Application of a dual deep eutectic solvent system in a pH-swing process for gypsum valorization and CO₂ sequestration: experimental, kinetic, and geochemical insights

The utilization of flue gas desulfurization (FGD) gypsum (CaSO 4 2H 2 O) for carbon dioxide (CO₂) mineralization is limited by its low solubility and the co-dissolution of impurities, which restrict calcium availability and reduce product purity. To address these challenges, this study develops a pH-swing process based on deep eutectic solvents (DES), employing a levulinic acid–choline chloride DES for dissolution and an L-arginine–ethylene glycol DES for pH adjustment and carbonation. Under optimized conditions (2 M acid, 75 °C, 120 min), the DES acid achieved high calcium extraction (∼84%), comparable to hydrochloric acid (HCl) used as a benchmark, while significantly suppressing Fe and Al dissolution. Subsequent pH adjustment to ∼9 using the DES base enabled selective precipitation of impurity species, producing a calcium-rich solution. Carbonation at CO₂ pressures ≥10 bar resulted in near-complete conversion (∼100%) and the formation of high-purity calcium carbonate (CaCO₃) (∼99%), predominantly as calcite. Kinetic analysis indicated a mixed control mechanism involving product layer diffusion and surface reaction, with lower apparent activation energy for the DES system compared to HCl. PHREEQC simulations further confirmed that calcite supersaturation is achieved at pH ≥9, consistent with the experimentally observed carbonation behavior. The results demonstrate that the DES-based pH-swing enables selective calcium extraction, effective impurity removal, and controlled CO₂ mineralization within an integrated framework. While no quantitative sustainability assessment is performed, the use of tunable DES systems provides a viable alternative to conventional reagents by improving process selectivity and reducing impurity carry-over.