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)34- to form the ternary mixed U(VI) peroxo-carbonato species UO2(O2)(CO3)24−. Orange rectangular plates of K4[UO2(CO3)2(O2)]·H2O were isolated and characterized by single crystal X-ray diffraction studies. Crystallographic data: monoclinic, space group P21/n, a = 6.9670(14) Å, b = 9.2158(10) Å, c = 18.052(4) Å, Z = 4. Spectrophotometric titrations with H2O2 were performed in 0.5 M K2CO3, with UO2(O2)(CO3)24− concentrations ranging from 0.1 to 0.55 mM. The molar absorptivities (M−1 cm−1) for UO2(CO3)34− and UO2(O2)(CO3)24− 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(O2)(CO3)24−, which has an apparent formation constant of log K′ = 4.70 ± 0.02 relative to the tris-carbonato complex.