Nanoscale UO2 and novel complex U(IV)-sulphate phase formation from electrolytically reduced uranyl sulphate solutions

Three different processes have been explored to determine the ranges of U-concentration and acidity (pH-value) for nanocrystalline U2+x precipitation from electrolytically reduced Uranyl sulphate Solutions. Precipitation of U2+x nanoparticles aggregates was found to occur Only ill the narrow range of pH similar to 4.5 to 5 and for U-concentrations of similar to 10(-6) to 10(-7) M. The solid phase crystallized with the typical UO2-fcc structure and with a crystallite size <= 3 nm. The average aggregate size was mostly <= 80 nm. At higher U-concentrations, ranging from [U] similar to 10(-1) M at pH similar to 1.5 to [U] similar to 5 x 10(-4) M at pH similar to 4.9, formation of microscale precipitates of it novel complex U(IV)-sulphate phase Occurred, which was characterized by SEM/EDX-WDX, ICP-OES and XRD-powder diffraction. The crystal Structure was identified with an orthorhombic cell (space group Cmca), with the following lattice parameters: a = 1.9740(2) nm, b = 1.3336(2) nm and c = 2.0643(2) nm. Further composition analyses indicated a basic sulphate hydrate of the type Na3U(SO4)(3)(OH)center dot nH(2)O.