Plutonium bicarbonate solutions

Plutonium dioxide microspheres (149 to 177-ym diam) were contacted with 10"3 M biocarbonate buffer (pH 7) for various periods of time. Some of the microspheres had been pre-incubated in pH 4 acetate buffer for two years prior to the bicarbonate experiments. Other microspheres were contacted with bicarbonate solution without any prior contact with water. The microspheres were primarily 239Pu (88.9% of Pu mass). The microspheres had been calcined at 1150°C and had a specific surface area of 0.012 m2/gram. 

Table III. Plutonium Species Identified in pH 7.0 Sodium Bicarbonate Solutions Contacting High-Fired Pu02 Microspheres...

In rats and dogs following absorption of plutonium from the gastrointestinal tract, up to 95X of the absorbed dose has been found to be distributed to the skeleton (Carritt et al. 1947 Larsen et al. 1981 Toohey et al. 1984). Plutonium was also distributed to a less extent to the liver, carcass, and soft tissues (Carritt et al. 1947 Katz et al. 1955 Larsen et al. 1981 Sullivan et al. 1984). The distribution of plutonium-237 in a bicarbonate solution administered via a gelatin capsule was greatest to the axial skeleton (Toohey et al. 1984). 

Most of plutonium administered to dogs in a bicarbonate solution by the oral route was eliminated in the feces, with an average excretion of 98% of the administered dose after 5 or 6 weeks (Toohey et al. 1984). In mice and rats total retention of plutonium varied from 0.17 to 0.24% of the administered... 

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). 

In experiments where Mono Lake water was acidified to remove carbonate and bicarbonate ions and again adjusted to pH 10, more than 90 percent of the soluble plutonium moved to the sediment phase. When carbonate ion concentration was restored, the plutonium returned to solution—strong evidence of the importance of inorganic carbon to solubility in that system(13). Early studies with Lake Michigan water, which has low DOC, had also implicated bicarbonate and carbonate as stabilizing ligands for plutonium at pH 8(14). This latter research characterized the soluble species as mainly anionic in character. 

To determine if bicarbonate has similar effects on the nature of the radiocolloid, the plutonium size distributions were studied at pH 7 as a function of bicarbonate concentration, using bicarbonate only from atmospheric CO.j and in solutions with [CH(V] equal to 10"2Af. The latter were prepared by adding NaHCOa. Two different ionic strengths were used to distinguish between the effects of added bicarbonate and ionic strength. The distributions of identical solutions which were shaken in flasks with and without granular silica were determined initially and... 

The bicarbonate ion, HC03, is a prevalent species in natural waters, ranging in concentrations up to 0.8 X 10 3. As was indicated previously, carbonate ions have the ability to form complexes with plutonium. Starik (39) mentions that, in an investigation of the adsorption of uranium, there was a decrease in the adsorption after reaching a maximum, which was explained by the formation of negative carbonate complexes. Kurbatov and co-workers (20) found that increasing the bicarbonate ion concentration in a UXi (thorium) solution decreased the amount of thorium which formed a colloid and became filterable. This again was believed to be caused by the formation of a soluble complex with the bicarbonate. 

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). 

The sorption and desorption of aqueous plutonium in the range of 10 7 to 10 8M was studied on quartz and other silica surfaces. Sorption continued typically for 12 to 15 days before apparent equilibrium was reached, and the distribution of plutonium particle sizes sorbed on the silica was different from that in solution. At pH 7, sorption increased with increasing ionic strength, but decreased when bicarbonate was added. The amount of sorption varied at pH 5 and 7, but differently at high and low ionic strengths, as well as with the age of the solution. Plutonium desorption indicated that there were two basically different sorbed species, and the rate and quantity of desorbed material increased at pH 5 compared with 7 and 9. 

In order to determine if bicarbonate may similarly affect colloidal plutonium, a comparison was made of sorption from solutions containing bicarbonate only from atmospheric CO2 and from those with added 10"2Af bicarbonate. Two different ionic strengths were used in order to distinguish adequately between and rule out the possibility of an ionic strength effect. The results of the study are given in Table IV. When the bicarbonate concentration was increased from the equilibrium atmospheric value to 10 2M, the sorption coefficient greatly decreased at both ionic strengths. 

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