Importance of the ligand-to-metal charge transfer (LMCT) pathway in the photocatalytic oxidation of arsenite by TiO2

Abstract

Photooxidation of As(III) by TiO₂ is a complicated process in which the oxidation mechanisms are always controversial. In this study, the enhanced photooxidation rates of As(III) with increasing pH values from 8.0 to 11.0 indicate the high photocatalytic reactivity of TiO₂ under alkaline conditions. Moreover, As(III) improves the production of H₂O₂, indicating H-abstraction from As(III) (soluble or adsorbed) for H₂O₂ production. Although O₂˙⁻, h⁺, ˙OH and –OOH are always regarded as the reactive oxygen species in the UV-TiO₂ system, the superoxo and peroxo species formed on the surface of TiO₂ also contribute to As(III) oxidation. The As(III)–O–Ti(IV) surface complexes formed on TiO₂, as well as the decreased bandgaps of TiO₂ with increasing concentrations of As(III) indicate that the ligand-to-metal charge transfer (LMCT) pathway also contributes to the oxidation of As(III) under alkaline conditions. Electrochemical analyses further reveal that As(III) enhances the electron density on the surface of TiO₂, thereby improving the catalytic reactivity of TiO₂. We therefore suggest that H-abstraction from As(III) or H₂O to the formed superoxo and peroxo species results in the formation of H₂O₂, accompanied by the oxidation of As(III). This enriches our knowledge on the oxidation of As(III), as well as other contaminants rich in –OH groups during the photocatalytic oxidation processes.