Redistribution of soil mercury species mediated by thiolated biochar under dry–wet cycles

Abstract Frequent heatwave events increase the frequency of soil dry–wet cycles, accelerating soil weathering and indirectly triggering mercury (Hg) activation risks. Thiol-modified biochar (TMB) has been widely demonstrated as an effective environmental material for immobilizing soil Hg. However, whether TMB can maintain long-term stability under frequent dry–wet conditions, and how it regulates key soil components to influence Hg behavior, remain unknown. In this study, 30 cycles of simulated dry–wet alternation confirm that TMB promotes soil mineral weathering to redistribute Hg species, thereby reducing total Hg leaching under acid rain, lowering Hg bioavailability, and decreasing net methylmercury accumulation. Specifically: (i) TMB accelerates the dissolution of soil calcium carbonate (CaCO 3 ) by 19.1%, reducing the total proportion of exchangeable and carbonate-bound Hg by 89.7%, while increasing soil pH by 8.5% and decreasing zeta potential by 1.7-fold, which favors Hg precipitation and electrostatic adsorption by soil particles; (ii) TMB facilitates the transformation of Fe/Al oxides (e.g., Fe 2 O 3 , Al 2 O 3 ) into hydroxylated forms with stronger binding capacity (e.g., FeO·OH, Al(OH) 3 ), leading to decreased residual-bound Hg and increased oxide-bound Hg; (iii) TMB enhances the release of soil organic matter (e.g., mineral-associated), which improves Hg complexation capacity and reduces its availability, thereby lowering net methylmercury accumulation. Additionally, TMB increases the relative abundances of Bacillales, Gemmatimonadales, and other taxa in soil microbial community, and enhances the species richness and evenness. This study highlights the critical role of TMB-mediated mineral weathering in promoting Hg redistribution, elucidates the intrinsic mechanism underlying long-term Hg immobilization by TMB under heatwave-accelerated dry–wet conditions, and provides a theoretical basis for the sustainable engineering application of TMB in Hg-contaminated soil remediation. Graphical Abstract