0 ) [BA]0 ) [formate]0 ) 500 µM, pHi ) 3, and air-equilibrated. thus less subject to reaction 5 and subsequently oxidized to As(V) through reaction 4. Effects of Competitive Substrates on As(III) PCO. Dutta et al. (26) claimed that OH radicals are the main oxidant of As(III) PCO on the basis of the observation that benzoic acid (BA) added as a hydroxyl radical scavenger was not degraded until most of the As(III) was oxidized to As(V). We reproduced their result in Figure 2a. Their interpretation is as follows: OH radicals are not available to BA since As(III) scavenges all OH radicals produced, therefore, As(III) should be oxidized by OH radicals, not by superoxides. A similar experiment using formate (a good VB hole scavenger) instead of BA also produced a similar result (Figure 2b). The photocatalytic degradation of formate was delayed until most As(III) was converted into As(V). The observed kinetics does not seem to be explained in terms of “the superoxide mechanism”. However, it should be noted that the presence of BA (or formate) little influenced the PCO rate of As(III). This cannot be easily understood since BA (or formate) should scavenge some fraction of OH radicals and subsequently reduce the PCO rate of As(III) if OH radicals were the main oxidant. This phenomenon can be more clearly understood by an alternative interpretation: the photocatalytic degradation of BA was delayed not because the OH radicals were directly scavenged by As(III), but because OH radicals (or VB holes) are depleted through the charge-recombining null cycles (reaction 5) as long as As(III) is present on the TiO2 surface. To support this alternative interpretation, we performed the photoelectrochemical test in the presence of formate. The level of photoanodic current (with formate) suddenly dropped when As(III) was spiked as in the case of Figure 1a (see Supporting Information Figure S1). That is, the presence of As(III) inhibited the PCO of formate. In essence, As(III) is a poison

FIGURE 3. Effects of Ag+ addition (as an alternative electron acceptor) on As(III) PCO. The experimental conditions were [TiO2] ) 0.5 g/L, [As(III)]0 ) [AgNO3]0 ) 500 µM, pHi ) 3, and air-equilibrated or N2-saturated. to normal TiO2-mediated PCO reactions that are based on the action of adsorbed OH radicals and VB holes. The above argument is contrasted with a recent study by Yoon and Lee (27) which reported that the addition of excess formate, oxalate, or iodide as a hole scavenger significantly retarded the photogeneration of As(V). However, in our investigation, 5 mM formate did not reduce the PCO rate of As(III) because the experimental conditions were quite different. It should be noted that the concentration ratios of [hole scavenger]/[TiO2] were very different between the two cases: for example, the [formate]/[TiO2] ratio employed in ref 27 was 30 times higher than ours. Therefore, the inhibition of PCO by formates appears to be not due to the hole scavenging, but rather to be related with the competitive adsorption on TiO2 between As(III) and formates. When the concentration ratio of hole scavengers to surface sites is higher than a critical value, the adsorption of As(III) is inhibited with retarding the PCO rate. In a similar context, Ferguson et al. (25) demonstrated that the addition of 0.1 M t-BuOH to the TiO2 suspension containing 2.5 µM arsenic decreased the As(III) PCO rate by half because the presence of t-BuOH inhibited the initial As(III) adsorption. Therefore, it should be taken into account that the effect of excess hole scavengers could be confounded especially when they interfere with the adsorption of substrates on TiO2. The apparent retardation in the As(III) PCO rate might be due to either the real hole scavenging effect or the inhibition of As(III) adsorption. Effects of Alternative Electron Acceptors. If As(III) adsorbed on TiO2 surface acts as a charge-pair recombination center, the As(III) PCO mechanism should be entirely altered when alternative electron acceptors that can compete with As(IV) are present. In the aerated suspension of pure TiO2, the only significant electron acceptor is the dissolved O2. Silver ion (Ag+) that is widely used as a good electron acceptor in TiO2 photocatalytic system was employed to see if As(III) could be oxidized in the absence of O2. The color of the TiO2 suspension changed during the photoreaction from white to dark brown as a result of Ag0 formation. Figure 3 shows that As(III) can be oxidized even in the absence of O2 and the PCO rate was greatly enhanced with Ag+. This obviously indicates that superoxides cannot be the oxidant in this case. This is quite contradictory to the previous case where CCl4 was used as an alternative electron acceptor in the de-aerated suspension of TiO2 (24, 25). In that case, As(III) PCO was insignificant in the presence of CCl4 (without O2), which supported that the presence of O2 was essential for the oxidation of As(III). The difference between Ag+ and CCl4 as an electron acceptor is ascribed to the fact that Ag+ is thermochemically a far better electron acceptor than CCl4.

