Abstract
Toxic metalliferous mine-tailings pose a significant health risk to ecosystems and neighboring communities from wind and water dispersion of particulates containing high concentrations of toxic metal(loid)s (e.g., Pb, As, Zn). Tailings are particularly vulnerable to erosion before vegetative cover can be reestablished, i.e., decades or longer in semi-arid environments without intervention. Metal(loid) speciation, linked directly to bioaccessibility and lability, is controlled by mineral weathering and is a key consideration when assessing human and environmental health risks associated with mine sites. At the semi-arid Iron King Mine and Humboldt Smelter Superfund site in central Arizona, the mineral assemblage of the top 2m of tailings has been previously characterized. A distinct redox gradient was observed in the top 0.5m of the tailings and the mineral assemblage indicates progressive transformation of ferrous iron sulfides to ferrihydrite and gypsum, which, in turn weather to form schwertmannite and then jarosite accompanied by a progressive decrease in pH (7.3–2.3).Within the geochemical context of this reaction front, we examined enriched toxic metal(loid)s As, Pb, and Zn with surficial concentrations 41.1, 10.7, 39.3mmolkg−1 (3080, 2200, and 2570mgkg−1), respectively. The highest bulk concentrations of As and Zn occur at the redox boundary representing a 1.7 and 4.2-fold enrichment relative to surficial concentrations, respectively, indicating the translocation of toxic elements from the gossan zone to either the underlying redox boundary or the surface crust. Metal speciation was also examined as a function of depth using X-ray absorption spectroscopy (XAS). The deepest sample (180cm) contains sulfides (e.g., pyrite, arsenopyrite, galena, and sphalerite). Samples from the redox transition zone (25–54cm) contain a mixture of sulfides, carbonates (siderite, ankerite, cerrusite, and smithsonite) and metal(loid)s sorbed to neoformed secondary Fe phases, principally ferrihydrite. In surface samples (0–35cm), metal(loid)s are found as sorbed species or incorporated into secondary Fe hydroxysulfate phases, such as schwertmannite and jarosites. Metal-bearing efflorescent salts (e.g., ZnSO4·nH2O) were detected in the surficial sample. Taken together, these data suggest the bioaccessibility and lability of metal(loid)s are altered by mineral weathering, which results in both the downward migration of metal(loid)s to the redox boundary, as well as the precipitation of metal salts at the surface.
Highlights
Beneficiation of massive sulfide deposits for economic base metals such as Cu, Zn, and Pb results in mine-wastes that can be enriched in unextracted metals and metalloids (HUDSONEDWARDS et al, 2011; LOTTERMOSER, 2010)
The fate of toxic metal(loid)s at IKMHSS is controlled by ion mobility and secondary mineral precipitation as a result of primary sulfide and gangue mineral dissolution in the gossan zone
Solid phase products are dominated by ferric iron and sulfate minerals that, as shown act to buffer the release of toxic elements to the environment
Summary
Beneficiation of massive sulfide deposits for economic base metals such as Cu, Zn, and Pb results in mine-wastes that can be enriched in unextracted metals and metalloids (HUDSONEDWARDS et al, 2011; LOTTERMOSER, 2010). Mine tailings are characterized by poor soil structure, high soluble salts, high concentrations of phytotoxic elements, and low pH (HAMMARSTROM et al, 2005; YE et al, 2002), resulting in a lack of vegetative cover and substantially increased vulnerability to particle dispersion due to wind and water erosion (MENDEZ and MAIER, 2008). Mine tailings from arid and semi-arid climates pose a risk to human health from fugitive dust emissions in to neighboring communities. Arid and semi-arid conditions diminish the prevalence of perennial surface water and, the dispersion of acid mine drainage. The site was added to the National Priorities List in 2008 due to its proximity to the town of Dewey-Humboldt and the elevated (>2000 mg kg-1) concentrations of several toxic metal(loid)s, of particular concern were As and Pb (USEPA, 2014)
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