Abstract
Abstract As a model for ion equilibrium in montmorillonite, the Poisson–Boltzmann (PB) equation was solved for two parallel charged surfaces in contact with an external NaCl/CaCl 2 mixed solution. The ion concentration profiles in the montmorillonite interlayer were obtained from the PB equation and integration of those gave the occupancy of Na + and Ca 2+ in the clay. That information together with the composition of the external electrolyte were then used for the calculation of the Gaines–Thomas selectivity coefficient K GT . The predictions from the model were compared to experimental data from batch as well as compacted conditions, and the agreement was generally good. With a surface layer-charge density of one unit charge per 145 A 2 , which is close to the value for Wyoming-type montmorillonite, the calculated selectivity coefficients were found to vary from about 4 in batch to 8 in compacted montmorillonite with dry density ∼1700 kg/m 3 . From the point of view of assessing the evolution, with regard to sodium–calcium ion exchange, of the bentonite buffer in a repository for spent nuclear fuel, these results justify the use of data obtained in batch experiments.
Published Version
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