Coprecipitation of Heavy Metals in Calcium Carbonate from Coal Fly Ash Leachate

Water that comes into contact with coal fly ash contains toxic elements, including heavy metals. CO₂ gas can be used for treatment by inducing precipitation of carbonates with alkaline earth metals, thereby coprecipitating toxic elements and neutralizing the pH. This process has the added benefit of sequestering CO₂ and contributing to negative emissions of greenhouse gases. This investigation used a novel synchrotron-based approach with X-ray fluorescence nanospectroscopy at the NSLS-II HXN beamline. Multielement incorporation of As, Ba, Cr, Cu, and Zn into calcium carbonate was measured for a simulated fly ash leachate. We discovered that the extent of trace element incorporation was consistently higher than predicted by a thermodynamic model of solid solution coprecipitation, with incorporation of Cr being 3000 times more than predicted, and incorporation of Zn being 3 times more. Enhancement of trace element incorporation was correlated to the solubility of the endmembers (CaCrO₄ > CaHAsO₄ > BaCO₃ > CuCO₃ > ZnCO₃). Iron inhibited trace element coprecipitation because the dominant process was surface adsorption. These results suggest that mineralization of heavy metals via coprecipitation in metastable carbonate phases is a more effective strategy for immobilizing toxic elements than solubility would suggest and is a promising environmental remediation strategy.