Regenerated Dye-Sensitized Photocatalytic Oxidation of Arsenite over Nanostructured TiO₂ Films under Visible Light in Normal Aqueous Solutions: An Insight into the Mechanism by Simultaneous (Photo)electrochemical Measurements

Abstract

TiO2 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 TiO2 film in the normal aerated aqueous solutions. The amount of oxidation of As(III) via the photo-oxidative (by dye cation, S+) and photoreductive (by electron-initiated reactive oxygen species, EIROS; O2•– 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 O2 and under an anodic potential bias where EIROS is absent indicate that As(III) can be highly efficiently oxidized by S+ via a two-electron reaction with ∼100% Coulombic efficiency, while the results in the dark and under a cathodic bias where S+ 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+ 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+ and indirect one-electron oxidation of As(III) by EIROS to generate As(IV), which further reacts with O2, producing As(V). The dye could be completely regenerated in situ through the oxidation of As(III) and consequently was photostable.