Toxic metal(loid) speciation during weathering of iron sulfide mine tailings under semi-arid climate |
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| Institution: | 1. Department of Soil Science and Agricultural Engineering, Federal University of Paraná State, Rua dos Funcionários, 1540, Cabral, 80.035-050 Curitiba, PR, Brazil;2. School of Geography and Environmental Science, M087, The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia;3. Brazilian Agricultural Research Corporation, Estrada da Ribeira km 111, C.P. 319, 83411-000 Colombo, PR, Brazil;1. Terrestrial Paleoclimatology Research Group, Department of Geology, One Bear Place #97354, Baylor University, Waco, TX 76798-7354 USA;2. Department of Earth & Planetary Sciences, Rutgers University, Piscataway, NJ 08854-8066, USA;1. Department of Environmental Geosciences, Faculty of Environmental Sciences, Czech University of Life Sciences Prague, Kamýcká 129, 165 00 Prague - Suchdol, Czech Republic;2. Technological Research Institute, University of Santiago de Compostela, Rúa Constantino Candeira s/n, Santiago de Compostela 15782, Spain;3. Institute of Geochemistry, Mineralogy and Mineral Resources, Faculty of Science, Charles University in Prague, Albertov 6, 128 43 Prague 2, Czech Republic;4. Regional Centre of Advanced Technologies and Materials, Palacký University, 17. listopadu 1192/12, 771 46 Olomouc, Czech Republic;5. Department of Civil and Environmental Engineering and Earth Sciences, University of Notre Dame, Notre Dame, IN 46556, USA;6. Department of Geosciences, Western Michigan University, Kalamazoo, MI 49008, USA |
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| 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 2 m of tailings has been previously characterized. A distinct redox gradient was observed in the top 0.5 m 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, 39.3 mmol kg?1 (3080, 2200, and 2570 mg kg?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 (180 cm) contains sulfides (e.g., pyrite, arsenopyrite, galena, and sphalerite). Samples from the redox transition zone (25–54 cm) 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 (0–35 cm), 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. |
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