Ciprofloxacin transformation in aqueous environments: Mechanism, kinetics, and toxicity assessment during •OH-mediated oxidation - 白凤杨

The initial reactions of organics with •OH are important to understand their transformations and fates in advanced oxidation processes in aqueous phase. Herein, the kinetics and mechanism of •OH-initiated degradation of ciprofloxacin (CIP), an antibiotic of fluoroquinolone class, are obtained using density functional and computational kinetics methods. All feasible mechanisms are considered, including H-abstraction, •OH-addition, and sequential electron proton transfer. Results showed that the H-abstraction is the dominant reaction pathway, and the product radicals P7single bondH, P9single bondH, and P10single bondH are the dominating intermediates. The aqueous phase rate coefficients for the •OH-triggered reaction of ciprofloxacin are calculated from 273 K to 323 K to examine the temperature dependent effect, and the theoretical value of 6.07 × 109 M−1 s−1 at 298 K is close to the corresponding experimental data. Moreover, the intermediates P7single bondH, P9single bondH, and P10single bondH could easily transform to several stable products in the presence of O2, HO2•, and •OH. The peroxy radical, which is generated from the incorporation of H-abstraction product radicals (P7single bondH, P9single bondH, and P10single bondH) with O2, prefers to produce HO2• into the surrounding through direct concerted elimination rather than the indirect mechanism. In addition, the peroxy radical could react with HO2• via triplet and singlet routes, and the former is more favorable due to its smaller barrier compared with the latter. The hydroxyl-substituted CIP has higher activity than its parent compound in their reactions with •OH due to its lower barrier and faster rate. In addition, the -NHC(O)-containing compound IM3-P10-H-4 is harmful to aquatic fish and is the primary product in the •OH-rich environment according to the ecotoxicity assessment computations. This study can improve our comprehension on CIP transformation in complex water environments.