Plutonium colloids, formation

In the BX part of the separation battery concentrations of 0,2 to 1,2 M HN0 and in the BS part of 1,2 to 1,5 M HNO must be considered as optimum acidities. In spite of that quite low acid conditions plutonium colloid formation have never been observed because plutonium exists mainly in the trivalent state. 

Natural colloid particles in aqueous systems, such as clay particles, silica, etc. may serve as carriers of ionic species that are being sorbed on the particulates (pseudocolloids). It seems evident that the formation and transport properties of plutonium pseudocolloids can not yet be described in quantitative terms or be well predicted. This is an important area for further studies, since the pseudocolloidal transport might be the dominating plutonium migration mechanism in many environmental waters. 

As trivalent americium has a smaller ionic potential than the ions of plutonium it hydrolyses to a much lesser extent than the various plutonium ions. However, like Pu3+, hydrolytic reactions and complex formation are still an important feature of the aqueous chemistry of Am3+. Starik and Ginzberg (25) have shown that Am(III) exists in its ionic form from pH 1.0 to pH 4.5 but above pH 4.5 hydrolysis commences and at pH 7.0 colloidal species are formed. The hydrolytic behaviour of Cm(III) resembles that of Am(III). 

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). 

Although it is important to understand the mechanisms in the formation of the distribution of sizes among the colloidal plutonium hydroxide particles, the distributions themselves will influence the behavior... 

Despite its ability to remove much of the soluble plutonium present in body fluids, DTPA is not an exceptional chelating agent for tetravalent actinides. The formation constant of its plutonium complex is too low to displace hydroxides from the colloids and polymers of hydrolyzed plutonium or solubilize compounds such asPuC>2 at physiological pH. In addition, the inability of DTPA to completely coordinate the... 

Pu(HP04)44 in 2M nitric acid. The formation of the phosphate complex, in effect, increases the average negative charge of the ionic or colloidal plutonium species. As in the case of the bicarbonate complex, this should reduce sorption onto the negatively charged silica surface. However, as is shown in Table V, the sorption increased. In another experiment in which the phosphate concentration was varied between zero and 1.25 X 10 2M, there was no significant difference in the sorption constants. 

Plutonium solubility in marine and natural waters is limited by the formation of Pu(OH)4(am) (for amorphous) or Pu02(c) (for crystalline). The K q of these species is difficult to measure, in part due to the problems of the polymer formation. A measured value for Pu(OH)4(am) is log = -56. This value puts a limit on the amoimt of plutonium present, even if Pu(V) or Pu(IV) are the more stable states in the solution phase. Moreover, hydrolyzed Pu(IV) sorbs on colloidal and suspended material, both inorganic and biological. 

The hydrolysis of tetravalent plutonium in aqueous solution is dominated by the formation of colloids. Even in 1 Af HCIO4 polymer formation occurs, and reliable hydrolysis constants are found only for the first step with n = 1 (5) ... 

On account of its large practical importance, polymer formation in hydrolyzed plutonium(iv) solutions has attracted much interest [203]. This polymer is formed fairly rapidly [204,205]. The reaction is faster, and more extensive, the higher the temperature. As long as ionic plutonium(iv) is present in detectable amounts, the rate of polymerization is proportional to the concentration of this component, and inversely proportional to the square of the acidity. When the ionic plutonium(iv) has been consumed, the rate depends in a rather complicated manner upon the concentration of other oxidation states present [205]. If the polymer is allowed to age, depolymerization becomes very slow even if the concentration of acid is fairly high [204]. The colloid behaves very differently from ionic plutonium(iv) in the extraction and ion-exchange procedures used in the processing of plutonium, and is also apt to transform into a precipitate. The conditions should therefore be chosen so that the formation of the colloid is... 

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