Interaction of U(VI) with bacterial strains isolated from uranium mining piles: spectroscopic and microscopic studies

Bacteria, ubiquitous in all aquatic and soil systems, can interact in many ways with actinides. They can mobilize or immobilize actinides in the environment, leading to their dissolution or precipitation. Knowledge of bacteria-actinide interactions is important for understanding the migration behaviour of the latter in the biogeosphere. In this work, a combination of Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy, Infrared (IR) spectroscopy, Transmission Electron Microscopy (TEM) and Energy Dispersive X-ray Spectroscopy (EDS) was used to conduct a molecular and atomic analysis of the uranium complexes formed by different bacterial strains isolated from uranium mining waste piles. EXAFS analysis showed that the cells of Stenotrophomonas maltophilia JG-2, Pseudomonas rhodesiae R5 and Myxococcus xanthus precipitate U(VI) as autunite-like phase at pH 4.5, probably due to the release of the inorganic phosphate from the cells. However, in the case of B. sphaericus JG-A12 the uranium bonding was consistent with the formation of a ternary complex with phosphate and carboxylate. These results are in agreement with those found by Infrared measurements. TEM and EDS analysis showed strain-specific extracellularl and/or intracellularl uranium accumulation to varying degrees. In B. sphaericus JG-A12 the uranium is located at the cell wall, while the cells of S. maltophilia JG-2 and P. rhodesiea R5 accumulate uranium both extracellularly on the cell surface and intracellularly as electron-dense granules.