Cu(II) inhibits the photolysis via complexation-regulated excited state of enrofloxacin in seawater

Abstract

Enrofloxacin (ENR) and Cu(II) usually coexist in seawater, particularly in wastewater from marine aquaculture. However, the effect of Cu(II) on the photolysis of ENR is still unclear, limiting the understanding of the fate of antibiotics in natural seawater. In this study, we investigated the role of Cu(II) in the photolysis of ENR. Cu(II) significantly inhibited the photolysis of ENR at environmentally relevant concentrations, reducing the degradation rate from 84.12 % to 61.45 %. Cu(II) is more inclined to form ENR-Cu complexes with a complex ratio of 1 via the oxygen atoms of the carbonyl and carboxyl groups on HENR^± and H₂ENR⁺, with intramolecular ligand-to-metal charge transfer of 0.794 e and 0.580 e, respectively. The complexation altered the original active reaction sites of ENR, reducing the nucleophilicity of the parent molecule and diminishing its reactivity with singlet oxygen (¹O₂). When the ENR:Cu(II) molar ratio was 1, the steady-state concentration of ¹O₂ was 4.069 × 10⁻¹⁴ M, which was 50.64 % lower than that in the absence of Cu(II). Besides, the excited state of the complexes enhanced the hole-electron separation. Additionally, Cu(II) can modify the partial photolysis pathway of ENR, thereby impacting the toxicity of the byproducts. This work contributes to a better understanding of the environmental fate of antibiotics in natural seawater.