Sulfidation Unlocks Dual Reductive Pathways in Uranium Immobilization by Iron Sulfide

Iron sulfide minerals are critical mediators of uranium (U) immobilization in anoxic environments, yet the electron transfer mechanisms across Fe- and S-containing phases remain incompletely understood. Here, we demonstrate that redox-driven structural transformations unlock a dual pathway for U(VI) reduction. Comparative experiments using pristine mackinawite (FeS), partially oxidized FeS (O-FeS), and sulfur-enriched FeS (S-FeS) revealed that FeS and O-FeS reduce U(VI) primarily through oxidation of structural S(-II), whereas sulfidation-induced structural alterations in S-FeS activate otherwise inert Fe(II) as a coreductant, as evidenced by Fe(III) formation. This dual electron-transfer pathway shows pH dependence. At pH 6.5, U(VI) reduction to U(V)/U(IV) is nearly complete in S-FeS, while the reduction extent decreases to 34% at pH 8.5, due to the formation of a passivated surface layer rich in Fe(III) and sulfur that inhibits further electron transfer. These findings demonstrate that oxidation- and sulfidation-driven variations in FeS stoichiometry and structure regulate uranium reduction and immobilization pathways, with important implications for predicting the fate of redox-sensitive metal contaminants in dynamic subsurface environments.