Transport Of Polystyrene Microplastics In Saturated Porous Media

Microplastics as the emerging persistent pollutants have attracted more attention in terrestrial environments.Due to the small size,large specific surface area,and strong hydrophobicity of microplastics,on the one hand,microplastics are easy to enter the soil under the action of external forces,thus affecting the movement of soil water and nutrients.On the other hand,microplastics can be used as the carrier of heavy metals and organic pollutants in the soil,which bringing potential harm to the soil environment.In the natural soil environment,metal oxides are ubiquitous on the surface of porous media,which dramatically affected the transport behavior of pollutants in soil.Moreover,the soluble organic matter in the soil environment also has a significant effect on the transport of micro/nanoparticles in the medium.However,the environment in natural soil is relatively complex,so it is difficult to directly study the migration of microplastics in soil.Therefore,the forming process of soil can be simply simulated by bare quartz sand,manganese oxides coated sand,and sandy soil,and then explore the transport behavior of polystyrene microplastics in the three different media.The effects of different p H,ionic strength,cation type,and humic acid conditions on the transport behavior of polystyrene microplastics in saturated porous media were investigated.The main research results of this study are as follows:1.In bare quartz sand,with the increase of p H value from 5 to 9,the effluent rate of polystyrene microplastics increased from 43.10%to 88.70%,and the mobility of polystyrene microplastics increased.The mobility of polystyrene microplastics under high ionic strength condition(10 mmol L^-1)is the lowest,and the deposition retention coefficient of k increased to0.0745 min^-1,the deposition of polystyrene microplastics in quartz sand is enhanced.The breakthrough curve under Ca²⁺condition is significantly lower than that under the K⁺condition,and the effluent rate of polystyrene microplastics decreased.When polystyrene microplastics cotransport with Cd²⁺,the breakthrough curve of polystyrene microplastics is significantly lower than that in the single transport of polystyrene microplastics.2.In the manganese oxides coated sand,the breakthrough curve of polystyrene microplastics is significantly lower than that of bare quartz sand under the same chemical conditions,and p H,ionic strength,and cation type conditions have little effect on the changes of the breakthrough curve,and the mobility is less affected by hydrochemical conditions.Specifically,the breakthrough peak of Cd²⁺decreases slightly when polystyrene microplastics cotransport with Cd²⁺,which is manifested by the effluent time is delayed and the breakthrough peak is reduced.In addition,the breakthrough curve of polystyrene microplastics in the cotransport with Cd²⁺is significantly lower than that in bare quartz sand,and the breakthrough peak is significantly reduced than that in the single transport of polystyrene microplastics.3.The breakthrough curve of polystyrene microplastics in sandy soil is also significantly affected by p H,ionic strength,and cation type as in the bare quartz sand.The breakthrough curve is higher under the high p H value(p H=9),and with the increase of ionic strength from 1mmol L^-1 to 10 mmol L^-1,the value of deposition retention coefficient of k increased from0.1044 min^-1 to 0.2070 min^-1,the deposition increased and the migration decreased.Furthermore,the inhibition of Ca²⁺on the breakthrough curve of polystyrene microplastics is greater than K⁺,and the retention rate under Ca²⁺conditions is higher.Besides,the breakthrough curve of polystyrene microplastics in sandy soil is significantly lower than that in bare quartz sand under the same chemical conditions.4.The presence of humic acid(1 mg L^-1)significantly promoted the breakthrough curve of polystyrene microplastics in manganese oxides coated sand,and the effluent rate increased.The breakthrough peak increased with the increase of humic acid concentration(0-10 mg L^-1),and the value of deposition retention coefficient of k decreased from 0.1051 to 0.0379 min^-1.Moreover,the presence of humic acid(1 mg L^-1)in manganese oxides coated sand also increased the changes of the breakthrough curve of polystyrene microplastics under different p H,ionic strength,and cation type conditions,and the breakthrough peak is higher at high p H value(p H=9)and low ionic strength(1 mmol L^-1),while the breakthrough curve at Ca²⁺conditions is lower than that at the K⁺conditions,and the mobility decreased.In summary,the mobility of polystyrene microplastics in the three media is as follows:bare quartz sand>sandy soil>manganese oxides coated sand.The p H,ionic strength,and ion type have significant effects on the transport of polystyrene microplastics in bare quartz sand and sandy soil,but they have little effect on the transport of manganese oxides coated sand.The presence of humic acid not only promoted the transport of polystyrene microplastics in manganese oxides coated sand,but also increased the effects of p H,ionic strength and ionic type on the transport in manganese oxides coated sand.