Oxidative degradation of fluoroquinolone antibiotics by ferrate(VI): Kinetics, reaction mechanism, and theoretical calculations

Abstract

The extensive use of fluoroquinolone antibiotics (FQs) and their low degradation efficiency pose significant threats to aquatic ecosystems and human health. However, the species-specific reactions and the key reaction mechanisms underlying their degradation by ferrate (Fe(VI)) based on density functional theory (DFT) calculations remain unclear. This study systematically examines the oxidation mechanisms of four FQs (enoxacin (ENO), ofloxacin (OFL), gatifloxacin (GAT), and fleroxacin (FLE)) by Fe(VI), through combined experimental and DFT methods. The results showed that the oxidation of FQs by Fe(VI) conformed to secondary reaction kinetics with second-order reaction rate constants following FLE (1.57 mM⁻¹·min⁻¹) > GAT (0.99 mM⁻¹·min⁻¹) > OFL (0.96 mM⁻¹·min⁻¹) > ENO (0.79 mM⁻¹·min⁻¹). While Fe(VI) species dominated the reaction, specific contributions from Fe(V)/Fe(IV) and hydroxyl radicals (·OH) were quantitatively verified, and DFT further proved that FeO₄^2-, as the predominant Fe(VI) species, governed the reaction at pH 8.0, the optimum reaction pH. Instrumental analysis detected the main products, and DFT predicted the reactive active sites, suggesting that the quinolone and piperazine rings cleavage on the FQs molecules was achieved through hydroxylation, decarboxylation, and other reactions, with the intermediates tending to be harmless. Both methods identified three distinct reaction mechanisms: ·OH attack, single-oxygen transfer (SOT), and double-oxygen transfer, with ·OH attack the most likely to occur and SOT the main reaction mechanism. This study combines DFT calculations with experimental observations to identify the mechanisms of Fe(VI)-mediated FQs degradation at the molecular structural level, and provide new insights into the treatment of FQs.