Speciation of uranium in seepage waters from a mine tailing pile studied by time-resolved laser-induced fluorescence spectroscopy (TRLFS)

Chemical speciation of U(VI) in natural seepage water and corresponding model solutions was investigated by time-resolved laser-induced fluorescence spectroscopy. Calculations of uranium speciation in this medium show that UO2(CO3)(3)(4-) and UO2(CO3)(2)(2-) should be the major individual components. Due to the very low fluorescence intensity, the pure uranyl carbonate complexes could not be measured directly by TRLFS. However, a uranium fluorescence spectrum was recorded from seepage water samples. The TRLFS investigations show that the main component of uranium in this seepage water is a calcium uranium carbonate complex. The main fluorescence wavelengths of this complex are at 463.9, 483.6, 502.8, 524.3 and 555.4 nm. The fluorescence lifetime of the species is 64+/-17 ns. This study shows that the calcium content of the water has a great influence on the uranium speciation. For the first time, the existence of a calcium uranium carbonate complex - {Ca-2[UO2(CO3)3]}((ag.)) - is reported. The stability constant was determined to be log beta = 26.8+/-0.7. Existing thermodynamic data bases do not contain this species and modeling calculation must lead to erroneous results in the medium under investigation.