Abstract
The structure of dioxouranium(VI) as a function of pH at different (CH3)4N-OH concentrations has been investigated with the aid of U LIII-edge EXAFS. Polynuclear hydroxo species were identified by an U-U interaction at 3.808Å at pH = 4.1. The precipitate formed at pH = 7 has a schoepite like structure. In solution at high pH [0.5 M (CH3)4N-OH], the EXAFS data are consistent with the formation of a monomeric four coordinated uranium(VI) hydroxide complex UO2(OH)42-of octahedral geometry. The first shell contains two O atoms with a U=O distance of 1.830Å, and four O atoms were identified at a U-O distance of 2.265Å.In strong alkaline solutions [>1 M (CH3)4N)-OH],17O-NMR spectra indicate the presence of two species, presumably UO2(OH)42-and UO2(OH)53-, the latter in low concentration, which are in rapid equilibrium with one another at 268 K in aqueous solution.
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