Trivalent uranium reduction

Halo-carboxylic acids, reduction by trivalent uranium, 34 80... 

Although the trivalent uranium species of the tetrapyrrolide polyanions display a very high reactivity, no evidence was found for the ability to interact with N2. However, the reduction of the trivalent [ [(—CH2 )5]4-calix[4]tetrapy-rrole U(dme)][K (dme)] with [K-(naphthalenide)] in DME afforded N2cleavage with formation of the mixed-valenceU(IV)-U(V) nitride below (Scheme 13) (36). The highly reactive species that performs the N2 cleavage in this case was not identified since reactions carried out under an Ar atmosphere promoted solvent deoxygenation, as well as depolymerization of polysilanol. 

Oxidation of the sterically crowded complex Cp 3U provided access to [Cp 2U]2(/i-0) as the first molecular trivalent uranium oxide. The U-O-U angle in this molecule is 171.5(6)°.85 Reduction of the uranium(iv) thiolates... 

Another most interesting and unusual result has been reported regarding the formation of an U(III) oxide complex, [(Cp )2U]2( x-0) (Fig. 5) obtained by the (r] -C5Me5)3U reduction system (Scheme 4) [107]. This has been claimed to be the first molecular trivalent uranium oxide so far reported. The complex was isolated from a reaction of (r -C5Me5)3U with KCg in toluene. Probably this is the first example of an SIR process in which the CsMes" reduction precedes the U(III) electron transfer. 

I.2.2. Actinides. The s5oithesis of trivalent uranium Cp3U can be carried out by different methods (Marks and Ernst 1982), for example by reduction of several derivatives or by photo-induced 3-hydride elimination of tris-cyclopentadienyl uranium(IV)isopropyl... 

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. 

An additional bulky amide ligand type, which supports novel coordination complexes of lower valent uranium has been developed. Complexes of the formula U(NRAr)3(THF) (R = Bu, adaman-tyl Ar = 3,5-Me2C6H3) cannot be generated directly from trivalent halide precursors instead, they are produced in the reduction of the uranium(fV) iodide complex by sodium amalgam. ... 

Several methods were proposed for Pu-U partitioning which are not based on a reduction of plutonium to the less extractable trivalent state. The separation is achieved by either forming aqueous PuH complexes which have a low solubility in the TBP-hydrocarbon solvent or by saturating the organic solvent with uranium, which depresses the extractability of plutonium. 

In the partition contactor, plutonium was converted to inextractable, trivalent Pu (N03)3 by a reductant solution of ferrous sulfamate containing aluminum nitrate to keep uranium in the hexone phase. Plutonium was thus separated from uranium and transferred back to the aqueous phase along with the aluminum nitrate. Impure plutonium nitrate was purified by additional cycles of solvent extraction, not shown. 

In partition, step 5, plutonium is separated from uranium by reducing plutonium to the organic-insoluble, trivalent state with a reductant strong enough to act on plutonium but not so... 

The next step in the Purex process after primary decontamination is separation of plutonium from uranium. This is done by reducing plutonium to the trivalent state, in which it is inextractable by TBP, while leaving the uranium in the extractable hexavalent condition. Reductants that have been used for this purpose include Fe, U, hydroxylamine, or cathodic reduction. 

Process selection. The processes just described recovered neptunium only partially and in variable yield because of the difficulty in controlling the distribution of neptunium valence between 5 and 6 in the primary extraction step with nitrite-catalyzed HNO3 and the incomplete reduction of neptunium from valence 5 to 4 in the partitioning step with feirous ion. This section describes a modified Purex process that could be used if more complete recovery of neptunium were required. It is based on process design studies by Tajik [Tl]. The principal process steps are shown in the material flow sheet Fig. 10.32. In the primary decontamination step, pentavalent vanadium oxidizes neptunium to the extractable hexavalent state. In the partitioning step, tetravalent uranium reduces plutonium to the inextractable trivalent state while converting neptunium to the still-extractable tetravalent state. 

The most prevalent form of uranium in aqueous solution is the tight yellow, fluorescent uranyl ion U02. The U cation (green in solution) can be obtained by strong reduction of U(V1), but readily oxidizes back to U02 in air. The pentavalent ion U02 can be reversibly formed by reduction of U02, but it readily disproportionates into U(IV) and U(VI). The trivalent U can be formed by reduction of U(IV) but is unstable to oxidation in aqueous solution. 

This is similar for mercury [15], caesium, technetium and uranium [16], as well as for molybdenum, which can be bound to iron oxides even in the form of the molybdate anion [17]. In some soils, hydrous oxides represent dominant substrates of copper [18] and cadmium [19]. The highly toxic, hexavalent chromium is by far more mobile than the trivalent, which makes its reduction in the environment especially important [20], although it also sorbs onto hydrous oxides [21]. 

Hydrazine is also used in conjunction with HNA to impede hydroxylamine oxidation by nitrous acid, always present in nitric acid media, which increases the HNA availability for the plutonium reduction. Both hydrazine and HNA interact with nitrous acid (HNO2), but the hydrazine oxidation is much faster. On the other hand, the use of HAN in conjunction with nitric acid introduces the possibility of an autocatalytic reaction resulting in the overpressurization of the system or explosion in a reprocessing facility as pointed out in Barney s report (Barney, 1998). The main function of HNA is to reduce plutonium from the tetravalent state to the trivalent state and, thus, separate the plutonium from the uranium, which is retained in the hexavalent oxidation state and, hence, in the organic phase. The reduction reaction by HNA is the result of two irreversible reactions (Equations 14.10 and 14.11) ... 

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