Surface Modifications on Photocatalytic Oxidation of Arsenite

Apr 6, 2004 - increased on Pt-TiO2, which could be ascribed to the enhanced superoxide generation through an efficient interfacial electron transfer f...
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Environ. Sci. Technol. 2004, 38, 2928-2933

Effects of TiO2 Surface Modifications on Photocatalytic Oxidation of Arsenite: The Role of Superoxides JUNGHO RYU AND WONYONG CHOI* School of Environmental Science and Engineering, Pohang University of Science and Technology, Pohang, 790-784, Korea

Using TiO2 photocatalyst, arsenite [As(III)] can be rapidly oxidized to arsenate [As(V)], which is less toxic and less mobile in the aquatic environment. Superoxides have been recently proposed as a main photocatalytic oxidant of As(III) whereas OH radicals are dominant oxidants in most TiO2 photocatalytic oxidation (PCO) reactions. This study confirms that superoxides are mainly responsible for the As(III) PCO by investigating PCO kinetics in pure and modified TiO2 systems. The rate of As(III) oxidation drastically increased on Pt-TiO2, which could be ascribed to the enhanced superoxide generation through an efficient interfacial electron transfer from the conduction band (CB) to O2. Since the addition of tert-butyl alcohol (OH radical scavenger) had little effect on the PCO rate in both naked and Pt-TiO2 suspensions, OH radicals do not seem to be involved. The addition of polyoxometalates (POMs) as an electron shuttle between TiO2 CB and O2 highly promoted the PCO rate whereas the POM alone was not effective at all in oxidizing As(III). 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. The arsenite oxidation proceeded under visible light with a similar rate to the case of Pt-TiO2/UV when dye-sensitized Pt-TiO2 was used. Since only superoxides can be generated as a photooxidant in this visible light system, their role as a main oxidant of As(III) is confirmed. In addition, the PCO rate was significantly reduced in the presence of superoxide dismutase.

Introduction Arsenic contamination of groundwater is emerging as an important environmental issue since it poses significant health risks to millions of people worldwide (1). In particular, the chronic arsenic poisoning due to drinking untreated groundwater is widespread among people in Bangladesh; can be associated with various health effects including skin, kidney, liver, and lung cancers; and calls for emergent actions (2-4). In the meantime, the U.S. Environmental Protection Agency (EPA) lowered the maximum contaminant level (MCL) for arsenic in drinking water from 50 to 10 µg/L (5). The raised concern about arsenic poisoning and the stricter regulation for arsenic pollution have accelerated many efforts in developing cost-effective methods for removing arsenic from drinking water (6-8). Arsenic contamination of water results from either anthropogenic sources such as fertilizers and wood preser* Corresponding author e-mail: [email protected]; phone: +82-54-279-2283; fax: +82-54-279-8299. 2928

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vatives (9) or natural weathering or dissolution of As-bearing minerals with its common oxidation state of As(III) or As(V) (10). The oxidation state of arsenic critically affects its mobility in natural systems and the removal efficiency in treatment processes (7, 8). As(III) species are more toxic and mobile than As(V) and prevalent in the anoxic condition. Therefore, the oxidation of As(III) species is required in common arsenic removal technologies to reduce toxicity and promote immobilization. According to published literature, As(III) can be oxidized to As(V) by O2 and/or O3 (11), H2O2 (12), synthetic MnO2 (13, 14), UV/iron (15, 16), and TiO2/UV (17, 18). TiO2 photocatalysis has been extensively studied for its environmental application and demonstrated as an effective method of remediating contaminated water and air (1923). In our previous study (18), the photocatalytic oxidation (PCO) of As(III) to As(V) in UV-illuminated TiO2 suspension has been carried out to understand the kinetics and mechanism. Arsenite could be efficiently oxidized in the presence of dissolved oxygen. In this case, the OH radicals that are main oxidants in most PCO processes (21-23) seemed not to take part in As(III) oxidation because the addition of excess tert-butyl alcohol (OH radical scavenger) had little effect on the rate of As(III) oxidation and the addition of humic acid (also OH radical scavenger) highly enhanced the oxidation rate on the contrary along with the production of H2O2. On the basis of these results, it was proposed that the superoxide is the main oxidant of As(III) in the TiO2/UV process. However, the arsenite photooxidation mechanism is complex, and the role of superoxide is not clearly understood. In addition, more direct experimental evidences for the involvement of the superoxide in As(III) oxidation are needed. In this study, we performed the PCO reactions of As(III) using various TiO2 photocatalysts that include pure TiO2, platinized TiO2, TiO2 with polyoxometalate (POM), fluorinated TiO2, and dye-sensitized TiO2 in order to seek for clearer evidences for the role of superoxides and to understand how the TiO2 surface properties are related with the As(III) oxidation mechanism. From this study, we could confirm that the superoxide preferentially reacts with As(III) more than any other photooxidants and that the surface property of TiO2 critically affects the PCO of arsenite mainly through altering the superoxide generation.

