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Henry Moll, Gerhard Geipel, T. Reich, Gert Bernhard, Thomas Fanghänel, I. Grenthe

Uranyl(VI) complexes with alpha-substituted carboxylic acids in aqueous solution

The complex formation in the binary uranium(VI)-glycolate, -α-hydoxyisobutyrate, -α-aminoisobutyrate systems in 1.0M NaClO₄ medium was studied by means of UV-vis, TRLFS, and EXAFS. An increase in absorption and a red shift of the spectra, 5nm compared to the free UO₂²⁺, indicate a complex formation between UO₂²⁺ and α-substituted carboxylic acids already at pH 2. 1:1 complexes dominate the uranyl speciation in the glycolate, α-hydoxyisobutyrate, and α-aminoisobutyrate system at pH 2 and 3, respectively. At higher ligand concentrations a 1:2 complex between UO₂²⁺ and α-aminoisobutyric acid was observed.

There is a very strong quenching of the U(VI) fluorescence in the
uranyl–α-hydroxycarboxylate systems that can be quantitatively described by the Stern–Volmer equation. As a result of the strong quenching it is not possible to detect fluorescence spectra for these complexes using TRLFS. The UO₂²⁺(aq) concentration calculated from the Stern–Volmer equation was used to determine equilibrium constants which are in good agreement with those obtained by potentiometry and NMR spectroscopy. No quenching was observed in the α-aminoisobutyrate system and their fluorescence spectra could be deconvoluted into components for the different species present. The following stability constants result from our TRLFS experiments: a) for the glycolate system log β_{UO2(HOCH2COO)⁺}=2.52&plusm;0.20, b) for the α-hydroxyisobutyrate system log β_{UO2[HOC(CH3)2COO]⁺}=3.40&plusm;0.21, and c) for the α-aminoisobutyrate system logβ_{UO2[NH3C(CH3)2COO]²⁺}=1.30&plusm;0.10 and logβ_{UO2[NH3C(CH3)2COO]2²⁺}=2.07&plusm;0.25. An increase of the fluorescence intensity connected with a red shift of the fluorescence emission spectra was observed in the system uranyl–α-aminoisobutyric acid. Fluorescence lifetimes and spectra were obtained for UO₂²⁺, UO₂[NH₃C(CH₃)₂COO]²⁺, and UO₂[NH₃C(CH₃)₂COO]₂²⁺.

Uranium L_III-edge EXAFS measurements yielded an U-O_eq distance of 2.40 to 2.43Å in the pH range from 2 to 4 in the α-hydroxyisobutyrate system showing a dominant bidentate coordination via the oxygens of the carboxylic group. Slightly shorter U-O_eq distances of 2.40 to 2.38Å and no evidence for U-C distances around 2.90Å in the glycolate system in this pH range may indicate a monodentate coordinated ligand via one oxygen from the carboxylic group. The decrease in the U-O_eq distance of the equatorial oxygens in both systems to 2.36-2.37Å at pH values ≥5 is a strong indication for the formation of a chelate complex due to the deprotonation of the α-OH-group of the ligand. In the glycolate system in the pH range 5.5 to 11, the EXAFS spectrum showed evidence of U-U interaction at 3.81Å indicating the formation of dimeric species.

Radiochimica Acta, Oldenbourg Wissenschaftsverlag

Print ISSN: 0033-8230
Volume: 91, 01/2003
Pages: 011

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