title: 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.
abstract: The aim of the present paper is to clarify previous results showing that selectivity coefficients determined for the exchange of Na(+) for Ca(2+) 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 Ca(2+) 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.
authors.affiliation: Université de Poitiers, HydrASA UMR 6269 CNRS, rue Albert Turpain, Bat. B8, 86022 Poitiers Cedex, France. emmanuel.tertre@univ-poitiers.fr
authors.forename: E
authors.initials: E
authors.lastname: Tertre
authors.forename: E
authors.initials: E
authors.lastname: Ferrage
authors.forename: I
authors.initials: I
authors.lastname: Bihannic
authors.forename: L J
authors.initials: LJ
authors.lastname: Michot
authors.forename: D
authors.initials: D
authors.lastname: Prêt