| Antibiotics have been increasingly detected in environmental waters as emerging contaminants.These pollutants are pseudopersistent and have been proved to induce bacterial resistance,making them of acute concern.Phenicol and fluoroquinolone antibiotics(FQs) are commonly used in aquaculture and ubiquitous in the environment,especially in fish farms and ambient waters,so it is of great significance to investigate their environmental transformation, fate and ecological risk.In surface waters,photochemical degradation is a central factor in determining the fate of antibiotics.Therefore,the present study selected 2 phenicols and 8 FQs as model compounds,investigated their photodegradation kinetics,photoproducts,pathways and mechanisms,and elucidated the effects of main aqueous environmental factors on the photodegradation so as to better understand their environmental photochemical behavior.Photodegradation experiments on thiamphenicol and florfenicol were performed in pure water under irradiation of different light sources.The two phenicols did not photodegrade under solar or simulated solar irradiation(λ＞290 nm),but photolyzed quickly when exposed to UV-vis irradiation(λ＞200 nm).The UV-vis photodegradation reactions followed the pseudo-first-order kinetics,and their quantum yields were 0.022±0.002 and 0.029±0.002, respectively.Electron paramagnetic resonance(EPR) measurements and scavenging experiments indicated that the phenicols underwent direct photolysis and self-sensitized photodegradation via singlet oxygen(1O2).The photodegradation intermediates were identified by HPLC-MS/MS and IC,and the proposed degradation pathways involve self-sensitized photo-oxidation,photoinduced hydrolysis,dechlorination and chlorination.Photodegradation kinetics of the two phenicols in different waters were investigated.It was found that under UV-vis irradiation,they photodegraded the fastest in seawater,followed by pure water and freshwater,whereas under solar or simulated sunlight,they photodegraded in freshwater only.The effects of Cl-(the dominant seawater constituent),humic acids(HA, main constituents in freshwater) and other water constituents(Fe(Ⅲ),NO3-,HCO3-,etc.) on the photodegradation of the antibiotics as a function of different light sources were studied so as to interpret the light-source-dependent effects of different waters.Under UV-vis irradiation,Cl- was found to promote 1O2 formation and accelerated the photodegradation of the two phenicols,whereas the phenicois did not photolyze under simulated solar irradiation, irrespective of Cl-.In contrast,the presence of HA inhibited phenicol photolysis under UV-vis irradiation through competitive photoabsorption,but HA photosensitized degradation under simulated solar irradiation.Assessing the role of all the water constituents showed that the light-source-dependent effects of Cl- and HA on the photodegradation explained most of the different photolytic kinetics in natural waters and pure water.To further understand the effects of aqueous dissolved matter on photodegradation of antibiotics,we explored the environmental photochemical behavior of eight FQs,such as sarafloxacin,gatifloxacin,etc.These FQs were exposed to simulated solar irradiation(λ＞290 nm) and their photodegradation followed apparent first-order kinetics.The quantum yields ranged from 4.7×10-3 to 7.0×10-2.Scavenging experiments revealed that the FQs underwent direct photolysis and self-sensitized photodegradation via·OH and 1O2.Product studies indicated that three main photodegradation pathways co-occurred and were highly parent-structure dependent.The three pathways are piperazinyl N4-dealkylation,photoinduced decarboxylation,and hydroxylated defluorination.The photodegradation solutions of the FQs exhibited photomodified toxicities to luminescent bacterium Vibrio fischeri,indicative of the formation of some hazardous products.The FQs exhibited a similar or less photodegradable potential in freshwater and seawater, compared to that in pure water.In order to elucidate the photochemical behavior in natural waters,sarafloxacin and gatifloxacin were selected as model compounds to examine the individual role of pH,HA and Cl- on the photodegradation kinetics of the two FQs.Moreover, the multivariate effects of Fe(Ⅲ),HA,NO3- and Cl- were investigated by a four-factor central composite design.It was observed that the FQs photodegraded the fastest around their isoelectric points.The photodegradation kinetics were not affected by Cl-(p＞0.05).HA and NO3- inhibited the photodegradation for they acted mainly as radiation filters and had an important role in scavenging reactive oxygen species.These results suggested that the similar or less photodegradable potential of FQs in natural waters was attributed to the integrative effects ofpH and the different aqueous dissolved matter.In conclusion,both the phenicols and the FQs underwent direct photolysis as well as self-sensitized photodegradation.Their photodegradation kinetics were affected by the aqueous dissolved matter.The study revealed the mechanisms for the aqueous dissolved matter affecting the photodegradation,which are of great significance to better understand the environmental photochemical behavior of the antibiotics.
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