Adsorption Of Antibiotics Onto Microplastic Surfaces

Both of Microplastics and antibiotics are two classes of emerging contaminants,which have been paid more and more worldwide attention.Microplastic usually is defined to be that which dimeter smaller than 5 mm in the world.Due to its small particle size and wide range of pollution,microplastic is easy to be eaten by the organism,endangering the growth and reproduction of the organism.In mairine environment,the pollution of microplastics is becoming more and more severe,as a result of the random stacking of garbage,the incomplete disposal of industrial sewage and the development of coastal tourism.In addition,antibiotics can also be found in offshore environments through the process of such as sewage discharge and be widely used in the coastal aquaculture industry.This is also one of the main causes of antibiotic residues in the coastal environment.Therefore,the adsorption of antibiotics on microplastics has become one of the hotspots in the research of microplastic pollution.This study conducted field survey of the microplastic pollution in the coastal zone of the Qinzhou Bay of Guangxi,and combined with the previous survey results of China's coastal zone.Batch equilibrium experiments were conducted to investigate adsorption and desorption characteristics of oxytetracycline(OTC)on polyethylene(PE)and polystyrene(PS).In order to further verify the reliability of the mechanism,we conducted to investigate adsorption characteristics of five kinds of antibiotics(SDZ?CIP?TMP?TC?AMX)on five kinds of microplastics(PS?PE?PP?PVC?PA).The experimental results can provide a theoretical basis for the transfer and transformation of antibiotics in the serious area of marine microplastic pollution.It also provides basic data for the pollution control of microplastics and antibiotics in the offshore environment.The main results are as follows:(1)The field survey of the tidal flats and mangrove areas in the Qinzhou Bay of Guangxi shows: The microplastic abundance of supralittoral zone is obviously larger than that of intertidal zone.The size of microplastics particle ranged mainly 2~5 mm.The main types of microplastics in order are debris,ball,fiber,particle and film.The polymer types are polystyrene,polypropylene,polyethylene,low density polyethylene and oxidized polyethylene.And the surface structure of microplastics was complex exposured to environment.(2)Using polystyrene and polyethylene microplastics to carry out the adsorption experiment of antibiotics,the results showed that the sorption amounts of OTC from large to small were as follows: PS-pure water > PE-purewater > PS-seawater > PE-seawater.The freundlich model has a good fitting result for the isothermal adsorption data,which shows a significant nonlinearity(n=0.25~0.77).SEM and ATR-FTIR were used to investigate the interaction mechanisms between OTC and microplastics.ATR-FTIR showed that no new molecular group appeared during the adsorption process.The van Edward force and the mechanism of micropore filling are the main reasons for adsorption of OTC on microplastics.The pseudo-second order kinetic model successfully described the adsorption kinetics process.Desorption experiments showed that the cumulative desorption of OTC as follows: PS-pure water > PE-pure water > PS-seawater > PE-seawater.(3)To investigate adsorption characteristics of five kinds of antibiotics(SDZ?CIP?TMP?TC?AMX)on five kinds of microplastics(PS?PE?PP?PVC?PA).Scanning Electron Microscope(SEM)and X-ray diffractometer(XRD)analysis revealed that microplastics had different surface characterizes and various degrees of crystalline.Adsorption isotherms demonstrated that PA had the strongest adsorption capacity for antibiotics with distribution coefficient(Kd)values ranged from 7.36 ± 0.257 to 756 ± 48.0 L/kg in the freshwater system,which can be attributed to the high specific surface area and hydrogen bonding.The adsorption amounts of 5 antibiotics decreased in the order of CIP > AMX > TMP > SDZ > TC with Kd correlated with octanol-water partition coefficients(Log Kow).Comparing to freshwater system,adsorption capacity in seawater decreased significantly and no adsorption was observed for CIP and AMX.Our result indicated that commonly observed polyamide particles can serve as a carrier of antibiotics in aquatic environment and release of antibiotics may occur as microplastic transport from rivers to estuaries.