Concrete as an important component of an engineered barrier system in deep geological repositories (DGR) for radioactive waste may come into contact with bentonite, or other clays, rich in indigenous microorganisms, with potentially harmful impacts on barrier integrity. Our study aimed to assess the effect of a concrete environment on indigenous bentonite and groundwater microbial communities as these particular conditions will select for the potentially harmful microorganisms to the concrete in the future DGR. The two-month experiment under anoxic conditions consisted of crushed, aged, low-pH concrete, Czech Ca-Mg bentonite, and anoxic groundwater, with control samples without concrete or with sterile groundwater. The microbial diversity and proliferation were estimated by qPCR and 16S rRNA gene amplicon sequencing. The presence of concrete had a strong effect on microbial diversity and reduced the increase in total microbial biomass, though low-pH concrete harbored indigenous bacteria. The growth of sulfate reducers was also limited in concrete samples. Several genera, such as Massilia, Citrifermentans, and Lacunisphaera, dominant in bentonite controls, were suppressed in concrete-containing samples. In contrast, genera such as Bacillus, Dethiobacter and Anaerosolibacter specifically proliferated in the presence of concrete. Genera such as Thermincola or Pseudomonas exhibited high versatility and proliferated well under both conditions. Because several of the detected bacterial genera are known to affect concrete integrity, further long-term studies are needed to estimate the effect of bentonite and groundwater microorganisms on concrete stability in future DGR.

Research Centre Rez Department of Nuclear Fuel Cycle Husinec Rez 130 25068 Czech Republic

Technical University of Liberec Institute for Nanomaterials Advanced Technologies and Innovations Bendlova 1407 7 461 17 Liberec 1 Czech Republic

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Dramatic loss of microbial viability in bentonite exposed to heat and gamma radiation: implications for deep geological repository

Survivability and proliferation of microorganisms in bentonite with implication to radioactive waste geological disposal: strong effect of temperature and negligible effect of pressure