Spectrophotometric investigation of U(VI) chloride complexation in the NaCl/NaClO_4 system

摘要

Post closure radioactive release scenarios from geologic salt formation, such as the WIPP (Waste Isolation Pilot Plant) (USA) include hydrologic transport of radionuclides through a chloride saturated aquifer. Consequently, the understanding of actinide solution chemistry in brines is essential for modeling requiring accurate knowledge of the interaction between AnO_2~(2+) and chloride ions. Complexation constants of two U(VI) chloride species UO_2Cl~+ and UO_2Cl_2~0 have been intensively studied for about 40 years using differnt methods. However, large uncertainties reflect the general difficulty in determining accurate stability constants of weak complexes. In order to model the behavior of U(VI) in brines, we studied the formation of its chloride complexes by UV-Vis spectroscopy as a function of the NaCl concentration at 25 deg C. The experiments were performed at constant ionic strength by varying the concentration ratio of NaCl and NaClO_4. Deconvolution resulted in single component absorption spectrum for UO_2Cl_2~0. The apparent stability constants of UO_2Cl~+ and UO_2Cl_2~0 are determined at different ionic strengths. The experimental data are used to parameterize using the SIT approach.