Plutonium aqueous oxidation-reduction reactions

Newton, T. W and F. B. Baker Aqueous Oxidation-Reduction Reactions of Uranium, Neptunium, Plutonium, and Americium. In if. F. Gould (Ed.), Lanthanide/Actinide Chemistry, Advances in Chemistry Series, Vol. 71, p. 268. Washington American Chemical Society 1967. 

Aqueous Oxidation-Reduction Reactions of Uranium, Neptunium, Plutonium, and Americium... 

T. W. Newton, The Kinetics of the Oxidation Reduction Reactions of Cranium, Neptunium, Plutonium, and Americium in Aqueous Solution, TID-26506, U.S. Energy, Research, and Development Administration (ERDA) Technical Information Center, Washington, D.C., 1975. 

Plutonium can exist simultaneously in four oxidation states in acidic, aqueous solutions. All four oxidation state species [Pu(III), Pu(IV), Pu(V), and Pu(VI)] can be present in solution to give thermodynamically stable systems (17). At environmental acidities, Pu(III) is probably unstable to oxidation (20) thus, the three higher oxidation states are more likely to E encountered, with Pu(IV) being the most stable (17). Because of the relatively slow oxidation-reduction reaction (Hue to the Pu-0 bond) occurring between Pu4+ and Pu022+, these two species should be the easiest to assay for by radiochemical methods. The presence of Pu02+... 

Table 6 presents a summary of the oxidation—reduction characteristics of actinide ions (12—14,17,20). The disproportionation reactions of UO2, Pu , PUO2, and AmO are very compHcated and have been studied extensively. In the case of plutonium, the situation is especially complex four oxidation states of plutonium [(111), (IV), (V), and (VI) ] can exist together ia aqueous solution ia equiUbrium with each other at appreciable concentrations. 

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]. 

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). 

The anode residues must be chemically processed to recover the plutonium remaining in the residues. This may amount to about 10% of the feed mass if delta alloy is the feed metal. Either aqueous or pyrochemical processes may be used for anode recovery. One pyrochemical process used for recovery utilizes oxidation of the plutonium with zinc chloride to form plutonium chloride salt, followed by calcium reduction of the PUCI3 contained in the salt phase to produce pure plutonium metal (the impurities follow the zinc metal obtained from the oxidation reaction and are discarded to waste). Impurities more stable than calcium chloride remain in the salt phase and are also... 

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