2001/08/01 by G. Bernhard, G. Geipel, Gerhard Geipel +8 · 2 citations
Chemistry · Computer Science · Environmental Science · #Atmospheric and Environmental Gas Dynamics #Geochemistry and Geologic Mapping #Radioactive element chemistry and processing
paper · doi:10.1524/ract.2001.89.8.511
crossref issued 2001/08/01 · crossref published 2001/08/01 · crossref published-print 2001/08/01 · openalex publication_date 2001/08/01 · crossref created 2006/08/29 · crossref published-online 2009/09/25 · crossref deposited 2025/04/08 · openalex created_date 2025/10/10 · crossref indexed 2026/07/29 · openalex updated_date 2026/08/03
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) logβ° 213 =30.45±0.35 and ii) logβ° 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.