Reaction Mechanism of Oxytetracycline Degradation by Electrogenerated Reactive Chlorine: The Influence of Current Density and pH

Abstract

A binary dimensionally stable anode Ti/TiO₂–RuO₂ electrode was used to abate the antibiotic oxytetracycline (OTC) (C₂₂H₂₄N₂O₉) in chloride water. The anode was prepared using the Pechini method and subsequently characterized by X-ray diffraction, scanning electron microscopy–energy-dispersive X-ray spectroscopy (SEM–EDS), and cyclic voltammetry (CV). The optimum values of the operational parameters affecting removal efficiency were determined using a 2 × 3 factorial design by screening j (6.0, 10, and 20 A m^–2) and pH (3, 6.5, and 10). The textural analysis revealed the formation of active oxides (RuO₂ and TiO₂ coating rutile-type P4₂/mnm, space group 136), with a cracked surface and good dispersion of metal components. A contour graph verified that the most suitable condition for contaminant degradation was 20 A m^–2 at a circumneutral pH of 6.5, resulting in approximately 97% degradation after 20 min of electrolysis according to pseudo-first-order reaction kinetics and the loss of the antibiotic activity of OTC. In addition, the results of oxidant formation and CV indicate that the best electrochemical activation of the anode to form Cl₂-active mainly depended on pH. Liquid chromatography–mass spectrometry (LC–MS) and density functional theory were employed to propose a reaction pathway for OTC degradation. Three byproducts with m/z 426, 256, and 226 were identified corresponding to the removal of amide and amine groups, which are susceptible sites to electrophilic attack by active chlorine species. The findings from this work stand out for prospective applications of anodic electrochemical oxidation to efficiently eliminate antibiotics with similar chemical structures in wastewater containing chlorides.