LEACHING OF POTENTIALLY TOXIC ELEMENTS FROM ALASKAN STOKER-BOILER FLY ASH

Nearly all coal burned in Alaska is derived from the Usibelli Coal Mine (Healy, AK), which produces sub-bituminous coal with a unique alkaline mineral assemblage. However, little is known about the fate of potentially toxic elements present in Alaskan fly ash (FA). In this study we examine FA collected from the University of Alaska Fairbanks stoker-boiler power plant. Analyses indicate this FA contains high concentrations of several potentially toxic elements, including As, Cu, Se, Sb, and Hg, which are enriched by factors >25 and Pb by >8, relative to the average upper continental crustal level. The goal of this study is to examine the elemental release and mineralogical transformation of this FA when reacted with aquatic media, including 18 MΩ cm^-1 H₂O (as a baseline) and simulated rainwater at room temperature for up to 90 days (d). In both experiments, leachate pH increased to >12.7 within 1 hour while the aqueous concentrations of several elements (e.g., Ca, Ba, Pb) reached maxima and then declined slowly. The concentrations of oxyanion-forming elements (e.g., As, Se, Mo, Sb) remained relatively constant, suggesting equilibrium with solid phase. Only aqueous Pb concentrations exceeded regulatory criteria over the 90 d. Lead concentrations consistently exceeded EPA drinking water treatment technique action levels (15 mg L^-1) until the last time point and, in one instance, exceeded the EPA freshwater criteria maximum concentrations (65 mg L^-1). The synthetic precipitation leaching procedure (SPLP; EPA method 1312) results were similar to our leaching experiment, also indicating Pb concentrations above regulatory limits. However, the SPLP fails to capture temporal trends, such the initial pulse of Pb released. X-ray diffraction analyses of unreacted and solid-phase leached residuals indicated the presence of ettringite [Ca₆Al₂(SO₄)₃(OH)₁₂•26H₂O] in all samples, with its abundance increasing at longer leaching times. Ettringite commonly forms in alkaline weathering environments and can act as a sink for oxyanion-forming potentially toxic elements, possibly explaining their low, invariant supernatant concentrations. These leaching experiments lend insight into the factors controlling elemental release from FA and may help inform environmental risk assessments of Alaskan coal combustion products.