Performance, mechanism and environmental effect evaluation of thiol-functionalized montmorillonites for Hg-contaminated paddy soil remediation

Mercury (Hg) contamination in paddy soils has attracted increasing attention due to the health risks of human exposure to methylmercury (MeHg) through rice ingestion. To safely utilize Hg-contaminated soil and effectively reduce total Hg (THg) and MeHg accumulation in rice grains and its exposure risks, two types of thiol-functionalized montmorillonites developed by covalent grafting (CG-Mt) and mechanochemical grafting (MG-Mt) were as amendments for remediation of Hg-contaminated paddy soil. Pot environment results suggested that CG-Mt and MG-Mt application to soil (0.1%–1%, m/m) significantly reduced the accumulation of total THg and MeHg in rice grains by 40.3%–61.9% and 43.9%–62.3%, with the acceptable daily intake and target hazard quotient value of MeHg reduction by 39.5%-65.1% and 40.7%-64.4%. Application of CG-Mt and MG-Mt decreased soil available Hg and MeHg by 14.6%–95.4% and 19.4%–71.1%, respectively, which are directly responsible for the inhibition of THg and MeHg accumulation in rice. Structural equation model revealed that the changes in Hg methylation and demethylation related functional genes hgcAB, merB and merA indirectly determined THg and MeHg level in rice. In view of the stabilized soil pH, enhanced enzyme activities and less disturbance in bacterial community, MG-Mt is a promising soil amendment for mitigating THg and MeHg exposure risks of rice produced from Hg-contaminated paddy soils.