Interfacial chemistry of mercury on thiol-modified biochar and its implication for adsorbent engineering

[Display omitted] •Binding mode, polymer morphology, and Hg ρatom are controlled by biochar structure.•The h2, K2 and qm are controlled by surface S/Si/O/N elements, charges, and defects.•Thiol groups promote the monolayer adsorption of CH3Hg+.•The oxides and precipitates are trapped by hydroxyl groups and π bonds. Mercury pollution in surface/groundwater has been a serious environmental problem that threatens human health. In recent years, thiol-modified biochars have exhibited excellent but unstable removal of mercury, which is likely to be affected by structure and interfacial chemistry, but the mechanism remains unclear. In this study, 3-mercaptopropyltrimethoxysilane (3-MPTS) was used as thiolation reagent, the interfacial chemical processes of mercury (Hg2+ and CH3Hg+) on thiol-modified biochar (porous and multilayer structures) were revealed, and the key molecular mechanisms were clarified. Specifically, the structure of biochar affected the binding mode of 3-MPTS (mainly through hydroxyl and π bonds), the morphology of the polymer (flocculent or spherical), and the surface Hg atomic density (e.g., ρatom was 1068.5 vs 3.0 #Hg/nm2 for CH3Hg+). Thiol-modified biochars exhibited more surface S, Si, O, N elements, more negative surface charges, and more surface defects, resulting in faster initial reaction rate (larger h2 and K2) and larger adsorption capacity (qm increased by 11–14 and 17–19 times, respectively) for Hg2+ and CH3Hg+ removal. The thiol groups played a major role, not only complexing with uncharged mercury species (Hg(OH)2 and CH3HgOH) but also promoting the monolayer adsorption of CH3Hg+. Although some thiol groups were oxidized or precipitated, the products (e.g., HgSO3 and HgS) were trapped by hydroxyl groups and π bonds. This study confirmed the potential of thiol-modified biochar in sorbent engineering with promising applications in mercury-contaminated surface and groundwater remediation.