Experimental Section Materials and Chemicals. NaAsO2 (As(III), Aldrich) and Na2HAsO4‚7H2O (As(V), Kanto) were used as the arsenic sources. Other chemicals used include H2PtCl6‚6H2O (Aldrich), humic acid (HA) (sodium salt, Aldrich), 2,9-dimethyl1,10-phenanthroline (DMP) (Aldrich), CuSO4‚5H2O (Shinyo), KH2PO4 (Kanto), NaF (Samchun Chemical, Korea), CCl4 (J. T. Baker), and tert-butyl alcohol (TBA) (Shinyo), all of which were of reagent grade and used as received. Tris(4,4′dicarboxy-2,2′-bipyridyl)ruthenium(II) complex (abbreviated as RuIIL3) was synthesized as previously described (24) and used as a visible light sensitizer of TiO2. Superoxide dismutase (SOD) from Escherichia coli (Sigma, manganese-containing enzyme, lyophilized) was added to the TiO2 suspension when the effect of superoxide scavenging by SOD on As(III) oxidation was investigated. SiW12O404- (Fluka) of reagent grade was used as a POM. Titanium dioxide (Degussa P25), a mixture of 80% anatase and 20% rutile with an average surface area of 50 m2/g, was selected as a base photocatalyst. Deionized water used was ultrapure (18 MΩ‚cm) and prepared by a Barnstead purification system. Modifications of TiO2 Photocatalyst. Platinized TiO2 (PtTiO2) was prepared using a photodeposition method as 10.1021/es034725p CCC: $27.50

 2004 American Chemical Society Published on Web 04/06/2004

reported previously (25). An aqueous TiO2 suspension containing 1 M CH3OH and 0.1 mM H2PtCl6 was irradiated with a 200-W medium-pressure mercury lamp for 30 min. After irradiation, the Pt-TiO2 sample was collected with a 0.45-µm filter and washed with deionized water. The Pt loading on TiO2 was estimated to be about 0.2 wt % by comparing the chloroplatinate concentrations remaining in solution before and after the photodeposition. The chloroplatinate concentration was determined by inductively coupled plasma-atomic emission spectroscopy (ICP-AES, Spectro Flame Modula). The deposited Pt particles were in the size range of 2-4 nm (from a TEM image). For the experiment using fluorinated TiO2, 10 mM NaF was added to the suspension with adjusting the pH to 3. In case of dyesensitized TiO2 system, the sensitizer was added to the Pt-TiO2 suspension to give [RuIIL3]i ) 10 µM, and the pH of the suspension was adjusted to 3 at which the sensitizer molecules were strongly adsorbed on TiO2 (24). Photolyses and Analyses. The concentrations of TiO2 and arsenite [As(III)]0 were fixed at 0.5 g/L and 500 µM, respectively, in most experiments. The pH of the suspension was adjusted with HCl or NaOH standard solution, and the suspension was stirred for 20 min to allow equilibrium adsorption of arsenite 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) was used as a light source. The light was passed through a 10-cm IR water filter and a UV cutoff filter (λ > 300 nm for most photolyses or λ > 420 nm for visible light photolyses) and focused onto a cylindrical Pyrex reactor (o.d. 45 mm, volume 90 mL) with a quartz window (45 mm diameter). The diameter of the focused beam was about 20 mm. The reactor was open to the ambient air (air-equilibrated condition) or closed with a rubber septum under continuously purging nitrogen gas (N2-saturated condition) and rapidly stirred magnetically during irradiation to ensure uniform illumination and mixing. The photooxidation experiment with SOD was carried out using a small UV spectrophotometer cell (3 mL) in order to minimize both the irradiation time and the amount of SOD added. 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 arsenate [As(V)] 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 total arsenic or As(III) concentration in TiO2 suspension was measured using ICPAES. The formation of H2O2 as a byproduct of arsenite photooxidation was also monitored. The concentration of photogenerated H2O2 in TiO2 suspension was measured by DMP method (18, 26). The detailed procedure of the DMP method is described in our previous paper (18).

