Leaching of As, Cr, and Cu from High-Carbon Fly Ash–Soil Mixtures

Three coal fly ashes (FA) with different acid-base characteristics and unburned carbon contents, and their mixtures with a typical embankment soil, were analyzed for arsenic, copper, and chromium leaching as a function of pH using batch-type water leach tests. Final leachate pH of the samples was linearly correlated ($R^2=0.84$) to their water-soluble Ca (a surrogate for soluble CaO) and oxalate-extractable Fe concentrations (a surrogate for amorphous iron hydroxides). The leaching behavior of Cu, Cr, and As appear to be primarily governed by leachate pH, and no significant difference in terms of leaching trends was observed among different fly ashes solely attributable to different unburned carbon content. At acidic pH, increased Cu and Cr (presumably in the form of ${\mathrm{Cr}}^{\mathrm{III}}$) leaching were observed. ${\mathrm{Cr}}^{\mathrm{VI}}$ was determined as the predominant species in leachates. As and ${\mathrm{Cr}}^{\mathrm{VI}}$ were strongly retained below pH 7 and leached more at alkaline pH. According to ${\mathrm{Cr}}^{\mathrm{VI}}$ sorption test results, ${\mathrm{Cr}}^{\mathrm{VI}}$ retention capacity was higher for fly ashes with higher oxalate-extractable iron content. Thus, minimum metal release occurred from mixtures that produced leachates with pH values near neutral range. The leachate pH of acidic fly ash could be increased by adding soil or ash with neutral characteristics. The fly ash-to-soil ratio should be carefully adjusted because soil addition to ashes improved compactibility of the mixtures but also reduced the retention capacity for As and ${\mathrm{Cr}}^{\mathrm{VI}}$. Regardless, pH of naturally alkaline fly ash with high soluble CaO content could not be decreased to a neutral range and leached more metal, especially toxic ${\mathrm{Cr}}^{\mathrm{VI}}$.