Impact of dissolved organic matter on mercury and methylmercury sorption to activated carbon in soils: implications for remediation

Activated carbon (AC) amendments have shown promise in reducing inorganic mercury (Hg(II) complexes, "Hg") and methylmercury (MeHg) risk in contaminated soils. However, the effectiveness of AC in Hg and MeHg immobilization has varied among studies, suggesting that site biogeochemistry might dictate efficacy. In this study, we examined the effect of dissolved organic matter (DOM) on MeHg and Hg sorption to AC. We evaluated the impact of Suwannee River Humic Acid (SRHA) on sorption to AC directly using an isotherm approach and in a soil/AC mixture using slurry microcosms. Aqueous sorption coefficients to AC (log K-AC) for Hg-SRHA and MeHg-SRHA complexes were one to two orders of magnitude lower (Hg-SRHA = 4.53, MeHgSRHA = 4.35) than those for chloride complexes (HgCl2 = 6.55, MeHgCl = 4.90) and more closely resembled the log K-AC of SRHA (3.64). In anoxic, sulfidic soil slurries, the K-AC for sulfide species appeared stronger than for chloride or SRHA species for both Hg and MeHg. AC significantly reduced porewater concentrations of both ambient MeHg and a fresh (MeHg)-Hg-199 spike, and the addition of up to 60 mg L-1 SRHA did not reduce sorption to AC. The AC also reduced ambient Hg and Hg-201 porewater concentrations, but as SRHA concentration increased, the magnitude of solid phase sorption decreased. Speciation modeling revealed that SRHA may have impacted Hg distribution to the solid phase by reducing HgS precipitation. This study highlights the need for site-specific evaluation of AC efficacy and the value in developing biogeochemical models of AC performance for Hg control.