Ciprofloxacin degradation in microbubble ozonation combined with electro-generated H2O2 process: Operational parameters and oxidation mechanism

Microbubble ozonation combined with electro-generated H2O2 process (EH2O2-MB-O3) was applied to degrade ciprofloxacin (CIP) as a typical antibiotic in wastewater in this study. The results showed that the performance of CIP degradation and mineralization in EH2O2-MB-O3 process was more efficient than other related processes. Microbubble ozonation and electro-generated H2O2 showed obvious synergistic effect on enhanced CIP removal in EH2O2-MB-O3 process. The O3 dosage, applied current and initial pH in EH2O2-MB-O3 process were optimized as 12.5 mg/min, 50 mA and 7.0 (uncontrolled). Under the optimal conditions, CIP could be degraded completely within 60 min and the TOC removal efficiency of 28.85% could be achieved within 90 min in EH2O2-MB-O3 process. The corresponding ozone utilization efficiency was close to 100%. More efficient ROS generation was observed in EH2O2-MB-O3 process, especially •OH, compared with other related processes. The contribution rates of •OH, 1O2, O3 and O2.- to CIP removal were determined as 66.79%, 23.57%, 6.67% and 2.98% in EH2O2-MB-O3 process, respectively. The electro-generated H2O2 was clarified to contribute to not CIP removal but ROS generation. Three CIP degradation pathways were proposed according to identified degradation intermediates, which indicated that pyridone structure of quinoline and piperazine ring in CIP molecule could be destroyed more easily than benzene ring of quinoline, and the hydroxylation seemed to prevail in these CIP degradation pathways due to •OH electrophilic addition reaction probably. The removal of other antibiotics in EH2O2-MB-O3 process also depended on not ozone direct oxidation but •OH oxidation mainly.