Environmental application of mineral sulfides for removal of gas-phase Hg(0) and aqueous Hg²⁺

Synthesized and commercially available metal sulfides were evaluated for their ability to adsorb elemental and ionic mercury. The coinage group metal sulfides adsorb elemental Hg(0) vapor stoichiometrically. The mechanism of Hg(0) uptake by the copper and gold sulfides is a redox process resulting in the formation of HgS. The mechanism in the case of Ag₂S involves redox but there is no HgS formation. The relative rates of Hg(0) adsorption increases in the order CuS > Ag₂S > Au₂S > Au₂S₃ corresponding to the metal ion reduction potentials. The relative rate of Hg(0) adsorption for commercial grade CuS is increased by an activation process which involves making a slurry of the metal sulfide in concentrated oxalic acid followed by drying and then exposure to Hg(0). Relative rates of Hg(0) adsorption were also increased by decreasing the particle size of the metal sulfides. Particle sizes were decreased by synthesis of the metal sulfide in the presence of the particle-size mediating agent CTAB. The metal sulfides remove ionic mercury from acidic solutions by precipitation with the dissolved sulfide forming HgS. In acidic solution there was; no evidence of physical or chemical adsorption between Hg(0) and the metal sulfide. The quantity of ionic Hg²⁺ removal from aqueous solutions is correlated with the solubility of the metal sulfide. Cu₂S was the most soluble metal sulfide tested and thus removed the most ionic mercury from solution by precipitating HgS.