Interlanthanide separations

N -diacetic acid (DACDA) in conjunction with TT A (chloroform). Ligands of this type offer interesting possibilities as they combine the characteristics of crown ethers with those of aminopolycarboxylic acid complexants. At pH 5, only R(TTA)3 is extracted, while a 1 1 1 complex R(DAPDA,DACDA)TTA is extracted at pH7.5-8.0. The extraction kinetics at pH 5 are slow, governed by the rate of dissociation of aqueous metal-crown complexes, but are relatively fast and independent of the aqueous macrocycle complex dissociation rate in the high-pH regime. These kinetic results suggest the possibility of interlanthanide separations based on differences in com-plexation rates. 

A chromatographic interlanthanide separation by cation exchange in the presence of 1,4,7,10-tetraazacyclododecane-N,N, N",N" -tetraacetic acid (DOTA), present as an aqueous complexant, has been reported by Merciny et al. (1986). In this system the lanthanides are separated into two groups based on the rate of decomposition of the R-DOTA complex. Separation factors are not large, but a chromatographic separation of Gd and Eu is suggested. 

Fig. 4. Interlanthanide separation factors (relative to Ce) for selected methods HDEHP and TTA data from Stary (1966), AHIB (a-hydroxyisobutyric acid/Dowex 50) data from Choppin and Silva (1956X and Smith and Hof nan (1956), DHDECMP (dihexyl -N, N-diethyl-carbamoylmethylphosphonate) data from Horwitz et al. (1981).

In these extractants, HNO3 interaction with the extractant occurs with the carbamoyl portion of the molecule (Horwitz etal. 1981), leaving the solvating phosphorus portion of the molecule to interact with the metal ion. These compounds are indeed more efficient extractants of the trivalent metal ions from acidic solutions, able to extract trivalent actinide and lanthanide ions from relatively dilute nitric-acid solutions. Horwitz et al. (1981) have studied the separation of the lanthanides and trivalent actinides from Am to Fm (table 2) using dihexyl-N, N-diethylcarbamoyl-methylphosphonate (DHDECMP) and aqueous nitrate solutions. Steadily decreasing distribution ratios are observed for the lanthanides, but nearly constant D s are found for the trivalent actinides. Group separation does not appear feasible while interlanthanide (but probably not interactinide) separations are possible. However, substitu-... 

A sampling of the methods which are (or could be) used for interlanthanide or interactinide separations are shown in figs. 4 and 5, respectively. These should not be... 

---