Actinides solution behavior

Schmidt, C., Saadioui, M., Bohmer, V. et al. 2003. Modification of calix[4]arenes with CMPO-functions at the wide rim. Synthesis, solution behavior, and separation of actinides from lanthanides. Org. Biomol. Chem. 1 4089 4096. 

Aqueous carbonate complexes of An ions, An(C03) ", =1—5, form stepwise with increasing solution pH and carbonate concentration." As with other oxoanionic ligand systems, the stability of the carbonate complexes decreases across the series, such that the pentacarbonato complex is well studied for Th and U. The tetracarbonato complex is more important for Np and Pu in solution, although salts of the pentacarbonato anion are known across the series. Most studies of Th, U, Np, and Pu do indicate that mixed hydroxyocarbonate complexes, An(0H) t(C03) " " e.g, Th(0H)3(C03) for Th, are important in describing the aqueous solution behavior. For the lower order carbonates the actinide is presumably nine or ten-coordinate with waters and bidentate carbonate in the inner coordination sphere. For the penta-and hexacarbonato complexes there is no evidence that any water molecules remain bound to the actinide center. 

With no supporting fundamental data, reactor technologists concerned with the chemistry of reactor fuels could merely speculate on the behavior of the actinide solutions should they be exposed to electromagnetic radiation. Then as fuel reprocessing became a topic of much interest during the past decade, the concern of possible photochemical activity began to occupy the thoughts of some researchers. 

Solution Behavior of Actinides in White Oak Lake Water... 

Results and Discussion. Changes in concentrations of the actinides in solution in the blank containers are shown in Figure 9. A large fraction of the Pu, Am and Cm was removed from solution while only a small amount of the U and none of the Np was lost from solution. Since the starting concentration of Pu exceeds the solubility for the hydroxide (or hydrated oxide), the Pu probably precipitated as colloidal-size particles (2) the behavior of Am and Cm cannot be explained by a similar mechanism. However, there is recent experimental evidence that indicates the solubility products for Am and Cm carbonates may be of the order 10 ( ). If this is the case, there was sufficient... 

Despite the extremely low concentrations of the transuranium elements in water, most of the environmental chemistry of these elements has been focused on their behavior in the aquatic environment. One notes that the neutrality of natural water (pH = 5-9) results in extensive hydrolysis of the highly charged ions except for Pu(V) and a very low solubility. In addition, natural waters contain organics as well as micro- and macroscopic concentrations of various inorganic species such as metals and anions that can compete with, complex, or react with the transuranium species. The final concentrations of the actinide elements in the environment are thus the result of a complex set of competing chemical reactions such as hydrolysis, complexation, redox reactions, and colloid formation. As a consequence, the aqueous environmental chemistry of the transuranium elements is significantly different from their ordinary solution chemistry in the laboratory. 

Other research has focused on a phenomenon that is important but outside the subject of this review the influence of radiolysis on the redox behavior of actinide ions in nitric acid aqueous solution or in extraction systems (116 119, 317-320). 

The oxidation-reduction behaviors of neptunium, plutonium and americium in basic solution have been determined via polarographic and coulometric studies (6-9). These studies, which showed that the more soluble (V), (VI), and (VII) oxidation states of these actinides are stable in alkaline solution under certain redox conditions, helped identify possible actinide species and oxidation states in our experiments. Actual identification of radioelement oxidation states was not done in the present experiments. 

Investigation of the amalgamation behavior of trivalent actinides in acetate and citrate solutions by treatment with sodium amalgam showed that Bk(III) does not readily form an amalgam. This behavior is in contrast to that of the heavier actinides californium, einsteinium, and fermium, which readily amalgamate (216, 217). 

Figure 1 shows the extraction behavior of the actinide elements when a HLLW solution is contacted with a solution of 1 M HDEHP (di(2-ethylhexyl)phosphoric acid) in an aliphatic diluent (Nysolvin 75A). It is evident from this Figure that a good extraction of all actinide elements can not be obtained at any single acidity. This led us to the decision to first extract Pa,... 

Figure 3 shows a possible separation and recovery scheme of actinides based on the precipitation behavior of individual elements. A part of this procedure could be good enough if the starting solution contains simpler constituents. 

Pu(VI) ions in the supernatant decrease rapidly to a minimum. They increase then gradually with increasing concentration of ammonium sulfate or hexamminecobalt(III) salt. The precipitation behavior of Pu(VI) ion is similar to that of Pu(IV) ion. However, the concentration of Pu(VI) ions in the supernatant is about 30 times as high as that of Pu(IV) ions when all other conditions are kept the same. In the sulfate system, Pu(IV) and U(VI) ions can be separated from Np(V),Am(III) and Cm(III) ions. Similarly Am(VI) ion can be separated from Cm(III) ion. Thus,at initial concentration as low as 1 mg/mL more than 99% of U(VI), Pu-(IV) and Am(VI) ions can be recovered from the solution. In the sulfate system, hexamminecobalt(III) salt is a separating agent as well as a precipitant of actinide(IV) and (VI) ions. 

Actinide(V) and (VI) ions form soluble complex ions with peroxide ion in slightly alkaline medium, whereas actinide(III) and (IV) ions precipitate as hydroxides. Actinide(VI) ions in slightly alkaline hydrogen peroxide solution precipitate upon addition of cobalt(III) complex salts. Figure 7 shows the precipitation behavior of U(VI) peroxo complex ion with the following kinds of cobalt(III) complex salts ... 

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