Using thiol-functionalized montmorillonites for chemically immobilizing Hg in contaminated water and soil: A comparative study of intercalation and grafting functionalization

Mercury (Hg) contamination in water and soil is of great concern due to its high toxicity and biomagnification in food chain. In this study, clay mineral montmorillonites (Mt) was thiol-functionalized via 2–mercaptoethylammonium intercalation (ISH–Mt) and 3–mercaptopropyltrimethoxysilane surface grafting (GSH–Mt) for enhancing remediation ability of Hg contamination. Characterization results indicated that surface grafting significantly changed surface morphology and porous structure, with rougher surface, larger specific surface area and more porous structure. The adsorption capacity of Hg ²⁺ was significantly enhanced by thiol functionalization, being 141.55 and 136.92 mg g ⁻¹ for ISH–Mt and GSH–Mt, respectively. Fitting results of pseudo-second-order kinetic models ( R ² = 0.879– 0.930) and Freundlich isotherms ( R ² = 0.900– 0.950) indicated that Hg ²⁺ was adsorbed on heterogeneous surfaces with chemical processes as the rate-limiting step. TCLP-Hg concentrations in soils after applying ISH–Mt and GSH–Mt (0.1%–1% dosage, m/m) were < 200 μg L ⁻¹ (The Leaching Standards of Hazardous Wastes, USEPA). Hg fractionation shifted from mobile fractions (exchangeable and carbonate-bound) to stable forms (organic matter-bound). XPS and Hg L _III-edge EXAFS studies revealed that the complexation of –SH toward Hg was the primary mechanism for Hg adsorption and stabilization. These results illustrated that thiol-functionalized Mts prepared by intercalation and surface grafting are promising for the remediation of Hg-contaminated water and soil.