Investigating the Transformations of Arsenic and Toxic Metal(Loid)S Speciation and the Impact on Arsenic Bioaccessibility in Particulate Matter from Mine Tailings Sites Distributed in Different Climates

Abstract

Legacy mine tailings are a persistent source of contaminant metal(loid) release, posing potential risks to human and ecosystem health. This study examines the solid-phase weathering, geochemical speciation, and bioaccessibility of arsenic (As), lead (Pb), zinc (Zn), and cadmium (Cd) in sulfidic mine tailings collected from 11 abandoned sites across a broad climatic gradient in the Western United States, ranging from arid to humid environments. Tailings were sampled from surface to 2 m depth and characterized for total metal concentrations, mineralogy, and As speciation using X-ray absorption spectroscopy. In vitro bioaccessibility assays (IVBA) were performed under gastric and intestinal conditions to assess human health risk. Arsenic weathering patterns varied with climate: arid sites exhibited surface enrichment correlated with Fe and S, while humid sites showed deeper As enrichment and near-surface depletion, reflecting oxidative translocation and secondary mineral retention. Arsenopyrite (As⁰) dominated in unweathered zones, while oxidized arsenate (As⁵⁺) was prevalent in weathered layers; As³⁺ was not detected. IVBA results revealed low As bioaccessibility (<3.5%) in humid regions and moderate values (10–36%) in arid to semi-arid sites, with site-specific mineralogy—particularly Fe-bearing phases like jarosite and ferrihydrite—playing a critical role. Pb exhibited low bioaccessibility due to transformation into stable phases (e.g., plumbojarosite), while Zn and Cd were more labile, especially under acidic conditions. Climatic conditions strongly influenced metal lability, weathering depth profiles, and associated risks, with moderate climates often presenting higher bioaccessibility. These findings underscore the importance of climate-driven geochemical processes in controlling contaminant mobility and provide insight for prioritizing remediation at legacy mining sites under future climate scenarios.