The hydrolysis of dioxouranium(VI) investigated using EXAFS and O-17-NMR

The structure of dioxouranium(VI) as a function of pH at different (CH3)(4) N-OH concentrations has been investigated with the aid of U L-III-edge EXAF S. Polynuclear hydroxo species were identified by an U-U interaction at 3.8 0(8) Angstrom at pH = 4.1. The precipitate formed at pH = 7 has a schoepite like structure. In solution at high pH [0.5 M (CH3)(4)N-OH], the EXAFS dat a are consistent with the formation of a monomeric four coordinated uranium (VI) hydroxide complex UO2(OH)(4)(2-) of octahedral geometry. The first she ll contains two O atoms with a U=O distance of 1.83(o) Angstrom, and four O atoms were identified at a U-O distance of 2.26(5) Angstrom. In strong alkaline solutions [>1 M (CH3)(4)N)-OH],O-17-NMR spectra indicate the presence of two species, presumably UO2(OH)(4)(2-) and UO2(OH)(5)(3-), the latter in low concentration, which are in rapid equilibrium with one a nother at 268 K in aqueous solution.