Degradation of Antiviral Drug Favipiravir Using UV, UV/H₂O₂, and Photocatalysis with Co-Doped ZnS Quantum Dots: Operational Parameters, Kinetic Studies, and Toxicity Assessment

Antiviral drugs, in particular those used to treat COVID-19, have been categorized as emerging pollutants in recent years due to their persistent presence in water/wastewater. They have been identified in environmental matrices all over the world, proving that current treatment methods cannot eliminate them from water/wastewater. In this study, the degradation of favipiravir (FAV) and ecotoxicity changes in different water matrices were investigated using the UV, UV/H₂O₂, and UV/Co-doped ZnS quantum dots (QDs) photocatalytic processes. The effects of initial FAV concentration, water matrices, pH, H₂O₂ concentration, and catalyst amount on the FAV degradation rate were evaluated. Initial FAV concentrations (50, 100, and 150 μg/L) have a slight effect on the FAV degradation rate. The matrix composition significantly reduced the degradation rates and efficiencies for the UV, UV/H₂O₂, and UV/Co-doped ZnS QDs processes in the following order: wastewater treatment plant (WWTP) effluent > tap water (TW) > distilled water (DW). The pH (4.0, 5.0, 7.0, and 9.0) had a remarkable effect on the degradation rate in all processes. The degradation rate enhanced from 86.3 to 98.1% with increasing pH from 4.0 to 9.0 due to increasing the ionization degree of FAV in the UV process. The maximum FAV degradation rate was obtained at pH 7.0 with removal efficiencies of 93.9 and 100% in UV/H₂O₂ and UV/Co-doped ZnS QDs processes, respectively. Transformation products of FAV were determined in UV and UV/Co-doped ZnS QDs processes. The toxicity to algae increased with increasing FAV concentrations from 50 to 150 μg/L in distilled water. Growth inhibition rates for 50, 100, and 150 μg/L FAV concentrations were 15.1, 33.3, and 36.3% at 96 h, respectively, without any treatment. After 60 min of the UV process, growth inhibition decreased below 0.5% regardless of concentration. Overall, the UV/Co-doped ZnS QDs process is effective in degrading FAV in all aqueous matrices; however, an initial treatment step is required to remove natural organic matter from actual matrices.