Birnessite enhanced Cr(III) oxidation during subsurface geochemical processes: Role of Mn(III)-induced nonphotochemical reactive oxygen species

Cr(III) oxidation by birnessite was the dominant geologic source of Cr(VI), which increases the environmental mobility and toxicity of Cr, threatening ecological safety. Photochemically hydroxyl radical (•OH) generated by birnessite was widely accepted to be the dominant reactive oxygen species (ROS) oxidating Cr(III). However, birnessite and Cr mainly co-exist in dark subsurface soils, with contribution of nonphotochemical ROS remaining unclear. In this work, free-radical quenching experiments, electrochemistry method and density functional theory (DFT) calculations were performed to elucidate ROS generation mechanisms during Cr(III) oxidation in simulated light-deprived environment. The results indicated that •OH was completely suppressed and nonphotochemical O₂•^- still accelerated Cr(III) oxidation in dark aerobic conditions with the contribution of 15.3%-19.1%. Moreover, DFT calculations proved that O₂•^- was produced by O₂ molecules adsorbed on oxygen vacancies in the structure, thus being generated spontaneously in the dark. The oxidation contribution of O₂•^- was undetectable after extracting Mn(III), indicating that electron transfer occurred between Mn(III) and O₂ to generate O₂•^-. Additionally, intervention of Cd²⁺ (for occupying oxygen vacancies) did not reduce participation of •OH, but resulted in suppression of electron transport which greatly reduced the production of O₂•^-, thereby affecting Cr(III) oxidation process. The above findings provide new insights on Cr(III) oxidation by manganese oxides and is able to have profound significance for predicting the fate of Cr in subsurface environments.