Plasma-catalytic degradation of ciprofloxacin in aqueous solution over different MnO2 nanocrystals in a dielectric barrier discharge system

The alpha-MnO2, beta-MnO2 and gamma-MnO2 samples were prepared by the hydrothermal method and were used for the degradation of ciprofloxacin (CIP) wastewater in a combined DBD-catalytic process. The physical and chemical properties of the samples were systematically studied by several analytical techniques including BET, XRD, SEM, HRTEM, XPS, and H-2-TPR. The combination of DBD with alpha-MnO2 showed the highest CIP degradation efficiency, and the efficiency could reach 93.1% after 50 min, which was 10.8% and 18.1% higher, respectively, than those of beta-MnO2 and gamma-MnO2 catalysts in the plasma-catalytic system. According to the model of response surface methodology, the contribution of key experimental parameters on the CIP degradation decreased in the order: peak voltage > air flow rate > initial concentration > initial pH. The optimum operating parameters were peak voltage 17 kV, air flow rate 2.5 L min(-1), an initial concentration 5 mg L(-1 )and an initial pH 6.9. The quenching experiments of active species showed that center dot OH and center dot O-2(-) were critical to the CIP degradation. The generated O-3 might be adsorbed by the alpha-MnO2 catalyst and resulted in more center dot OH generation. The intermediate products of CIP degradation in DBD+alpha-MnO2 system were analyzed by LC-MS, and three possible degradation pathways were proposed. This research provides an insight into the use of the crystallographic structures in discharge plasma system for antibiotics in water. (C) 2020 Elsevier Ltd. All rights reserved.