Tetravalent actinides

The lacunary anions [PWnOsg] ", [SiWn039] " and (a2-) [P2Wi706i] °" were first shown to form complexes with a variety of lanthanide cations (Ln POM =1 1 and 1 2) by Peacock and Weakley in 1971.[41] At that time only the 1 2 complexes could be isolated, and the ai isomer of [P2Wi706i] ° had not been discovered. The first actinide (tetravalent Th, U) analogs of the Peacock-... 

Hydroxides. Thorium (TV) is generally less resistant to hydrolysis than similarly sized lanthanides, and more resistant to hydrolysis than tetravalent ions of other early actinides, eg, U, Np, and Pu. Many of the thorium(IV) hydrolysis studies indicate stepwise hydrolysis to yield monomeric products of formula Th(OH) , where n is integral between 1 and 4, in addition to a number of polymeric species (40—43). More recent potentiometric titration studies indicate that only two of the monomeric species, Th(OH) " and thorium hydroxide [13825-36-0], Th(OH)4, are important in dilute (<10 M Th) solutions (43). However, in a Th02 [1314-20-1] solubiUty study, the best fit to the experimental data required inclusion of the species. Th(OH) 2 (44). In more concentrated (>10 Af) solutions, polynuclear species have been shown to exist. Eor example, a more recent model includes the dimers Th2(OH) " 2 the tetramers Th4(OH) " g and Th4(OH) 2 two hexamers, Th2(OH) " 4 and Th2(OH) " 2 (43). 

Previous studies of the hydrothermal hydrolysis of tetravalent Th, U and Np (1-4) have shown a remarkable similarity in the behavior of these elements. In each case compounds of stoichiometry M(0H)2S0i, represent the major product. In order to extend our knowledge of the hydrolytic behavior of the actinides and to elucidate similarities and differences among this group of elements, we have investigated the behavior of tetravalent plutonium under similar conditions. The relationships between the major product of the hydrothermal hydrolysis of Pu(IV), Pu2(OH)2(SO.,)3 (H20) t, (I)> and other tetravalent actinide, lanthanide and Group IVB hydroxysulfates are the subject of this re-... 

The only crystalline phase which has been isolated has the formula Pu2(OH)2(SO )3(HaO). The appearance of this phase is quite remarkable because under similar conditions the other actinides which have been examined form phases of different composition (M(OH)2SOit, M=Th,U,Np). Thus, plutonium apparently lies at that point in the actinide series where the actinide contraction influences the chemistry such that elements in identical oxidation states will behave differently. The chemistry of plutonium in this system resembles that of zirconium and hafnium more than that of the lighter tetravalent actinides. Structural studies do reveal a common feature among the various hydroxysulfate compounds, however, i.e., the existence of double hydroxide bridges between metal atoms. This structural feature persists from zirconium through plutonium for compounds of stoichiometry M(OH)2SOit to M2 (OH) 2 (S0O 3 (H20) i,. Spectroscopic studies show similarities between Pu2 (OH) 2 (SOO 3 (H20) i, and the Pu(IV) polymer and suggest that common structural features may be present. 

In contrast to the situation observed in the trivalent lanthanide and actinide sulfates, the enthalpies and entropies of complexation for the 1 1 complexes are not constant across this series of tetravalent actinide sulfates. In order to compare these results, the thermodynamic parameters for the reaction between the tetravalent actinide ions and HSOIJ were corrected for the ionization of HSOi as was done above in the discussion of the trivalent complexes. The corrected results are tabulated in Table V. The enthalpies are found to vary from +9.8 to+41.7 kj/m and the entropies from +101 to +213 J/m°K. Both the enthalpy and entropy increase from ll1 "1" to Pu1 with the ThSOfj parameters being similar to those of NpS0 +. Complex stability is derived from a very favorable entropy contribution implying (not surprisingly) that these complexes are inner sphere in nature. 

The Table shows a great spread in Kd-values even at the same location. This is due to the fact that the environmental conditions influence the partition of plutonium species between different valency states and complexes. For the different actinides, it is found that the Kd-values under otherwise identical conditions (e.g. for the uptake of plutonium on geologic materials or in organisms) decrease in the order Pu>Am>U>Np (15). Because neptunium is usually pentavalent, uranium hexavalent and americium trivalent, while plutonium in natural systems is mainly tetravalent, it is clear from the actinide homologue properties that the oxidation state of plutonium will affect the observed Kd-value. The oxidation state of plutonium depends on the redox potential (Eh-value) of the ground water and its content of oxidants or reductants. It is also found that natural ligands like C032- and fulvic acids, which complex plutonium (see next section), also influence the Kd-value. 

Although the D s for U(VI) and tetravalent actinides are very high, the data in Table VII show that formic acid (HC00H) will readily back-extract these elements as well as Am(III) from all the extractants except in the case of U(VI) with 0 D[IB]CMP0. 

This latter situation affords a good method for separating uranium from plutonium. Hydroxylammonium formate (HAF) and hydrazium formate (NHF) were added to the formic acid to reduce Pu(IV) to Pu(III) to aid in plutonium recovery, although formic acid alone will strip tetravalent actinides, e.g., Th(IV) from 0D[IB]CMP0, once excess HNO3 present in the organic phase is removed. Thus, formic acid with HAF and NHF affords an excellent method for stripping all the actinides from these very powerful CMP extractants. Under the above conditions Am(III) and Cm(III) present in... 

