Oxidation states of plutonium

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. 

An important experiment was the establishment of the +4 oxidation state of plutonium ... 

Investigations of the chemical properties of plutonium have continued in many laboratories throughout the world as it has become available. This has led to the situation where the chemistry of this relative newcomer is as well understood as is that of most of the well-studied elements. The four oxidation states of plutonium—III, IV, V, and VI—lead to a chemistry which is as complex as that of any other element. It is unique among the elements in that these four oxidation states can all exist simultaneously in aqueous solution at appreciable concentration. As a metal, also, its properties are unique. Metallic plutonium has six allotropic forms, in the temperature range from room temperature to its melting point (640 C), and some of these have properties not found in any other known metal. 

The known oxidation states of plutonium present a 5f -series, starting from f1 [Pu(VII)] up to f5 [Pu(III)]. But contrary to the 4f - and 5f series across the period table, where the properties can be described by some smooth varying parameters, changing of the oxidation states influences the electronic properties drastically. Due to the large range of available oxidation states plutonium represents a favorable element among the actinides to study these effects. 

The investigation of plutonium chemistry in aqueous solutions provides unique challenges due in large part to the fact that plutonium exhibits an unusually broad range of oxidation states -from 3 to 7-and in many systems several of these oxidation states can coexist in equilibrium. Following the normal pattern for polyvalent cations, lower oxidation states of plutonium are stabilized by more acidic conditions while higher oxidation states become more stable as the basicity increases. 

In studies where different oxidation states of plutonium have been complexed by the same ligand, the sequence of complex-ing strength most commonly observed is that described for the... 

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). 

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. 

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. 

Research into the aquatic chemistry of plutonium has produced information showing how this radioelement is mobilized and transported in the environment. Field studies revealed that the sorption of plutonium onto sediments is an equilibrium process which influences the concentration in natural waters. This equilibrium process is modified by the oxidation state of the soluble plutonium and by the presence of dissolved organic carbon (DOC). Higher concentrations of fallout plutonium in natural waters are associated with higher DOC. Laboratory experiments confirm the correlation. In waters low in DOC oxidized plutonium, Pu(V), is the dominant oxidation state while reduced plutonium, Pu(III+IV), is more prevalent where high concentrations of DOC exist. Laboratory and field experiments have provided some information on the possible chemical processes which lead to changes in the oxidation state of plutonium and to its complexation by natural ligands. 

These various broad research observations generated questions about the influence of chemical environments in aquatic systems upon plutonium and what chemical species might be present. The oxidation states of plutonium, its associations with DOC, and its complexation by inorganic ions all seemed interrelated and important to the understanding of environmental transport. 

The ability to identify the oxidation states of plutonium and measure their concentrations in environmental samples has improved markedly in resolution since the start of our research program. 

The Oak Ridge technique employs Ti02 powder to adsorb Pu(V). This procedure has been used to characterize the oxidation state of plutonium in Pond 3513 as predominantly Pu(V). 

An important feature of both flowsheets is that the oxidation state of plutonium does not have to be adjusted because all three states, III, IV, and VI, extract. Another feature of the flowsheets is that Am(III) and Cm(III) follow Pu through the separation scheme and thus behave as denaturing nuclides. 

The various oxidation states of plutonium exhibit characteristic absorption spectra in the ultraviolet, visible and infrared regions. Each oxidation state is sufficiently distinct that its reaction can be monitored during hydrolysis and complex formation. Various research groups have studied the relationship between oxidation and absorption spectra (6-9). The absorption spectra may respond to complex formation or hydrolysis Nebel (10) has shown that the absorption peak of Pu(IV) shifts from 470 nm to 496 nm when Pu(IV) complexed with two molecules of citrate. 

Other factors which regulate disproportionation reactions include the presence of complexing ions which stabilise one particular oxidation state of plutonium. 

The ionic species corresponding to the four oxidation states of plutonium vary wifli the acidity of the solution. In moderately strong < one-molar) acid the species are Pu11, Pu4+, PuO ,1, and PuO 2 E The 10ns are hydrated but it is not possible at present to assign a definite hydration to each ion. The potential scheme of these ions in one-molar perchloric acid is tile following ... 

Peretrukhin, V F. Alekseeva, D. P., "Polarographic Properties of Higher Oxidation States of Plutonium in Aqueous Alkali Solutions, Sov. Radiochem.. 1974, 16 823-828... 

Wahlgren, M A., Alberts, J.J., Orlandini, KA. and Kucera, E.T. (1977) Study of the occurrence of multiple oxidation states of plutonium in natural water systems. Argonne National Laboratory, Chicago. Annual Report ANL 77-65 Pt III, pp. 92-94. 

The data for Pu(III)8 are approximate estimates since the authors faced the problem of maintaining the trivalent oxidation state of plutonium. 

Hydration and Hydrolysis. The various oxidation states of plutonium form strong ion-dipole bonds with water to become strongly hydrated in aqueous solution. To a first approximation, we can expect the hydration numbers of the first coordination sphere to be the same as the most probable coordination numbers suggested in the preceeding section. This means values of 8 or 9 for Pu(lll), 7 or 8 for Pu(Vl), and, perhaps, 4 for PuOj and 6 for PuOj. However, the polarization of the water dipoles of the primary hydration layer leads to attraction of additional waters of hydration. Estimates of the total number of waters of hydration for trivalent lanthanides and actinides have been given as 12 - 15 model of a small number of... 

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