Spectroscopic characterization of alkaline earth uranyl carbonates

A series of alkaline uranyl carbonates, M[UO ₂(CO ₃) ₃]· nH ₂O ( M=Mg ₂, Ca ₂, Sr ₂, Ba ₂, Na ₂Ca, and CaMg) was synthesized and characterized by inductively coupled plasma mass spectrometry (ICP-MS) and atomic absorption spectrometry (AAS) after nitric acid digestion, X-ray powder diffraction (XRD), and thermal analysis (TGA/DTA). The molecular structure of these compounds was characterized by extended X-ray absorption fine-structure (EXAFS) spectroscopy and X-ray photoelectron spectroscopy (XPS). Crystalline Ba ₂[UO ₂(CO ₃) ₃]·6H ₂O was obtained for the first time. The EXAFS analysis showed that this compound consists of (UO ₂)(CO ₃) ₃ clusters similar to the other alkaline earth uranyl carbonates. The average U-Ba distance is 3.90±0.02 Å.Fluorescence wavelengths and life times were measured using time-resolved laser-induced fluorescence spectroscopy (TRLFS). The U-O bond distances determined by EXAFS, TRLFS, XPS, and Raman spectroscopy agree within the experimental uncertainties. The spectroscopic signatures observed could be useful for identifying uranyl carbonate species adsorbed on mineral surfaces.