Uranyl(vi) complexation with fluoride and chloride was investigated with luminescence spectroscopy, and the strong quenching by chloride was overcome by freezing.

The safe disposal of high-level radioactive waste in a deep geological repository is a huge social and technical challenge. So far, one of the less considered factors needed for a long-term risk assessment, is the impact of microorganisms occurring in the different host rocks. Even under the harsh conditions of salt formations different bacterial and archaeal species were found, e. g. Halobacterium sp. GP5 1–1, which has been isolated from a German rock salt sample. The interactions of this archaeon with uranium(VI), one of the radionuclides of major concern for the long-term storage of high-level radioactive waste, were investigated. Different spectroscopic techniques, as well as microscopy, were used to examine the occurring mechanisms on a molecular level leading to a more profound process understanding. Batch experiments with different uranium(VI) concentrations showed that the interaction is not only a simple, but a more complex combination of different processes. With the help of in situ attenuated total reflection Fourier-transform infrared spectroscopy the association of uranium(VI) onto carboxylate groups was verified. In addition, time-resolved laser-induced luminescence spectroscopy revealed the formation of phosphate and carboxylate species within the cell pellets as a function of the uranium(VI) concentration and incubation time. The association behavior differs from another very closely related halophilic archaeon, especially with regard to uranium(VI) concentrations. This clearly demonstrates the importance of studying the interactions of different, at first sight very similar, microorganisms with uranium(VI). This work provides new insights into the microbe-uranium(VI) interactions at highly saline conditions relevant to the long-term storage of radioactive waste in rock salt.

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<p>A multi-barrier system in which the radioactive waste is encapsulated in metal containers surrounded by a geotechnical barrier (e.g. compacted bentonite) deep underground in a stable geological formation is one of the internationally accepted options for the disposal of highly radioactive waste. Bentonites have good properties such as high swelling capacity and low hydraulic conductivity, which makes them favorable as backfilling material. However, indigenous microorganisms may affect these properties. Bentonite samples were collected from the Full-scale Engineered Barrier Experiment (FEBEX) - Dismantling Project [1] at the Grimsel Test Site (Switzerland) to study their microbial diversity. For that, total DNA was extracted directly from the cores and from enrichments of sulfate- and iron-reducing microorganisms as well as microorganisms were isolated from those enrichments. The microbial communities of the bentonites, the enrichments, as well as the isolates were analyzed by 16S rRNA gene sequencing. The results showed that Actinobacteria and Alphaproteobacteria dominated the FEBEX bentonite microbial population, while the dominant phylum in both enrichments was Firmicutes: concretely, Bacilli and Clostridia classes. In addition, bacteria from the genera <em>Desulfitobacterium</em>, <em>Desulfosporosinus</em> and <em>Clostridium</em> were isolated from the enrichments. <em>Desulfosporosinus hippei</em> DSM 8344 as a phylogenetic close relative was selected to study its interactions with uranium and especially its potential to reduce U(VI) to U(IV). This study revealed that microorganisms are present in bentonite samples after a long-term continuous heating. Sulfate- and iron-reducing microbes were enriched by using favorable conditions in specific media and the potential of the sulfate-reducing microorganisms on the reduction of uranium was verified. Therefore, it is important to characterize the microbial population of the bentonite, because microbes might compromise the safety of the deep geological repository of highly radioactive waste.</p><p>[1]http://www.grimsel.com/gts-phase-vi/febex-dp/febex-dp-introduction</p>