Chemical stability of Hg-adsorbed pyrite under different pH and redox conditions: A fundamental study for stable management of Hg-bearing wastes

Abstract

Along with the worldwide awareness of mercury (Hg) toxicity, safe disposal and preservation of Hg-bearing waste becomes a matter of concern. In this study, Hg adsorbed pyrite, which is a type of Hg-bearing wastes, was synthesized and subjected to leaching tests at different pH (2, 6, and 10) and dissolved oxygen concentrations (aerobic and anaerobic) for 168 h in order to evaluate the chemical stability of the adsorbed Hg. Hg solubilization after 168 h leaching was negligibly small (0.13 µmol/L) at pH 10 under aerobic conditions while robust sulfur solubilization here (29.4 mmol/L), resulting from oxidative dissolution of pyrite, was expected to trigger the Hg desorption. Under anaerobic conditions, 35 times more Hg was released at pH 10 than pH 2, which was the opposite trend from aerobic conditions. X-ray photoelectron spectroscopy (XPS) and X-ray absorption fine structure (XAFS) analyses found that Fe (oxy)hydroxides were formed on the pyrite surface through its oxidative dissolution, contributing to the re-immobilization of once-desorbed Hg via the formation of surface complexation. Little Fe (oxy)hydroxide formed under anaerobic conditions, thus leading to the enhanced Hg release at alkaline than acidic pH values. Numerical calculation with a geochemical modeling confirmed that this hypothesis was valid and allowed the Hg distribution during the leaching of Hg-bearing pyrite to be quantified. Overall, we developed some suggestions for the stable management of Hg-bearing waste at the final waste disposal site based on the findings of this study.