The reduction potentials of Ag+/Ag0, O2/O2•-, and CCl4 (polarographic E1/2) are 0.80 (30), -0.33 (30), and -0.51 V (31) (vs NHE), respectively. CCl4 as a weak electron acceptor cannot compete efficiently for CB electrons with As(IV) and hence reaction 5 is hardly inhibited. As long as reaction 5 is in action, superoxides are required as a main oxidant of As(III). Therefore, As(III) PCO cannot be efficient in the deaerated suspension with CCl4. On the other hand, the presence of Ag+ as an alternative electron acceptor drastically changes the PCO mechanism by inhibiting reaction 5. Although Ag+ (E0 ) 0.8 V) is weaker than As(IV) (E0 ) 2.4 V) in its electron accepting power, much higher concentration of Ag+ than the transient As(IV) species allows the silver ions to act as the dominant electron scavenger quenching reaction 5. When reaction 5 is blocked by Ag+, As(III) can be now oxidized irreversibly by adsorbed OH radicals or VB holes. The initial PCO rate is markedly higher in the presence of Ag+, which indicates that the intrinsic As(III) oxidation rate by VB holes (or surface-bound OH radicals) is indeed much faster than that induced by superoxides unless hampered by reaction 5. It is noted that the production of As(V) in the Ag+/N2 system is terminated at lower As(V) concentrations than that in the air-equilibrated system. This seems to be due to the depletion of Ag+. That is, the anoxic PCO of As(III) proceeds as long as the powerful electron acceptor (like Ag+) that is able to block reaction 5 is available. This observation demonstrates that the mechanism of As(III) PCO is clearly dependent on the kind of CB electron acceptors. On the other hand, the fact that As(III) can be oxidized to As(V) in the absence of O2 implies that As(IV) is further oxidized by other paths than reaction 4. As(IV) might be oxidized by VB holes (reaction 6) or alternative electron acceptors (reaction 7). As(IV) can be also oxidized by injecting an electron directly into TiO2 CB (reaction 8) or disproportionation (reaction 9). The reduction potential of As(V)/As(IV) couple was reported to be -1.2 VNHE, which is far above the TiO2 CB edge potential (21). All reactions 6-9 may contribute although we are not sure which path is dominant.

As(IV) + hVB+ (or surface-bound OH•) f As(V) (6) As(IV) + Ag+ (or alternative e- acceptor) f As(V) + Ag0 (7) As(IV) (adsorbed on TiO2) f As(V) + eCB-

(8)

As(IV) + As(IV) f As(V) + As(III)

(9)

