Plutonium oxidation state studies

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). 

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. 

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). 

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 only crystalline phase which has been isolated has the formula Pu2(OH)2(SO )3(HaO). The appearance of this phase is quite remarkable because under similar conditions the other actinides which have been examined form phases of different composition (M(OH)2SOit, M=Th,U,Np). Thus, plutonium apparently lies at that point in the actinide series where the actinide contraction influences the chemistry such that elements in identical oxidation states will behave differently. The chemistry of plutonium in this system resembles that of zirconium and hafnium more than that of the lighter tetravalent actinides. Structural studies do reveal a common feature among the various hydroxysulfate compounds, however, i.e., the existence of double hydroxide bridges between metal atoms. This structural feature persists from zirconium through plutonium for compounds of stoichiometry M(OH)2SOit to M2 (OH) 2 (S0O 3 (H20) i,. Spectroscopic studies show similarities between Pu2 (OH) 2 (SOO 3 (H20) i, and the Pu(IV) polymer and suggest that common structural features may be present. 

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... 

Measurement of the stability constants of plutonium complexes is hampered by difficulties of maintaining a particular oxidation state. Formation of complexes of Pu+3, except in very acid solutions, is accompanied and often obscured by complexation catalyzed oxidation to Pu+lt. Study of complexation of Pu+lt is often confused by competition with hydrolysis above pH 1-2. 

Prediction of the chemistry of plutonium in near-neutral aqueous media is highly dependent on understanding reactions that may be occurring in such media. One of the most important parameters is the stability and nature of complexes formed by plutonium in its four common oxidation states. Because Pu(III), Pu(IV), and Pu(VI) are readily hydrolysed, complexation reactions generally are studied in mildly to strongly acidic media. Data determined in acid media (and frequently at high concentrations of plutonium) then are used to predict the chemical speciation of plutonium at near-neutral pH and low concentrations of the metal ion. 

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,, ]) ... 

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. 

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. 

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. 

Many oxidation states of the actinides are poorly stable or stable only under certain conditions. Great care must thus be taken in preparing samples for relaxometry studies. Working under the same chemical conditions with different actinides in the same oxidation state is sometimes impossible. Plutonium is particularly noteworthy because it is the only element in the Mendeleev table that can exist simultaneously in solution in four different oxidation states. This unusual situation stems from the fact that the ions and PuO have a tendency to undergo dismuta-... 

The oxidation-reduction behaviors of neptunium, plutonium and americium in basic solution have been determined via polarographic and coulometric studies (6-9). These studies, which showed that the more soluble (V), (VI), and (VII) oxidation states of these actinides are stable in alkaline solution under certain redox conditions, helped identify possible actinide species and oxidation states in our experiments. Actual identification of radioelement oxidation states was not done in the present experiments. 

Studies of the oxidation-state behaviour of plutonium in environmental waters illustrate this situation very clearly. 

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. 

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. 

Since the uranyl ion is so obviously its own category, it is very interesting to compare with the analogous species formed by transuranium elements. M = Np, Pu and Am form all three MO 72 and MO which are, by no means, the most stable oxidation states of their elements, and which tend toward reduction by the radiochemical products concomitant with the high specific radioactivity of the isotopes normally studied of plutonium and americium (whereas e.g. 244Pu with the half-life 82 million years would not present this problem). Contrary to some reports in literature, it does not seem that curium (and the subsequent elements) form such dioxo complexes. 

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