Lanthanide/actinide separation factors

The potential of diluent modification in enhancing lanthanide/actinide separation factors has barely been tapped. The absence of a reliable predictive theory of solvation of both the extractant molecules and the extracted complexes hampers the development of this area. In view of the small energies required to reverse extraction order (a few hundred joules), subtle alteration of the organic diluent (or the aqueous medium in ion exchange procedures) also has the potential for significantly improving (at least) group separations. 

Quaternary ammonium compounds are analogous to tetramethylammonium-based anion-exchange resins. They offer additional possibilities over anion-exchange resins for trivalent lanthanide/actinide separation (over resins) in the ability to use solvation effects to enhance separation factors. The first application of quaternary amines to lanthanide/actinide group separation was made by Moore (1964), who examined the system Aliquat 336/xylene/H2S04-NH4SCN. A principal advantage of this method is the relatively low concentration of salts required to attain a usable separation (as compared with tertiary amine/nitrate systems). Several other applications of the method have been summarized by Weaver (1974). 

Cation exchange resins in general resemble the sulfonic acid extractants most closely. Separation factors for cation exchange separations of the trivalent cations are extremely small. Successful lanthanide/actinide separation using cation exchange resins depend primarily on the use of aqueous complexants and/or alteration of the aqueous medium for the separation. 

There is also evidence that non-cyclic polyethers can function as synergists in lanthanide/actinide separation. Ensor and Shah (1983,1984) report that 1,13-bis(8-quinolyl)-l,4,7,10,13-pentaoxatridecane (Kryptofix-5, or K-5) enhances the extraction of Ce(III), Eu(III), Tm(III), Am(III), Cm(III), Bk(III) into chloroform solutions of TTA. The extracted complex has the stoichiometry R(TTA)3-K-5, and the synergistic enhancement is comparable to that of TBP under the conditions studied. Intra-group separation factors for lanthanides and actinides are approximately two while the usual synergistic leveling effect is observed for Am/Eu separation (i.e., they are worse than those for TT A alone). 

Alkyl pyrocatechol extractants provide greater group separation factors. Eu/Am separation factors of 70 have been reported for a non-equilibrium extraction in the system 4-(a,(z-dioctylethyl)-pyrocatechol/NaOH/DTPA (diethylenetriamine-N, N, N, N, N-pentaacetic acid) (or DTPP - diethylenetriamine-N, N, N, N", N"-pentameth-ylenephosphonic acid) (Karalova et al. 1982). The separation factors are based mainly on the difference in the rates of the metal-DTPA (or DTPP) complexation equilibria for Eu and Am. They are therefore highly dependent on the contact time. A principal limitation of the practical application of such a separation scheme is the relatively long contact times (> 10 min) required for extraction. However, the slow equilibration compares favorably with that for other lanthanide-actinide separation processes (e.g. TALSPEAK, section 7). 

The character of the diluent in a solvent extraction separation scheme generally can have a profound effect on the overall degree of extraction, but the effect of diluent on separation factors is usually much more subtle. Alteration of the diluent modifies metal-ion extraction equilibria primarily by its effect on the solvation of the hydro-phobic metal complex. Although few recent studies have been made of diluent effects in lanthanide/actinide separation (with no truly systematic investigations), enough historical reports exist to make some general observations of the effect of diluent on lanthanide/actinide separation. 

Solutions of a-hydroxycarboxylic and aminopolycarboxylic adds are commonly used to elute americium from cation-exchange resins. When these reagents are used in a displacement elution system, they provide excellent separation of americium from trivalent lanthanides and other trivalent actinides. Separation factors, acS, for americium from curium range from 1.2 to 1.4 for a-hydroxycarboxylic adds and from 1.2 to 2 for the separation of americium from curium with aminocarboxylic adds [16]. 

In this way, it is possible, if necessary, to let the mixture migrate over a long distance relative to the solvent and so to obtain a very high separation factor. In most cases, non-isotopic ions are separated in very high purity. In some rare cases, in which the relative difference of the mobility is less than 1% (e.g. Lanthanides and some actinides) pure components are only obtained if the mobility difference is increased by additional effects, such as different complexation degree of the components by means of a suitable ligand. 

