Electron microscopy of biogenic minerals: structure and sizes of uranium dioxide nanoparticles with Mn$^{2+}$ impurities

Methodological aspects of the extraction of structural and chemical information from transmission electron microscopy (TEM) of uranium dioxide (UO$_2$) biogenic nanoparticles are presented. Nanoparticles were formed via the bacterial reduction of water-soluble uranyl acetate with U (VI) in the presence of Mn$^{2+}$ ions and cultures Shewanella oneidensis MR-1 in the medium. The particles of 1.2–3.5 nm in diameter and particle agglomerations were visualized in conventional TEM, high resolution TEM, scanning TEM modes. Their phase and chemical composition were investigated with electron diffraction, X-ray energy dispersive spectrometry and electron energy loss spectroscopy with high spatial resolution. Maintenance of the element balance helped to find the composition of the mixture of UO$_2$ and Mn acetates. The interpretation of TEM data and modeling allowed to propose the mechanism for the suppression of UO$_2$ particle growth and higher resistance to dissolution of smaller UO$_2$ particles with adsorbed Mn acetate compared to the larger pure particles.