Uranyl complex ions

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... 

Figure 3. Distribution of uranyl complexes as a function of pH at 25°C in presence of typical ligands in surface and ground waters (PCO2 = 10 atm, E F = 0.3 ppm, E Cl = 10 ppm, E SO4 = 100 ppm, E PO4 = 0.1 ppm, E Si02 = 30 ppm). Below pH 4-5 uranyl (U02 ) ion and uranyl fluoride complexes predominate, at intermediaiy pHs (4.5 < pH < 7.5) U02(HP04)2 is the predominant species, whereas at higher pH uranyl is complexed with carbonates. [Used with permission of Elsevier Science, from Langmuir (1978) Geochim Cosmochim Acta, Vol. 42, Fig. 11, p. 558].

Vidali, M. et al J. Inorg. Nucl. Chem., 1975, 37, 1715-1719 A series of uranyl complexes of macrocyclic azomethines were used as ligands for transition metal ions, with perchlorate anions. Raman spectra of the uranyl-metal complexes could not be recorded because the samples exploded during attempted measurements. 

At higher temperatures the monomer is the predominant species although the rate of hydrolysis to U03 is increased. U03 dissolves in uranyl solutions to give U02OH+ and polymerised hydroxo-bridged species. Polynuclear species could arise from U4+ as it hydrolyses in dilute acid solutions. Complex ions are formed with thiocyanate, phosphate, citrate and anions of other organic acids. 

The U(IV) chemistry is similar to that of Th4+, except for the difference in the charge/radius ratio of the ions. U4+ solutions are green in color, stable, and slowly oxidized by air to U02+. Solutions of U4+ are generally prepared by reduction of solutions of the uranyl (U02+) ion. U(IV) forms complexes with many anions (C204-,C2H302, C03-, Cl-, and NO3"). The chlorides and bromides of U(IV) are soluble while the fluorides and hydroxides are insoluble. In aqueous solution, U(IV) hydrolyzes via the reaction,... 

One of the most important uranyl complexes is the nitrate, which crystallizes with six, three, or two molecules of water, depending on whether it is obtained from dilute, concentrated, or fuming nitric acid. In each case, there are two bidentate nitrate ions and two water molecules coordinated equatorially. The anhydrous... 

Appendix C contains the chemical formulae for the minerals used in this book. There are very few minerals that have the ideal crystalline structures discussed above. There are sufficient substitutional impurities, crystal defects, and distortions that make the CBPC structure significantly different from the models discussed above. Several well-established minerals exhibit these features, as are many of those listed in Appendix C. For example, Ca(UO2)2(PO4)2T0H2O is formed by the substitution of Ca in autunite by uranyl (UO2) ions, making the autunite a mineral of radioactive uranium. Similarly, (Ce,Th)P04 is formed by the substitution of the Ce in monazite by Th. Numerous minerals can be formed by substitutions and provide a researcher sufficient degree of freedom to synthesize very complex minerals to produce useful CBPCs. 

The oxidation-reduction behavior of plutonium is described by the redox potentials shown in Table I. (For the purposes of this paper, the unstable and environmentally unimportant heptavalent oxidation state will be ignored.) These values are of a high degree of accuracy, but are valid only for the media in which they are measured. In more strongly complexing media, the potentials will change. In weakly complexing media such as 1 M HClOq, all of the couples have potentials very nearly the same as a result, ionic plutonium in such solutions tends to disproportionate. Plutonium is unique in its ability to exist in all four oxidation states simultaneously in the same solution. Its behavior is in contrast to that of uranium, which is commonly present in aqueous media as the uranyl(VI) ion, and the transplutonium actinide elements, which normally occur in solution as trlvalent... 

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