MERCURY MOBILIZATION BY OXIDATIVE DISSOLUTION OF CINNABAR (α-HGS) AND METACINNABAR (β-HGS)

Abstract

Cinnabar (α-HgS) and metacinnabar (β-HgS) dissolved at environmentally significant rates in oxygenated slurry experiments simulating a low-flow fluvial system. Based on SO42- production, cinnabar dissolution rates were 2.64 to 6.16 μmol (SO42-) m- 2 day- 1, and metacinnabar dissolution rates were 1.20 to 1.90 μmol (SO42-) m- 2 day- 1. Monodentate-bound thiosulfate (S2O32-) was identified as an oxidation product on the HgS surface by ATR-IR spectroscopy based on strong infrared absorption bands in the 1140-1145 cm- 1 and 1006-1014 cm- 1 regions. The presence of sulfide oxidation intermediates on the HgS surface indicates that SO42- concentration underestimates α-HgS and β-HgS dissolution in this setting. Mercury release rates during dissolution were more than two orders of magnitude less than SO42- production, but were significant: 0.47 mg (Hg) m- 2 y- 1 from cinnabar [6.45 nmol (Hg) m- 2 day- 1], and 0.17 mg (Hg) m- 2 y- 1 from metacinnabar [2.29 nmol (Hg) m- 2 day- 1]. The Hg mobilized during α-HgS and β-HgS dissolution is sufficient to form natural Au-Hg amalgam in downstream placer settings. The proportion of mercury that is not remobilized during α-HgS and β-HgS dissolution likely adsorbs to the dissolving mercuric sulfide. Adsorption of Hg2+ to cinnabar was detected in situ by anodic stripping voltammetry using a cinnabar-modified carbon paste electrode following accumulation of Hg2+ on the electrode at open circuit potential. © 2007 Elsevier B.V. All rights reserved.