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

Abstract

The α-MnO₂, β-MnO₂ and γ-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 H₂-TPR. The combination of DBD with α-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 β-MnO₂ and γ-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 OH and O₂^- were critical to the CIP degradation. The generated O₃ might be adsorbed by the α-MnO₂ catalyst and resulted in more OH generation. The intermediate products of CIP degradation in DBD+α-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.

Keywords: Ciprofloxacin degradation; MnO(2); Non-thermal plasma; Response surface methodology; Wastewater.