WEATHERING BEHAVIOR OF WASTE GLASSES GENERATED BY COAL COMBUSTION POWER PLANTS

Coal combustion by-products (CCBs) are high-volume wastes produced by the electrical power industry. These wastes contain trace element pollutants concentrated by the combustion process and are typically disposed of in unlined landfills and lagoon impoundments. It is therefore important to understand the long-term weathering behavior of CCB wastes to model the possible release of these contaminants to the environment.

Samples of fresh fly ash, weathered CCB materials and ash pond pore water were collected from several closed coal ash disposal facilities where coal combustion by-products have been exposed to water corrosion for 14 to ~50 years. The microscopic features, mineralogy, and semi-quantitative chemical homogeneity and composition of granular ash samples were determined by XRD and SEM-EDS analyses. Electron microprobe analysis (EMPA) was used to examine the quantitative chemical composition and micro-scale elemental distribution of several glass particles. Fresh and weathered ash samples were microwave-assisted nitric-acid digested. The digestate solutions and pore water samples were analyzed by ICP-OES.

EMPA transects of fresh glass particles show compositional zoning similar to that proposed, but not demonstrated, by Warren and Dudas (1984). The compositional zonation is likely the result of temperature gradients during the combustion process. In contrast, weathered glass particles do not retain any compositional zoning, suggesting that the outer portion of fresh glass particles is more reactive and the likely source of ash pond pore water solutes such as Ca, Mg, K, and Na. A thermodynamic model designed by Paul in 1977 has been widely used to describe the corrosion of glass in aqueous solutions and assumes that the initial release of species from the glass matrix is a congruent process far from equilibrium. This study suggests that CCB glass dissolution during chemical weathering is not a congruent process, even over the short-term.