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

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 pi bonds), the morphology of the polymer (flocculent or spherical), and the surface Hg atomic density (e.g., patom 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 pi bonds. This study confirmed the potential of thiol-modified biochar in sorbent engineering with promising applications in mercury-contaminated surface and groundwater remediation.

Automated summary

This study reveals the key molecular mechanisms of thiol-modified biochar in mercury removal and confirms its potential for remediation of mercury-contaminated surface and groundwater.