Photolytic degradation of ciprofloxacin in solid and aqueous environments: kinetics, phototransformation pathways, and byproducts

Many lipophilic pharmaceuticals may be sorbed in solid phases, leading to different photochemical behaviors. This study investigated the photochemistry of ciprofloxacin in a solid-phase system and compared it to that in a water-phase system. Kaolinite was used as the model solid matrix. The photolysis of ciprofloxacin in kaolinite fits pseudo-first-order kinetics for thicknesses less than 199 μm, and the rate constants kp decreased from 0.0154 to 0.0016 min−1 as the thickness of the layer increased. Unlike the aqueous phase, two-step degradation processes were observed for all kaolinite layer thicknesses (14–199 μm), and the pseudo-first-order constant at the surface of the kaolinite layer was smaller than that in the water phase. Comparatively, a similar photolysis rate constant of ciprofloxacin in a kaolinite suspension was also observed, and it was an order of magnitude smaller than that of the direct photodegradation (0.035 min−1) in water. The results indicate that ciprofloxacin is likely more stable when it is adsorbed on kaolinite and that the half-lives of ciprofloxacin in kaolinite and a kaolinite suspension are 2–25 times longer than that in deionized water (20 min) under simulated sunlight. Direct photolysis is proposed to be the main photodegradation mechanism for ciprofloxacin in kaolinite, and the cleavage of a piperazine ring is the main degradation pathway. However, the interaction between ciprofloxacin and kaolinite reduces the direct photolysis and leads to a higher light stability. In association with the reduction in photolysis, the yields of norfloxacin and defluorinated byproduct decreased significantly. Consequently, the interaction increases the persistence of ciprofloxacin and thus the ecological risk to the environment.