Pore-scale simulation of micro and nanoparticle transport in porous media

The transport and deposition of colloidal particles in saturated porous media are processes of considerable importance in many fields of science and engineering, including the propagation of contaminants and of microorganisms in aquifer systems and the use of micro- and nano-particles as reagents for groundwater remediation interventions. Colloid transport is a peculiar multi-scale problem: pore-scale phenomena and inter granular dynamics have an important impact on the larger-scale transport. In this thesis a microscale approach was used to gain a better understanding of the mechanisms underlying colloidal processes, such as deposition and aggregation. The research activity was carried out by performing numerical simulations through the FEM software, COMSOL Multiphysics®. The first part of the study focuses on the development of a new correlation equation to predict single collector efficiency, a key concept in filtration theory, which allows predicting particle deposition on a single spherical collector. By performing Eulerian and Lagrangian simulations in a simple geometry and by using an innovative approach to interpret the results, a new correlation equation to predict single collector efficiency has been formulated. A hierarchical approach to interpret the results was exploited. The proposed correlation equation presents innovative features, such as the validity for a wide range of parameters (also at very small Peclet numbers), the prediction of efficiency values always lower than unity, the total flux normalization and the analysis of the mutual interactions between the main transport mechanisms (advection, gravity and diffusion) and the steric effect. The final formula was also extended to include porosity and a reduced model was proposed. The second part of the study focuses on more realistic systems, characterized by a column of spherical collectors in series. The numerical simulations performed show the limits of the existing models to interpret the experimental data. Therefore, a more rigorous procedure to evaluate the filtration processes in presence of a series of collectors was developed.