Selective binding of ions of uranium and of transuranium and rare-earth metals with functionally substituted crown ethers

Abstract

The extraction of uranium and of transuranium and rare-earth elements from nitric acid solutions with solutions of 4,4′(5′)-bis(dialkylphosphoryl)-,-bis(diphenylphosphoryl)-, and-bis[(O-alkyl)phosphoryl]-benzo-n-crown-m ethers (n = 18, m = 6; n = 21, m = 7; n = 24, m = 8) in 1,2-dichloroethane and chloroform was studied. The distribution ratios of U(VI), Pu(IV), Am(III), and Eu(III) in the extraction depend both on the experimental conditions and on the structural features of the ligands. In the range [HNO3] = 0.02–1.0 M, the metal distribution ratios DM depend on the mutual steric orientation of the phosphoryl groups. With the cis (4,4′) isomers, DM is higher than with the trans (4,5′) isomers and with the isomer mixtures. With the compound containing the (n-BuO)2P(O) groups in the cis position as example, it was found that the highest distribution ratios DU = 0.98 were observed in 3 M HNO3; DPu = 5.1, in 0.5 M HNO3; and DAm = 0.007, at pH 2. For the trans conformer under the same conditions, DM of U(VI), Pu(IV), and Am(III) is 0.091, 1.8, and 0.003, respectively. The macroring size (number of carbon atoms n), under other conditions equal, also significantly affect the extraction activity of the reagent. With the extraction of Am(III) from 0.01–3 M HNO3 as example, it was shown that the crown ethers with n = 21 are the most efficient with all the substituents. The maximum of DAm shifts toward higher HNO3 concentrations with increasing n. The coordinating power of phosphoryl-containing ligands based on functionally substituted benzo-21-crown-7 ether in the extraction of Am(III) from 0.01–3 M HNO3 decreases in the order (n-BuO)(OH)P(O)-≫ (n-BuO)2P(O)-> Ph2P(O)-, under other conditions equal. The extraction ability of di(n-BuO)(OH)P(O)-dibenzo-21-crown-7 (DAm = 814 in extraction from 0.1 M HNO3) exceeds that of the other compounds studied by more than two orders of magnitude. Conditions were found for the selective extractive separation of Am(III) and Eu(III) using bis[(O-alkyl)(OH)phosphoryl]dibenzo-21-crown-7, with a separation factor of >90 at [HNO3] = 0.01 M.