Influence of trace metal distribution on its leachability from coal fly ash

The risks associated with the reuse of coal fly ash in natural environmental settings in terms of their mobility and ecotoxicological significance is largely determined by: (1) the physicochemical conditions the fly ash is placed under; (2) the total leachable metal content in fly ash and; (3) the distribution or mineralogical fractionation of metals. In this study, we report the mobility of As, Cr, Pb, Fe, Cu and Zn from a single Class F fly ash (CFFA). The influence of pH on metal release was compared to the total leachable metal content, as determined by single and sequential chemical extractions. The results show that the CFFA sample is environmentally safe under natural pH conditions, with metal leaching less than the mandated RCRA limits. The elements Fe, Pb and Cr were moderately soluble at acidic pH and sparingly soluble beyond neutral pH. Arsenic release from CFFA was higher under aggressive pH conditions (pH < 4 and pH > 9) and consistent with its oxyanionic behavior. Partial dissolution of the acid soluble (exchangeable) fraction at acidic pH; desorption of oxyanions at alkaline pH; adsorption and or co-precipitation of metals with iron (hydr) oxides at neutral pH appeared to be the probable mechanisms controlling metal release. While simple EDTA extractions provided good indications of the total leachable amounts, a direct correlation with pH leaching data was impossible as the mineralogical distribution of the metals in the fly ash appeared to play a significant role in their leachability. In the case of Class F fly ash, metal association with Fe-oxide appeared to play a more dominant role in metal release.