New Efficient Calixarene Amide Ionophores for the Selective Removal of Strontium Ion from Nuclear Waste: Synthesis, Complexation, and Extraction Properties

Three novel lower rim hexamide derivatives 5(6), 7(6), and 9(6) of p-hydroxycalix[6]arene and four octamides 5(8), 7(8)-9(8) derived from the corresponding p-hydroxycalix[8]arene were synthesized, and their potential as extractants in radioactive waste treatment was evaluated, in comparison with upper rim analogues 12(6) and 12(8) and other existing selective neutral ionophores currently used in radioactive waste treatment. Extraction of alkali and alkaline earth metal picrates from water to dichloromethane, and of the corresponding nitrates from acidic water solution simulating radioactive waste. to 2-nitrophenyl hexyl ether (NPHE), showed that the lower rim amides extract divalent cations much better than monovalent ones. The upper rim hexa-12(6) and octamide 12(8) are very inefficient ligands, hardly extracting any cation. In all cases, p-alkoxy octamides are more efficient and selective extractants than the corresponding hexamides. In the case of simulated waste solutions, the distribution coefficients for strontium removal by octamides (6.5 < D-Sr < 30) are much higher than the corresponding value (Ds,) found for dicyclohexyl-18-crown-6 (DC18C6), and the same applies for the strontium/sodium selectivity, which is 6500 < D-Sr/D-Na < 30 000 for octamides and 47 for DC18C6. ESI-MS, UV-vis, and X-ray crystal structure studies give consistent results and indicate the formation of 2:1 (cation/ligand) strontium complexes for all octamides tested. Stability constants were determined in homogeneous methanol solution for alkali metal (log beta (11) less than or equal to 2), calcium (4.3 less than or equal to log beta (11) less than or equal to 6.0; 9.4 less than or equal to log beta (21) less than or equal to 12.0), and strontium (5.6 less than or equal to log beta (11) less than or equal to 12.3) ions using a UV-vis competition method with 1-(2-pyridylazo)-2-naphthol (PAN). They confirm the high efficiency and high divalent/monovalent selectivity found in metal ion extraction experiments for the new octamide ligands. Evidence for a positive cooperative effect between the two metal ion binding sites was obtained in the case of the Ca2+ complex of octamide 1(8).