Enhanced generation of reactive oxygen species by iron minerals for Sb(III) photooxidation: the vital role of Fe(II)

Preliminary studies have demonstrated that light can facilitate the biogeochemical cycling of iron, influencing the valence state of heavy metals, solidifying them, and ultimately achieving detoxification effects. However, few studies have focused on the oxidation of Sb(III) in different iron minerals and the differences in mechanisms. This study explored the photooxidation of Sb(III) in the presence of various iron minerals (hematite, pyrite and goethite), focusing on how different iron minerals influenced the oxidation process and the mechanisms involved. Among the minerals tested, pyrite exhibited exceptional photooxidation performance, achieving an Sb(III) oxidation efficiency of 67.03% after 300   minutes. This superior photooxidation activity correlated with pyrite’s significant Fe(II) yield (11.70   μg/L). Our study demonstrated that light irradiation enhanced Sb(III) oxidation through three iron-based minerals. Light exposure drove the generation of Fe(II) within these minerals, initiating Fe(II)/Fe(III) redox cycling. This process promoted reactive oxygen species (ROS) generation, including hydroxyl radicals (•OH), superoxide radicals (O 2 •− ) and hydrogen peroxide (H 2 O 2 ), which collectively mediated Sb(III) oxidation to the less toxic Sb(V). In the photooxidation of Sb(III), •OH played a critical role in reactions involving hematite and pyrite, while O 2 •− dominated in goethite and pyrite systems. H 2 O 2 was found to have a pivotal role across all three iron mineral systems. Additionally, photogenerated holes ( h + ) significantly enhanced Sb(III) oxidation in the pyrite system. This research highlighted the impact of light on the biogeochemical cycling of iron and its role in facilitating the oxidation of Sb(III) through reactions that involve photo-generated electrons and free radicals.