Uranium pentavalent

For vanadium solvent extraction, Hon powder can be added to reduce pentavalent vanadium to quadrivalent and trivalent Hon to divalent at a redox potential of —150 mV. The pH is adjusted to 2 by addition of NH, and an oxyvanadium cation is extracted in four countercurrent stages of mixer—settlers by a diesel oil solution of EHPA. Vanadium is stripped from the organic solvent with a 15 wt % sulfuric acid solution in four countercurrent stages. Addition of NH, steam, and sodium chlorate to the strip Hquor results in the precipitation of vanadium oxides, which are filtered, dried, fused, and flaked (22). Vanadium can also be extracted from oxidized uranium raffinate by solvent extraction with a tertiary amine, and ammonium metavanadate is produced from the soda-ash strip Hquor. Fused and flaked pentoxide is made from the ammonium metavanadate (23). 

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. 

Renshaw JC, LJC Butchins, FR Livens, I May, JR Lloyd (2005) Bioreduction of uranium environmental implications of a pentavalent intermediate. Environ Sci Technol 39 5657-5660. 

Margolis [203] confirms such results for antimonates and reports the existence of a surface compound containing Sb3+—O—C. Aykan and Sleight [34] examined the system U—Sb—O in air up to 1000°C by different techniques (e.g. ESR) and found the ternary components USbOs and USb3Oi0. Since USb03 is paramagnetic, the formal oxidation state of U must be 5+, hence Sb must also be in the 5+ state. The authors conclude that USb3O10 also contains pentavalent uranium. 

In the reduction of hexavalent uranyl ions, one electron is involved the product, pentavalent uranyl, disproportionates spontaneously into tetra- and hexavalent uranium... 

The pentavalent antimony ion in an octahedral environment was postulated as necessary for the partial oxidation of propylene, and the uranium ion was indicated to be important in stabilizing the Sb5+ structurally and in aiding its regeneration with gaseous oxygen during the course of catalytic operation. 

Pentavalent. The pentafluorides of Pa, U, and Np have been reported. The synthetic routes to obtain the pentafluorides are given in equations (11) and (12). Uranium pentafluoride, UF5, gives two different modifications that have been structurally characterized, a (grayish white solid) and (yellowish white). The a-phase, which is produced from the reaction of UFs with HBr or mildly heating a mixture of UFs and UF4, has infinite chains of UFe units. The /3-phase produced by heating a mixture of UFe and UF4 to... 

Hydroxides. The hydrolysis of Np has been studied more than that of any other pentavalent actinide because it is the most stable oxidation state for Np and it is an actinide ion of significant concern for environmental migration. Pentavalent uranium disproportionates in aqueous solution at pH values where hydrolysis would occur. Hydrolysis products for Pa, Pu, and Am are very similar to, but much less stable than those of Np, so only Np hydroxides will be described in detail. Neptunyl hydrolyzes at about pH 9, to form the stepwise products, Np02(0H) and Np02(0H)2 ", which have been identified by optical absorbance and Raman spectroscopy. " In addition to the hydroxide these complexes likely have two or three inner-sphere waters in the equatorial plane and pentagonal bipyramidal coordination geometry. 

The pentavalent oxidation state is accessible for the early actinides uranium, protactinium, neptunium, and plutonium. Pentavalent species with neutral Group 16 bases can include either adducts of AnXg or complexes incorporating oxo-containing cations, AnO " or An02". ... 

Binary halides. A number of homoleptic halides of pentavalent protactinium, uranium, and neptunium have been reported. In particular, the fluoride complexes AnFs are prepared by high... 

Wyartite is the only mineral known to contain pentavalent uranium, although several synthetic compounds are known to contain uranium in this valence state... 

In the case of pentavalent cations, the conditional interaction constant (Table I) obtained for Np02" (log P = 4.6) shows a relatively low affinity of the neptunyl cation with the humic acids as it could be predicted from the charge of the ion. The interaction constant obtained for the hexavalent cation (U) with humic acid (Table I) is independent of pH (4-5) in the non-hydrolysis pH-range but some variation with uranium concentration is observed as for trivalent cations. Moreover, the complexation of uranium to humic substances is of the same order of magnitude than the complexation of trivalent actinides which corroborates chemical analogy between both cations. 

