Characterization of uranium (VI) complexes formed by different bacteria recovered from uranium mining waste piles

Uranium and other actinides may be present initially as soluble or insoluble forms in nature and, after disposal, they may be converted from one to the other form by microorganisms. Under appropriate conditions, actinides can be mobilized or immobilized by direct (enzymatic) or indirect (nonenzymatic) microbial actions. These include: (i) oxidation-reduction reactions, (ii) changes in pH and Eh, (iii) chelation, or production of specific sequestering agents, (iv) formation of stable minerals, (v) biodegradation of actinides-organic complexes, and (vi) biosorption by microbial biomass and biopolymers. In this work, we investigated uranium sorption efficiency of different bacterial strains isolated from uranium mining wastes such as Acidithiobacillus ferrooxidans, Leptospirillum ferrooxidans Pseudomonas stutzeri and Pseudomonas migulae, and also of some reference strains under various conditions (pH, amount of biomass, temperature). In addition, from desorption studies we obtained information about the kind of interaction (sorption, surface complexation, or uptake) and the binding strength. In order to conduct atomic and molecular scale studies on U(VI) interaction with these bacteria, we used a combination of Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy, Infrared Spectroscopy (IRS), Time-Resolved Laser-Induced Fluorescence Spectroscopy (TRLFS) analysis. Transmission electron microscope (TEM) was used for imaging the location of uranium accumulated by the bacterial cells, and for determining the active binding sites of the examined different types of biomass, and consequently to elucidate the possible mechanism of uranium accumulation. Electron-dispersive X-ray (EDX) analysis allowed to perform an elemental analysis of the uranium accumulates.