Structural analysis of the aqueous (UO2)2CO3(OH)3−-complex – A combined approach using ATR FT-IR spectroscopy and DFT calculations

The migration behavior of heavy metal contaminants like actinyl ions (UO22+) in ground water aqui-fers is mainly controlled by sorption processes at water-mineral interfaces. Sorption and therewith the retardation of uranium in the environment depends predominately on its aqueous speciation.

From our recent spectroscopic study, the formation of a dimeric uranyl carbonate hydroxo complex on the surface of gibbsite was derived [1]. The found interatomic distances and coordination numbers for the surface complex are in line with the results of Szabo et al. [2] for the mixture of aqueous (UO2)2CO3(OH)3−-complexes. However, an unequivocal verification of the molecular structure is still lacking. According to previous investigations [2, 3] and predicted aqueous speciation of uranium(VI), a uranyl carbonate hydroxo complex is predominant over a broad pH range. Under near neutral pH conditions, similar to those of natural waters, the (UO2)2CO3(OH)3−-complex is the dominating species over a wide concentration range. Maya et al. [3] determined the stability constant (−log β= 18.63 ± 0.08) and measured the Raman spectrum [4] of the complex. The stoichiometry of this complex and the coordination geometry of the uranyl ion was discussed by means of four possible structures [2] of which the preferred structure.
Up to date, no vibrational spectra of the aqueous complex are available in the literature potentially providing further structural information particularly about the configuration of the carbonate ion. We prepared the dimeric complex under well defined atmospheric conditions and recorded IR spectra. In combination with DFT calculations, the assignment of the spectroscopic data to the respective isomer is accomplished.

1. K. Gückel, A.R., V. Brendler, H. Foerstendorf, Chemical Geology, 2012. under revision.
2. Szabo, Z., H. Moll, and I. Grenthe, Structure and dynamics in the complex ion (UO2)(2)(CO3)(OH)(3)(-). Journal of the Chemical Society-Dalton Transactions, 2000(18): p. 3158-3161.
3. Maya, L., HYDROLYSIS AND CARBONATE COMPLEXATION OF DIOXOURANIUM(VI) IN THE NEUTRAL-PH RANGE AT 25-DEGREES-C. Inorganic Chemistry, 1982. 21(7): p. 2895-2898.
4. Maya, L. and G.M. Begun, A RAMAN-SPECTROSCOPY STUDY OF HYDROXO AND CARBONATO SPECIES OF THE URANYL(VI) ION. Journal of Inorganic & Nuclear Chemistry, 1981. 43(11): p. 2827-2832.