Immobilization of uranium and neptunium by microorganisms in subsurface crystalline rock environments

In crystalline rock, the dominant transport medium for radionuclides is groundwater flowing through subsurface fractures. Since groundwater is containing microorganisms, fracture surfaces support biological growth of microbial communities, the so-called biofilms. The microbial diversity of these biofilms depends on the microbial consortia and the chemical composition of the fracture water. Subsurface biofilms have a significant effect on the adsorption capacity of host rock formations by forming a barrier between the rock surface and the groundwater. They can significantly affect subsurface biogeochemical interactions, leading to the immobilization and adsorption of radionuclides.

Microbial studies were performed to evaluate the relevance of microbial processes for the immobilization of radionuclides in a deep crystalline repository for high-level radioactive waste. Studies were performed in Olkiluoto, in the underground rock characterisation facility ONKALO in Finland, and in the Äspö Hard Rock Laboratory (HRL) in Sweden.
Massive 5–10-mm thick biofilms were observed in both sites attached to tunnel walls where groundwater was seeping from bedrock fractures. In experiments the effect of uranium on biofilms was studied on site in the ONKALO tunnel by adding UO2(ClO4)2 with a final U-concentration of 1.0×10-5 M to the fracture water in a self constructed flow cell by using detached biofilm samples. Biofilm specimens collected for transmission electron microscopy studies indicated that uranium in the biofilm was immobilized intracellularly in microorganisms as needle-shaped uranyl phosphate minerals, similar to meta-Autunite (Ca[UO2]2[PO4]2•10-12H2O). In contrast, thermodynamic calculation of the theoretical predominant fields of uranium species and time-resolved laser fluorescence spectroscopy showed that the formation of aqueous uranium carbonate species Ca2UO2(CO3)3 and Mg2UO2(CO3)3 was predicted due to the high concentration of carbonate in the groundwater.
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. In laboratory sorption experiments UO2(ClO4)2 and NpO2ClO4 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). The results of the experiments showed that 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. Under the given pH conditions the uptake of uranium (85%) and Np (95%) depends predominantly on the high amount of ferrihydrite precipitated onto the stalks. The results showed that the combination of the biological material and Fe oxides created an abundant surface area for bioaccumulation and adsorption of radionuclides.