Plutonium oxidation-reduction potentials

Oxidation-reduction potentials for plutonium. Oxidation reduction potentials [Al] for plutonium... 

Ions of different valences of a metal behave like different elements with respect to extract-ability. The difference between Ce and Ce in Table 4.2 is one example. Another is afforded by Pu and Pu 02, which are readily extracted by TBP in kerosene, whereas Pu has a very low distribution coefficient [G31. Consequently, by adjusting the oxidation-reduction potential of the aqueous phase to control the proportion of an element in different valence states, it is possible to vary its distribution coefficient between wide limits. This is the mearts by which plutonium is stripped from aqueous solutions containing plutonium and uranium in sections C and D of Fig. 4.5 illustrating the Purex process. Addition of a reducing... 

Pentaralent neptunium is the most stable state in solution. It hydrolyzes only in basic solutions, disproportionates only at high acidity, and forms no polynuclear complexes. As shown by the oxidation-reduction potentials of Table 9.6, hexavalent neptunium is much less stable in solution than is hexavalent plutonium in fact, hexavalent neptunium is a strong oxidizing agent and is easily reduced in the presence of oxidizable substances, such as those present in ion-exchange and solvent extraction separations [K2]. 

What are the relative concentrations of trivalent, tetravalent, and hexavalent plutonium when a small amount of PuOj is dissolved in a large excess of 1 V HNO3 Repeat for 3 N HNO3. Assume that activities are proportional to molarities. For oxidation-reduction potentials use Fig. 9.1, sixth diagram from the top. 

Other Coordination Complexes. Because carbonate and bicarbonate are commonly found under environmental conditions in water, and because carbonate complexes Pu readily in most oxidation states, Pu carbonato complexes have been studied extensively. The reduction potentials vs the standard hydrogen electrode of Pu(VI)/(V) shifts from 0.916 to 0.33 V and the Pu(IV)/(III) potential shifts from 1.48 to -0.50 V in 1 Tf carbonate. These shifts indicate strong carbonate complexation. Electrochemistry, reaction kinetics, and spectroscopy of plutonium carbonates in solution have been reviewed (113). The solubiUty of Pu(IV) in aqueous carbonate solutions has been measured, and the stabiUty constants of hydroxycarbonato complexes have been calculated (Fig. 6b) (90). 

In comparison with uranium, the (-1-3) state has become much more stable (Table 9.5), witness the standard reduction potential of -1-0.98 V for Pu" + -I- e -> Pu +, the comparative value for uranium being —0.63V. This means that quite strong oxidizing agents such as manganate(vii) are needed to effect this oxidation in the case of plutonium. 

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... 

Calcinm glectrowinning in piutniiium production. A potential application of inorganic membranes in radioactive waste treatment is in the industrially practiced direct oxide reduction process. In this process plutonium oxide is calciothermit y reduced to plutonium in the presence of calcium chloride according to the following reaction ... 

The rather similar values for the plutonium reduction If reactions at pH 8 indicate that plutonium may exist in oxic waters in more than one oxidation state. The reduction potential at pH 8 of the Pu(III)/Pu(IV) couple indicates that Pu(III) is unlikely to exist in oxic waters in the absence of a reductant, but may be present in anoxic waters. Each oxidation state of plutonium differs in chemical behavior from that of the other states so modelling the geochemical behavior of plutonium must include the correct oxidation state, or states, of plutonium which are likely to be present in a particular system. 

R. E. Connick, "Oxidation States, Potentials, Eqxailibria, and Oxidation-Reduction Reactions of Plutonium."in The Actinide Elements, Natl. Nucl. Energy Series, Div. 

Uranium and mixed uranium—plutonium nitrides have a potential use as nuclear fuels for lead cooled fast reactors (136—139). Reactors of this type have been proposed for use ia deep-sea research vehicles (136). However, similar to the oxides, ia order for these materials to be useful as fuels, the nitrides must have an appropriate size and shape, ie, spheres. Microspheres of uranium nitrides have been fabricated by internal gelation and carbothermic reduction (140,141). Another use for uranium nitrides is as a catalyst for the cracking of NH at 550°C, which results ia high yields of H2 (142). 

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. 

It has already been stated that plutonium can exist in aqueous solution in four oxidation states and an examination of Table 3 shows that the potentials of the couples Pu3+/Pu4+ and PuOj/PuO are less negative than the Pu /PuOJcouple. As a result of these lower couples it is possible for a simultaneous oxidation and reduction of Pu4+ and PuOj- This process of simultaneous oxidation and reduction is termed disproportionation and the process is represented thus ... 

Disposal of spent nuclear fuel and other radioactive wastes in the subsurface and assessment of the hazards associated with the potential release of these contaminants into the environment require knowledge of radionuclide geochemistry. Plutonium (Pu), for example, exhibits complex environmental chemistry understanding the mechanism of Pu oxidation and subsequent reduction, particularly by Mn-bearing minerals, is of major importance for predicting the fate of Pu in the subsurface. 

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