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 L-111-edge extended X-ray absorption fine structure (EXAFS) spectroscopy. The oxidation state of uranium was changed in situ using a gastight spectroelectrochemical cell, specifically designed for the safe use with radioactive solutions. For U(VI) we observed the complexes UO2(H2O)(5)(2+), UO2(H2O)(4)Cl+, UO2(H2O)(3)Cl-2(0), and UO2(H2O)(2)Cl-3(-) with [Cl-] increasing from 0 to 9 M, and for U(IV) we observed the complexes U(H2O)(9)(4+), U(H2O)(8)Cl3+, U(H2O)(6-7)Cl-2(2+), and U(H2O)(5)Cl-3(+). The distances in the U(VI) coordination sphere are U-O-ax = 1.76 +/- 0.02 angstrom, O-eq = 2.41 +/- 0.02 angstrom, and U-Cl = 2.71 +/- 0.02 angstrom; the distances in the U(IV) coordination sphere are U-O = 2.41 +/- 0.02 angstrom and U-Cl = 2.71 +/- 0.02 angstrom.