Environmental impacts of mercury contamination associated with mining

Approximately 7,500 tons of mercury (Hg) was discharged into the Carson River system, Nevada during precious metal mining processing associated with the Comstock Lode (1865-1895). At contaminated areas, the concentrations of Hg are some of the highest reported values in North America. Despite the fact that total mercury concentrations (Hg{dollar}rmsb{lcub}T{rcub}){dollar} in surface sediments of the Carson River system were in the ug/g range, levels of MeHg varied from about 2 to 28 ng Hg/g dry weight, representing less than 3% of Hg{dollar}rmsb{lcub}T{rcub}.{dollar} Concentrations of MeHg were well correlated with both the biotic (r = 0.95) and abiotic activity (r = 0.85) of the sediment. The reduction of methylation rate by high concentrations of added inorganic Hg ({dollar}>{dollar}15.3 ug/g) was found to be associated with an inhibition of microbial activity. The addition of group VI anions to sediment slurries result in the inhibition of the rate of Hg methylation. The ranking of inhibition is as follows: tellurate (TeO{dollar}sb4sp{lcub}2-{rcub})>{dollar} selenate (SeO{dollar}sb4sp{lcub}2-{rcub})>{dollar} molybdate (MoO{dollar}sb4sp{lcub}2-{rcub})>{dollar} tungstate (WO{dollar}sb4sp{lcub}2-{rcub}).{dollar} Increasing the sulfate (SO{dollar}sb4sp{lcub}2-{rcub}){dollar} concentration while using fixed concentrations of inhibitors led to the partial re-establishment of some MeHg production in WO{dollar}sb4sp{lcub}2-{rcub}{dollar} treated slurries, whereas, no such significant change (p {dollar}<{dollar} 0.05) was noticed in sediment slurries treated with MoO{dollar}sb4sp{lcub}2-{rcub}{dollar} and TeO{dollar}sb4sp{lcub}2-{rcub}.{dollar} Selenate and SO{dollar}sb4sp{lcub}2-{rcub}{dollar} showed a qualitatively similar effect on Hg methylation at concentrations tested in that, each showed stimulation at low concentrations and inhibition at high concentrations. The depression of MeHg formation by group VI anions was not accompanied by an inhibition of general microbial activity, suggesting that only particular microorganisms, such as sulfate-reducing bacteria, are responsible for Hg methylation.; Mercury cyanide complexes, primarily dicyanomercury Hg(CN){dollar}sb2,{dollar} are formed as by-products during extraction of precious metals from mercury-containing ore using cyanide solutions. In our studies, a new liquid chromatographic method was developed for the determination of Hg(CN){dollar}sb2{dollar} in water without decomposition of complexes. By using this method, we found that although Hg(CN){dollar}sb2{dollar} is unionized in water, its 1-octanol/water partition coefficient (Kow) at pH 4-7 is small {dollar}({lcub}<{rcub}10sp{lcub}-3{rcub}).{dollar} Dicyanomercury was found to be stable over a wide pH range of 3.6-8.0. At neutral and/or high pH, the addition of SO{dollar}sb4sp{lcub}2-{rcub},{dollar} Cl{dollar}sp-,{dollar} and CN{dollar}sp-{dollar} results in the conversion of Hg(CN){dollar}sb2{dollar} to other complexes.