TRENDS IN METAL SPECIATION AND DISTRIBUTION WITHIN MINE WASTES AS A FUNCTION OF PARTICLE SIZE

Decades of mining have left a legacy of metal-contaminated mine wastes across California. Characterizing trends in metal concentrations, metal speciation, and correlations between metals as a function of particle size can provide insight into the physical and chemical processes that distribute metals in the environment and affect their bioavailability. A combination of mechanical size separations, bulk chemical analyses, and X-ray spectroscopic methods aids the identification of such trends as well as pathways that control metal transport from mine impacted sites.

Samples of mine wastes from several sites throughout California were collected and weighed prior to particle size separation using stainless steel sieves to generate 11 size range-specific fractions (from >2830 μm down to <20 μm). Each fraction was weighed and analyzed using ICP-MS for concentrations of a standard set of 48 elements. Elemental concentrations were then plotted as a function of particle size and categorized based on their size-dependent behavior, resulting in groupings of elements that share similar trends and therefore possible geochemical associations.

Microspectroscopic methods were then applied to selected size fractions of mine wastes, utilizing X-ray fluorescence, X-ray absorption spectroscopy, and X-ray diffraction to assess the speciation, distribution, and correlation of metals of interest. These methods in particular allow the analysis of samples previously inaccessible by bulk spectroscopic techniques due to sample concentration limitations.

Results demonstrate that many heavy metals are highly concentrated in the finer grain size fractions, often by over an order of magnitude, increasing their mobility, reactivity, and bioavailability by ingestion or inhalation. However, changes in metal speciation corresponding to removal of more soluble phases may offset the potential toxicity of such metals in fine-grained fractions as demonstrated through selective leach extractions. Correlations between certain metals, such as iron and arsenic, were identified and further investigated with XRF, where specific As:Fe ratios could be associated with discrete phases. Such species appear to increase in diversity with decreasing particle size, suggesting secondary mineralization or sorption processes.