Trivalency actinides and lanthanides

In what follows we briefly review some of the previous attempts to analyze the available spectra of plutonium (6). In addition, we estimate energy level parameters that identify at least the gross features characteristic of the spectra of plutonium in various valence states in the lower energy range where in most cases, several isolated absorption bands can be discerned. The method used was based on our interpretation of trivalent actinide and lanthanide spectra, and the generalized model referred to earlier in the discussion of free-ion spectra. 

These considerations lead, for example, to the assignment of a predominantly outer sphere character to Cl, Br, F, CIO3, NO3, sulfonate, and trichloro-acetate complexes and an inner sphere character to F", IO3, SO, and acetate complexes of trivalent actinides and lanthanides. The variation in AH° and AS° of complexation of related ligands indicates that those whose pA), values are <2 form predominantly outer sphere complexes, while those for whom > 2 form predominantly inner sphere complexes with the trivalent lanthanides and actinides. As the pK increases above 2, increasing predominance of inner sphere complexation is expected for these metals. 

This scheme of steps reflects the ability of some metals, like the trivalent actinides and lanthanides, to vary their coordination number since the trivalent Ln and An may go from 9 to 8 and, finally, back to 9. The last step reflects the operation of the third mechanism proposed for synergism. 

The present work (1) was motivated by the desire to produce isotopes such as 247Cm in gram or multigram amounts, but the same methods are applicable to all the chemically similar trivalent actinide and lanthanide elements. Curium-247 has the highest atomic number, four beyond ura-... 

In the work reported here, which was directed toward the attainment of an isotopic enrichment of the trivalent actinide and lanthanide elements, the problem was compounded by the fact that these elements do not readily form appropriate compounds, like iodine in ethyl iodide. They do form some stable organic chelates, and, indeed, it is possible to obtain a Szilard-Chalmers reaction with such compounds. However, their radiation damage resistance does not appear adequate to permit useful production of an isotope like 247Cm, which requires a thermal neutron exposure ap-... 

Modolo, G., Asp, H., Schreinemachers, C., Vijgen, H. 2006. Development of aTODGA-process for co-separation of trivalent actinides and lanthanides from a high active raffinate. 9th OECD/NEAIEM on An and FP P T, Nimes, France, September 25-29. 

Miguirditchian, M., Guillaneux, D., Guiflaumont, D. et ah 2005. Thermodynamic study of the complexation of trivalent actinide and lanthanide cations by ADPTZ, a tridentate N-donor ligand. Inorg. Chem. 44 (5) 1404—1412. 

However, due to the chemical similarities of the trivalent actinide and lanthanide elements, historically, it has been easier to develop step-by-step processes first, An(III) + Ln(III) coextraction processes, which also address the problem of waste alpha decontamination, and second, An(III)/Ln(III) separation processes, which can only be implemented on the solutions produced by the first-step processes. Today, however, a few processes are available that allow recovery of the trivalent actinides in a single step from highly active liquid waste. 

The SETFICS process (Solvent Extraction for Trivalent /-elements Intragroup Separation in CMPO-Complexant System) was initially proposed by research teams of the former Japan Nuclear Cycle Development Institute (JNC, today JAEA) to separate An(III) from PUREX raffinates. It uses a TRUEX solvent (composed of CMPO and TBP, respectively dissolved at 0.2 and 1.2 M in -dodecane) to coextract trivalent actinides and lanthanides, and a sodium nitrate concentrated solution (4 M NaN03) containing DTPA (0.05 M) to selectively strip the TPEs at pH 2 and keep the Ln(III) extracted by the TRUEX solvent (239). However, the DFs for heavy Ln(III) are rather poor. An optimized version of the SETFICS process has recently been proposed as an alternative process to extraction chromatography for the recovery of Am(III) and Cm(III) in the New Extraction System for TRU Recovery (NEXT) process. NEXT basically consists of a front-end crystallization of uranium, a simplified PUREX process using TBP for the recovery of U, Np, and Pu, and a back-end Am(III) + Cm(III) recovery step (240, 241). 

Manohar, S., Sharma, J.N., Shah, B.V., Wattal, P.K. 2007. Process development for bulk separation of trivalent actinides and lanthanides from radioactive high-level liquid waste. Nuclear Science and Engineering 156 96-102. 

