TiO2 Photocatalytic Oxidation Mechanism of As(III) - Environmental

Dec 23, 2008 - ... Radiation Technology Institute, Korea Atomic Energy Research Institute, Jeongeup, Jeonbuk 580-185, South Korea, and Graduate School...
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Environ. Sci. Technol. 2009, 43, 864–869

TiO2 Photocatalytic Oxidation Mechanism of As(III) S U N G - H W A N Y O O N , † S A N G - E U N O H , * ,† JAE E. YANG,† JAI H. LEE,‡ MYUNJOO LEE,§ SEUNGHO YU,§ AND DAEWON PAK| Department of Biological Environment, Kangwon National University, Chuncheon 200-701, South Korea, Department of Environmental Science and Engineering, Gwangju Institute of Science and Technology, Gwangju 500-712, South Korea, Advanced Radiation Technology Institute, Korea Atomic Energy Research Institute, Jeongeup, Jeonbuk 580-185, South Korea, and Graduate School of Energy and Environment, Seoul National University of Technology, Seoul 139-743, South Korea

Received May 28, 2008. Revised manuscript received October 24, 2008. Accepted October 24, 2008.

There has been a controversy over the TiO2 photocatalytic oxidation (PCO) mechanism of As(III) for the last several years. The key argument has been whether superoxide (HO2•/O2-•) is the main oxidant of As(III) in the UV/TiO2 system. Previously we and other groups have refuted the claim that superoxide plays the main role in the TiO2 PCO of As(III). Nevertheless, thereafter, the superoxide-mediated As(III) oxidation mechanism has been repeatedly claimed, making it difficult to draw a clear conclusion regarding this mechanism. The objective of this study is to draw a unanimous conclusion on the TiO2 PCO mechanism of As(III) and thus finish the controversy regarding this issue. To investigate the correlation between As(III) oxidation and superoxide, both As(V) and H2O2 were measured simultaneously. When excess formic acid (FA) was added as a scavenger of valence band (VB) hole (or •OH) in UV/TiO2 or vacuum-UV lamp irradiation (λ ) 185 + 254 nm), As(III) oxidation was greatly inhibited while H2O2 generation was promoted. Since H2O2 is photochemically produced through the disproportionation of superoxide, this result definitely shows that superoxide has little role in the oxidation of As(III) not only in UV/TiO2 but also in other advanced oxidation processes (AOPs). Interestingly, not only FA (a scavenger of VB hole) but also methanol (a scavenger of adsorbed •OH) showed an inhibitory effect on the TiO2 PCO of As(III). Excess methanol retarded the TiO2 PCO of As(III) moderately but not completely, which indicates that adsorbed •OH also plays a significant role along with VB hole in the TiO2 PCO of As(III). Although the conclusion is not based on the rate constant between As(III) and superoxide but derived from indirect inference from the experimental data, this study provides convincing evidence to support that adsorbed •OH and VB hole are the main oxidants in the TiO2 PCO mechanism of As(III).

