Simulation of Colloid Transport and Retention Using a Pore-Network Model With Roughness and Chemical Heterogeneity on Pore Surfaces

Colloid transport and retention in porous media is a common phenomenon in both nature and industry. However, many questions remain on how to obtain colloid transport and retention parameters. Previous work usually assumed constant transport parameters in a medium under a given physicochemical condition. In this study, pore-network modeling is employed to upscale colloid transport and retention from the pore-scale to the macro-scale. The pore-scale transport parameters including the collection efficiency (eta), the sticking efficiency (alpha), and the fraction of the solid-water interface that contributes to the colloid attachment (S-f) are obtained using numerical simulation and probability analysis for each pore throat. The influence of roughness and charge heterogeneity on the distribution of pore-scale parameters is discussed. Breakthrough curves and the retention profiles under different roughness and charge heterogeneity conditions are also analyzed. Results show that pore-scale parameters eta, alpha, and S-f have various distributions in porous media that may not be accurately described using single-valued effective parameters. The value of eta decreases with velocity and exhibits a wide distribution under low-velocity conditions. The parameter alpha tends to decrease with the colloid size and the pore water velocity and increased with the charge heterogeneity fraction. Nanoscale roughness alters alpha in a non-monotonic fashion but tends to increase for lower roughness fractions and zeta potential. Microscopic roughness increases values of alpha for colloids that would otherwise be susceptible to hydrodynamic removal. Breakthrough curves and retention profiles show that more retention occurs for smaller particles, which reflects the influence of blocking.