Microbial
communities occurring in reference materials for artificial
barriers (e.g., bentonites) in future deep geological repositories
of radioactive waste can influence the migration behavior of radionuclides
such as curium (CmIII). This study investigates the molecular
interactions between CmIII and its inactive analogue europium
(EuIII) with the indigenous bentonite bacterium Stenotrophomonas bentonitica at environmentally relevant
concentrations. Potentiometric studies showed a remarkably high concentration
of phosphates at the bacterial cell wall compared to other bacteria,
revealing the great potential of S. bentonitica for
metal binding. Infrared spectroscopy (ATR-FTIR) and X-ray photoelectron
spectroscopy (XPS) confirmed the role of phosphates and carboxylate
groups from the cell envelope in the bioassociation of EuIII. Additionally, time-resolved laser-induced fluorescence spectroscopy
(TRLFS) identified phosphoryl and carboxyl groups from bacterial envelopes,
among other released complexing agents, to be involved in the EuIII and CmIII coordination. The ability of this
bacterium to form a biofilm at the surface of bentonites allows them
to immobilize trivalent lanthanide and actinides in the environment.
To date, the increasing production of radioactive waste due to the extensive use of nuclear power is becoming a global environmental concern for society. For this reason, many countries have been considering the use of deep geological repositories (DGRs) for the safe disposal of this waste in the near future. Several DGR designs have been chemically, physically, and geologically well characterized. However, less is known about the influence of microbial processes for the safety of these disposal systems. The existence of microorganisms in many materials selected for their use as barriers for DGRs, including clay, cementitious materials, or crystalline rocks (e.g., granites), has previously been reported. The role that microbial processes could play in the metal corrosion of canisters containing radioactive waste, the transformation of clay minerals, gas production, and the mobility of the radionuclides characteristic of such residues is well known. Among the radionuclides present in radioactive waste, selenium (Se), uranium (U), and curium (Cm) are of great interest. Se and Cm are common components of the spent nuclear fuel residues, mainly as 79Se isotope (half-life 3.27 × 105 years), 247Cm (half-life: 1.6 × 107 years) and 248Cm (half-life: 3.5 × 106 years) isotopes, respectively. This review presents an up-to-date overview about how microbes occurring in the surroundings of a DGR may influence their safety, with a particular focus on the radionuclide-microbial interactions. Consequently, this paper will provide an exhaustive understanding about the influence of microorganisms in the safety of planned radioactive waste repositories, which in turn might improve their implementation and efficiency.
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