Microplastics Influence The Adsorption And Desorption Characteristics Of Heavy Metals In Soil

In recent years,the pollution of microplastics,as a derivative of plastic products,in the marine environment has attracted widespread attention worldwide.With the development of research,microplastics in terrestrial ecosystems particularly agroecosystems are attracting increasing attention.The terrestrial soil environment has also been contaminated by heavy metals.In this case,the two pollutants will contact and affect each other,posing a potential threat to the ecological environment.Therefore,the nature and source of microplastics and the relationships between soil and heavy metals have important theoretical significance for evaluating the ecological risks of microplastic pollution in soil and preventing and controlling heavy metal pollution in farmland soil.This study selected a typical farmland soil in Shandong Province to study the effect and mechanism of microplastics on the adsorption and desorption of Cd in the soil.In this study,the aging experiment of polyethylene agricultural film was carried out to study the changes in surface characteristics and chemical properties of aging agricultural film and their effects on the adsorption and desorption of Cd.The main results are as follows.Here,batch experiments were conducted to study the effects of polyethylene microplastics on Cd adsorption and desorption in a farmland soil under varying conditions.The adsorption process was rapid,both Cd adsorption and desorption in soils reached equilibrium within 120 min.Cd adsorption kinetics followed the pseudo-second order model,and the adsorption isotherm fitted to the Langmuir model more precisely than the Freundlich model.Overall,addition of microplastics decreased Cd adsorption but increased desorption,and the effects varied with microplastics dose and particle size,and solution pH.Microplastics-induced decrease in Cd adsorption and increase in Cd desorption were more pronounced at higher MPs dose and larger particle size.EDS analysis confirmed Cd adsorption on MPs surface.Similar XRD patterns showed a high crystallinity of MPs before and after adsorption,which may explain their opposite impacts on Cd adsorption and desorption.The results of Zeta potential,N₂-BET,and SEM prove that the adsorption of Cd on the surface of microplastics is affected by electrostatic interaction and physical adsorption.Microplastics affect soil adsorption characteristics through dilution and changing the properties of soil.The results of the microplastic aging experiment indicate that more folds and cracks appear on the surface of the aging microplastic agricultural film.FTIR results confirmed that the number of oxygen-containing functional groups on the surface of microplastics had increased,and the CI value increased after aging.XRD results confirmed the trend of microplastics'crystallinity decreasing after aging.Kinetics of Cd adsorption on microplastics followed the pseudo-second order model,and the adsorption isotherm fitted to the Langmuir model.The results of adsorption experiments show that aging microplastics have stronger adsorption capacity than brand new microplastics,but due to insufficient aging time,this effect is not significant enough.According to the adsorption data and characterization results,it is confirmed that the adsorption of Cd on the microplastics is affected by physical adsorption,and also by electrostatic force interaction and functional group complexation.In conclusion,the addition of polyethylene microplastics reduced the adsorption characteristics of Cd on farmland soil,and the polyethylene agricultural film increased the adsorption capacity of Cd after aging.Therefore,the widespread use of polyethylene agricultural film will lead to an increase in the bioavailability of Cd in the farmland environment,and the aging microplastic agricultural film will become the carrier of Cd adsorption and transfer,which will pose a threat to the security of farmland environment and human health.Thus,people should pay attention to and take measures for the pollution of microplastics in the farmland environment.