Effects of natural colloidal particles derived from a shallow lake on the photodegradation of ofloxacin and ciprofloxacin.

Natural colloidal particles (NCPs), which are ubiquitous and abundant in surface waters, may play a crucial role in the sunlight-driven transformation of organic contaminants. This research focused on the effects of NCPs on the photodegradation of two fluoroquinolone antibiotics (FQs), ofloxacin (OFL) and ciprofloxacin (CIP), and assessed the photosensitivity of colloidal organic matter (COM). Results showed that the photodegradation rate constants (k) of OFL and CIP in NCP solutions ranged from 9.28 × 10 h to 15.98 × 10 h and 63.88 × 10 h to 196.59 × 10 h, respectively, and NCPs can significantly accelerate the photodegradation rate of OFL and CIP. Indirect photodegradation (IP) accounted for >50% of the overall observed degradation in most treatments and was the dominant degradation pathway for the two FQs, especially for CIP, for which IP reached 82%-94%. In the IP process, the contributions of triplet states of colloidal organic matter (COM) to the photolysis of OFL and CIP were close to 42% and 46%, respectively. The compositions of COM played an important role in the IP of the FQs, among which terrestrial sources of COM tended to have higher photoreactivity than biological sources. This study is essential in predicting the photochemical effect of FQs and also allows for a better understanding of the real environmental fate of antibiotic contaminants.