We recently discovered a neutral dicalcium uranyl tricarbonate complex, Ca 2 UO 2 (CO 3 ) 3 (aq.), in uranium mining related waters [1]. We are now reporting a further validation of the stoichiometry and the formation constant of this complex using two analytical approaches with time-resolved laser-induced fluorescence spectroscopy (TRLFS) species detection: i) titration of a non-fluorescent uranyl tricarbonate complex solution with calcium ions, and quantitative determination of the produced fluorescent calcium complex via TRLFS; and ii) variation of the calcium concentration in the complex by competitive calcium complexation with EDTA 4− .Slope analysis of the log (fluorescence intensity) versus log [Ca 2+ ] with both methods have shown that two calcium ions are bound to form the complex Ca 2 UO 2 (CO 3 ) 3 (aq.). The formation constants determined from the two independent methods are: i) log β • 213 = 30.45 ± 0.35 and ii) log β • 213 = 30.77 ± 0.25. A bathochrome shift of 0.35 nm between the UO 2 (CO 3 ) 3 4− complex and the Ca 2 UO 2 (CO 3 ) 3 (aq.) complex is observed in the laser-induced photoacoustic spectrum (LIPAS), giving additional evidence for the formation of the calcium uranyl carbonate complex.EXAFS spectra at the L II and L III -edges of uranium in uranyl carbonate solutions with and without calcium do not differ significantly. A somewhat better fit to the EXAFS of the Ca 2 UO 2 (CO 3 ) 3 (aq.) complex is obtained by including the U-Ca shell. From the similarities between the EXAFS of the Ca 2 UO 2 (CO 3 ) 3 (aq.) species in solution and the natural mineral liebigite, we conclude that the calcium atoms are likely to be in the same positions both in the solution complex and in the solid.This complex influences considerably the speciation of uranium in the pH region from 6 to 10 in calcium-rich uranium-mining-related waters.
The use of XAFS spectroscopy and related synchrotron radiation techniques for the molecular-level speciation of environmental contaminants including actinides has led to an improved understanding of the fundamental chemical and biological processes determining their behavior in complex systems. Several recent applications of XAFS spectroscopy to actinides in model systems and more complex environmental samples are reviewed to highlight the impact these studies have on our knowledge about the bioavailability of actinides, the development of remediation strategies, and predictive models for risk assessment. XAFS studies of actinide ion sorption at solid/aqueous solution interfaces are presented in greater detail. Representative examples include XAFS studies in combination with batch-type experiments of U(VI), Np(V), Pu(III), and Pu(IV) sorption on kaolinite.
The uranium carbonate andersonite Na2Ca[UO2(CO3)3] x 6H2O was synthesized and identified with classical analytical and spectroscopic methods. The classical methods applied were powder X-ray diffraction (XRD), nitric acid digestion, and scanning electron microcopy combined with energy-dispersive spectroscopy (SEM/EDS). To characterize andersonite spectroscopically, time-resolved laser-induced fluorescence spectroscopy (TRLFS), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FT-IR) were used. Natural and synthetic andersonite samples were characterized with the nondestructive 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). XPS measurements verified the composition of the synthetic andersonite sample. The measured intensity ratios of the XPS lines agree with the stoichiometry of Na2Ca[UO2(CO3)3] x 6H2O. The XPS features of the inner valence molecular orbitals are characteristic of the [UO2(CO3)3]4- structural moiety. These spectroscopic methods can be used to identify in a fingerprinting procedure secondary U(VI) phases in mixtures with other phases or as thin coatings on mineral and rock surfaces.
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