TRLFS study on U(VI) chemical species at various temperatures

Understanding of the chemical behaviors of radionuclides in a deep geological system is a matter of cardinal importance for the safety assessment of nuclear waste repository since the mobility of radionuclides is closely dependent of various geochemical reactions in aquifer systems. Therefore, knowledge of reliable chemical thermodynamic data is a prerequisite for the prediction of radionuclide and/or pollutant migration. Hydrolysis behaviors of uranium(VI) have been studied in the temperature range from 15 .deg. C to 75 .deg. C by utilizing TRLFS (time-resolved laser fluorescence spectroscopy). Fluorescence properties (fluorescence spectrum shape, position and lifetime) of the uranium(VI) hydroxide species, namely, UO22+, (UO2)2(OH)22+, (UO2)3(OH)5+, and (UO2)3(OH)7- were investigated with the time-resolved fluorescence intensity and a numerical fit method. With increasing temperature, the spectrum peak width was broadened, and the fluorescence lifetime was rapidly decreased due to dynamic quenching. Spectroscopic properties resulted from the present work were in good agreement with other literature data. The formation constants for the uranium(VI) hydroxide species were calculated to be log K'2,2 = -4.3 ± 0.6 for (UO2)2(OH)22+, log K'3,5 = -15.4 ± 0.3 for (UO2)3(OH)5+, and log K'3,7 = -28.2 ± 0.7 for (UO2)3(OH)7- at I = 0.5 M H/ NaClO4 and at 25 .deg. C. As expected, strong hydrolysis reactions of uranium(VI) chemical species were observed at elevated temperature, and the temperature-dependent formation constants were compared with some computational methods like the DQAUNT equation and the constant enthalpy equation. Both results are in good agreement with each other. In addition, the activation energies based on the Arrhenius equation were obtained from the temperature-dependent fluorescence lifetimes. Finally, the formation of Ca-UO2-CO3 complexes as being one of the dominant uranium complexes in natural groundwater systems was also investigated as a function of the uranium and calcium concentrations at room temperature. Based on the fluorescence characteristics, four different binary/ternary uranium(VI) carbonate and hydroxide species, Ca(UO2)(CO3)32-, Ca2(UO2)(CO3)3(aq), UO2(CO3)22-, and UO2(OH)3- were identified in the pH range of 7 - 12. These fluorescence results were discussed and compared to the findings in natural groundwater sample