Thermodiffusion of Solutes in Nanoporous Materials

Thermodiffusion describes the motion of solutes along a temperature gradient, also called the Soret effect. With respect to the geological disposal of high-level radioactive wastes, a better understanding of the thermodiffusion of solutes in geomaterials is essential for the evaluation of radionuclide migration for scenarios of early canister failure during the thermal phase of the repository. In nanoporous (geo)materials (e.g., clay rocks, cementitious materials), the interaction of ions with mineral surfaces plays an important role for the motion of ions, but its theoretical understanding is still incomplete. In this work, we investigated the thermodiffusion of charged tracers in saturated nanoporous silica using complementary experiments and simulations. The fluxes of potassium tracers in the porous solid under different external temperature gradients were determined by through diffusion experiments. To reveal the mechanisms of thermodiffusion of nanoconfined ions, a coupled thermal-electrochemical model was developed to analyze the different contributions to the ion flux. Our model shows that the gradient of the surface charge density induced by the applied temperature field leads to electrophoretic ionic mobility, which significantly promotes ionic transport compared to the classical thermophoretic motion. This study improves the understanding of the underlying mechanisms that govern the transport of solutes in nanoporous geomaterials under an external temperature gradient.