Photocatalytic oxidation of Mn(ii) on the surface of Bi2.15WO6via the ligand-to-metal charge transfer (LMCT) pathway

Abstract

Biotic and abiotic oxidation of Mn(II) in aqueous environments is an important process for the cycling of many elements. However, the mechanism involved in photocatalytic oxidation of Mn(II) has not been clearly elucidated yet. In this study, the photocatalytic oxidation of Mn(II) on the surface of self-doped Bi_2+xWO₆ (Bi_2.15WO₆) under visible light was conducted. Kinetics results show that visible light apparently accelerates the oxidation of Mn(II) to Mn(III, IV) oxides on Bi_2.15WO₆. The average oxidation states (AOS) of manganese reach 2.18 after 80 min of reaction under visible light at pH 8.50. Characterizations indicate the formation of Bi(III)–O–Mn(II) surface complexes between Mn(II) and surface Bi(III) on Bi_2.15WO₆, which then decreases the bandgap of [Bi_2.15WO₆ + Mn(II)]_light (2.53 eV) compared with those of [Bi_2.15WO₆ + Mn(II)]_dark (2.72 eV) and pure Bi_2.15WO₆ (2.86 eV), suggesting the contribution of the ligand-to-metal charge transfer (LMCT) pathway to the photocatalytic oxidation of Mn(II). Moreover, the addition of inorganic oxidants with strong oxidizing capacities (such as Cr₂O₇²⁻, NO₃⁻ or NO₂⁻) significantly increases the oxidation rate of Mn(II), further verifying the contribution of the LMCT pathway to Mn(II) oxidation. We therefore suggest that the LMCT pathway is one of the important oxidation routes for Mn(II) oxidation on Bi_2.15WO₆ under visible light.