Nuclear waste containing radionuclides including selenium isotopes, Se⁷⁹, will be disposed of in future deep geological repositories. Due to the long lifetime of these radioisotopes, studies on the impact of microbial processes on their chemical speciation would contribute significantly to understanding the risks associated with these repositories. Here we report a green method for biogenic reduction of Se(IV), production of amorphous Se(0) (a-Se) nanospheres and their subsequent transformation to one-dimensional (1D) trigonal selenium (t-Se) nanostructures using a combination of methods (XRD, STEM/HAADF, HRTEM/EDX, ESEM, etc.). The bacterial strain used, Stenotrophomonas bentonitica, was isolated from Spanish bentonites considered as artificial barriers for future Spanish repositories. After 24 h of incubation, 30–200 nm sized biogenic individual a-Se nanospheres were synthesized and then started to coalesce, forming aggregates after 48 and 72 h of incubation. The 144 h sample presented a mixture of single crystal and polycrystalline 1D t-Se nanostructures with different shapes (e.g. nanowires, hexagonal, polygonal, etc.) and diameters of 30–400 nm, in addition to a-Se nanospheres. The HRTEM analysis showed that the 1D nanostructures presented different lattice spacings corresponding to the (100), (101) and (111) planes of t-Se. Thus, a-Se nanospheres were initially synthesized and then would transform into t-Se nanostructures. STEM/HAADF and ESEM revealed that the cells and their extracellular proteins play an important role in this transformation process. Due to the low solubility of t-Se nanostructures compared to that of a-Se nanospheres and Se(IV), the mobility of selenium in future repositories may be significantly reduced.

Summary: Radionuclides (RNs) generated by nuclear and civil industries are released in natural ecosystems and may have a hazardous impact on human health and the environment. RN‐polluted environments harbour different microbial species that become highly tolerant of these elements through mechanisms including biosorption, biotransformation, biomineralization and intracellular accumulation. Such microbial–RN interaction processes hold biotechnological potential for the design of bioremediation strategies to deal with several contamination problems. This paper, with its multidisciplinary approach, provides a state‐of‐the‐art review of most research endeavours aimed to elucidate how microbes deal with radionuclides and how they tolerate ionizing radiations. In addition, the most recent findings related to new biotechnological applications of microbes in the bioremediation of radionuclides and in the long‐term disposal of nuclear wastes are described and discussed.

The deep geological repository (DGR) system is widely accepted as the solution for the disposal of radioactive wastes in the future. This concept is based on several natural and engineered barriers such as bentonite clays, which will encase the metal containers holding the radioactive waste. Microorganisms living therein can influence the mobility of the radionuclides (e.g. selenium, uranium, etc.) present in such residues. In this work the bentonite isolate Stenotrophomonas bentonitica is shown to reduce selenite (Se^IV) to elemental Se (Se⁰) nanostructures (amorphous and trigonal) and to volatile methylated Se^−II species. Electron microscopy (HAADF-STEM) analysis of purified Se nanostructures supported the transformation process from amorphous to trigonal Se, proposed in previous studies. Infrared spectroscopy (ATR-FTIR) and X-ray photoelectron spectroscopy (XPS) revealed the presence of amine rich organic matter, covering the nanostructures, suggesting the role of proteins in their synthesis and transformation. In addition, X-ray absorption spectroscopy (XAS) of SeNPs associated to the cells confirmed the formation of different Se⁰ structures (amorphous and crystalline). Finally, the reduction of Se^IV to volatile methylated species (DMDSe and DMSeS) was detected using a gas chromatography-mass spectrometry (GC-MS) system. The oxidation state and molecular coordination of Se in the purified Se nanostructures as well as the volatile Se species, by means of microscopic, spectroscopic, and gas chromatographic techniques, indicated their lower mobility and chemo-toxicity. This study thus highlights the potential environmental significance of microbial processes for the mobility and toxicity of selenium in future repositories, which in turn contribute to their safe implementation.

Elemental selenium (Se ⁰ ) nanomaterials undergo allotropic transition from thermodynamically‐unstable to more stable phases. This process is significantly different when Se ⁰ nanoparticles (NPs) are produced via physico‐chemical and biological pathways. While the allotropic transition of physico‐chemically synthesized Se ⁰ is fast (minutes to hours), the biogenic Se ⁰ takes months to complete. The biopolymer layer covering biogenic Se ⁰ NPs might be the main factor controlling this retardation, but this still remains an open question. Phylogenetically‐diverse bacteria reduce selenium oxyanions to red amorphous Se ⁰ allotrope, which has low market value. Then, red Se ⁰ undergoes allotropic transition to trigonal (metallic grey) allotrope, the end product having important industrial applications (e.g. semiconductors, alloys). Is it not yet clear whether biogenic Se ⁰ presents any biological function, or it is mainly a detoxification and respiratory by‐product. The better understanding of this transition would benefit the recovery of Se ⁰ NPs from secondary resources and its targeted utilization with respect to each allotropic stage. This review article presents and critically discusses the main physico‐chemical methods and biosynthetic pathways of Se ⁰ (bio)mineralization. In addition, the article proposes a conceptual model for the resource recovery potential of trigonal selenium nanomaterials in the context of circular economy.

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.

A Gram-stain negative, rod-shaped, aerobic bacterial strain, BII-R7^T, was isolated during a study targeting the culture-dependent microbial diversity occurring in bentonite formations from southern Spain. Comparative 16S rRNA gene sequence analysis showed that BII-R7^T represented a member of the genus Stenotrophomonas (class Gammaproteobacteria ), and was related most closely to Stenotrophomonas rhizophila e-p10^T (99.2 % sequence similarity), followed by Stenotrophomonas pavanii ICB 89^T (98.5 %), Stenotrophomonas maltophilia IAM 12423^T, Stenotrophomonas chelatiphaga LPM-5^T and Stenotrophomonas tumulicola T5916-2-1b^T (all 98.3 %). Pairwise sequence similarities to all other type strains of species of the genus Stenotrophomonas were below 98 %. Genome-based calculations (orthologous average nucleotide identity, original average nucleotide identity, genome-to-genome distance and DNA G+C percentage) indicated clearly that the isolate represents a novel species within this genus. Different phenotypic analyses, such as the detection of a quinone system composed of the major compound ubiquinone Q-8 and a fatty acid profile with iso-C_15 : 0 and anteiso-C_15 : 0 as major components, supported this finding at the same time as contributing to a comprehensive characterization of BII-R7^T. Based on this polyphasic approach comprising phenotypic and genotypic/molecular characterization, BII-R7^T can be differentiated clearly from its phylogenetic neighbours, establishing a novel species for which the name Stenotrophomonas bentonitica sp. nov. is proposed with BII-R7^T as the type strain (=LMG 29893^T=CECT 9180^T=DSM 103927^T).