Adsorption Kinetics Of Alkali Metal Ions On Variable Charge Mineral Surface

The adsorption and exchange capacity of ions is one of the most important physical and chemical properties in soil,which is closely related to soil self-purification ability,soil fertility retention ability,soil erosion and the formation and stability of soil aggregates.Previous studies have proposed many ion exchange adsorption models for soil/clay minerals interface,but these semi-empirical or semi-empirical models cannot well explain the co mplex process of several single reactions in soil or clay mineral,this is because the asymmetric strong electric field up to 10⁸ V m^-1 which formed by the charge on the mineral particles near the solid/liquid interface was ignored.In recent years,researchers have found that the electric field can cause the asymmetric hybridization of the outer nsnp orbital of oxygen atom and adsorbed metal ions on the mineral surface on the basis of considering solid/liquid interface has a asymmetric strong electric field,and produced new force in the interface,including nonclassical polarization force,polarization induced covalent force,and affecting the microscopic process from the soil.In addition,most soils in nature are the co-existence of permanent-charge minerals and variable-charge minerals,but studies on ion adsorption mechanism on the surface of charged mineral particles mainly rely on permanent-charge minerals.Therefore this research consider the orbital asymmetric hybridization,through the adsorption kinetics of alkali metal ions（Li⁺,Na⁺,K⁺,Cs⁺）on the surface of variable charge minerals at different p H（p H=4,p H=6,p H=8）,to analysis the types and intensities of the adsorption forces of alkali metal ions on clay minerals with variable charge under different conditions.The following conclusions were obtained according to the experimental results:（1）Under different p H conditions（p H4,p H6 and p H8）,the ion exchange adsorption process of Li⁺,Na⁺and Cs⁺on the surface of K⁺-kaolinite and K⁺-kaolinite/hematite mixed mineral all showed first-order rate characteristics,indicating that the adsorption of Li⁺,Na⁺and Cs⁺on the variable charge mineral particles surface is carried out under weak electrostatic force.With the increase of ion concentration,it takes less time to reach adsorption equilibrium of ions on the mineral particles surface,and the equilibrium adsorption capacity,adsorption rate constant are all increased.Under low ion concentration conditions(1×10^-4 mol L^-1 and 1×10^-3 mol L^-1),the equilibrium adsorption amount of Li⁺,Na⁺and Cs⁺is presented as Cs⁺>Na⁺>Li⁺,The equilibrium adsorption amounts of Li⁺on the surface of the K⁺-kaolinite/hematite mixed mineral was greater than the K⁺-kaolinite surface,while the equilibrium adsorptio n amounts of Cs⁺on the surface of K⁺-kaolinite/hematite mixed mineral is smaller than the K⁺-kaolinite surface.Under higher ion concentration conditions(1×10^-2 mol L^-1),the equilibrium adsorption amount of Li⁺,Na⁺and Cs⁺is presented as Cs⁺>Li⁺>Na⁺.It’s indicating that the adsorption kinetics of Li⁺,Na⁺and Cs⁺on the surface of mineral particles is closely related to the ion concentration and mineral constituent.（2）By analyzing the actual position of alkali metal ions in the diffusion double layer（d）on kaolinite surface found that under low ion concentration conditions(1×10^-4mol L^-1 and 1×10^-3 mol L^-1),the value of d is presented as d（Li⁺）>d（Na⁺）>d（Cs⁺）,under higher ion concentration conditions(1×10^-2 mol L^-1),ions are closer to the surface of mineral particles,and the value of d is presented as d（Li⁺）=d（Na⁺）>d（Cs⁺）.This is because the ion polarization capacity is different,so that alkali metal ion is different in the actual position in the diffusion double layer,eventually lead to different surface potentials of the kaolinite particles under different conditions.（3）On the basis of orbital asymmetric hybridization,we researched the adsorption energy of alkali metal ion on the K⁺-kaolinite and K⁺-kaolinite/hematite mixed mineral surface.It’s found that under the effect of electric fieldthe,the total adsorption energy of Li⁺is only composed of electrostatic energy,the total adsorption energy of Na⁺is composed of electrostatic energy and non-classical polarization energy,the total adsorption energy of Cs⁺consists of electrostatic energy,non-classical polarization energy and polarization induced covalent energy,because of their different nsnp orbital.Furthermore,as ion concentration changes,the proportion of different types of adsorption energy of different ions is different.（4）Through analyzes the effect of different p H on alkali metal ion adsorption kinetics on K⁺-kaolinite and K⁺-kaolinite/hematite mixed mineral surface,we found that the equilibrium adsorption capacity of ions increased with the increases of p H.At the same time,different p H will also affect the electrostatic adsorption energy of alkali metal ions on K⁺-kaolinite and K⁺-kaolinite/hematite mixed mineral surface,especially,under ion concentration of 1×10^-4 mol L^-1,the absolute value of the electrostatic adsorption energy of Li⁺and Na⁺on the surface of the kaolinite is significantly increased.In addition,compared with p H4,the absolute value of the non-classic polarizing adsorption energy under p H8 have a large extent,and the polarization induced covalent adsorption energy of Cs⁺also increased in varying degrees.