Results and Discussion Photocatalytic Oxidation of As(III) on Pt-TiO2. Surface modification through the deposition of noble metals such as Pt, Au, and Ag on semiconductor particles enhances the photocatalytic efficiency by trapping conduction band (CB) electrons and consequently retarding the fast charge-pair recombination (27-30). The rate of CB electron transfer to O2 is known to be much faster on platinized TiO2 than on bare TiO2 surface (reaction 1) (31, 32):

ecb-(Pt) + O2 f O2•-

(1)

This implies that more superoxides should be produced on Pt-TiO2 and that the photocatalytic As(III) oxidation should be enhanced on Pt-TiO2 provided that the superoxides are

FIGURE 1. Photocatalytic oxidation of arsenite on naked and platinized TiO2. The experimental conditions were [TiO2] ) 0.5 g/L, [As(III)]0 ) 500 µM, air-equilibrated, and [TBA] ) 0.53 M (added in the indicated case only).

FIGURE 2. Effects of electron acceptors (O2 or CCl4) on the arsenite PCO rate. The experimental conditions were [TiO2] ) 0.5 g/L, [As(III)]0 ) 500 µM, and continuously purged with O2 or N2 or airequilibrated. When using CCl4 as an alternative electron acceptor, excess CCl4 was added to the N2-saturated suspension to ensure its saturation (about 5 mM). the main oxidant as suggested previously (reaction 2) (18): O2

As(III) + O2•- f As(IV) 98 As(V)

(2)

Figure 1 shows that the rates of arsenite photooxidation significantly increase in Pt-TiO2 suspension at both pH 3 and pH 9, which could be ascribed to the enhanced superoxide generation on Pt-TiO2. Although the enhanced production of OH radicals on Pt-TiO2 (as a result of retarded recombination) could accelerate the As(III) oxidation rate as well, the addition of excess TBA as an OH radical scavenger only slightly decreased the initial PCO rate (Figure 1). This confirms that OH radicals are not major oxidants of As(III) in the Pt-TiO2 as well as in the pure TiO2 suspension. Direct photooxidation of As(III) in the absence of TiO2 was insignificant. In addition, it is interesting to note that the arsenite oxidation was observed in dark Pt-TiO2 suspension (air-equilibrated), which seems to be due to the catalytic effect of platinum itself. Since this dark oxidation on PtTiO2 did not take place under the N2-saturated condition (data not shown in Figure 1 for clarity), an O2-activation path on Pt surface should be responsible for this dark activity. However, the enhanced photooxidation rates on Pt-TiO2 are much higher than the sum of the PCO rate on bare TiO2 and the dark oxidation rate on Pt-TiO2 and should be ascribed to a photoeffect. Figure 2 shows that the initial As(III) oxidation rates are faster in the O2-saturated suspension VOL. 38, NO. 10, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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A similar result was also observed in a recent photoelectrochemical experiment that compared the POM-mediated current generation between bare TiO2 and Pt-TiO2 suspensions (36), where the POM-mediated current in Pt-TiO2 suspension was smaller than that in bare TiO2 suspension. The As(III) oxidation by the homogeneous POM photocatalyst alone is negligible, although POM can generate the superoxide in the presence of electron donors such as alcohols and organic acids (reactions 7-9) (36, 37):

FIGURE 3. Effects of a POM (SiW12O404-) addition on the photocatalytic oxidation of As(III) in naked or platinized TiO2 suspensions. The experimental conditions were [TiO2] ) 0.5 g/L, [As(III)]0 ) 500 µM, [SiW12O404-] ) 5 mM, pHi 3, and air-equilibrated. than in the air-equilibrated suspension and are markedly reduced in the N2-saturated suspension. Dissolved oxygen should be essential for As(III) oxidation not only as a precursor of superoxides but also as a CB electron scavenger to inhibit fast charge pair recombination. Therefore, the role of O2 cannot be clearly resolved from the kinetic data. To test whether As(III) photooxidation can proceed in the presence of other electron acceptors, CCl4 was used as an alternative electron acceptor. CCl4 is known to accept CB electrons on TiO2 through the reaction of CCl4 + ecb- f •CCl3 + Cl- (21, 24). Figure 2 shows that the addition of CCl4 to the N2-satuared suspension did not increase the PCO rate at all although about 80 µM Cl- was generated after 150 min of irradiation. This clearly supports the role of oxygen as a precursor of superoxides in oxidizing As(III). The slow oxidation rate observed in the N2-saturated suspension might be ascribed to reaction with adsorbed (or residual) O2 or lattice oxygen. Photocatalytic Oxidation of As(III) on TiO2 in the Presence of POMs. If the superoxide is the main oxidant of arsenites, the presence of electron shuttles that mediate electron transfers from CB to dissolved O2 should enhance the PCO rate. Polyoxometalates (POMs) can act as an electron acceptor in the TiO2 photocatalytic systems to enhance the rate of CB electron transfer and to retard recombination (33-37). The ability of a POM as an electron shuttle in UVilluminated TiO2 suspensions has been recently demonstrated using a photoelectrochemical method (36). The reduced POMs rapidly react with O2 to produce superoxides (reactions 3-5):

