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

Authors: LIN, DANTONG - Tsinghua University; HU, LIMING - Tsinghua University; Bradford, Scott; ZHANG, XINGHAO - Tsinghua University; LO, IRENE - Hong Kong University Of Science

Submitted to: Journal of Hydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/27/2021
Publication Date: 10/2/2021
Citation: Lin, D., Hu, L., Bradford, S.A., Zhang, X., Lo, I.M. 2021. Pore-network modeling of colloid transport and retention considering surface deposition, hydrodynamic bridging, and straining. Journal of Hydrology. 603 Part B. Article 127020. https://doi.org/10.1016/j.jhydrol.2021.127020.
DOI: https://doi.org/10.1016/j.jhydrol.2021.127020

Interpretive Summary: Knowledge of colloid transport and fate in soils is needed to protect water resources from contaminants like pathogenic microorganisms. A colloid transport model that represents soils as a network of interconnected pores (capillary tubes) of different lengths and sizes was extended to account for various physical and chemical retention mechanisms. This modeling approach allowed the relative importance of these retention mechanisms to be quantified. Physical retention mechanisms were found to increase with the ratio of the colloid to the pore size, the colloid concentration, and the water velocity. This information will be of interest to scientists and engineers concerned with predicting the fate of colloid and colloid-associated contaminants in soils and groundwater.

Technical 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.