Publikationsrepositorium - Helmholtz-Zentrum Dresden-Rossendorf

In order to evaluate the fate and transport of radionuclides in the environment, knowledge about complexation behaviour with inorganic ligands is mandatory. The complex stability constants (log10K) which are required for thermodynamic calculations are mostly determined at ambient conditions (293-303 K). However, high level radioactive waste is expected to considerably increase the temperature in the vicinity of waste disposal sites up to 373 K. The temperature dependence of the log10K value can be calculated if all necessary thermodynamic parameters (log10K(T0), ΔrSo(T0), ΔrHo (T0), and the temperature dependence of ΔrCpo) are known. However, reliable thermodynamic data for most actinide complexes with inorganic ligands, e.g. SO42− or CO32− are still lacking. Theoretical approximations may be helpful to estimate log10K values for higher temperatures, with the actual methods depending on the investigated temperature range and the chemical system.
In this study of the U(VI)−SO42− system, we compare two approximation methods (constant enthalpy of reaction and Ryzhenko-Bryzgalin model - RBM) for the calculation of log10K at different temperatures. Both models show an increase of log10K with increasing temperature for both the 1:1 and 1:2 complex. However, at the lowest and highest temperatures, the RBM gives slightly higher values than the constant enthalpy approach.
These predictions are compared to experimentally determined log10K values as f(T). They are based on various spectroscopic techniques (TRLFS, fluorimeter, UV-vis, conventional Raman and surface-enhanced Raman) and yielded with increasing sulfate-concentrations a stepwise complexation from the free UO22+, to the 1:1, 1:2 and 1:3 complex. This illustrated that a combination of different techniques is helpful for the distinct discrimination of the different complexes.