Study On Adsorption-Migration Of Polystyrene Nanoplastics In Soil Porous Media

Nanoplastics are characterized bysmall size,large specific surface area,and strong colloidal mobility,leading to their wide distribution in soil environment and increasing ecological risks.Ralated studies have shown that the input concentration,flow rate,pH and Ionic Strength（IS）of nanoplastics affect their adsorption-migration behavior in soil porous media.A large number dissolved organic matter（DOM）in soils,is also susceptible to interact with nanoplastics,and then affecting their sorption-migration behavior in soils.Iron oxide is an active mineral widely present in soil,but there is a lack of research on nanoplastics in the presence of iron oxide encapsulation and soil humic acid.Most of the studies only use quartz sand as a porous medium to study the adsorption-migration behavior of nanoplastics,which has certain limitations.In this study,quartz sand andgoethite-coated quartz sand were used as porous media by means of adsorption experiments,indoor column migration experiments and model fitting.The adsorption-migration behaviors of polystyrene nanoplastics（PSNPs）in quartz sand（QS）and goethite-coated quartz sand（GQS）were investigated under different pH,ionic strength and humic acid concentration conditions.The main conclusions drawn are:（1）As the concentration of polystyrene nanoplastics increases,the maximum adsorption of PSNPs on quartz sand and goethite-coated quartz sand gradually tends to equilibrate.The reason for this phenomenon is that the number of adsorption sites on the surface of porous media is limited and PSNPs need to compete for the adsorption sites on the surface of porous media.When the concentration of PSNPs increases,its zeta potential value decreases continuously,and the energy potential barrier value generated between PSNPs and quartz sand increases continuously,this leads to an increase in the C/C₀value of the PSNPs penetration curve in quartz sand.The fitted parameters obtained by fitting the migration data to the Two-Site Model（TSM）showed that the fitted values of the distribution coefficient K_dand the hindrance factor R_ddecreased continuously as the concentration of PSNPs increased.The value of K_dreflected the adsorption capacity of quartz sand to PSNPs,while the value of R_dreflected the resistance to PSNPs during migration.（2）As the pH increased from 4 to 6,the maximum adsorption of quartz sand on polystyrene nanoplastics decreased from 0.668 mg/g to 0.479 mg/g,and the maximum adsorption of goethite-coatedquartz sand on PSNPs decreased from 0.979 mg/g to 0.875mg/g.Therefore,the adsorption capacity of both GQS and QS on PSNPs decreased.The Zeta potential decreases with the increase of pH,which leading to an increase in the value of its energy potential barrier with QS and GQS,and an increase in the C/C₀value of PSNPs penetration curves in QS and GQS.The results of the migration experiments were fitted by TSM,and the fitted correlation parameters showed that the fitted partition coefficient K_dand blocking factor R_dtendedto decrease withthe increase ofpH value.（3）When the ionic strength increased from 10 mmol/L to 50 mmol/L,the Zeta potential of both quartz sand and PSNPs increased,resulting in a decrease in the electrostatic repulsion generated by their interaction.Accordingly,the adsorption capacity of quartz sand was enhanced and its maximum adsorption of PSNPs increased.The absolute value of the zeta potential increased,resulting in a decrease in the value of the energy barrier between quartz sand and PSNPs and a decrease in the C/C₀value of the PSNPs penetration curve in quartz sand.Fitting the PSNPs penetration curves using TSM leads to the fact that the fitted partition coefficient K_dand the hindrance factor R_dincrease with the increase of ionic strength.（4）After humic acid was added,it combined with the adsorption sites on the surface of quartz sand and goethite-coated quartz sand,occupying a large number of adsorption sites on their surfaces,causing the adsorption capacity of PSNPs on the surfaces of QS and GQS to be significantly weakened,and the maximum adsorption amount of PSNPs on the surfaces of GQS and QS was continuously reduced.When humic acid was added to the PSNPs solution,the Zeta potential value of the solution decreased,and the electrostatic repulsive force generated by the interaction between PSNPs and QS and GQS increased,and the energy potential barrier increased.Therefore,the mobility of PSNPs in porous media increases with increasing humic acid concentration.The correlation parameters obtained by fitting the PSNPs penetration curves using TSM showed that the fitted value assignment coefficient K_dand the hindrance factor R_ddecreased continuously with the increase of humic acid concentration.（5）After the goethite wrapped quartz sand,the surface of quartz sand became rough and uneven,and the adsorption sites increased,resulting in a stronger adsorption capacity of GQS to PSNPs than QS,and the maximum adsorption amount of GQS to PSNPs was higher than that of QS.After the quartz sand was wrapped by goethite,its absolute value of Zeta potential decreased,and the electrostatic repulsive force generated by the interaction with PSNPs decreased.Therefore,the mobility of PSNPs in GQS was lower than that of QS when the solution chemistry conditions were the same.The results of the migration experiments by TSM fitting revealed that the fitted values of the PSNPs penetration curves in GQS with the distribution coefficient K_dand the hindrance factor R_dwere larger than those of QS.