Complexes of plutonium

Plutonium(III), (IV), and (VI) complex stabiUty 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). 

Raymond DP, Dufield JR, Williams DR. 1987. Complexation of plutonium and thorium in aqueous environments. Inorg Chim Acta 140 309-313. 

Although the general effect of the addition of bicarbonate was to increase the size of the colloidal species, Lindenbaum and Westfall obtained the opposite effect with citrate addition over the pH range 4-11, as measured by the percent of plutonium (IV) that was ultrafilterable (22). However, their plutonium concentrations were 2 X 10 5Af, and the solutions probably contained true colloids, rather than pseudocolloids, if one accepts Davydovs analysis. Lindenbaum and Westfall concluded that the mechanism of the citrate action was the complexation of plutonium, thereby preventing the formation of hydrolytic polymers. It should be noted, however, that even with a citrate-plutonium molar ratio of 1800 (3.4 X 10 4Af citrate), about 10% of the plutonium still could not pass through the ultrafilter for solutions aged up to four days (22). 

Some authors have studied the effects of treatment with solid sorbents. Activated alumina was found to be very effective for secondary cleanup after alkaline scrubbing to remove compounds responsible for the decrease of interfacial tension and the complexing of plutonium. The drying of the solvent improves the capacity of activated alumina (102, 150, 151). 

Step 9. Add 1 mL of 50% NaNOz solution to the dissolved sample in the flask [or to the small sample from Step 2] to adjust the Pu oxidation state to +4 and form the anionic nitrate complex of plutonium. Swirl to mix and let sample stand overnight, or at least two hours if the sample must be rushed. The sample is now approximately 50 mL of 8 M HN03, and ready for the plutonium column. 

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... 

Sulfate Complexes. Knowledge of the sulfate complexes of plutonium(III) and (IV) has advanced somewhat in recent years. 

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... 

Nitrate Complexes. Although there is spectrophotometric evidence for the existence of nitrate complexes of plutonium(III),... 

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. 

Table III Stability Constants of Citrate Complexes of Plutonium(IV) at I = 0.5 M and 25°...

EDTA Complexes. Ethylenediaminetetraacetic acid (EDTA) and its homologues form the most stable known complexes of plutonium. This discussion will be limited to EDTA, which is most likely to be found in the environment as a result of its use as a medium for the addition of soluble iron to soils. The equilibrium constant for formation of the 1 1 chelate of plutonium(III), as given by the expression... 

Moskvin, A. I., and Gel man, A. D. Determination of the composition and instability constants of oxalate and carbonate complexes of plutonium(IV), Russ. J. Inorg. Chem., 2>... 

Grenthe, I., and Noren, B. On the stability of nitrate and chloride complexes of plutonium(IV), Acta Chem. Scand., 14, 2216 (1960). 

Ghosh Mazumdar, A. S., and Swaramakrlshnan, C. K. A study of the nitrate and the chloride complexes of plutonium(VI) by solvent extraction technique using TTA as the chelating agent, Inorg. Nucl. Chem., 21, 2423 (1965). ... 

Cauchetier, P., and Guichard, C. Electrochemical and spectro-photometric study of the complexes of plutonium ions with EDTA. I. Plutonium(III) and (IV), Radiochim. Acta, 19, 137 (1973). 

A similar scheme has been used to remove Zr and Nb while concentrating plutonium [15]. In this case use is made of the strong dependence of the stability of nitrate complexes of plutonium on the concentration of HNO3. Nitrate complexes are sorbed from concentrated (6 mol/L) solution of HNO3 on anion exchanger introduced in the NO3 form. At lower concentrations of HNO3 the complexes dissociate and the sorbed ions are displaced by nitrate ion in the diluted solution (1 mol/1 of HNO3) employed for flow reversal in column II. 

The extraction chromatography of the actinides has been reviewed and the treatment of zirconium phosphate sorbent for use in the separation of actinides has been described. An ion exchange study of the sulphate complexes of plutonium(iii) at 25 °C in IM and 2M perchloric/sulphuric add media found no evidence for the participation of protonated hgands in the complex formation. In the range... 

---