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Separating traces of cesium from aqueous medium containing large sodium amounts is a difficult problem because cesium and sodium ions have a similar behavior in aqueous medium. We selected a highly cesiumselective ligand in aqueous medium, and we determined the best conditions for using this ligand in a nanofiltration-complexation process in order to achieve Cs + /Na + separation. We also tried to find a relation between the ligand structure and its activity toward cesium complexation. Tetrahydroxylated bis-crown-6 calix[4]arene was found to be very cesium selective (S ) (Cs + )/ (Na + ) ) 6600). Combined with a nanofiltration process, this ligand helped reaching 90% cesium retention in a highly concentrated aqueous medium ([NaNO 3 ] ) 3 mol/L). IntroductionAmong the different available separative methods (liquidliquid and solid-liquid extractions, ion-exchange resins), nanofiltration (NF) is a new pressure-driven membrane process which produces very few additional wastes. Located between ultrafiltration and reverse osmosis, nanofiltration membranes " cutoffs " are in the range of 300-1000 g/mol molecular weight. 1-3 Moreover, to increase ionic separation, nanofiltration can be combined with a selective complexation step: retention of the target element is improved because the complex with the ligand, of larger size and mass than the target ion, prevents this ion from passing through the membrane. 4 For such a purpose, water-soluble ligands with molecular weight of about 500 g/mol can be used in the case of the nanofiltrationcomplexation association, whereas micro-or ultrafiltrationcomplexation systems need larger ligands and induce some specific problems of increasing viscosity and precipitation. 5 Our laboratory has been developping the nanofiltrationcomplexation technique since 1993. [6][7][8][9] We showed that using EDTA with a Desal 5 DK or a Nanomax 50 membrane allowed a 100% retention (rejection) of Sr 2+ and other polyvalent ions, and only a 10% retention of Na + and Cs + ions. Then, with the use of a cesium-selective ligand such as resorcinarene, Cs + / Na + separation was also improved but not satisfactorily. In fact, to remove radioactive traces of cesium in highly salted aqueous medium, a minimum of 99% Cs + retention with a maximum of 10% Na + retention is required for industrial applications.So the aim of the present study was to select a highly cesiumselective ligand in aqueous medium, and then to determine the

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