title: Diffusion of sorbed cations in clays: Development, improvement and application of new and existing models
Creators: name: Krejci, Philipp
Publisher: name: Universität Bern
Year: 2023
Subjects: subject: 550 Earth sciences &amp; geology

description: Clays and clay rocks play an important part in the foreseen multi-barrier system of the Swiss radioactive waste repository. The understanding of the transport behavior of ions through clays and clay rocks is crucial to assess the long-term safety of clay barrier systems. A low permeability and a high capacity to adsorb positively charged contaminants make clays and clay rocks suitable materials for such barriers. Many experimental studies revealed that diffusion of cations and anions in clays cannot be explained by pore diffusion alone, as diffusion coefficients derived from experiments were higher for cations and lower for anions than expected for pure pore diffusion. These observations can be attributed to the interactions of cations and anions with the negatively charged clay surfaces. Anions are (partially) excluded from the pore space close to the clay surfaces, while cations are enriched and maintain a certain mobility. These phenomena are called anion exclusion and surface diffusion.
The latter is subject to detailed investigations in this thesis. The main focus of this thesis lies on the development of new reactive transport models for the diffusion of sorbed cations in clays and their application to experimental data. The goal is to gain further insights into the diffusion processes of cations at the clay surfaces. The different modeling approaches used in this thesis are briefly summarized below.
A multi-site surface diffusion model was implemented in the continuum-scale reactive transport code Flotran (chapter 2). This model accounts for pore and surface diffusion by combining their contributions in a single diffusion coefficient that includes surface mobilities as model parameters. The surface mobilities are a measure of how mobile sorbed cations on different adsorption sites are compared to those in bulk pore water. They are used as fit parameters to match experimental data. This model was applied to diffusion data of Cs in Opalinus Clay (chapter 2). A set of surface mobilities could be derived, which describe the concentration-dependent diffusion of Cs consistently over a large range of Cs concentrations. The model results revealed that Cs remains a significant mobility on the so-called frayed-edge sites. The model was then also successfully applied to Na and Sr diffusion in Opalinus Clay (chapter 4), and to Na, Sr and Cs diffusion in Volclay bentonite (chapter 3) using a one-site cation exchange model with a respective cation-specific mobility. However, it became clear that the derived mobilities are not universal parameters, but are dependent on the specific experimental conditions (e.g., ionic strength).
A more detailed model that accounts for diffusion in ‘free’ pore water, in the diffuse layer, in the Stern layer and in interlayers (DL-SL-IL model) was implemented in Flotran (chapter 3). The model is parametrized by data of the clay microstructure and mobilities of cations in the Stern layer and in the interlayers. From this model, a combined diffusion coefficient can be derived, which can be directly compared to experimentally determined diffusion coefficients. Predictions of the DL-SL-IL model for Na, Sr and Cs diffusion were compared to experimental data of Volclay bentonite (chapter 3) and Opalinus Clay (chapter 4). The results of the comparison showed that the DL-SL-IL model is capable of predicting diffusion coefficients of cations under varying bentonite dry density as well as for different Opalinus Clay samples with different pore waters.
Molecular dynamics simulations were performed in order to investigate the mobility of Cs at illite edge surfaces (chapter 5). The simulations were carried out using the code LAMMPS. Various adsorption sites for Cs in the edge region could be identified. Constrained forces MD simulations were used to calculate the potential of mean force at the different sites, from which activation energies and attempt frequencies were determined. The latter were used as input for a jump diffusion model. Based on an effective medium approach a jump diffusion coefficient of Cs at the various illite edge surface sites was calculated. A comparable but somewhat larger mobility of Cs on the illite edges was found than the mobility on the frayed-edge sites determined in chapter 2.