Comparative EXAFS investigation of Uranium(VI) and -(IV) aquo chloro complexes in solution using a newly developed spectroelectrochemical 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)4Cl+, UO2(H2O)3Cl20 and UO2(H2O)2Cl3 with [Cl] increasing from 3 to 9 M, and for U(IV) the complexes U(H2O)8Cl3+, U(H2O)6-7Cl22+ and U(H2O)5Cl3+. In the uranium coordination sphere U-O distances are 2.41-0.02 Å and U-Cl distances are 2.71-0.02 Å, independent of oxidation state and chloride concentration.