K. S. Sanakulov, CEO, Navoi Mining and Metallurgical Combinat, Navoi, Uzbekistan
M. A. Meretukov, Research Adviser, Independent Expert, Professor, Doctor of Engineering Sciences, Prague, Czech Republic, mamerat@gmail.com

Nanoparticulate uraninite is found in natural settings such as ore deposits, U-contaminated soils and sediments associated with such natural deposits, spent nuclear fuel waste repositories, and uranium processing plants. Nanoparticulate UO₂ may act as a significant carrier of U (IV) in various environments, including emissions from coal-fired power plants, U mine sediments and systems that have undergone U bioremediation. Reduction of U(VI) and precipitation of nanoparticulate UO₂ can result from both abiotic and biological processes. Abiotic reduction generally proceeds by heterogeneous reduction at Fe (II) sites on the surfaces of Fe (II)-bearing minerals including biotite, magnetite and green rust. Bacteria mediate the reduction of U (VI) to nanoparticulate UO₂ by enzymatically controlled process. In this case U (VI) is used as the terminal electron acceptor during cell metabolism of dissimilatory metal reducing bacteria. Bacterially mediated reduction of uranyl in natural sediments indicates that U (IV) is dominantly present in the form of nanometer-sized particles of UO₂, on the order of 2–10 nm in diameter. Similar sizes are typical of formation of a UO₂ crystalline nanophase in reduction of U (VI) to U (IV) by green rust. Organic matter coating on biogenic, nanoparticles of UO₂ produced by the reduction of aqueous U (VI) by Shewanella putrefaciens, which could potentially affect the surface reactivity of biogenic, nanoparticulate UO₂. The case-study of Anaeromyxobacter bacteria show that uraninite particles formed in sediments by bacterial reduction are typically less than 2 nm, which impedes their stabilization and increases mobility in oil, sediments and water solutions. Another way for the mobilization of the uranium (IV) is colloids formation.

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