Coal combustion products (CCPs) are the solid wastes generated at coal-fired power plants and represent one of the largest industrial waste streams in the U.S. CCPs are usually disposed into holding ponds or landfills adjacent to power plants. Recent failures and discoveries of leaks at coal ash disposal sites have generated concern for the leaching and bioaccumulation of toxins that are typically enriched in CCPs. Here, we investigated the transformations of mercury (Hg), arsenic (As), and selenium (Se) associated with coal ash using laboratory sediment slurry experiments to mimic an ash spill scenario. Sediment microcosms were prepared using sediment and surface water from locations upstream of a major coal ash spill site. The microcosms were cultured in an anaerobic chamber and a selection of the slurries was amended with coal ash. The experiments tested the effects of microbial growth (stimulated with a carbon substrate) and two types of coal ash materials. The results demonstrated the production of methylmercury (MeHg) by anaerobic microorganisms in all the slurries. However, the coal ash provided a stimulating effect for MeHg production due the leachable sulfate from the ash that likely enhanced the growth of methylating microorganisms. The coal ash also increased the amount of dissolved As and Se in the slurries. However, during the five days of incubation, dissolved As concentrations continued to increase while dissolved Se decreased in the ash-amended slurries. These results suggested that arsenic was converting from arsenate to more soluble arsenite species in the slurries while selenium was converting from oxidized forms (e.g. selenate or selenite) to less soluble, reduced forms (e.g. elemental Se, selenide). Overall our work highlights the need to consider redox-dependent transformation that may occur in the environment when assessing the risks posed by coal ash contaminants.

Trace element toxins in coal ash: New considerations for identifying environmental risks

248th National Meeting (2014)

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