Gnillanmont R, Bouissieres G, Muxart R (1968) Chimie du Protactinium. I. Solutions aqueuses de protactinium penta- et tetravalent. Actinides Rev 1 135-163 Harvey BG (1962) Introduction to Nuclear Physics and Chemistiy. Prentice Hall Inc, New Jersey Henderson GM, Anderson RF (2003) The U-series toolbox for paleoceanography. Rev Mineral Geochem... 

One of the first bed materials was based on the extractant diamyl amylphosphonate (DAAP marketed under the name U-TEVA-Spec ) and was designed for purification of the tetravalent actinides (U (IV), Th (IV), Pu (IV)) and hexavalent uranium (U(VI)). This material is characterized by high (>10-100) distribution coefficients for U and Th in significant (>3 M) concentrations of both nitric and hydrochloric acids, and so is useful for both U and Th purification (Horwitz et al. 1992 Goldstein et al. 1997 Eikenberg et al. 2001a). 

Guillanmont R, Bonissieres G, Muxart Y (1968) Protactinium chemistiy. 1. Aqueous solutions for penta and tetravalent protactinium. Actinid Rev 1 135... 

In the trivalent state the stereochemistry of the actinides is similar to the lanthanides, that is, eight coordinate. However, higher coordination states are known for some trivalent actinides, for instance, UC13 exists in the nine coordinate state. In the tetravalent state the species normally encountered are eight coordinate. 

The actinide ions which are of importance in any discussion on their role in the biosphere are the trivalent (Pu3+, Am3+, Cm3+) tetravalent (Np4+, Pu4+) and dioxo species (U02+, NpOf, Pu02+, AmOf). 

The trivalent actinide state resembles that of the lanthanides. In an aqueous solution some M3+ ions exist (Am3+, Cm3+) ions the U3+ ions is readily oxidised by air or more slowly by water. Tetravalent U and Pu are reasonably stable in solution, whereas Am(IV) and Cm(IV) are readily reduced and exist only as complex ions in... 

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. 

Several difficulties arise from the use of TBP in the Thorex pro-cess.Thorium (IV) is the major element present, and the loading capacity of a 30% TBP-dodecane solution is considerably lower for the tetravalent actinides than it is for U(VI) thus, a larger organic/aqueous phase ratio... 

The use of HDEHP for extraction of actinides from waste solutions also has several drawbacks, including extraction of trivalent ions at a pH at which tetravalent ions such as Zr(IV) and Pu(IV) are hydrolyzed, and difficulties in stripping tetravalent and hexavalent actinides. All in all, monofunctional organophosphorus reagents are vastly inferior to their bifunctional counterparts for extracting Am(III) and other actinides from strong HNO3 media. 

In Fig. 1, we have plotted the oxidation numbers of the actinides and of the lanthanides. We see that for the lanthanides the valence 3 is the most stable valence throughout the series. There are exceptions Ce displays for instance tetravalency in many compounds Eu and Yb display divalency. These exceptions are understood e.g., Eu and Yb are at the half-filling and at the filling of the 4f shell, which are stable electronic configurations. There is a tendency for both to share just the two outer 5 s electrons in bonding, displaying therefore, divalency, and preserve these stable configurations. 

C. Licour, L. Lopes, Contribution to the Knowledge of the Electrochemical Properties of Actinides in Non-Aqueous Media III. The Reduction of Tetravalent Thorium and Tetravalent Neptunium in Various Organic Solvents, Pages 6-12, 1995, with permission from Elsevier). 

Th-oxyhydroxide species readily dissolve upon dilution below the solubility limit, it is not veiy likely that such actinide(IV) colloids play a role away from the source in the far field of a repository. In the near field of a repository, however, they may be predominant species controlling the solubility of tetravalent actinide species such as U(IV) and Pu(IV) and thus the source term. Unusual stability at high ionic strength has been also reported for amorphous SiOz colloids (Iler 1979 Healy 1994) which also cannot be explained solely by electrostatic repulsion. Formation of oligomeric or polymeric silicate species at the colloid-water interface are thought to exert additional steric stabilization by preventing close approach of those particles. 

Artinger, R., Buckau, G., Zeh, P., Geraedts, K., Vancluysen, I, Maes, A. Kim, J. I. 2003. Humic colloid mediated transport of tetravalent actinides and technetium. Radiochimica Acta, 91, 743-750. 

This behaviour is not restricted to Th(IV) and U(IV) as similar patterns have been found for all tetravalent actinides, An(IV). A more detailed discussion and comparison of An(IV) solubility and hydrolysis is given by Neck Kim (2001). These authors conclude from solubility data... 

Neck, V. Kim, J. I. 2001. Solubility and hydrolysis of tetravalent actinides. Radiochimica Acta, 89, 1-16. 

For vanadium and chromium the first ionization energies are much lower than the first ionization energies of phosphorus and sulphur, respectively. This explains the high heats of formation of VC13 and CrCl3. In uranium, the tetravalent state is more stable than that in tungsten because uranium as an actinide has a different electron configuration. 

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