Iron-Mediated Molecular Degradation and Macromolecular Transformation of Quinolone Antibiotics: From Enhancement Mechanisms to Transformation Product Profiles and Ecotoxicity Evolution

Abstract Quinolone antibiotics, a class of persistent emerging contaminants, are difficult to completely remove with conventional wastewater treatment processes. Iron-based materials have been widely applied to enhance quinolone antibiotic removal through diverse physicochemical and biological pathways. This review systematically compares iron-mediated advanced oxidation processes (AOPs), biological treatment, and abiotic humification for quinolone antibiotic removal, with an emphasis on enhancement mechanisms, transformation-product profiles, and ecotoxicity evolution. Iron enhanced quinolone removal through oxidant activation and reactive oxygen species generation in AOPs, regulation of microbial metabolism in biological systems, and nucleophilic addition and coprecipitation during abiotic humification. Integration of reported transformation pathways, predicted toxicity, and experimental bioassays indicates that iron involvement not only promotes parent-compound removal but also reshapes transformation products and associated ecological risks. In degradation-dominated systems, quinolones mainly undergo side-chain modification, defluorination, piperazine ring cleavage, quinolone-core destruction, and further degradation. Sufficient oxidation generally favors toxicity reduction in AOPs, whereas toxicity evolution in biological systems is less predictable, because antibacterial intermediates may persist. In abiotic humification systems, macromolecular transformation may reduce predicted ecotoxicity by promoting quinolone sequestration into humic-like products. Overall, this Review provides a comprehensive understanding of iron-mediated quinolone antibiotic removal by linking enhancement mechanisms, transformation pathways, and ecological risk evolution.