Introduction Arsenic contamination is a serious worldwide environmental problem (1). A lot of work has been conducted to cope with the lowered maximum contaminant level of 10 µg/L (2). Arsenic exists mainly as arsenite (As(III), H3AsO3) and arsenate (As(V), H3AsO4) in water. The pKa values of H3AsO3 and H3AsO4 are 9.2, 12.1, 12.7 and 2.3, 6.9, 11.5, respectively (3). Thus, in most natural waters, As(III) exists as neutral species (H3AsO3), whereas As(V) exists as anionic species (H2AsO4- or HAsO42-). It is well-known that anionic As(V) is easier to remove by adsorption or coagulation/precipitation than neutral As(III) (4). Accordingly, the oxidation of As(III) to As(V) is recommended in arsenic removal processes, and various methods have been studied for that purpose: UV/TiO2 (3-13), H2O2 (14), O3 (15), MnO2 (16), UV/H2O2 (5), UV/Fe(III)-complexes (17-19), Fe(II)/H2O2 (20), Fe(VI) (21), etc. Among them, TiO2 photocatalytic oxidation (PCO) of As(III) has been studied by several groups (3-13). However, there has been a controversy over the TiO2 PCO mechanism of As(III), which originated from the claim that HO2•/O2-• (hydroperoxyl/superoxide radical, hereafter referred to as superoxide) is the main oxidant of As(III) in the UV/TiO2 system (3, 6, 7, 13). This claim has been doubted and refuted by other research groups (8-10), because TiO2 PCO reactions are always initiated by valence band hole (VB hole, hVB+) or tTiO•){Ti(IV)sO2adsorbed OH radical (•OHad, sTi(IV)}s•OHT{Ti(IV)sO •sTi(IV)}sOH-) (22-24). VB hole behaves as an electron-transfer oxidant, and it is also called a positive hole. Adsorbed •OH is formed via the reaction between VB holes and surface OH-groups (tTiOH + hVB+ftTiO•+H+), and it acts like OH radical in that it abstracts H and adds to C-C multiple bonds (22). In the literature, the term “trapped hole” is sometimes used to mean •OHad: trapped hole)adsorbed •OH (25, 26). Some of adsorbed OH radicals might be able to diffuse away from TiO2 surface into the bulk solution and contribute to PCO reactions. These OH radicals are called free OH radicals (•OHfree) (24). If a TiO2 PCO reaction is caused mainly by •OHad and/or •OHfree, the mechanism is called “•OH mechanism”. If VB hole acts as the main oxidant in a TiO2 PCO reaction, the mechanism is called “hole mechanism”. Superoxide radical (HO2•/O2-•) is known to be an insignificant oxidant compared to hVB+ and •OHad/•OHfree. It has a lower oxidation potential than hVB+, •OHad/•OHfree, and even H2O2: E(hVB+ or •OH) ) 2.8 VNHE, E(H2O2) ) 1.77 VNHE, and E(HO2•) ) 1.7 VNHE (27). Thus, to the best of our knowledge, there is no reported case where superoxide plays the principal role in the UV/TiO2 system. This is the universal principle of TiO2 PCO chemistry. Previously we have refuted the claim that superoxide is the main oxidant in the TiO2 PCO of As(III), and proposed that VB hole plays the principal role in that reaction (8). Other research groups have also disputed against the superoxide-mediated As(III) oxidation mechanism, and concluded that •OHad plays the main role in As(III) oxidation (9, 10). Nevertheless, the superoxide-mediated As(III) oxidation mechanism in the UV/ TiO2 system has again been claimed as follows (reactions 1-5) (13). + f As(IV) As(III) + hVB

(1)

As(III) + · OHad(or · OHfree) f As(IV) + OH- (k ) 1.8 × 109 M-1 s-1) (ref 32) (2)

* Corresponding author e-mail: [email protected]; phone: +82-033-250-6440; fax: +82-033-241-6640. † Kangwon National University. ‡ Gwangju Institute of Science and Technology. § Korea Atomic Energy Research Institute. | Seoul National University of Technology. 864

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- 2(k ) As(III) + HO2 · ⁄O2 · f As(IV)+HO2 ⁄O2

3.6 × 109 M-1 s-1) (ref 10) (3) f As(III) As(IV)+eCB

10.1021/es801480u CCC: $40.75

(4)

 2009 American Chemical Society

Published on Web 12/23/2008

As(IV) + O2 f As(V)+O2 ·

(k ) 1.1 × 109 M-1 s-1) (ref 28) (5)

