Effect of Cinnabar Age on Release of Aqueous and Volatile Mercury

Mercury release from cinnabar (α-HgS) in aqueous solutions was evaluated as a function of the age of cinnabar after pretreatment. Seventy-three batch experiments were conducted with powdered cinnabar aged from <2 h up to 7.4 y after cleaning in 10% nitric acid to remove fines and suspected surface alteration products. Cinnabar was reacted with water and the amounts of released aqueous and volatile mercury were quantified. Solutions were in equilibrium with either air or nitrogen/argon atmospheres, and some air-equilibrated solutions were also adjusted with sodium chloride and/or nitric acid or sodium hydroxide. Mercury release from cinnabar of a known age was 10 times higher after 30 min than after 1 d, indicating that a pulse of mercury is released initially upon solution/sample contact, and then re-adsorbed to the cinnabar surface. At near-neutral pH, in the presence of oxygen, the amount of total released Hg increased systematically with age to a value 100 times higher at 656 d age (1.58 x 10-4 mol Hg m-2 HgS) than at < 0.1 d age (1.85 x 10-6 mol Hg m-2 HgS) in 30 min reactions. Released Hg decreased from 656 d to 2453 d age (9.89 x 10-6 mol Hg m-2 HgS). Mercury release was enhanced by two times in the absence of oxygen. Mercury(II) was the dominant species released in most experiments and generally followed the same pattern as total Hg with cinnabar age. Mercury(0) peaked at 6 d and decreased systematically with increasing age up to 656 d, and was variable from 2453 to 2709 d aged cinnabar. These observations suggest that changes in surface composition caused the variation in mercury release over time. Mercury release normalized to initial surface area of the cinnabar decreased with increasing mass. This relationship along with observations of hydrophobic aggregation and particle dispersion also indicate that the surface/solution interaction changed with increasing cinnabar age and may have caused the high variability observed in some replicate experiments. The patterns of released mercury in these experiments imply that cinnabar may not be a permanent sink for mercury in the environment.