An intercomparison campaign was organized at the beginning of the European FP6 EUROPART Integrated Project, among the partners involved in the assessment of the extracting properties of novel ligands, in order to check their accuracy and reliability when determining metal cation distribution ratios. Three different extracting systems, presenting various selectivity and affinity towards trivalent actinides were selected: (i) CMPO, (ii) TPTZ and (iii) BTP. Although some differences could be pointed out in the implementation of liquid-liquid extraction tests, the distribution ratios measured by the nine partners were fairly reproducible (< 20% deviation) for the first two extracting systems. However, large discrepancies occurred when testing the third extracting system, due to its strong hydrolytic instability, thus pointing out the difficulty of assessing extraction properties of instable ligands.
The syntheses of new calix[4]arenes featuring CMPO groups on the wide rim are reported and the extraction of Am(III) and Eu(III) from concentrated HNO 3 aqueous phases are discussed with reference to the properties of the symmetric tetra-CMPO derivative 1. All extraction studies were conducted in the same experimental conditions which allows to directly compare the dependence of the distribution coefficients of various calixarenes on the acid concentration (0.1 M < [HNO 3 ] < 5 M). Calix[4]arene 1 becomes a very poor extractant if the length of the aliphatic chain between the amide and phosphine oxide groups of CMPO is increased, if the bridging methylene groups are replaced by sulfur atoms or if the macrocyclic cavity size is increased. By contrast, mixed amide -CMPO calix[4]arenes are nearly as effective than 1. Moreover, Am(III)/Cm(III) separation coefficients between 1.5 and 3 have been obtained with unsymmetrical calix[4]arenes of type 1 with different aliphatic chains grafted on the narrow rim. Guidelines to anticipate the extraction ability of calix[4]arenes remain elusive because of the intricate solution behavior of these compounds.
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