Trivalent lanthanide/actinide group separation

The CMPO derivatives have been designed as efficient extractants for the trivalent lanthanide/ actinide group separation in the TRUEX process studied in the United States and Russia, see, for example, the review by Paiva and Malik (2004). Danonstration tests on the use of synergic mixtures of CMPO (Af,A-di-isopropyl carbamoyl methyl alkyl phenyl phosphine oxide) and chloroderivative CCD" for lanthanide/actinide extraction have been carried out in the development of the UNEX process, see Section 19.2.2 of this review. 

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). 

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-... 

The thermodynamic data for complexation of trivalent lanthanide and actinide cations with halate and haloacetate anions are reported. These data are analyzed for estimates of the relative amounts of inner (contact) and outer (solvent separated) sphere complexation. The halate data reflected increasing inner sphere character as the halic acid pKa increased. Use of a Born-type equation with the haloacetic acid pKa values allowed estimation of the effective charge of the carboxylate group. These values were, in turn, used to calculate the inner sphere stability constants with the M(III) ions. This analysis indicates increasing the inner sphere complexation with increasing pKa but relatively constant outer sphere complexation. 

The redox reaction has been utilized in the separation of light actinide elements (U, Np, and Pu) with both ion-exchange process and solvent extraction process. For trivalent heavy actinides with Z> 94 (except No), separation of these actinide ions from lanthanide ions is required for safe storage of long-lived nuclear waste and transmutation of these nuclides. Fundamental researches have widely been carried out by several groups for the purpose of quantitative separation of transuranium elements. Recent topics on the development and application of solvent extraction for the separation of transuranium elements are briefly summarized below. 

Stary (1966) cites the work of Street and Seaborg (1950) who found that group separation of the lanthanides and trivalent actinides can be done by cation exchange eluting with 20% ethanol saturated with HCl. The presence of the alcohol enhances the difference in chloride complex stability between the lanthanides and actinides as a result of partial dehydration of the cation in the 20% ethanol solution. 

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). 

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. 

Musikas (1985) has drawn attention to the use of complexation by softer ligands to obtain improved group separations of trivalent lanthanides from trivalent actinides. Such separations have been attributed to an enhanced covalency in the actinide-ligand complex. The selectivity between analogous actinide-lanthanide pairs by a ligand was compared using the selection factor, Sp defined by... 

Publications on the analysis of soil samples by Smith et al. [78] and Crain et al. [79] summarized two possible routes for the analysis of aqueous samples in chromatography extraction columns, with detection by conventional radiometric techniques such as ICP-MS. In this procedure, TRU-Spec SPS columns were used for group separation of actinides and TEVA-Spec columns were used to isolate the trivalent actinides from the lanthanide elements. A reduced solution (with ascorbic acid) was passed through a 1 mL TRU-Spec column equilibrated with 2 M nitric acid and 0.5 M aluminum nitrate. The trivalent actinides including americium and the lanthanide elements were eluted from the column with 12 mL of 4 M HCl. Plutonium and thorium were removed with 30 mL of... 

In the Talspeak process, the separation of trivalent actinides and lanthanides is accomplished by coextracting the two groups of elements into di(2-ethyl hexyl)phosphoric acid (HDEHP) from a carboxylic acid solution and then partitioning the acti-... 

The problems of practical separation of these elements are (1) the separation of actinide elements fi om the lanthanide group (trivalent actinide ions behave like lanthanide ions), and (2) the separation of the actinide elements from each other. Ion-exchange and solvent extraction methods have so far been extensively studied. 

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