Uranyl carbonate, aqueous solutions

The elucidation of actinide chemistry in solution is important for understanding actinide separation and for predicting actinide transport in the environment, particularly with respect to the safety of nuclear waste disposal.72,73 The uranyl CO + ion, for example, has received considerable interest because of its importance for environmental issues and its role as a computational benchmark system for higher actinides. Direct structural information on the coordination of uranyl in aqueous solution has been obtained mainly by extended X-ray absorption fine structure (EXAFS) measurements,74-76 whereas X-ray scattering studies of uranium and actinide solutions are more rare.77 Various ab initio studies of uranyl and related molecules, with a polarizable continuum model to mimic the solvent environment and/or a number of explicit water molecules, have been performed.78-82 We have performed a structural investigation of the carbonate system of dioxouranyl (VI) and (V), [U02(C03)3]4- and [U02(C03)3]5- in water.83 This study showed that only minor geometrical rearrangements occur upon the one-electron reduction of [U02(C03)3]4- to [U02(C03)3]5-, which supports the reversibility of this reduction. 

Goishi and Libby have investigated the extraction of pertechnetate from alkali solutions with pyridine. Later work showed that a better extraction is obtained using a mixture of sodium hydroxide and sodium carbonate as the aqueous phase. Since the uranyl carbonate complex is not extracted into pyridine, this system may be used for the separation of technetium from uranium. Distribution coefficients of fission products in pyridine are given in Table 4. Substituted pyridine such as 2,4-dimethylpyridine or 4-(5-nonyl)pyridine ) are useful for separating technetium from solutions containing appreciable amounts of aluminum nitrate. 

This solid-phase disappears at pH 7.5 and U species including carbonate is formed above pH 7. The dominant species in the pH range of 7-9 and above 9 are (U02)2C03(0H)3" and 1102(003)3, respectively. The aqueous phases of uranyl ion, uranyl hydroxyl carbonate and uranyl carbonate are formed as the pHs of solution increase. The solid-phase is uranyl hydroxide around pH 7. It is found the equilibrium model calculations that the dominant species at pHs 5.5 or below is uranyl ion although the CO2 conditions were varied. Uranium is precipitated as a hydroxide form of 3 H02(0H)2(s) at a neutral pH. The aqueous phase of uranium hydroxide, hydroxyl carbonate and carbonate are dominant species at a high pH. These species have anionic charge. 

Similarly, many tris(R-carbamate)dioxouranium(VI) complexes (exemplified by K[U02(Et2NCS2)3] H20) have been reported. The complexes are prepared by reacting uranyl acetate with the R-carbamate salt obtained by addition of R-amine to an aqueous solution containing equimolar amounts of carbon disulfide and potassium hydroxide. ... 

The U02 - XOs - H2O (X= C or N) systems have been chosen for analysis for two reasons. First, they are well studied experimentally, since carbonate and nitrate uranyl complexes are important from technological standpoints. Second, the isoelectronic anions XOs have the same planar structure (symmetry Dsh) and form bidentate coordination around uranyl ions (the type). However, their electron-donor characteristics are different El = 3.1 and 3.4 for the NOs and COs ions, respectively. From this viewpoint, it would be of interest to understand how the difference in the electron-donor properties influences complex formation in the U02 - XOs " - H2O systems. As in the case of aqua-complexes, we shall use the 18-electron rule to obtain answers to the following questions (a) what is the composition of stable complexes in aqueous solutions containing carbonate or nitrate uranyl complexes (b) what is the coordination number of U(VI) in these complexes. 

Ammonium uranyl carbonate (AUC) process This process was developed in the 1960 s in the Federal Republic of Germany. It comprises the simultaneous feeding of uranium(Vl) fluoride, carbon dioxide and ammonia into an aqueous ammonium carbonate solution at 70°C, whereupon tetra-ammonium tricarbonato-dioxo-uranate (ammonium uranyl carbonate) precipitates out ... 

Addition of an alkali carbonate to an aqueous solution of a thorium salt first precipitates a basic thorium carbonate of variable composition. Like uranyl carbonate, thorium carbonate dissolves in an excess of alkali carbonate, in this case forming the complex ion [Th(COs)4(OH)2] -. 

BAN/GLA] Banyai, I., Glaser, J., Micskei, K., Toth, I., Zekany, L., Kinetic behavior of carbonate ligands with different coordination modes Equilibrium dynamics for uranyl(2+) carbonato complexes in aqueous solution. A C and 0 NMR study, Inorg. Chem., 34, (1995), 3785-3796. Cited on page 349. 

Dispersion of artificial amphiphile molecules for electron microscopy were carried out by dissolving several mg of artificial amphiphiles in deionized water. This water solution was added to an almost equal volume of 2% aqueous solution of uranyl acetate. The solution mixture was placed on a carbon grid for an electron microscopic observation. Positively charged artificial amphiphiles were negatively stained by uranyl acetate. 

The starting material for the fabrication of UO2 standard fuel is enriched UFe, which is supplied by the enrichment plant in pressure cylinders (the preceding steps of fabrication starting from uranium ore have been described, among others, by Peehs, 1996). From this material, the UO2 compound is made either by a wet or by a dry process. Among the wet processes, the ADU (ammonium diuranate) process is mainly used, in which UFe gas is hydrolyzed in aqueous ammonia solution to form ammonium diuranate. Besides ADU, the AUC (ammonium uranyl carbonate) process is applied in which UFe is hydrolyzed in an aqueous solution... 

Complicating the development of ISEs for higher actinide ions is their inherent radioactivity. They also have chemistry tiiat often differs from that of the uranyl cation. Actinides from americium to lawrencium display solution-phase chemical features that resemble those of the trivalent lanthanides. Conversely, in certain oxidation states, the early actinides (thorium through neptunium) often mimic transition metals. Also, as mentioned above, many of the actinides can exist in a large number of oxidation states. For instance, in the case of plutonium, four oxidation states can exist simultaneously in aqueous solution. Finally, as true for the lanthanides, complex salts with hydroxide, halogens, perchlorates, sulfates, carbonates, and phosphates are well known for most of the actinides. 

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