Study On The Transport Characteristics Of Microplastics In Saturated Porous Media

Microplastics(MPs)are a new type of environmental pollutants with complex composition.They have the characteristics of small particle size and large specific surface area,and have strong adsorption to environmental pollutants.In this paper,polystyrene(100 nm)was selected as the research object.Based on the saturated soil column migration experiment and the soil physical Hydrus model,the transport of polystyrene particles in saturated porous media(quartz sand)were analyzed with different pH,ionic strength,initial concentration,flow rate,water flow interruption and other factors;In addition,exploring the interaction between polystyrene particles and naphthalene in saturated porous media;At the same time,isothermal adsorption of polystyrene to naphthalene under different ionic strength conditions was observed by isothermal adsorption experiments feature.Finally,the Hydrus model and DLVO/XDLVO theory are used to analyze the transport mechanism,which provides a theoretical basis for comprehensive evaluation of soil microplastic pollution risk.By comparing the different Hydrus models,the site-one was selected as the attachment-detachment kinetics,and the site-two was the depth-dependent blocking kinetic model for data fitting to analyze the transport mechanism of microplastics under different environmental conditions.In the experiment of the influence of different solvent chemical factors on the transport of microplastics,the effects of pH,ionic strength and initial concentration on the migration of microplastics in saturated porous media were studied.The results show that the penetration rate of microplastics increases with the increase of pH.This is because the increase of pH leads to an increase in the energy barrier between the particles,making it difficult for microplastics to deposit on the surface of quartz sand.The increase of the initial concentration promotes the transport of the microplastics because the microplastic particles occupy a limited attachment site and electrostatic repulsion with the same type of charge,resulting in a higher recovery rate of the microplastic.The model results show that the first-order attachment coefficient and detachment coefficient of the site-one increased,while the first-order attachment coefficient of the site-two decreased with the increase of pH.With the increase of initial concentration,the first-order attachment coefficient decreases while the detachment coefficient increases.It indicates that the alkaline environment and higher initial concentration conditions promote the migration behavior of microplastics in saturated porous media.With the increase of ionic strength,the recovery rate of microplastics is significantly reduced.The model results show that the first-order attachment coefficient increases to a greater extent,so the microplastics are more prone to coagulation and the penetration rate is reduced.In the experiment of the influence of different hydraulic factors on the transport of microplastics,the effects of influencing factors such as flow rate and flow interruption on the transport of microplastics were studied.The results show that the breakthrough rate of microplastics decreases with the decrease of flow rate,mainly because the hydrodynamic torque balance and the shadow area formed on the surface of quartz sand inhibit the transport of microplastics to sole extent.The flow interruption condition leads to a decrease in the C/Co value of the microplastics.The downward trend is more obvious as time goes on.This is due to the deposition of microplastics into larger particles,or the deposition of adhesion.Observing the isothermal adsorption characteristics of naphthalene by quartz sand and microplastics under different ionic strength conditions,the Freundlich model can be used to fit the experimental data well.The fitting results show that the increase of ionic strength enhances the adsorption capacity of micro-plastics for naphthalene.It is indicated that the adsorption of naphthalene by quartz sand and microplastics is usually not a single layer adsorption on the surface of the adsorbent,but may be the interaction of microplastics surface and π-π caused by naphthalene polycyclic structure.In the experiment of co-transport of microplastics with organic pollutants,the interaction of microplastics and naphthalene during transport and the effect of ionic strength on the transport process of mixed systems were studied.The addition of microplastics promotes the transport of naphthalene.The addition of naphthalene will reduce the transport ability of microplastics.When the microplastics concentration is 10,50,100 mg/L,the presence of naphthalene reduces the breakthrough rate of microplastics.62.48%,44.42%and 31.36%.Changing the ionic strength of the mixed system found that under the condition of low ionic strength(0.5 mM),the addition of microplastics promoted the migration of naphthalene.While the ionic strength was high(5 and 50mM),the addition of microplastics had little effect on the transport of naphthalene.