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The metal activating sulfite system has attracted widespread attention as a novel advanced oxidation process recently. In this study, a silica-supported highly dispersed cobalt catalyst was prepared using Co ammine complex as precursor, characterized by transmission electron microscope, X-ray diffraction, Brunauer–Emmett–Teller surface area, X-ray photoelectron spectroscopy and so forth, and used to activate sulfite for the removal of organic pollutants. The as-prepared catalyst displayed a durable and remarkable catalytic activation of sulfite for organic degradation in sequential experiments. The mechanistic study corroborated the dual roles of sulfite as a complexing ligand on catalyst surface and a precursor of oxysulfur radicals. The surface-bound and free SO 4 •− were found to be the dominating radicals responsible for organic degradation. The steady-state approximation of SO 4 •− were examined and the apparent reaction rate constant of SO 4 •− generation was 0.60 ± 0.07 M−1 s−1 at 0.25 g L−1 catalyst. The influencing factors and operating parameters, including catalyst dosage, sulfite/contaminant ratio, pH, temperature, anions and natural organic matter were investigated and optimized systematically. As conclusion, the as-prepared catalyst to efficient activation of sulfite can be proposed as a promising advanced oxidation process based on oxysulfur radicals for the treatment of wastewater. Image 1 • The silica-supported cobalt catalyst is synthesized via a complexation process. • CoASi shows an effective performance in sulfite activation for organic degradation. • Sulfite acts as a complexing ligand and a radical precursor in activation process. • Surface-bound and free sulfate radicals are responsible for dye degradation. • The optimal operating conditions are obtained for further application. [ABSTRACT FROM AUTHOR]