Hydroxyl radicals produced from oxidation of ferrous sulfides promote mobilization of mercuric sulfide in soil-water system.

Mercuric sulfide nanoparticles (HgS-NPs) are recognized as a significant source of bioavailable mercury in paddy fields. The factors influencing the mobilization and bioavailability of HgS-NPs formed in flooded or drained paddy field-like systems are complicated and remain unexplored to date. Here, we show that ferrous sulfide (FeS) as an important mineral substance plays a crucial role in the dissolution and transformation of HgS-NPs in overlying water or during the drainage stage, as well as their bioavailability toward rice. Specifically, we found that oxidation of FeS significantly enhances the dissolution of HgS-NPs, with the degree of activation intensified with increasing FeS concentrations. This activation was further evidenced to be driven by the generation of hydroxyl radicals (•OH) during FeS oxidation, leading to the release of Hg(Ⅱ). The enhanced dissolution of HgS-NPs increases its bioavailability, as verified by the augmented accumulation of Hg in rice upon FeS oxidation. This study underscores the overlooked yet important role of FeS in affecting the fate of HgS-NPs and offers valuable insights for pollution control of Hg-contaminated paddy fields and wetlands.