TRANSFORMATIONS OF MERCURY, ARSENIC AND SELENIUM IN RIVER SEDIMENTS CONTAMINATED WITH COAL ASH: SEDIMENT MICROCOSM STUDIES

Coal combustion products, including coal ash, represent the largest industrial waste stream in the United States and contain elevated levels of toxic elements such as mercury (Hg), arsenic (As), and selenium (Se). Much of this waste is stored in unlined holding ponds and landfills that are not always monitored for their discharge to adjacent waters. Moreover, these holding ponds are susceptible to failures such as the disaster at the Tennessee Valley Authority (TVA) Kingston Fossil Plant in 2008 that caused more than 1 billion gallons of coal ash slurry to spill into the adjacent Emory River. In such cases, the fate of toxic elements associated with coal ash is greatly influenced by environmental conditions such as redox potential and microbial activities that induce transformations and leaching of contaminants. Here, we investigated the mobilization of coal ash contaminants in sediments through laboratory sediment slurry experiments. We cultured sediment slurries in an anaerobic chamber to determine how the addition of coal ash could influence porewater chemistry and Hg speciation. The microcosms were prepared using sediment and surface water from a location upstream of the TVA spill site. A selection of the slurries was amended with coal ash obtained from the TVA Kingston plant. Results showed that the coal ash increased the amount of dissolved As and Se in the slurries at the initial time point. Over 7 days of incubation, dissolved As continued to increase while dissolved Se decreased in the slurries. These results suggested that arsenic was converting from As(V) to more soluble As(III) 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). The concentration of dissolved sulfate also decreased during the experiment, consistent with low redox potential in the slurries. The effect of coal ash on Hg speciation was mixed, with the coal ash providing a stimulating effect for MeHg production in some slurries and no effect in others. Further work will include sediment-coal ash slurries cultured under aerobic conditions and slurries amended with different varieties of ash. Overall, our research highlights the need to consider environmental conditions in assessing the potential hazards of coal ash contaminants.