Characterization of the Binding Mechanisms of Uranium to Different Isolated Bacteria in Function of pH

The mining and processing of uranium during the last decades for nuclear fuel and nuclear weapon production resulted in generation of significant amounts of radioactive wastes. The mobility of this radionuclide is controlled by its interactions with ions, minerals and microorganisms present in nature. As a consequence of their small size and diverse metabolic activities, bacteria are able to interact intimately with uranium and other metal ions present in their environment. Assessment of the potential hazard posed by the presence of uranyl-bacterial complexes, as well as the development and application of accurate contaminant transport models, requires an understanding of the speciation of uranium associated with bacteria. This paper summarizes the effect of pH on the speciation of uranium bound by about fifteen bacterial strains isolated from uranium mining wastes. Microbiological methods in combination with Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy, Infrared (IR) spectroscopy, Transmission Electron Microscopy (TEM) and Energy Dispersive X-ray Spectroscopy (EDS) were applied. EXAFS analysis showed that the cells of the most bacterial strains studied precipitate U(VI) as m-autunite-like phase at pH 4.5, probably due to the release of the inorganic phosphate from the cells as result of the acidic phosphatase activity. However, at pH 2 uranium formed complexes with organically bound phosphates of the cell surface. At pH 3, both organic and inorganic phosphate uranyl species occur together. No structural differences of the uranium complexes formed by three types of Acidithiobacillus ferrooxidans at above mentioned pH values were found, indicating the implication of organic phosphate moieties in the complexation. However, in the case of B. sphaericus JG-A12 the uranium bonding was consistent with the formation of complexes with organic bound phosphate and carboxylate groups of the cell surface. We applied Iterative Target Test Factor Analysis to determine the speciation at different pH values quantitatively from the EXAFS spectra. Transmission electron microscopy and energy dispersive X-ray analysis revealed species and even strain-specific extracellular and/or intracellular uranium accumulations to varying degrees. Different hypothesis explaining the different coordination chemistry of uranium to bacteria in function of the pH of the uranium solution in terms of solubility of m-autunite and/or microbial activity will be discussed.