Roles of direct and indirect photodegradation in the photochemical fates of three 3rd generation fluoroquinolones.

Fluoroquinolones (FQs) are widely prescribed antibiotics that are commonly detected in aquatic environments, but the persistence, fates, and ecotoxicities of new generation FQs have yet to be fully investigated. We investigated the direct and indirect (hydroxyl radical (·OH), singlet oxygen (O21), and excited stated of organic matter (CDOM)) photodegradation of three 3rd generation FQs, moxifloxacin (MOX), gatifloxacin (GAT), and sparfloxacin (SPAR). The photodegradation rates and photolytic quantum yields (Φ) of the FQs depended on their dissociation species at different pH in a range of 1×10 to 1×10 M mol-photon. Unlike MOX and GAT whose zwitterions had the highest Φ, the anionic form of SPAR had the highest Φ. The k values were in the order of: k > k ≈ k with the 10Ms order of magnitude. The k values were in the order of: k (∼10Ms) > k (∼10Ms) > >> k (insignificant). Higher k values were observed for MOX (10 to 10Ms) compared to GAT and SPAR (10 to 10Ms). The zwitterions had the highest reactivities with ·OH and the lowest reactivities with O21 and CDOM. Reactions with ·OH enhanced the formation of transformation products (TPs) from decarboxylation and sidechain oxidation pathways, whereas reactions with O21 and CDOM enhanced the formation of TPs from sidechain oxidation pathways. Some of the TPs were predicted to exhibit aquatic ecotoxicity and environmental persistence. The half-lives of the FQs were estimated to be 0.42 to 0.67 h for MOX and SPAR, and 4.6 to 4.9 h for GAT. Their half-lives and main photochemical fates depended on the surface water pH and water column depth. These results highlight the key roles that photodegradation plays in removing new generation FQs from aquatic environments, though this might lead to the formation of TPs that are harmful to aquatic ecosystems.