When only O2 is present as an electron acceptor, As(III) is initially oxidized to As(IV) by hVB+ or •OH (either adsorbed or free) (reactions 1 and 2), and then the transient As(IV) is immediately reduced back to As(III) by conduction band (CB) electron (eCB-) (reaction 4). Ryu and Choi (13) have hypothesized that reaction 4 occurs much faster than reaction 5 enough to mask the contribution of reactions 12. They have repeatedly claimed that As(III) is oxidized mainly by superoxide (reaction 3) (13). In other words, the redox couple of As(III)/As(IV) immediately mediates the recombination of hVB+ (or •OH) and eCB- pairs in null cycles, and thus the longer-lived superoxide governs the overall TiO2 PCO of As(III). However, when electron acceptors that are more efficient than O2 are present, they agreed that hVB+ (or •OHad) is the main oxidant in the TiO2 PCO of As(III). Regarding this phenomenon, they have stated that an alteration in the experimental conditions entirely changes the TiO2 PCO mechanism of As(III) (13). The above-mentioned mechanism is hard to believe. Since As(IV) is a reducing species that reacts with O2 almost at a diffusion-controlled rate (reaction 5) (28), the hypothesis that reaction 4 occurs much faster than reaction 5 seems not to be true. A pair of rebuttal and response letters (29, 30) has been published after ref 13. Leng et al. (29) have disputed against the validity of the photoelectrochemical approach employed in ref 13. Especially, they have pointed out that superoxide itself could not be formed if the As(III) oxidation by UV/TiO2 occurred as described in ref 13. Regarding this contradiction, Ryu and Choi (30) have replied that there are some electron-hole pairs which survive the null cycle and produce pairs of superoxide and •OH. If this is the case, the controversy is the same before the publication of ref 13. How can superoxide act as the main oxidant of As(III) instead of •OH (or VB hole), when superoxide and •OH are photogenerated in pairs (at the same concentration)? Although Ryu and Choi (30) have stated that superoxide oxidizes As(III) while •OH recombines to H2O2, this statement also is not true. The steady-state concentration of •OH in normal advanced oxidation processes (AOPs) is generally 10-13 to 10-11 M, and •OH reacts with another •OH to form H2O2 at 5.2 × 109 M-1 s-1 (31). During As(III) oxidation, As(III) is initially present at much higher concentrations (e.g., at least 10-8 M) than •OH, and •OH reacts with As(III) at 1.8 × 109 M-1 s-1 (32). Kinetically, the recombination pathway of •OH (•OH + •OHfH2O2) is inhibited, and •OH (or VB hole) plays the major role in the As(III) oxidation by various AOPs including UV/TiO2. In summary, some previous studies (3, 6, 7, 13, 30) have concluded that the superoxide mechanism works in the environment that contains only dissolved oxygen and As(III) in water. The authors of the papers have stated that the superoxide mechanism is the most environmentally relevant As(III) PCO mechanism, saying that most natural waters do not have alternative electron acceptors or organic matters in concentrations high enough to change the superoxide mechanism (13, 30). They seem to claim that the •OH- or hole-induced As(III) PCO mechanism works in special conditions where As(III) concentrations are very high or there are some additional substrates (e.g., formate, polyoxometalate (POM), Ag+, Cu2+, and BrO3-). In our opinion, however, their claim on this issue seems not to be a scientific truth. First of all, to our knowledge, there is no solid evidence that superoxide is a kinetically meaningful oxidant of As(III) in various AOPs including UV/TiO2. On the contrary, there are several papers of different groups showing that superoxide is a negligible oxidant of As(III) compared with •OH (or hole) (8, 10, 20, 44). To disprove the superoxide-mediated As(III)

oxidation mechanism, more compelling and definitive evidence is necessary. Also, it is not clear which one between VB hole and adsorbed •OH plays the principal role in the TiO2 PCO of As(III) (8-10). Accordingly, the objective of this study is to provide a clear and unanimous conclusion on the TiO2 PCO mechanism of As(III).

Experimental Section Materials. NaAsIIIO2, Na2HAsVO4 · 7H2O, HCOOH, KO2, and CuSO4 · 5H2O were purchased from Aldrich. CH3OH was supplied from Merck. All the other chemicals were of reagent grade and used as received. In all the experiments, Degussa P25 TiO2 and Milli-Q water (18.2 MΩ · cm) were used. A 4-W UV-A lamp (Sankyodenki, F4T5BLB, 300 < λ < 400 nm) or a 10-W vacuum-UV (VUV) lamp (Lighttech, G12T5VH, λ ) 185 + 254 nm) was used as the UV irradiation source. Experiments. Every experiment was conducted in a cylindrical glass reactor (5 cm i.d. × 16 cm height). Inside the reactor, the UV irradiation source was positioned at the center within a quartz sleeve. The distance between lamp sleeve and reactor wall was 1.25 cm. The reactor was open to ambient air through a sampling hole (i.d. ) 1 cm) and contained 180 mL of TiO2 suspensions (or homogeneous solution). The suspensions (or homogeneous solution) were magnetically stirred during irradiation. TiO2 PCO experiments were conducted under air-equilibrium, O2-sparging, or N2sparging. O2- or N2-sparging was applied for at least 10 min before UV irradiation and was continued throughout PCO experiments. The pH of TiO2 suspensions was adjusted with 1 M HCl and 1 M NaOH. Sample aliquots (3 mL) were intermittently withdrawn and TiO2 particles were removed by 0.45-µm syringe filters. Analyses. As(V) was measured by the molybdenum blue method (33). Photogenerated H2O2 was measured using the 2,9-dimethyl-1,10-phenanthroline (DMP) method (34). A much higher phosphate buffer solution (600 mM NaH2PO4 · H2O) was used to ensure the final pH condition (pH g 7) of the DMP method. All experiments were performed two or three times. The standard deviations were mostly less than 10% of the average values.

