Mechanistic insight into the degradation of ciprofloxacin in water by hydroxyl radicals

Ciprofloxacin (CIP), an effective antibacterial drug, is widely used to treat bacterial infections in humans and animals. However, drug pollution from residues and the development of resistant genes may pose serious ecological risks. Among the known methods of CIP degradation, advanced oxidation technology initiated by hydroxyl radicals exhibits great potential. However, an in-depth study of the degradation mechanism is difficult because of the limitations of the testing methods. In this study, CIP oxidation by hydroxyl radicals was evaluated using density functional theory (DFT), and the thermodynamics, kinetics, and toxicity were investigated. The results show that CIP oxidation occurs mainly through the piperazine ring, benzene ring, and C--C. High reactivity is achieved in the initial reactions, where only five reactions are not thermodynamically spontaneous. Reactions involving direct hydrogen abstraction by oxygen in this system are superior to the indirect reactions. Some theoretically predicted products, such as P6 and P11, are consistent with those reported in previous experiments, indicating that the theoretical study can provide supplementary information about the oxidation paths. The branching ratios for the hydrogen atom abstraction and addition reactions were 37. 45% and 62.55%, respectively. Finally, this reaction system is completely nontoxic based on toxicity assessment.

Automated summary

In this study, the oxidation of ciprofloxacin (CIP) by hydroxyl radicals was investigated using density functional theory (DFT), and the thermodynamics, kinetics, and toxicity were evaluated. The results showed that CIP was mainly oxidized through the piperazine ring, benzene ring, and C--C bond. High reactivity was observed in the initial reactions, with only five reactions being thermodynamically nonspontaneous. Reactions involving direct hydrogen abstraction by oxygen were found to be more favorable than indirect reactions. The theoretically predicted products were consistent with previous experimental findings, supporting the use of theoretical studies to provide supplementary information on oxidation pathways. The branching ratios for hydrogen atom abstraction and addition reactions were 37.45% and 62.55%, respectively. Finally, the toxicity assessment revealed that this reaction system is completely nontoxic.