Elevated concentrations of Hg in a range of environmental compartments (e.g., rivers, sediment, lakes, and oceans) are a world-wide concern, especially for sediment that is a sink for Hg accumulation. Different remediation methods have been evaluated for mercury removal or stabilization. These methods include sediment dredging, amendment using activated carbon or biochar, etc. However, most of the remediation methods are expensive and the long-term fate of Hg has not been well investigated.

Here we investigated the effectiveness of activated carbon and four types of biochar, including (switchgrass (300°C and 600°C), poultry manure (600°C), and oak (~700°C)) to stabilize Hg in contaminated sediment under anaerobic conditions for 1030 days. The feed stocks of these biochars are widely available and can be used for biochar production at low cost. The results indicate aqueous concentrations of total Hg and methylmercury generally decreased in the presence of biochars, with the exception of a spike in methylmercury concentration observed at ~440 days in the switchgrass at 600°C and oak biochar systems. The change of total Hg and methylmercury in sediment controls and amended systems corresponded to the observation of potential mercury methylators and change of carbon sources for the microbes, and total Fe and sulfate concentrations in solution.

Identification of the mechanisms controlling Hg uptake by these biochar particles was determined using X-ray fluorescence imaging (XRFI) and X-ray absorption spectroscopy (XAS) techniques, including micro-X-ray fluorescence (micro-XRF) mapping, confocal micro-X-ray fluorescence imaging (CMXRFI), Hg extended X-ray absorption fine structure (EXAFS), and S X-ray absorption near-edge structure (XANES). Micro-XRF maps and CMXRFI show that Hg co-exists with S, Cu, Fe, Mn, and Zn on the surface and inside the biochar particles. EXAFS modeling shows that Hg is in an oxide form on the surface of an iron (hydro)oxide particle from fresh sediment and in Hg-sulfide forms in biochar samples. S XANES analyses show that sulfide is present within the biochar particles. After amendment with biochars, a fraction of the Hg originally present in unstable forms (dissolvable, HgO, colloidal, nano, etc.) in the sediment was likely stabilized as less soluble Hg-sulfide phases on the surface or within the biochar particles.

Based on the aqueous and solid phase analysis, three primary geochemical stages were assigned to the microcosm experiment, including nitrate reduction, overlapped iron and sulfate reduction, and methanogenic stages. Methylmercury was primarily generated during the overlapped iron and sulfate reduction stages. Total Hg was under control after the overlapped iron and sulfate reduction stage. The Hg that accumulated in the biochar through this process is expected to remain stable for a prolonged period. Further, this process may also be extended to the accumulation of other hazardous metal elements.