Results and Discussion Effects of Formic Acid and Methanol on the TiO2 PCO of As(III). Under air-equilibrium (i.e., the most environmentally relevant condition), excess formic acid (FA) almost completely prevented the TiO2 PCO of As(III), whereas excess methanol moderately retarded the TiO2 PCO of As(III). This indicates that the TiO2 PCO of As(III) is caused by both VB hole and adsorbed •OH. Previously we have reported that FA has an inhibitory effect on the TiO2 PCO of As(III) (8), whereas Ryu and Choi (13) have reported that formate did not show an inhibitory effect on the TiO2 PCO of As(III) in their experimental condition. This discrepancy prompted us to start by reinvestigating the effect of FA more thoroughly. As shown in Figure 1, it is sure that FA has an inhibitory effect on the TiO2 PCO of As(III) and the inhibiting effect increases with an increase in the FA concentration. In ref 13, formate was added at a concentration equivalent to As(III) ([formate]/[As(III)] ) 1). This concentration seemed to be too low to cause a significant effect. The great inhibitory effect of excess FA is an indisputable evidence which disproves the superoxidemediated As(III) oxidation mechanism, since adding excess FA in the UV/TiO2 system enables As(III) to react only with superoxide but not with hVB+ (or •OHad) (kformate/superoxide < 0.01 M-1 s-1) (35). This point will be discussed more thoroughly in the subsequent section. Interestingly, excess methanol retarded the TiO2 PCO of As(III) moderately but not completely. The As(V) formation rates in the absence of methanol and in the presence of 75 VOL. 43, NO. 3, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Effects of formic acid (FA) and methanol on the TiO2 PCO of As(III). Experimental condition: [TiO2] ) 100 mg/L, [As(III)]i ) 75 µM, pH ) 3, volume ) 180 mL, and under air-equilibrium.

FIGURE 2. Effects of formic acid (FA) and methanol on the anoxic TiO2 PCO of As(III). Experimental condition: [TiO2] ) 100 mg/L, [As(III)]i ) 75 µM, [Cu(II)]i ) 500 µM, pH ) 3, and volume ) 180 mL. mM methanol were 2.38 × 10-8 and 1.52 × 10-8 M/s, respectively. Since methanol reacts mainly with •OHad in TiO2 photocatalysis (26), this result indicates that not only VB holes but also adsorbed OH radicals contribute to the TiO2 PCO of As(III). This contrasts somewhat with our previous study (8) in which methanol did not retard the TiO2 PCO of As(III). In that study (8), the As(V) generation rate with methanol under O2-sparging was carelessly compared with the As(V) generation rate without methanol under airequilibrium; the As(V) yield in the presence of methanol was increased by O2-sparging in ref 8. Thus, the imperfect conclusion that the TiO2 PCO of As(III) occurs almost entirely by VB hole was withdrawn. Possibly, the different As(V) measurement methods (ion chromatography vs molybdenum blue method) might be responsible for this discrepancy. However, when the molybdenum blue method is used for measuring As(V), it is sure that excess methanol retards the TiO2 PCO of As(III) to some extent under the same condition of air-equilibrium (the most environmentally relevant condition). Hence, as concluded by Dutta et al. (9) and Xu et al. (10), adsorbed OH radical also plays a significant role in the As(III) oxidation by UV/TiO2. In the anoxic TiO2 PCO also, 75 mM methanol showed an inhibitory effect on As(III) oxidation (Figure 2). Cu(II) was added as an alternative electron scavenger and N2-sparging was applied to induce an anoxic condition. In 20 min, the As(V) concentrations in 866

