Plutonium aqueous

Plutonium(III) in aqueous solution, Pu " ( 4)> is pale blue. Aqueous plutonium(IV) is tan or brown the nitrate complex is green. Pu(V) is pale red-violet or pink in aqueous solution and is beUeved to be the ion PuO Pu(VI) is tan or orange in acid solution, and exists as the ion PuO. In neutral or basic solution Pu(VI) is yellow cationic and anionic hydrolysis complexes form. Pu(VII) has been described as blue-black. Its stmcture is unknown but may be the same as the six-coordinate NpO (OH) (91). Aqueous solutions of each oxidation state can be prepared by chemical oxidants or reductants... 

Evidence foi the anionic complex PuCP is the precipitation of complex halides such as Cs2PuClg from concentrated HCl (aq). The ability of Pu(IV) to form stable nitrate complexes provides the basis for the Purex and ion-exchange (qv) process used in the chemical processing of Pu (107). Pu(VI) is similar to Pu(IV) in its abihty to form complex ions. Detailed reviews of complex ion formation by aqueous plutonium are available (23,94,105). 

Table 10. Spectral Absorption Data for Aqueous Plutonium Ions at 25°C ...

Aqueous plutonium photochemistry is briefly reviewed. Photochemical reactions of plutonium in several acid media have been indicated, and detailed information for such reactions has been reported for perchlorate systems. Photochemical reductions of Pu(VI) to Pu(V) and Pu(IV) to Pu(III) are discussed and are compared to the U(VI)/(V) and Ce(IV)/(III) systems respectively. The reversible photoshift in the Pu(IV) disproportionation reaction is highlighted, and the unique features of this reaction are stressed. The results for photoenhancement of Pu(IV) polymer degradation are presented and an explanation of the post-irradiation effect is offered. 

The authoritative documents on plutonium 0 >2) do not include photo-chemical reactions of plutonium in aqueous systems. The first papers in Western world literature on studies that were dedicated to aqueous plutonium photochemistry appeared in 1976 (3, 4 ), even though photochemical changes in oxidation states were indicated as early as 1952 (5,, ]) ... 

Burger and coworkers (5) in 1952 reported that some distribution coefficients for Pu022+ in organic-aqueous systems at lighted conditions were different from those observed for dark conditions, and those authors believed that some Pu022+ had been photochemically reduced. That reduction was confirmed by others (6) in 1965, and in 1969 a report suggested that most aqueous plutonium reactions were affected by light (7 ). 

Studies of actinide photochemistry are always dominated by the reactions that photochemically reduce the uranyl, U(VI), species. Almost any UV-visible light will excite the uranyl species such that the long-lived, 10-lt seconds, excited-state species will react with most reductants, and the quantum yield for this reduction of UQ22+ to U02+ is very near unity (8). Because of the continued high level of interest in uranyl photochemistry and the similarities in the actinyl species, one wonders why aqueous plutonium photochemistry was not investigated earlier. 

The possible application of aqueous plutonium photochemistry to nuclear fuel reprocessing probably has been the best-received justification for investigating this subject. The necessary controls of and changes in Pu oxidation states could possibly be improved by plutonium photochemical reactions that were comparable to the uranyl photochemistry. 

The primary reason for studying aqueous plutonium photochemistry has been the scientific value. No other aqueous metal system has such a wide range of chemistry four oxidation states can co-exist (III, IV, V, and VI), and the Pu(IV) state can form polymer material. Cation charges on these species range from 1 to 4, and there are molecular as well as metallic ions. A wide variety of anion and chelating complex chemistry applies to the respective oxidation states. Finally, all of this aqueous plutonium chemistry could be affected by the absorption of light, and perhaps new plutonium species could be discovered by photon excitation. 

Visible and UV spectrophotometric techniques are most convenient for studying the polymer and various oxidation states of plutonium. The spectra of the plutonium states and the procedure for resolution of the concentrations were previously described (9 ). Changes in the relative concentrations of the oxidation states and of the polymer generally are determined from corresponding changes in the spectra and a comparison of the changes to standard spectra of the various states. These techniques have been used exclusively for studying the photochemistry of aqueous plutonium. 

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. 

Silver, G. L. Minor Problems in Aqueous Plutonium Chemistry, U.S. AEC Report MLM-2075, Mound Laboratory, Miamisburg, OH, 1973. 

In the present paper the chemistry of plutonium is reviewed, with particular reference to the ambient conditions likely to be encountered in natural waters. In addition, experimental work is presented concerning the effects of such variables as pH, plutonium concentration, ionic strength, and the presence of complexing agents on the particle size distributions of aqueous plutonium. In subsequent papers it will be shown that these variables, as they influence the particle size distribution of the aqueous plutonium, greatly affect its interaction with mineral surfaces. The orientation of these studies is the understanding of the likely behavior and fate of plutonium in environmental waters, particularly as related to its interaction with suspended and bottom sediments. 

Table I. Selected Equilibrium Constants for Aqueous Plutonium Reactions0...

Schramke J. A., Rai D., Fulton R. W., and Choppin G. R. (1989) Determination of aqueous plutonium oxidation states by solvent extractions. J. Radioanalyt. Nuclear Chem. 130(2), 333-346. 

The photochemical effects on four aqueous plutonium perchlorate solutions have been reported. The photochemical reductions of PuOl to PUO2 and of Pu to Pu were observed. A reversible photochemical shift in the equilibrium of the Pu disproportionation was also recorded and the equilibrium coefficient found to increase by a factor of three when the sample was irradiated. Light was found to increase the rate and extent of depolymerization of plutonium(iv) polymer. The preparation of the new plutonium-palladium phase Pu3Pds has been reported and the structure found to resemble that of Ga5Zr3. 

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