Although it is well-known that As(III) is oxidized to As(V) in the UV/TiO2 system, the main oxidant for that reaction is still not clear. Accordingly, the present study aims at reinvestigating the TiO2-photocatalyzed oxidation mechanism of As(III). We performed a series of As(II) oxidation experiments by using UV-C/H2O2 and UV-A/TiO2, focusing on the effects of competing compounds. The experiment with UV-C/H2O2 indicated that HO2*/O2-* is not an effective oxidant of As(III) in the homogeneous phase. The effects of oxalate, formate, and Cu(II) on the photocatalytic oxidation of As(III) contradicted the controversial hypothesis that HO2*/ O2-* is the main oxidant of As(III) in the UV/TiO2 system. The effect of As(III) on the TiO2-photocatalyzed oxidations of benzoate, terephthalate, and formate was also incompatible with the superoxide-based As(II) oxidation mechanism. Instead, the experimental observations implied that OH* and/or the positive hole are largely responsible forthe oxidation of As(III) in the UV/TiO2 system. To determine which species plays a more significant role, the effects of methanol and iodide were tested. Since excess methanol did not retard the oxidation rate of As(III), OH* seems not to be the main oxidant. Therefore, the best rationale regarding the oxidation mechanism of As(III) in the UV/TiO2 system seems to be the direct electron transfer between As(III) and positive holes. Only with this mechanism, it was possible to explain the data of this study. Besides the mechanistic aspect, an application method for this technology was sought. The usage of UV/TiO2 for oxidizing As(II) requires a posttreatment in which both As(V) and TiO2 should be removed from water. For this objective, we applied FeCl3 and AIK(SO4)2 as coagulants, and the result implied that the combined usage of TiO2 and coagulation might be a feasible solution to treat arsenic contamination around the world.

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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 (lambda = 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).