Reactive oxygen species generated in iron sulfide mediated advanced oxidation systems: A critical review of mechanisms and implications for geochemistry and environmental remediation

Iron sulfides (FexSy), including natural FexSy minerals and artificially prepared sulfide modified zero valent iron (S-ZVI), have been widely concerned in the field of geochemistry and environmental remediation due to their strong reducibility, high reactivity, earth-abundant, low-cost and environmentally friendly. In this work, the generation mechanism of reactive oxygen species (ROS) in FexSy mediated advanced oxidation processes (FexSy-AOPs) are reviewed. Structural Fe(II) in FexSy is considered to be the main active species responsible for activating oxidants (eg., O2, hydrogen peroxide, persulfate, calcium peroxide, etc.) to generate multiple ROS (e.g., •OH, •O2－, FeⅣO2+, Fe(III)-OOH, SO4•−, 1O2, etc.), accounting for both toxic effects on microorganisms and oxidative removal of various organic/inorganic pollutants in the environment. Importantly, both the acid-producing effect caused by the oxidation reaction of FexSy and the Fe(II) regeneration process induced by soluble S intermediates (eg., S2-, S22- and SO32-) improve the generation process of ROS and promote the removal of toxic pollutants. The doping state, crystallinity, surface composition and physicochemical properties of solution (eg., solution pH, coordination state of dissolved Fe, coexisting organic matter and various complex ligands) have a significant impact on the electron transfer process, which is influential on the generation mechanism of ROS. This review also discussed the controversial issues existing in the current literatures, such as the ambiguous in free radical and non-free radical mechanisms, and the controversy of the contribution of homogeneous and heterogeneous reactions. Finally, the key knowledge gaps are identified and the potential challenges encountered in practical applications of the FexSy-AOPs are addressed.