T194 Frontiers in Coal Science: From Basic Research to Applied Technology

Tuesday, 6 November 2012
1:30 PM - 5:30 PM
201AB

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

Paper number: T194-4
Program: Technical Sessions
Time: 2:20 PM – 2:35 PM

Authors

Abstract

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 with the liquid to solid ratio of 10 mL/g. Selenium speciation in solid phase and aqueous samples was quantified using X-ray absorption near edge structure (XANES) spectroscopy and HPLC-ICP-MS, respectively.

Dissolved Se showed a minimum concentration at pH 3, and increased as pH increased. The pH-dependent Se leaching pattern was relatively similar for all materials tested. While leached at coal ash's natural pH (7.6 - 9.4), selenite is the major species in dissolved Se as indicated by HPLC-ICP-MS analysis. This results agree with that of XANES analysis, wherein the decrease of Se oxyanions (~85% of selenite and ~15% of selenate) in the post-leached residuals showed a 1:1 relationship (regression slope = 1.08 ± 0.15; r2 = 0.91**) with the total concentration of dissolved Se. At ~pH12, however, the leaching of Se appeared to derive from both desorption of Se oxyanions and oxidative dissolution of reduced Se compounds (e.g. Se0) from the original coal ash materials. Our results indicated that Se oxyanion is the most labile species accounting for Se release from coal ash materials and that the leachability of Se will increase if coal ash materials are subjected to alkaline treatments such as limestone amendments.