Assessing the risks of the migration of radiotoxic or chemical markers in the environment relies on our ability to predict the behavior of these pollutants in complex environments where physico-chemical conditions can vary in time and space. Knowledge of the chemical reactions in solution and at solid/solution interfaces must implicitly be linked to the transport properties of the medium. A detailed understanding of the behavior of radioelements in natural environments is therefore essential for the development of predictive reactive transport codes. In real cases of radiological and/or chemical contamination of groundwater and rivers, the source term is generally complex. Interactions between radioelements (“cocktail” effect) can alter their retention properties on solid phases in the surrounding medium. Similarly, the physico-chemical conditions in the environment will influence the speciation of elements in solution, and in particular, their retention properties on reactive phases.
To improve the knowledge of radioelement behavior in soil and groundwater, particularly in a multi-contaminant source (e.g., U, I, Cs, Sr, Ru, Tc), it is essential to understand the behavior of individual radioelements. Ruthenium (Ru), for example, has been identified in the literature as either strongly or weakly mobile, depending on the physico-chemical context. Its behavior in the environment in particular remains poorly understood. In these contexts, Ru occurs primarily in oxidation states +2 to +4, which vary with three main factors: pH, redox potential, and the presence of complexing ligands in solution. To predict the speciation of Ru in natural waters, it is necessary to have complexation constants for the dominant ions in the environment, including ammonium (NH4+), carbonate (HCO3-/CO32-), chloride (Cl-), sulfate (SO42-), nitrate (NO3-), hydroxide (OH-), and phosphate (PO43-). However, equilibrium constants for ruthenium (II-IV) complexes with ligands present under natural conditions are highly variable and limited in the literature. Depending on its speciation in solution, ruthenium can also sorb to or co-precipitate on reactive mineral phases such as clays and carbonates. This chemical reactivity, which depends on the physico-chemical context, is essential for predicting the migration of Ru and other radioelements in the environment.
This thesis aims to fill the gaps in thermodynamic data (complexation in solution, sorption, etc.) for the geochemical modeling of radioelements of interest (in particular ruthenium and technetium) in a natural physicochemical context. It also aims to assess the competitive effects on sorption, both with respect to anions and cations in solution and to mineral phases in the solid medium. This work will include an experimental and geochemical modelling approach.