Ion-Exchange Equilibria between Montmorillonite and Solutions of Moderate-to-High Ionic Strength

Abstract

Prediction of the migration rates of ions in geologic formations is of extreme importance in the fields of nuclear waste disposal and enhanced oil recovery. Demonstration of the safety of nuclear waste repositories requires that the repository be so designed and situated that migration rates from the repository be acceptably low in the event of water flow through the repository. On the other hand, the principal criterion for selection of tracers to monitor fluid flow in the flooding techniques of enhanced oil recovery is that the tracers be not retained on the formation, i.e., migration rates must be high. Migration rates are dependent on the distribution coefficient of the species between fluid and the geologic media such that the greater the distribution coefficient the slower the rate of migration. Therefore, for reliable prediction of migration rates, accurate knowledge of the adsorption behavior of the nuclides and tracers must be known as a function of all of the pertinent variables.Obtaining the necessary data for this complex situation is complicated by both the large number of different minerals which may be present and the wide range of solution compositions. In order to accumulate a systematic body of information pertinent to this complex situation, we are measuring adsorption of ions of interest on minerals typical of classes, from a wide range of aqueous solution compositions. Initially, montmorillonite was selected for attention because its high adsorptive capacity for ions may cause it to dominate the adsorptive proportion of the formation in which it is present. Reported here are distribution coefficients of alkali metal and alkaline earth ions, and of one rare earth, between montmorillonite predominantly in the sodium and calcium forms and aqueous solutions of controlled pH, at moderate to high salinities (>0.01MNaCl). The ions were selected for initial study, not only because of their intrinsic interest for these applications but also because they allow evaluation of the extent to which equilibria can be described by conventional ion-exchange equations with minimum difficulties from hydrolysis, precipitation, and complexing. Correlation of results as a 0-8412-0498-5/79/47-100-297$07.00/0

Adsorption of Cadmium and Cobalt on Montmorillonite as a Function of Solution Composition

“…The addition of up to 0.1 M acetate buffer to the solution of strontium had no effect on the distribution coefficient (Shiao et al, 1978). The adsorbabilities of Co and Cd from 0.1 M Na solution as a function of acetate concentration are given in Figure 4.…”

“…where FAB is the activity coefficient quotient (see, e.g., Shiao et al, 1978) and D g is the distribution coefficient of ion A between the clay and the solution, defined as the concentration of A in the solid phase (moles/kg dry solid) divided by its concentration in the solution (moles/liter). When only traces of A are present, that is, when its concentration in the solid phase is much smaller than the capacity of the clay, when FAB is constant, and when electrolyte invasion is negligible, it follows from Eq, (2) that:…”

“…As the values obtained fell close to the values of the adsorption test (solid circles in Figure 1), it is concluded that the adsorption is Figure 2 shows plots of the distribution coefficients of Sr, Co, and Cd as a function of their concentration in the clay (loading on the clay). The results for Sr were taken from Shiao et al (1978). This plot shows that the distribution coefficient of Sr at pH 5 is constant for loading values as high as 10 -z moles/kg.…”

“…All comparisons of adsorbabilities for different adsorbates, pH, etc., were carried out on samples from the same preparation. noncomplexing media (chloride for Sr, taken from Shiao et al, 1978, and nitrate for Co and for Cd). The log D-log(Na) function for Sr is linear, and its slope is -1.6, in comparison with -2 expected for an ideal divalent/monovalent ion exchange.…”

“…This paper reports the results of a study of the adsorption of Cd 2+, Co 2+, and Sr 2+ on montmorillonite from solutions of a wide range of salt concentrations and at pHs from 5.0 to 6.5. The adsorption of Sr on montmorillonite (see Shiao et al, 1978, and the references contained therein) can be explained in terms of simple ion-exchange mechanisms and therefore serves as a reference to which the adsorbability of Cd and Co can be compared. Cd and Co are transition metals that differ in their ability to form chloride anionic complexes.…”