Traditionally, antibiotics have been used to treat human and animal diseases caused by pathogenic bacteria. The aquaculture industry, which is massively expanding currently, also makes use of several antibiotic classes, resulting in potential antibiotic residues in the surrounding aquatic environment, as well as the cultured products raising bacterial resistance. The aim of this study was the optimization, validation, and application of a solid-phase extraction (SPE) method in combination with liquid chromatography (LC)-LTQ/Orbitrap mass spectrometry in order to determine the most commonly used antibiotics in waters sampled from fish farms, both saltwater and freshwater, located in Greece. Under optimum conditions, the method was validated, achieving recoveries in the range of 57.7% (for sulfamethoxazole in river water) to 95.8% (for florfenicol in river water). The method quantification limits were within the range of 0.25 and 10 ng·L⁻¹ in all cases, with relative standard deviations (RSDs) < 15.9%. The application of the proposed methodology revealed the presence of oxytetracycline and trimethoprim traces. Finally, an assessment of the environmental risk posed by the detected antibiotics was performed, calculating either the risk quotient (RQ) for three trophic levels (8.013 × 10⁻⁶ < RQ < 0.496) or the mixture RQ (0.005 < RQ < 0.682), proving that in all cases, the risk was medium to low.

Photocatalysis is a prominent alternative wastewater treatment technique that has the potential to completely degrade pesticides as well as other persistent organic pollutants, leading to detoxification of wastewater and thus paving the way for its efficient reuse. In addition to the more conventional photocatalysts (e.g., TiO₂, ZnO, etc.) that utilize only UV light for activation, the interest of the scientific community has recently focused on the development and application of visible light-activated photocatalysts like g-C₃N₄. However, some disadvantages of g-C₃N₄, such as the high recombination rate of photogenerated charges, limit its utility. In this light, the present study focuses on the synthesis of WO₃ fibers/g-C₃N₄ Z-scheme heterojunctions to improve the efficiency of g-C₃N₄ towards the photocatalytic removal of the widely used insecticide clothianidin. The effect of two different g-C₃N₄ precursors (urea and thiourea) and of WO₃ fiber content on the properties of the synthesized composite materials was also investigated. All aforementioned materials were characterized by a number of techniques (XRD, SEM-EDS, ATR-FTIR, Raman spectroscopy, DRS, etc.). According to the results, mixing 6.5% ^W/_W WO₃ fibers with either urea or thiourea derived g-C₃N₄ significantly increased the photocatalytic activity of the resulting composites compared to the precursor materials. In order to further elucidate the effect of the most efficient composite photocatalyst in the degradation of clothianidin, the generated transformation products were tentatively identified through UHPLC tandem high-resolution mass spectroscopy. Finally, the detoxification effect of the most efficient process was also assessed by combining the results of an in-vitro methodology and the predictions of two in-silico tools.

Furazolidone, a nitrofuran antibiotic, has been broadly used in aquaculture and veterinary medicine, and its presence in water poses considerable environmental and health hazards due to its toxicity. This study investigated a hybrid photocatalytic process for the removal of furazolidone, employing graphitic carbon nitride (g-C₃N₄) and persulfate anions (PS) under both laboratory and pilot-scale conditions. The synergistic effect of g-C₃N₄ and PS enhanced the generation of reactive species, facilitating the efficient degradation of FZ in two different aqueous matrices. Through scavenging studies, positive holes were determined to be the dominant reactive species, followed by sulfate radicals. Seven transformation products of FZ were tentatively identified via UHPLC-LTQ/Orbitrap MS analysis. The optimized photocatalytic system (g-C₃N₄/PS) achieved a 100% removal of furazolidone in less than 60 min under simulated solar light, demonstrating its potential for large-scale application in wastewater remediation. Furthermore, pilot-scale experiments using real secondary treated municipal wastewater proved that the applied process is capable of achieving an 86.2% removal of furazolidone (k = 0.017 min⁻¹) as well as a 90% decrease in effluent ecotoxicity within 120 min of UV_A irradiation. This study provides insights into sustainable processes for the removal of antibiotic contaminants from wastewater and underscores the role of g-C₃N₄-based photocatalytic approaches in upper-scale applications.