An XAS study of Hg(II) sorption to Al-based drinking water treatment residuals

Drinking water treatment residuals (DWTRs) are produced from the coagulation and flocculation processes in conventional drinking water treatment. The abundant metal oxide content of these materials resulting from the use of coagulants, like alum and ferric chloride, has driven strong research interest into the reuse of DWTRs as sorptive materials. Using a suite of aluminum-based DWTRs, we provide new insights into Hg(II) sorption mechanisms. Experiments performed at circum-neutral pH show that sorption capacities are related to the amount of organic carbon/matter present in DWTRs. We found that carbon rich samples can scavenge about 9000 mg/kg of Hg, in contrast to 2000 mg/kg for lime based DWTRs. X-ray absorption spectroscopy (XAS) at the Hg L3 edge further characterizes mercury coordination. X-ray absorption near edge structure (XANES) and extended x-ray absorption fine structure (EXAFS) results point to a partial association of mercury with sulfur at low mass loadings, transitioning to a full association with oxygen/carbon at higher concentrations of sorbed Hg(II) and in DWTRs with limited sulfur content. These results suggest that sorption of Hg(II) is primarily controlled by the carbon/organic matter fraction of DWTRs, but not by the coagulants.