EXAFS investigation of U(VI) and U(IV) aquo chloro complexes with a newly developed spectro-electrochemical cell

The coordination of the U(IV) and U(VI) ions as a function of the chloride concentration in aqueous solution has been studied by U LIII-edge extended X-ray absorption fine structure (EXAFS) spectroscopy. The oxidation state of uranium was changed in situ using a gas-tight spectro-electrochemical cell, specifically designed for the safe use with radioactive solutions. For U(VI) we observed the complexes UO2(H2O)52+, UO2(H2O)4Cl+, UO2(H2O)3Cl20 and UO2(H2O)2Cl3 with [Cl] increasing from 0 to 9 M, and for U(IV) the complexes U(H2O)94+, U(H2O)8Cl3+, U(H2O)6-7Cl22+ and U(H2O)5Cl3+. The distances in the U(VI) coordination sphere are U-Oax = 1.760.02 Å, Oeq = 2.410.02 Å and U-Cl = 2.710.02 Å, the distances in the U(IV) coordination sphere are U-O = 2.410.02 Å and U-Cl = 2.710.02 Å.