Solar irradiation accelerates the oxidation of Cr(III) by δ-manganese dioxide

Cr(VI) has been observed to be released from Cr(III)-bearing natural sources or residues when they are found alongside manganese and manganese oxides. However, relevant mechanism studies normally ignore the effect of simulated solar irradiation on this oxidation reaction. Therefore, we investigated the photochemical reaction between Cr(OH)₃ and δ-MnO₂, the common species of chromium and manganese oxide found in the environment. At pH 11, the oxidation of Cr(OH)₃ by δ-MnO₂ was accelerated under simulated solar irradiation, which had an oxidation rate 2.7-fold greater than that in the dark condition. Further investigation revealed that δ-MnO₂, an n-type semiconductor with a 2.7 eV band gap, can be excited by light with wavelengths < 459 nm and produce photogenic electrons and holes. These photo-induced carriers reacted with surrounding molecules to form free radicals and participate the redox reactions. Free-radical quenching experiments indicated that hydroxyl radicals (•OH) are the main oxidants of Cr(III) under simulated solar irradiation. This work provides new mechanistic insight into the oxidation of Cr(III) to Cr(VI), which may help clarifying the environmental fate of Cr and the potential solar-triggered release of Cr(VI).