Solution coordination chemistry of uranium in the binary UO₂²⁺-SO₄^2-and the ternary UO₂²⁺-SO₄^2--OH^-system

Abstract

Uranyl / Sulfate / 17 O-NMR / EXAFS / Dynamics / StructureSummary. The structure and reaction dynamics in the systems UO 2 2+ -SO 4 2− and UO 2 2+ -SO 4 2− -OH − were investigated using EXAFS and 17 O-NMR spectroscopy. Uranium L III edge EXAFS indicated a bidentate coordination mode of sulfate to uranyl. In solution, this is characterized by an U−S distance of 3.11 Å. Approximately 5 oxygen atoms were observed in the equatorial plane at 2.39-2.43 Å. The kinetics in the binary uranyl sulfate system can be described by four dominant exchange reactions: (1) , and (4) UO 2 SO 4 + SO 4 2− UO 2 (SO 4 ) 2 2− (k 4 ). These reactions have rate constants indicating that the exchange is not of the simple Eigen-Wilkins type.Ternary uranyl sulfate hydroxide species were characterized by their 17 O chemical shift and by potentiometry. There are no separate signals for the possible isomers of the ternary species indicating that they are in fast exchange with each other.

On the structure and relative stability of uranyl(VI) sulfate complexes in solution

“…A characteristic feature of the bond distances in solid compounds that contain both mono-and bidentate sulfate coordination is that the UeO sulfate bond distance is about 0.15 Å shorter in the former than in the latter. Structure studies of uranyl(VI) sulfate complexes in solution have been made by Moll et al using EXAFS [4] and by Neuefeind et al using large-angle X-ray diffraction (LAXS) [5]. Both methods provide accurate information on bond distances, but much less precise information on coordination numbers.…”

Sorption of Uranium(VI) onto Schwertmannite - EXAFS investigations

“…Such a uranium -iron distance is indicative of a bidentate inner-sphere complexation (Reich et al 1998). In contrast, the EXAFS of uranium(VI) sorbed onto Schwertmannite at pH 4.2 in sulfate-rieb solutions shows only a sulfur backscatterer at a distance of 3.66 A, indicating a monodentate uranium(VI) -sulfate coordination (Moll et al 2000). This could be interpreted either as uranium(VI) surface complexes with the structural sulfate of the Schwertmannite lattice (=SO-UO/, =(S0)2=U02) or as a temary surface complex (=FeO-SOcU02).…”

Uranium Bioremediation: Approaches and Challenges

“…In acidic sulphate-rich water, such as seepage waters from uranium mines after in situ leaching by sulfuric acid, uranyl ion forms binary sulphate complexes: UO 2 SO 4 (aq), UO 2 (SO 4 ) 2 2À and UO 2 (SO 4 ) 3 4À . However, increase in pH and/or uranium concentration leads to several ternary sulphate complexes with hydroxo bridging such as UO 2 (OH)SO (Moll et al 2000). Nitrate does not complex with uranium (VI) and has no role in the presence of other inorganic ligands in environmentally relevant waters.…”