Ion Exchange Reactions of Clays

I t has been recognized for many ypars that many aspects of clay technolofcy including soil treatment and drilling nnul treatment must remain in an essentially empirical state until a basis for the understanding of ion exchange reactions is established. Much of the work on ion exchange reactions of clays in the i)ast has been directed toward establishing total exchange capacities or determining the ionic distribution empirically. This information in general is not suitable for the evaluation of hypotheses designed to provide a basis for understanding the exchange reaction. When the techninues for characterizing the various clay minerals offered the possibilit.v of quantitative study, the solution and exchanger phase contributions to the ionic distribution covdd be experimentally evaluated in principle. The particular experimental techniques which have been used to measure ionic distril)uti()n, however, frequently neglected observations which are essential if the data are to he used for testing and developing theories of ion exchange. It is now well recognized that molecular adsorption, complex ion formation in solution, and ion-pair formation between a mobile solution ion and a fixed exchanger group may occur in addition to the ion exchange reaction. Therefore, if the data are to be useful to develop theories of ion exchange, the whole system must be selected to minimize such extraneous contributions. On the basis of recent tlieoreticsil worli, various experimental techniiiues art; evaluated from the point of view of their suitability for equilibrium ion distribution studies. The mass action, adsorption isotherm, and Gibbs-Donnan equilibrium formulations of the ion exchange theory are discussed as they may apply to clay systems. Recent progress is summarized in (1) solution thermodynamics of mixed electrolytes as it is relevant to ion exchange processes of clays, (2) the contributions of non-ideality of the clay exchanger phase, and (S) the work of swelling of clays which affects the ionic distributions in ion exchange reactions. It is concluded that the parameters which relate to the solid phase of the exchanger and those which relate to the solution are now sufficiently well recognized that future experiments can be planned which may more realistically provide an experimental basis for iinderstanding the process of equilibrium ion exchange distributions in aqueous clay-electrolyte systems.