First identification and thermodynamic characterization of the ternary U(VI) species, UO2(O2)(CO3)24-, in UO2-H2O2-K2CO3 solutions

In alkaline carbonate solutions, hydrogen peroxide can selectively replace one of the carbonate ligands in UO2(CO3)(3)(4-) to form the ternary mixed U(VI) peroxo-carbonato species UO2(O-2)(CO3)(2)(4-). Orange rectangular plates of K-4[UO2(CO3)(2)(O-2)] center dot H2O were isolated and characterized by single crystal X-ray diffraction studies. Crystallographic data: monoclinic, space group P2(1)/n, a = 6.9670(14) angstrom, b = 9.2158(10) angstrom, c = 18.052(4) angstrom, Z = 4. Spectrophotometric titrations with H2O2 were performed in 0.5 M K2CO3, with UO2(O-2)(CO3)(2)(4-) concentrations ranging from 0.1 to 0.55 mM. The molar absorptivities (M-1 cm(-1)) for UO2(CO3)(3)(4-) and UO2(O-2)(CO3)(2)(4-) were determined to be 23.3 +/- 0.3 at 448.5 nm and 1022.7 +/- 19.0 at 347.5 nm, respectively. Stoichiometric analyses coupled with spectroscopic comparisons between solution and solid state indicate that the stable solution species is UO2(O-2)(CO3)(2)(4-), which has an apparent formation constant of log K' = 4.70 +/- 0.02 relative to the tris-carbonato complex.