From bentonite to rock salt – microbial studies for nuclear waste disposal


From bentonite to rock salt – microbial studies for nuclear waste disposal

Cherkouk, A.

Clay and crystalline rock as well as rock salt formations are considered as potential host rocks for the long-term storage of high-level radioactive waste in a deep geological repository. In all three host rocks are particular microbial communities present that could potentially influence the properties of the host rock or materials used in the repository due to their metabolic activity.
Within the EU project MIND the potential influence of natural occurring microorganisms within the bentonite on the bentonite barrier, which is placed as a part of the multi barrier concept between the steel-canister (containing the HLW) and the surrounding host rock (e.g. clay rock), is currently under investigation. Therefore, microcosms were set up with two different Bavarian bentonites (a natural and an industrial one), which were supplied with an anaerobic, synthetic Opalinus-clay pore water solution under an N2/CO2-atmosphere and incubated for one year at 30 °C and 60 °C. To some set ups organics (acetate or lactate) or H2 were supplemented. During the incubation time samples were taken and analysed for several bio-geochemical parameters and the evolution of microbial diversity. Our results clearly demonstrate that natural occurring microbes affect geochemical parameters. Set ups containing the industrial bentonite supplemented with lactate show the most striking effects, which resulted in the dominance (up to 81 %) of Desulfosporosinus spp. – spore-forming, strictly anaerobic, sulphate-reducing organisms, able to survive under very harsh conditions. Concomitantly, an increase of ferrous iron and a simultaneous decrease of ferric iron were observed. Furthermore, the lactate and sulphate concentration decreased, whereas pyruvate and acetate were formed. Similar observations were also made in setups containing H2. Samples from selected batches were analysed regarding its mineralogy with Scanning Electron Microscopy (SEM), at the University of Greifswald. The SEM mapping analysis indicated a higher number of iron-sulphur accumulations in lactate-containing B25-samples compared to the raw material and samples including hydrogen or no substrate. Thus, microbial activity under the applied conditions led to an alteration of mineral phases when the conditions were favourable.
Moreover, the microbial potential in rock salt as a potential host rock for nuclear waste disposal is currently being investigated. Extremely halophilic archaea, e.g. Halobacterium species, dominate this habitat. For long-term risk assessment it is of high interest to study how these microorganisms can interact with radionuclides if released from the waste repository. Therefore, the interactions of the extremely halophilic archaeon Halobacterium noricense DSM 15987T with uranium were investigated in detail in batch experiments. A multi-spectroscopic and microscopic approach was used to decipher the interaction mechanisms on a molecular level. H. noricense DSM 15987T showed a multistage bioassociation of uranium. By using time-resolved laser-induced fluorescence spectroscopy and X-ray absorption spectroscopy the formation of U(VI) phosphate minerals, such as meta-autunite, was observed. The retardation of uranium as phosphate minerals highlight the potential significance of the microbial life in deep geological hypersaline environments and offer new insights into the microbe-actinide interactions at highly saline conditions relevant to the disposal of highly radioactive waste.

Related publications

  • Invited lecture (Conferences)
    THEMENFELD-KOLLOQUIUM „ENERGIE“, Topic: Results and outcomes of European programme on Microbiology In Nuclear waste Disposal (MIND) – View s and perspectives over future research, 19.09.2018, Berlin, Deutschland

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