Publikationsrepositorium - Helmholtz-Zentrum Dresden-Rossendorf

The migration of contaminants though the environment can be retarded by various processes – one of them being sorption onto mineral phases along the flow paths. This process is itself a variable combination of surface complexation, ion exchange, surface precipitation, diffusion and others. However, the data accuracy is currently still limited due to a restricted understanding of molecular events decisive for the binding onto and incorporation into solid phases. In particular, underlying surface speciation is often inconsistent and range from assumed unrealistic postulations up to spectroscopically evident species with great impact on the corresponding thermodynamic data. Respective mechanistic models required for prognostics based on reactive transport are often lacking an evaluated, consistent set of species and thermodynamic parameters. This work provides answers to these problems.
Recently, surface-sensitive spectroscopic methods developed significantly, permitting the derivation of thermodynamically consistent sorption data sets. In combination with binding site densities (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 realistic surface speciation and the internal consistency, these models are more robust to varying chemical and environmental conditions (pH, pe, composition of the aqueous phase).
This work aims on a re-evaluation of already published protolysis and sorption raw data. As examples, the sorption of uranium(VI) onto various iron(III) and iron (II) containing mineral phases (ferrihydrite, goethite, hematite, magnetite), ubiquitous in nature and also being corrosion products of waste containers, will be presented. There, the use of the spectroscopically verified, bidentate bound uranyl surface complex is sufficient to fully describe the radionuclides sorption. Even with simple models like the diffuse double-layer model only one surface species is necessary for the sorption calculation. In most cases also the CO₂ containing ternary system does not call for additional species. 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.