Modeling the coupled effects of pore space geometry and velocity on colloid transport and retention

Recent experimental and theoretical work has demonstrated that pore space geometry and hydrodynamics can play an important role in colloid retention under unfavorable attachment conditions. Conceptual models that only consider the average pore water velocity and a single attachment rate coefficient are therefore not always adequate to describe colloid retention processes, which frequently produce nonexponential profiles of retained colloids with distance. In this work, we highlight a dual-permeability model formulation that can be used to account for enhanced colloid retention in low-velocity regions of the pore space. The model accounts for different rates of advective and dispersive transport, as well as first-order colloid retention and release in fast and slow velocity regions of the pore space. The model also includes provisions for the exchange of colloids from fast to slow regions in the aqueous phase and/or on the solid phase. A sensitivity analysis performed with the dual-permeability model parameters indicated that low rates of advective transport to low-velocity regions had a pronounced influence on colloid retention profiles, especially near the inlet. The developed model provided a good description of measured colloid breakthrough curves and retention profiles that were collected for a variety of conditions.