The proposition that the presence of alternative electron acceptors changes the As(III) PCO mechanism can provide a more consistent interpretation for other similar observations that have been understood differently. When Cu2+, polyoxometalate (POM), or bromate was employed as an alternative electron acceptor to probe the role of O2 in As(III) PCO, it was observed that the anoxic PCO rate was comparable to or even higher than the PCO rate under the airequilibrated condition (22, 27). On the basis of these observations, Xu et al. (22) and Yoon and Lee (27) claimed that the primary role of oxygen is to serve as an electron acceptor and dismissed the role of superoxides as a main oxidant of As(III). However, their investigation with Cu2+, POM, or bromate is essentially identical to our case with Ag+ in that alternative electron acceptors were used in the anoxic suspension. Therefore, the logical interpretation is not that superoxides are not involved in As(III) PCO, but that the presence of Cu2+, POM, or bromate opened up an alternative path that enabled the anoxic oxidation of As(III). Although such alternative electron acceptors were added as a probe into the PCO mechanism, they actually changed the prevailing mechanism. VOL. 40, NO. 22, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 4. Change of photocurrent generated with a TiO2/ITO electrode immersed in the solution of POM or Cu2+ upon spiking an aliquot of As(III). The current profile with Cu2+ was offset by -30 µA/cm2 for visual clarity. The experimental conditions were [LiClO4] ) 10 mM (electrolyte), [As(III)]spike ) 500 µM, [SiW12O404-]0 ) [Cu2+]0 ) 5 mM, applied potential ) +0.1 V (vs SCE), pHi 1.8 (w/POM) and 3 (w/Cu2+), and air-equilibrated. To probe the influence of Cu2+ and POM on the interfacial electron-transfer event, the change of photocurrent upon spiking As(III) was monitored in the presence of Cu2+ or POM in the same way we did in Figure 1. The results shown in Figure 4 are in marked contrast with those of Figure 1. First of all, it is noted that the background photocurrents are much lower with POM or Cu2+ than those shown in Figure 1. This indicates that the alternative electron acceptors are scavenging CB electrons from the TiO2 electrode and lowering the background photocurrent. Second, the addition of As(III) immediately increased the photoanodic current, which is contrary to the case of Figure 1 where the photocurrent was suppressed upon spiking As(III). As expected, As(III) is indeed oxidized by VB holes in the presence of Cu2+ or POM. In our previous study (24), we also observed that the addition of a POM (SiW12O404-) to a pure TiO2 suspension significantly increases the PCO rate. At that time, we interpreted the role of the POM as an electron-transfer mediator between the TiO2 CB and O2 to enhance the generation of superoxides. But now we realize that another important role of POM is to shut down the As(III)/As(IV)-mediated recombination path. This evidence clearly indicates that more efficient electron acceptors (e.g., Ag+, Cu2+, POM, bromate) effectively scavenge CB electrons thus blocking the recombination pathway (reaction 5). Under this situation, As(III) is oxidized by VB holes, not by superoxides. Now the discrepancy between our interpretation (supporting the role of superoxides) and others (supporting the role of VB holes or surface-bound OH radicals) in As(III) PCO mechanism is more clearly understood. The alternative electron acceptors that were used as a mechanistic probe actually changed the mechanistic path of As(III) oxidation! Photocatalytic Oxidation Mechanisms of As(III) that Depend on the Reaction Conditions. The photocatalytic oxidation of As(III) provides a unique example in which the mechanism is sensitively dependent on the reaction and catalyst conditions. Superoxides seem to be the main oxidant in aqueous TiO2 suspension with dissolved O2 as a sole electron acceptor. However, VB holes or OH radicals can play the role of oxidants when electron acceptors stronger than O2 are present. However, in most natural waters contaminated with arsenic, it is unlikely that an impurity species that could serve as an alternative electron acceptor is present in concentrations high enough to change the superoxide-driven oxidation mechanism. PCO mechanisms in a few cases are comparatively discussed below. 7038

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In most TiO2 PCO reactions, the role of superoxides as an oxidant is minor or insignificant because they are simply much weaker oxidants than OH radicals and VB holes. However, in the presence of As(III) adsorbed on TiO2, the redox couple of As(III)/As(IV) rapidly mediates the recombination of charge pairs while depleting VB holes and adsorbed OH radicals in null cycles with CB electrons (Scheme 1a). Therefore, under such condition, superoxides that are much longer-lived and are able to diffuse farther into the solution bulk govern the overall oxidation process. On the basis of the rate constant of reaction 3 (3.6 × 106 M-1 s-1) and the observed rate of As(V) generation (0.05 × 10-6 M s-1), the concentration of superoxides at the photostationary state is estimated to be around 2.8 × 10-11 M. The photostationary concentration of superoxides ([O2•-]ss) in the P25 TiO2 suspension (4.3 gTiO2/L at pH 11.5) was previously reported to be around 1 µM by using a luminol chemiluminescence method (29). In the presence of As(III), [O2-]ss could be much lower than the µM level because a significant fraction of CB electrons is consumed by the As(III)/As(IV) couple-mediated recombination as we proposed in Scheme 1a. In any case, the concentration of superoxides generated in the illuminated TiO2 suspension should be high enough to account for the observed rate of As(V) generation although the intrinsic reactivity of superoxides with As(III) (reaction 3) is lower than that of OH radicals (reaction 1) by 3 orders of magnitude. To make another confirmation, 500 µM of KO2 was added as a direct source of superoxides into 100 µM As(III) solution. About 60% conversion to As(V) was achieved immediately upon spiking KO2 regardless of the presence of TiO2, which reassures that superoxides are able to oxidize As(III) to As(V). On the other hand, the situation changes when alternative electron acceptors (e.g., Ag+, Cu2+, POM) are present. In such cases, the charge-recombining null cycles mediated by the As(III)/As(IV) couple are terminated and the VB holes or adsorbed OH radicals are able to take part in the oxidation mechanism as illustrated in Scheme 1b. The charge-recombining null cycles mediated by the As(III)/As(IV) couple also can be suppressed by inhibiting the adsorption of As(III) on TiO2. One way of doing this is to fluorinate the surface of TiO2 (F-TiO2) (32, 33). The surface adsorption of As(III) was hindered on F-TiO2 as demonstrated in our previous study (24). With little As(III) adsorbed on the fluorinated surface, the As(III)/As(IV)-mediated recombining null cycles are less operative. In such case, the role of free OH radicals as an oxidant becomes important since the generation of free OH radicals is enhanced on F-TiO2 (32, 33). We indeed observed that the addition of excess t-BuOH significantly retarded the As(III) PCO rate in the presence of fluorinated TiO2, whereas it did not affect the PCO rate at all in the absence of fluorination (24). This indicates that OH radicals do contribute to As(III) oxidation on F-TiO2 and that the surface modification changes the PCO mechanism.

