Natural photodegradation characteristics of enrofloxacin in the aqueous phase and aqueous-montmorillonite systems: Insight into the effect of copper and its complex behavior

The complexation between antibiotics and heavy metals significantly influenced the environmental photochemical behavior. Studying the evolution of complexation patterns in real environments was essential for the evaluation of composite ecological risk. Consequently, the photochemical behavior of enrofloxacin (ENR) and its interaction mechanism with Cu²⁺ were investigated in both the aqueous phase and aqueous-montmorillonite (MMT) system in this study. In the aqueous phase, ENR underwent CN bond cleavage and piperazine ring opening via direct and self-sensitized degradation pathways, ultimately degrading into small molecules. The presence of Cu²⁺ induced the complexation of two ENR molecules with one Cu²⁺ through the CO groups of the pyridine ring and carboxyl groups, forming ENR-Cu²⁺ complexes. At Cu²⁺ concentrations below 40 mg·L⁻¹, the Cu(OH)₂ precipitation competed with ENR for photons and inhibited photodegradation. And at concentrations above 40 mg·L⁻¹, the photoexcited electron transfer on the ENR-Cu²⁺ surfaces promoted Cu²⁺ reduction to Cu⁺ and accelerated •OH generation via the Cu²⁺/Cu⁺ cycle, and •O₂⁻ synergistically facilitating photodegradation and oxidation intermediate ([ENR-Cu²⁺]⁻•). In the aqueous-MMT system, ENR degradation primarily depended on •OH and •O₂⁻, driving defluorination and cleavage of polycyclic structures. In the presence of Cu²⁺, MMT-Cu²⁺-ENR ternary complexes formed through bridging. At Cu²⁺ concentrations below 40 mg·L⁻¹, complexation inhibited ENR photodegradation, whereas concentration above 40 mg·L⁻¹ markedly enhanced degradation efficiency. The Cu²⁺-mediated bridging effect on MMT surfaces increased ENR binding sites, further promoting Cu⁺ generation and sustaining reactive oxygen species (ROS) cycling. Synergistic effects of complexation and photodegradation collectively dominated the photochemical behavior of ENR-Cu²⁺ in aqueous and aqueous-MMT systems, providing a theoretical foundation for environmental photochemical transformation of combined contaminations.