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Thin-layer and paper chromatography have been used for the determination of radiochemical yields of lS6Re complexation with three selected ligands methylenediphosphonic acid (MDP), ethylenediamminotetraacetic acid (EDTA), and citrate convenient for the radiopharmaceutical applications. The combination of selected two chromatographic systems has been chosen due to the satisfactory separation of free perrhenate, corresponding complex with lS6Re and reduced hydrolyzed rhenium. Rhenium complexes with studied ligands were prepared by reduction of perrhenate at presence of suitable ligand. Stannous chloride together with ascorbic acid (antioxidant) was used for perrhenate reduction. The effect of pH of reaction mixture, reaction time and concentration of reducing agent on the radiochemical yield of complexation is described and the optimal conditions for synthesis of rhenium complexes with MDP, EDTA and citrate have been found. Under optimal condition the radiochemical yield of complexation lS6Re-MDP, lS6Re-EDTA and lS6Re-citrate reached more than 90%, 80%, and 80%, respectively.

The mechanisms of transport and retardation processes, chemistry and migration behaviour of radionuclides of fission products and actinides in engineered barriers, especially bentonites, have been summarised. A "critical group of radionuclides" is proposed for thorough investigation of their retardation properties in natural sorbents. The evaluation of accessible data of retardation and transport parameters relevant for the conditions of underground deep repository to be projected is carried out from point of view their possible inclusion into a data-bank. The effect of solubilities and dissolution reactions, diffusion and sorption/desorption, complexation and variations in the aqueous phase composition, pH-value and oxidation-reduction properties and other phenomena on Kd values is discussed. Attention is devoted to thermodynamic modelling and its validation for evaluation of various phenomena in the buffer/backfill ground water systems.

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