Significance of Electrostatic Sorption in the Retardation of Radionuclides Released from a Repository
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SIGNIFICANCE OF ELECTROSTATIC SORPTION IN THE RETARDATION OF RADIONUCLIDES RELEASED FROM A REPOSITORY
H. MEIER, E. ZIMMERHACKL, G. ZEITLER AND P. MENGE Staatliches Forschungsinstitut fir Geochemie, D-96049 Bamberg, Germany
ABSTRACT
Results of zeta potential measurements by electroosmosis with site-specific waters and sedimentary rocks from strata overlying the Gorleben salt dome are given and compared with sorption data of radionuclides obtained in batch experiments. It is shown that zeta potentials of sediment-groundwater systems in spite of being lower than the corresponding surface potentials are a sensitive indicator of interface reactions depending on the type of the sediment, ionic strength of waters, pH and temperature. Moreover, it is demonstrated that sorption data and, additionally, the influence of various parameters can often be understood by taking into account variable electrostatic forces between charged surfaces and nuclide species. INTRODUCTION Long-term calculations of radionuclide migration may be often based on the simulation of the hydrologic flow-path from a repository to the biosphere by homogenous geochemical zones with constant sorption coefficients for each nuclide. For ensuring this "constant K, model" in the safety assessment it is necessary to identify the sensitivity of sorption to changes in geochemical variables of the rock-groundwater zones and, additionally, to elucidate the influence of processes which control sorption. Among reactions which are effective in the retention of radioelements electrostatic interaction between charged surfaces and nuclide species can be considered as a key process. However, little overall analysis about the extent of electrostatic interaction compared with specific chemical sorption [1,21 for quantifying sensitivity of sorption to changing conditions in site-specific systems is available [3]. Therefore, in order to obtain an insight into the effect of coulombic forces on the retardation of nuclides correlations between sorption data and surface charge properties have been studied. Results for corresponding studies with site-specific waters and sedimentary rocks from strata overlying the Gorleben salt dome are given in this paper.
EXPERIMENTAL
Samples As samples sediments from selected strata overlying the Gorleben salt dome have been investigated. The geological samples were taken between surface and 200m depth from exploratory shafts by procedures which avoid the presence of Mat. Res. Soc. Symp. Proc. Vol. 353 01995 Materials Research Society
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drilling agents and any change of natural particle size distribution [4]. According to X-ray diffraction analysis the sediments consists of sandy loose rocks of about 95% quartz and 5% clays ( mainly illite, kaolinite and montmorillonite ), marlstones and clay beds. Water samples used were natural groundwaters with different salt contents taken from the same site as the sediments. In the following, numerical designation corresponds to the site-specific nomenclature. Radionuclides The radionuclides in
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