Nanoscale Distribution and Chemical Speciation of Potentially Toxic Elements In Coal Fly Ash.

Hundreds of millions of tons of coal fly ash are produced worldwide each year. Coupled with its large annual production, the possibility of toxic elements leaching makes fly ash storage and disposal a significant environmental concern.  A limiting factor in our ability to predict the mobility of toxic metals in fly ash is uncertainty in our understanding of their solid phase chemical speciation. To resolve these major uncertainties, we characterized fly ash samples representative of 2008 spill from Kingston Valley plant, TN. Total elemental analysis and X-ray diffraction showed that the major constituents were quartz (SiO₂) and the aluminoslilicate sillimanite (Al₂SiO₅). Extended X-ray absorption fine structure (EXAFS) data on Ca, Fe, Sr and S and synchrotron based-XRD data on samples were collected to further study the other major phases. The X-ray absorption near edge structure (XANES) spectroscopic data determined that As(V), Cr(III), and Se(IV) are the dominant forms of these redox active  elements. XANES also indicated the major phases for Fe, Cu and Sr are FeOOH, CuO and SrCO₃, respectively. X-ray fluorescence maps collected using JEOL JXA 8530F Hyperprobe indicated that Si, Al, Fe, Ca and Mg occur in clusters (grains) of sizes up to 20 μm; conversely, the trace metals Cu, Mn, Se and Sr are scattered and appear to occur in adsorbed form or in coatings of the grain surfaces. The results of this characterization will help us to understand the toxicity, mobility and bioavailability of the various elements present in fly ash and may contribute to new  management and remediation strategies.