Thiol-modified biochar synthesized by a facile ball-milling method for enhanced sorption of inorganic Hg2+ and organic CH3Hg+

Modification of thiol on biochar often demands complex synthetic procedures and chemicals. In this work, a simple and environment friendly thiol-modified biochar (BMS-biochar) was successfully synthesized by ball milling pristine biochar with 3-mercaptopropyltrimethoxysilane (3-MPTS). The resultant BMS-biochar was characterized and tested for aqueous inorganic Hg2+ and organic CH3Hg+ removal. Characterization results showed that 3-MPTS was loaded on the surface of biochar through oxygen-containing functional groups (i.e., OH and CO) and π–π bond. Ball milling method improved the properties of BMS-biochar, namely, more efficient SH load, a larger surface area, more functional groups, more negatively charged surface, which resulted in higher removal efficiency of Hg2+ and CH3Hg+ (320.1 mg/g for Hg2+ and 104.9 mg/g for CH3Hg+) compared to the pristine biochar (105.7 mg/g for Hg2+ and 8.21 mg/g for CH3Hg+) and thiol-modified biochar through chemical impregnation (CIS-biochar) (175.6 mg/g for Hg2+ and 58.0 mg/g for CH3Hg+). Ball milling increased the sorption capacities of Hg2+ and CH3Hg+ through surface adsorption, electrostatic attraction, ligand exchange, and surface complexation. Modeling results suggested that the surface diffusion was the rate-limiting adsorption step for BMS-biochar. This work gave prominence to the potential of ball milling for the preparation of thiol-modified biochar to remove mercury especially organic CH3Hg+ by adsorption.