Uranyl(VI) carbonate complex formation: Validation of the Ca2UO2(CO3)(3)(aq.) species

We recently discovered a neutral dicalcium uranyl tricarbonate complex, Ca2 UO2(CO3)(3)(aq.), in uranium mining related waters [1]. We are now reportin g a further validation of the stoichiometry and the formation constant of t his complex using two analytical approaches with time-resolved laser-induce d fluorescence spectroscopy (TRLFS) species detection: i) titration of a no n-fluorescent uranyl tricarbonate complex solution with calcium ions, and q uantitative determination of the produced fluorescent calcium complex via T RLFS; and ii) variation of the calcium concentration in the complex by comp etitive calcium complexation with EDTA(4-). Slope analysis of the log (fluorescence intensity) versus log [Ca2+] with b oth methods have shown that two calcium ions are bound to form the complex CaUO2(CO3)(3)(aq.). The formation constants determined from the two indepen dent methods are: i) log beta degrees (213) = 30.45 +/- 0.35 and ii) log be ta degrees (213) = 30.77 +/- 0.25. A bathochrorne shift of 0.35 nm between the UO2(CO3)(3)(4-) complex and the Ca2UO2(CO3)3(aq.) complex is observed in the laser-induced photoacoustic s pectrum (LIPAS), giving additional evidence for the formation of the calciu m uranyl carbonate complex. EXAFS spectra at the L-II and L-III-edges of uranium in uranyl carbonate so lutions with and without calcium do not differ significantly. A somewhat be tter fit to the EXAFS of the Ca2UO2(CO3)3(aq.) complex is obtained by inclu ding 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 conclu de that the calcium atoms are likely to be in the same positions both in th e solution complex and in the solid. This complex influences considerably the speciation of uranium in the pH re gion from 6 to 10 in calcium-rich uranium-mining-related waters.