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FIGURE 3. Photochemical generation of As(V) and H2O2 by UV/ TiO2. Experimental condition: [TiO2] ) 100 mg/L, [As(III)]i ) 75 µM, pH ) 3, volume ) 180 mL, and under O2-sparging. the absence of methanol and in the presence of 75 mM methanol were 71.7 and 36.2 µM, respectively. Therefore, the best conclusion on the TiO2 PCO mechanism of As(III) is that both VB holes and adsorbed OH radicals cause the oxidation of As(III) to As(V). Photochemical Generation of As(V) and H2O2. Superoxide is inherently not a significant oxidant of As(III) in advanced oxidation processes (AOPs) including UV/TiO2, so the superoxide-mediated As(III) oxidation mechanism is not a true mechanism regardless of experimental conditions. The best way to evaluate the oxidation potential of superoxide for As(III) is to observe the As(III) oxidation efficiency under the conditions where As(III) reacts only with superoxide while •OHad (or hVB+) is scavenged by other substrates. The addition of excess FA in the As(III) oxidation by UV/TiO2 or vacuum-UV (VUV) lamp irradiation (λ ) 185 + 254 nm) is an appropriate approach for that purpose (36-39). Since H2O2 is generated by the disproportionation of superoxide (HO2•/O2-•) in TiO2 photocatalysis (36-38), the measurement of H2O2 is sufficient for the verification of superoxide generation. In the presence of excess FA and O2, the TiO2-photocatalyzed generation of H2O2 occurs through the following reactions (reactions 6-13). + f HCO2 · +H+ (in the valence band of TiO2) HCOOH + hVB (6)

HCO2 · f H++CO2 · (pKa ) 1.4)(ref 40)

(7)

CO2 · + O2 f CO2 + O2·

(8)

O2 + eCB f O2 ·

(in the conduction band of TiO2)

+ O2 · +H T HO2 ·

(pKa ) 4.8) (ref 35)

(10)

O2 · + HO2 · f O2 + HO2

(11)

HO2 · + HO2 · f O2 + H2O2 O2 · +O2 ·

+ 2H2O f O2 + H2O2 + 2OH

(9)

(12) -

(13)

The recombination pathway of two OH radicals (•OH + •OHfH2O2) is prevented by excess FA. The generation curves of As(V) and H2O2 during the TiO2 PCO of As(III) are shown in Figure 3. In this set of experiments, O2-sparging was necessary to obtain a moderate yield of H2O2. In the absence of FA, the TiO2 PCO of As(III) occurred efficiently, and a low level of H2O2 generation was observed. Without FA, the As(V) generation rate was 4.73 × 10-8 M/s, and the H2O2 concentration in 20 min was 13.7 µM. The H2O2 formation in the absence of FA occurred through the disproportionation of

FIGURE 4. Photochemical generation of As(V) and H2O2 by VUV lamp irradiation. Experimental condition: [As(III)]i ) 75 µM, pH ) 3, volume ) 180 mL, and under air-equilibrium. superoxide radicals which were formed in reactions 5 and 9, not through the recombination of OH radicals. When 75 mM FA was added ([FA]/[As(III)] ) 1000), the TiO2 PCO of As(III) was greatly inhibited whereas H2O2 generation was enhanced. In the presence of FA, the As(V) generation rate was 5.42 × 10-9 M/s and the H2O2 concentration in 20 min was 42.4 µM. Since superoxide radicals disappear by secondorder kinetics to form H2O2 (41), a higher yield of H2O2 in the presence of FA implies an enhanced generation of superoxide radicals. That is, the addition of 75 mM FA decreased the As(III) oxidation greatly but simultaneously increased the superoxide generation. As shown in reactions 6-8, excess FA in the UV/TiO2 system converts VB hole (or •OH) into superoxide at the ratio of 1:1. Thus, if superoxide were able to cause the oxidation of As(III) to As(V), the TiO2 PCO efficiency of As(III) would not be greatly decreased by 75 mM FA. On the contrary, the TiO2 PCO efficiency of As(III) should be increased by the addition of 75 mM FA, if superoxide were an effective oxidant for As(III). Although Ryu and Choi (13) stated that the inhibitory effect of excess FA seems to be not due to the hole scavenging but rather to be related with the competitive adsorption on TiO2 between As(III) and FA, excess FA does not affect the possible reaction between As(III) and superoxide regardless of whether it decreases the adsorption of As(III) on TiO2 (kformate/superoxide < 0.01 M-1 s-1) (35). Thus, the fact that excess FA inhibits As(III) oxidation while promoting superoxide generation in the UV/TiO2 system is clear evidence showing that superoxide is a negligible oxidant for As(III) compared with •OH (or hole). The effects of FA and methanol on the As(III) oxidation by vacuum-UV (VUV) lamp irradiation (λ ) 185 + 254 nm) also proved that superoxide is an insignificant oxidant for As(III) (Figure 4). Since As(III) is little oxidized to As(V) by germicidal lamp irradiation (λ ) 254 nm), the As(III) oxidation by VUV lamp irradiation occurred via free OH radicals which were produced from the photosplitting of H2O (42). The following are major reactions to be considered in this set of experiments. H2O + hv (λ ) 185 nm) f · OH + H·

