黄壤颗粒表面离子吸附动力学中的离子特异性效应

Hofmeister Effect in Ion Adsorption Kinetics on Surface of Yellow Earth Particles

基于考虑离子非经典极化作用的离子吸附动力学模型,以可变电荷黄壤颗粒为代表材料,研究了锂离子(Li+)、钠离子(Na+)和铯离子(Cs+)在黄壤颗粒表面的吸附动力学特征,进一步计算了离子在土壤/水界面的扩散距离与表面电化学参数。结果表明:(1)Li+、Na+和Cs+在黄壤颗粒表面吸附仅存在一级动力学过程并存在明显的离子特异性差异,这种差异随着电解质浓度降低而增大;(2)离子非经典极化与体积效应的共同作用决定了离子在固/液界面的扩散距离,在较高浓度电解质中,Na+和Li+的离子体积和极化效应基本平衡,导致Na+和Li+离子间差异不显著,但Cs+由于强烈的非经典极化作用,扩散距离表现为:Cs+>Na+≈Li+;在低浓度电解质中,离子非经典极化作用占主导,扩散距离表现为:Cs+> Na+> Li+,表明离子扩散距离在低浓度下差异大,而高浓度下差异不显著;(3)离子扩散距离的差异导致固/液界面电位的不同,Stern电位、电荷密度和电场强度均受到离子扩散距离的影响,表面电位(绝对值)在各个浓度下均表现为:Li+> Na+> Cs+,说明表面电位仅受非经典极化作用的影响。本研究将对土壤/水界面反应理论的完善提供新思路。

【Objective】A number of studies have reported that specific ion effects possibly exist universally in cation adsorption processes. In this study, based on the ion adsorption kinetics model that takes into account non-classical polarization of ions, adsorption kinetics of alkali metal ions on the surface of yellow earth particles (a surface with variable charges) was characterized. In addition, origins of the Hofmeister effects in the ion adsorption kinetics on surface of yellow earth particles were investigated. 【Method】 Kinetics of Li^+, Na^+ and Cs^+ adsorptions in the K^+ -saturated yellow soil was studied with the miscible displacement technique under a steady flow condition. In the flowing liquid Li^+, Na^+ and Cs^+ was spiked at 1×10^-2 mol·L^-1, 1×10^-3 mol·L^-1 and 1×10^-4 mol·L^-1 respectively and pH adjusted to 4. 【Result】(1) Li^+、Na^+ and Cs^+ varied sharply in adsorption rate and equilibrium adsorption capacity, when they were the same in concentration, which shows apparent ion specificity in the ions exchange process. The three metals exhibited an order of Cs^+ > Na^+ > Li^+, in adsorption rate and equilibrium adsorption capacity, when in solutions low in electrolyte concentration (1×10^-4 mol·L^-1), and an order of Cs^+ > Na+ ≈ Li^+ when in solutions high in electrolyte concentration (1×10^-2 mol·L^-1 and 1×10^-3 mol·L^-1), which suggests that volume is a major factor affecting equilibrium adsorption capacity in solutions high in electrolyte concentration. (2) It was also found that d (thickness of the Stern layer) decreased with increasing electrolyte concentration in the same electrolyte system, exhibiting an order of dNa >dLi > dCs in solutions high in electrolyte concentration (1×10^-2 mol·L^-1 and 1×10^-3 mol·L^-1), and a different order ofdLi >dNa > dCs in solutions low in electrolyte concentration (1×10^-4 mol·L^-1). The Stern layer of Cs+ was the lowest in d, regardless of electrolyte concentration, and too thin for the more softer electric cloud of Cs+, 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 dNa was found higher than dLi; 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 dLi > dNa. Therefore diffusion distance and then equilibrium adsorption capacity is determined by volume effect and non-classical polarization. (3) The order of KdiffCs/H > KdiffNa/H > KdiffLi/H≈1 means that the sequence of Li^+,Na^+ and Cs^+ in electric adsorption in the K^+-saturated yellow soil is Cs^+ > Na^+ > Li^+≈H^+, and also demonstrates that volume does not have much effect on ion adsorption in the double diffusion layer. And in terms of surface potential (absolute value), the three metals follows an order Li^+ > Na^+ > Cs^+ in solutions the same in concentration, indicating that the surface potential is only affected by non-classical polarization.【Conclusion】The interaction of ion-surface is determined by ion non-classical polarization and ion volume effect.