Oxidation Mechanism of As(III) in the UV/TiO₂ System: Evidence for a Direct Hole Oxidation Mechanism

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Abstract

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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