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The ion exchange adsorption of tracer-level strontium by the clay minerals kaolinite and montmorillonite from solutions containing two competing cations was investigated. The exchange of strontium in these systems was successfully predicted from the results of the exchange adsorption of strontium from one competing cation solutions. INTRODUCTION Previous studies in this laboratory have dealt with the exchange of radioisotopes with single competing cations on single clays (Wahlberg and Fishman, 1962; Wahlberg and others, 1965). Data obtained in these studies will b& useful if they enable prediction of the movement of specific radioisotopes in streams and soils in the presence of several competing cations. The present investigation was performed to evaluate the effect of two competing cations on the exchange of strontium by clay minerals. The study chosen was the exchange of approximately 10-1W strontium-85 by kaolinite and montmorillonite from the following chloride solutions: sodium-potassium, sodiumcalcium, sodium-magnesium, potassium-calcium, magnesium-calcium, and magnesium-potassium. The clays were left in contact with the solutions for a period of 4 days to insure equilibrium conditions. The distribution coefficient, Kd (Tompkins and Mayer, 1947), expresses the distribution of an ion between solution and solid phases. If fs represents the fraction of a substance present that is sorbed by the clay and (1 /«) represents the fraction remaining in solution, then where V is the solution volume in milliliters and M is the mass of the Di D2 ION EXCHANGE ON MINERAL MATERIALS clay in grams. For a particular clay-solution system, the Kd remains approximately constant as the V/M ratio varies; however, as V/M changes, the fraction adsorbed will vary. The constancy of Ka with varying V/M ratios means that if a system is sufficiently defined with regard to salt concentrations and types of earth materials present to assign a distribution coefficient to the system, then by determining either the fraction of radio cation adsorbed or the V/M ratio, the other parameter may be calculated. Although distribution coefficients are useful for the prediction of radiocation exchange, they apply only at the specific salt concentration at which they were determined. To enable predictions of distribution coefficients at various concentrations, it is necessary to know the relationship between the distribution coefficient and the salt concentrations. To a first approximation, this relationship is given by the mass-action equation. Therefore we chose to use the mass-action equation to correlate the results of the one and two competing cation systems. To determine the relation between the salt concentration and distribution coefficient, one must consider the exchange reactions involved. For the reactions (2) and (3) the mass-action equilibrium constants are given by the following equations : (SrX.XNa*)' (Sr++)(NaX)3 (SrX2)(Ca++) Al~ and, since K( _ f, V^(SrX2) 3

Comparison of distribution coefficients for strontium exchange from solutions containing one and two competing cations