Plutonium , stability constants

Despite the problems of direct experimental evaluation of plutonium stability constants, they are needed in modeling of the behavior of plutonium in reprocessing systems in waste repositories and in geological and environmental media. Actinide analogs such as Am+3, Th+, NpOj and UOj2 can be used with caution for plutonium in the corresponding oxidation states and values for stability constants of these analogues are to be found also in reference 20. 

Considering the anion concentration ranges in natural waters (Table II) and the magnitude of the corresponding plutonium stability constants (Table III), the chemistry of plutonium, as well as that of uranium and neptunium, is almost entirely dominated by hydroxide and carbonate complexation, considering inorganic complexes only (41, 48, 49). ... 

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. 

Stability Constants, Enthalpies, and Entropies of Plutonium(III) and Plutonium(IV) Sulfate Complexes... 

The physical nature of the sulfate complexes formed by plutonium(III) and plutonium(IV) in 1 M acid 2 M ionic strength perchlorate media has been inferred from thermodynamic parameters for complexation reactions and acid dependence of stability constants. The stability constants of 1 1 and 1 2 complexes were determined by solvent extraction and ion-exchange techniques, and the thermodynamic parameters calculated from the temperature dependence of the stability constants. The data are consistent with the formation of complexes of the form PuSOi,(n-2)+ for the 1 1 complexes of both plutonium(III) and plutonium(IV). The second HSO4 ligand appears to be added without deprotonation in both systems to form complexes of the form PuSOifHSOit(n"3) +. ... 

K3 is the conditional stability constant for the association of plutonium with the solid adsorber. 

Acetate complexes with Pu(III),Pu(IV) and Pu(VI). In the case of Pu(III) and Pu(IV) five acetate molecules interact with the cations. In the case of trivalent plutonium the stability constant of Pu(acetate) + has been estimated to be 5 x 1016(i<5) while that of tetravalent plutonium, Pu(acetate)j has been estimated to be between 8.06 x 1022 and 3,98 x 1022 (18). Evidence indicates that Pu(VI) forms mono, di-, tri- and tetra-acetate complexes (19). 

There have been many studies of the complex formation of plutonium with ethylene-diaminetetraacetic acid (EDTA) and DTPA. Over the range pH 1.0 to 3.0 Pu(III) formed 1 1 complex with EDTA with a stability constant of 1.3 x 1018 (22). At pH 3.3 Pu(IV) reacted with EDTA to form two complexes with Pu EDTA ratios of 1 1 and 1 2 with stability constants of 4.5 x 1012 and 1.6 x 1024, respectively. 

Hafez (24) has investigated the interactions of plutonium with DTPA. Trivalent plutonium was shown to form a complex with DTPA which was stable at pH values greater than pH 8.5. This complex had a Pu(III) DTPA ratio of 2 3. The interaction of Pu(IV) with DTPA resulted in formation of three Pu(IV) DTPA complexes which existed over a wide range of pH. From pH 1.0 to pH 5.8 the 1 1 Pu(IV) DTPA complex was quite stable and had a stability constant of 2.8 x 1023 while over the range pH 5.8 to pH 8.5 the complex existed as a 2 3 complex with a stability constant of 1018. Above pH8.5 the complex existed as an unstable 1 2 complex with a stability constant of 6.3 x 1023. 

Ammonium plutonium(iv) fluorides were obtainedby reaction of Pu with NH F in HF. The stability constants for the formation of BkCF and EsCP are very similar to those reported for AmCP", and it was concluded that the monochloro-complexes of tervalent actinides up to Es are weak and of the outer-sphere type. " ... 

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 M carbonate. These shifts indicate strong carbonate complexation. Electrochemistry, reaction kinetics, and spectroscopy of plutonium carbonates in solution have been reviewed (113). The solubility of Pu(IV) in aqueous carbonate solutions has been measured, and the stability constants of hydroxycarbonato complexes have been calculated (Fig. 6b) (90). 

Plutonium(III), (IV), and (VI) complex stability constants have been determined for some oxygen-donor (carboxylate) (114) and a few nitrogen-donor (115,116) ligands. Complexes of plutonium with natural complexants such as humic acids have also been studied extensively (89). 

Figure 7. Solubility of Plutonium as a Function of Eh at 25°C. (The upper patterned area represents plutonium solubility in Grande Ronae groundwaters using the published value for the stability constant of PuCO,2+. The lower patterned area represents plutonium solubility in the absence of PuC032+ species.)...

Ki and K2 are conditional stability constants for the 1 1 and 2 1 complexes of plutonium with organic ligands. 

Carbonate Complexes. Of the many ligands which are known to complex plutonium, only those of primary environmental concern, that is, carbonate, sulfate, fluoride, chloride, nitrate, phosphate, citrate, tributyl phosphate (TBP), and ethylenediaminetet-raacetic acid (EDTA), will be discussed. Of these, none is more important in natural systems than carbonate, but data on its reactions with plutonium are meager, primarily because of competitive hydrolysis at the low acidities that must be used. No stability constants have been published on the carbonate complexes of plutonium(III) and plutonyl(V), and the data for the plutoni-um(IV) species are not credible. Results from studies on the solubility of plutonium(IV) oxalate in K2CO3 solutions of various concentrations have been interpreted to indicate the existence of complexes as high as Pu(C03) , a species that is most unlikely from both electrostatic and steric considerations. From the influence of K2CO3 concentration on the solubility of PuCOH) at an ionic strength of 10 M, the stability constant of the complex Pu(C03) was calculated (10) to be 9.1 X 10 at 20°. This value... 

Numerous determinations have been made of the stability constants of sulfate complexes of plutonium(IV), and the results vary by three orders of magnitude. The most plausible values (14) have come from careful ion exchange studies at I = 2 M and 25°, which indicate the presence of two species, Pu(S0i+) and Pu(S0i )2, with stepwise stability constants of 6.6 X 10 and 5.8 X 10, respectively. This Ki is in satisfactory agreement with the value 4.6 X 10 obtained potentiometrically at I = 1 M and 25° ( ). Spectrophotometric and electrophoretic studies indicate that plutonyl(VI) forms complexes containing as many as four sulfate groups, with anionic species predominating at sulfate concentrations above 1 M (15). The monosulfato complex has a reported stability constant of 14.4 as determined by extraction studies at I - 2 M and 25° (16). 

Fluoride Complexes. Fluoride is known to complex plutonium strongly, but quantitative data on these environmentally important complexes are limited. Cation exchange studies (17) yielded values of 4.5 X 10 at I = 1 M and 7.9 X 10 at I = 2 M for -the stability constant of the monofluoro complex of plutonium(IV), which are in satisfactory agreement with the value 1.2 X 10 obtained from... 

Phosphate Complexes. Of all the systems described in this paper, the most difficult to evaluate is that of the phosphate complexes of plutonium. Many of the stability constants were calculated from solubility measurements, which often yield data of dubious accuracy. The values that have been obtained are reported in Table II, and many appear to be too high, a common error in solubility determinations. The ion exchange values for the acid phosphate complexes of plutonium(III) are the most plausible. The data for plutonium(IV) and plutonyl(VI) should be used with caution. 

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