引用本文:潘小丽,刘新敏,李 航,李 睿.碱金属离子在蒙脱石-Cu2+表面吸附的离子特异性[J].土壤学报,2020,57(2):370-380. DOI:10.11766/trxb201904110079
PAN Xiaoli,LIU Xinmin,LI Hang,LI Rui.Specificity of Alkali Metal Ions Absorbed on Surface of Montmorillonite-Cu2+[J].Acta Pedologica Sinica,2020,57(2):370-380. DOI:10.11766/trxb201904110079
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碱金属离子在蒙脱石-Cu2+表面吸附的离子特异性
潘小丽, 刘新敏
西南大学资源环境学院, 土壤多尺度界面过程与调控重庆市重点实验室
摘要:
研究离子交换中的离子特异性效应有助于揭示离子-带电表面相互作用机制。以蒙脱石Cu2+饱和样为研究对象,采用恒流法研究不同浓度碱金属离子Li+、Na+和K+的吸附动力学过程,并建立1:1型(LiNO3、NaNO3、KNO3)电解质溶液中离子平衡吸附量与体系吸附活化能之间的关系。结果发现:(1) Li+、Na+和K+在蒙脱石-Cu2+表面的吸附过程仅呈现出弱静电力作用下的一级动力学特征,并存在明显的离子特异性效应。(2) 离子非经典极化作用与体积效应共同决定了离子在双电层中的位置,从而导致表面电位存在差异;并且表面电位(绝对值)随着电解质浓度降低而增加,表现为Li+ > Na+ > K+。(3) 根据新建立的模型可预测吸附离子在双电层中的位置,进而求出体系的吸附活化能,并发现离子特异性效应产生的根本原因是由活化能决定的,同时本研究表明建立的新模型在固/液界面反应中具有普适性。本研究将对固/液界面反应理论的完善提供新思路。
关键词:  带电表面  离子吸附  表面电位  非经典极化  活化能  离子特异性
基金项目:国家自然科学基金项目(41530855,41877026)和重庆市自然科学基金项目(cstc2018jcyjAX0318)资助
Specificity of Alkali Metal Ions Absorbed on Surface of Montmorillonite-Cu2+
PAN Xiaoli, LIU Xinmin
Chongqing Key Laboratory of Soil Multi-scale Interfacial Process, College of Resources and Environment, Southwest University
Abstract:
【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-Cu2+ 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 (LiNO3, NaNO3, KNO3) 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 dNa > dLi , while in solutions low in electrolyte concentration, non-classical polarization would play a leading role, weakening the volume effect, and reversing the order as dLi > dNa. 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.
Key words:  Charged surface  Ion adsorption  Surface potential  Non-classical polarization  Activation energy  Ion specific effects