FeS Colloids Trigger Antimony Redox Cycling, Colloid Formation, and Ultimate Fate during the Anoxic-Oxic Transition.

Antimony (Sb) mobility in subsurface environments is tightly coupled to iron (Fe) geochemistry, yet the roles of ferrous sulfide (FeS) colloids in controlling Sb fate across anoxic-oxic interfaces is not well understood. Here, we systematically elucidate how FeS colloids trigger Sb redox transformation, colloidal formation, and ultimate sequestration during anoxic-oxic transition at circumneutral pH. Experiments and theoretical calculations indicate that FeS both reduce Sb(V) and promote Sb(III) oxidation, with the dominant pathway determined by system redox conditions. We identify that transient labile Fe(III) intermediates, generated during FeS oxidation, serve as the oxidants for Sb(III) oxidation. During early-stage FeS oxidation, a substantial fraction of Sb is associated with stable colloidal particles (150-350 nm) composed of FeS and newly formed ferrihydrite, underpinning a transient, colloid-facilitated transport pathway that may enhance Sb mobility. Upon complete oxidation, all colloidal Sb exists in larger aggregates (>1000 nm), thus reducing mobility. Spectroscopic analyses confirm that complete FeS oxidation results in the production of secondary Fe(III) (oxyhydr-)oxides, which sequester Sb(V) through combined surface adsorption and structural incorporation via isomorphous substitution. Collectively, this work reveals previously unrecognized multifunctionality of FeS colloids at anoxic-oxic interfaces, providing a mechanistic framework for more accurate prediction of Sb mobility and fate in FeS-rich, redox-dynamic subsurface environments.