Uranium(VI) reduction by a sulphate-reducing microorganism in Opalinus Clay pore water

1 Introduction
Clay formations are potential host rocks for the long-term storage of high-level radioactive waste in a deep geological repository in Germany, besides salt and crystalline rock. A multi-barrier system is fa-vored, consisting of the technical (container with the waste), the geotechnical (sealing and backfilling material, e.g. bentonite) and the geological barrier (host rock) to isolate it from the biosphere.
Different studies showed that sulphate-reducing microorganisms, especially Desulfosporosinus species, occur in various clay formations, as well as in bentonite [1,2]. Desulfosporosinus hippei DSM 8344 is an anaerobic spore-forming microorganism isolated from permafrost soil [3] and a close phylogenetic relative of the Desulfosporosinus species detected in clay formations. Therefore, this strain was selected to study the reduc-tion of uranium(VI) to the less mobile uranium(IV).

2 Results
A time-dependent experiment in artificial Opalinus Clay pore water [4] (100 µM uranium(VI), pH 5.5) revealed a 95 % removal of uranium from the supernatant within 24 h. The corresponding microscopy of live/dead stained cells showed the formation of agglomerates and an increasing number of dead cells within the incubation time. The black colouring of the agglomerates already provided hints of the occur-ring reduction of uranium(VI).
Different aqueous species including uranyl(VI) lactate and uranyl(VI) carbonate complexes are present in the supernatant, as determined by time-resolved laser-induced luminescence spectroscopy. The assign-ment of the different species was possible by comparison with reference spectra. While the amount of the uranyl(VI) lactate complex decreased with the incubation time, the uranyl(VI) carbonate fraction re-mained almost constant. This leads to the assumption, that the cells reduce only the uranyl(VI) lactate complex. This conclusion can be supported by the fact that the reduction process did not take place in bicarbonate buffer, where the uranyl(VI) carbonate complexes are dominant, using the same microor-ganism.
The comparison of the UV/VIS band positions of the dissolved cell pellets with the spectra of pure uranium(IV) and uranyl(VI) samples provides clear evidence of the formed uranium(IV). Furthermore, bands of uranyl(VI) occur in the spectrum, as well. Therefore, a combination of a sorption and reduction processes is assumed. These findings offer new insights into the microbe-actinide interactions relevant to high-level radioactive waste disposal in clay rock.

The authors gratefully acknowledge the funding provided by the German Federal Ministry of Education and Research (BMBF) (Grant 02NUK053E) and The Helmholtz Association (Grant SO-093).

References
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[3] A. Vatsurina et al., “Desulfosporosinus hippei sp. nov., a mesophilic sulfate-reducing bacterium isolated from permafrost”, Int. J. Syst. Evol. Microbiol. 58, 1228–1232 (2008).
[4] P. Wersin et al. “Biogeochemical processes in a clay formation in situ experiment: Part A - Overview, experimental design and water data of an experiment in the Opalinus Clay at the Mont Terri Underground Research Laboratory, Switzerland”, Appl. Geochemistry 26, 931–953 (2011).