scCO₂ mineralization of fly ash: enhanced carbonation and acidification-driven heavy metal remobilization

Supercritical CO₂ (scCO₂) mineralization offers a promising route to enhance fly ash carbonation, yet its effects on heavy metal immobilization remain unclear. Four typical fly ash samples collected from different coal-fired power plants were used. Experiments used a high-pressure autoclave at 40 °C, liquid-to-solid ratio 10 mL/g, 100 rpm, and 1 h, with pressure varied from 1 to 8 MPa. Efficiency was determined by thermogravimetric analysis (TGA), and products were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Leaching of Pb, Cu, Cr, As, Zn, and Cd was evaluated by HJ/T 300, and fractions were determined by modified European Community Bureau of Reference (BCR) sequential extraction. scCO₂ promoted both surface and diffusion reactions; penetration depth increased from 108.6 μm at 5 MPa to 136.1 μm at 8 MPa.. Heavy metal immobilization falls into two categories: Pb, Cu, Cr, and As are continuously immobilized via direct precipitation or coprecipitation; Zn and Cd are immobilized at 1-7 MPa but re-released at 8 MPa due to severe acidification, which induces complexation-dissolution of ZnCO₃ and CdCO₃ into soluble bicarbonates. BCR sequential extraction and pollution index evaluation corroborate these mechanisms. Energy penalty was 2.6-3.8 GJ/t-CO₂, competitive with conventional carbon capture, utilization, and storage (CCUS) systems (3.5-4.5 GJ/t-CO₂). Multiple revenue streams derivable from the mineralization process building materials, high-purity CO₂ commodities, carbon trading credits, and avoidable landfill disposal fees. Overall, scCO₂ mineralization effectively immobilizes most heavy metals and reduces integrated pollution index, but risk of Zn/Cd remobilization due to acidification requires pH regulation.