Publications Repository - Helmholtz-Zentrum Dresden-Rossendorf

The ferrous iron-oxidizing and stalk-forming bacterium Gallionella ferruginea was cultivated in laboratory experiments. Since this bacterium is gaining energy for its growth from the oxidation of ferrous iron, ferric iron is precipitating quickly and forming biogenic ferrihydrite. UO2(NO3)2 and NpO2(ClO4) was added to these samples under anaerobic conditions in the neutral pH range, adjusting a final U and Np concentration of 0.08 mM, respectively. The results showed an uptake of 91 mg U and 38 mg Np/g dry mass by the abundant surface area of the samples.
At the ROBL Beamline of the European Synchrotron Radiation Facility in Grenoble Extended X-ray absorption fine structure (EXAFS) spectroscopy at the uranium LIII–edge and the neptunium LIII–edge were carried out. The k3-weighted chi-spectrum and its Fourier transform magnitude of the studied biogenic ferrihydrite sample bears close resemblance to the bidentate edge-sharing innersphere sorption complex (1E), which is the most prominent surface species in the absence of carbonate and the main sorption species on abiotic ferrihydrite [1]. As a second species a smaller portion of the aqueous type-B ternary uranyl-carbonato complex was determined as a result of the addition of a carbon source during the cultivation of the Gallionella ferruginea strain. By iterative target test factor analysis (ITFA), using the spectra of the two endmember species, we determined that the 1E complex is in fact predominant with 95%, while the ternary uranyl-carbonato complex is present only to 5%. Based on the shell fit analysis, the distances of the coordination shells U–Oeq ~ 2.34 Å, U–Oax ~ 1.79 Å, and U–Fe ~ 3.44 Å are similar to those determined of abiotic ferrihydrite samples [2]. The data of the biogenic Np ferrihydrite sample were compared to Np interaction with a hematite surface and showed similar distances of the coordination shells, also indicating a bidentate edge-sharing coordination [3].