Temperature-Dependent Hydrolysis Reactions of U(VI) Studied by TRLFS

Abstract

Temperature-dependent hydrolysis behaviors of aqueous U(VI) species were investigated with time-resolved laser fluorescence spectroscopy (TRLFS) in the temperature range from 15 to 75°C. The formation of four different U(VI) hydrolysis species was measured at pHs from 1 to 7. The predominant presence of UO2 2+ , (UO2)2(OH)2 2+ , (UO2)3(OH)5 + , and (UO2)3(OH)7 - species were identified based on the spectroscopic properties such as fluores- cence wavelengths and fluorescence lifetimes. With an increasing temperature, a remarkable decrement in the flu- orescence lifetime for all U(VI) hydrolysis species was observed, representing the dynamic quenching behavior. Furthermore, the increase in the fluorescence intensity of the further hydrolyzed U(VI) species was clearly observed at an elevated temperature, showing stronger hydrolysis reactions with increasing temperatures. The formation constants of the U(VI) hydrolysis species were calculated to be log K0 2,2 = -4.0 ‚ 0.6 for (UO2)2(OH)2 2+ , log K 0 3,5 = -15.0 ‚ 0.3 for (UO2)3(OH)5 + , and log K 0 3,7 = -27.7 ‚ 0.7 for (UO2)3(OH)7 - at 25°C and I = 0 M. The specific ion interaction theory (SIT) was applied for the extrapolation of the formation constants to infinitely dilut- ed solution. The results of temperature-dependent hydrolysis behavior in terms of the U(VI) fluorescence were compared and validated with those obtained using computational methods (DQUANT and constant enthalpy equa- tion). Both results matched well with each other. The reaction enthalpies and entropies that are vital for the com- putational methods were determined by a combination of the van't Hoff equation and the Gibbs free energy equa- tion. The temperature-dependent hydrolysis reaction of the U(VI) species indicates the transition of a major U(VI) species by means of geothermal gradient and decay heat from the radioactive isotopes, representing the necessity of deeper consideration in the safety assessment of geologic repository.