Metal(loid) liberation from Alaskan coal combustion products as a function of time in various aqueous media

Abstract

Little is known about the fate and potential toxicity of metal(loid)s that could be leached from coal combustion products by a (sub- )Arctic environment. Several potentially toxic elements are enriched in coal combustion products relative to the average crustal abundance including As, Cu, Se, and Sb. The overarching goal of this project is to examine the release of these and other metal(loid)s from early stage coal ash and fly ash from the University of Alaska Fairbanks (UAF) power plant and identify transformations in the presence of aqueous environmental media. Bioaccessibility experiments performed indicate that early stage coal ash and fly ash contain bioaccessible Cr, As, Se, Sb, and Pb. Bioaccessible concentrations of these commonly known toxic metal(loid)s were found to exceed EPA drinking water and freshwater regulations. Early stage coal ash and fly ash was reacted with 18 MΩ H₂O (control) or simulated rainwater to quantify metal(loid) liberation as a function of time. Leachate pH increased to ca. 12.5 within the first hour. Some metal(loid)s quickly reached the maximum measured concentration and consistently decreased in concentration with time such as Ba, Pb, and Zn, while other metal(loid)s increased in concentration with increased reaction time (e.g., Al, V, and Cr). Leaching behavior of between early stage coal ash and fly ash may be controlled by total initial concentrations present in the two ashes, differences in particle size, dissolution and precipitation reactions, and heterogeneity of metal(loid) distribution within the particles. Early stage coal ash and fly ash were also reacted with reconstituted dissolved organic matter solutions to simulate possible environmental interactions. It was found that for some elements (e.g., Ca), dissolved organic matter did not affect the mobility. Other metal(loid) mobilities were affected by the presence of dissolved organic matter, such as that of Sb, As, Zn, Se, Mo, and V. Some metal(loid) concentrations decreased while others increased with increasing dissolved organic carbon concentrations. Through these experiments, we have obtained a quantitative understanding of the kinetic controls of metal(loid) release from coal ash leaching with various aqueous media. Results from these experiments can help to improve storage and remediation processes for coal combustion products in an effort to protect human and the ecosystem health.