During the last five or six years Siekierski and his coworkers (50—54) have investigated a large number of data involving the partition coefficients (separation factors), the influence of enthalpy and entropy on the separation factors, temperature effect on the extraction and have plotted these quantities and others, like the unit cell volumes, radii etc. for the lanthanides and a few of the actinides to enumerate their double-double (tetrad) hypothesis. In many of these plots they have used the... 

The possibility of separation of actinides and lanthanides in carbonate media was studied by introducing two complexants in the system, one as an extractant and the other as a masking reagent. Am /Eu , Am /Tb, and Am /Ce pairs were studied, and the best results were obtained from an XO/HEDPA combination with separation factors of 4.5—5-6 for the first two pairs and 7.8—12.4 for the third pair [191-... 

The dyes were also studied for the separation of actinides in higher oxidation states. In contrast to the result for trivalent actinides and lanthanides, the extraction of actinides in the oxidation states +4, +5, and +6 (Pu, UOi, NpOi, and Pa ) is negligible except for AtnO in the presence of 0.02 M dyes [22. Separation factors of 10 —10 can be achieved for trivalent actinides, UOl", NpOi, and Pu. AC and XO have been shown to be the most effective extractants from carbonate media for the higher oxidation state elements. 

Separation factors (relative to Am) for solvent extraction of trivalent lanthanides and actinides with 0.817 M DHDECMP/ DIPB/l.OM HNOj from Horwitz et al. (1981). 

Tertiary amines are poor extractants for lanthanides and actinides from dilute nitrate media, but extract these metal ions strongly from concentrated nitrate solutions of low acidity (as was true of TBP). Similar observations have been made for extraction from chloride media. Figure 1 indicates that for 30% Alamine 336/xylene/ll M LiCl group separations are good, some interactinide separations are possible, but lanthanide separation factors are small. Weaver briefly discusses the application of the TRAMEX (tertiary amine extraction) process for the purification of... 

Separation factors for TTA extraction of trivalent lanthanides and actinides from Stary (1966) (TTA/toluene) and Alstad etal. (1974) (TTA/Caj. 

Separation factors for trivalent lanthanide/actinide extraction by HDEHP/ toluene (Stary 1966) and HDEHP/kerosine (Sato 1989). 

Because the lanthanide and actinide metal ions are readily hydrolyzed (and precipitated) in alkaline solutions, these studies require the presence of water-soluble com-plexants. Both solvating and chelating extractants have been used in these studies. Primary and quaternary amines, alkylpyrocatechols, /J-diketones, pyrazolones, and N-alkyl derivatives of aminoalcohols are the extractants indicated as useful for alkaline extraction processes. A variety of diluents have been used, but their nature seems to have little effect on the extraction efficiency or separation factors. 

Group separation factors for the trivalent actinides and lanthanides are a 2 for extraction by Aliquat 336-EDTA in contact with an alkaline EDTA aqueous solution. Such a separation factor may be sufficient if the system is operated in the extraction chromatographic mode (Bukhina 1983). 

Separation factors (relative to Cm) for ammonium a-hydroxyisobutyrate cation exchange of trivalent lanthanides and actinides from Smith and Hoffman (1956), Choppin et al. (1956), and Choppin and Silva... 

In a recent application of HDEHP to actinide/actinide and actinide/lanthanide separations, Novikov and Myasoedov (1987) report the use of a supported liquid membrane impregnated with HDEHP for the separation of Am/Cm, Eu/Tb, and Am/Eu using DTPA, citric acid, and the potassium salt of a heteropolyacid (KioP2Wi706i - KPWO) as aqueous complexants. The optimum separation factors reported are = 5.0, Sj = 10.8, and > 10. ... 

The emphasis of work by French researchers has been the use of soft-donor extractants and complexants to enhance actinide/lanthanide group separation (Vitorge 1985, Musikas et al. 1980, Musikas 1985). The relative stability constants for lanthanide and actinide azide complexes reported by Musikas etal. (1980) suggest that hydrazoic acid could function as a useful reagent for this separation. This is confirmed in a later report for Am/Eu separation (Musikas 1985) in which americium extraction is suppressed by complex formation with azide. The separation factors are not very different from those reported by Sekine (1965) using SCN as the complexant in TBP extraction. However, Choppin and Barber (1989) find that, while the trivalent actinide-azide stability constants are somewhat larger than those of the trivalent lanthanides, the complexation enthalpies calculated from the temperature coefficient of the / s do not support the existence of a covalent bonding contribution. 

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