Quantitatively understanding the mechanism of highly enhanced regenerated dye sensitized photooxidation of arsenite over nanostructured TiO2 electrodes under visible light by I−

We reported previously that the regenerated dye-sensitized photocatalytic oxidation (SPCO) of As (III) over nanostructured TiO2 films under visible light could be highly accelerated by I− in aqueous solutions using a model ruthenium dye as sensitizer. Yet the mechanism remains to be determined. In this Article, the impact of I− upon the oxidation of As (III) via the photo-oxidative (by dye cation, S+) and photo-reductive pathways (by electron-initiated reactive oxygen species, EIROS) was quantified and the mechanism of the enhanced oxidation was evaluated by a combination of (photo) electrochemical measurements. The results under an anodic potential bias and without O2 indicate that I− could facilitate the regeneration of S+ through one-electron reaction to produce I 2 • - which could efficiently oxidize As (III) with ∼100% of columbic efficiency along with S+, thereby highly accelerating the oxidation of As (III) via S+. The regeneration of S+ was first-order reaction in surface-sorbed iodide. Under normal open-circuit conditions, although I−promoted the reduction of O2, it decreased both the oxidation rate of As (III) by EIROS and the columbic efficiency of EIROS, because EIROS could also react with I− to produce less efficient oxidants such as I 2 • - for the conversion of As (III) to As (V). Such negative effect of I− on the EIROS-induced oxidation of As (III) partly offset its positive influence on the S+-induced oxidation. Hence the enhancement by I− at open-circuit conditions was mainly resulted from the enhanced photo-oxidative pathway.