Effects of Particle Size on Arsenic Speciation, Distribution, and Bioavailability in Mine Wastes

Decades of mining have left a legacy of metal-contaminated mine wastes across California, particularly in the western Mojave Desert where mining activities have generated large volumes of tailings and waste rock containing high concentrations of naturally-occurring arsenic. Characterizing trends in arsenic concentration, speciation, and correlations with other metals as a function of particle size can provide insight into the physical and chemical processes that control arsenic transport from mine-impacted sites.

Mine waste samples from the Randsburg mining complex in Southern California were collected and weighed prior to particle size separation to generate 11 size range-specific fractions (from >2830 um down to <20 um). Each fraction was weighed and analyzed using ICP-MS for concentrations of 48 separate elements. Elemental concentrations were plotted as a function of particle size and categorized based on their size-dependent behavior.

Microspectroscopic methods were then applied to selected size fractions of mine wastes, utilizing X-ray fluorescence, X-ray diffraction, and X-ray absorption spectroscopy to assess the speciation, distribution, and correlation of metals of interest. Finally, the environmental and biological availability of arsenic from mine wastes through exposure to water or passive ingestion were examined by leach extractions using water and a simulated gastric fluid (SGF) medium.

Results demonstrate that arsenic is typically more highly concentrated in the finer grain size fractions, often by over an order of magnitude, potentially increasing its mobility, reactivity, and bioavailability. Correlations with metals such as iron were identified and conclusively associated with discrete mineral phases. Changes in arsenic speciation with particle size corresponding to the removal of more soluble phases may partially offset the potential toxicity of such metals in fine-grained fractions as demonstrated through the water leach extractions. However, SGF studies show that in acidic conditions arsenic is still preferentially released in the finer fractions, even when corrected for surface area effects.