Pore-network modeling of colloid transport and retention considering surface deposition, hydrodynamic bridging, and straining

Abstract

Colloid transport and retention in porous media is a common phenomenon in nature. However, retention mechanisms are not fully revealed based on macroscale experimental observations. The pore-network model (PNM) is an effective method to account for the pore structure of a porous medium and provides a direct connection between pore-scale retention mechanisms and macroscale phenomenon. In this study, PNMs with cylindrical pore throats and spherical pore bodies are used to upscale water flow and colloid transport from pore- to macro-scales, taking into consideration surface deposition, hydrodynamic bridging, and straining. Numerical experiments were conducted to investigate the effect of colloid size, initial concentration, and flow velocity of pore water on colloid transport and retention behavior. Results show that hydrodynamic bridging and straining produce hyper-exponential retention profiles, whereas surface deposition due to nanoscale roughness and charge heterogeneity yields exponential or uniform retention profiles. Hydrodynamic bridging will not happen when the colloid size, initial concentration and flow velocity are lower than some threshold value (rp ≤ 500 nm, C0 ≤ 7.1 × 1014 Nc/m3, U0 ≤ 0.1 m/d under the conditions of this study). The relative importance of hydrodynamic bridging in comparison to surface deposition increases with an increase in the colloid size, initial concentration, and flow velocity. The PNM is a useful tool to discriminate different retention mechanisms and to predict colloid transport and retention behavior in porous media.