TiO2 + hv f ecb- + hvb+

(3)

POM + ecb- f POM-

(4)

POM- + O2 f POM + O2•-

(5)

The reported bimolecular rate constant for SiW12O405- (POM-) + O2 reaction is 127 M-1 s-1 (33). The addition of a POM (SiW12O404-) to the naked TiO2 suspension significantly increases the PCO rate as shown in Figure 3. This strongly supports that the presence of POMs enhances the generation of superoxides, which are the main oxidant of As(III). However, the addition of the POM to the Pt-TiO2 suspension reduces the PCO rate, which can be ascribed to the fact that the reduced POM (POM-) is efficiently reoxidized on the Pt surface with reducing H+ into H2 (reaction 6) (38): Pt

POM- + H+ 98 POM + 1/2H2 2930

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(6)

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POM + hv f POM*

(7)

POM* + D f POM- + D+

(8)

POM- + O2 f POM + O2•-

(9)

This seems to imply that the concentration of superoxides generated in the homogeneous POM solution is much smaller than that in the heterogeneous TiO2 suspension. Photocatalytic Oxidation of As(III) on Fluorinated TiO2. The photocatalytic oxidation of arsenite employing fluorinated TiO2 (F-TiO2) was performed in order to check out the possibility that As(III) reacts directly with valence band (VB) holes without the involvement of superoxides. The direct VB hole transfer can be facilitated through the innersphere complexation between the arsenite and the surface OH group (reactions 10 and 11):

tTiOH + As(OH)3 f tTisOsAsIII(OH)2 + H2O (10) hvb+ + tTisOsAsIII(OH)2 f tTisOsAsIV(OH)2 (11) Since the fluoride adsorption on TiO2 surface is wellknown to replace the surface hydroxyl groups with Ti-F species (39, 40), the inner-sphere complexation between the arsenite and the surface OH group should be hindered on F-TiO2. As a result, the direct VB hole path should not be favored on F-TiO2 (reactions 12 and 13):

tTiOH + F- f tTisF + OH-

(12)

hvb+ + tTiF + As(OH)3 f no reaction

(13)

Figure 4 shows that the amount of As(III) and As(V) species adsorbed on F-TiO2 was much smaller than that on bare TiO2, which confirms that the surface fluorination reduces the adsorption of arsenic species on TiO2 and that the direct hole oxidation on TiO2, if any, should be retarded on F-TiO2. Incidentally, the deficit in the quantitative production of As(V) from As(III) oxidation (shown in Figures 1-3) is mainly ascribed to As(V) adsorbed on TiO2 (as shown in Figure 4a). Figure 5 shows that the initial PCO rate with F-TiO2 was similar to the case of naked TiO2. The As(V) concentration produced on F-TiO2 is even higher after 1-h irradiation, which could be ascribed partly to easier desorption of As(V) products. This ensures that the direct hole oxidation is not responsible for the photooxidation of As(III). On the other hand, the PCO rate is markedly reduced on F-TiO2 in the presence of excess TBA whereas it is not affected by TBA on naked TiO2. This indicates that OH radicals do take part in the PCO reaction in the F-TiO2 suspension but not in the bare TiO2 suspension. The preferential formation of homogeneous free OH radicals from the F-TiO2 surface has been recently proposed (39, 40). Diffusing free OH radicals, if any, should be efficiently scavenged by TBA (reaction 14). The resulting carbon-centered radicals immediately react with O2 to be fully oxidized eventually:

(CH3)3COH + •OHfree f •CH2C(CH3)2OH + H2O (14)

FIGURE 4. (a) As(V) and (b) As(III) adsorption isotherms on bare TiO2 and F-TiO2 under the dark condition. The experimental conditions were [TiO2] ) 0.5 g/L, [F-] ) 10 mM, pH 3, and airequilibrated.

