MERCURY SPECIATION AND TRANSFORMATION IN MINE WASTES COLLECTED FROM ABANDONED MERCURY MINES IN THE USA

Mercury mines worldwide contain highly elevated Hg contents, but more problematic environmentally are elemental Hg and soluble Hg salts produced during ore retorting, which typically remain in wastes at mine sites. Under certain conditions, these inorganic Hg compounds convert to bioavailable, highly toxic organic Hg forms, which are subsequently taken-up by aquatic organisms. Speciation and transformation of Hg was studied in mine wastes collected from abandoned Hg mines at McDermitt, Nevada, and Terlingua, Texas, which are of moderate size on an international scale and produced about 10,000 and 5,000 t of elemental Hg, respectively. In mine waste samples, we measured total Hg and methyl-Hg contents, identified various Hg compounds using Hg-thermo-desorption pyrolysis, and determined rates of Hg methylation and methyl-Hg demethylation using isotopic-tracer methods.

Mine wastes contain as much as 14,000 mg/g total Hg, and methyl-Hg concentrations as high as 88 ng/g. Pyrolysis analysis of mine wastes showed variable amounts of cinnabar, metacinnabar, Hg salts, elemental Hg, and elemental Hg sorbed onto particulates such as clay and Fe-oxides. Mine wastes with the highest methyl-Hg contents correspond to those with elemental Hg and particulate-sorbed elemental Hg. Mine wastes with high methyl-Hg contents also produced the highest laboratory estimated potential Hg methylation rates, as much as 4.8 %/day. Samples containing dominantly cinnabar showed little or no Hg methylation. Mine wastes with high methyl-Hg contents generally showed low methyl-Hg demethylation, suggesting that Hg methylation was dominant. Both of these mines are located in semiarid climates, and during this study, there was no water runoff from the mines and streambeds below the mines were dry. Compared to Hg contents in mine wastes, total Hg contents in stream sediments collected below the mines show significant dilution, and methyl-Hg contents were typically below the limit of determination. Methylation of Hg downstream from Hg mines is probably lower in arid climates due to the lack of mine-water runoff and lower microbial activity. The correspondence of mine wastes containing elemental Hg and high methyl-Hg contents suggests that Hg(0) oxidizes to Hg(II), which is subsequently bioavailable for microbial Hg methylation.