Uranyl hydroxide

The hydrolysis of the uranyl(VI) ion, UO " 2> has been studied extensively and begins at about pH 3. In solutions containing less than lO " M uranium, the first hydrolysis product is the monomeric U02(OH)", as confirmed using time-resolved laser induced fluorescence spectroscopy. At higher uranium concentrations, it is accepted that polymeric U(VI) species are predominant in solution, and the first hydrolysis product is then the dimer, (U02)2(0H) " 2 (154,170). Further hydrolysis products include the trimeric uranyl hydroxide complexes (U02)3(0H) " 4 and (1102)3(OH)(154). At higher pH, hydrous uranyl hydroxide precipitate is the stable species (171). In studying the sol-gel U02-ceramic fuel process, O nmr was used to observe the formation of a trimeric hydrolysis product, ((U02)3( -l3-0)(p.2-0H)3) which then condenses into polymeric layers of a gel based on the... 

Uranyl hydroxides, 25 430 Uranyl metaphosphates, 25 433 Uranyl nitrate, 25 428 Uranyl perchlorate crystals, 25 429 Uranyl sulfate system, 25 429 Urban air quality, 17 814—815 Urea(s), 8 206-207... 

Kim, C. W., Wronkiewicz, D. J., Finch, R. J. BUCK, E. C. 2002. Incorporation of cerium and neodymium in a uranyl hydroxide solid. Materials Research Society Symposium Proceedings, 713, 663-670. 

Addition of aqueous NH3 to U02(N03)2 solutions affords the so-called ammonium diuranate. This is mainly a hydrated uranyl hydroxide containing NHJ. Below 580°C it gives U03 and above, U3Og. 

When alkali metal bases are used to raise the solution pH to moderate levels, the uranium will precipitate from the solution in the form of hydrous uranyl hydroxides or uranates, for example, Na2U207. However, through judicious choice of a base, for example, tetramethylammonium hydroxide, (TMA)OH, or tetramethylaimnoirium trifluoromethansulfonate, the study of the amphoteric behavior of uranyl hydroxides can be undertaken. Polynuclear anions of the form (U02)3(0H)7, (U02)3(0H)g, and (U02)3(OH)io are examples of soluble species in solutions where the pH < 14. When the concentration of the (TMA)OH is increased (>0.6 M OH ), highly soluble ( 0.1M) monomers ofthe form U02(0H) "(n = 3, 4,5) have been reported. These three species are in equilibrium with each other however, in solutions where the [OH ] is greater that 1M, the pentahydroxo complex predominates the speciation. 

Equilibrium distributions of uranium species by the model calculation are shown in Figures 1 and 2. Figure 1 is the calculated result of U speciation under 0% CO2 condition. Uranium almost exists as U02 at pHs 5.5 or below. Uranium also exists as a form of UO2OH and (U02)3(0H)s between pH 5 and 6.5. Uranium is precipitated as a form of 0 -U02<0H)2(s) between pH 6 and 9. The distribution percentage of the solid-phase is over 80%. Uranium exists as U02(0H)3 at pHs 9 or above. The dominant solid-phase is uranyl hydroxide because of the CO2 free condition. The distribution of U species under air condition is shown in Figure 2. Uranium exists as U02 at pHs 5.5 or below and as U020H and (U02)3(0H)s between pH 5 and 6.5. Uranium is precipitated as species of 0 -U02(0H)2(s) between 6 and 7.5. The maximum percentage of the solid-phase is 54%. 

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. 

Orloff, Ohem. Zeit., 1907, 31, 375. Pormanek Anndkn, 1890,257,102), using uranyl hydroxide, obtained similar crystals, to which he gave the formula UOjOrOi.llHjO. 

Uranyl Selenate, UO2Se04.xH2O, is present in the glassy mass obtained by evaporating a solution of uranyl hydroxide in selenic acid. It has not been obtained in a pure state. Such solutions also yield acid salts, of which the following have been isolated ... 

Potassium Uranyl Carbonate, K4U02(C03)a, may be prepared by the action of carbon dioxide on potassium uranyl cyanide, or by evaporating at moderate temperature a solution containing potassium bicarbonate and potassium uranate. It crystallises in small hexagonal prisms, which are stable in dry air and dissolve in cold water without decomposition. The solution is hydrolysed on warming, and the addition of alkali causes j recipitation of uranyl hydroxide. 

Reaction with Water. Uranyl metaborate is virtually insoluble in water at 25°C., but the salt hydrolyzes slowly over several days to form uranyl hydroxide. 

There are a number of structurally interesting mixed-ligand uranyl hydroxides. For example, the basic compound of composition Zn(U02)2S04(00)4 1.5020, has a structure based on chains of 1102(011)302 pentagonal bipyramids containing tridentate bridging OH groups. Species of this type have also been studied in solution, but the complexity of the system has precluded structural characterization. There are many hydrated binary and ternary uranium oxides, such as the uraninites, that contain uranyl hydroxide complexes within their structure. 

Uranyl sulfate, UO2SO4 3 H20, is prepared by the crystallization of a solution of uranyl hydroxide in dilute sulfuric acid or by heating uranyl nitrate with sulfuric acid. It forms yellow-green crystals, which under the microscope show a beautiful fluorescence. On exposure to air they loose water slowly, and at 115° a monohydrate is formed while at 175° the anhydrous salt is produced. Both acid salts and double alkali sulfates are formed. 

Uranyl acetate, UOtCCbHiOi) 2 H2O, is next to the nitrate the most important uranyl salt of commerce. It is prepared by the solution of uranyl hydroxide or oxide in acetic acid. It is soluble in water, forming a solution with a density of 2.89 from which it crystallizes in fluorescent prisms. It displays the phenomenon of photalysis. 

The extraordinary stability of the UO + ion is also clear in its aqueous chemistry. If acid solutions are neutralised then, near pH 3.5, first dimers and then trimers are formed [10]. These have bridging hydroxide groups, but Raman spectroscopy shows that there is little change within the dioxo group as the condensation proceeds [11]. At pH 5 uranyl hydroxide is precipitated, the solid state structure confirming that the oxycation is intact [12]. 

One of the major outputs of these model simulations is a prediction of a sequence of secondary phases. These phases arc important because they can act as sinks for radionuclides and other toxic components of the waste, limiting their migration away from the point of emplacement. Even phases not containing such components affect the overall course of reaction via effects on pH, redox conditions, etc. Figure 1 shows the results of one simulation for reaction of spent fuel and J-13 well water at 25°C. Haiweeite (a calcium uranyl silicate), soddyitc (a uranyl silicate), and schoepite (a uranyl hydroxide) are successive (and overlapping) sinks for uranium, the dominant constituent of spent fuel. 

Red, mobile liquid d 1.563. Monomeric in benzene. Readily soluble in benzene, ether, petroleum ether, etc. Extremely sensitive to moisture forms uranyl hydroxide when hydrolyzed. Strong oxidizing agent. Readily reduced to uranium (V) ethoxide. Syn-proportionates with U (IV) ethoxide according ... 

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