Acknowledgments This work was supported by the SRC/ERC program of MOST/ KOSEF (grant R11-2000-070-080010), the Basic Research program of KOSEF (grant R01-2003-000-10053-0), and the Brain Korea 21 program. We thank Chad Vecitis for his help in manuscript editing.

Supporting Information Available The change of photoanodic current in the presence of formate (Figure S1) and the case investigation of 4-chlorophenol surface complex-mediated oxidation of As(III) on TiO2 under visible light (Figure S2). This material is available free of charge via the Internet at http://pubs.acs.org.

Literature Cited (1) Tondel, M.; Rahman, M.; Magnuson, A.; Chowdhury, I. A.; Faruquee, M. H.; Ahmad, S. A. The relationship of arsenic levels in drinking water and the prevalence rate of skin lesions in Bangladesh. Environ. Health Perspect. 1999, 107, 727-729. (2) Berg, M.; Tran, H. C.; Nguyen, T. C.; Phan, H. V.; Schertenleib, R.; Giger, W. Arsenic contamination of groundwater and drinking water in Vietnam: A human health threat. Environ. Sci. Technol. 2001, 35, 2621-2626. (3) Smedley, P. L.; Kinniburgh, D. G. A review of the source, behaviour, and distribution of arsenic in natural waters. Appl. Geochem. 2002, 17, 517-568. (4) Smith, A. H.; Lopipero, P. A.; Bates, M. N.; Steinmaus, C. M. Arsenic epidemiology and drinking water standards. Science 2002, 296, 2145-2146. (5) Cullen, W. R.; Reimer, K. J. Arsenic speciation in the environment. Chem. Rev. 1989, 89, 713-764. (6) Pettine, M.; Campanella, L.; Millero, F. J. Arsenite oxidation by H2O2 in aqueous solutions. Geochim. Cosmochim. Acta 1999, 63, 2727-2735. (7) Kim, M.; Nriagu, J. Oxidation of arsenite in groundwater using ozone and oxygen. J. Sci. Total Environ. 2000, 247, 71-79. (8) Hug, S. J.; Canonica, L.; Wegelin, M.; Gechter, D.; Gunten, U. V. Solar oxidation and removal of arsenic at circumneutral pH in iron containing waters. Environ. Sci. Technol. 2001, 35, 21142121. (9) Kocar, B. D.; Inskeep, W. P. Photochemical oxidation of As(III) in ferrioxalate solutions. Environ. Sci. Technol. 2003, 37, 15811588. (10) Yang, H.; Lin, W. Y.; Rajeshwar, K. Homogeneous and heterogeneous photocatalytic reactions involving As(III) and As(V) species in aqueous media. J. Photochem. Photobiol. A 1999, 123, 137-143. (11) Bissen, M.; Vieillard-Baron, M.; Schindelin, A. J.; Frimmel, F. H. TiO2-catalyzed photooxidation of arsenite to arsenate in aqueuos samples. Chemosphere 2001, 44, 751-757. (12) Lee, Y.; Um, I.-H.; Yoon, J. Arsenic(III) oxidation by iron(VI) (ferrate) and subsequent removal of arsenic(V) by iron(III) coagulation. Environ. Sci. Technol. 2003, 37, 5750-5756. (13) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Environmental applications of semiconductor photocatalysis. Chem. Rev. 1995, 95, 69-96. (14) Choi, W.; Hong, S. J.; Chang, Y.-S.; Cho, Y. Photocatalytic degradation of polychlorinated dibenzo-p-dioxins on TiO2 film under UV or solar light irradiation. Environ. Sci. Technol. 2000, 34, 4810-4815. (15) Choi, W.; Ko, J. Y.; Park, H.; Chung, J. S. Investigation on TiO2coated optical fibers for gas-phase photocatalytic oxidation of acetone. Appl. Catal. B 2001, 31, 209-220. (16) Kim, S.; Choi, W. Kinetics and mechanisms of photocatalytic degradation of (CH3)(n)NH4-n+ (0ene4) in TiO2 suspension: The role of OH radicals. Environ. Sci. Technol. 2002, 36, 2019-2025. (17) Cho, S.; Choi, W. Solid-phase photocatalytic degradation of PVCTiO2 polymer composites. J. Photochem. Photobiol. A 2001, 143, 221-228. (18) Herrmann, J. M. Heterogeneous photocatalysis: State of the art and present applications. Topics Catal. 2005, 34, 49-65.

