The Structure of Trimeric Uranyl Hydroxo Complexes in Aqueous Solution

The solubility and the speciation of uranium(VI) in water at neutral pH with mM total uranium concentrations (Utot) is dominated by polymeric hydroxo species (e.g., (UO2)2(OH)22+ and (UO2)3(OH)5+). Thermodynamic data of various polymeric species had been reviewed and were published as OECD/NEA database [1]. However, direct structural information of polymeric uranyl hydroxo species are scarce. Here EXAFS in combination with quantum chemical calculations can serve as a powerful tool to explore the structure of polymeric uranyl hydroxo species in order to correlate thermodynamic speciation [1] with structural information from EXAFS. For instance, this method can provide both U-U distances and U-U coordination numbers, and can be a direct proof of the presence of dimeric and trimeric complexes. (UO2)3(OH)5+ and (UO2)3(O)(OH)3+ are stoichiometric equivalent but they significantly differ in the U-U distance and can be distinguished by EXAFS. In this work, the structures of uranyl trimeric hydroxo complexes in aqueous solution have been studied by the B3LYP hybrid density functional theory (DFT) calculations and were compared with EXAFS structural information and thermodynamic speciation.
We studied the structure of trimeric uranyl(VI) hydroxo species including (UO2)3(OH)5+ and (UO2)3(O)(OH)3+ by DFT calculations, and obtained that the U-U distances vary between 3.83 and 4.30 Å. Among them, two species were found to be energetically favorable. One is (UO2)3(O)(OH)3+ having oxo bridging in the center with an average U-U distance of 3.83 Å. The other is (UO2)3(OH)5+ with no bridging oxygen in the center and an average U-U distance of 4.30 Å. A previous EXAFS and 17O-NMR study on the trimeric uranyl hydroxo complex by Moll et al.[2] reports the U-U distance of 3.80 Å and coincides well with the structure of (UO2)3(O)(OH)3+ obtained by DFT calculations. The EXAFS/DFT results are also discussed together with thermodynamic speciation of uranyl(VI) under the given condition (pH, Utot, I, etc.) and with Raman spectroscopic data of various polymeric hydroxo species obtained by Nguyen-Trung et al. [3]. Not only the structural information, but also the kinetics of the proton transfer in (UO2)3(O)(OH)3+ to yield (UO2)3(OH)5+ was studied.
[1] Guillaumont, R.; Fanghänel, T.; Fuger, J.; Grenthe, I.; Neck,V.; Palmer, D.A.; Rand, M.H. Update on the Chemical Thermodynamics of Uranium, Neptunium, Plutonium, Americium and Technetium; Elsevier Science Publishing Company, Inc.: New York, 2003; Vol. 5.
[2] Moll, H.; Reich, T.; Szabo, Z. Radiochim. Acta 2000, 88, 411.
[3] Nguyen-Trung, C.; Palmer, D.A.; Begun, G.M.; Peiffert, C.; Mesmer, R.E. J.Sol.Chem. 2000, 29, 101.