Cations, trivalent actinide

Jensen, M.P., Bond, A.H. 2002. Influence of aggregation on the extraction of trivalent lanthanide and actinide cations by purified Cyanex 272, Cyanex 301, and Cyanex 302. Radiochim. Acta 90 (4) 205-209. 

Neutral extracting agents possessing oxygen-donor atoms (hard bases) in their structure easily coordinate trivalent lanthanide and actinide cations, but do not discriminate between the two families of elements, because the ion-dipole (or ion-induced dipole type) interactions mostly rely on the charge densities of the electron donor and acceptor atoms. As a result, the similar cation radii of some An(III) and Ln(III) and the constriction of the cation radius along the two series of /elements make An(III)/Ln(III) separation essentially impossible from nitric acid media. They can be separated, however, if soft-donor anions, such as thiocyanates, SCN-, are introduced in the feed (34, 35, 39, 77). 

There are various types of organic proton exchangers (34, 35, 38). Diesters of phosphoric acid, (RO)2P = 0(0H), phosphonic acids, R(RO)P = 0(0H), and phos-phinic acids, R2P = 0(0H), where R represents linear or branched alkyl or phenyl substituents, are the most common cation exchangers developed in liquid-liquid extraction for the extraction of trivalent 4/and 5/elements. They were initially developed for the American TALSPEAK and the Japanese DIDPA processes and have recently been introduced in the French DIAMEX-SANEX process. As for previously described NOPCs, these organophosphorus acids present oxygen-donor atoms (hard bases) in their structures and therefore will easily coordinate trivalent lanthanide and actinide cations, but they will not allow complete discrimination of the two families of elements. However, contrary to previously described neutral organophosphorus... 

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. 

Trivalent lanthanide and actinide cations form labile, ionic complexes of both inner and outer sphere character. Consequently, they are useful probes to study inner-outer sphere complexation competition due to ligand properties. Two earlier papers have reported complexation of these cations by two series of related anions, the halates (9) and the chloroacetates (10). In this paper we offer a more extensive analysis of the inner-outer sphere competition in these complexes. 

Four of the possible six possible skeletal isomers having the Wells-Dawson structure, as exemplified by [EgWigOgz] (E = P or As), have been structurally identified. The so-called Preyssler anion was structurally identified as [ Na(H2 0) P5W3oOiio] , itbeing shown that the Na(H2 0)+ group was tightly bound in the center of the cluster. However, under forcing conditions, the aqua-sodium cation could be replaced by Ca +, trivalent lanthanide cations and some tetravalent actinide cations, all of which have similar ionic radii... 

While the idea that ligand 7 could prove useful for the coordination of other, non-uranyl actinide cations, has yet to be tested by experiment, it is important to note that this ligand has so far proved less than satisfactory for the coordination of other large, non-actinide cations. Indeed, in spite of extensive efforts devoted to the problem, no stable, non-labile complexes of the trivalent lanthanides (ionic radii 0.86 - 1.36 A ) have as yet been characterized with this system. Nor have 1 1 complexes with other large cations, e.g., Cd + (ionic radius 1.0 A ) or Pb + (ionic radius 1.2 A ), been documented.This has proven to be the case even though mass spectrometric evidence consistent with metal coordination has been obtained in certain instances. 

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 multiplicity of oxidation states of the light actinides can be utilized to accomplish very efficient separation of these elements from the lanthanides. Except for actinium (only trivalent), the actinide ions to plutonium either exist predominantly in higher oxidation states [Th(IV), Pa(IV, V)] or can be interconverted with relative ease among any of four oxidation states (III, IV, V, VI). The upper two oxidation states exist in aqueous solutions as the dioxocations AnOj or AnO - The relative strength of complexes formed by the actinide cations in these oxidation states is An(IV) > An(VI) > An(III) > An(V), which order also applies to the separation reactions involving these cations. The dominant oxidation states for the light actinides are Ac(III), Th(IV),Pa(IV or V),U(IV or VI), Np(IV or V). For plutonium, the redox potentials indicate nearly equal stability for all four oxidation states in acidic solutions. The tri-, tetra-, and hexavalent oxidation states are most important in separations. 

Trivalent lanthanide and actinide cation radii [from Shannon (1976) and Marcus (1983)]. 

Fig. 21. Logarithm of upstroke-transition pressure versus cation/anion radius ratio for the B1 to B2 transition in lanthanide and actinide compounds. Trivalent ionic radii are used for all cations. Some transitions to other high-pressure structures are included for comparison and have been marked as such in the graphs, (a) Mono-pnictides with As, Sb, and Bi. The isolated data for the phosphides CeP and ThP are not included. No mononitrides have been observed to transform to the B2 type under pressure, (b) Mono-chalcogenides with S, Se, and Te. The EuO transition is outside the graph.

