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 UO 2 2+ -SO 4 2- and UO 2 2+ -SO 4 2- -OH - were investigated using EXAFS and 17 O-NMR spectroscopy. Uranium L III 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) UO 2 2+ + SO 4 2- ⇔ UO 2 SO 4 (k 1 ), (2) U * O 2 2+ + UO 2 SO 4 ⇔ U * O 2 SO 4 + UO 2 2+ (k 2 ), (3) UO 2 2+ + UO 2 (SO 4 ) 2 2- ⇔ 2 UO 2 SO 4 (k 3 ), and (4) UO 2 SO 4 + SO 4 2- ⇔ UO 2 (SO 4 ) 2 2- (k 4 ). 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 17 O 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.