Photo-transformation of acetaminophen sensitized by fluoroquinolones in the presence of bromide.

Fluoroquinolones (FQs) are a class of antibiotics with emerging concern. This study investigated the photochemical properties of two representative FQs, i.e., norfloxacin (NORF) and ofloxacin (OFLO). Results showed that both FQs could sensitize the photo-transformation of acetaminophen under UV-A irradiation, during which excited triplet state (3FQ*) was the main active species. In the presence of 3 mM Br‾, the photolysis rate of acetaminophen increased by 56.3% and 113.5% in the solutions with 10 μM NORF and OFLO, respectively. Such an effect was ascribed to the generation of reactive bromine species (RBS), which was verified by 3,5-dimethyl-1H-pyrazole (DMPZ) probing approach. 3FQ* reacts with acetaminophen through one-electron transfer, producing radical intermediates which then couple to each other. Presence of Br‾ did not lead to the formation of brominated products but the same coupling products, which suggests that radical bromine species, rather than free bromine, were responsible for the accelerated acetaminophen transformation. According to the identified reaction products and assisted with the theoretical computation, the transformation pathways of acetaminophen under UV-A irradiation were proposed. The results reported herein suggest that sunlight-driven reactions of FQs and Br‾ may influence the transformation of coexisting pollutants in surface water environments.