Effect Of Ionic Strength On Transport Of Colloid And As(Ⅲ) In Saturated Porous Media

There are a large number of colloids in the undersurface environment.The colloids can adsorb and promote transport of pollutants in soil and groundwater.And some of the colloids themselves are pollutants,these pollutants will do harm to human health after they enter into water.In recent years,the behavior of colloids in soil and groundwater has been an active research area,indicating that the changes of undersurface environment would cause the release and deposition of soil colloid.It is of great significance to study the deposition and remobilization of soil colloids in groundwater for the migration and remediation of pollutants in the vadose zone and aquifer.In this study,we choose polystyrene latex microspheres with diameter of 1 μm as model colloid,quartz sand as porous medium.We studied the effect of ionic strength on the attachment and remobilization of colloid and the impact of ferrihydrite colloid and ionic strength on the transport of heavy metal As³⁺ in saturated steady flow by laboratory column experiments.According to the obtained concentration breakthrough curve,the attachment coefficient and remobilization coefficient of colloid were calculated.The potential energy diagram was drawn according to the DLVO theory,and the experimental results were quantitatively analyzedThe results shown that（1）with the increase of the ionic strength of the solution,the number of colloids adsorbed on the surface of porous media increases.When the ionic strength of the solution was 50 mM,60 mM,70 mM,the corresponding percentage of colloids retained in sand to the total colloid mass injected to the column was 92.96%,94.62% and 94.78% respectively.（2）The instant change of the ionic strength of the solution could cause the remobilization of colloid.The variation of ionic strength were same,the number of colloid remobilized when the ionic strength was transformed from 50 mM to 10 mM was the biggest.The remobilization coefficient was 3.88%.When the initial ionic strength was 60 mM,the number of colloid remobilized was the biggest when the ionic strength was decreased to 10 mM.The remobilization coefficient was 1.75%.The number of colloid remobilized when the ionic strength was changed from electrolyte solution to ultrapure water directly was bigger than it was changed to electrolyte solution with low ionic strength first,then to ultrapure water.It revealed that the transport and remobilization of colloid in saturated porous media was not only affected by the size of the ionic strength,but also related to the variation rate of ionic strength and the historical conditions of solution chemistry.（3）When ferrihydrite colloid was present in the sand column and did not migrate,the ferrihydrite colloid could adsorb As³⁺ and inhibit the transport of As³⁺ in the saturated sand column,and the adsorption capacity increased with the increase of ionic strength.（4）When the ferrihydrite colloids were distributed dispersedly in the sand column,the quantity of As³⁺ remained in the sand column was larger than the quantity of As³⁺ remained when ferrihydrite colloid was distributed intensively in the top of the column,for the increased adsorption sites and contact time.The results shown that the effect of colloid on the transport of As³⁺ in saturated porous media was greater than that of ionic strength.The above results can provide theoretical and decision basis for making water and soil health standards,preventing and controlling groundwater pollutant by organic colloid such as pathogenic microorganisms and all kinds of organic and inorganic pollutants carried by colloid particles.