Environmental Impact of Fluoroquinolones and Their Photocatalytic Transformation Products: Degradation with Activated Sludge and in Surface Waters, Change in Antimicrobial Activity and Ecotoxicity

Fluoroquinolone antibiotics are widely used in medicine and in veterinary medicine. Due to their stable chemical structure, their residues have been detected in the aquatic environment. Photocatalytic degradation is one of the promising methods for the removal of antibiotics, but the formed organic by-products can exhibit antimicrobial activity and still be toxic to aquatic organisms. The purpose of this study was to identify intermediates formed during norfloxacin and enrofloxacin photocatalysis with TiO₂ and to evaluate in vitro their ecotoxicity towards selected bacteria strains and, in silico, towards algae, daphnids, and fish. Furthermore, changes in the ecotoxicity and composition of solutions after photocatalysis were studied in activated sludge and samples from various natural aquatic ecosystems. After degradation, 24 and 27 intermediates of norfloxacin and enrofloxacin, respectively, were identified, and most of them retained a preserved biologically active pharmacophore. Some intermediates were persistent in the natural environment and may pose long-term ecological risks. In the presence of activated sludge, the majority of fluoroquinolones, as well as most of the intermediates, were degraded. This indicates that the combination of photocatalysis and biological treatment significantly reduces the ecotoxicity of solutions containing fluoroquinolone residues. One of the toxic degradation products was resistant to biodegradation under the conditions used.