Ozone-enhanced TiO nanotube arrays for the removal of COVID-19 aided antibiotic ciprofloxacin from water: Process implications and toxicological evaluation.

The purpose of this study was to evaluate the performance of synthesized TiO nanotube arrays (NTAs) for the removal of the COVID-19 aided antibiotic ciprofloxacin (CIP) and the textile dye methylene blue (MB) from model wastewater. Synthesis of TiO NTAs showed that anodization potential and calcination temperatures directly influence nanotube formation. The increased anodization potential from 10 to 40 V resulted in the development of larger porous nanotubes with a diameter of 36-170 nm, while the collapse of the tubular structure was registered at the highest applied potential. Furthermore, it was found that the 500 °C calcination temperature was the most prominent for the formation of the most photocatalytically active TiO NTAs, due to the optimal anatase/rutile ratio of 4.60. The degradation of both model compounds was achieved with all synthesized TiO NTAs; however, the most photocatalytically active NTA sample was produced at 30 V and 500 C. Compared to photocatalysis, CIP degradation was greatly enhanced by 5-25 times when ozone was introduced to the photocatalytic cell (rates 0.4-4.2 × 10 min versus 0.07-0.2 × 10 min). This resulted in the formation of CIP degradation by-products, with different mass-to-charge ratios from [M+H] 346 to 273 m/z. Even though the CIP degradation pathway is rather complex, three main mechanisms, decarboxylation, hydroxylation reaction, and piperazine ring cleavage, were proposed and explained. Furthermore, treated samples were placed in contact with the crustaceans Daphnia magna. It was found that 100% mortality was achieved when approximately 60% of the remaining TOC was present in the samples, indicating that toxic degradation by-products were formed.