Speciation, Distribution, and Mobility of Hazardous Trace Elements in Coal Fly Ash: Insights from Cr, Ni, and Cu

Coal fly ash (CFA) has gained increasing societal concerns as hazardous trace elements (HTEs) in CFA might pose environmental and public health risks. To develop better management and remediation strategies, it is highly important to obtain an in-depth understanding of the occurrence and mobility of HTEs in CFA. This study systematically characterized the speciation, distribution, and mobility of Cr, Ni, and Cu in class F and class C CFA samples using scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDX), X-ray absorption near-edge spectroscopy (XANES), and sequential chemical extraction. SEM-EDX results show that Cr, Ni, and Cu are present in a range of 2-15 pm sized particles either as discrete particles or encapsulated in the glass phase. XANES results suggest that Cr is mainly present in Fe oxides or silicates and spinel minerals; Ni mainly in Fe oxides and as NiO; and Cu mainly as Cu20/CuO or in Fe oxides. Cr(III) and Cu(I) are the dominant oxidation states for Cr and Cu, respectively. Sequential extraction generally shows higher mobility of trace elements (Cr, Mn, Co, Ni, Cu, Zn, Cd, and Pb) in class C CFA, as compared to the mobility of those in class F CFA, implying potentially higher environmental risks for class C CFA.