Specificity of Alkali Metal Ions Absorbed on Surface of Montmorillonite-Cu~(2+)

PAN Xiaoli;LIU Xinmin;LI Hang;LI Rui;Chongqing Key Laboratory of Soil Multi-scale Interfacial Process, College of Resources and Environment, Southwest University;

Charged surface;;Ion adsorption;;Surface potential;;Non-classical polarization;;Activation energy;;Ion specific effects

【Objective】The interactions between ions and charged particles determine microscopic properties of particles, interface reaction processes and interactions between particles. Ion exchange adsorption is an important physicochemical process of the reactions at solid/liquid interfaces. In this study, based on the newly established ion adsorption kinetics model, adsorption kinetics of alkali metal ions on the surface of montmorillonite particles(a surface with permanent charges) was characterized, in an attempt to analyze specificity of the ions in the adsorption process and to provide theoretical support to studies on interactions between ions and charged particle surface. 【Method】Kinetics of the adsorption of alkali metal ions, Li~+, Na~+ and K~+ on the surface of montmorillonite-Cu~(2+) particles was studied with the aid of the constant flow method, relative to ion concentration of the soluttion. Relationship between ion equilibrium adsorption capacity and system activation energy in 1︰1 electrolyte(LiNO_3, NaNO_3, KNO_3) solution was established. 【Result】(1) Li~+, Na~+ and K~+ varied sharply in adsorption rate and equilibrium adsorption capacity, in solutions the same in concentration, and in the ions adsorption process, they exhibited apparent ion specificities. Adsorption selectivities of the ions were subject to concentration of the electrolyte, and in solutions, 1×10~(–4) mol·L~(–1) and 1×10~(–3) mol·L~(–1) in electrolyte concentration, the metal ions exhibited an order of K~+ >> Na~+ > Li~+ in equilibrium adsorption capacity, while in solutions, 1×10~(–2) mol·L~(–1) in electrolyte concentration, they did an order of K~+ >> Li~+ > Na~+, which suggests that volume of the ions is a major factor affecting equilibrium adsorption capacity in solutions high in electrolyte concentration. The adsorption processes of Li~+, Na~+ and K~+ appeared to be of first-order kinetics as affeted by weak force. Their desorption processes did too, but with apparent ion specificities;(2) In the same electrolyte system, d(the distance between alkali metal ions and clay mineral surface when the adsorption reaches equilibrium) decreased with increasing electrolyte concentration, exhibiting an order of dNa > dLi > dK in solutions high in electrolyte concentration(1×10~(–2) mol·L~(–1)), and an order of dLi > dNa > dK in solutions low in electrolyte concentration(1×10~(–4) mol·L~(–1) and 1×10~(–3) mol·L~(–1)). Obviously d of K~+ is always the lowest regardless of electrolyte concentration because it has a layer of softer outer electron cloud around, and hence is much stronger in non-classical polarization than Na~+ and Li~+, but for Na~+ and Li~+, in solutions high in electrolyte concentration volume might be a main factor affecting ion adsorption processes, thus resulting in d_(Na) > d_(Li), while in solutions low in electrolyte concentration, non-classical polarization would play a leading role, weakening the volume effect, and reversing the order as d_(Li) > d_(Na). Therefore the joint effect of non-classical polarization and volume effect of the ions determines position of the ions in the double electric layer and then equilibrium adsorption capacity, thus leading to differences in surface potential(absolute value), which increases with decreasing electrolyte concentration, exhibiting an order of Li~+>Na~+>K~+ in solutions the same in concentration, which indicates that surface potential is mainly influenced by non-classical polarization; and(3) As the newly established model can be used to predict positions of the ions in the double electric layer, and further to calcuate activation energy of the system. The activation energy decreases with increasing electrolyte concentration, and in solutions regardless of eletrolyte concentration and type of cation, both adsorption saturation and activation energy of cations follow a similar law. 【Conclusion】 The occurrence of ion specificities is caused and determined mainly by activation energy. All the findings in this study demonstrate that the newly established model of cation exchange adsorption is universally applicable to researches on solid/liquid interface reaction.