Stoichiometry and Structure of Uranyl(VI) Hydroxo Dimer and Trimer Complexes in Aqueous Solution

Abstract

We studied the structure and stoichiometry of aqueous uranylVI hydroxo dimers and trimers by spectroscopic (EXAFS, FTIR, UV-vis) and quantum chemical (DFT) methods. FTIR and UV-vis spectroscopy were used for the speciation of uranyl complexes in aqueous solution. DFT calculations show that (UO2)2(OH)22+ has two bridging hydroxo groups with a U-U distance of 3.875 A. This result is in good agreement with EXAFS, where a U-U distance of 3.88 A was found. For the hydroxo trimer complex, DFT calculations show that the species (UO2)3(O)(OH)3+ with oxo bridging in the center is energetically favored in comparison to its stoichiometric equivalent (UO2)3(OH)5+. This is again in line with the EXAFS result, where a shorter U-U distance of 3.81-3.82 A and evidence for oxo bridging in the center were found. Several stable intermediates which lie several tens of kJ/mol above that of (UO2)3(O)(OH)3+ were identified, and their structures, energies, and intramolecular proton-transfer reaction are discussed.

The structure of monomeric and dimeric uranyl adsorption complexes on gibbsite: A combined DFT and EXAFS study

“…For both minerals no U-U backscattering was observed at acidic pH. The distance and CN at alkaline pH, S-2, are in line with the findings of Sylwester et al and can be explained by sorption of the aqueous uranyl dimer, (UO 2 ) 2 (OH) 2 , for which Tsushima et al (2007) determined a U-U distance of 3.875 Å . The significantly longer U-U distance of the acidic samples S-1 and S-3 are not in agreement with those of dimeric and trimeric aqueous complexes, but are in line with those of uranyl hydroxides such as b-hydroxide (4.22 Å ) (Taylor and Bannister, 1972), c-hydroxide (4.23 Å ) (Siegel et al, 1972) and the peroxide mineral studtite (4.21 Å ) (Burns and Hughes, 2003), all not very likely phases in our system.…”

“…In the calculations with polymeric uranyl units, the most diffuse basis functions on uranium with the exponent of 0.005 (s-, p-, d-, and f-type functions) were omitted, as otherwise the geometry optimizations did not easily converge. Macak et al (2006) and Tsushima et al (2007) used a similar approach to calculate uranyl dimers and trimers, pointing out that this basis set without the diffuse functions had only a minor effect (less than 1 kJ/mol) on the total energies. All calculations were performed with the Gaussian03 (Frisch et al, 2004).…”

A surface structural model for ferrihydrite II: Adsorption of uranyl and carbonate

“…See also http://migrationdb.jaea.go.jp/tdb_e/d_page_e/d_0500_e.html. (Tsushima et al, 2007). c From Sverjensky et al (1997).…”

An Amidoximated-UHMEPE Fiber for Selective and High Efficient Removal of Uranyl and Thorium from Acid Aqueous Solution

“…The , and also shows a significantly difference in the intensity of the FT peak at ~2 Å (without phase shift). With the results of EXAFS fitting parameters as shown in Table 1 [35] crystal and uranyl hydroxo complex [36]. Compared with these previous results, it is suggested that the uranyl ion is surrounded by hydroxyl groups and water molecules but for the amidoxime group on the surface of the fiber.…”