Efficient degradation of sulfamethazine with CuCo₂O₄ spinel nanocatalysts for peroxymonosulfate activation

CuCo₂O₄ spinel nanoparticles (NPs) synthesized using a solvothermal method were used as catalysts to activate peroxymonosulfate (PMS) with sulfamethazine (SMZ) as the target pollutant. A degradation efficiency of 87.2% was achieved in 20min with 20mgL^-1 PMS and 0.01gL^-1 CuCo₂O₄ catalyst. In contrast, only 51.1%, 11.3%, 12.5%, and 7.9% degradations of SMZ were observed with Co₃O₄, CuFe₂O₄, CuO, and Fe₃O₄, respectively, as the catalysts. The superior catalytic reactivity of CuCo₂O₄ was explained with the presence of Co²⁺ on the catalyst surface and the combined catalytic reactivity of copper and cobalt towards PMS. Based on the XPS results and the relative catalytic reactivity of Cu²⁺ and Cu⁺, it was proposed that the Cu²⁺/Cu⁺ circulation was least likely the key reaction steps. Instead, a complex reaction mechanism involving the generation of Cu³⁺ was used to explain the activation of PMS by Cu²⁺. The investigation on the reaction parameters showed that the SMZ degradation efficiency responded positively to increases in the PMS dose and the scavenger effect. A mild alkaline condition favored the degradation of SMZ, and an optimized operational condition was found to achieve 98% SMZ degradation with 20mgL^-1 PMS, 0.04gL^-1 CuCo₂O₄, and 5mgL^-1 SMZ at pH 7.7. The activation energy of SMZ degradation was thus estimated to be 21.0kJmol^-1 for the CuCo₂O₄/PMS system and 38.4kJmol^-1 for the CuFe₂O₄/PMS system. Finally, a degradation mechanism on the basis of analyzing the degradation products of SMZ was proposed and the stability and reusability of the CuFe₂O₄ NPs were evaluated.