Statistical Optimization of Alkaline-Activated Carbonation of Coal Fly Ash for CO ₂ Sequestration and Heavy Metal Stabilization

This study evaluates process parameters to enhance carbonation efficiency in the direct gas–solid carbon mineralization of coal fly ash (CFA). The effects of NaOH concentration and carbonation temperature were analyzed using regression analysis and response surface methodology (RSM) based on central composite design (CCD) comprising nine experimental runs. NaOH concentration has a stronger effect (83.55%) than temperature, and optimal carbonation efficiency (>40%) was achieved with NaOH concentrations above 1.91 M and temperatures above 92.2 °C. Heavy metal leaching from raw and carbonated CFA was also assessed. Carbonation under alkaline conditions formed a carbonate layer containing calcite (CaCO3) and gaylussite (Na2Ca(CO3)2·5H2O), which immobilized heavy metals through sorption and coprecipitation, thereby reducing the leachability and enhancing the long-term chemical stability of the material. This study offers critical insights into optimizing carbon mineralization for simultaneous CO2 sequestration and heavy metal stabilization.