Sequential Extraction Insights into the Mobility of Arsenic (As), Lead (Pb), Chromium (Cr), and Copper (Cu) in Coal Fly Ash

Coal combustion generates coal fly ash (CFA) as a solid byproduct, with nearly 16 million tons produced in 2023 alone. CFAs contain harmful trace metals, including arsenic (As), lead (Pb), chromium (Cr), and copper (Cu), whose mobility and environmental impact depend on their chemical speciation. This study examined a set of 16 representative CFAs across the U.S. with trace element speciation and mobility operationally defined by chemical sequential extraction. A principal component analysis (PCA) was performed on CFA bulk properties, and trace element partitioning data were projected onto the PCA space to explore associations. The residual and reducible fractions are the primary hosts for As, Pb, Cr, and Cu, accounting for 8%–95% and 2%–82% of total concentrations, respectively. Subbituminous CFAs were associated with a higher CaO content, whereas bituminous CFAs contained a higher Fe2O3 content. Interestingly, subbituminous CFAs were primarily associated with the more labile fractions (i.e., exchangeable, acid-soluble, reducible, and oxidizable). This suggests that Cu, Cr, and Pb may be more mobile in subbituminous CFAs, which are characterized by higher CaO and lower Fe2O3 contents in this study. A partial least squares discriminant analysis (PLS-DA) revealed that CaO and Fe2O3 content are statistically significant in determining the mobility of Cr.