Iron sulfides mediated abiotic and biotic purification of wastewater: Reaction mechanisms and environmental effects

Iron sulfides (Fe_xS_y), a key carrier of geochemical elements cycling, have attracted much attention in investigating their environmental chemical behaviors and pollutants transformation mechanisms due to the unique redox cycles of iron and sulfur species. However, the unclear interface reaction process‌, complex environmental factors, and multi-pollutants synergism ‌bring enormous challenges to reveal the roles of iron and sulfur species in environmental pollutants abatement. This review first introduces the classification, distribution regularities, and geochemical cycle mechanisms of Fe_xS_y in nature. Subsequently, we summary the reduction and oxidation behaviors of Fe_xS_y in activating environmental dioxygen, water molecules and common peroxides, mainly focusing on the electron transfer pathways along with elemental cycles of Fe and S, and generation of reactive species (i.e., reductive reactive species (RRSs) and reactive oxygen species (ROSs)). We noted that reactive species generation of specific Fe_xS_y phases under different redox conditions is co-determined by their crystal structure and defect chemistry. Then, we demonstrate that engineering reactive sites of Fe_xS_y enables to promote the decomposition and transformation of pollutants, including the removal of heave metal ions, RRSs or ROSs-involved pollutants removal, as well as the cooperation of RRSs and ROSs in a reductive and oxidative coupling (ROC) process for the degradation of refractory organic pollutants. We also discuss the challenges and perspectives of Fe_xS_y in the detoxification and decontamination of environmental pollutants in aspects of fundamental research and industrial application.