Actinides transplutonium

Table 89 Some Hydrates of Transplutonium Actinide(III) Compounds...

Table 90 Complexes of Transplutonium Actinide(III) /3-Ketoenolates with P-Oxides...

Table 91 Transplutonium Actinide(III) Halides and Halogeno Complexes...

Today, it is accepted that lanthanides (4/elements) and transplutonium actinides (5/ elements) possess relatively similar physical and chemical properties (28-31, 63) including ... 

The oxidation-reduction behavior of plutonium is described by the redox potentials shown in Table I. (For the purposes of this paper, the unstable and environmentally unimportant heptavalent oxidation state will be ignored.) These values are of a high degree of accuracy, but are valid only for the media in which they are measured. In more strongly complexing media, the potentials will change. In weakly complexing media such as 1 M HClOq, all of the couples have potentials very nearly the same as a result, ionic plutonium in such solutions tends to disproportionate. Plutonium is unique in its ability to exist in all four oxidation states simultaneously in the same solution. Its behavior is in contrast to that of uranium, which is commonly present in aqueous media as the uranyl(VI) ion, and the transplutonium actinide elements, which normally occur in solution as trlvalent... 

Of the divalent transplutonium actinides, only Am compounds have been prepared in any significant quantity. 

In contrast to the early actinide compounds already discussed, the iodates of the transplutonium actinides that have been prepared contain trivalent oxidation states for the actinide elements. For Am, several higher oxidation states are possible, but attempts to prepare Am(V) or Am(VI) analogs of these iodates... 

The chemistry of actinide ions is generally determined by their oxidation states. The trivalent, tetravalent and hexavalent oxidation states are strongly complexed by numerous naturally occurring ligands (carbonates, humates, hydroxide) and man-made complexants (like EDTA), moderately complexed by sulfate and fluoride, and weakly complexed by chloride (7). Under environmental conditions, most uncomplexed metal ions are sorbed on surfaces (2), but the formation of soluble complexes can impede this process. With the exception of thorium, which exists exclusively in the tetravalent oxidation state under relevant conditions, the dominant solution phase species for the early actinides are the pentavalent and hexavalent oxidation states. The transplutonium actinides exist only in the trivalent state under environmentally relevant conditions. 

It is no surprise that the majority of the noble gases, krypton and xenon, have been lost, nor that there are still traces trapped in some of the core minerals. The relatively soluble alkali and alkaline earth elements have also been lost to a large extent, as have molybdenum, cadmium and iodine. The elements zirconium, technetium, lead, and to some extent ruthenium have at least been redistributed in the core. The rare earth elements, cerium, neodymium, samarium, and gadolinium as well as the actinides, thorium, uranium, neptunium, and plutonium show little evidence of migration, except possibly near the periphery of the core. By analogy to the rare earth elements it is probable that the transplutonium actinides, americium, curium, etc. would not migrate in this same environment. 

Bode, Separation Chemistry of the Lanthanides and Transplutonium Actinides in MTP Int. Rev. Scir Inorg. C/ierw., Ser. One+ vol 7, K. W. Bagnall Ed, (University Park Press Baltimore, 1972) pp 1-45 Moetler, 44The Lanthanides in Comprehensive Inorganic Chemistry, vol, 4 J, C. Bailar, Jr, et al, Eds. (Pergamon Press Oxford. 1973) pp l-101. 

Although, at first sight, the use of two phase liquid systems may not appear attractive, if the partition coefficient of the sample is known and sufficiently different from its contaminating solutes, considerable purification and concentration can be achieved into a non-polar scintillant-rich phase. This principle has been extensively exploited in the field of inorganic chemistry and is typified by the analysis of the transplutonium actinides by extracting the lipo-phyllic complex formed with 1-nonyl-decylamine sulphate into the scintillant phase (McDowell, 1972). 

The actinides uranium and thorium occur in nature as primordial matter. Actinium and protactinium occur in nature as daughters of thorium and uranium, while small amounts of neptunium and plutonium are present as a result of neutron-capture reactions of uranium. All other members of the series are man-made. Separation chemistry has been central to the isolation and purification of the actinides since their discovery. The formation of the transplutonium actinides was established only as a result of chemical-separation procedures developed specifically for that purpose. The continued application of separation science has resulted in the availability of weighable quantities of the actinides to fermium. Separation procedures are central to one-atom-at-a-time chemistry used to identify synthetic trans-actinide (superheavy) elements to element 107 and above (Report of a Workshop on Transactinium Science 1990). 

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. 

On the topic of lanthanide/actinide separation, few reviews have dealt in detail with the most difficult aspect of this field, separation of the lanthanides from the trivalent transplutonium actinides. Jenkins (1979,1984) reviewed ion exchange applications in the atomic-energy industry. Relatively short sections of these reviews dealt with the separation of the trivalent metal ions. Symposium volumes entitled Actinide Separations (Navratil and Schulz 1980) and Lanthanide/Actinide Separations (Choppin et al. 1985) are collections of papers from several authors covering all aspects of lanthanide/actinide separation, some of which deal with the purification of the trivalent metal ions. 

Thorium, neptunium and plutonium are usually (although not exclusively) extracted or adsorbed onto resins in the tetravalent oxidation state while uranium (VI) is the most common oxidation state in separations. In most separations of the actinides, there exists a great difference in the extractability of the (IV) and (VI) oxidation states relative to the trivalent oxidation state. It is reasonable to expect, therefore, that the removal of the lanthanides and transplutonium actinides from the light actinides is readily accomplished. In fact, the low extractability of the trivalent state, and the ease of reduction of plutonium forms the basis for the isolation of Pu from the other... 

The strong similarity in the solution chemistry of the 4f and the 5f elements is most evident for the trivalent oxidation state of both families. The discovery experiments of the transplutonium actinides depended directly on this similarity as it allowed very accurate predictions of the chemical properties of the to-be-discovered elements. However, the actinide series is not an exact analog of the lanthanide elements. For example, while the stability of the trivalent oxidation state is a primary characteristic of all the lanthanide elements, trivalency is not the most stable state for the actinide elements of Z = 90-94 and 102. The greater stability of Nof q, relative to No, j,+ is not observed in the 4f analog although Yb " can be present in reducing systems. Such differences are related to the dififerences in the relative energies of the (n)f, (n -I- l)d and (n -I- 2)s orbitals when n = 4 (Ln) and 5 (An). 

Thermodynamic measurements that have been made on the actinide metals are low- and high-temperature heat capacities, properties of phase transitions, and vapor pressures. At least one of these measurements has been made on each element through einsteinium unfortunately, however, none has been made on actinium, so that even its enthalpy of vaporization must be estimated. As of the time of writing (February 1986) vapor-pressure measurements have been made through einsteinium [22] and low-temperature heat-capacity measurements through americium [23] by innovative microscale methods, innovative microscale methods have been applied to determine vapor pressures and very low-temperature heat capacities of transplutonium actinides. 

---