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 U 2+x precipitation from electrolytically reduced uranyl sulphate solutions. Precipitation of U 2+x nanoparticles aggregates was found to occur only in the narrow range of pH ∼ 4.5 to 5 and for U-concentrations of ∼10 -6 to 10 -7 M. The solid phase crystallized with the typical UO 2 - fcc structure and with a crystallite size ≤3 nm. The average aggregate size was mostly ≤80 nm. At higher U-concentrations, ranging from [U]∼10 -1 M at pH∼1.5 to [U]∼5×10 -4 M at pH∼4.9, formation of microscale precipitates of a 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 Na 3 U(SO 4 ) 3 (OH)· n H 2 O.