Highly efficient removal of aqueous Hg2+ and CH3Hg+ by selective modification of biochar with 3-mercaptopropyltrimethoxysilane

Biochars pyrolyzed at different temperatures were thiol-modified using 3-mercaptopropyltrimethoxysilane (3-MPTS) and applied on aqueous Hg2+ and CH3Hg+ removal with high efficiencies. The structure and physicochemical properties of biochar before and after thiol-modification were characterized. The biochar pyrolyzed at low temperature had more active sites (-OH, C=O, C-O-C, and pi bond) for surface modification with 3-MPTS. Pseudo-second-order kinetic model and Langmuir isotherm model fitted well with the sorption data. Among all the raw and thiol-modified biochars, 3BS (thiol-modified biochar from pyrolysis at 300 degrees C) had the largest Langmuir adsorption capacities of Hg2+ (126.62 mg/g) and CH3Hg+ (60.76 mg/g). The introduction of -SH enhanced the selectivity of biochar for mercury sorption (especially CH3Hg+). Ligand exchange and complexation between surface active sites (-SH, -OH, -COOH, and -NH2) and mercury were the dominant removal mechanisms. Among them, -SH played a major role. With the increase of 3-MPTS content in the preparation of 3BS, the adsorption capacity of Hg2+ and CH3Hg+ increased first and then decreased (the optimal content of 3-MPTS was 2%). Natural organic matter (NOM), glucose and humic acid (0-24 mg/L) had little effect on the adsorption of Hg2+ and CH3Hg+ by 3BS (the mercury removal rate only changed by 1.2-9.4%). This study demonstrates potential of thiol-modified biochar for mercury (especially CH3Hg+) removal from water.