Evidence of U(VI) sorption on Acidovorax facilis by TRLFS and EF-TEM/EELS

Acidovorax facilis, an aerobic Gram-negative Betaproteobacteria which is commonly found in soil, was used for U(VI) sorption experiments. Experiments were performed in batch cultures under aerobic conditions at 25 °C using nutrient broth. For U(VI) sorption experiments UO2(NO3)2 was added to the culture to achieve an initial U concentration of 0.05 and 0.1 M, respectively, at a neutral pH range. The duration of the sorption experiments were limited to 8 h and 48 h, respectively. By time-resolved laser fluorescence spectroscopy (TRLFS) the measured emission spectrum of the cells is characterized by four emission bands with peak maxima at 497.8, 519.5, 544.1 and 568.6 nm ± 0.5 nm. In addition, the spectra of the Uranyl-lipopolysaccharide-complexes R-O-PO3-UO2 and [R-O-PO3]2-UO22-, reported by Barkleit et al. (2008), were used for comparison. They show only a small deviation from those observed in our studies. Hence, it can be concluded that phosphoryl groups are the main binding sites for uranyl, located in the lipopolysaccharide (LPS) unit in the outer membrane by Gram-negative Acidovorax facilis cells. After the U(VI) biosorption experiments, Acidovorax facilis cells were prepared for Energy-filtered transmission electron microscopy (EF-TEM) and electron energy-loss spectroscopy (EELS). The results provide microscopically and spectroscopically evidence of U(VI) sorbed at the outer membrane of Acidovorax facilis cells by showing high electron density and U ionization intensity peaks.