Sub-nanometer FeOOH clusters drive mercury immobilization and methylation inhibition in paddy soil-water systems via iron-modified biochar.

Mercury (Hg) contamination in paddy systems poses severe environmental and public health threats due to the microbial transformation of inorganic Hg into highly toxic methylmercury (MeHg). Although biochar (BC) has been widely applied for heavy metal remediation, its limited capacity to immobilize Hg constrains its practical effectiveness. Here, we present a comprehensive study combining batch experiments and density functional theory (DFT) calculations to elucidate the molecular mechanisms by which iron-modified biochar (Fe-BC) enhances Hg stabilization and inhibits MeHg formation in paddy systems. Our results reveal that Fe modification induces the coordination of FeOOH clusters with oxygen-containing functional groups and aromatic domains on the BC surface, significantly enhancing its Hg-affinity. Notably, the structural stability and adsorption performance of Fe-BC are strongly size-dependent, with the FeOOH dimer (FeOOH) showing the most robust binding to BC and the highest capacity for Hg immobilization. Fe-BC application markedly reduces both MeHg production and bioavailability in the contaminated paddy by strengthening Hg adsorption through interactions with sub-nanometer FeOOH clusters, especially (FeOOH) anchored on the aromatic structures of BC. Furthermore, we demonstrate that coexisting soil ions modulate Hg adsorption: Na reduces Hg binding via electrostatic competition, whereas Cl and SO promote stable complex formation (Fe-BC-Hg-Cl and Fe-BC-Hg-SO), further enhancing Hg retention. These findings provide molecular-level insight into Fe-BC's stabilization mechanisms and highlight the importance of optimizing Fe cluster structures and ion interactions to maximize remediation efficiency. This study offers both theoretical and practical guidance for advancing Fe-BC-based strategies for sustainable Hg immobilization in contaminated paddy systems.