Plutonium, redox reactions

The photochemical reduction of a solution containing both uranium(VI) and plutonium(IV) is also of interest for reprocessing applications. Early experiments (12a) showed a significant reduction of plutonium(IV) by light in Purex-type process solutions. Since the quantum yield for plutonium redox reactions is about one-tenth that for uranyl reduction (7b,c) the most likely path of plutonium(IV) reduction in these experiments appears to have been by uranium(IV) or uranium(V) generated by photochemical reduction of uranyl by other components of the solutions. Further experiments in this area would be useful. 

Studies of ligands which might provide specificity in binding to various oxidation states of plutonium seems a particularly promising area for futher research. If specific ion electrodes could be developed for the other oxidation states, study of redox reactions would be much facilitated. Fast separation schemes which do not change the redox equilibria and function at neutral pH values would be helpful in studies of behavior of tracer levels of plutonium in environmental conditions. A particularly important question in this area is the role of PuOj which has been reported to be the dominant soluble form of plutonium in some studies of natural waters (3,14). 

Only the obvious studies of aqueous plutonium photochemistry have been completed, and the results are summarized below. The course of discussion will follow the particular photochemical reactions that have been observed, beginning with the higher oxidation states. This discussion will consider primarily those studies of aqueous plutonium In perchloric acid media but will include one reaction in nitric acid media. Aqueous systems other than perchlorate may affect particular plutonium states by redox reactions and complex formation and could obscure photochemical changes. Detailed experimental studies of plutonium photochemistry in other aqueous systems should also be conducted. 

Grate, J. W. and Egorov, O., Investigation and optimization of on-column redox reactions in the sorbent extraction separation of americium and plutonium using flow injection analysis, Anal. Chem., 70, 3920-3929, 1998. 

Amacher, M.L., and Baker, D, E. (1982). Redox Reactions Involving Chromium, Plutonium, and Manganese in Soils, DOE/DP/OY515.1. Inst. Res. Land and Water Resour. Pennsylvania State University, University Park. 

A further problem is that compounds in the 4-5 and 4-6 oxidation states tend to be reduced (autoradiolysis), as Pu is a strong a-emitter (1 mg Pu emits over a million a-particles a second) decomposing water molecules into H, OH and 0 radicals which, in turn, participate in redox reactions. In acidic solution, decomposition of the solvent to H2O2 and the acid can occur. In nitric acid, for example, both HNO2 and nitrogen oxides are formed, so that, starting with plutonium(vi), ions in the lower oxidation states are generated... 

The following is a redox reaction that occurs for plutonium ... 

In the Electropulse Column, plutonium reduction is achieved both through the redox reaction of plutonium(IV) with uranium(IV), produced electrolytically from uranium(VI) in the cathode chamber, and by direct reduction of the plutonium(IV) at the cathode. The basic reactions involved in the electro-lytic method are as follows ... 

Reduction and oxidation (redox) steps are major process steps in the Purex process. Use is made of redox reactions to alter the valency of plutonium, uranium or neptunium with the object of producing these metals with a high degree of purity. 

Amacher, M.C., and D.E. Baker. 1982. Redox reactions involving chromium, plutonium, and manganese in soils. DOE/DP/045I5-1. USDOE, Las Vegas, NV, and Inst, for Res. on Land and Water Resour., Pennsylvania State Univ., University Park, PA. 

Several other useful reviews of reactions involving metal ions have also been published. Redox reactions of chromium(m)-amine species have been described and a survey has been made of the solution chemistry together with reaction paths involved in the redox reactions of various plutonium species. Oxidation reactions of thallium(m) have also been described. Developments in the redox chemistry of peroxides have been reviewed, the nature of the reactions which involve iron(iii) in various complexed forms providing a fascinating example of the manner in which geometry and co-ordination to the metal centre greatly affect the reactivity of the system. Redox properties of cobalt chelates, with delocalized... 

A summary of qualitative information about the oxidation-reduction characteristics of the actinide ions is presented in Table 14.6. The disproportionation and redox reactions of UO2, Pu, PuO, and Am02 are especially complex, and, despite extensive study, many aspects of these reactions still remain to be explored. In the case of plutonium, the situation is especially complicated, for ions in all four oxidation states iii, iv, v, and vi can exist simultaneously in aqueous solution in equilibrium with each other in comparable concentrations. The kinetics of the redox reactions of the actinide elements have been ably summarized by Newton [22]. 

The redox chemistry of the actinide elements, especially plutonium, is complex (Katz et al., 1980). Disproportionation reactions are especially important for the +4 and +5 oxidation states. Some of the equilibria are kinetically slow and irreversible. All transuranium elements undergo extensive hydrolysis with the +4 cations reacting most readily due to their large charge/radius ratio. Pu (IV) hydrolyzes extensively in acid solution and forms polymers. The polymers are of colloidal dimensions and are a serious problem in nuclear fuel reprocessing. 

In principle, the calculation of the oxidation states of plutonium requires knowledge of the redox potential. Eh, of the aqueous phase. However, the Eh measured with a certain type of electrode may not be the potential for the particular redox couple with which the plutonium reacts. One of the reasons for this is that the Eh-electrode usually catalyses the reaction rate for its specific redox couple. For example, in surface sea water, the measured Eh is about 0.8 V and is due to the O2/H2O couple. In the log Eh versus pH diagram,... 

The multiplicity of oxidation states of the light actinides can be utilized to accomplish very efficient separation of these elements from the lanthanides. Except for actinium (only trivalent), the actinide ions to plutonium either exist predominantly in higher oxidation states [Th(IV), Pa(IV, V)] or can be interconverted with relative ease among any of four oxidation states (III, IV, V, VI). The upper two oxidation states exist in aqueous solutions as the dioxocations AnOj or AnO - The relative strength of complexes formed by the actinide cations in these oxidation states is An(IV) > An(VI) > An(III) > An(V), which order also applies to the separation reactions involving these cations. The dominant oxidation states for the light actinides are Ac(III), Th(IV),Pa(IV or V),U(IV or VI), Np(IV or V). For plutonium, the redox potentials indicate nearly equal stability for all four oxidation states in acidic solutions. The tri-, tetra-, and hexavalent oxidation states are most important in separations. 

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