EXAFS investigation of uranium(VI) complexes formed at Acidithiobacillus Ferrooxidans types

Bacteria express a wide variety of complex molecules on their surfaces, which, at physiological pH values, contain numerous charged chemical groups (such as phosphoryl, carboxyl, and amino groups) that usually give the cell surface a net of anionic (negative) charge density. Since the cell surface is in direct contact with the environment, the charged groups within the surface layers are able to interact with ions or charged molecules present in the external milieu. As a result, metal cations can become electrostatically attracted and bound to the cell surface [1].

Uranium (VI) complexes formation at surfaces of three Acidithiobacillus ferrooxidans types was studied using uranium LIII-edge extended x-ray absorption fine structure (EXAFS) spectroscopy. In all samples uranium is coordinated by two axial oxygen atoms (Oax) at distance of 1.77-1.78 Å. The average distance between uranium and the equatorial oxygen atoms (Oeq) is 2.35 Å. The coordination number for Oeq is 5-6. In comparison to the uranium crystal structure data, the U-Oeq distance indicates a coordination number of the equatorial oxygen of 5.

Within the experimental error, there are no differences in the U-O bond distances between samples from the 3 types of A. ferrooxidans investigated. The fit to the EXAFS data of samples measured as wet pastes gave the same results as for dried samples. No significant structural differences were observed for the uranium complexes formed by the three eco-types of A. ferrooxidans. However, the EXAFS spectra are indicating a formation of uranium complexes which are different from those formed by Bacilli [2] where the bond length of 2.28 Å indicates a coordination number of 4 for the equatorial oxygen atoms.