| Antibiotics have been widely applied to treat human and veterinary infectious diseases and to promote growth in aquaculture,agriculture,and,livestock.Following application to the target organisms,antibiotics can not be absorbed or metabolized completely.Thus,large fraction of antibiotics could be released into the environment.The residue of antibiotics in aquatic environment can cause potential threat to aquatic organisms and the spread of resistance genes,which finally result in adverse effects on human health.Therefore,the research on the migration and transformation of antibiotics has become a hot topic in the environmental field in recent years.Sediment is an essential element involved in the substance migration and transformation process.It has been reported that sediment exposed in aquatic environment can be colonized by microorganisms rapidly.Microorganisms attach to sediment surface and secrete extracellular polymeric substances(EPS)to form biofilms.It has been proved that biofilms can sorb and degrade some organic containments.Thus,biofilms colonized on sediment surface will change the sorption and degradation characteristics of primary sediments.However,whether biofilms can affect sorption and degradation of antibiotics in water-sediment system is need to be investigated.Based on the mentioned research background above,ofloxacin(OFL)is selected as a model antibiotic to investigate its migration and transformation.The ongoing wide usage of OFL make it pseudo-persistent in aquatic environments.Because of resistance to hydrolyzation and volatilization,the environmental fate of OFL is influenced by sorption and degradation.Generally,sorption is an important process controlling behavior of organic contaminants in the environment.Therefore,it is necessary to do comprehensive research on OFL sorption mechanism in aquatic environment and investigate the effect of sorption on the degradation of OFL.In this paper,the method of OFL determination by fluorescence spectrophotometry was optimized firstly,which laid the foundation for the accurate and rapid quantitative analysis of OFL in subsequent researches.Secondly,batch sorption experiments were conducted on intact and EPSfree biofilms.The sorption mechanisms of OFL onto biofilms were investigated by comparing changes in physicochemical properties of different biofilm samples along with the interaction between EPS and OFL.On the basis of understanding sorption mechanism of OFL onto biofilm,the batch sorption experiments were conducted on primary sediments and biofilm/sediment coexisting solids to investigate the role of biofilms in OFL sorption.The effects of biofilms on sorption process were discussed with differences in sorption characteristics between primary sediments and biofilm/sediment coexisting solids along with the changes in physicochemical properties of different sediment samples.Finally,OFL migration and transformation simulation system was constructed to investigate the photodegradation and biodegradation of OFL in water-sediment system and water-biofilm/sediment system when sorption of OFL coexisted.Besides,the contributions of sorption to total decreased amount of OFL from water phase were studied.The results showed that OFL could be determined at the excitation wavelength of 293 nm under the condition of 20 ? and pH = 7 by fluorescence spectrophotometer.The concentrations of OFL should be quantified by the matrix-matched calibration method to avoid the matrix effect.The method is simple and appropriate for OFL quantification when investigating the environmental behavior of OFL in the lab.For OFL sorption onto biofilms,the existence of EPS could restrict the diffusion of OFL into biofilm cells by forming EPS-OFL complexes.During sorption process,OFL was sorbed onto the outer layer of biofilm and linear partitioning and nonlinear Langmuir adsorption were involved in sorption.The linear partitioning capacities of OFL into EPS-free biofilms was 1.0 to 8.2 times higher than that of intact biofilms,indicating that the partitioning of EPS was inhibited by EPS.Besides,removal of EPS could affect cation exchange between biofilms and OFL as well as the formation of hydrogen bond.Thus,EPS could inhibit the sorption of OFL onto biofilms.For the effect of biofilm on sediment sorption capacity,the linear partitioning capacities of OFL onto biofilm/sediment coexisting solids were 50 % and 60 % of that onto primary sediments,while the Langmuir adsorption capacities were 1.69 and 2.04 times higher after incubated for 30 and 45 days.The combined effect of both linear partitioning and nonlinear Langmuir adsorption resulted in that the sorption isotherm of OFL onto primary sediments and biofilm/sediment coexisting solids had a crossover point.For the migration and transformation characteristics of OFL in water-sediment and water-biofilm/sediment systems,the decrease rate of OFL from aqueous phase in water-biofilm/sediment system was 1.5 times higher than that in water-sediment system when sorption coexisted with photodegradation.The faster decrease rate of OFL resulted from the changes of DOM and the sorption characteristics of OFL onto sediments after the growth of biofilms.When sorption coexisted with biodegradation,the decrease rate of OFL from aqueous phase in water-biofilm/sediment system was 80 % of that in water-sediment system.The lower decrease rate of OFL in water-sediment system was due to the sorption of OFL onto biofilm and utilization of EPS as the carbon source for microorganisms' activities.According to the discussions above,biofilm could influence the photodegradation and biodegradation of OFL in water-sediment system by affecting OFL sorption process.Meanwhile,the contribution of sorption to total decreased amount of OFL was 77.6 to 99.7 %.In conclusion,the existence of biofilm can influence the environmental behavior of OFL in the water-sediment system and sorption is the key to control the decrease of OFL from aqueous phase in water-sediment and water-biofilm/sediment system.In sorption process,biofilm can influence the sorption of OFL onto sediments by affecting the linear partitioning and nonlinear Langmuir adsorption.The results of this study are of great environmental significance to understand the migration and transformation process of OFL in water-natural solid system,which can also provide theoretical support for the research on environmental behavior prediction and risk assessment of OFL in aquatic environment. |
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