Simulation of colloid transport and retention using a pore-network model with roughness and chemical heterogeneity on pore surfaces

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

Submitted to: Water Resources Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/23/2020
Publication Date: 1/6/2021
Citation: Lin, D., Hu, L., Bradford, S.A., Zhang, X., Lo, I.M. 2021. Simulation of colloid transport and retention using a pore-network model with roughness and chemical heterogeneity on pore surfaces. Water Resources Research. 57(2). Article e2020WR028571. https://doi.org/10.1029/2020WR028571.
DOI: https://doi.org/10.1029/2020WR028571

Interpretive Summary: An understanding and ability to predict colloid transport and fate in soils is needed for many industrial, agricultural, and environmental applications. A mathematical model was developed to simulate colloid migration in soils by representing it as a network of interconnected pores (capillary tubes) of different lengths and sizes. This approach allowed colloid transport and retention processes in individuals pores to be accurately captured and described in the pore network. Results demonstrate that colloid retention parameters are not constant as commonly assumed, but changes with the pore size and length, the velocity, the type and amount of charge variations and roughness properties to create a distribution in the soil network. This information will be of interest to scientists and engineers concerned with predicting the fate of colloid contaminants, like pathogenic microorganisms, in soils and groundwater.

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