Efficient degradation of antibiotics by non-thermal discharge plasma: Highlight the impacts of molecular structures and degradation pathways

Pollution of antibiotics in water has aroused global concerns. In this study, non-thermal discharge plasma oxidation was applied to eliminate three antibiotics with different substituent groups and chemical properties, tetracycline (TC), sulfadiazine (SD), and ciprofloxacin (CIP). All the three antibiotics could be effectively degraded but with different reaction kinetics, and the reaction rate of TC was always the highest, followed by SD and CIP. Reactive oxygen species (ROS) including center dot O-2(-), O-1(2), and center dot OH all contributed to the removal of the three antibiotics. The theoretically calculation using Gaussian with DFT indicated that the ionization potential (IP) of the three antibiotics was in the order of TC < SD < CIP. Since the compounds with lower IP values were more vulnerable to ROS attack, the lowest IP of TC suggested that it was the easiest to be degraded among the three target compounds. In addition, the Gaussian calculations predicted that the chemical bonds with low energies were susceptible for oxidation. Integrating the results of theoretical calculation and those of UV-vis spectra, three-dimensional fluorescence spectroscopy, gas chromatography-mass spectrometry and ion chromatography, the degradation pathways of the three antibiotics were figured out, mainly including hydroxylation, decarboxylation, dealkylation, and ring-opening reactions. The results provide clues to predict the potentials and pathways of oxidative degradation of antibiotics in water.