Uranyl polynuclear complexes

This paper is devoted to the sorption of uranyl, which exhibits a complex aqueous and surface chemistry. We review briefly the sorption behaviour of An in the environment, and illustrate the variety of environmental processes using published data of uranyl sorption in the Ban-gombe natural reactor zone. After summarizing the general findings of the mechanisms of An sorption, we then focus particularly on the current knowledge of the mechanisms of uranyl sorption. A major area of research is the influence of the aqueous uranyl speciation on the uranyl surface species. Spectroscopic data of U(VI) sorbed onto silica and alumina minerals are examined and used to discuss the role of aqueous uranyl polynuclear species, U02(0H)2 colloids and uranyl-carbonate complexes. The influence of the mineral surface properties on the mechanisms of sorption is also discussed. 

The Fourier transform moduli of EXAFS data of three silica samples that sorbed U(VI) appeared to be very similar (Fig. 3-5). The uranyl U=0 distance of 1.78 0.01 A is typical of U(VI) compounds. Shells for hydroxide coordinated in the equatorial plane appeared in the range 2.22 to 2.31 A, while bond distances for coordinated water molecules were observed from 2.43 to 2.51 A. The presence of a U-Si shell (between 3.1 and 3.3 A) in all samples investigated suggests the formation of inner-sphere uranyl surface complexes, and there was no evidence for a U-U shell, which would indicate surface precipitation or polynuclear uranyl species. The short U-Si bond length of about 3.2 A and coordination number of 1 at pH around 3 suggests bidentate complexation to a single silica atom. At pH 5, the coordination number of 2 for the Si shell suggests the coordination of the uranyl complex to two Si atoms (Sylwester et al., 2000). 

Fig. 213 Hydrolysis of uranyl(yi) in 3m (Nd)Cl methum at 25°C [50], (a) The hgand number as a function of pH for values of Q, varying from 0.62S to 80 him. Circles denou experimental data, curves are calculated from the constants found (listed in Table 21.3). Signs in circles indicate the sign of the difference between calculated and experimental values of n. (b) If all the polynuclear complexes (l/O2) (Of0n"" had had n 2(m-l), as have (U02)2(0H)1 (UO UOIDV, and (U02U0H)l the curves (logCM-2lQg[H+]) would have coincided. They do in their lower parts where such complexes (especially (1/02)2 (0H)2 ) predominate but not in their upper parts where (U02)3(OH)s and possibly also (U02)4(0H)t, not belonging to this series, become important.

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. 

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. 

In aqeous systems, containing uranyl ions, there are either mono- or polynuclear [U02(L)n] complexes with compositions determined by the chemical nature and concentration of acidic or/and electroneutral ligands L, their electron donor characteristics and their possible types of coordination to the U atoms. 

Even in very dilute solutions, the hydrolysis of UO results almost exclusively in polynuclear species. This is immediately evident from the marked dependence of the degree of hydrolysis on the uranyl concentration, C, even at fairly low values of (Fig. 21.3(a)). Within wide ranges of pH and C , the predominant complex is the dimer (1102)2(OH) " all the media studi (Table 21.3). The dimer does not dissociate very readily, so the monomer UO2OH is formed in appreciable amounts only in very dilute solutions, CM lniM.For higher values... 

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