COMPETITIVE ADSORPTION OF CESIUM, COBALT AND STRONTIUM IN CONDITIONED CLAYEY SOIL SUSPENSIONS

Competitive adsorption of the ions (solutes) cesium, cobalt and strontium by soil samples from Hudspeth County, Texas, was investigated in laboratory experiments. Binary and ternary mixtures containing same weight percentage of each ion were placed in contact with the soil, at constant soil: solution ratio, temperature and pressure, until equilibrium was reached. Once it was determined that the adsorption of single adsorbates was well represented by the Freundlich equation, the Sheindorf-Rehbun-Sheintuch (SRS) equation was used to obtain the competitive coefficients for each component of the binary mixtures. The SRS-equation for ternary mixtures predicts the adsorption of each ion based on the parameters of its respective single-analog isotherm and the competitive coefficients obtained for binary mixtures. Predicted values were very close to those obtained experimentally for ternary mixtures. Competition coefficients vary from 0.15 to 0.20 for cobalt in the presence of strontium and 1.0 to 1.3 in the presence of cesium; 0.3 to 0.6 for cesium in the presence of strontium and 0.4 to 0.8 in the presence of cobalt, 3.0 to 6.3 for strontium in the presence of cesium, and 4.5 in the presence of cobalt. These values suggest heterogeneous interactions between ions and adsorption sites: cobalt and cesium do not compete for adsorption sites as much as cobalt does with strontium, or cesium with strontium. The predictive capability of these coefficients has the potential to reduce experimental effort and to provide a more realistic representation of radionuclide adsorption in soils than single-component isotherms. Experimental work needs to be extended in three general areas at least: a lower range of solute concentrations than in this study (below 1 mg/liter) to simulate other scenarios of possible contamination; additional concentration ratios among competing ions to further test the applicability of the competitive coefficients; and evaluation of the response of the coefficients to changes in water and soil composition, pH and temperature, among other important variables.