(19) Agrios, A. G.; Pichat, P. State of the art and perspectives on materials and applications of photocatalysis over TiO2. J. Appl. Electrochem. 2005, 35, 655-663. (20) Buxton, G. V.; Greenstock, C. L.; Helmen, W. P.; Ross, A. B. Critical review of rate constants for reactions of hydrated electrons, hydrogen atoms and hydroxyl radicals (•OH•O-) in aqueous solution. J. Phys. Chem. Ref. Data 1988, 17, 513-886. (21) Klaning, U. K.; Bielski, B. H. J.; Sehested, K. Arsenic(IV). A pulseradiolysis study. Inorg. Chem. 1989, 28, 2717-2724. (22) Xu, T.; Kamat, P. V.; O’Shea, K. E. Mechanistic evaluation of arsenite oxidation in TiO2 assisted photocatalysis. J. Phys. Chem. A 2005, 109, 9070-9075. (23) Lee, H.; Choi, W. Photocatalytic oxidation of arsenite in TiO2 suspension: Kinetics and mechanisms. Environ. Sci. Technol. 2002, 36, 3872-3878. (24) Ryu, J.; Choi, W. Effects of TiO2 surface modifications on photocatalytic oxidation of arsenite: The role of superoxides. Environ. Sci. Technol. 2004, 38, 2928-2933. (25) Ferguson, M. A.; Hoffmann, M. R.; Hering, J. G. TiO2-photocatalyzed As(III) oxidation in aqueous suspensions: Reaction kinetics and effects of adsorption. Environ. Sci. Technol. 2005, 39, 1880-1886. (26) Dutta, P. K.; Pehkonen, S. O.; Sharma, V. K.; Ray, A. K. Photocatalytic oxidation of Arsenic(III): Evidence of hydroxyl radicals. Environ. Sci. Technol. 2005, 39, 1827-1834. (27) Yoon, S.-H.; Lee, J. H. Oxidation mechanism of As(III) in the UV/TiO2 system: Evidence for a direct hole oxidation mechanism. Environ. Sci. Technol. 2005, 39, 9695-9701. (28) Mills, A.; Elliott, N.; Hill, G.; Fallis, D.; Durrant, J. R.; Wills, R. L. Preparation and characterisation of novel thick sol-gel titania film photocatalysts. Photochem. Photobiol. Sci. 2003, 2, 591596. (29) Hirakawa, T.; Nosaka, Y. Properties of O2•- and OH• formed in TiO2 aqueous suspensions by photocatalytic reaction and the influence of H2O2 and some ions. Langmuir 2002, 18, 32473254. (30) Bard, A. J., Parsons, R., Jordan, J., Eds. Standard Potential in Aqueous Solution; Marcel Dekker: New York, 1985. (31) Choi, W.; Hoffmann, M. R. Photoreductive mechanism of CCl4 degradation on TiO2 particles and effects of electron donors. Environ. Sci. Technol. 1995, 29, 1646-1654. (32) Minero, C.; Mariella, G.; Maurino, V.; Pelizzetti, E. Photocatalytic transformation of organic compounds in the presence of inorganic anion. 1. Hydroxyl-mediated and direct electrontransfer reactions of phenol on a titanium dioxide-fluoride system. Langmuir 2000, 16, 2632-2641. (33) Park, H.; Choi, W. Effects of TiO2 surface fluorination on photocatalytic reactions and photoelectrochemical behaviors. J. Phys. Chem. B 2004, 108, 4086-4093. (34) Weinstock, I. A. Homogeneous-phase electron-transfer reactions of polyoxometalates. Chem. Rev. 1998, 98, 113.

Received for review May 23, 2006. Revised manuscript received September 6, 2006. Accepted September 8, 2006. ES0612403

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