Petal-like CuCo2O4 spinel nanocatalyst with rich oxygen vacancies for efficient PMS activation to rapidly degrade pefloxacin

Herein, for the first time, we report the design and preparation of petal-like spinel structure CuCo2O4−σ with rich oxygen vacancies (Ov) derived from CuCo-zeolitic imidazolate framework (ZIF) for activating peroxymonosulfate (PMS) to quickly and efficiently degrade pefloxacin (PFX), one of important second-generation fluoroquinolone antibiotics. The petal-like CuCo2O4-σ was fabricated via carbonizing bimetal CoCu-ZIF precursor at 700 °C in air, followed by simple NaBH4 reduction strategy. Various characterization outcomes prove that the CuCo2O4-σ possesses rich Ov, hierarchical meso- and macro-porous microstructures, and stable spinel structure. The CuCo2O4-σ/PMS system exhibited approximate 95% PFX degradation during 5 min reaction, while only 4% PFX removal did in the presence of control product C-700. The PFX degradation rate constant (kapp) in the CuCo2O4-σ/PMS system was 0.572 min−1, which was about 78 times that of the C-700/PMS system (0.0073 min−1) and about 34 times that of the CuCo2O4/PMS system (0.017 min−1). Similarly, the kapp values for the Co3O4-σ/PMS and CuO1-σ/PMS systems are about 7.7 and 3.2 times those of the Co3O4/PMS and CuO/PMS systems, respectively, indicating that Ov significantly accelerates PFX degradation. In addition, the kapp value for the CuCo2O4-σ is much higher than the sum of kapp values for the Co3O4-σ and CuO1-σ, indicating important role of synergy between Cu and Co in PFX degradation. The electrochemical impedance and XPS trials demonstrate that, in addition to accelerate the electron transfer ability of CuCo2O4, the introduction of Ov also leads to the generation of low valence Co2+ ions and Cu+ ions on the surface of CuCo2O4-σ, driving the redox cycles of Co2+/Co3+ and Cu+/Cu2+ to accelerate PMS activation and realize fast PFX degradation. The CuCo2O4-σ exhibited good PMS activation capability in the 3.0 to 10.0 pH range, and good recyclability with over 92% PFX removal after the fourth-cycle run, and low leaching of Co and Cu during catalytic reaction. In addition, the CuCo2O4-σ/PMS system can rapidly degrade other organic pollutants (i.e. levofloxacin, ciprofloxacin, tetracycline hydrochloride, and rhodamine B), indicating its good universality. The active reactive species investigation reveals the generation of SO4•–, •OH, O2•− and 1O2 in the catalytic degradation of PFX. Effects of various factors, i.e. the calcination temperature for the preparation of CuCo2O4-σ, initial solution pH, catalyst dosage, PMS concentration, reaction temperature, inorganic anions, and humic acid on the degradation of PFX in the CuCo2O4-σ/PMS system were explored. Relative to other nanocatalysts for PMS activation, the CuCo2O4-σ exhibits low activation energy of 27.0 KJ/mol. Combined with LC-MS determination of PFX degradation intermediates, the possible PMS activation mechanism by the CuCo2O4-σ and possible PFX degradation pathways were proposed.