Degradation of enoxacin with different dissociated species during the transformation of ferrihydrite-antibiotic coprecipitates

Abstract

Ferrihydrite acts as a natural reservoir for nutrient elements, organic matter, and coexisting pollutants through adsorption and coprecipitation. However, the degradation of emerging fluoroquinolone antibiotics during the transformation of ferrihydrite coprecipitates, especially those with various dissociated species, remains insufficiently explored. In this study, Enoxacin (ENO), employed as a model antibiotic, was introduced to prepare ferrihydrite-ENO coprecipitates. The influence of coprecipitated ENO on the transformation of the ferrihydrite-ENO coprecipitate was investigated across different pH conditions. The results revealed that ferrihydrite-ENO coprecipitates thermodynamically transformed into more stable goethite and/or hematite under all pH conditions. In neutral and alkaline conditions, ENO promoted the transformation of coprecipitates into goethite while hindering hematite formation. Conversely, under acidic conditions, ENO directly obstructed the transformation of coprecipitates into hematite. Different dissociated species of ENO displayed distinct degradation pathways. The cationic form of ENO exhibited a greater tendency for hydroxylation and defluorination, while the zwitterion form leaned toward piperazine ring oxidation, with limited preference for quinolone ring oxidation. The anionic form of ENO exhibited the fastest degradation rate. It is essential to emphasize that the toxicity of the degradation products was intricately connected to the specific reaction sites and the functional groups they acquired post-oxidation. These findings offer fresh insights into the role of antibiotics in coprecipitation, the transformation of ferrihydrite coprecipitates, and the fate of coexisting antibiotics.

Keywords: Antibiotic; Coprecipitation; Degradation; Ferrihydrite; Toxicity.