Solution coordination chemistry of uranium in the binary UO22+}-SO42- and the ternary UO22+-SO42--OH- system

The structure and reaction dynamics in the systems UO22+-SO42- and UO22+-SO42--OH- were investigated using EXAFS and 17O-NMR spectroscopy. Uranium LIII edge EXAFS indicated a bidentate coordination mode of sulfate to uranyl. In solution, this is characterized by an U-S distance of 3.11 Å. Approximately 5 oxygen atoms were observed in the equatorial plane at 2.39-2.43 Å. The kinetics in the binary uranyl sulfate system can be described by four dominant exchange reactions: (1) UO22+ + SO42- ⇔ UO2SO4 (k1), (2) U*O22+ + UO2SO4 ⇔ U*O2SO4 + UO22+ (k2), (3) UO22+ + UO2(SO4)22- ⇔ 2 UO2SO4 (k3), and (4) UO2SO4 + SO42- ⇔ UO2(SO4)22- (k4). These reactions have rate constants indicating that the exchange is not of the simple Eigen-Wilkins type. Ternary uranyl sulfate hydroxide species were characterized by their 17O chemical shift and by potentiometry. There are no separate signals for the possible isomers of the ternary species indicating that they are in fast exchange with each other.