Characteristics And Mechanism Of Enoxacin Degradation In Water Using Heterogeneous Photocatalysis Based On Ferrihydrite-Containing Fenton System

In recent years,The residues of fluoroquinolone antibiotics have been detected in various water bodies worldwide due to the widespread use and discharge of fluoroquinolone antibiotics.The residual fluoroquinolone antibiotics in the environment pose a potential threat to human health and the ecological environment.Ferrihydrite is a natural iron mineral.It has characteristics such as a large specific surface area and abundant surface hydroxyl groups,which allow ferrihydrite to affect the migration and transformation of coexisting pollutants through adsorption,co-precipitation,and redox reactions.Previous studies have shown that ferrihydrite can efficiently catalyze the decomposition of H₂O₂ to degrade organic pollutants.Additionally,the catalytic activity of iron-based materials is improved by the introduction of light into the Fenton-like systems,leading to the production of new active species to enhance the degradation efficiency of organic pollutants.However,the study on using ferrihydrite-catalyzed heterogeneous Fenton reaction to remove fluoroquinolone antibiotics is not yet sufficient.In this study,the second-generation fluoroquinolone antibiotic,enoxacin,was used as the model.Different reaction systems were constructed using various combinations of ferrihydrite,H₂O₂,and simulated sunlight to analyze the differences in enoxacin degradation characteristics under different systems.The main influencing factors on the degradation efficiency were determined by investigating the effects of enoxacin concentration,ferrihydrite dosage,H₂O₂ concentration,Cl^-concentration,and initial p H of the reaction system.The types of active species and enoxacin degradation products in the system were identified by a combination of free radical analysis,theoretical calculations,and mass spectrometry detection.The degradation pathways and mechanisms of enoxacin in the ferrihydrite/H₂O₂/simulated sunlight were elaborated.Finally,the toxicity of enoxacin and its degradation products was evaluated using the Ecological Structure Activity Relationship（ECOSAR）and Toxicity Estimation Software Tool（TEST）models.The main results are as follows:In the six different systems including the individual simulated sunlight,individual H₂O₂,H₂O₂/simulated sunlight,ferrihydrite/simulated sunlight,ferrihydrite/H₂O₂,and ferrihydrite/H₂O₂/simulated sunlight,the highest degradation efficiency of enoxacin was achieved in the ferrihydrite/H₂O₂/simulated sunlight system.The degradation efficiency of enoxacin in the ferrihydrite/H₂O₂/simulated sunlight system decreased with increasing initial concentration of enoxacin,but increased with the increase of ferrihydrite dosage and H₂O₂ concentration.When the initial p H value of the system was 3.0,5.0,7.0,and 9.0,the degradation efficiencies of enoxacin were 89.7%,73.9%,87.0%,and 88.9%,respectively.The presence of Cl^-inhibited the degradation of enoxacin.In the reaction system,hydroxyl radicals（·OH）and superoxide radicals were detected.The accumulated·OH concentration in the reaction system increased with the increase of the ferrihydrite dosage and the H₂O₂ concentration.The order of the accumulated·OH concentration at different p H values was p H 3.0>p H 7.0>p H 5.0>p H 9.0,and the consumption order of H₂O₂ was p H 7.0>p H 5.0>p H 9.0>p H 3.0.When the p H of the system was 3.0,the decomposition rate of H₂O₂ was the lowest,but the accumulated·OH concentration was the highest.The accumulated·OH concentration of the untreated reaction system was higher than that of the N₂ treated and O₂ treated reaction systems.During the degradation of enoxacin in the ferrihydrite/H₂O₂/simulated sunlight system,the conversion of Fe（III）and Fe（II）on the surface of the ferrihydrite was observed.The results of theoretical calculations showed that the highest occupied molecular orbital of enoxacin was mainly located in the quinolone and piperazine rings.The piperazine ring of enoxacin had a positive electrostatic potential,and the sites with larger Fukui function values were mainly distributed in the quinolone ring.Eleven degradation products of enoxacin were generated in the ferrihydrite/H₂O₂/simulated sunlight system.The defluorination,as well as the cleavage and oxidation of piperazine and quinolone rings were its main degradation pathways.After the degradation of enoxacin in the ferrihydrite/H₂O₂/simulated sunlight system,the overall toxicity of degradation products of enoxacin decreased.This study can provide a reference for the development of new materials and technologies based on ferrihydrite for removing emerging contaminants represented by fluoroquinolone antibiotics.