Publikationsrepositorium - Helmholtz-Zentrum Dresden-Rossendorf

At a depth of 70 m of the ONKALO tunnel in Finland, 771 m from the tunnel entrance, massive biofilms were observed attached to the tunnel walls where groundwater was seeping from bedrock fractures. During a sample campaign 2010 samples of the biofilms and of the groundwater seeps were taken for laboratory experiments. Microelectrode measurements in the biofilms and in the groundwater seeps indicated that the studied biofilms constituted their own microenvironments, which differed significantly from that of the groundwater seeps. A pH of 5.37 was recorded inside the biofilm, approximately 3.5 units lower than the pH observed in the groundwater seeps. Similarly, the Eh of +73 mV inside the biofilm was approximately 420 mV lower than the Eh measured in the groundwater seeps. In flow cells with detached biofilms uranium was added to the circulating groundwater with a final uranium concentration of 4.25 × 10–5 M. After 42 h the pH increased in the biofilm to 7.27 and the Eh was reduced to –164 mV. The changes of Eh and pH influenced the bioavailability of uranium. EF-TEM investigations indicated that uranium in the biofilm was immobilized intracellularly in microorganisms by the formation of metabolically mediated uranyl phosphate, similar to needle-shaped Autunite (Ca[UO2]2[PO4]2•2-6 H2O) or meta-Autunite (Ca[UO2]2[PO4]2•10-12H2O).

In contrast, TRLFS studies of the uranium-contaminated groundwater identified aqueous uranium carbonate species, likely (Ca2UO2[CO3]3), formed due to the high concentration of carbonate in the uranium contaminated circulating groundwater. The results agreed with thermodynamic calculations of the theoretically predominant field of uranium species, formed in the uranium-contaminated groundwater at the measured geochemical parameters.