Wet catalytic oxidation upcycling semi-dry flue gas desulfurization ash to sustainable gypsum.

Semi-dry flue gas desulfurization ash (SFGDA), rich in calcium sulfite (CaSO3) and highly alkaline, poses challenges for direct reuse due to its instability and slow oxidation. This review synthesizes recent advances in wet catalytic oxidation (WCO) that transform CaSO3 into construction-grade calcium sulfate (CaSO4). We frame homogeneous and heterogeneous radical-chain routes and lattice-defect activation, underscoring how Mn/Co/Fe redox cycles and oxygen-vacancy engineering accelerate conversion. Performance comparisons span conventional salts, metal-organic frameworks (MOFs), perovskites, and nanostructured catalysts, with Co-MOFs achieving 96% oxidation in 3 h and MnTiO3 lowering activation energy by 40%. Process optimizations ultrasound assisted nanobubbles, micro-scale mass-transfer intensification, and pH control at 5-6 address diffusional and alkaline limitations, while layered double hydroxides and anti-scaling surfaces mitigate chloride poisoning and fouling. By merging mechanistic insights with technological progress, this work maps out a sustainable pathway for SFGDA management.