Species-Dependent Degradation of Ciprofloxacin in a Membrane Anodic Fenton System

The anodic Fenton treatment method (AFT) has been successfully applied to the removal of ciprofloxacin (CIP), a widely used fluoroquinolone antibiotic, from aqueous solution. Degradation kinetics were found to be species dependent. At initial pH 3.2, CIP remained in its cationic form and the kinetics followed a previously developed AFT model. At an initial near-neutral pH, CIP speciation changed during the degradation, due to pH changes over the process, and no obvious model fit the data. Density functional theory (DFT) calculations indicated a protonated species-dependent reaction affinity toward hydroxyl radicals. A new model based on the AFT model with the addition of species distribution during the degradation was derived, and it was shown to describe the degradation kinetics successfully. Degradation of reference compounds further confirmed that the free carboxylic acid group, which contributes to the species changes, plays a key role in the observed degradation pattern. Furthermore, degradation of reference CIP metal complexes confirmed that the formation of these complexes does not have a major effect on the degradation pattern. Optimization of CIP degradation was carried out at pH 3.2 with an optimal H2O2/Fe2+ ratio found between 10:1 and 15:1. Three degradation pathways based on mass spectrometry data were also proposed: (1) hydroxylation and defluorination on the aromatic ring; (2) oxidative decarboxylation; and (3) oxidation on the piperazine ring and dealkylation. By the end of the AFT treatment, neither CIP nor its degradation products were detected, indicating successful removal of antibacterial properties.