Towards a better knowledge of U(VI) speciation in weakly alkaline solution through an in-depth study of U(VI) intrinsic colloids.

The formation of U(VI) intrinsic colloids has a non-negligible impact on the dissemination of actinides in the environment. It is therefore essential to better identify their nature, formation conditions, and stability domains. These specific points are especially important since the behavior of these elements in environment is generally estimated by geochemical transport modeling. This modeling relies on the accurate prediction of their speciation in various situations based on thermodynamic databases that have to be continuously updated. In this work, we identified and characterized the U(VI) intrinsic colloids that grow in model solutions having simplified composition that consists of NaCl and NaCl with NaSiO electrolytes at pH comprised between 7 and 10. Using a simple, reliable and systematic method based on geochemical simulations, elemental analyses by ICP-AES and in situ Small and Wide Angle X-ray Scattering characterizations, several characteristics of U(VI) colloids such as their sizes, volumes, concentration and formula were determined and used to calculate the distribution ratio of U(VI) between intrinsic colloids and solution. This distribution ratio is crucial to enhance predictive geochemical calculation, especially to predict the fraction of U(VI) present as colloids that may migrate into natural waters.