| In recent years,the pollution of microplastics in the environment has become more and more serious,which has attracted increasing attention worldwide.With the in-depth study of microplastics pollution in terrestrial environment,increasing attention has been paid to microplastics in agroecosystem.In agroecosystem,there may be both microplastics and heavy metals in the soil,and the two kinds of pollutants may affect each other’s environmental behavior in the soil and interact to affect the yield and quality of crops.Therefore,it is of great significance to study the combined pollution of microplastics and heavy metals in farmland soil,which is worthy of further discussion.In this thesis,polyethylene microplastics and zinc were taken as the research objects.First,polyethylene agricultural film,which was widely used in farmland,was selected for aging experiment to compare the differences in the adsorption and desorption effects of new and aging polyethylene agricultural film on zinc,and to characterize the polyethylene agricultural film and compare the differences in its surface microstructure and chemical properties,so as to understand the impact of microplastics aging on its adsorption performance.Second,the influence and mechanism of zinc adsorption and desorption after adding microplastics in farmland soil were studied,and the influence of p H of solution,particle size of microplastics,ion coexistence and humic acid and other factors were discussed.（1）The adsorption and desorption of zinc for both new and aged polyethylene agricultural films reached equilibrium within two hours,aged（HNO3）microplastics had the highest adsorption capacity and adsorption capacity,followed by aged（H2O2）microplastics and new microplastics.The desorption capacity of aged（HNO3）microplastics was weaker,and the higher the aged degree,the more difficult it was to desorb.The adsorption of the three kinds of microplastics on zinc was affected by the particle size.The smaller the particle size was,the greater the adsorption amount of zinc was and the smaller the desorption amount was.As the p H value of the solution increased,the adsorption capacity also increased.Scanning electron microscopy（SEM）,Fourier transform infrared spectrometer（FTIR）,X-ray diffraction（XRD）,X-ray photoelectron spectroscopy（XPS）and other methods were used to analyze the changes in property of aged microplastics.The results of characterization also confirmed that the higher the aged degree of microplastics,the greater the adsorption amount of zinc.Although the adsorption capacity of aged microplastics on zinc showed an overall increasing trend,the increasing trend of the three kinds of microplastics was not significant,which might be because the aging experiment time of microplastics was short,leading to insufficient aged degree of microplastics.（2）The rate of adsorption and desorption of zinc in farmland soil was faster,and the equilibrium of adsorption and desorption was reached within two hours.The addition of microplastics significantly reduced the adsorption capacity of zinc,but increased the desorption capacity of zinc.The particle size of microplastics and the p H value of the solution affected the adsorption of zinc in soil.Smaller particle size led to greater adsorption amount of zinc and smaller desorption amount.At lower p H values,especially in the soils with microplastics,more zinc could be desorbed.The addition of copper ions significantly reduced the adsorption of zinc in the soil.Soil water content and humic acid had little effect on zinc adsorption and desorption.The maximum adsorption capacity of the microplastics to zinc was 39.50 mg/kg,and the adsorption occurred rapidly in one hour,and the adsorption equilibrium occurred in two hours.The results of SEM and XRD analysis showed that the adsorption process of zinc in farmland soil was complex,which was affected by organic matter and minerals in soil.The results of energy dispersive X-ray spectrometer（EDS）and FTIR analysis showed that the adsorption capacity of microplastics to zinc was much lower than that of farmland soil,and the adsorption capacity of zinc was mainly affected by the physical adsorption mechanism.In conclusion,the polyethylene agricultural film with higher aged degree had stronger adsorption capacity of zinc.The addition of microplastics could reduce the adsorption capacity of soil to zinc and increase the desorption capacity.In the farmland environment,the long-term use of polyethylene agricultural film led to the accumulation of microplastics and pollution aggravation,and polyethylene agricultural film occurred in the soil environment of natural aging,aged polyethylene agricultural film was more likely to become the carrier of heavy metal accumulation and migration,affect soil quality and productivity,and indirectly harm human health. |