Cordier, P.Y., Francois, N., Boubals, N., Madic, C., Hudson, M.J., Liljenzin, J.O. 1999. Synergistic systems for the selective extraction of trivalent actinides from mixtures of trivalent actinides and lanthanides. ISEC 99 Conference on Solvent Extraction for the 21st Century, July, Barcelona, Spain. 

The wide-rim CMPO calix[4]arenes are the most efficient compounds for the extraction of trivalent actinides and lanthanides. They are also the compounds that display the highest selectivity along the lanthanide series, provided that phosphorus atoms are linked to phenyl groups. They also display a higher selectivity than the calixarene-bearing diphosphine oxide, where the phosphorus atom is linked to phenyl units CPo21.176- ... 

Koma, Y., Watanabe, M., Nemoto, S., Tanaka, Y. A counter current experiment for the separation of trivalent actinides and lanthanides by the SETFICS process. Solvent Extr. IonExeh. (1998), 16 (6), 1357-1367. 

Mathur, J. N. and Khopker, P. K. (1988) Use of crown ethers as synergists in the solvent extraction of trivalent actinides and lanthanides by l-phenyl-3-methyl-4-trifluoroacetyl-5-pyrazolone, Solvent Extr. IonExch, 6(1), 111-124. 

The Visible and Near Infrared Absorption Spectra of Some Trivalent Actinide and Lanthanide Elements in DCIO4 and in Molten Salts, W.T. Camall and P.R. Fields, Developments in Applied Spectroscopy 1, 233-247 (1962). 

We have calculated sets of theoretical energy levels for the trivalent actinides and lanthanides and correlated these levels with transitions observed in the solution absorption spectra of these elements. Using the eigenvectors resulting from this energy level calculation, we have computed the theoretical matrix elements required to account for the observed band intensities in the two series of elements. The extent to which the theoretical calculations can be correlated with experimental results has been discussed, and some applications for the intensity relationships are pointed out. 

Table I. Parameters Used to Calculate Energy Levels Observed in the Solution Absorption Spectra of the Trivalent Actinides and Lanthanides...

The trivalent actinides and lanthanides form only weak chloride complexes in aqueous solution. Although the trivalent actinides are absorbed moderately strongly by anion exchange resins from concentrated LiCl, the MX2 complex appears to be the highest complex present to a measurable extent in the aqueous chloride solutions (2). The bromide complexes appear to be even weaker (3). The only other evidence for anionic trivalent actinide chloro or bromo complexes has been in the system of UCI3 in fused CsCl (4). Ryan and J0rgensen have recently prepared the trivalent lanthanide hexachloro and hexabromo complexes and studied their absorption spectra (8). 

Bhattacharyya, A., Mohapatra, P.K., and Manchanda, V.K., Separation of trivalent actinides and lanthanides using a flat sheet supported hquid membrane containing Cyanex-301 as the carrier, Sep. Purif. Tech., 2006, 50 278-281. 

In alkaline solutions trivalent actinides and lanthanides form rather stable complexes with tartaric acid and DOBTA. The complexes undergo no appreciable decay during long keeping of the solutions for instance, Eu and Am complexes with DOBTA in 0.5-1 M NaOH are stable for more than a month. 

Azide complexes. The aqueous trivalent actinide and lanthanide azides complexes were examined by two techniques ... 

Figure 1. Absorption spectra of trivalent actinide and lanthanide ions in the...

The acid dependency observed in practice hal been only approximately inverse third power. Impurities in Cleanex feed solutions often cause a departure from ideality (e.g., by common-ion effect or by consumption of some of the HDEHP), and we have not been able to control the extraction of the actinide elements solely by monitoring the aqueous-phase acidity. Fortunately, when processing transplutonium elements, the high specific activity of 21+I+Cm facilitates the detection of that isotope in both phases, thus permitting a rapid determination of the degree of extraction. The extraction coefficients of the trivalent actinides and lanthanides are all quite similar, so the 21+1+Cm serves as an excellent marker for all the extracted ions. 

First, the trivalent actinide and lanthanide elements are separated from the other elements in the waste. In the second step, americium and curium are then separated from the lanthanide elements. Experimental studies have largely been laboratory-scale in which synthetic waste solutions and tracer levels of radioactivity were utilized. A few laboratory-scale experiments were made in hot cells on the coextraction of trivalent actinides and lanthanides. The two most promising methods investigated for co-removal of trivalent actinides and lanthanides are ... 

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-... 

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