Mineralogical Characterization of Tailings and Respirable Dust From Lead-Rich Mine Waste and its Control on Bioaccessibility
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One of the main risks that exposed and unvegetated tailings pose to the environment and human health is airborne dust that may be inhaled or ingested. In the case of Pb-bearing dust, both particle size and the identity of the Pb-hosting minerals affect the degree of risk. Finer dusts (<5µm diameter) have a higher potential of being ingested deeper into the human lung causing possible tissue damage and toxic effects. We have collected size-fractionated airborne dust and near-surface pH-neutral tailings at New Calumet Mine, Quebec, Canada, a former Pb-Zn mine. Bioaccessibility describes how much of a substance can be dissolved by body fluids and become available for absorption by the body. The most bioaccessible Pb compounds are PbCO3 (cerussite), Pb3(CO3)2(OH)2 (hydrocerussite) and PbO, followed by PbSO4 (anglesite), PbS (galena) and Pb5(PO4)3Cl (pyromorphite). Airborne dust samples were collected on the tailings piles using a PIXE Cascade Impactor which separates aerosol fractions onto nine impactor stages ranging from 16µm to 0.06µm. These stages were then analyzed by PIXE to obtain elemental concentrations. Samples of non-vegetated and vegetated near-surface tailings were collected for bioaccessibility tests as well as for total metal content, grain size distribution, and Pb speciation using ESEM and synchrotron techniques. Both airborne dust and near-surface tailings samples underwent synchrotron microanalysis including microXRD for identification of microcrystalline compounds and microXRF for element mapping and metal ratio evaluation. Despite extensive oxidation of iron sulfide minerals in the near-surface tailings, galena persists as the most abundant Pb-bearing phase in the pH-neutral tailings. However, rims of cerussite and hydrocerussite forming alteration rims on galena grains have been identified throughout the tailings. In vitro bioaccessibility testing of Pb in the tailings resulted in 0-0.05% bioaccessible Pb in lung fluid and 23-69% bioaccessible Pb in the gastric fluid.