Photochemical behavior of antibiotics impacted by complexation effects of concomitant metals: a case for ciprofloxacin and Cu(ii)

Abstract

Many water bodies, especially those adjacent to aquaculture and livestock breeding areas, are contaminated by both antibiotics and transition metals. However, the effects of the interaction between antibiotics and transition metals on the environmental behavior and the ecotoxicology of antibiotics are largely unknown. We hypothesized that antibiotics may coordinately bind with metal ions, and this complexation may affect the environmental photochemical behavior of antibiotics. We took ciprofloxacin (CIP) and Cu(II) as a case, and employed simulated sunlight experiments and density functional theory calculations to investigate the underlying reaction mechanisms. The results showed that monovalent cationic ciprofloxacin (H₂CIP⁺) that is predominant in the normal pH range (6–9) of surface waters can chelate with hydrated Cu(II) to form [Cu(H₂CIP)(H₂O)₄]³⁺. Compared with H₂CIP⁺, [Cu(H₂CIP)(H₂O)₄]³⁺ has different molecular orbitals, and atomic charge distribution. As a result, [Cu(H₂CIP)(H₂O)₄]³⁺ showed dissimilar light absorption properties, slower direct photolytic rates, lower ¹O₂ generation ability and weaker reactivity towards ¹O₂. Due to the Cu(II) complexation, the apparent photodegradation of H₂CIP⁺ was inhibited, and the photolytic pathways and product distribution were altered. This study implies that for an accurate ecological risk assessment of antibiotics under transition metal co-contamination conditions, the effects of metal complexation should be considered.