Reductive Dissolution Mechanisms of Manganese Oxide Mediated by Algal Extracellular Organic Matter and the Effects on 17α-Ethinylestradiol Removal

Reductive dissolution of manganese oxide (MnO_x) is a major process that improves the availability of manganese in natural aquatic environments. The extracellular organic matter (EOM) secreted by algae omnipresent in eutrophic waters may affect MnO_x dissolution thus the fate of organic micropollutants. This study investigates the mechanisms of MnO_x reductive dissolution mediated by EOM and examines the effects of this process on 17α-ethinylestradiol degradation. The influences of EOM concentration (1.0–20.0 mgC/L) and pH (6.0–9.0) in both dark and irradiated conditions were assessed. In the dark, EOM was found to facilitate MnO_x reductive dissolution via the ligand-to-metal charge transfer (LMCT). The dissolution was further enhanced under irradiation, with the participation of superoxide ions (O₂^•–). Higher EOM concentrations increased the contents of available reducing substances and O₂^•–, accelerating the reductive dissolution. Higher pH slowed the photoreductive dissolution rates, while O₂^•–-mediated reduction became more important. Polyphenols and highly unsaturated carbon and phenolic formulas in EOM were found to drive the reductive dissolution. Soluble reactive Mn(III) formed through reductive dissolution of MnO_x effectively removed 17α-ethinylestradiol in solution. Overall, the findings regarding the mechanisms behind reductive dissolution of MnO_x have broad implications for Mn geochemical cycles and organic micropollutant fate.