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

Abstract

Summary The complex formation in the binary uranium(VI)-glycolate, -α-hydoxyisobutyrate, -α-aminoisobutyrate systems in 1.0 M NaClO4 medium was studied by means of UV-vis, TRLFS, and EXAFS. An increase in absorption and a red shift of the spectra, 5 nm compared to the free UO2 2+, indicate a complex formation between UO2 2+ 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 UO2 2+ and α-aminoisobutyric acid was observed. There is a very strong quenching of the U(VI) fluorescence in theuranyl–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 UO2 2+(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±0.20, b) for the α-hydroxyisobutyrate system log β UO₂[HOC(CH₃)₂COO]⁺=3.40±0.21, and c) for the α-aminoisobutyrate system logβUO₂[NH₃C(CH₃)₂COO]²⁺=1.30±0.10 and log β UO₂[NH₃C(CH₃)₂COO]₂²⁺=2.07±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 UO2 2+, UO2[NH3C(CH3)2COO]2+, and UO2[NH3C(CH3)2COO]2 2+. Uranium L III-edge EXAFS measurements yielded an U-Oeq distance of 2.40 to 2.43 Å 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-Oeq distances of 2.40 to 2.38 Å and no evidence for U-C distances around 2.90 Å 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-Oeq distance of the equatorial oxygens in both systems to 2.36-2.37 Å 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 3.81 Å indicating the formation of dimeric species.