From KD to Surface Complexation Models: Present State of Actinide Data Supply

Almost all risk assessment studies related to uranium mining and milling legacies rely on the wide-spread but simplistic KD concept of distribution coefficients, which subsumes all relevant physico-chemical phenomena into only one parameter. To overcome the resulting limitations, more realistic chemical speciation patterns are needed, including complex formation, heterogeneous phase equilibria and surface complexation. Following this strategy, the paper presents a digitized version of a thermodynamic sorption database as required for the parametrization of Surface Complexation Models (SCM). It is mineral-specific and hence also suitable for additive models of more complex solid phases such as rocks or soils. All major electrostatic variants of SCM are integrated. Also the concept of strong and weak binding sites, and the 1-pK approach is applicable. Surface species include ternary ones.
Data records comprise of mineral properties, specific surface areas, surface binding site concentrations and protolysis constants, sorption ligand information, and surface complexation reactions, all coupled to an extensive bibliography. Based on them, a detailed picture about the distribution of actinide SCM data amongst minerals, SCM subtypes and ligands is presented. Derived from that and focused on uranium, critical data gaps are identified, existing parameter sets are evaluated, and consistency tests are performed. Final goal is the establishment of recommended reference data sets.
Only few of the surface complexes proposed in the literature (mostly resulting just from best-fit considerations) are actually validated by spectroscopic evidence. If such information is available it is stored in the database and used for data evaluation.
Originally implemented as a stand-alone relational database under MS Access on a PC, the database RES³T (Rossendorf Expert System for Surface and Sorption Thermodynamics) is now connected to the WWW and accessible through common web browser interfaces.