Specific Ion Effects Of Alkali Metal Ions Absorbed On Montmorillonite Particle Surface

It is necessary to study the ion interface reaction in order to understanding the mechanism of various important processes in soil,and ion exchange adsorption is an important manifestation of ion interface reaction.As an important physical and chemical process in soil,it has been widely concerned by researchers.By studying the ion exchange adsorption experiments in soil/clay minerals,it is helpful to clarify the microscopic mechanism of the ion interface reaction.At the same time,it also provides an understanding of the environmental problems of the increasingly serious farmland non-point source pollution in order to understand the preservation and leaching of nutrients in the soil.In recent years,great progress has been made in the research of ion adsorption mechanism in soil/clay system:?1?the distinction between the mass transfer process of ion adsorption and the interface reaction process;?2?the relationship between ion diffusion and electrostatic adsorption driven by electric field;?3?Kinetic description of adsorption processes under different forces;?4?the non-electrostatic adsorption caused by electrostatic field and so on.A series of classical models used to describe ion exchange adsorption have been proposed gradually.However,the ion exchange in soil is usually a complex process in which multiple adsorption forces interlace and multiple reactions occur simultaneously,so these classical models are difficult to reflect the interface process of ions in the actual reaction.Recent studies have shown that the non-classical polarization of ions in the strong electric field near the soil surface has a profound effect on their exchange and adsorption in the soil.Therefore,this study based on the existing research results,researched adsorption kinetics of different types of alkali metal ions on different montmorillonite particles under different concentration conditions.Using the established ion adsorption model,explore the essential reason of ion-specific effects,and clearing the influence degree of the effects of different electric field on the ion specificity,energy changes and its mechanism of action.Through these studies,the following results are obtained:?1?when alkali metal ions exchange montmorillonite-Zn²⁺,montmorillonite-Cu²⁺and montmorillonite-Ca²⁺,the adsorption process only presents the first-order kinetic characteristics under weak electrostatic force and has obvious ion-specific effects,which can be characterized by the adsorption rate and equilibrium adsorption quantity.The equilibrium adsorption amount and adsorption rate of alkali metal ions decrease with the decrease of alkali metal concentration.?2?Under the condition of the same ion type and electrolyte concentration,the difference of the saturated ions on the mineral surface will strongly affect the adsorption capacity of other ions on its surface.The adsorption capacity of the mineral surface after being saturated with low-valent ions is much greater than that after being saturated by high-valent ions.The equilibrium adsorption capacity of Li⁺and Na⁺on the surface of montmorillonite-Zn²⁺was significantly lower than that on the surface of montmorillonite-K⁺at the corresponding concentration,and the equilibrium adsorption capacity of Li⁺and Na⁺ions on the surface of saturated samples of montmorillonite-K⁺was 2⁸ times higher than that on the surface montmorillonite-Zn²⁺at the corresponding concentration.It indicates that the adsorption of alkali metal ions on montmorillonite has strong selectivity,and the adsorption selectivity order is affected by the alkali metal concentration.The adsorption quantity of ions on either montmorillonite-Cu²⁺or montmorillonite-Ca²⁺surfaces showed K⁺>>Na⁺>Li⁺in the electrolyte concentration of 0.0001 mol·L^-11 and 0.001 mol·L^-1;while the concentration increased to 0.01 mol·L^-1,the equilibrium adsorption quantity presented as K⁺>>Li⁺>Na⁺.?3?The non-classical polarization and volume effect jointly determine the diffusion distance of ions at the solid/liquid interface.No matter montmorillonite-Cu²⁺or montmorillonite-Ca²⁺,the distance of K⁺was the lowest in d regardless of electrolyte concentration because of its softer outer electron cloud,thus possessing relatively stronger non-classical polarization than Na⁺and Li⁺,but for Na⁺and Li⁺,volume might play a dominant role affecting ion adsorption processes in solutions high in electrolyte concentration,and consequently d_Na>d_Li;Otherwise the non-classical polarization would play a dominant role in solutions low in electrolyte concentration,weakening the volume effect,and reversing the order as d_Li>d_Na.Therefore position of ions in the double electric layer is determined by ionic non-classical polarization and volume effect.And the position leads to the difference of surface potential?absolute value?,which increases with the decrease of electrolyte concentration,presenting as?0?Li?>?0?Na?>?0?K?.?4?The position of ions in the double electric layer was estimated according to the estimated new model,and then the activation energy of the system can be obtained.It is found that the activation energy directly determines ion adsorption saturation.It is also found that the activation energy of the three ions has obvious ion-specific effects,but the adsorption saturation and activation energy of different ions on different surfaces are in the same law.The activation energy predicted by the ion adsorption model established in this study is consistent with that predicted by Arrhenius'law in chemical reactions,which further verifies the correctness of the new model theory.This study shows that the established ion-surface interaction model has a universal application in the solid/liquid interface reaction.It provides a theoretical basis for further clarifying the mechanism of ion-charged surface interaction.