Spectroscopic characterization of the uranium carbonate andersonite Na₂Ca[UO₂(CO₃)₃]*6H₂O

The uranium carbonate andersonite Na₂Ca[UO₂(CO₃)₃]*6H₂O was synthesized and identified with classical analytical and spectroscopic methods. The classical methods applied were X-ray powder diffraction (XRD), nitric acid digestion, and Scanning Electron Microcopy combined with Energy-Dispersive Spectroscopy (SEM/EDS). To spectroscopically characterize andersonite, Time-Resolved Laser-induced Fluorescence Spectroscopy (TRLFS), X-ray Photoelectron Spectroscopy (XPS), and Fourier-Transformed InfraRed spectroscopy (FT-IR) were used. Natural and synthetically prepared andersonite samples were characterized with the non-destructive TRLFS by six fluorescence emission bands at 470.6, 486.1, 505.4, 526.7, 549.6, and 573.9 nm. In addition, andersonite was characterized by FT-IR measurements by the appearance of the asymmetric stretching vibration of the uranyl cation (v₃ UO₂ ²⁺) at 902 cm^-1 with a shoulder at 913 cm^-1 and by XPS measurements. The intensity of the measured XPS lines was used to verify the composition of the synthetic andersonite sample. The measured intensity ratios and the calculated atomic ratios agree with the stoichiometry of Na₂Ca[UO₂(CO₃)₃]*6H₂O. The results of both the qualitative and quantitative XPS analysis confirmed the formation of andersonite. The spectral features of the U 4f electrons and the distinct splitting of the U 6p3/2 electrons can be used to identify the [UO₂(CO₃)₃]^4- structural moiety.

These spectroscopic methods proved to be suitable to identify in a fingerprinting procedure secondary U(VI) phases in mixtures with other phases or as thin coatings on mineral and rock surfaces. The obtained spectroscopic information are of fundamental importance for uranyl(VI) adsorption studies, since such information are useful to identify the coordination environment and structure of unknown adsorbed uranium(VI) surface species on minerals surfaces.