Mercury speciation transformation mediated by thiolated biochar in high salinity groundwater: Interfacial processes, influencing factors, and mechanisms

Groundwater, a vital freshwater source, faces mercury contamination threats, impacting ecosystems and human health. Removing low concentrations of mercury from high-salinity groundwater has always been a challenge. This study demonstrates the effectiveness of thiol-modified biochar (BCS) in decontaminating mercury from saline groundwater by mediating mercury speciation transformation. Initially, mercury species (e.g., HgClOH, Hg(OH)2, HgCl2, HgOHCO3–) selectively attach to thiol groups, leading to an increase in surface zeta potential and a reduction in electrostatic attraction to calcium species (e.g., CaCO3 and CaSO4). Then, cations bound to thiol groups (e.g., Na+, Ca2+, and CaHCO3+) undergo ion exchange with mercury species (e.g., Hg2+, HgOH+, and HgCl+). Finally, mercury species (e.g., Hg(OH)2, HgSO4, and HgS) gradually precipitate or co-precipitate with calcium minerals (e.g., calcite, aragonite, and vaterite). Compared with biochar (BC), BCS exhibits a larger mercury adsorption capacity (qm = 4.30 ± 0.13 mg g−1) and a faster initial reaction rate (h2 = 4.12 × 10–2 mg g−1 min−1). BCS withstands pH variations, small organic molecules, and coexisting heavy metal ions but exhibits slightly reduced performance in higher ionic strength and with large organic molecules. Additionally, BCS serves as an effective permeable reactive barrier against flowing mercury-contaminated groundwater, the Adams-Bohart and Yan models effectively describe the breakthrough curves. These findings provide scientific support for BCS's application in remediating high-salinity and trace mercury-contaminated groundwater.