The overlooked role of Co(OH)₂ in Co₃O₄ activated PMS system: Suppression of Co²⁺ leaching and enhanced degradation performance of antibiotics with rGO

Cobalt oxide (Co₃O₄) activated peroxymonosulfate (PMS) system was extensively studied due to its excellent catalytic performance. However, the relatively high cobalt (Co²⁺) leaching (up to 2 mg/L) would pose high ecotoxicological risks. Herein, we identified the existence of Co(OH)₂ on the surface of Co₃O₄ were the major source of Co²⁺ leaching in the Co₃O₄ activated PMS system. Furthermore, the Co²⁺ leaching was effectively suppressed by converting Co(OH)₂ to Co₃O₄ via pyrolysis treatment. In addition, reduced graphene oxide (rGO) was engaged to enhance the degradation performance of antibiotics in the Co₃O₄ activated PMS system. The oxygen functionalities of rGO would catalyze PMS to generate sulfate radicals (SO₄^−[rad]) and trigger the non-radical pathway of singlet oxygen (¹O₂). We have achieved outstanding catalytic performance for carbamazepine (CAZ) degradation with low Co²⁺ leaching, as CAZ (5 mg/L) could be completely degraded in 30 min. Combining experimental investigation and theoretical calculation, we also revealed the degradation pathways and mechanisms that CAZ would be oxidized and detoxified by ¹O₂ and SO₄^−[rad]. We have provided a simple approach to inhibit the Co²⁺ leaching and enhance the catalytic performance of Co₃O₄ activated PMS system for the effective control of antibiotics.