Geological and anthropogenic factors influencing mercury speciation in mine wastes: an EXAFS spectroscopy study

The speciation of Hg is a critical determinant of its mobility, reactivity, and potential bioavailability in mine-impacted regions. Furthermore, Hg speciation in these complex natural systems is influenced by a number of physical, geological, and anthropogenic variables. In order to investigate the degree to which several of these variables may affect Hg speciation, extended X-ray absorption fine structure (EXAFS) spectroscopy was used to determine the Hg phases and relative proportions of these phases present in Hg-bearing wastes from selected mine-impacted regions in California and Nevada. The geological origin of Hg ore has a significant effect on Hg speciation in mine wastes. Specifically, samples collected from hot-spring Hg deposits were found to contain soluble Hg-chloride phases, while such phases were largely absent in samples from silica-carbonate Hg deposits; in both deposit types, however, Hg-sulfides in the form of cinnabar (HgS, hex.) and metacinnabar (HgS, cub.) dominate. Calcined wastes in which Hg ore was crushed and roasted in excess of 600 °C, contain high proportions of metacinnabar while the main Hg-containing phase in unroasted waste rock samples from the same mines is cinnabar. The calcining process is thought to promote the reconstructive phase transformation of cinnabar to metacinnabar, which typically occurs at 345 °C. The total Hg concentration in calcines is strongly correlated with particle size, with increases of nearly an order of magnitude in total Hg concentration between the 500–2000 μm and <45 μm size fractions (e.g., from 97–810 mg/kg Hg in calcines from the Sulphur Bank Mine, CA). The proportion of Hg-sulfides present also increased by 8–18% as particle size decreased over the same size range. This finding suggests that insoluble yet soft Hg-sulfides are subject to preferential mechanical weathering and become enriched in the fine-grained fraction, while soluble Hg phases are leached out more readily as particle size decreases. The speciation of Hg in mine wastes is similar to that in distributed sediments located downstream from the same waste piles, indicating that the transport of Hg from mine waste piles does not significantly impact Hg speciation. Hg L_III-EXAFS analysis of samples from Au mining regions, where elemental Hg(0) was introduced to aid in the Au recovery process, identified the presence of Hg-sulfides and schuetteite (Hg₃O₂SO₄), which may have formed as a result of long-term Hg(0) burial in reducing high-sulfide sediments.