Since 1958, more than 20 nuclides of actinides ranging from neptunium to einsteinium were identified and prepared for tracer studies. From neutron-irradiated uranium samples 2 9Np was adjusted to the pentavalent state and separated by TBP extraction from perchloric acid media. Plutonium-239 was separated by TBP extraction from nitric acid solution followed by anion exchange in a system of Dowex-1 resin and nitric acid. Neptunium-237 was separated from a spent fuel solution of JRR-1 (Japan Research Reactor -1) using anion exchange and TBP extraction. The TBP extraction in the hydrochloric acid medium is a simple and effective technique to purify neptunium from plutonium contamination. On the other hand, both anion exchange and solvent extraction with HDEHP could be used to separate tracer scale plutonium from irradiated neptunium targets. 

A laboratory study was undertaken to determine the behaviour of neptunium in the WAK flowsheet, and to devise a procedure for its recovery. Based on static ( ) and counter-current experiments (J5), the conclusion was reached that about half of the Np is co-extracted with the U and Pu in the HA-HS mixer-settlers of WAK while the other half is rejected to the HAW, see Fig.1. It could also be shown that an increase of the aqueous acidity, or the addition of pentavalent vanadium as an oxidant into the lower stages of the HA mixer-settler (6), would increase the Np yield in the organic solvent. In the 1BX-1BS mixer-settlers where the partitioning of U and Pu is carried out by use of uranium (IV)nitrate - hydrazine nitrate, a splitting of the coextracted Np between the two product streams was observed the proportions of the (co-extracted) Np which ended up in the 1CU (uranium product) stream fluctuated from 30 to 93 % while the difference amount (from 7 to 70 %) ended up in the 1 BP (plutonium product) stream. 

No definite reason for these fluctuations could be identified, but it is known that neptunium, due to its complicated redox chemistry, reacts in a very sensitive way to even minor process variations (7,8). Based on these results the proposal was made (J5) to recover the "co-extracted" portion of the neptunium by running the second plutonium and uranium purification cycles under conditions where the Np is directed into the aqueous raffinates (2AW and 2DW streams). In the Pu purification cycle, this can be done by adding sufficient nitrous acid to keep the Np pentavalent, while in the U purification cycle (which is run under slightly reducing conditions) a low acidity and a high loading help to reject Np into the aqueous 2DW stream. The two raffinate streams are combined in WAK in the 3W evaporator, and the Np is thus collected in the concentrate from this unit (3WW stream). Consequently the proposal was made to recover the Np from this 3WW stream by use of the well-known anion exchange process (9,J ). 

At one time the preferred catalyst for propylene ammoxidation was a uranium-antimony oxide composition whose active phase was USb3O2 Q. We have found that the partial substitution of certain tetravalent metals for the pentavalent antimony in this phase greatly increases catalytic activity. 

Figure 11 shows the atomic configurations of the uranate centres proposed by Runciman (a) and by Kaplyanskii (b). In contrast to these proposals Pant et al. ) ascribe the luminescence properties to square-planar U04-groups. Recently, E.P.R. measurements have been performed on X- or 7-irradiated crystals of uranium-activated lithium fluoride and uranium-activated sodium fluoride " ). The authors propose (UOsF) ), (UOs—or (UOs)" ) to be the uranium centres, containing pentavalent uranium. 

In the absorption spectra of the crystals no indication was found for the presence of pentavalent uranium. Therefore it is concluded that the uranate centres contain hexavalent uranium ions. 

NH4, Rb, and Cs, are isostructural with KgPaF (37, 39), but the analogous uranium(V) complexes possess diiferent structures. The rubidium salts of pentavalent uranium, neptunium, and plutonium are, in fact, isostructural with K2NbF7, being therefore 7-coordinate. 

No structural data are available for any of the pentavalent nitrates. Tetravalent protactinium nitrates are unknown. Fuming nitric acid oxidizes protactinium(IV) and obviously N.2O4 and N2O5 will do the same. However, it may be possible to prepare hexanitratoprotactinates(IV), M2Pa(NO j)a, or even complexes of the type Pa(N03)4>a L (L = oxygen donor ligand), analogous to those known for uranium(IV), by metatheses in nonaqueous solvents. 

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