Plutonium citrate

Fig. 7-11. Effect of raindrop size and acidity on the retention of (a) plutonium-citrate and (b) plutonium-nitrate by Phaseolus vulgaris foliage (redrawn from Cataldo et at., 1981).

Nebel, D. Potentiometric studies of the equilibrium of plutonium citrate in an aqueous solution, Physik. Chem. (Leipzig), 232, 368 (1966). 

Investigations of the radiation effects of plutonium in laboratory animals indicated that translocation of plutonium from the lungs to other tissues was dependent on several factors including the solubility of the plutonium isotope or compound. Translocation to the bone occurred with plutonium citrate and with plutonium nitrate (Bair et al. 1973). By 4,000 days post-exposure, osseous atrophy and radiation osteodystrophy occurred in dogs given a single inhalation exposure to plutonium-238 dioxide (Gillett et al. 1988). The dose which resulted in these specific effects was not reported. For further discussion of this study see Section 2.2.1.8. 

Absorption of plutonium was slightly increased when administered in a citrate or nitrate solution and when administered as a very acidic solution (Weeks et al. 1956). Absorption of 0.003 to 0.01% of the administered plutonium citrate or nitrate has been reported in rats and hamsters (Carritt et al. 1947 David and Harrison 1984 Katzetal. 1955 Stather etal. 1981). 

Gastrointestinal absorption increased when plutonium was administered on an empty stomach. In hamsters that had been fasted for 8 to 24 hours, absorption increased to 0.1 to 0.15% of the administered plutonium citrate or ascorbate compared to 0.01 % in animals which had not been fasted (Harrison et al. 1986). 

From injection studies in laboratory animals it was found that retention was dependent on the isotope, chemical form, and sex. In dogs plutonium-239 was retained longer than plutonium-237 (Bair et al. 1974). The retention of plutonium-242 and plutonium-244 was similar, and was longer than the retention time for plutonium-236 and plutonium-239 (Guilmette et al. 1978). In mice no difference was seen in fractional retention at low and high doses (Andreozzi et al. 1983). In hamsters more plutonium administered intravenously in an insoluble form (plutonium dioxide) was retained than plutonium administered in a soluble form (plutonium citrate) (Brooks et al. 1976b). Retention after intraperitoneal injection of mice and hamsters may be sex-dependent females retained more in the liver than males (Smith et al. 1976, 1978). However, retention after intravenous injection was not sex-dependent (Smith et al. 1978). Total retention and liver retention increased with age (Bruenger et al. 1980 David and Harrison 1984). 

Certain of the actinide compounds, e.g., plutonium citrate complexes, are soluble, are absorbed (although even here absorption is very low) and dissociate in the body giving a relatively high fractional retention so that actinides in these initial physicochemical forms are bioavailable to a measurable extent. 

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. 

With the aid of ultrafiltration techniques Lindenbaum and Westfall (15) showed that 92% of the plutonium solution (1.96 x 10 sm) was ultrafilterable in the presence of 3.4 x 10 2m citrate over a pH range of pH 4.0 to 10.0. However, when the citrate concentration was lowered to 1.96 x 10-SM the ultrafilterability was only 77 % for the range pH 4.0 to 8.5 and at pH 11.0 only 5 %. Lindenbaum and Westfall (14) have also demonstrated that citrate could bring about a resolubilisation of the colloid. Equimolar concentrations of Pu(IV) and citrate were adjusted to pH 11.0 and then one hour later the solution was adjusted to either pH 7.8 or pH 4.0. Within... 

In all cases a significant fraction of the plutonium remained immobilised on the leaves, both citrate and nitrate showed a 60—90% loss in solubility. It is possible that the reduced leachability of the plutonium may have been due to the entrapment of the compounds in crevasses on the leaf surface, engulfment by expanding epidermal plates, or by adsorption onto unsatisfied valences. 

Catalado and Craig (134) demonstrated that the maximum accumulation of plutonium occurred when the plant had attained its maximum growth. However, Catalado and Craig did not demonstrate that this maximum accumulation of plutonium was the result of elevated citrate levels in the plant tissue. The presence of citrate in the leaves could exert a significant alteration in the foliar transport of Pu02 since it has been shown that citrate (10-4m) can bring about a rapid solubilisation of small particles of Pu02 (38). 

At pH 7 33% of the plutonium was not readily desorbed from particles. However, it is possible that complexing agents could alter the affinity of silicates for different cations. The natural affinity of kaolins is Th > La > Ca > K, but in the presence of citrate or fluoride the order is reversed (156). Presumably these complexing agents will also alter the binding of plutonium to the silica particles. 

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). 

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... 

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

Yang W (2008) An equivalent metal ion in one- and two-metal-ion catalysis. Nat Struct Mol Biol 15 1228-1231 Yong P, Macaskie EE (1997) Removal of lanthanum, uranium and thorium from the citrate complexes by immobilized cells of Citrobacter sp. in a flow-through reactor implications for the decontamination of solutions containing plutonium. Biotechnol Lett 19 251-255... 

The limited success with sodium citrate led to the trial and error testing of other chelating agents. Despite the fact that hard Lewis acids such as plutonium do not bind strongly to sulfur ligands, the success of 2,3-dimercapto-l-propanol, BAL, as an efficient chelator for arsenic79) led to testing its ability to remove actinides. As expected on a chemical basis, excretion of plutonium was not enhanced by treatment... 

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