Reaction of montmorillonite with acid aqueous solutions: solute activity control by a secondary phase

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Al, Ca, Mg and Na montmorillonites were reacted with acid aqueous solutions in order to test the assumption of equilibrium underlying solubility determinations of thermodynamic properties of montmorillonite. The value calculated for the equilibrium constant varied with pH over the pH range 1.5-4.2. Hence montmorillonite was not in a unique state of equilibrium with all solutions over this range. Therefore the derivation of thermodynamic properties of montmorillonite by the application of a single step dissolution equation to results of solubility studies in mildly acidic solutions is invalid. Indications of solute activity control by a secondary solid phase were given by: (1) the dependence of the solution concentrations of some, but not all, montmorillonite constituent elements upon each other; and (2) a lack of congruency in the dissolution of montmorillonite. These indications were confirmed by a principal components statistical analysis, which showed that the solution activities of Si, Al and H after long-term treatments of the mineral from undersaturation were always related by the same expression. This particular expression is equivalent to an equilibrium constant for the reversible dissolution of a secondary solid phase that is enriched in silicon by comparison with montmorillonite. An aluminous solid phase was also shown to be formed.

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