Abstract

The study aims to stabilize elemental mercury to mercury sulphide and investigate the effect of various groundwater interferences like chloride, nitrate, sulphate and bicarbonate on the dissolution of mercury from mercury sulphide and elemental mercury. Elemental mercury was stabilized using sodium polysulphide solution in a one-step batch experiment in a period of 96 h. Mercury sulphide was formed as a black fine powder with average particle size of 10–500 nm. Mercury sulphide was tested under different pH conditions and under different concentrations of Cl⁻, NO₃⁻, SO₄²⁻, and HCO₃⁻ for dissolution of mercury. At pH ≤ 4 and pH ≥ 12, dissolved mercury concentrations from mercury sulphide were 70.47 μg/L and 41.81 μg/L, respectively. At pH 10, mercury dissolution was the lowest with dissolved mercury as 8 μg/L proving that mild alkaline pH is necessary for stability of mercury. At low concentration of the anions, interfering effects were in the order of NO₃⁻ > Cl⁻ > SO₄²⁻ > HCO₃⁻, whereas under high concentrations, the order was Cl⁻ > NO₃⁻ > SO₄²⁻ > HCO₃⁻. Unlike NO₃⁻ and Cl⁻, SO₄²⁻ and HCO₃⁻ ions were reported to cause potential leaching only if they are present in significantly high concentrations. Nevertheless, mercury sulphide was found to be the preferred chemical state for permanent storage of mercury in the subsurface when compared to elemental mercury.

Graphic abstract