Uranium oxidation-reduction potentials

Fig. 2. Oxidation-reduction potentials of uranium ions tin 1-molar hydrochloric acid)...

Uranium deposited by wet or dry precipitation will be deposited on land or in surface waters. If land deposition occurs, the uranium can be reincorporated into soil, resuspended in the atmosphere (typically factors are around 10 ), washed from the land into surface water, incorporated into groundwater, or deposited on or adsorbed onto plant roots Gittle or none enters the plant through leaves or roots). Conditions that increase the rate of formation of soluble complexes and decrease the rate of sorption of labile uranium in soil and sediment enhance the mobility of uranium. Significant reactions of uranium in soil are formation of complexes with anions and hgands (e.g., COj, OH ) or humic acid, and reduction of U" " to U. Other factors that control the mobility of uranium in soil are the oxidation-reduction potential, the pH, and the sorbing characteristics of the sediments and soils (Allard et al. 1979, 1982 Brunskill and Wilkinson 1987 Herczeg et al. 1988 Premuzie et al. 1995). 

The oxidation-reduction potential of water is important in controlling the mobility of uranium. In anoxic waters where the aquatic environment is reductive, U(VI) will be reduced to U(IV) (e.g., changed from a soluble compound to an insoluble one). The U(IV) will be deposited into the sediment due to the insolubility of the resulting U(IV) salts (Allard et al. 1979 Herczeg et al. 1988). Mobilization and... 

The mobility of uranium in soil and its vertical transport (leaching) to groundwater depend on properties of the soil such as pH, oxidation-reduction potential, concentration of complexing anions, porosity of the soil, soil particle size, and sorption properties, as well as the amount of water available (Allard et al. 

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... 

To leach the more acid-resistant minerals containing tetravalent uranium, steam is fed to the second tank to bring the temperature to 49 to 60°C, and sodium chlorate NaQOj is added to bring the oxidation-reduction potential e, measured relative to the calomel electrode, to from —0.47 to —0.51 V. At —0.51 V, the equilibrium ratio of ferric iron to ferrous iron in the solution is 0.52. Ferric iron catalyzes the oxidation of insoluble tetravalent uranium to the soluble hexavalent uranyl form ... 

The electron configuration of the gaseous uranium atom is 5f 6dls, and its atomic weight is 238.07. As might be expected, uranium shows valence states that range between 2 -(-and 6 +. In minerals, however, only the valences 4 +, 5 + and 6+ occur. Oxidation-reduction potential data for uranium are summarized in Table 1. Oxidation states 4+ and 6+ are the most important from a geochemical point of view. 

The solution chemistry of uranium is that of the +4 and +6 oxidation states, that is, U4+ and U02+. The formal reduction potential of uranium in aqueous solution (i.e., 1 M HC104) is... 

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... 

From the Frost diagram in Figure 23.15, it can be seen that the most stable oxidation state of uranium in aqueous acid is U (i.e., it has the most negative free energy of formation, the quantity plotted on the y axis). However, the reduction potentials for the U02 /U02 and U02 /U couples are quite small, +0.170 and +0.38 V, respectively (hence, the U02 /U potential is +0.275 V). Therefore, b ause the O2/H2O reduction pwtential is 1.229 V, the most stable uranium ion is U02 if sufficient oxygen is present ... 

For example, if the amount of ferrous iron minerals present in repository backfill and fracture minerals (represented by FeC03(s) in Fig. 1(a)) is much greater than the amount of O2 remaining after closure, then with time, all O2 will be reduced to H2O by these minerals, producing iron hydroxide in the process. This would ensure that the reducing intensity would return to values at least as low as the redox potential of the Fe(0H)3(s)/FeC03(s) couple (near —0.05 V). This is below the threshold for corrosion of either copper or uranium oxide by O2. It is also shghtly above the threshold for sulphide production by sulphate reduction (—0.2 V). The presence of ferrous minerals thus buffers the redox intensity of the repository to conditions that are favourable for repository performance. 

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. 

Cathodic stripping voltammetry has been used [807] to determine lead, cadmium, copper, zinc, uranium, vanadium, molybdenum, nickel, and cobalt in water, with great sensitivity and specificity, allowing study of metal specia-tion directly in the unaltered sample. The technique used preconcentration of the metal at a higher oxidation state by adsorption of certain surface-active complexes, after which its concentration was determined by reduction. The reaction mechanisms, effect of variation of the adsorption potential, maximal adsorption capacity of the hanging mercury drop electrode, and possible interferences are discussed. 

For the rapid determination of Tc in a mixture of uranium fission products. Love and Greendale have used the method of amalgam polarography. It consists in a selective reduction of technetium at a dropping mercury electrode at a potential of —1.55 V vs. SCE in a medium of 1 M sodium citrate and 0.1 M NaOH. Under these conditions, technetium is reduced to an oxidation state which is soluble in mercury. The amalgam is removed from the solution of fission fragments and the amount of Tc determined in nitric acid solution of the amalgam by a y count. For Tc the measurement accuracy is within 1 %, and the decontamination factor from other fission products 10 . 

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