Uranium and thorium sequestration by a Pseudomonas sp.: mechanism and chemical characterization

The mechanism and chemical nature of uranium and thorium sequestration by a Pseudomonas strain was investigated by transmission electron microscopy, energy dispersive X-ray (EDX) analysis, FTIR spectroscopy and X-ray diffractometry. Atomic force microscopy (AFM) used in the tapping mode elucidated the morphological changes in bacterial cells following uranium and thorium binding. Transmission electron microscopy revealed intracellular sequestration of uranium and thorium throughout the cell cytoplasm with electron dense microprecipitations of accumulated metals. Energy dispersive X-ray analysis confirmed the cellular deposition of uranium and thorium. EDX and elemental analysis of sorption solution indicated the binding of uranium and thorium by the bacterial biomass via displacement of cellular potassium and calcium. The strong involvement of cellular phosphate, carboxyl and amide groups in radionuclide binding was ascertained by FTIR spectroscopy. X-ray powder diffraction (XRD) analyses confirmed cellular sequestration of crystalline uranium and thorium phosphates. Overall results indicate that a combined ion-exchange-complexation-microprecipitation mechanism could be involved in uranium and thorium sequestration by this bacterium. Atomic force microscopy and topography analysis revealed an undamaged cell surface with an increase in cell length, width and height following radionuclide accumulation. The arithmetic average roughness (R(a)) and root mean square (RMS) roughness (R(q)) values indicated an increase in surface roughness following uranium and thorium sequestration.