(14)

HCOOH + · OH + O2 f HCO2 · + H2O + O2 f H3O++CO2 + O-2 · (15) CH3OH + · OH + O2 f · CH2OH + H2O + O2 f HCHO + H2O + HO2 · (16) In the absence of FA or methanol, As(III) was efficiently oxidized to As(V) at 1.39 × 10-7 M/s, because photoproduced •OHfree was consumed only by As(III) and not by other

FIGURE 5. As(III) oxidation by the addition of KO2 and H2O2. Experimental condition: [As(III)]i ) 75 µM, [KO2]i ) 500 µM (or [H2O2]i ) 200 µM), pH ) 2.6-10.6, volume ) 180 mL, and under air-equilibrium. competitive substrates. Since the transient As(IV) reduces O2 to produce superoxide at 1.1 × 109 M-1 s-1 (reaction 5), H2O2 was also produced at 1.05 × 10-7 M/s. In the presence of 75 mM FA, the generation rates of As(V) and H2O2 were 4.68 × 10-9 and 2.14 × 10-7 M/s, respectively. When 75 mM methanol was added as an •OH-scavenger, As(V) and H2O2 were produced at 9.25 × 10-9 and 1.08 × 10-7 M/s, respectively. The H2O2 generation in the presence of FA or methanol occurred through the disproportionation of superoxide radicals which were produced by reaction 15 or 16. The effect of FA indicates that superoxide generation was enhanced but As(III) oxidation was almost completely inhibited by excess FA. The effect of methanol indicates that superoxide generation changed little but As(III) oxidation was almost completely inhibited by excess methanol. So, the data shown in Figure 4 are also indisputable evidence that superoxide is an insignificant oxidant for As(III). Although the VUV lamp experiments were conducted in homogeneous phase, the data shown in Figure 4 are actually sufficient to disprove the superoxide-mediated TiO2 PCO mechanism of As(III) because the reactivity of superoxide for As(III) oxidation does not change depending on the oxidation method. Oxidation of As(III) by KO2. The As(III) oxidation by the addition of KO2 occurs not by superoxide but by H2O2, so it does not support the superoxide-mediated As(III) oxidation mechanism. In ref 13, the As(III) oxidation by the addition of KO2 was mentioned as evidence supporting that superoxide causes the oxidation of As(III) to As(V). We reinvestigated this point, and the result is shown in Figure 5. Since As(III) is much more easily oxidized at higher pHs (pH > 9) (14), the experiment was conducted by adding 35.5 mg of KO2 powder into a 1 L aqueous solution spiked with 75 µM As(III) at pH 2.6. With this low pH, an abrupt pH increase due to the addition of KO2 could be prevented. The solution pH was maintained at 2.6, or increased to 6.6 (buffered by 3 mM HCO3-), and 10.6. After the pH adjustment, As(V) was measured in 20 min. The As(III) oxidation efficiencies at pH 2.6 and 6.6 were 4.07 and 6.28%, respectively. At pH 10.6, however, As(III) was completely oxidized by the addition of 500 µM KO2. If superoxide were able to oxidize As(III) to As(V), the As(III) oxidation efficiencies at pH 3 and 6.6 would not be so low. This result indicates that the As(III) oxidation by the addition of KO2 is not due to superoxide but due to H2O2 whose performance for As(III) oxidation increases with the increase in solution pH (14). When dissolved in water, KO2 produces not only superoxide but also H2O2 (43). Adding 500 µM KO2 indeed produced ca. 200 µM H2O2. When 200 µM H2O2 was added into a 75 µM As(III) solution, the As(III) VOL. 43, NO. 3, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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oxidation efficiencies were very similar with those by the addition of 500 µM KO2. Another control experiment was conducted as follows. 