Mechanistic Studies On Adsorption And Charge Reversal Of Metal Ions At Clay Interfaces

Clay minerals are distributed widely on the Earth surfaces and represent the main components of soils,sediments,and atmospheric dusts.They have complex porous structures,tiny particles,large specific surface areas and expansibility,causing them to rank as one of the commonly used adsorption materials.Adsorption of ions at clay surfaces has always been the focus for a number of research fields such as soil science,environmental chemistry,and colloidal and interfacial science.Adsorption of ions at clay surfaces not only profoundly affects the dissolution,surface precipitation,ion exchange and catalytic performance of clay minerals,but also can control the migration and enrichment of nutrients,metal ions and pollutants in soil systems.Most of ion adsorption and exchange in soils occurs between soil solutions and electric double layers.The structure of electric double layers determines the amount of adsorption and charge reversal of ions at clay-water interfaces.In recent years,the phenomenon of charge reversal has gained enough attention,from charged mica surfaces to charged lipid membranes,actin and etc.However,mechanism of charge reversal is still a mystery.Therefore,studying the adsorption,electric double layer,adsorption capacity and charge reversal of metal ions at clay interfaces is of great significance to understand the properties and functions of soil clay minerals,adsorb and transport of nutrient element,and remediation of heavy metal pollutions.Isomorphous substitution causes clay minerals to have a large amount of permanent charges,which can effectively adsorb oppositely charged ions from solutions.Beidellite and montmorillonite are often used as adsorptive materials because of their small particles,outstanding colloidal characteristics,and huge number of surface charges.In this thesis,two common soil clay minerals,beidellite and montmorillonite,were studied in-depth and systematically by molecular dynamics simulations.A simple method based on molecular dynamics was established to estimate the thickness of electric double layers.On this basis,adsorption capacities of metal ions at clay surfaces,including the influences of charge distribution and charge density,were addressed.In the end,the phenomenon and mechanisms of excessive adsorption and charge reversal for different metal ions at clay interfaces were discussed.The results indicate:（1）A simple method for estimating the thickness of electric double layers is proposed and verified.The thickness of electric double layers at clay interfaces is calculated by molecular dynamics simulations,considering these factors:different electrolyte solutions,different charge densities,and charge locations.We found that both molecular dynamics and empirical Debye length at very low concentrations can correctly estimate the thickness of electric double layers,and predict the compression of electric double layers in relatively concentrated conditions.Molecular dynamics simulation have the obvious advantages,mainly in:1)More reasonable trends for thicknesses of electric double layers with increase of concentrations,especially under the condition of high concentration;2)Thickness of double electric layers reflects ion-specific effects.For example,the thickness of electric double layers shows significant difference for various alkali metal ions;3)Dependence of the thickness of double electric layers on charge locations is rationalized.For example,at the identical concentrations,the thickness of electric double layers is significantly different for montmorillonite and beidellite,and the thickness of electric double layers for montmorillonite reduces more slowly with increase of ionic concentrations.（2）Studies of adsorption of different metal ions at clay surfaces help us understand the capacities of ion adsorption and influencing factors at the molecular scale.With increase of electrolyte concentrations,the total adsorption capacities of five ions(K⁺,Na⁺,Cs⁺,Pb²⁺and Ca²⁺)all increase significantly,and show the pronounced ion-specific effects.At low concentrations,the total adsorption capacity sequence of K⁺,Na⁺and Cs⁺follows as K⁺<Cs⁺<Na⁺;With increase of concentration,the total adsorption capacity sequence changes to Na⁺<Cs⁺<K⁺,and the sequence has undergone reversal,which is caused by increase of concentration,increase of ion adsorption capacity of inner-sphere and outer-sphere,strengthening of interactions between ions and clay surfaces,and difference in the radius and screening charge ability of each ion.On the other hand,the screening ability of Pb²⁺and Ca²⁺ions to surface electric fields is stronger than that of monovalent ions.The equivalent adsorption capacity of Pb²⁺and Ca²⁺ions is significantly larger than that of monovalent ions.The adsorption capacity of inner-sphere is greatly affected by electrolyte concentrations,while adsorption capacities of outer sphere and diffusion layer are much less affected.In addition,charge location and charge density affect the total ion adsorption capacity by changing adsorption type and adsorption intensity of ions.（3）Adsorption of different metal ions at clay/water interfaces may cause charge reversal and exhibit distinct characteristics.The excessive adsorption of K⁺,Na⁺,Cs⁺,Pb²⁺and Ca²⁺ions all become more significant with increase of electrolyte concentrations.The excessive adsorption of K⁺,Na⁺and Cs⁺is caused by co-adsorption of anions.As the concentration of Pb²⁺and Ca²⁺increases,the surface potential of clay minerals is reversed.During the adsorption process of Pb²⁺and Ca²⁺,the tendency of surface potential reversal becomes more evident with increase of concentrations,consistent with the performances of the ion correlation model.Multivalent ions form strongly correlated liquids at clay interfaces whose shot order is similar to a Wigner crystal.Cohesive energy of this liquid leads to strongly additional attraction of counterions,resulting in the charge reversal.In addition,charge location affects the type and amount of ion adsorption,while causes no influences on charge reversal.Charge density can significantly affect charge reversal by changing the major adsorption mode and adsorption amount of ions and enhancing the interaction force between ion and clay surfaces.