Sustainable Photodegradation of Ciprofloxacin Using Green-Synthesized Co₃O₄ Nanoparticles: Catalytic Performance and Antimicrobial Resistance Risk

The persistence of antibiotics such as ciprofloxacin (CIP) in water systems is a major environmental challenge, as residual concentrations may exert selective pressure and contribute to antimicrobial resistance. Co₃O₄NPs were synthesized using aqueous extracts of Coffea arabica L. leaves, followed by structural and morphological characterization via XRD, SEM, EDS, and UV–Vis spectroscopy. Catalytic performance in CIP degradation was evaluated under dark and light conditions (fluorescent), both with and without Co₃O₄NPs. CIP and its degradation products (DPs) were quantified using a validated high-performance liquid chromatography method. Kinetic parameters were determined, and antimicrobial resistance risk was operationally defined as the phenotypic resistance-selection potential of residual CIP or its DPs, based on their ability to maintain antimicrobial selective pressure and reduce bacterial susceptibility. This was assessed by exposing Escherichia coli ATCC 25,922 to subinhibitory concentrations equivalent to 1/2 MIC of CIP and DPs for 10 days, followed by MIC reassessment. The process followed second-order kinetics in the absence of light (R² = 0.9924) and first-order under illumination (R² = 0.9869), achieving 90.4% and 98.5% CIP removal in 72 h, respectively. Catalytic performance remained effective in the presence of humic acid and chloride ions, indicating robustness under environmentally relevant matrices. HPLC–MS/MS analysis identified dominant DPs (m/z 306, 362, and 263), consistent with piperazinyl oxidation and quinolone-ring disruption, which supports the reduced antibacterial activity observed for the DPs. CIP exposure increased the MIC of E. coli 15-fold, whereas DPs maintained low MIC values, indicating reduced antimicrobial selective pressure and lower phenotypic resistance-selection potential after catalytic treatment. Overall, green-synthesized Co₃O₄NPs showed high efficiency for CIP degradation, supporting their potential application as photocatalysts in advanced water treatment systems.