Coprecipitation of ferrihydrite with coexisting substances is more prevalent than their subsequent sorption, as organic matter and antibiotics typically exist in the environment before ferrihydrite forms. However, the influence of functional groups in antibiotics and organic matter on coprecipitate transformation and antibiotic degradation is still not fully explored. Enoxacin (ENO) and piperic acid (PPA) were selected to investigate the influence of fluorine substitution, while citric acid (CA) served as a model natural organic matter for carboxyl groups. The results revealed that the influence of carboxyl groups and fluorinated substituents on the transformation of coprecipitates varied with pH. During the transformation of the ferrihydrite-enoxacin coprecipitate, degradation ENO at pH 10.0 and 7.0 was approximately 2.0 and 1.8 times higher, respectively, than at pH 3.0. The most significant difference in the transformation pathways of coprecipitates was that at pH 3.0, direct transformation to hematite occurred without passing through the goethite intermediate. The transformation of coprecipitates to hematite at pH 10.0 and 7.0 was 85.0% and 28.2%, respectively. At pH 10.0, the carboxyl groups chelated with ferrihydrite coprecipitate forming relatively fixed grids, hindering the collision of nanoparticles and suppressing the formation of hematite. At pH 3.0, the carboxyl groups formed ligand-exchange complexes with Fe(III) and hydrogen bonds with ENO, producing larger aggregates and suppressing particle rearrangement. Compared with PPA, at pH 10.0, ENO containing fluorine substituents formed inner sphere complexes with coprecipitates, weakening Fe-O bonds and promoting transformation of coprecipitates. At pH 3.0, fluorine substituents inhibited the transformation of coprecipitates by influencing the p K a and charge distribution. At pH 7.0, the transformation of Fh-ENO coprecipitates to hematite was 1.7 times that of Fh-PPA. In Fh-ENO, fluoride ions formed Fe III Fn³⁻ polymers promoting goethite formation. This study provides a theoretical insight for predicting the environmental behavior of antibiotics containing different functional groups. • C–F bond and carboxyl group influence antibiotic degradation varying with pH. • C–F bond promote coprecipitate transformation, whereas carboxyl groups inhibit it. • Functional groups influence antibiotic degradation via coprecipitate transformation. • Under acidic conditions, the carboxyl group inhibited enoxacin degradation.
Wang et al. (Wed,) studied this question.