Abiotic degradation of methylmercury mediated by hydroxyl radicals generated during ferrous oxidation in rice paddy soils

Rice consumption is a primary exposure pathway for neurotoxic methylmercury (MeHg), with paddy soils acting as critical hotspots for net MeHg accumulation. Although microbial and photolytic demethylation are well-documented, dark abiotic demethylation mechanisms within light-impenetrable soil matrices remain poorly understood. This study investigated the dark abiotic MeHg degradation driven by hydroxyl radicals (·OH) generated during the oxygenation of different ferrous species: surface adsorbed Fe(II) on ferric oxides (≡Fe 3 O 4 -Fe(II)), ferrous sulfide (FeS), and ferrous disulfide (FeS 2 ). Upon air exposure, substantial ·OH accumulation occurred during the oxidation of ≡Fe 3 O 4 -Fe(II) (428.4 ± 5.82 μM) and FeS (561.9 ± 26.0 μM) systems over 72 h, whereas low-spin FeS 2 remained inert under our experimental conditions. In autoclaved paddy soil microcosms under oxic conditions, this oxidation cascade led to a significant dark demethylation of 35.45% (≡Fe 3 O 4 -Fe(II)) and 27.22% (FeS) of spiked MeHg over 48 h, while anoxic controls remained stable. Mechanistic analyses revealed divergent interfacial electron transfer pathways: surface passivation by amorphous Fe(III) limited the long-term reactivity of ≡Fe 3 O 4 -Fe(II), whereas the destruction of the FeS lattice and potential generated core–shell structure sustained internal electron pumping and prolonged MeHg demethylation (maximum MeHg demethylation rate constant of 0.192 h −1 ). These findings elucidate a previously overlooked dark abiotic MeHg sink in redox-fluctuating rice paddy soils, demonstrating that Fe(II)-mediated Fenton-like reactions under alternating redox conditions can mitigate MeHg accumulation, a process that has been overlooked in current Hg transport models.