Calix[6]arene-picolinamide extractants for radioactive waste treatment: effect of additional carboxy binding sites in the pyridine 6-positions on complexation, extraction efficiency and An/Ln separation

The effect of the presence of an additional ester or amide carboxy group in the 6-position of the pyridine nuclei of calixarene-based picolinamide ligands on the extraction and complexation properties of lanthanide(III) and actinide(III) metal ions was studied. For this purpose, six new ligands 1-6 were synthesized; their conformational properties were studied both in solution and in the solid state, and their binding properties towards lanthanide (Ln(III)) and actinide (Ant(III)) metal ions were determined under extracting conditions simulating those present in radioactive waste. In the presence of BrCosan as synergizer, a rather high efficiency in the extraction of trivalent metal ions by these ligands was observed even at [HNO(3)] > 2-3 M. Complexation of Ln(III) metal ions was also studied under homogeneous conditions (methanol solution), both in chloride and nitrate media, by using spectrophotometry and calorimetry. A comparison with the data obtained with ligands L1-L3, lacking the additional binding sites in the 6-position of the pyridine nuclei, is reported and the effects of the structure of the ligands on the stoichiometry of the complexes, coordination of the metal ions, basicity of the pyridine binding groups and efficiency and selectivity in the extractions are also discussed.