Selective extraction of lanthanides and actinides with ionizable crown ethers

Sym-dibenzo-16-crown-5 - oxyacetic acid and its analogues are highly selective for the extraction of the f-block elements. The extraction does not require specific counter-anions and is reversible with respect to pH. Spectroscopic evidence indicates that both the cavity and the carboxylate group of the macrocycles are involved in the complexation. Under substoichiometric extraction conditions, the distribution coefficient (D) ratio of Lu3+/La3+ can be as large as 30. From plots of log D versus log L (L = ionizable crown ether), the ratio of Lu3+ to ligand has been determined to be 1:2 indicating a possible sandwich formation. Experimental results show that the efficiencies of extracting trivalent lanthanide ions are much greater (> 102) than those of the alkali metal and alkaline earth metal ions of similar ionic radii. These particular ionizable crown ethers also show high extraction efficiencies for the actinides such as thorium, uranium, americium, and californium. Applications of the ionizable crown ethers as extracting agents for preconcentration of lanthanides and actinides for analytical purposes and for separation of radioactive rare earth isotopes from their alkaline earth precursors will be discussed. Transport of lanthanides across liquid membranes containing the ionizable crown ethers will also be discussed