Kronenberg et al. [39] observed that the values of trivalent lanthanide (Tb ) and actinide cations Am and in mixed HF/0.1 M HNO3 solutions on the... 

The study of these polyanionic platforms in liquid-liquid extraction of various trivalent lanthanide and actinide cations from high-level radioactive acidic waste solutions was tested." It is interesting to emphasize that these novel ionic ligands have shown a dramatic enhanced extraction ability for trivalent actinides and lanthanides, particularly for Am + a thorough extraction (>99%) has been achieved in a single extraction step." " " ... 

The overall distribution of lanthanides in bone may be influenced by the reactions between trivalent cations and bone surfaces. Bone surfaces accumulate many poorly utilized or excreted cations present in the circulation. The mechanisms of accumulation in bone may include reactions with bone mineral such as adsorption, ion exchange, and ionic bond formation (Neuman and Neuman, 1958) as well as the formation of complexes with proteins or other organic bone constituents (Taylor, 1972). The uptake of lanthanides and actinides by bone mineral appears to be independent of the ionic radius. Taylor et al. (1971) have shown that the in vitro uptakes on powdered bone ash of 241Am(III) (ionic radius 0.98 A) and of 239Pu(IV) (ionic radius 0.90 A) were 0.97 0.016 and 0.98 0.007, respectively. In vitro experiments by Foreman (1962) suggested that Pu(IV) accumulated on powdered bone or bone ash by adsorption, a relatively nonspecific reaction. On the other hand, reactions with organic bone constituents appear to depend on ionic radius. The complexes of the smaller Pu(IV) ion and any of the organic bone constituents tested thus far were more stable (as determined by gel filtration) than the complexes with Am(III) or Cm(III) (Taylor, 1972). 

Another area where titration calorimetry has found intensive application, and where the importance of heat flow versus isoperibol calorimetry has been growing, is the energetics of metal-ligand complexation. Morss, Nash, and Ensor [225], for example, used potenciometric titrations and heat flow isothermal titration calorimetry to study the complexation of UO "1" and trivalent lanthanide cations by tetrahydrofuran-2,3,4,5-tetracarboxylic acid (THFTCA), in aqueous solution. Their general goal was to investigate the potential application of THFTCA for actinide and lanthanide separation, and nuclear fuels processing. The obtained results (table 11.1) indicated that the 1 1 complexes formed in the reaction (M = La, Nd, Eu, Dy, andTm)... 

The apparent failure of trivalent and tetravalent cations to enter plants could result from the interaction of the cations with the phospholipids of the cell membranes. Evidence for such interactions is provided by the use of lanthanum nitrate as a stain for cell membranes (143) while thorium (IV) has been shown to form stable complexes with phospholipid micelles (144). However, it is possible that some plant species may possess ionophores specific to trivalent cations. Thomas (145) has shown that trees such as mockernut hickory can accumulate lanthanides. The proof of the existence of such ionophores in these trees may facilitate the development of safeguards to ensure that the actinides are not readily transported from soil to plants. 

In studies of the concentrations of arsenic, bromine, chromium, copper, mercury, lead and zinc in south-eastern Lake Michigan, it was shown that these elements concentrated near the sediment water interface of the fine-grained sediments. The concentration of these elements was related to the amount of organic carbon present in the sediments (161). However, it was not possible to correlate the concentration of boron, berylium, copper, lanthanum, nickel, scandium and vanadium with organic carbon levels. The difficulty in predicting the behaviour of cations in freshwater is exemplified in this study for there is no apparent reason immediately obvious why chromium and copper on the one hand and cobalt and nickel on the other exhibit such variations. However, it must be presumed that lanthanium might typify the behaviour of the trivalent actinides and tetravalent plutonium. 

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. 

To increase the distribution ratios, a solution of lithium nitrate 1M was used. This salt, which has a common anion with europium and americium to be extracted but a cation which is usually negligibly extracted by other calixarenes, should increase the distribution ratios according to the relation Du = A (JU "[N03- ". It seems that these calixarenes, as several nitrogen ligands do, present a certain affinity for this lithium cation. The lipophilic dicarbollide anion (BrCosan), which is known to facilitate cation extraction, was implemented and led to a strong increase of the extraction of cations from 10 3 M HN03 solutions. Under these conditions, only thiopicolinamide was not able to significantly extract trivalent actinides.187... 

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