Plutonium species

Of all plutonium species, Pu02 should have the lowest tendency to form complexes. The only reported value, log ei(PuOoCl) =... 

The uncertainty of the proper coordination number of any particular plutonium species in solution leads to a corresponding uncertainty in the correct cationic radius. Shannon has evaluated much of the available data and obtained sets of "effective ionic radii" for metal ions in different oxidation states and coordination numbers (6). Unfortunately, the data for plutonium is quite sparse. By using Shannon s radii for other actinides (e.g., Th(iv), U(Vl)) and for Ln(III) ions, the values listed in Table I have been obtained for plutonium. These radii are estimated to have an uncertainty of 0.02 X ... 

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. 

After observing the photochemical reduction of Pu(VI) and Pu(IV), it seems obvious that reaction (3) should be light-sensitive. However, it is not obvious how photons would affect the equilibrium concentrations of the plutonium species. The experimental results [3,4] are very interesting and are described below, but a complete explanation is yet to be developed. 

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. 

The glass phase furnished an active surface for the possible deposition of dispersed plutonium species. 

Silver was critical of the lack of use by plutonium chemists of a-coefficients. Assuming that Silver was referring to a-coeffi-cients defined as the fraction of the total concentration of a substance that exists as a particular species, he was wrong to say that plutonium chemists have not used them. Phil Horwitz at ANL has used them. Publications from ORNL have reported them to easily show relative concentrations of plutonium species, and L. M. Toth used such a-coefficients as percent of Pu(IV) polymer in his symposium talk Tuesday. Alpha coefficients are a commonly used, simple concept - certainly since Ringbom s article in the Journal of Chemical Education in 1958."... 

Extractions and Laser Photoacoustic-Spectroscopy for the Determination of Plutonium Species in Near-Neutral Carbonate Solutions. 

It has been established that plutonium hydrolysis products exhibit colloidal behaviour (147-151) and may adsorb onto minerals and other surfaces to form radiocolloids. However, it is difficult to determine whether a radiocolloid is a true colloid or a pseudocolloid formed by adsorption of the plutonium species onto other colloidal impurities in the solution (152). In some cases both forms may be present... 

Monomeric plutonium species deposited in the liver become concentrated in the liver ferritin, the principal iron repository (191). On analysis of plutonium deposition in bone a dichotomy becomes immediately apparent. Monomeric plutonium no longer follows an iron transport/deposition mechanism, for bone contains little or no iron complexed within the bone matrix. Calcium phosphate as a chromatographic media does, of course, retain iron. 

To understand the nature of the plutonium species likely to be present in natural waters, it is useful to compare graphically the solubilities... 

Figure 13.3 Alpha spectrum of oxidised and reduced plutonium species as separated by the neodymium fluoride technique (oxidised yield tracer 236Pu, reduced yield tracer 242Pu).

An alternative procedure for the study of neptunium oxidation states at trace concentrations has been described by Inoue and Tochiyama (1977). They showed that, in the pH range 6-7, Nplv may be quantitatively absorbed on silica gel whilst Npv remains in solution. In acid solution, however, a precipitate of barium sulfate selectively absorbs Nplv leaving the higher oxidation states in solution. The authors gave no environmental data for neptunium in their publication but Nelson and Orlandini (1979) subsequendy adapted the procedure to demonstrate that the dominant oxidised plutonium species in natural waters is Puv and not Puvl. 

Step 12. Elute the plutonium from the column into a clean 100-mL beaker with 50 mL of freshly prepared 0.05 N NH4I solution in 10 M HC1 at a flow rate of 2 mL per minute. The column will turn dark brown due to the formation ofI2t this step destroys the anionic plutonium species complex that had been sorbed on the anion exchange resin. 

The alpha activity of plutonium in the wastewater sample was too low to be measured conveniently. A spike solution (with 95% Pu activity and 5% iPu activity) was added to the wastewater as a tracer for plutonium. The Pu spike solution was chemically treated to assure that all the Pu was in the (IV) oxidation state before adding it to the wastewater. This was done by first reducing all the plutonium to Pu(III) with hydrogen peroxide (in 1.0 M HNO3). The Pu(III) was then oxidized to Pu(IV) by adding sodium nitrite. The plutonium spike was assumed to react with the wastewater to form a plutonium species distribution similar to that of the original wastewater. Nitsche (7) has shown that Pu(IV) spikes added to groundwater react in a relatively short time to form mainly Pu(V) and Pu(VI) species. 

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