A simulation study of mercury immobilization in estuary sediment microcosm by activated carbon/clay-based thin-layer capping under artificial flow and turbation.

In-situ thin layer capping (TLC) is a promising sediment remediation approach that has been shown effective in immobilizing contaminants from releasing to natural biotas and human beings. This research intended to comprehend the effectiveness of Hg immobilization by TLC under turbation condition via a microcosm study. Three TLC caps with different activated carbon (AC)/clay combinations were applied to actual Hg-contaminated estuary sediment (76.0 ± 2.6 mg-Hg/kg). The caps with AC (3%) + bentonite (3%) and AC (3%) + kaolin (3%) were efficient in reducing both total mercury (THg) and methylmercury (MeHg) concentrations in overlying water by 75-95% and 64-98%, respectively, in the later stage of 75-d operation. In contrast, the AC (3%) + montmorillonite (3%) cap did not show a significant reduction on THg and MeHg in the overlying water, probably due to the unstable, suspension property of montmorillonite. The stable caps showed higher resistance to Hg breakthrough under occasional turbation events; however, a labile cap appeared to have dramatic Hg breakthrough when turbation occurred. It is therefore essential to note that with unstable caps, turbation events may result in unwanted secondary resuspension of contaminants.

Using raw and sulfur-impregnated activated carbon as active cap for leaching inhibition of mercury and methylmercury from contaminated sediment.

Sulfur-impregnated activated carbon (SAC) has been reported with a high affinity to Hg, but little research has done on understanding its potential as active cap for inhibition of Hg release from contaminated sediments. In this study, high-quality coconut-shell activated carbon (AC) and its derived SAC were examined and shown to have great affinity to both aqueous Hg2+ and methylmercury (MeHg). SAC had greater partitioning coefficients for Hg2+ (KD = 9.42 × 104) and MeHg (KD = 7.661 × 105) as compared to those for AC (KD = 3.69 × 104 and 2.25 × 105, respectively). However, AC appeared to have greater inhibition in total Hg (THg) leaching from sediment (14.2-235.8 mg-Hg/kg-sediment) to porewater phase as compared to SAC. 3 wt% AC amendment in sediment (235.8 mg/kg Hg) was the optimum dosage causing the porewater THg reduction by 99.88%. Moreover, significant inhibition in both THg and MeHg releases within the 83-d trial microcosm tests was demonstrated with active caps composed of SAC + bentonite, SAC + clean sediment, and AC + bentonite. While both AC and SAC successfully reduce the porewater Hg in sediment environment, the smaller inhibition in Hg release by SAC as compared to that by raw AC may suggest that possibly formed HgS nanoparticles could be released into the porewater that elevates the porewater Hg concentration.