Effects of Mineral Weathering and Plant Roots on Contaminant Metal Speciation and Lability in Arid Lead-Zinc Sulfide Mine Tailings at the Klondyke Superfund Site, Graham County, AZ

Abstract

Historic mine tailings pose a significant health risk to surrounding ecosystems and communities because of high residual concentrations of contaminant metals. The initial tailings mineral assemblage, metal sulfides, silicates, and carbonates are unstable at earth surface conditions and undergo oxidative and proton-promoted weathering. The weathering of metal sulfides generally produces acid that, if not balanced by protonconsuming dissolution of silicates and carbonates, leads to progressive acidification. The Klondyke State Superfund Site in Graham County, Arizona contains high concentrations of Pb (up to 13 g kg⁻¹) and Zn (up to 6 g kg⁻¹), and remains unvegetated 50 years after mining cessation. Field-scale investigation revealed a wide range of pH (2.5-8.0) and plant-available (DTPA-extractable) metals in the near surface of the tailings pile. Four samples were chosen for in-depth characterization ranging in pH, as denoted by subscript, from 2.6 to 5.4. The mineral transformations occurring in these four samples were investigated using a variety of techniques and the data indicated an increase in tailings weathering extent with increasing acidification (decreasing pH). Lead speciation, studied by a combination of chemical sequential extraction and X-ray absorption fine structure (XAFS) spectroscopy, was found to vary with tailings depth. The principle lead-bearing mineral was plumbojarosite (PbFe₆(SO₄)₄(OH)₁₂), with smaller amounts of anglesite (PbSO₄) and lead-sorbed iron-oxide. Anglesite, the most bioavailable mineral form of Pb in the tailings, was found to accumulate at the tailings surface, which has important implications for health risks. Total Zn content decreased by an order of magnitude (from 6 to 0.4 g kg⁻¹) and showed a change in molecular speciation with decreasing pH. Zinc-rich phyllosilicates and Zn-containing manganese oxides predominate at high pH, whereas low pH samples contained principally Zn-sorbed iron oxides. One of the overarching goals of the project is to remediate the Klondyke site using phytostabilization to keep contaminant metals from eroding offsite either by wind or water transport mechanisms. However, the impacts of plant growth on metal bonding environment are unknown. To address that gap in knowledge, we have developed a technique for the study of root-microbe-mineral-metal interactions that occur in the rhizosphere, the volume of soil surrounding, and affected by, plant roots. This technique involves the conjunctive use of fluorescence in-situ hybridization, X-ray fluorescence elemental mapping, XAFS and Raman micro-spectroscopies, and electron microscopy on single roots. Manganese and iron root plaques collocalized with elevated Pb, Zn, and Cr demonstrate that the rhizosphere can affect metal speciation. Metal speciation is an important factor in determining metal bioavailability, and thus is critical for understanding the health risk associated with mine tailings. The results of this research provides site-specific information about Pb and Zn speciation, which will be used to evaluate the effectiveness of site remediation within the context of metal toxicity.