Removal of antibiotic cloxacillin by means of electrochemical oxidation, TiO2 photocatalysis, and photo-Fenton processes: analysis of degradation pathways and effect of the water matrix on the elimination of antimicrobial activity

Abstract

This study evaluates the treatment of the antibiotic cloxacillin (CLX) in water by means of electrochemical oxidation, TiO₂ photocatalysis, and the photo-Fenton system. The three treatments completely removed cloxacillin and eliminated the residual antimicrobial activity from synthetic pharmaceutical wastewater containing the antibiotic, commercial excipients, and inorganic ions. However, significant differences in the degradation routes were found. In the photo-Fenton process, the hydroxyl radical was involved in the antibiotic removal, while in the TiO₂ photocatalysis process, the action of both the holes and the adsorbed hydroxyl radicals degraded the pollutant. In the electrochemical treatment (using a Ti/IrO₂ anode in sodium chloride as supporting electrolyte), oxidation via HClO played the main role in the removal of CLX. The analysis of initial by-products showed five different mechanistic pathways: oxidation of the thioether group, opening of the central β-lactam ring, breakdown of the secondary amide, hydroxylation of the aromatic ring, and decarboxylation. All the oxidation processes exhibited the three first pathways. Moreover, the aromatic ring hydroxylation was found in both photochemical treatments, while the decarboxylation of the pollutant was only observed in the TiO₂ photocatalysis process. As a consequence of the degradation routes and mechanistic pathways, the elimination of organic carbon was different. After 480 and 240 min, the TiO₂ photocatalysis and photo-Fenton processes achieved ∼45 and ∼15 % of mineralization, respectively. During the electrochemical treatment, 100 % of the organic carbon remained even after the antibiotic was treated four times the time needed to degrade it. In contrast, in all processes, a natural matrix (mineral water) did not considerably inhibit pollutant elimination. However, the presence of glucose in the water significantly affected the degradation of CLX by means of TiO₂ photocatalysis.

Keywords: Advanced oxidation process; Antimicrobial activity removal; Degradation pathways; Electrochemical oxidation; Matrix effects; Pollutant mineralization; Water treatment; β-Lactam antibiotic.