Combined UV-vis and EXAFS study on the complex formation of uranium(VI) with several carboxylic acids


Combined UV-vis and EXAFS study on the complex formation of uranium(VI) with several carboxylic acids

Lucks, C.; Roßberg, A.; Scheinost, A.

Natural organic matter (NOM) released by microorganisms and plants is ubiquitous in waters, soils and sediments. NOM can form relatively stable aqueous complexes with U(VI) and may interact like U(VI) with mineral surfaces. In systems containing U(VI), NOM and mineral surfaces binary and ternary sorption complexes have to be considered. These intricate interactions may profoundly influence the mobility of U(VI). However, the study of such a ternary system requires first its decomposition into simpler binary subsystems. Here we show the interaction of U(VI) with relatively simple carboxylic acids. Acetic, succinic and DL-tartaric acid were used as model compounds to simulate the different -COOH and -OH functionalities of the structurally more complicated, polyelectrolytic NOM.

Aqueous complexes of U(VI) with the model compounds were investigated by applying UV-VIS and EXAFS measurements. UV-VIS pH titration experiments were performed to identify the binary complexes formed in large excess of organic ligands and to derive their speciation. Uranium LIII-edge EXAFS spectra of pH series were then measured. Based on the speciation derived from UV-VIS spectroscopy, we were able to derive the EXAFS spectra of the pure U(VI) complexes from the EXAFS spectral mixtures by means of iterative target transform factor analysis [1]. Shell fit and an inversion method was then applied to validate their structures.
The uranium acetate and succinate complexes are characterized by a bidentate coordination of the carboxylic group. Furthermore, the formation of seven-membered rings can be rejected in the uranium succinate system. In contrast, the tartaric acid system is much more complicated because tartaric acid is able to form tridentate chelates with uranium using two carboxylic and one alcoholic groups. These chelates were identified to be dimeric at lower pH (3-5) and trimeric at higher pH (6-7). In addition, the structure of the trimer is also validated by the U-O radial pair distribution function (PDF) which is directly calculated from the EXAFS spectrum by an inversion method. The PDF of the oxygen atoms in the equatorial plane of U(VI) is asymmetric and gives evidence for one central oxygen atom linking the three uranyl ions of the trimer in a much shorter U-O distance than the remaining equatorial U-O distances.
In conclusion, we can provide structural models for the aqueous complexes of U(VI) with acetate, succinate and tartrate – the substances used as model compounds for NOM in this study.

[1] A. Rossberg, T. Reich, G. Bernhard, Anal. and Bioanal. Chem. 376, 631 (2003).

Keywords: uranium; complex formation; EXAFS

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    Migration Conference 2009, 20.-25.09.2009, Kennewick, Washington, USA

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Publ.-Id: 12534