Regenerated Dye-Sensitized Photocatalytic Oxidation of Arsenite over Nanostructured TiO<sub>2</sub> Films under Visible Light in Normal Aqueous Solutions: An Insight into the Mechanism by Simultaneous (Photo)electrochemical Measurements

Abstract

TiO<sub>2</sub> photocatalysis has been demonstrated as an alternative pretreatment method for arsenic-contaminated water by oxidizing As­(III) to less toxic and less mobile As­(V). However, the lack of visible light absorption of the catalyst limits the utilization of sunlight. In this article, we report that As­(III) could be efficiently oxidized by visible light (λ ≥ 420 nm) over a typical ruthenium dye N719-sensitized nanostructured TiO<sub>2</sub> film in the normal aerated aqueous solutions. The amount of oxidation of As­(III) via the photo-oxidative (by dye cation, S<sup>+</sup>) and photoreductive (by electron-initiated reactive oxygen species, EIROS; O<sub>2</sub><sup>•–</sup> considered to be the dominant species) pathways was quantified by simultaneously measuring the oxidation rate and interfacial charge transfer rate of the film electrodes. The results in the absence of O<sub>2</sub> and under an anodic potential bias where EIROS is absent indicate that As­(III) can be highly efficiently oxidized by S<sup>+</sup> via a two-electron reaction with ∼100% Coulombic efficiency, while the results in the dark and under a cathodic bias where S<sup>+</sup> is absent suggest that EIROS could also efficiently oxidize As­(III) via a one-electron reaction with ∼100% Coulombic efficiency as well. Under open circuit and in the normal aerated aqueous solutions, nearly all of the interfacial transferred charge was utilized for the As­(III) photo-oxidation, and 33 and 67% of As­(V) production resulted from S<sup>+</sup> and EIROS-initiated oxidation, respectively. The mechanism of As­(V) formation under this situation was the direct two-electron oxidation of As­(III) by S<sup>+</sup> and indirect one-electron oxidation of As­(III) by EIROS to generate As­(IV), which further reacts with O<sub>2</sub>, producing As­(V). The dye could be completely regenerated in situ through the oxidation of As­(III) and consequently was photostable

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