Effects Of Polarization On Ion Exchange Adsorption In Clay Mineral

Ion exchange adsorption is the ubiquitous physicochemical process occurring in soil/clay mineral.It not only has a direct impact on nutrient cycling in soil/water environment,heavy metal pollution and farmland non-point source pollution,but also closely related to the occurrence of macroscopic phenomena,such as soil erosion,the formation and stabilization of soil aggregates,soil water infiltration and soil material migration.For a long time,ion exchange adsorption has been widely concerned by researchers,and a series of ion exchange adsorption models for describing the complex soil system have been proposed.However,the exchange adsorption of ions in the soil is a complex process with multiple adsorption forces and multiple reactions simultaneously occurring,while the present models of soil ion exchange adsorption are mostly empirical or semi-empirical models,so it’s difficult to apply them to reveal the complex mechanism of ionic interface reaction.In recent years,some important progress has been made in the study of the mechanism of ion adsorption in soil or clay systems,including:（1）distinguish the mass transfer process and the interfacial reaction process of ion adsorption;（2）the relationship between ion diffusion and electrostatic adsorption driven by electric field;（3）weak electrostatic adsorption and strong interactions（electrostatic and non electrostatic）to describe the kinetics of the adsorption process;and（4）the non-electrostatic adsorption caused by the electrostatic field and so on.Especially with respect to the"electrostatic field-induced non-electrostatic attraction"effect,which may play an important role in the ion interface adsorption in soil or clay systems.It is well known that the surface of the soil or clay particles carries a large amount of charges,and these charges together with the adsorbed counterions in the diffusion layer can form an electric field with an intensity of 10⁸ to 10⁹ V m^-1 near the interface.According to the principle of quantum mechanics,the energy and quantum states of extranuclear electron of atoms（or ions）（especially outer electrons）in such strong electric field will undergo a significant change,which will lead to new interactions between ions and soil particles.Changes in the energy and quantum states of extranuclear electron of atoms can cause changes in the apparent polarization or dipole moment.In other words,the change in the energy and quantum state of extranuclear electron of atoms can be reflected from the atomic polarization or dipole moment in some extent.Recent studies have found that the polarizability of metal ions adsorbed on the surface of soil or clay particles has changed dramatically,and the polarizability of ions can be several hundred times higher than their classical polarizability.Currently,this polarization is referred as non-classical polarization.Due to this non-classical polarization,the ions generate new additional adsorption energies,and it is found that this new additive effect is the origin of the specific ion effects,or the Hofmeister effect of the clay system.So the new additional energy generated by ion polarization can also be called Hofmeister energy.It is no doubt that the polarization effect of the ions in the strong electric field near the surface of the soil particles must profoundly affect the ion adsorption characteristics in the soil.Therefore,the scientific objective of this study were to:（1）establish an ion adsorption kinetics model that considering polarization effects,preliminarily clarify the influence of ionic polarization on the ion adsorption kinetics;（2）According to the kinetic model of ion adsorption and corresponding experimental data,present a method to quantitatively characterize the ion polarization effect;（3）Based on the kinetic analysis,clarify the effects of ion polarization on the parameters of ion adsorption,such as ion adsorption rate,adsorption activation energy,ion diffuse average distance and ionic apparent diffuse coefficient;（4）initially elucidate the ion adsorption mechanisms taking polarization effects into account.Through these studies,we have obtained the following results:1.The ion adsorption kinetics model based on ion non-classical polarization has been established.Influence by the surface electric field,the random fluctuation of the ionic outer electrons will be sharply amplified in the process of ions approach to the charged surface,and thus the non-classical polarization effect of ions produced.In the process of establishing the adsorption kinetic model,substituting the ion non-classical polarization energy（also referred to Hofmeister energy,represented by w_H（0））into the basic equation of ion nonlinear diffusion（flux equation and nonsteady diffusion equation）to obtain the basic equation of ion adsorption based on polarization.The theoretical equations of ion adsorption kinetics for ion polarization effects can be further obtained by solving these two basic equations,and the theoretical expressions of ion adsorption rate,ion equilibrium adsorption capacity and rate constants are given.The theoretical model shows that the strength of Hofmeister energy determine the possible kinetics characteristics of the ion adsorption process.The theoretical model shows that:（1）the kinetics characteristics of ion adsorption will be different even if for the isovalent ions,such as the adsorption kinetics characteristics of Cs⁺and Li⁺were different due to their different w_H（0）（ie,the polarizability）;（2）For cations with larger w _H（0）value,the adsorption of ions will exhibit the zero order kinetic characteristics in the early stage and then turn into the first-order kinetics;（3）For cations with smaller w _H（0）value,the theoretical model indicates that only the first order kinetics could be exhibited during ion adsorption;（4）The ion adsorption rate and the equilibrium adsorption capacity（metastable equilibrium）will increase with the increase of w_H（0）.2.The established theoretical model has been verified by ion adsorption kinetics experiments.The w _H（0）value for Cs⁺,Na⁺and Li⁺were w _H（0）_Cs=1.40×Fψ（0）,w _H（0）_Na=0.18×Fψ（0）and w_H（0）_Li=0.063×Fψ（0）respectively.The experimental results showed that:（1）the adsorption for Cs⁺,Na⁺and Li⁺would exhibit the different kinetics characteristics due to the different w _H（0）.In the range of 0.0001-0.01 mol L^-1electrolyte concentration,Cs⁺adsorption exhibited the zero and first order kinetics characteristics,Li⁺adsorption only exhibited the first order kinetics characteristics,Na⁺adsorption showed the first order kinetics characteristics except at the electrolyte concentration of 0.0001 mol L^-1 that the intensity of electric field is the strongest.