INVESTIGATION OF CHANGES IN LEAD SPECIATION DURING MINERAL WEATHERING IN ARID MINE TAILINGS

Mine tailings frequently host elevated concentrations of toxic metals, such as lead (Pb), and may represent a significant hazard to surrounding communities and ecosystems. Tailings deposited in arid and semi-arid environments are particularly vulnerable to wind (and water) erosion that transports Pb into communities for inhalation or ingestion. Since exposure concentrations do not directly correlate with blood Pb levels, assessing speciation is a critical step toward understanding health risks associated with Pb in mine tailings. Speciation of Pb in tailings at the Klondyke State Superfund Site (Graham County, AZ) was studied using chemical extractions, micro-focused X-ray fluorescence mapping and X-ray absorption fine structure (XAFS). Lead-bearing phases identified in the tailings by XAFS were: plumbojarosite (PbFe₆(SO₄)₄(OH)₁₂), anglesite (PbSO₄), and Pb associated with iron oxides. Bulk XAFS-derived estimates of the fraction of plumbojarosite and anglesite to be 63-100% and 0-36%, respectively, with the largest fraction of anglesite found in tailings collected from the top 30 cm. Lead, initially deposited as galena (PbS), was quickly (within 50 years) transformed to plumbojarosite and continued weathering resulted in iron oxide-associated Pb and anglesite. The clay size fraction was also found to be highly (2-3 times by mass) enriched in Pb relative to the bulk tailings, suggesting the Pb in surficial tailings would be readily transported offsite. Surficial enrichment of Pb in bioavailable anglesite indicates the Pb associated with these tailings represent a larger potential health hazard than would be indicated by analysis of bulk tailings.