Determinations of uranium structures by EXAFS: Schoepite and other U(VI) oxide precipitates

We have investigated the structures of U(VI) oxides precipitated from room temperature aqueous solutions at low ionic strength as a function of pH. Using the uranium L(III)-edge extended X-ray absorption fine structure (EXAFS) and infrared (IR) spectroscopies as probes of the local structure around the uranium, a trend is observed whereby the axial oxygen bond lengths from the uranyl groups increase from 1.80 Angstrom at pH = 7 to 1.86 Angstrom at pH = 11. Shifts in the IR spectral frequencies support this assignment. A concomitant decrease in the equatorial oxygen and nearest-neighbor uranium bond lengths also occurs with increasing pH. Expansion of the linear O=U=O group is seen directly at the L(III) absorption edge where multiple scattering resonances systematically shift in energy. EXAFS curve-fitting analysis on these precipitates and a sample of synthetic schoepite indicate that the structure of the species formed at pH = 7 is similar to the structure of schoepite. At pH = 11, the precipitate structure is similar to that of a uranate.