Clarification of the sorption behavior of uranium onto mica and feldspar

As part of long-term safety assessments for radioactive waste disposals, scenarios have to be considered, which lead to the mobilization of radionuclides from the waste and to their transport through the repository system. Any repository sites (such as Gorleben) the sedimentary overburden is an important barrier for radionuclide transport. For most radionuclides the transport is retarded by sorption on mineral phases. Mica and feldspars are major components of the Gorleben sediments. However, almost no sorption parameters are available (www.hzdr.de/res³t [1]) for these systems.
The WEIMAR-project (Further Development of the Smart Kd-Concept for Long-Term Safety Assessment) shall address these shortcomings. Batch sorption experiments and spectroscopic investigations will allow the assessment, the evaluation as well as the processing of sorption data for U, Np, La(III) onto muscovite and orthoclase. Generally, the amount of sorption can depend on the pH-value, ionic strength, redox potential, concentrations of the contaminant as well as of the sorbent, of complexing ligands and competing ions. All batch experiments are carried out under ambient atmosphere and with 0.01 M NaClO4 as background electrolyte.
For muscovite and orthoclase the background concentration of the sorbing elements, the tendency of wall sorption and the dissolution behavior has to be determined. For both minerals an optimal solid-to liquid ratio has to be optimized. This means that the sorption is neither too low nor too high, so the sorption has to be between 5 and 95%. All those problems will be illustrated for the case of U(VI) sorption.

This project is funded by the German Federal Ministry of Economics and Technology (BMWi) under contract number 02 E 11072B

[1] Brendler, V. et al. (2003), J. Cont. Hydrol. 61, 281-291.