SPECIATION AND RELEASE OF SELENIUM IN COAL ASH FROM THE TENNESSEE VALLEY AUTHORITY KINSTON FOSSIL PLANT

The unintended release of coal ash to the aquatic environment, such as the 2008 coal ash spill at the Tennessee Valley Authority (TVA) Kingston Fossil Plant, is a great concern due to the elevated levels of selenium (Se) and other toxic elements in the associated watersheds. Selenium, in particular, is capable of bioaccumulating in food webs and causing toxicological effects in aquatic organisms. The mobility and bioavailability of Se in the environment is mainly controlled by partitioning among different valence states of Se. Therefore, an understanding of the Se species that exists in coal ash and leachates is needed to assess the environmental impacts and health risk posed by coal ash spills. The objective of this research was to determine the potential release of Se in relation to the Se species in the original ash materials. We performed pH-dependent leaching experiments by exposing a variety of coal ash materials from the TVA Kingston plant to water. Selenium speciation in solid phase and aqueous samples was quantified using X-ray absorption near edge structure (XANES) spectroscopy and HPLC-ICP-MS, respectively.

In general, dissolved Se in leachates showed a minimum concentration at pH 5 to 6, and increased as pH decreased below 5 and as pH increased above 6. The pH-dependent Se leaching pattern was relatively similar for all materials tested. The release of Se at low pH could be presumably ascribed to the dissolution of coal ash materials under acidic conditions. The increase in Se in leachates at pH > 6 was likely caused by the desorption of Se oxyanions (e.g. selenite and selenate) and the release of organo-Se (e.g. selenocystine and selenomethionine) as indicated by the results of HPLC-ICP-MS analysis. With an exception of one fly ash sample (out of 6), the XANES results showed that the sum of selenite, selenate, selenocystine, and selenomethionine in original coal ash materials is positively correlated (r² = 0.92) with the total concentration of Se released at their natural pH condition (pH 7.6 – 9.5). Our results demonstrated that both Se oxyanions and organo-Se should be considered as labile Se. Conclusively, Se speciation is a potential method to evaluate the environmental risk pertaining to Se release from coal ash spills.