A novel heterogeneous system for sulfate radical generation through sulfite activation on a CoFe₂O₄ nanocatalyst surface

Highlights: • CoFe₂O₄−S(IV)−O₂ system is proposed as a new system of sulfate radical based AOPs. • Alkaline pH favors the activation of sulfite on CoFe₂O₄ surface to produce oxysulfur radicals. • Generation of Co−OH complexes on the surface of CoFe₂O₄ is the main factor for sulfite activation. • Degradation of organic contaminants by CoFe₂O₄−S(IV)−O₂ system were achieved. • Main intermediates and pathways for the degradation of metoprolol were identified. - Abstract: Heterogeneous catalytic activation is important for potential application of new sulfate-radical-based advanced oxidation process using sulfite as source of sulfate radical. We report herein a heterogeneous system for sulfite activation by CoFe₂O₄ nanocatalyst for metoprolol removal. Factors that influence metoprolol removal were investigated, including pH and initial concentrations of components. The CoFe₂O₄ nanocatalyst was characterized by X-ray diffractometry (XRD) and transmission electron microscopy (TEM), and the catalytic stability was tested by consecutive runs. Radicals generated in the CoFe₂O₄−S(IV)−O₂ system were identified through radical quenching experiments and by electron spin resonance (ESR). The catalytic mechanism was elucidated further by X-ray photoelectron spectroscopy (XPS). The catalytic process was dependent on initial pH, and more than 80% of the metoprolol can be removed at pH 10.0 following the Langmubir-Hinshelwood equation. The generation of Co-OH complexes on the CoFe₂O₄ surface was crucial for sulfite activation. SO₄^·− was verified to be the main oxidative species responsible for metoprolol degradation. Other organic pollutants, such as sulfanilamide, sulfasalazine, 2-nitroaniline, sulfapyridine, aniline, azo dye X-3B and 4-chloroaniline, could also be removed in this CoFe₂O₄−S(IV)−O₂ system. The results suggest that the CoFe₂O₄−S(IV)−O₂ system has good application prospects in alkaline organic wastewater treatment.