Uranyl(VI) Carbonate Complex Formation: Validation of the Ca2UO2(CO3)3(aq.) Species

We recently discovered a neutral dicalcium uranyl tricarbonate complex, Ca2UO2(CO3)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 Na2EDTA.
Slope analysis of the log (fluorescence intensity) versus log [Ca2+] with both methods have shown that two calcium ions are bound to form the complex Ca2UO2(CO3)3(aq.). The formation constants determined from the two independent methods are: i) log ßo213 = 25.26 ± 0.71 and ii) log ßo213 = 25.57 ± 0.37.
A bathochrome shift of 0.35 nm between the UO2(CO3)34- complex and the Ca2UO2(CO3)3(aq.) complex was observed in the UV-vis absorption spectrum and in the laser-induced photoacoustic spectrum (LIPAS), giving additional proof for the formation of the calcium uranyl carbonate complex.
EXAFS spectra at the LII and LIII - edges of uranium in uranyl carbonate solutions with and without calcium did not differ significantly. A somewhat better fit to the EXAFS of the Ca2UO2(CO3)3(aq.) complex was obtained by including the U-Ca shell. From the similarities between the EXAFS of the Ca2UO2(CO3)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.