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Abstract: Na/Ca ion-exchange isotherms were performed on a Na-saturated montmorillonite for different constant normalities of the aqueous chloride solution and at two solid/solution ratios. The experimental data suggest that the affinity of Na+ for the solid increases with total normality and m/V ratio and that a significant proportion of CaCl+ is also sorbed. A thermodynamic modeling procedure with one sorption site and three sorbed cations (Na+, Ca2+ and CaCl+) was applied to interpret the data. We show that by accounting for the activities of aqueous species, the experimental data obtained at different total normalities for a given solid/solution ratio can be fitted using a unique set of selectivity coefficient values. However, when the m/V ratio is decreased from 25 to 2.5g/L, an increase in the logKc(Na+/Ca2+) of up to 0.4±0.05 was required to interpret the data with a constant logKc(Na+/CaCl+) value. The same behavior concerning the increase in the logKc(Na+/Ca2+) with a m/V ratio decrease was observed when using a multi-site model taking into account the specific sorption of H+. The results clearly indicate that the Na+/Ca2+ selectivity coefficients strongly depend on the solid/solution ratio but are independent of the exchanger composition and the total normality of the solution. Such findings provide possible pitfalls when using selectivity coefficients obtained in diluted suspension to assess the sorption in compacted clayrocks and allow a better prediction of the sorption of trace elements in competition with major cations. [ABSTRACT FROM AUTHOR]

Abstract: The aim of the present paper is to clarify previous results showing that selectivity coefficients determined for the exchange of Na+ for Ca2+ in montmorillonite were dependent on the solid/solution ratio. The organization of montmorillonite suspensions upon Na+/Ca(II) exchange was analyzed by combining optical microscopy, small-angle X-ray scattering and X-ray diffraction. All samples displayed flocculated characteristics, eliminating the possibility of contrasting accessibility of sorption sites with the solid/solution ratio. Modeling of experimental X-ray diffraction patterns was used to quantify the relative proportions of interlayer Ca2+ and Na+ cations along the exchange isotherm. The results further confirmed the influence of the solid/solution ratio on the degree of interlayer Ca(II)-for-Na+ exchange, and specific selectivity coefficients for interlayer sites were determined. The effect of the solid/solution ratio was finally interpreted by the resulting local changes in the solution chemistry. We demonstrated that by accounting for the Donnan effect, the different data can be interpreted using a single selectivity coefficient. The obtained Kc constant was successfully applied to interpret existing hydrogeochemical data on a natural aquitard. This most likely represents a more constrained and valid approach for the modeling of reactive element transport in natural media than does the poorly defined Kd parameter. [Copyright &y& Elsevier]