Influence of the ionic strength and solid/solution ratio on Ca(II)-for-Na+ exchange on montmorillonite. Part 2: Understanding the effect of the m/V ratio. Implications for pore water composition and element transport in natural media

The aim of the present paper is to clarify previous results [1] 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.

Influence of the Donnan effect on the apparent Na+/Ca2+ selectivity coefficient value of a montmorillonite: apparent log Kc(Na+/Ca2+) are plotted as a function of the solid/solution ratio and clay matrix porosity calculated as explained in the text.Figure optionsDownload high-quality image (58 K)Download as PowerPoint slideHighlights
► Description of Na+/Ca(II) ion exchange on clay minerals.
► Characterization of the solids in suspension by XRD, SAXS and optical microscopy.
► Same accessibility of sorption sites for contrasted m/V (from 2.5 to 1500 g/L).
► Independence of selectivity coefficients with the m/V and the clay matrix porosity.
► Significant Donnan effect when the m/V is superior to 10 g/L.