Revisiting the CoO_x/sulfite process for pollutant degradation: The overlooked role of surface Co(IV)

Sulfite (S(IV))-based advanced oxidation processes have lately shown strong potential for water purification, with cobalt oxides (CoO_x) recognized as highly effective activators. Current research predominantly attributes pollutant removal in CoO_x/S(IV) systems to free radicals, while the possible involvement of highly reactive Co(IV) is overlooked. In this study, the reactive species in CoO_x/S(IV) systems were re-evaluated, with CoO, Co₂O₃, and Co₃O₄ selected as representative catalysts. Surface Co(IV) was consistently detected across these CoO_x/S(IV) systems, with its yield remaining stable under most conditions, while excessive S(IV) hindered its formation. Density functional theory (DFT) calculations suggested that Co(IV) can form via two distinct pathways: the first pathway involved a two-electron transfer originating from Co(II)-PMS complexes, and the second pathway proceeded through hydrogen atom transfer that came from Co(III)-OH complexes. Co(IV), SO₄^•-, SO₅^•-, and HO^• were the principal reactive species in the CoO/S(IV) system. Among them, Co(IV) exhibited strong selectivity in degrading nine representative organic micropollutants. Meanwhile, real water matrix experiments confirmed that natural organic matter (NOM) inhibited PMSO removal, yet increased the relative contribution of Co(IV). These findings clearly demonstrate that surface Co(IV) plays a critical role in CoO_x/S(IV) systems and may also be applicable to other heterogeneous cobalt/S(IV) systems.