Retention of U(VI) and Np(V) in bacteriogenic iron oxide-producing biofilms from Äspö HRL (Sweden)

At the Äspö HRL (Sweden) Gallionella ferruginea dominated biofilms associated with bacteriogenic iron oxides (BIOS) and groundwater were sampled from an in situ continuous flow cell, which has been installed in a cavity of the main access tunnel at 2200 A site, 300 m below sea level (Anderson & Pedersen, 2003). In laboratory sorption experiments UO2(ClO4)2 and NpO2(ClO4) were added to the BIOS biofilms in groundwater under aerobic conditions adjusting a final U(VI) concentration of 1.9×10-5 M.U(VI) and 3.27×10-5 M Np(V). At the end of the experiments the groundwater/BIOS biofilm samples were ultra centrifuged and the pH and Eh of the supernatants were defined to be 7.52 0.1 and 355 +- 30 mV in the uranium contaminated samples and 7.02 +- 0.1 and 367 +- 30 mV in the neptunium contaminated samples. The analysis showed a substantial decrease of uranium and neptunium in the groundwater of approximately 85 % and 95 %, respectively. Thermodynamic calculation of the theoretical predominant field of uranium species was performed using the analytical data of the uranium-contaminated groundwater. Under the given pH and Eh the formation of the aqueous uranium carbonate species Ca2UO2(CO3)3(aq) is predicted due to the high concentration of carbonate in the groundwater. In the BIOS biofilm the ferrous iron-oxidizing and stalk-forming bacterium Gallionella ferruginea is dominating the sorption process. The stalk represents an organic surface upon which Fe oxyhydroxides can precipitate. Due to the high concentration of ferric iron in the BIOS biofilm the portion of iron oxyhydroxides (ferrihydrite) amounts approximately to 70 wt%. Under the given pH conditions the uptake of U and Np depends predominantly on the high amount of ferrihydrite precipitated onto the stalks. Conclusively, the combination of this biological material and iron oxides creates an abundant surface area for adsorption of radionuclides.