Effects Of Different Ionic Strength/Type And PH On The Co-transport Of Cu And Colloids In Porous Media

Heavy metal pollution in the underground environment is of great concern.Copper（Cu）is a common metal element in the environment,and its own risk index in the environment is high,and it is more likely to exceed the environmental threshold,causing pollution to the environment and endangering human health.The different types of colloids in soil can play a carrier role in the transport of heavy metals because of their huge specific surface area and rich functional groups.Therefore,understanding the transport characteristics and mechanism of Cu under the action of colloids is important to clarify the enrichment and transformation of heavy metals in groundwater and soil,and provide a theoretical basis for the remediation and treatment of heavy metal contaminated soil.In this paper,the transport processes of montmorillonite colloids and humic acid colloids in cooperation with Cu under different ionic strength/type and pH conditions were investigated by indoor sand column experiments with Cu as the research object.The results show that colloids not only promote the migration of pollutants,but also inhibit or retard the transport of pollutants under certain conditions and change the migration pattern of pollutants;meanwhile,the presence of heavy metal ions will change the charge and particle size of the colloids,which will have an impact on the release migration of colloids,and then affect the migration diffusion of pollutants.The main research results are as follows.（1）Measurement of Cu concentration in colloidal stateAqua regia（concentrated nitric acid and concentrated hydrochloric acid in a volume ratio of 1:3）has the best extraction effect on colloidal Cu,reaching more than 90%.The method was used to ablate the mixed effluent of colloidal and Cu,and the concentration of total Cu in the effluent was obtained,and the concentration of colloidal Cu was obtained by subtracting the concentration of dissolved Cu.（2）Transport characteristics of montmorillonite colloids in quartz sand in cooperation with CuThe deionized water drenching reduced the potential of quartz sand and montmorillonite,the electrostatic repulsion was enhanced,and the sudden release of colloids occurred,and the release process was kinetically controlled.The effect of colloid on Cu transport was different under different pH conditions.At pH 4.0,Cu was mainly transported as dissolved state,and montmorillonite colloid inhibited Cu transport;at pH6.0,Cu was mainly transported as colloidal Cu,and montmorillonite colloid promoted Cu transport;and the increase of pH promoted Cu transport under the action of montmorillonite colloid.When the background solution was NaCl,the abrupt release of co-transported montmorillonite colloids occurred;when the background solution was CaCl₂,the colloids were mainly adsorbed in the primary potential well,which was insensitive to the change of ionic strength,and no release of colloids occurred due to the deformation blocking effect（straining）of the colloids.The presence of NaCl inhibits the migration of total Cu and colloidal Cu and promotes the migration of dissolved Cu due to competitive adsorption and compressed double layer effects;a large amount of Cu migrates with the release of colloids,which increases the risk of heavy metal contamination in groundwater.When the background solution was CaCl₂,the presence of colloids inhibited the Cu transport,and the inhibition diminished with increasing Ca²⁺concentration due to competitive adsorption.Under high ionic concentration（0.001 mol/L）conditions,the ability of both the divalent cation Ca²⁺to compress the double layer and compete for adsorption was greater than that of the monovalent cation Na⁺,leading to an increase in the mobility of dissolved Cu.（3）Transport characteristics of humic acid colloids in quartz sand in cooperation with CuThe increase of pH not only enhanced the electrostatic repulsion between quartz sand and humic acid colloids,but also increased the dissociation of colloidal acidic functional groups,broke the internal hydrogen bonds and reduced the particle size,which promoted the transport of humic acid colloids;no colloidal release peak appeared in the drenching stage.The effect of humic acid colloids on Cu transport was different at different pH.When the pH was 4.0,most of the Cu was transported in the dissolved state,and humic acid colloids did not promote or inhibit the transport of total Cu;when the pH was 6.0,the adsorption of colloids on Cu increased,and Cu was transported in both the dissolved and colloidal states,and humic acid colloids promoted the transport of Cu.When the background solution was NaCl,the increase of ionic strength inhibited the migration of humic acid colloids,and no colloid release peak appeared when the ionic strength was reduced;when the background solution was CaCl₂,the colloids basically did not flow,and the release of colloids occurred when the ionic strength was reduced.Due to the competitive adsorption of Na,the humic acid colloid carried little Cu,and the effect of colloid on Cu migration was not significant,and Cu was mainly transported in the form of dissolved state.When the background solution was 0.001 mol/L CaCl₂,the retention-carrying effect of colloids was greater than the competitive adsorption thus inhibiting the Cu transport;when the background solution was 0.01 mol/L CaCl₂,the competitive adsorption effect of high concentration of Cawas greater than the retention-carrying effect of colloids,thus promoting the Cu transport.Under the condition of low ion concentration（0.001 mol/L）,the competitive adsorption of Cawas lower than the retention-carrying effect of humic acid colloids,resulting in lower recoveries of total Cu,dissolved Cu,and colloidal Cu than at NaCl;while the opposite was true under the condition of low ion concentration（0.01 mol/L）.