Insights into the enhanced calcium sulfite oxidation by air assisted with manganese/cigarette filter-based porous carbon catalysts: Valence cycle and radical/non-radical synergistic pathways

Calcium sulfite (CaSO₃) oxidation represents a crucial step in the resource utilization of calcium-based desulfurization ash (CDA) from semidry flue gas desulfurization processes for SO₂ emission control. Porous carbon-based materials for heterogeneous catalytic oxidation have received attention due to their frequent applications in advanced oxidation reactions involving sulfites [S(Ⅳ)] and persulfates (PS) activation. Herein, a manganese/cigarette filter-based porous carbon catalysts (Mn@CF-600) with a large S_BET (479.16 m²·g⁻¹) and various active sites (ketone group, Mn species, and oxygen vacancies) was synthesized by the impregnation-assisted pyrolysis method and applied to wet catalytic oxidation of CaSO₃. The results showed that an oxidation efficiency of CaSO₃ up to 91.30 % within 3 h were achieved under the air-assisted catalysis of Mn@CF-600, and the highest oxidation rate was 0.0566 mmol·L^-1·s⁻¹ at 338 K. Oxidation kinetics indicated that 1.0 g·L^-1 Mn@CF-600 approached the critical dosage for diffusion-controlled reactions, with an apparent activation energy of 18.70 kJ·mol⁻¹. The air flow rate and CaSO₃ concentration reaction orders were 0.26 and −0.31, respectively. Moreover, the immobilized CG-Mn@CF-600 maintained an oxidation efficiency of 78.08 % after five cycles of use. Based on quenching experiments, ESR, and XPS analysis, the high stability and catalytic activity could be attributed to the synergistic involvement of specific reactive oxygen species (ROS, including •SO₃^2-, •O₂^–, ¹O₂, and •SO₅^-), HSO₃⁻, and mixed valence Mn(II/III/Ⅳ). This work provides new insights into the carbon-supported Mn-based catalyst/CaSO₃ oxidation mechanism by the valency cycle and radical/non-radical synergistic pathway, and hence develops an efficient approach for practical CDA treatment.