Building a reference database for thermodynamic sorption modelling

Any safety assessment of waste disposal concepts requires comprehensive and consistent thermodynamic data for the respective reactive transport modelling. This includes sorption, ion exchange or surface precipitation as major retardation processes. The current lack of respective quality-assured databases for these interface phenomena (invalid reaction formulation, missing consistency, restricted application ranges, and contradictory data) severely hampers a reliable modelling.
This work aims on a re-evaluation of already published sorption raw data based on spectroscopically verified surface complexes and their formation reactions. This shall help to transform the free-for-use digitized sorption data collection RES³T (http://www.hzdr.de/res3t) into a true thermodynamic reference database be used for complex real systems such as rocks or soils following the “Component Additivity” approach. Coupled to this is an extension of RES³T allowing also for the storage of sorption raw data sets. Eventually, a full integration with the thermodynamic reference database THEREDA (http://www.thereda.de) is envisaged to provide a comprehensive database for a holistic geochemical modeling.
Sorption speciation calculations of radionuclides on various mineral surfaces will be presented, showing the actual consequences of inconsistent sorption data that can be found in literature, as well as the possibilities using a validated surface speciation. The latter is mainly based on a combination of ATR FT-IR, TRLFS, and EXAFS which allows to create chemically realistic surface complexing models. In combination with the site-density data (including ones from crystallographic measurements), surface complexation models are deduced that describe the sorption of radionuclides accurately and with less surface species then assumed in a vast number of literature references published in the past. Due to the correct description of the realistic surface chemistry and the internal consistency, these models are more robust to other chemical and environmental conditions (pH, pe, composition of the aqueous phase).
As examples, the sorption of uranium(VI) onto various mineral phases (Al-, Fe- and Si-phases), ubiquitous in nature will be presented.