The complexes of uranium(VI) with salicylhydroxamate, benzohydroxamate, and benzoate have been investigated in a combined computational and experimental study using density functional theory methods and extended X-ray absorption fine structure spectroscopy, respectively. The calculated molecular structures, relative stabilities, as well as excitation spectra from time-dependent density functional theory calculations are in good agreement with experimental data. Furthermore, these calculations allow the identification of the coordinating atoms in the uranium(VI)-salicylhydroxamate complex, i.e. salicylhydroxamate binds to the uranyl ion via the hydroxamic acid oxygen atoms and not via the phenolic oxygen and the nitrogen atom. Carefully addressing solvation effects has been found to be necessary to bring in line computational and experimental structures, as well as excitation spectra.

The complex formation of uranium(VI) with salicylhydroxamic,benzohydroxamic, and benzoic acid in 0.1 M NaClO

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Fluorescent Pseudomonas species secrete pyoverdin-type siderophores with a high potential to dissolve, bind, and thus transport uranium in the environment. The formation of complexes of UO 2+ 2 with pyoverdins released by the groundwater bacterium Pseudomonas fluorescens (CCUG 32456) isolated at a depth of 70 m in theÄspö Hard Rock Laboratory, Sweden, was studied. Mass spectrometry indicated that the cells produce a pyoverdin-mixture with four main components: pyoverdin with a succinamide side chain, pyoverdin with a succinic acid side chain, ferribactin with a succinamide side chain, and ferribactin with a glutamic acid side chain. Three pK values could be determined from the pHdependent changes in the absorption spectra of the pyoverdin mixture: log ß 012 = 22.67 ± 0.15 (pK 1 = 4.40), log ß 013 = 29.15 ± 0.05 (pK 2 = 6.48), and log ß 014 = 33.55 ± 0.05 (pK 3 = 10.47). The fluorescence properties of the pyoverdin mixture were pH-dependent. The emission maximum changed from 448 nm at pH = 2.1 to 466 nm in the pH 3.8-8.9 range. At pH > 4 a mono-exponential fluorescence decay dominates with a decay time of 5865 ± 640 ps. A drastic change in the intrinsic fluorescence properties, e.g., static fluorescence quenching, occurred due to the complex formation with UO 2+ 2 . Species containing UO 2+ 2 of the type M p L q H r were identified from the dependencies observed in the ultraviolet visible and time-resolved laser-induced fluorescence spectroscopy spectra at pyoverdin concentrations below 0.1 mM. The following average formation constants were determined: log ß 112 = 30.00 ± 0.64 and log ß 111 = 26.00 ± 0.85 at ionic strength I = 0.1 M (NaClO 4 ). The determined stability constants can be used directly in safety calculations of the mobilizing effect of released pyoverdins on uranium, in uranium-contaminated environments such as mine waste disposal sites.