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Theory to describe incomplete ion exchange in charged heterogeneous systems

Du, Wei, Liu, Xinmin, Li, Rui, Tian, Rui, Ding, Wuquan, Li, Hang
Journal of soils and sediments 2019 v.19 no.4 pp. 1839-1849
activation energy, adsorption, cation exchange, cation exchange capacity, cations, cesium, desorption, electric field, lithium, montmorillonite, potassium nitrate, sodium, sodium nitrate
PURPOSE: Incomplete ion exchange state in charged heterogeneous systems has been observed for a long time, but no general theory to describe this seemingly ordinary yet important phenomenon has been developed. The purposes of this study were to verify the effects of specific ions on the incomplete ion exchange and establish a theory to describe the incomplete ion exchange state. MATERIALS AND METHODS: Permanently charged montmorillonite were pre-saturated with KNO₃. K⁺-saturated montmorillonite (~ 0.5 g) were evenly spread in the exchange chamber. Exchange solutions (CsNO₃, NaNO₃, and LiNO₃) with electrolyte concentrations of 0.0001, 0.001, 0.01, 0.02, and 0.03 mol L⁻¹ concentrations flowed across the sample layer with a constant flow rate of 0.5 mL min⁻¹, respectively. The adsorption amount for each cation species was calculated based on the concentration difference before and after each experiment. RESULTS AND DISCUSSION: There were great differences between the adsorption amounts of Cs⁺, Na⁺, and Li⁺. The adsorption amounts of different ions were smaller than the cation exchange capacity of montmorillonite, aside from the Cs⁺ adsorption at the maximum Cs⁺ concentration of 0.03 mol L⁻¹. Therefore, the observed adsorption equilibrium of cations was actually in an incomplete ion exchange state rather than a complete ion exchange state. The amounts of adsorbed cations at the incomplete ion exchange state displayed the order of Cs⁺ > Na⁺ > Li⁺, which means that the activation energies of cation exchange adsorption possessed the order of Cs⁺ < Na⁺ < Li⁺. CONCLUSIONS: The potential barrier of diffusion for the adsorption and desorption ions in the electric field resulted in the incomplete ion exchange state. There is a basic rule for the incomplete ion exchange-exhibited specific ion effects: the ratio of the adsorption rate constants of an ion under two ionic concentrations will equal the ratio of the two concentrations.