Oxidative weathering decreases bioaccessibility of toxic metal(loid)s in PM emissions from sulfide mine tailings

Abstract

Environmental contamination from legacy mine-waste deposits is a persistent problem due to the long history of hard-rock mining. Sulfide ore deposits can contain elevated levels of toxic metal(loid)s that, when mobilized by weathering upon O2 and H2O infusion, can result in groundwater contamination. Dry-climate and lack of vegetative cover result in near-surface pedogenic processes that produce fine-particulate secondary minerals that can be translocated as geo-dusts leading to ingestion or inhalation exposure in nearby communities. In this study, in vitro bioassays were combined with synchrotron-based x-ray spectroscopy and diffraction to determine the potential risk for toxic element release from dust (PM10) samples into biofluid simulants. PM10 were isolated from across the oxidative reaction front in the top meter of tailings subjected to 50 years of weathering under semi-arid climate, and introduced to synthetic gastric- and alveolar-fluids. Aqueous concentrations were measured as a function of reaction time to determine release kinetics. X-ray diffraction and absorption spectroscopy analyses were performed to assess associated changes in mineralogy and elemental speciation. In vitro bioaccessibility of arsenic and lead was highest in less-weathered tailings samples (80-110 cm) and lowest in samples from the sub-oxic transition zone (40-52 cm). Conversely, zinc release to biofluids was greatest in the highly-weathered near-surface tailings. Results indicate that bioaccessibility of As and Pb was controlled by (i) the solubility of Fe2+-bearing solids, (ii) the prevalence of soluble SO42-, and (iii) the presence of poorly-crystalline Fe(III) oxide sorbents, whereas Zn bioaccessibility was controlled by the pH-dependent solubility of the stable solid phase.