Potential of mercapto montmorillonites for immobilization remediation of Hg-contaminated paddy soil: Perspective from soil environmental quality

Consideration of the high exposure risk of mercury (Hg) in paddy system, the development highly-efficient and environmental-friendly amendments for the immobilization remediation of Hg-contaminated paddy soil is of significant importance for mitigating Hg-related health risks. In this study, the potential of 2-mercaptoethylam-monium intercalation, 3-mercaptopropyltrimethoxysilane covalent grafting and mechanochemical grafting mercapto montmorillonites (referred as ISH-Mt, GSH-Mt and BSH-Mt, respectively) for immobilization remediation of Hg contaminated paddy soil and its effects on soil environmental quality were investigated. Results indicated that the application of 0.1-1% (m/m) mercapto Mts exhibited much higher immobilization efficiency for Hg than pristine Mt., in which soil HCl and Na2S2O3 extractable Hg content were decreased by 5.13-62.44% and 8.03-82.24% as compared to CK. Hg fractions in mercapto Mts amended soils shifted form exchangeable and carbonate-bound to organic-bound as results of complexation by the loaded thiol groups. The response of soil environmental quality indicators to amendments varied with the preparation methods of mercapto Mts. Compared to CK, ISH-Mt exhibited greater effects on soil physicochemical properties than GSH-Mt and BSH-Mt, such as the decreased pH and the substantially increased EC (9.85-29.00%) and available S content (5.13-41.09%), implied the potential risk of soil acidification and thiol modifier leaching. Application of mercapto Mts enhanced soil enzyme activities of dehydrogenase, arylsulfatase and fluorescein diacetate hydrolase by 14.63-27.55%, 2.39-5.41% and 25.64-39.72%, respectively. While, GSH-Mt caused greater disturbance on bacterial community than ISH-Mt and BSH-Mt for the reason that GSH-Mt changed the composition of pre-dominant phyla, increased the abundance of methylators Anaerolinea and Geobacter and exhibited the most biomarkers (15) and differential metabolic pathways (44). Overall consideration Hg immobilization potential and secondary environment risk, BSH-Mt was a promising immobilization amendment for the remediation of Hg -contaminated paddy soil.