title: First identification and thermodynamic characterization of the ternary U(VI) species, UO2(O2)(CO3)2(4-), in UO2-H2O2-K2CO3 solutions.
abstract: 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(O2)(CO3)2(4-). 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 P2(1)/ n, a = 6.9670(14) A, b = 9.2158(10) A, c = 18.052(4) A, Z = 4. Spectrophotometric titrations with H 2O 2 were performed in 0.5 M K 2CO 3, with UO2(O2)(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(O2)(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(O2)(CO3)2(4-), which has an apparent formation constant of log K' = 4.70 +/- 0.02 relative to the tris-carbonato complex.
authors.affiliation: Chemistry and Material Physics and Applications Divisions, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA.
authors.forename: George S
authors.initials: GS
authors.lastname: Goff
authors.forename: Lia F
authors.initials: LF
authors.lastname: Brodnax
authors.forename: Michael R
authors.initials: MR
authors.lastname: Cisneros
authors.forename: Shane M
authors.initials: SM
authors.lastname: Peper
authors.forename: Stephanie E
authors.initials: SE
authors.lastname: Field
authors.forename: Brian L
authors.initials: BL
authors.lastname: Scott
authors.forename: Wolfgang H
authors.initials: WH
authors.lastname: Runde