Effects of nitrate and humic acid on enrofloxacin photolysis in an aqueous system under three light conditions: kinetics and mechanism

Environmental context Photolysis is one of the most important transformation pathways in natural ecosystem for enrofloxacin (Enro), which is a hazard for humans and other living organisms. The effects of NO3– and humic acid on Enro photolysis were found to be light-source dependent. These results are of significance toward the goal of providing insight into the transformation and fate of Enro in the environment. Abstract The light-source-dependent effects of NO3– and humic acid (HA) on enrofloxacin (Enro) photolysis kinetics in aqueous solutions were investigated under solar, UV-254 and UV-365 lamp irradiation. NO3– was found to suppress Enro photolysis through competitive photoabsorption under UV-365 irradiation, whereas it accelerated Enro photolysis under UV-254 and solar irradiation as a result of NO3– photosensitisation. Similarly, HA enhanced, inhibited or had no obvious effect on Enro photolysis under different light irradiation conditions. Even under the same light irradiation conditions, the effect of HA on Enro photolysis varied with HA concentration. The reactive oxygen species (ROS) scavenger experiments demonstrated that Enro photolysis undergoes •OH- and 1O2-mediated self-sensitised photolysis. The photolysis pathway of Enro involved decarboxylation, defluorination and piperazinyl N4-dealkylation reactions. The toxicity towards Vibrio fischeri luminescent bacteria under solar irradiation was different from that under UV irradiation. The 90-min toxicity of Enro and its photoproducts increased under solar irradiation but decreased under UV-365 and UV-254 irradiation compared to the initial Enro toxicity, which indicated that UV light not only had higher photolysis efficiency but also posed less toxicity towards bacteria than solar.