Radionuclide solubility control in solid solution – aqueous solution systems: Radium solubility in the presence of barite

摘要

The migration of radionuclides in the geosphere is to a large extend controlled by sorption processes onto minerals and colloids. On a molecular level, sorption phenomena involve surface complexation, ion exchange as well as co/precipitation reactions. Coprecipitation leads to the formation of structurally incorporated radionuclides in a host structure. Such solid solutions are ubiquitous in natural systems - most minerals in nature are atomistic mixtures of elements rather than pure compounds. Deriving a thermodynamic model requires a molecular level concept including information such as the actual substitution mechanism, site occupancies and related ordering phenomena. However, radionuclide solubility controlled by solid solutions is currently not considered in long term safety assessments for a nuclear waste repository system. One reason is related to the fact that only a limited number of rather simple solid solution systems have been studied to a sufficient level.The control of Radium solubility in (Ra,Ba,Sr)S04 solid solution - aqueous solution systems under relevant conditions will be discussed. ln general, its solubility is significantly reduced if it is controlled by a binary (Ra,Ba)S04 solid solution or an even more complex ternary (Ra,Ba,Sr)S04 solid solution. New experimental data as well as computational approaches using first principle calculations will be discussed. ln particular with respect to the conditions of interest here, one quest always needs to be considered: Do measured data of the solid solution and the aqueous solution represent equilibrium conditions or do they represent a metastable situation? The challenge to derive thermodynamic data from experiments in aqueous solution at rel. low temperatures with potentially metastable states will be discussed.