Specific Ion Effects On Adsorption Dynamics Process Of Alkali Metal Drived By Soil Electric Field

Ion adsorption is one of the most important chemical properties in soil, which is the primary cause of the soil having environmental capacity to supply soil nutrient, storage nourishing elements, self-purification ability for pollution elements and pollutants. The existing researches show that the adsorption capacity of alkali metal ion on the soil surface is Cesium ion> Rubidium ion> Potassium ion> Sodium ion> Lithium ion, which was called Hofmeister series, the different physical and chemical phenomenons caused by this sequence in system called Hofmeister effects or ion specific effects.Current researches deem that the main inducement reason for ion specific effects result from the difference of ionic radius, ion hydration radius and dispersion force. The ion specific effects affected by volume effect from the ionic radius, ion hydration radius and dispersion force only appear in higher electrolyte concertration (>0.1molL-1) where the distance between ions shorten in the bulk solution.Meanwhile,the particle surface electric field was shielded sufficiently by adsorbed counter-ion, the dispersion force would be dominant.However, the identical valence ions present the drastical ion specific effects when the concertration was(10-4-10-2) mol L-1in our research, which convinced that the main inducement reason are not the difference of ionic radius, ion hydration radius and dispersion force.The lucubration of Hofmeister series in ion adsorption during decade years found that the adsorption capacity sequence may contain in concealed and profound theoretical basis.The current research imply that on the interface, the nonclassical polarization of extranuclear electron occurred in strong electrical field could be the basis of ion specific effects.The soil surface contain a large amount of charge, which will form a strong electric field as much as108V/m in nanoscale space around soil, that will affect the kinetics and thermodynamics of ion adsorption process. However,"electric field"and "ion specific effects" were ignored for a long time during the research of soil ion adsorption kinetics process.This paper based on the theory of soil electric field, with considering the soil electric field of ion adsorption dynamics model as the theoretical basis, researched adsorption process of ion specific effects of alkali metal ions (Li+, Na+, K+, Cs+) on montmorillonite and purple soil surface quantitatively. Finding that the surface electric field affects the dynamic adsorption process of alkali metal ions profoundly.We researched the difference of alkali metal ions quantitatively. The relevant results could be summarized as follows: (1)The adsorption kinetic parameters of different alkali metal ions under the same concentration on the surface of montmorillonite/purple soil are different, which showed the significant ion specificity effects. The adsorption rate of Li+,Na+and Cs+on the surface of the montmorillonite/purple soil were64.54mmol min-1m-2,87.38mmol min-1m-2,161mmol min-1m-2and56.31mmol min-1m-2,71.42mmol min-1m-2,110.7mmolmin-1m-2respectively when the concentration was1×10-3mol L-1. Equilibrium adsorption quantity were114.2mmol kg-1,194.5mmol·kg-1,647.4mmol·kg-1and63.41mmol·kg-1,70.96mmol·kg-1,168.2mmol·kg-1respectively. Adsorption rate constant and equilibrium adsorption quantity increases with increases of ionic radius. The identical valence ions appeared strong ion specific effects in the process of dynamic adsorption under low concentration condition, which was not explained by ionic radius, the radius of ion hydration and dispersion force.(2) In the early process of dynamic adsorption on the surface of the montmorillonite/purple soil appeared the zero-order kinetics characteristics under strong electrostatic force when the concentration was1x10-4mol L-1.When strong electrostatic adsorption saturated, adsorption process shifted to a weak electrostatic adsorption force and appeared the first order kinetics characteristics.When the concentration increased tol×10-3mol L-1,electric double layer was compressed, the surface electric field was strong shielded, the first order kinetics characteristics appeared on surface of K+-purple soil under the weak adsorption. The surface electric field is dominant inducement for Hoffmeister effects of identical valence ions.(3) the slope of kinetic adsorption rate curves increased along with the increased of ion concentration for identical ions.The diffusion distance from ion to particles surface in the electric double layer would shorten with the concentration increased of the ion concentration.The different ions under identical concentration appeared the tendency which the slope of kinetic adsorption rate curves decreased with the increased of ion radius.This is because the polarization effect of large ion is significant, which leading the diffusion coefficient decreases within ion kinetic adsorption process.(4) By compared the study without consider the surface electric field and within consider the surface electric field in ion adsorption dynamics mechanism model, we found that without considering the surface electric field, ion adsorption rate constant and equilibrium adsorption quantity was one of the thousands of points compared with considering the surface electric field.Thus, the effect of surface electric field for alkali metal ion specificity in the dynamic adsorption process must be considered.(5) Equilibrium adsorption quantity of each ion in adsorption kinetics experiments was far less than the cation exchange capacity of the sample, so the metastable adsorption state exists in the process of alkali metal ions adsorption dynamics.This is due to the chemical potential of ion is lower than the activation energy of the adsorbed ion in system within low ion concentration. When the reaction energy of system barriered by activation energy, the reaction process would incompletely according to the chemical thermodynamics point.When the ions concentration increased, metastable adsorption quantity on the surface of the particles decreased, and the chemical thermodynamics reaction reduced the limits of kinetics reaction, the adsorption rate accelerated, equilibrium adsorption quantity increased, metastable adsorption quantity decreased.(6) The metastable adsorption quantity of ions was decreased with the increased chemical potential of system, declined with the decreased of particle surface potential in system. With identical valence ions and concentration the metastable adsorption quantity are unequal, thus the coupling effects of soil surface electric field, ion size and metastable is the primary cause of special ion effect in alkali metal ions dynamic adsorption process.