Formation of reactive oxidants from Fe(II)-bearing minerals in oxic environments: A review

Iron is the fourth most abundant element in the earth’s crust. It plays a pivotal role in regulating reactive oxidants including reactive oxygen species (ROS) formation and other geochemical processes. The generation mechanisms of reactive oxidants, the influencing factors and their environmental occurrence under oxic conditions are systematically reviewed. In oxic aquatic environments, Fe(II)-containing minerals can drive the production of reactive oxidants, including ROS, tetravalent iron (Fe(IV)), carbon-centered free radicals (R-C•) and other uncharacterized reactive species. The key formation processes include one-electron reduction and two-electron reduction of oxygen reduction mediated by Fe(II) minerals, solar irradiation, microbial activity, three sulfur defect-mediated pathways (production of •OH from reaction between Fe(III) and H₂O, and reaction of Fe(II)/O₂ with sulfur intermediates, as well as formation of H₂O₂ in the reaction between S(-II) and O₂) and carbon-centered radicals formed during Fe(II)-oxalate oxidation. Furthermore, Fe(IV) formation at pH ≥ 7 and metal ion-mediated ¹O₂ generation play significant roles. The environmental factors, Fe(II) minerals speciation (eg., crystallinity, surface defects) and ligand chemistry that together modulate oxidant yields are reviewed. Field evidence is compiled from riparian zones, lake sediments, coastal systems, and paddy soils, demonstrating ROS roles in pollutant degradation and microbial metabolism regulation. Significant gaps in in-situ quantifying the contributions of non-hydroxyl radical species, synergies, reaction pathways and interfacial reaction dynamics under natural conditions are proposed with suggestions for future research. It can provide a comprehensive understanding for pollution control and environmental remediation involving reactive oxidants in Fe(II)-rich environments.