Assessment of surface area normalisation for interpreting distribution coefficients (Kd) for uranium sorption

Adsorption of radionuclides on soils and sediments is commonly quantified by distribution coefficients (K-d values). This paper examines the relationship between K-d values for uranium(VI) adsorption and the specific surface area (SSA) of geologic materials. We then investigate the potential applicability of normalising uranium (U) K-d measurements using the SSA, to produce 'K-a values' as a generic expression of the affinity of U for the surface. The data for U provide a reasonably coherent set of K-a values on various solid phases, both with and without ligands. The K-a representation provides a way of harmonising datasets obtained for materials having different specific surface areas, and accounting for the effects of ligands in different systems. In addition, this representation may assist in developing U sorption models for complex materials. However, a significant limitation of the K-a concept is that sorption of radionuclides at trace levels can be dominated by interactions with specific surface sites, whose abundances are not reflected by the SSA. Therefore, calculated K-a values should be interpreted cautiously. (C) 2010 Elsevier Ltd. All rights reserved.