The role of surface reactivity in geochemical processes: numerical investigations of adsorption and dissolution mechanisms

Surface reactivity is a fundamental driving force of geochemical processes such as adsorption or dissolution reactions at the mineral-fluid interface. This dissertation combines numerical approaches to simulate and predict mineral surface processes at scales ranging from single nanometers to hundreds of micrometers. By focusing on the interaction between intrinsic surface reactivity heterogeneity and extrinsic factors such as fluid transport, this thesis provides new and fundamental insights and tools for applications in both natural and engineered settings.
The influence of heterogeneous surface reactivity on adsorption processes is investigated on nanotopographic surfaces. Focusing on sheet silicates as important mineral component in host rocks for nuclear waste repositories, the study uses density functional theory (DFT) calculations to quantify the energy barrier for adsorption at surface sites such as steps, kinks, and terraces. Resulting energy values are used to parameterize a kinetic Monte Carlo (KMC) simulation to study the adsorption on nanotopographic mineral surfaces. Low-barrier sites such as steps and kinks are shown to enhance radionuclide retention by up to three times compared to flat surfaces, while desorption occurs primarily at terrace sites. This mechanistic understanding of adsorption variability offers new insight into sorption efficiency, which is important in applications such as nuclear waste containment.
Pulsating dissolution is a recently discovered, dynamic process driven by the self-organization of reactive surface features. KMC simulations in combination with high-resolution rate maps demonstrate that rhythmic variations in local dissolution rates are caused by intrinsic surface-controlled mechanisms. This study rules out extrinsic factors such as transport control, emphasizing the importance of nanotopographic surface architecture as an intrinsic factor controlling surface reactions. This phenomenon challenges conventional steady-state dissolution models.
The influence of surface topography on reactivity and its integration into predictive models is a key problem. Dissolution processes influenced by varying surface reactivity can be parameterized by a nano- or microtopographic surface slope parameter in reactive transport models (RTM) where the simulation of individual surface atoms is no longer feasible. By correlating the surface slope with the density of reactive sites, this parameterization captures the spatial variability of dissolution rates and accurately predicts the evolution of topography, surface reactivity, and geometry. This description of reactivity is critical for an accurate simulation of dissolution at the pore scale. These results demonstrate the importance of including intrinsic parameters for surface-controlled
processes in RTMs where transport processes are not sufficient to describe mineral dissolution. This significantly improves the predictive capabilities of RTMs for complex geochemical systems.
By unifying these insights, this doctoral thesis establishes a methodology for modeling surface reactivity in adsorption and dissolution systems across multiple scales. The integration of nanoscale heterogeneity into RTMs bridges atomistic mechanisms with macroscopic predictions, addressing key challenges in applied fields such as nuclear waste management or carbon capture. These advances provide a foundation for the further development of predictive tools that enhance the understanding and management of complex geochemical processes across time and length scales.