Oxidant-Free Thiol–Ene Grafting of Granular Activated Carbon for Mercury(II) Adsorption from Aqueous Solution and Synthetic Gas Condensate

Abstract

Mercury removal from aqueous and hydrocarbon media requires sorbents that combine strong binding affinity with sufficient pore accessibility and mass transfer. In this study, granular activated carbon (GAC) was treated with 1,2-ethanedithiol through an oxidant-free, thermally initiated thiol–ene-type radical route to produce sulfur-functionalized GAC (S-GAC) with different sulfur loadings. The sulfur content increased from 3.38 to 15.07 wt %, while the BET specific surface area decreased from 859 to 25 m2 g–1, indicating progressive pore occupation and blockage. At low initial Hg(II) concentration, sulfur functionalization enhanced Hg(II) removal in both aqueous solution and synthetic gas condensate, with 10–30%S-GAC achieving very low residual Hg(II) concentrations. At elevated aqueous Hg(II) concentrations, pristine GAC exhibited faster initial uptake and approached a plateau earlier, whereas S-GAC showed slower but sustained adsorption. At approximately 40 mg L–1 Hg(II) concentration, 5%S-GAC and 10%S-GAC ultimately achieved slightly higher observed uptakes than pristine GAC, while at approximately 120 mg L–1 pristine GAC retained the highest uptake over the monitored period. These results show that the effect of sulfur functionalization depends on Hg(II) loading, contact time, and the balance between Hg–S affinity and pore accessibility. XPS, Hg L3-edge XAS, and DFT calculations were consistent with sulfur-containing Hg coordination environments and more favorable interaction near sulfur-rich edge configurations. Sulfur functionalities remained strongly retained over pH 2–10, although regeneration performance decreased with repeated cycles. Overall, Hg(II) capture reflects the combined effects of sulfur-site affinity, pore accessibility, mass transfer, Hg(II) loading, and the treatment matrix.