500 µM KO2 solutions were prepared at pH 2.6 and left intact in darkness for 1 day. Considering the second-order disproportionation kinetics (41) and the decay constant (35), 1 day is enough to remove superoxide virtually completely. Thereafter, As(III) was added at 75 µM into the superoxide-free KO2 solution, and the solution pH was adjusted to 2.6, 6.6 (buffered by 3 mM HCO3-), and 10.6. In this control experiment also, the As(III) oxidation efficiencies were very similar with those by the addition of KO2 powder. Therefore, the As(III) oxidation by the addition of KO2 is caused not by superoxide but by H2O2. Refutation of Superoxide-Mediated As(III) Oxidation Hypothesis. The superoxide-mediated As(III) oxidation mechanism in TiO2 photocatalysis is a misleading conclusion. There is no reason why As(III) is oxidized by superoxide instead of VB hole (or •OHad), when electrons (or superoxide) and VB holes (or •OH) survive in pairs (at the same concentration). Other research groups have already implied that superoxide is not a meaningful oxidant for As(III) in AOPs (20, 44). Hug and Leupin (20) have pointed out that superoxide is an insignificant oxidant for As(III) using Fe(II)/ H2O2 process and 2-propanol. Xu et al. (44) have also concluded that superoxide has little or no role in the oxidation of As(III) to As(V) by using γ-radiolysis and formate. This study provides more compelling evidence disproving the superoxide-mediated As(III) oxidation mechanism by showing that the TiO2 PCO of As(III) is suppressed despite enhanced superoxide generation by excess FA (see Figure 3). Since the reactivity of superoxide for As(III) is independent of the reaction phase (homogeneous or heterogeneous), the inhibitory effect of •OH-scavengers on the As(III) oxidation in the homogeneous phase is also sufficient to disregard the superoxide-mediated As(III) oxidation mechanism (see Figure 4). The rate constant for the possible reaction of As(III) with superoxide has been determined to be 3.6 × 106 M-1 s-1 by Xu et al. (10), using pulse radiolysis and the competitive reactions of 1,4-benzoquinone (BQ) and As(III) with superoxide. The value is 500-times lower than the rate constant for the reaction of As(III) with •OH (k ) 1.8 × 109 M-1 s-1 (32)), which supports that superoxide is an insignificant oxidant of As(III) compared with •OH (or hole) and thus cannot be the main oxidant of As(III) in the UV/TiO2 system where •OH (or hole) and superoxide are generated in pairs. In our opinion, the superoxide-mediated As(III) oxidation mechanism in TiO2 photocatalysis seemed to be based on seemingly supportive evidence (3, 6, 7, 13, 30). To claim the validity of the superoxide-mediated As(III) oxidation mechanism, however, it is first necessary to provide unquestionable evidence showing that superoxide directly oxidizes As(III) to As(V) similarly with •OH (or hole) and to explain why As(III) oxidation is greatly inhibited under superoxide generation (see Figures 3 and 4). Without this, other seeming evidence appear not to have significance enough to validate the superoxide mechanism. The photoelectrochemical approach using a TiO2 electrode in ref 13, which has been refuted in detail by Leng et al. (29), does not provide any compelling evidence supporting the superoxide-mediated As(III) oxidation mechanism. Furthermore, we do not agree with the interpretation that the slightly enhanced PCO of As(III) by fluorinated TiO2 (F-TiO2) despite the reduced As(III) adsorption is evidence disproving the hole mechanism (6). As reported by Minero et al. (23, 24), the PCO efficiency of phenol was higher on F-TiO2 than on naked TiO2. Thus, it seems that the PCO of As(III) is also somewhat promoted on F-TiO2 due to the favored formation of •OHfree on F-TiO2, although the As(III) adsorption is reduced on F-TiO2. In summary, since superoxide is inherently a negligible oxidant of As(III) compared with •OH (hole) in various AOPs 868

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including UV/TiO2, the superoxide-mediated TiO2 PCO mechanism of As(III) is not a scientific truth regardless of experimental conditions. Although there has been a controversy in the literature, the truth regarding this mechanism is simple and clear. The TiO2 PCO of As(III) follows a general PCO mechanism in which the oxidation of As(III) to As(V) is caused by both hVB+ and •OHad. That is, As(III) is initially oxidized to As(IV) by hVB+ and •OHad (reactions 1 and 2), and then As(IV) is immediately converted to As(V) by oxygen, hVB+, or •OHad.

Acknowledgments This study was supported by the Research Institute of Agricultural Sciences at Kangwon National University and the Nuclear R&D program of the Ministry of Science and Technology (MOST).

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