Immobilization of uranium in biofilm microorganisms exposed to groundwater seeps over granitic rock tunnel walls in Olkiluoto, Finland
Artikel i vetenskaplig tidskrift, 2012

In an underground rock characterization facility, the ONKALO tunnel in Finland, massive 5-10-mm thick biofilms were observed attached to tunnel walls where groundwater was seeping from bedrock fractures at a depth of 70 m. In laboratory experiments performed in a flow cell with detached biofilms to study the effect of uranium on the biofilm, uranium was added to the circulating groundwater (CGW) obtained from the fracture feeding the biofilm. The final uranium concentration in the CGW was adjusted to 4.25 x 10(-5) M, in the range expected from a leaking spent nuclear fuel (SNF) canister in a future underground repository. The effects were investigated using microelectrodes to measure pH and E-h, time-resolved laser fluorescence spectroscopy (TRLFS), energy-filtered transmission electron microscopy (EF-TEM), and electron energy-loss spectroscopy (EELS) studies and thermodynamic calculations were utilized as well. The results indicated that the studied biofilms constituted their own microenvironments, which differed significantly from that of the CGW. A pH of 5.37 was recorded inside the biofilm, approximately 3.5 units lower than the pH observed in the CGW, due to sulfide oxidation to sulfuric acid in the biofilm. Similarly, the E-h of +73 mV inside the biofilm was approximately 420 mV lower than the E-h measured in the CGW. Adding uranium increased the pH in the biofilm to 7.27 and reduced the E-h to -164 mV. The changes of E-h and pH influenced the bioavailability of uranium, since microbial metabolic processes are sensitive to metals and their speciation. 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)center dot 2-6H(2)O) or meta-autunite (Ca[UO2](2)[PO4](2)center dot 10-12H(2)O). In contrast, TRLFS studies of the contaminated CGW identified aqueous uranium carbonate species, likely (Ca2UO2[CO3](3)), formed due to the high concentration of carbonate in the CGW. The results agreed with thermodynamic calculations of the theoretically predominant field of uranium species, formed in the uranium-contaminated CGW at the measured geochemical parameters. This investigation clearly demonstrated that biological systems must be considered as a part of natural systems that can significantly influence radionuclide behavior. The results improve our understanding of the mechanisms of biofilm response to radionuclides in relation to safety assessments of SNF repositories. (C) 2012 Elsevier Ltd. All rights reserved.

p661

complex-formation

electron-microscopy

fluorescence spectroscopy

hexavalent

sp-nov

reduction

microbial communities

ns mp

v39

IPEL G

radionuclide

1994

uranium

applied microbiology and biotechnology

RADIOCHIMICA ACTA4th International Conference on Chemistry and Migratory

adsorption

desulfovibrio-desulfuricans

bacteria

1993

Författare

E. Krawczyk-Barsch

H. Lunsdorf

Karsten Pedersen

Göteborgs universitet

T. Arnold

F. Bok

R. Steudtner

A. Lehtinen

V. Brendler

Geochimica et Cosmochimica Acta

0016-7037 (ISSN) 0046-564X (eISSN)

Vol. 96 94-104

Ämneskategorier

Biologiska vetenskaper

DOI

10.1016/j.gca.2012.08.012