（2）for the Cs⁺with larger w _H（0）value,zero-order kinetics exhibited at the initial stage of the adsorption experiment,and subsequently turned to first-order kinetics;（3）for the Na⁺and Li⁺with smaller w _H（0）value,in addition to the strongest electric field that Na⁺adsorption showed the zero-order kinetics and then turned to the first-order kinetics,the Na⁺and Li⁺adsorption processes showed only a first-order kinetics at other electrolyte concentrations;（4）the adsorption rate and equilibrium adsorption capacity for ions in the order of Li⁺<Na⁺<<Cs⁺at same electrolyte concentration.For example,at the electrolyte concentration of 0.0001 mol L^-1,the adsorption rate and equilibrium adsorption capacity for Li⁺,Na⁺and Cs⁺were 8.479,9.509,13.39mmol kg^-1 min^-1 and 79.32,103.9,523.0 mmol kg^-1 respectively.The ion adsorption rate and equilibrium adsorption capacity increase with the increase of w_H（0）.3.A new method for characterizing the Hofmeister energy produced by ion non-classical polarization is proposed by using the new theory of ion adsorption kinetics.The mathematical relationship between the Hofmeister energy and ionic equilibrium adsorption capacity was established,and the Hofmeister energy of the ions in the clay-water system was quantitatively calculated using the measured ion equilibrium adsorption capacity.The results showed that the ion size,hydration or dispersion force can not reasonably explain the Hofmeister energy measured in the experiment,and Hofmeister energy can be reasonalized by the non-classical polarization of ions.It is found that the Hofmeister energy of adsorbed ions have two important characteristics:（1）it increases with the increase of the electric field intensity;（2）the more softer electric cloud of Cs⁺possesses higher Hofmeister energy than the relatively hard Na⁺.These two characteristics imply that the possibility of the deformation of the ionic electron cloud conformation（polarizability）in the ion-clay mineral interaction determines the intensity of the Hofmeister energy.Since the Hofmeister energy produced by the polarization can induce the apparent charge of Cs⁺increases from the classical+1 value to the+2.354 in the clay mineral system;in contrast,the apparent charge of Na⁺increases from+1 to+1.112.4.The ion adsorption equilibrium problem based on ion non-classical polarization has been discussed.The constant flow method was adopted to investigate the exchange adsorption equilibrium of monovalent cation X⁺（X⁺=Cs⁺,Na⁺and Li⁺）on the surface of montmorillonite particles in X⁺/K⁺exchange system.Theoretically,the equilibrium adsorption capacity of X⁺is not limited by the cation exchange equilibrium,the adsorbed K⁺at t→∞can be completely exchanged by Cs⁺,Na⁺and Li⁺respectively.However,the experimental results show that the adsorption capacity of other X⁺ions at t→∞was much smaller than that of K⁺in the diffusion layer,except that the adsorption amount of Cs⁺at the electrolyte concentration is higher than 0.03 mol L^-1.The order of adsorption capacity was Cs⁺>>Na⁺>Li⁺.For example,when the electrolyte concentration was 0.0001 mol L-1,the adsorption capacity of Cs⁺,Na⁺and Li⁺at t→∞was only account for 45.5%,9.69%and 6.90%of the adsorption amount at the true equilibrium state respectively.When the electrolyte concentration was 0.001and 0.01 mol L^-1,the adsorption capacity of Cs⁺,Na⁺,Li⁺at t→∞was 56.3%,14.4%,10.7%and 95.5%,33.1%,30.3%account for the adsorption amount at real equilibrium state,respectively.These results showed that there is a significant metastable equilibrium state in the X⁺/K⁺-clay mineral exchange adsorption experiment,and the metastable equilibrium showed specific ion effects.Further analysis showed that the non-classical polarization of ions caused by the surface electric field of charged clay minerals leads to the generation of the metastable equilibrium states5.The theory of ion adsorption metastable equilibrium based on non-classical polarization has been presented.For ion adsorption in a heterogeneous charged system,diffusion is the rate limiting step.At the same time,the diffusion of ions in the heterogeneous charged system is the diffusion of ions in the electric field.Therefore,the Smoluchoski equation was used to describe the diffusion process instead of the Fick’s equation.According to the solution of the Smoluchoski equation under the given boundary conditions and initial conditions in the constant flow experiments,a theory of ion adsorption metastable equilibrium considering the ion polarization effect has been presented.The mathematical formula given by this theory showed that:（1）the equilibrium adsorption capacity of ions will depend on the adsorption rate or the activation energy of adsorption,so the ion adsorption equilibrium must be a metastable equilibrium rather than a true equilibrium;（2）The activation energy of adsorption is closely related to the Hofmeister energy produced by ion polarization;（3）The ratio of ion adsorption rate constant at two electrolyte concentrations is equal to the corresponding concentration ratio in the presence of ion metastable adsorption equilibrium process.The theory has been verified by the exchange adsorption experiments of X⁺/K⁺（X⁺=Cs⁺,Na⁺and Li⁺）that conducted at the surface of the montmorillonite.Combined with the above-mentioned research results,the following conclusions were drawn:（1）ion polarization affects the kinetic characteristics of ion adsorption;（2）different non-classical polarizability of isovalent ions is the fundamental reason for different kinetics characteristics of ion exhibited;（3）the adsorption of strongly polarized ions will show zero-order rate characteristics at the initial stage of adsorption,and then turn into first-order rate characteristic,but the weakly polarized ions exhibit only first rate characteristic;（4）)Ion polarization will result in a metastable equilibrium of ion adsorption,and the rate of ion adsorption in the presence of metastable equilibrium follows a simple rule:the ratio of the ion adsorption rate constant at the two electrolyte concentrations is equal to the corresponding concentration ratio.The relevant theories presented in this study have been verified by the experimental results.