The relationship of cation-adsorption capacity to exchange adsorption in selected feldspar minerals

Abstract

The transport of radioisotopes by ground water has been the subject of numerous investigations. An important factor controlling the rate of transport is the distribution coefficient, K[subscript d], as defined by Mayer and Tompkins. The K[subscript]d is a measure of the amount of a specific cationic species distributed between the solution phase and the solid phase. The distribution will depend upon the adsorbent capacity of the solid material. The purpose of this investigation is to determine the relationship between the distribution and the adsorbent capacity of granular microcline, albite, and bytownite feldspar. Adsorption by earth materials is commonly expressed as cation - exchange capacity. This value represents the maximum amount of cations that a mineral is capable of adsorbing. Earth materials adsorb cations from solution regardless of whether or not sufficient cations are available for complete saturation. The maximum adsorption under these conditions is referred to as cation-adsorption capacity. Cation-adsorption capacities were measured for 20-30 mesh size granular microcline, albite, and bytownite feldspars. Bytownite has a larger adsorption capacity than does microcline, and microcline has still a larger adsorption capacity than albite. The cation-adsorption capacity is related to the magnitude of the residual negative charge on the feldspar grains which is in turn related to the structural configuration of the mineral. Adsorption during the distribution reaction is termed exchange adsorption. The magnitude of the exchange adsorption will depend upon the nature of the adsorbent and the nature of the cations in solution. It was found that in a distribution involving cesium and sodium, calcium, or potassium that cesium and potassium will be distributed according to their respective concentrations in the solution. Distributions involving cesium and calcium or sodium will react as if the cesium is present in larger concentrations than is actually the case. This is due to the influence of the double diffuse layer on the electrical charge distribution on the calcium and sodium ions. The double diffuse layer is related to the hydration characteristics of the ion. Cesium and potassium have similar hydration characteristics, therefore, the double diffuse layer does not induce relative diffusion in the distribution reaction. The distribution reaction is determined by the competing cation, the trace cation, and the adsorption capacity of the mineral adsorbent. For a particular solution-solid system the adsorption reaction for the competing cation and the mineral adsorbent cannot be predicted. The influence of the competing cation upon the distribution reaction prohibits the use of cation-adsorption capacity to determine K[subscript d] values.