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)2　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)2　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　SO42-　promote　stable　complex　formation　(Fe-BC–Hg–Cl　and　Fe-BC–Hg–SO4),　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.　©　2025　Elsevier　Ltd