A persulfate oxidation system for removing acid orange from aqueous solution: Evaluation and degradation mechanism

Peroxymonosulfate-based advanced oxidation (PMS-AOP) is a promising technology for the degradation of environmental pollutants. PMS can be activated by various transition metals, especially cobalt-based catalysts, but pure cobalt catalyst suffers from severe metal leakage and poor cyclicality. This study synthesized NiCo₂O₄ using a co-precipitation hydrothermal method. The structures, morphologies, and chemical states of the prepared catalysts were hexagonal sheet structures. The activation of PMS by catalyst (NiCo₂O₄) is investigated in a PMS/carbonate (PC) system for Orange II degradation. The observed pseudo-first-order rate constants (k₁) were assessed by the effects of different water matrices and operation conditions. The results show that k_obs with NiCo₂O₄ were increased by 13 times than that of treatment without NiCo₂O₄. This was mainly due to Co³⁺ and Ni³⁺ act as electron acceptors to capture electrons from the PMS/PC system, forming a good redox cycle with HSO₅^-/SO₅^- and oxidizing Co²⁺/Ni²⁺ to produce a large amount of more active components (e.g., ¹O₂ and SO₄^⋅-). The good reusability and high stability of NiCo₂O₄ were demonstrated by five recycle tests. These results suggest that the NiCo₂O₄/PC system is an efficient and stable method of pollution remediation.