Molecular Insights into Mercury Sequestration by the Sulfate and Biochar Combined Application Strategy Guide Pollution Treatments

Sulfur and biochar are two important agents for remediating heavy metal-polluted soils. Nevertheless, the double-edged sword effect of sulfur and the relatively limited efficacy of biochar in addressing soil mercury (Hg) contamination have constrained its practical use in Hg-polluted areas. This study developed an optimized strategy involving the combined application of biochar and sulfate to reduce soil methylmercury (MeHg) production and grain MeHg accumulation and revealed the underlying molecular mechanisms governing the interactions among sulfate, biochar, and Hg, based on both batch experiments and density functional theory calculations. Our findings demonstrate that the combined use of biochar and sulfate at optimal proportions can effectively promote rice crop growth while mitigating grain MeHg accumulation. This positive effect is attributed to the strong adsorption capacity of carboxylic and phenolic hydroxyl groups in biochar for soil Hg. Sulfate can further enhance the stability of adsorbed Hg­(II), forming a highly stable biochar-Hg­(–SO 4 )–Cl structure in soil, thereby mitigating MeHg production. Meanwhile, the suppression of Hg methylator activity, coupled with alterations in soil pH, and the chemical speciation of Fe and Mn upon biochar addition, indirectly counteracts sulfate-stimulated microbial Hg-methylation in paddy soils. This research highlights the effectiveness of a combined biochar and sulfate application strategy in Hg sequestration and reduction of MeHg accumulation in grains. By unveiling the molecular mechanisms underlying the interactions among sulfate, biochar, and Hg in soil, our study offers fresh insights for designing more efficient and safe strategies aimed at enhancing rice yields while mitigating the risks associated with soil Hg pollution.