Superoxide-Mediated Photocatalytic Degradation of Azithromycin by Cellulose Nanocrystal-TiO2 Nanocomposites: From Synthetic Solutions to Hospital Wastewater

Abstract

Trace amounts of azithromycin in aquatic systems exert selective pressure on microbial populations, accelerating the emergence and dissemination of antibiotic-tolerant bacteria. Consequently, treatment approaches that eliminate the parent compound are critical for mitigating resistance risks, with photocatalytic oxidation offering a highly effective solution. A simple and novel cellulose nanocrystal-supported TiO₂ photocatalyst was synthesized, in which superoxide radicals dominate the oxidative degradation of azithromycin. The nanocomposites were produced via a precipitation-based method and characterized using FESEM, XRD, FTIR, and UV-visible diffuse reflectance spectroscopy. Operational factors, including catalyst dosage, irradiation time, and solution pH, were systematically optimized, achieving near-complete (~100%) pollutant removal under acidic conditions and high efficiency (~98%) under near-neutral pH. Reactive species quenching experiments identified superoxide radicals as the primary contributors to the degradation mechanism. Degradation products were identified through LC-MS analysis, enabling the proposal of a transformation pathway. Substantial mineralization was confirmed by a significant reduction (~71%) in total organic carbon, while bioassays revealed diminished antibacterial activity, indicating effective detoxification. The catalyst maintained strong performance in real hospital wastewater and exhibited minimal metal leaching over multiple cycles, demonstrating good stability and reusability. These findings highlight the potential of cellulose-supported titanium dioxide nanocomposites for practical antibiotic removal from wastewater.