Water Activation on Sulfur Vacancies Enables Pyrite-Mediated Methylmercury Demethylation and Immobilization.

The accumulation of neurotoxic methylmercury (MeHg) in rice paddies presents a major global food safety challenge, yet effective in situ remediation strategies for flooded, anoxic soils are lacking. Here, we report that sulfur vacancy (SV)-rich pyrite efficiently degrades MeHg under dark, anoxic conditions via an innovative pathway involving vacancy-activated water (H2O). Experimental and theoretical evidence reveals that SVs serve a dual function: they enhance MeHg adsorption, weakening the carbon-mercury bond, while simultaneously activating ambient H2O to generate surface-bound hydroxyl radicals (•OH) that drive demethylation. The reaction rapidly converts MeHg into inorganic Hg(II), which is subsequently sequestered as insoluble, nanocrystalline metacinnabar (β-HgS) epitaxially anchored to the pyrite surface. Crucially, this β-HgS byproduct demonstrates negligible bioavailability in incubation experiments with Geobacter sulfurreducens PCA, a key mercury-transforming bacterium in paddy soils. This work establishes a "degrade and sequester" mechanism that transforms a mobile neurotoxin into a geochemically stable mineral phase. Our findings provide a sustainable, mineral-based strategy for the in situ remediation of MeHg-contaminated anoxic environments, closing the mercury loop and mitigating its entry into the food chain.