Radionuclides iti the geosphere /Sorption ratios /Colloid formation/Radioactive Cs + ¡Radioactive Sr 2+ /Radioactive Ce 3+ Summary Natural systems comprising sediments and groundwaters of low and of high salinity are investigated with respect to the formation and the influence of coarsely dispersed particles and finely dispersed particles (colloids). Equations are derived for the sorption equilibria in such systems and it is shown that the efficiencies of mutual separation of coarsely dispersed particles, finely dispersed particles and sediments (efficiencies of filtering) strongly influence the sorption ratios that are observed.In groundwater without sediments the formation of colloids of radioactive Ce 3+ is demonstrated, whereas Cs + and Sr ì+ do not form colloids under these conditions, as expected.The amounts of elements and of colloids given off by the sediments into the groundwaters are measured for the samples of low and of high salinity and the influence of agitation is shown.Sorption ratios of radioactive Cs + , Sr î+ , and Ce 3+ are investigated as function of the ratio of the groundwater volume to the mass of the sediment without filtration, after filtration through 0.45 Mm filters and after ultrafiltration. The results are discussed on the basis of the equations derived. It is shown that Cs + ions are sorbed mainly on the clay particles and that the larger amount of these clay particles has grain sizes greater than 0.45 Mm under the experimental conditions of smooth shaking. Sr 2+ ions are mainly sorbed on the sediments with a relatively low sorption ratio. Ce 3+ ions form colloids ("Eigenkolloide") that are retained by ultrafiltration.
Sorption of radionuclides / Sorption ratio / Sorption mechanisms / Radioactive Cs + ¡Radioactive Sr 2+ /Radioactive Ce 3+ Summary A general formula for the sorption ratio that is observed experimentally in natural systems is presented taking into account three kinds of processes: sorption at the outer surface of the solid particles, sorption in porous particles and one-directional processes such as precipitation or coprecipitation. These various processes as well as the mechanisms and the kinetics of sorption are discussed.The sorption of radioactive Cs + , Sr 2+ and Ce 3+ is studied in two selected natural sediment/groundwater systems, one of low and one of high salinity, under various conditions. Cs + ions are mainly sorbed in clay minerals that are suspended in the groundwater with relatively high sorption ratios, Sr 2+ ions are mainly sorbed on the sediments with relatively low sorption ratios, and Ce 3+ is mainly found in form of colloids or in suspended particles or in coprecipitates. Coprecipitation leads to very high sorption ratios for Ce 3+ . The behaviour of Cs + , Sr 2+ and Ce 3+ is typical for all elements with similar properties.
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