The hydrolysis of uranium(VI) has been the subject of extensive studies for the last 50 years because uranyl forms strong complexes with OH- in solution. A comprehensive discussion and review of the thermodynamic data is published in [1]. We investigated the structure of UO22+ as a function of pH with the aid of U LIII-edge EXAFS spectroscopy. The experiments were carried out at beamline BM20. The equipment allows excellent XAS measurements up to high k-values, 17.5 Å­1 in our case.

The speciation of uranium(VI) at a total concentration of 0.05 M in slightly acidic solutions (pH 3 to 4) is dominated by the two polynuclear complexes (UO2)2(OH)22+ and (UO2)3(OH)5+. Sample A is an example from this pH region, where (UO2)3(OH)5+ is the dominant species. Structural investigations of these polymeric cations in solution at such uranium concentrations are rare. The formation of polynuclear complexes is clearly confirmed by the U­U interaction at 3.81 Å (Figure 22). Approximately five oxygen atoms are coordinated in the equatorial plane at 2.41 Å.

Figure 22
Fig. 22: Raw k 3-weighted EXAFS spectra for samples A to D in the binary uranyl hydroxide system including the best theoretical fits and the FT filtered U­U scatter from the (UO 2) 3(OH) 5 + complex. The uranium concentration in solutions A, C, and D was 0.05 M. 

In the pH region from 6 to 11, the U(VI) speciation is dominated by the precipitation of schoepite phases, UO2(OH)2xH2O (sample B). A longer U­U distance of 3.87 Å was measured. Similar U bond lengths of around 3.9 Å were found in schoepite phases. Their structure consists of a network of UO2(OH)2 sheets, where the uranyl centres are connected via a double OH bridge.

In the alkaline pH region (sample C and D), monomeric uranium species are formed. As compared to our previous study [2], we were able to extend the k-space region from 15 Å­1 in [2] up to 17.5 Å­1 and thereby to increase the precision. The EXAFS measurements confirm the speciation calculations indicating that UO2(OH)42­ is the major species in 0.5 M tetramethylammonium hydroxide (TMA-OH). There are two trends in the EXAFS data, the U­Oaxial bond length increases 1.79 Å, 1.81 Å, and 1.83 Å moving from pH 4.1 to 13.7, while the average U­Oequatorial bond length decreases, 2.41 Å, 2.34 Å, 2.26 Å, respectively. This indicates a stronger bonding of equatorial OH groups with increasing pH. Clark et al. [3] have presented spectroscopic evidence for the formation of a pentahydroxide complex at high TMA-OH concentrations, however with no information about the equilibrium constant. We have tested the hypothesis of Clark et al. using 17O-NMR spectroscopy with 17O-enriched "yl" oxygens. The spectrum recorded at 258 K (Figure 23) shows only one peak for UO2(OH)22­ in 1 M TMA-OH. However, when increasing the hydroxide concentration to 3 M two peaks were obtained, one with the same shift as in the 1 M TMA-OH solution, 1132.2 ppm, the other at 1135.8 ppm presumably due to UO2(OH)53­ (Figure 23). To conclude, the complex UO2(OH)42­ has a very broad range of existence in strongly alkaline solution. At very high total concentrations of hydroxide (> 1 M TMA-OH), an additional OH­ ligand may coordinate in an associative reaction.

Figure 23
Fig. 23: 17O-NMR spectra of a 0.05 M UO 2 2+ solution in aqueous methanol at different TMA-OH concentrations; a) 1 M and b) 3 M (at 258 K).

References
[1] I. Grenthe, J. Fuger, R.J.M. Konings, R.J. Lemire, A.B. Muller, C. Nguyen-Trung and H. Wanner, in Chemical Thermodynamics of Uranium, H. Wanner, I. Forrest, (Eds.), NEA OECD, Issy-les-Moulineaux, France, p. 241 (1992).
[2] U. Wahlgren, H. Moll, I. Grenthe, B. Schimmelpfennig, L. Maron, V. Vallet and O. Gropen, J. Phys. Chem. A, 103, 8257-8264 (1999).
[3] D.L. Clark, S.D. Conradson, R.J. Donohoe, D.W. Keogh, D.E. Morris, P.D. Palmer and R.D. Rogers, C.D. Tait, Inorg. Chem., 38, 1456-1466 (1999).

Principal Publication and Authors
H. Moll (a, b), T. Reich (b) and Z. Szabó (a), Radiochim. Acta, 88, 411-415 (2000).
(a) The Royal Institute of Technology (KTH), Stockholm (Sweden)
(b) Forschungszentrum Rossendorf e.V., Dresden (Germany)