SPECIATION AND BIOACCESSIBILTY OF PARTICULATE MATTER FROM LEGACY MINE TAILINGS (Invited Presentation)

Communities nearby mine wastes in arid and semi-arid regions are potentially exposed to high concentrations of toxic metal(loid)s from fugitive dusts deriving from impoundments. To assess the relation between lofted particulate matter and human health risk, we studied the bioaccessibility (BAc) and metal(loid) speciation for mine tailings from a Superfund site in central Arizona, USA with high levels of As and Pb (up to 58.7 and 34.0 mmole kg^-1 respectively) by coupling in vitro bioassay with X-ray absorption spectroscopy. Bio-simulant lung and gastric fluid were reacted with respirable and ingestible fine (10-150 μm dia.) particulate matter (PM) isolated from: 1) PM₁₀, 2) PM₁₅₀, 3) surface crust (<1 cm), and 4) efflorescent salts, under physiological conditions (e.g. dark, 37^o, etc.) for 30 sec to 7 days to examine the pharmacokinetic release of toxins. The BAc fractions of As and Pb released to gastric fluid were about 10 and 100 times greater than in lung fluid, respectively. Gastric fluid showed release of 10-100% of the maximum BAc for both As and Pb at the shortest exposure times (30 sec) for all dust types, attributed to dissolution of fine, highly-reactive particles followed by slower dissolution of less soluble particles, indicating a single time step may not adequately describe release dynamics. In lung fluid, the kinetic release of As was continuous to 7 days, indicating residence time must also be considered for exposure risk determination. Pre- and post-gastric assay solid phase speciation in the low pH simulant showed As^(V) was released from bidentate-binuclear coordination at ferrihydrite surfaces and partially reincorporated into jarosite-like coordination, as indicated by a relative increase in the contribution of longer range corner-sharing Fe octahedra (~ 3.6 Å), and consistent with a relative decrease in the mass fraction of ferrihydrite compared to jarosite. The bioaccessibility of Pb was limited due to robust sequestration and inaccessibility in plumbojarosite. Results indicate that BAc of As and Pb was controlled by (i) the solubility of Fe²⁺-bearing solids, (ii) the prevalence of soluble SO₄^2-, and (iii) the presence of poorly crystalline Fe(III) oxide sorbents. Targeting exposure sites within the human body and interrogating the release and speciation of aerosolizable particles with the combined application of in vitro bioassay and X-ray techniques elucidates potential risk to human health from mine tailings derived geo-dusts.