Andrew Thomas

Soil, Water, and Environmental Science

In vitro Contaminant Bioaccessibility Across Tailings Samples with Varying Stages of Oxidation
     Sulfide mine tailings, or low-grade ore discarded during mineral processing, tend to have high concentrations of toxic metal (loid) contaminants. The in vitro bioaccessibility (IVBA), or solubility of the contaminants in synthetic aqueous biofluids, determines their toxicity in biological systems and depends strongly on their molecular speciation and bonding environment. Therefore, chemical and mineralogical changes occurring to the tailings during the oxidative weathering process have a strong effect on metal (loid) contaminant IVBA, as contaminant release is related to the dissolution kinetics of minerals in the tailings. So far, there have been few studies of the IVBA of toxic metal (loids) in mine tailings that relate contaminant release to mineral dissolution kinetics, and such studies may allow scientists to predict the health risks posed by tailings sites using the elemental composition and mineralogy of these sites.
     The tailings site chosen for study is the Iron King Mine and Humboldt Smelter Superfund site, a legacy mine near Dewey-Humboldt, Arizona. To measure the dependence of arsenic and lead IVBA on the extent of oxidative weathering of the tailings, samples were taken from successive depth ranges in a 1 meter-deep pit, and the PM10 fraction (particles with a diameter < 10 μm) of each sample was isolated. Separate samples of PM10 from each depth range were then treated with synthetic gastric and lung fluid and the elemental concentrations of iron, arsenic, and lead were measured over time. These concentrations were normalized to the mass of the original sample to yield the time-dependent IVBA. X-ray diffraction and x-ray absorption fine structure spectroscopy (XAFS) were also used to determine the changes in the sample’s mineralogical composition and the contaminant oxidation states and bonding environments.
     This experiment found that arsenic gastric IVBA was higher in tailings materials with higher levels of poorly-crystalline iron oxides because these minerals have greater solubility in acidic media. Lead gastric IVBA was found to depend strongly on the level of oxidation, as oxidized lead-bearing minerals such as anglesite (PbSO4) and adsorbed elemental lead are much more soluble under acidic conditions. Because of the low solubility of many metal(loid)-bearing minerals at near-neutral pH, contaminant IVBA in lung fluid was found to depend largely on adsorption-desorption processes.
     The most important finding from this experiment is that oxidative weathering of sulfide ore tailings increases lead and arsenic IVBA relative to unaltered tailings because it concentrates arsenic and lead in more bioaccessible minerals. Because the dissolution of certain minerals (especially poorly-crystalline iron oxides) controls the release of arsenic and lead into synthetic biofluids, knowledge of the mineralogy of tailings surfaces across the region can potentially be used to predict the IVBA of arsenic and lead at these sites. This would lead to better knowledge of the health risks posed to communities located near legacy mine sites.