Transport Behavior And Synergistic Interaction Of Microplastics And Lead In Saturated Porous Media

In recent years,the emergence of microplastics and their co-exposure with other pollutants in water environment have brought new challenges to water environment safety.Heavy metals are common pollutants in the water environment,it is of great significance to study their transport with microplastics in porous media for understanding the exposure risks of these two pollutants in the water environment.In this paper,glass beads were selected as the model porous media to study the single transport behavior and co-transport behavior of polystyrene microplastics and lead in the porous media,and the effects of ionic strength,media particle size and functional groups of microplastics on the transport of pollutants were also considered.Column experiments,deposition experiments,DLVO calculation and model fitting were carried out to analyze the transport rule of the pollutants.The main research results are as follows:(1)When microplastics transported alone,the increase of ionic strength inhibited the transport of microplastics through double electric layer compression.The presence of carboxyl and sulfonic groups might increase the transport of microplastics due to the increase of electronegativity and hydrophilicity of microplastics.Media with small particle size might inhibit the transport of microplastics by increasing the straining effect.When lead transported alone,the increase of ionic strength might promote the transport of lead due to the competition of background cations and the decrease of the electronegativity of the media.The media with small particle size could provide more adsorption sites and higher electronegativity,thus more lead was trapped.(2)When the microplastics and lead transported together,the transport of the two pollutants were also affected by ionic strength,media particle size and functional group of microplastics,respectively,and the change rule of the two pollutants was similar to that when they transported alone.However,the interaction between the two affected their transport behavior.Under the condition of low ionic strength,microplastics and lead inhibit the transport of each other.At this time,the addition of lead led to the destabilized agglomeration and decreased electronegativity of microplastics,which reduced the electrostatic repulsion and increased the straining effect of microplastics in the media thus inhibiting the transport of microplastics.The addition of microplastics increased the adsorption sites of lead in the system,thus inhibiting the transport of lead.In addition,microplastics with carboxyl and sulfonic groups might have a greater adsorption capacity with lead complexation,and so inhibited the transport of lead to a higher degree.And under the high ionic strength,the transport of the two promoted each other,at this point,lead complexes with other ions in the system might compete with microplastics for adsorption sites on the media,and the lead adsorbed on the media might generate electrostatic repulsion with the cations wrapped around the microplastics,these interactions might contribute to the transport of microplastics.Microplastics could also occupy part of the adsorption sites on the media,and form competition with lead ions,so as to increasing the transport of lead.(3)The deposition experiments could correspond to the adsorption and deposition of microplastics on the surface of the media during the transport process.The effects of ionic strength and functional group changes on microplastics obtained by deposition experiments were consistent with the results of column experiment.The classical DLVO theory could explain the single transport of microplastics,but could not predict the co-transport of microplastics and lead.In addition,the above two methods could not explain and predict the influence of media particle size on pollutant transport and deposition.The HYDRUS 1D software could well simulate the transport of microplastics and lead in porous media,and the fitted parameters provided a quantitative reference for the interpretation of pollutant transport behavior.Therefore,in the study and prediction of pollutant transport,a variety of experimental and simulation methods could be used to combine analysis,so as to better understand pollutant transport behavior.These conclusions provided a data reference for a deeper understanding of the deposition of microplastics and lead on the surface of the porous media and their transport in the media,had a guiding role in predicting the exposure risk of pollutants in the water environment,and at the same time had certain reference significance for the effect prediction and improvement of the water treatment filtration process.