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 20 min with 20 mg L−1 PMS and 0.01 g L−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 20 mg L−1 PMS, 0.04 g L−1 CuCo2O4, and 5 mg L−1 SMZ at pH 7.7. The activation energy of SMZ degradation was thus estimated to be 21.0 kJ mol−1 for the CuCo2O4/PMS system and 38.4 kJ mol−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.