Effect Of Microplastic Structure Onto Adsorption-Desorption Performance Of Pollutants In Aqueous Environments

Microplastics have become a new type of environmental pollutants,known as’PM_2.5in the ocean’,which has brought potential risks to the environment and attracted great attention from the public and the scientific community.Microplastics such as polyethylene（PE）,polypropylene（PP）,polystyrene（PS）and polyvinyl chloride（PVC）account for more than 50%of the total amount of microplastics in the environment and are easy to coexist with various pollutants.Therefore,the study on the interaction between microplastics and environmental pollutants and their migration and transformation is of great significance to evaluate the direct or indirect effects of microplastics on the ecological environment.However,different types of microplastics have different structures.Their structures would change under the action of environmental factors.And the interaction characteristics between different types of microplastics and different types of pollutants in the environment would also change accordingly.The adsorption-desorption behaviors of various pollutants onto microplastics with different structures and different aging time have not been reported.In this paper,four typical microplastics（PE,PP,PS and PVC）in the environment were used as the research objects.Xenon lamp artificial accelerated aging experiment was used to study the effects of different irradiation time on the surface properties and structural characteristics of different microplastics.The adsorption-desorption behaviors of different microplastics on different types of pollutants(methylene blue（MB）,antibiotic rifampicin（RIF）and heavy metals Cr（VI）and Cu（II）were investigated,respectively.The effects of different water quality environments on the adsorption behavior of microplastics.The following conclusions were drawn:（1）Aging has a great influence on the surface morphology,crystallinity and other physical and chemical properties of MPs.The surface roughness and specific surface area of MPs increased with the extension of aging time（0～20 d）,and the aging process is accompanied by the formation of functional groups（such as carbonyl and hydroxyl）.The surface hydrophilicity of MPs was also changed.The order of carbonyl index value is:PP>PVC>PE>PS,which can preliminarily judge the aging speed of four microplastics.And the existence period of microplastics in the environment during natural aging is judged by establishing relevant model fitting equations.The crystallinity,melting temperature,glass transition temperature,thermal decomposition temperature and molecular weight of the microplastics also decreased regularly due to aging,and the change rule was consistent with the change trend of carbonyl index.Those changes in these properties will affect the adsorption properties of different types of pollutants in the water environment.（2）The order of adsorption capacity was PP-20 d>PE-20 d>PS-20 d>PVC-20 d.The adsorption equilibrium time of four microplastics for MB was about 48 h.The maximum adsorption capacity of PP-20 d was 6.8 mg/g.Different from PP-20 d,PE-20d and PS-20 d,the adsorption isotherms of PVC-20 d for MB conformed to Freundlich model,and the adsorption mechanism was mainly multilayer adsorption（physical adsorption）formed by uneven surface,while the adsorption mechanism of PP-20 d,PE-20 d and PS-20 d for MB was mainly electrostatic and chemical bond（physical and chemical adsorption）.The MB adsorption capacity of the four kinds of microplastics varies greatly with the environmental conditions.Increasing the p H of the solution can increase the MB adsorption capacity of the four kinds of microplastics,but increasing the salt concentration of the solution significant decrease in the adsorption capacity.The PS-20 d saturated adsorption of MB had the highest desorption rate of MB in simulated gastrointestinal fluid,up to 58.1%,indicating that the aging microplastics may become an important carrier of MB into the water environment and biological tissues,thereby aggravating the harm of MB and other pollutants to the environment and aquatic organisms.（3）The adsorption equilibrium time of four kinds of microplastics for RIF antibiotics was about 72 h,and the order of adsorption capacity was PP-20 d>PE-20 d>PVC-20 d>PS-20 d.The adsorption capacity of all microplastics for RIF antibiotics decreased with the increase of p H,salinity and FA concentration,and the adsorption isotherms were well consistent with the Freundlich model.However,PP-20 d showed higher affinity to RIF antibiotics,which may be due to the higher aging degree of PP than other types of microplastics under the same light conditions,and its surface structure is more conducive to improving the adsorption of RIF.The desorption experiment results showed that the desorption rate of PS-20 d saturated with RIF antibiotics was the highest,reaching 56%.It can be seen that the microplastics in the environment may become the migration carrier of antibiotics including RIF in the environment due to their structure caused by environmental factors.（4）In the single metal solution system,the adsorption amount of Cr（VI）on the four microplastics was greater than that on Cu（II）.The adsorption process of Cr（VI）and Cu（II）on the surface of microplastics has multiple stages,and the adsorption driving forces mainly include van der Waals force,micropore filling mechanism and coordination reaction.The adsorption process was greatly affected by p H,salt concentration and FA concentration.In the mixed metal solution system,metal ions have competitive adsorption on the surface of microplastics.In the RIF antibiotic-heavy metal mixed solution system,the presence of RIF can significantly improve the adsorption capacity of microplastics for various metal ions.Although the mechanism of RIF and heavy metals on microplastics needs to be further explored,this result provides an important basis for evaluating the environmental behavior and ecological risk of microplastics during combined pollution.