Colloid-Facilitated Hg Transport from New Idria and Sulfur Bank Mercury Mine Tailings

Mercury contamination in aquatic environments poses significant health risks for both human and aquatic life. Mercury release from inoperative mines in the California Coast range has been documented, but little is known about the release and transport mechanisms. Transport of Hg and other heavy metals by colloids has been implicated in a number of studies, so colloidal Hg may constitute a significant fraction of total Hg released from mines. In this study, mine wastes were characterized using analytical and spectroscopic techniques, and column experiments were performed using selected Hg-mining wastes. Our objectives were to determine (1) the significance of colloid-facilitated Hg transport from selected mine sites, and (2) the effect of primary mineral composition and ore processing on the Hg release potential. Tailings (calcines and waste rock) from the New Idria (NI) and Sulfur Bank (SB) Hg mines in CA were dry sieved into 9 size fractions, and the physical and chemical properties of each fraction were determined using CVAFS, ICP-AES, XRD, BET, SEM/EDAX, ATEM, and EXAFS. Total Hg concentration increased with decreasing particle size in calcines (NI and SB), but reached a maximum at an intermediate particle size range in the SB waste rock. Also, there was not a distinct inverse relationship between particle surface area and Hg concentration, suggesting that Hg is not present primarily as a sorbed species. XRD results indicate differences in the primary matrix minerals for samples from each location, and ATEM analysis of the d_p<45 micron size fraction indicated the presence of HgS as well as an amorphous Si/Al-containing phase. SEM and ATEM revealed that the NI particles are aggregates of sub-micron size particles, suggesting that colloid release and migration through the column are likely. Hg-EXAFS results indicate that more than 50% of the Hg in the NI samples and more than 89% of the Hg in the SB samples consists of low-solubility HgS species rather than soluble or sorbed Hg species. Chromatographic columns filled with mine wastes were preconditioned by leaching with 100 pore volumes of 0.1M NaCl. Colloid generation was then initiated by leaching with 5-mM NaCl. NI calcines released significant quantities of Hg-bearing colloids that accounted for greater than 95% of the Hg exiting the column. Effluent Hg concentrations ranged from 75 mg/L during preconditioning to greater than 2000 mg/L during colloid generation. Colloid and Hg releases from SB waste rock were similar to that observed in experiments using NI calcines. The SB calcines behaved differently however, as no observable colloids were released from the SB calcine material during the preconditioning step or during the colloid generation step. Release of colloidal Hg from mine wastes can be a significant component of mercury loadings to watersheds impacted by mercury mining. The primary mineral composition of the mine waste as well as ore processing (calcining) affect the quantity and speciation of Hg transported from mine waste sites.