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Sodium-K, Na-Ca, Na-Mg, and Na-Sr exchange isotherms were performed at a total Cl concentration of about 0.005 mol L^sup -1^sup on the fine fraction (<2 µm) of MX80 montmorillonite, mixed-layer illite smectite minerals (IS), and clayey stones from European underground research laboratories. The derived selectivity coefficients for Na-K, Na-Ca, Na-Mg, and Na-Sr were found to vary very significantly as a function of the exchanger composition. A model was built to reproduce the data, taking into account the change of selectivity coefficient as a function of the exchanger composition of smectite. Model parameters were fitted using data from the present study and data from the literature obtained on smectite minerals. The addition of parameters for cation-anion pairs was found to improve the fitting results. The model was then successfully tested to reproduce published cation-exchange features on natural clayey materials and soils suspended in fresh or saline water. Finally, it is shown that the simple combination of this smectite exchange model with a previously published illite exchange model is quite successful although perfectible in reproducing exchange data on IS.
Abbreviations: IS, illite and smectite minerals; OM, organic matter.
(ProQuest: ... denotes formulae omitted.)
Cation exchange is considered as a major process governing cation availability in soils for plants and cation delays during transport in porous media. For these reasons, many studies have focused on this topic over a period of several decades to obtain cation-exchange reaction equilibrium constants, especially on clay minerals (e.g., see exchange database by Benson, 1982 and references therein).
An exchange reaction usually describes a stoichiometric replacement of ions on a surface according to the following generic equation (e.g., Fletcher and Sposito, 1989):
... Reaction 1
where z^sub A^ and z^sub B^ are the charges of ions A and B, X is the solid exchanger and (s) denotes ions in the solution phase. A thermodynamic equilibrium constant K^sup B[arrow right]A^sup is associated with Reaction 1:
... (1)
where N^sub A^ and N^sub B^ refer to the mole fractions of species ... their respective activity coefficients and [A^sub (s)^] and [B^sub (s)^] the activities of solute species A^sub (s)^ and B^sub (s)^, respectively. In given experimental conditions, one can define a conditional equilibrium coefficient ...
... (2)
K ... refers also to the Vanselow...