Light trivalent actinides

Figure 5.14. Compound formation capability in the binary alloys of Sc, Y, light trivalent lanthanides (as exemplified by La), heavy trivalent lanthanides (exemplified by Gd) and of the actinides (exemplified by Th, U and Pu). The different partners of the 3rd group metals are identified by their position in the Periodic Table. Notice that a sharper subdivision between compound-forming and not forming metals will result from a shifting of Be and Mg from their position in the 2nd group towards the 12th group (see 5.12.3). The behaviour of the divalent lanthanides Eu and Yb is shown in Fig. 5.7 where it is compared with that of the alkaline earth metals.

Trivalent actinides and perhaps lanthanides are potential adsorbates that can be studied using this method. Application of TRLFS to adsorbents other than silica is limited in view of the requirement that the dispersion should be transparent to visible light. 

An examination of the trivalent actinide energy level schemes reveals several possibilities for laser action. These are discussed in light of the general properties cited above. Only conventional broadband optical pump sources are considered. Obviously with selective laser excitation and cascade lasing schemes, stimulated emission from many more states should be possible, but these special situations are too numerous to be considered in detail here. 

In a two-cycle Purex procedure, the recoveries of both Pu and U are on the order of 99.9% the Pu/U separation factor is 10 . The literature reports that plutonium decontamination factors from the fission products are 10 , and that the only long-lived impurities detected in the final product are Zr, Tc, and ° Ru. However, the authors experience has been that the light rare earths (mainly La and Ce), thorium, neptunium, and the trivalent actinides (Am and Cm), which exhibit some degree of complexation in nitrate media (Guseva and Tikhomirova 1979), are present in a gram-sized plutonium sample at concentrations that are detectable by radiochemical means. 

The separation of the lanthanides from thorium, uranium, plutonium, and neptunium can fairly readily be achieved by exploiting the greater extractability of the higher oxidation states of the light-actinide elements. However, the transplutonium actinides do not have stable higher oxidation states. In this case, separation of the lanthanide fission products from the transplutonium actinides must exploit the small differences in the solution chemistry of the lanthanides and actinides in the trivalent oxidation state. It is the separation of the lanthanides from the trivalent actinide cations that is the focus of this chapter. 

The concept of molecular recognition has its roots in coordination chemistry in the binding of metal ions by crown ethers. In this class of ligands, the size of the cavity is often the principle determinant of metal-ion selectivity. For the light lanthanides and trivalent actinides, the optimum cavity size is attained in the 1,4,7,10,13,16-hexa-oxocyclooctadecane (18-crown-6) configuration. 

While actinide metals are easily soluble in some mineral acids such as HCl an.d HCIO4, they are typically found in solution in valences IV to VI. However, Baluka and co-workers first reported in 1981 that dissolution of U and Np metals in superacids produces initially trivalent actinides (Baluka et al. 1981). The UV-visible spectra of the U and Np solutions, shown in fig. 2, are characteristic of the trivalent actinides found in aqueous acidic or basic systems. Avens and colleagues (1988) have dissolved both Pu and Am metal and observed trivalent species by UV-visible spectroscopy. The stability of oxidation state (111) for U, Np and Pu is all the more striking especially in light of the lower stability of the series U (lll)-Pu (111) relative to higher oxidation states e.g., trivalent uranium (111) is able to reduce water under normal conditions. In addition to the unusually low oxidation state obtained, another feature of actinide dissolution in... 

The intensity of an absorption band can be defined in terms of the area under the band envelope as normalized for concentration of the absorbing ion and the path length of light in the absorbing medium. A proportional quantity, oscillator strength P, has been tabulated for trivalent actinide-ion absorption bands in aqueous solution. The experimentally determined oscillator strengths of transitions can in turn be related to the mechanisms by which light is absorbed [47] ... 

The trivalent lanthanide metals include lanthanum(III) through lutetium(III). The lanthanide oxides have a variety of uses from semiconductors, glasses, solid-state lasers and catalysts. There is quite a difference in the extent to which the hydrolysis reactions of the lanthanide metals have been studied. Typically, the light lanthanides have been studied to a greater extent than the heavier lanthanides. Neodymium(III) has received the most attention due to the perception that it can be used as an analogue for the trivalent actinide metals, in particular, americium (III). All of the isotopes of promethium are radioactive. 

On the contrary, there is a spread of oxidation numbers for the light actinides (at least up to Cm), which, for Pu and Np, range from 3 to 7 After Cm, however, the trivalent oxidation state is always met, and this second half of the actinide series approaches more the behaviour of the lanthanides. 

In this way, Ecoh (trivalent) for all actinide metals are derived and for light actinides found in large discrepancy with the experimental ones. For them, a valence higher than three is suggested by the discrepancies. 

The (dashed) right part corresponds to the case in which both the metal and the aqueous ions are trivalent this case is the unrealistic case for the light actinides. 

Light actinide metals (U, Np, Pu) react with elemental iodine or bromine in donor solvents forming trivalent AnX3L4 (X = Br, I) complexes (Table 5.12), for example (5.55) [359,360] ... 

The majority of the chemistry that has been investigated for the actinide elements has been in aqueous solutions. For the light actinides in acidic solutions, four types of cations persist trivalent, tetravalent, pentavalent, and hexavalent. The later two ions are always found to have trans oxo ligands, making up a linear dioxo unit. Actinide ions of this type are typically referred to yls and have the names, uranyl (1, U02+/ +), neptunyl (2, Np02+ +), plutonyl (3, Pu02+ +), (4, Am02+/ ) and so on. 

The values of F2 for the trivalent lanthanides and actinides are plotted vs. Z (atomic number) in Figure 7, and those of f are shown graphically in Figure 8. Values of F2 and 5/ for actinides above curium were extrapolated from the light half of the series assuming a linear relationship for the parameters (9). These parameters, in turn, were used to calculate the expected energy levels for Es ", and Fm-" ". ... 

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. 

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