Thiol-functionalized montmorillonite prepared by one-step mechanochemical grafting and its adsorption performance for mercury and methylmercury

The preparation of low-cost and highly efficient functional materials for the cleanup of mercury-contaminated water by adsorption in an environmentally friendly way is of great significance. In this study, thiol-functionalized montmorillonite (BSH-MMT) was prepared by a novel one-step mechanochemical grafting method and applied to aqueous Hg2+ and CH3Hg+ adsorption. Characterization results showed that thiol groups were successfully grafted by chemical bonding with Si-OH or broken SiO bonds. The maximum adsorption capacities of BSH-MMT for Hg2+ and CH3Hg+ were 104.79 mg g-1 and 39.27 mg g-1, which were approximately seven- and nine-fold that of pristine MMT, respectively. Adsorption kinetics and isotherm fitting indicated that Hg2+ adsorbs heterogeneously, while CH3Hg+ proceeds through monolayer adsorption, both with chemical adsorption as the rate-limiting step. BSH-MMT maintained high adsorption performance over a wide pH range and in the presence of humic acid because of the high affinity of thiol groups toward mercury. The primary adsorption mechanism of thiol-ligand complexation was confirmed by the results of X-ray photoelectron spectroscopy and X-ray absorption spectra, in which a complex structure of bis-coordinated S-Hg-S (2.30 Å distance) was observed. These results demonstrated that mechanochemical grafting is a promising one-step method to prepare thiol-functionalized montmorillonite for effective cleanup of Hg2+/CH3Hg+ contamination in water.