Cesium Distribution Coefficients in Unconsolidated Geological Materials

Abstract

Distribution coefficients (K_d's) for cesium were determined by batch procedures in fifteen Canadian unconsolidated geologic materials with a wide range in texture, cation-exchange capacity, clay-mineral composition and carbonate-mineral content. The K_d values varied over three orders of magnitude with no discernable correlation between the magnitude of the K_d and measured soil properties such as cation-exchange capacity, major-cation concentrations, clay-mineral composition, carbonate-mineral content, organic-matter content and pH. In more than half of the samples, however, the K_d values were related significantly to the natural exchangeable cesium content of the geologic material. When the amount of natural exchangeable cesium in the sample greatly exceeds the amount added in the test solution, the partitioning of the cesium between the solution and solid phases in the batch test is governed by the partitioning of the natural cesium. The results demonstrate that the retardation of ¹³⁷Cs in hydrogeologic systems can be governed by the concentrations of natural exchangeable cesium that occur in the system, and that in order for laboratory K_d values to provide an accurate representation of the field retardation potential, the total exchangeable cesium concentration in the batch test must duplicate the field condition.