FIGURE 5. Photocatalytic oxidation of arsenite on F-TiO2. The experimental conditions were [TiO2] ) 0.5 g/L, [F-] ) 10 mM, [As(III)]0 ) 500 µM, [TBA] ) 0.53 M, [HA] ) 10 ppm, pHi 3, and airequilibrated. These evidences suggest that the free OH radicals in the solution phase, if they are present, initiate the As(III) oxidation whereas VB holes and surface-bound OH radicals do not compete with the superoxides in the photooxidation of As(III). It is consistent with the fact that the literature bimolecular rate constant (kOH) of As(III) + OH reaction is near diffusion limited (kOH ∼109 M-1 s-1) (41). On the other hand, the addition of HA to the F-TiO2 suspension increases the initial PCO rate as in the case of naked TiO2 (18), and the addition of TBA to the HA/F-TiO2 system has little effect on the PCO rate. In our previous study (18), the HA-enhanced effect was ascribed to the superoxide production on TiO2 by sensitization, and the arsenite photooxidation with HA only in the absence of TiO2 was negligibly small. The same

FIGURE 6. Time-dependent profiles of H2O2 production in the course of As(III) photooxidation in (a) TiO2 and (b) F-TiO2 suspensions. The experimental conditions were [TiO2] ) 0.5 g/L, [F-] ) 10 mM, [As(III)]0 ) 500 µM, [TBA] ) 0.53 M, [HA] ) 10 ppm, pHi 3, and airequilibrated. sensitizing mechanism should apply to the case of HA/FTiO2 system. The arsenite oxidation through this sensitization path should not be affected by the presence of TBA. The production of H2O2 has been observed during the As(III) photooxidation when HA was present (18). It was suggested that H2O2 is a byproduct of As(III) oxidation in the HA-sensitized process (reactions 15-19):

HA + hv f HA*

(15)

HA* + TiO2 f HA+ + ecb-

(16)

O2 + ecb- f O2•-

(17)

O2•- + ecb- + 2H+ f H2O2

(18)

2O2•- + 2H+ f H2O2 + O2

(19)

HA that can serve as not only a sensitizer but also a hole scavenger should enhance the As(III) photooxidation and the production of superoxides and H2O2 under UV illumination. Therefore, the HA-enhanced effects should be ascribed to both the sensitizing effect and the hole scavenging effect. The H2O2 production profiles during the As(III) photooxidation are compared between the case of naked TiO2 and F-TiO2 in Figure 6. In both cases, the production of H2O2 in the presence of both As(III) and HA drastically increases during the initial stage and then gradually decreases due to its reaction with OH radicals (reaction 20):

H2O2 + OH• f HO2• + H2O

(20)

Therefore, the decaying profile of H2O2 was not observed when TBA was added as an OH radical scavenger. This also VOL. 38, NO. 10, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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SCHEME 1. Schematic Illustrations of Various As(III) Photooxidation Pathways in Naked and Modified TiO2 Systems

supports that the H2O2 production comes through the reductive path (reactions 17-19), not from the recombination of two OH radicals. In addition, it is noted that the F-TiO2 system without HA shows a moderate level of H2O2 production, which might be explained by the fact that photoproduced H2O2 is easily desorbed from F-TiO2 surface whereas H2O2 forms strong complexes on the naked TiO2 surface (42). Photocatalytic Oxidation of As(III) on Dye-Sensitized Pt-TiO2. All the above evidences support that the photooxidation of As(III) on TiO2 proceeds through the CB electron pathway that does not need OH radicals or VB holes. If this is true, the arsenite oxidation should take place on the dyesensitized TiO2 under visible light, where the TiO2 VB is not involved in the photoinduced electron-transfer processes (24). The visible light-induced electron transfer on dyesensitized TiO2 could be greatly enhanced by depositing Pt nanoparticles on TiO2 (25). To test the idea, the visible lightinduced oxidation of As(III) on Pt-TiO2/RuIIL3 was carried out. Figure 7 shows the As(V) production profiles in this dyesensitized TiO2 system. The visible light-induced oxidation on dye-sensitized Pt-TiO2 is as fast as the UV photooxidation on Pt-TiO2. In this case, the excited sensitizer injects electrons to TiO2 CB and the subsequent generation of superoxides is initiated by transferring CB electrons to O2 (reactions 2123) (25): hv(λ > 420 nm)

