Direct and indirect photolysis of oxolinic acid in surface waters and its inhibition by antioxidant effects

Oxolinic acid is a quinolone antibiotic used in aquaculture to prevent and treat animal diseases. Because of its application and the large expansion of aquaculture in the latest decades, oxolinic acid enters environmental waters through the effluents of aquaculture facilities, posing concerns due to its potential adverse effects on aquatic ecosystems. It is thus important to study the fate of this antibiotic in water bodies. This work investigated the reactivity of the anionic form of oxolinic acid (OxA) by direct and indirect photolysis. The quantum yield of direct photolysis and the bimolecular rate constants of OxA reactions with reactive species photochemically produced in fresh- and seawater (i.e., HO^•, CO₃^•-, triplet states of dissolved organic matter, ¹O₂, and Br₂^•-) were determined through steady-state irradiation experiments and laser flash photolysis measurements. Results showed that OxA photoreactivity is significant, in particular towards HO^• and CO₃^•- radicals. However, the direct photolysis and reactions with CO₃^•- and the triplet states of dissolved organic matter were found to be significantly inhibited in the presence of phenol, here used as a representative compound for antioxidant dissolved organic matter, most likely because of a back-reduction process. Photochemical modeling predicted an antibiotic half-life time of some days in fresh- and seawater, showing that OxA degradation is mainly due to direct photolysis in both environments plus reactions with CO₃^•- (freshwater) and Br₂^•- (seawater).