Characterization of Trace Metal Leaching from Maryland Coal Fly Ashes

Abstract

Three coal fly ashes with different acid-base characteristics and their mixtures with an embankment soil were analyzed for arsenic, copper, and chromium leaching as function of pH using batch-type water leach tests (pH 4-10). Leach tests results showed that significant Cu release occurred only at pH ~4.3 and dissolved Cr concentrations typically increased with increasing pH. Cr(VI) was determined as the predominant oxidation state in leachates and results from WLTs spiked with Cr(VI) suggest that Cr was strongly sorbed below pH 7. Sorption affinity of fly ashes for Cr(VI) seemed to be to correlated to their oxalate-extractable Fe content, which is presumably a surrogate for amorphous iron (hydr)oxide content. Arsenic release typically followed a similar leaching pattern observed for Cr, with the exception of 100% alkaline fly ash; decreased As release above pH 9 for this sample was attributed to precipitation of Ca-As phases due to its high CaO content.