(RuIIL3)-TiO2 98 (RuIIL3)*-TiO2

(21)

(RuIIL3)*-TiO2 f (RuIIIL3)-TiO2 (ecb-)

(22)

ecb- + O2 f O2•-

(23)

Since other oxidants such as OH radicals and VB holes cannot be generated under visible light, the superoxides should be 2932

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FIGURE 7. Photocatalytic oxidation of As(III) on dye-sensitized PtTiO2 under visible light irradiation. The experimental conditions were [TiO2] ) 0.5 g/L, [As(III)]0 ) 500 µM, [RuIIL3] ) 10 µM, λ > 420 nm, pHi 3, and air-equilibrated or N2-saturated. responsible for the arsenite oxidation. Accordingly, the visible light activity is significantly reduced in the N2-saturated suspension. The visible light-induced oxidation on Pt-TiO2 (in the absence of RuIIL3) was observed only in the presence of dissolved O2 and should be ascribed to the intrinsic dark oxidation reaction on Pt-TiO2 (see Figure 1). The fact that the oxidation was observed even in the Pt-TiO2/RuIIL3/N2/ visible system should be ascribed to in-situ photogeneration of O2 through the reaction of water with oxidized sensitizer (reaction 24). The visible light-induced O2 generation on PtTiO2/RuIIL3 has been experimentally confirmed in a recent study (25). Therefore, the dark arsenite oxidation in the PtTiO2/RuIIL3/N2 system where the in-situ O2 generation was absent did not take place at all.

2RuIIIL3 + H2O f 2RuIIL3 + 2H+ + 1/2O2

(24)

FIGURE 8. Effect of superoxide dismutase (SOD) addition on the photocatalytic oxidation of arsenite. The experimental conditions were [TiO2] ) 0.5 g/L, [As(III)]0 ) 400 µM, [SOD] ) 66.7 mg/L, pHi 7.2, reactor volume ) 3 mL, and air-equilibrated. Role of Superoxides in As(III) Oxidation. To directly confirm the role of superoxides, the arsenite photooxidation experiment was carried out in the presence of SOD, a metalloenzyme that dismutates the superoxides to hydrogen peroxide and oxygen (reaction 19) (43, 44). Figure 8 shows that the As(V) production rate in the presence of SOD is reduced by half from that in the absence of SOD, which strongly supports the fact that superoxides are largely responsible for the As(III) oxidation. However, the photooxidation could not be completely inhibited even with increasing the SOD concentration up to 333 mg/L. This might be due to the instability and rapid inactivation of SOD under UV irradiation. The mechanistic pathways of the arsenite photooxidation in the naked and surface-modified TiO2 systems, which have been discussed so far, are comparatively illustrated in Scheme 1. The rate of As(III) oxidation markedly increases under the conditions that could enhance the generation of superoxides such as in the cases of Pt-TiO2 (Scheme 1b) and TiO2 + POM (Scheme 1c). On F-TiO2 where the adsorption of As(III) is reduced, the PCO rate is even higher than that on naked TiO2. In this case, free homogeneous OH radicals that could be generated from the F-TiO2 surface contribute to the As(III) oxidation (Scheme 1d), whereas the surface-bound OH radicals do not seem to be efficient in initiating the photooxidation reaction (Scheme 1a). On the other hand, the homogeneous photocatalytic oxidation of As(III) using POM only was negligible (Scheme 1e). Finally, the visible light-induced arsenite oxidation on Pt-TiO2/RuIIL3 where only reductive electron transfers are allowed confirms that the superoxide is largely responsible for the As(III) photooxidation (Scheme 1f). Superoxides are generally considered as a weak oxidant. However, in this specific case of arsenite photooxidation, they seem to be very efficient oxidants that can be easily photogenerated under sunlight conditions. Visible lightinduced photocatalytic oxidation of arsenites in the presence of natural sensitizer such as humic acids is highly probable in the natural aquatic environment and can be utilized in the development of a simple solar remediation technology.

Acknowledgments This work was supported by Korea Industrial Technology Foundation (KOTEF) through the program for the training of graduate students for regional strategic industry.

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Received for review July 7, 2003. Revised manuscript received December 5, 2003. Accepted March 2, 2004. ES034725P VOL. 38, NO. 10, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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