Green synthesis and biotransformation of amorphous Se nanospheres to trigonal 1D Se nanostructures: impact on Se mobility within the concept of radioactive waste disposal

Abstract

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.