Predicting colloid transport through saturated porous media: A critical review

Abstract

Understanding and predicting colloid transport and retention in water‐saturated porous media is important for the protection of human and ecological health. Early applications of colloid transport research before the 1990s included the removal of pathogens in granular drinking water filters. Since then, interest has expanded significantly to include such areas as source zone protection of drinking water systems and injection of nanometals for contaminated site remediation. This review summarizes predictive tools for colloid transport from the pore to field scales. First, we review experimental breakthrough and retention of colloids under favorable and unfavorable colloid/collector interactions (i.e., no significant and significant colloid‐surface repulsion, respectively). Second, we review the continuum‐scale modeling strategies used to describe observed transport behavior. Third, we review the following two components of colloid filtration theory: (i) mechanistic force/torque balance models of pore‐scale colloid trajectories and (ii) approximating correlation equations used to predict colloid retention. The successes and limitations of these approaches for favorable conditions are summarized, as are recent developments to predict colloid retention under the unfavorable conditions particularly relevant to environmental applications. Fourth, we summarize the influences of physical and chemical heterogeneities on colloid transport and avenues for their prediction. Fifth, we review the upscaling of mechanistic model results to rate constants for use in continuum models of colloid behavior at the column and field scales. Overall, this paper clarifies the foundation for existing knowledge of colloid transport and retention, features recent advances in the field, critically assesses where existing approaches are successful and the limits of their application, and highlights outstanding challenges and future research opportunities. These challenges and opportunities include improving mechanistic descriptions, and subsequent correlation equations, for nanoparticle (i.e., Brownian particle) transport through soil, developing mechanistic descriptions of colloid retention in so‐called “unfavorable” conditions via methods such as the “discrete heterogeneity” approach, and employing imaging techniques such as X‐ray tomography to develop realistic expressions for grain topology and mineral distribution that can aid the development of these mechanistic approaches.

Subsurface Flow and Moisture Dynamics in Response to Swash Motions: Effects of Beach Hydraulic Conductivity and Capillarity

“…Despite the difficulty in simulation, high-frequency flow and moisture dynamics forced by wave swash are important for understanding aspects of beach aquifer systems such as subsurface geochemical conditions, particulate transport, chemical fate, surface evaporation dynamics, and aeolian sediment transport (Geng et al, 2015(Geng et al, , 2016bHuettel et al, 1996;Kim et al, 2017;Malott et al, 2017;Molnar et al, 2015;Nickling & Davidson-Arnott, 1990;Wu et al, 2017). Wave swash strongly controls the flux of water into beach aquifers, as well as flow paths, residence times, and mixing of various chemical species beneath the swash zone (Geng et al, 2014;Malott et al, 2016;Robinson et al, 2014;Xin et al, 2010).…”

Influence of Biochar on Deposition and Release of Clay Colloids in Saturated Porous Media

“…The functional groups cause charge properties locally different from those of bulk biochar surfaces. Various studies (Bradford and Torkzaban, 2013; Johnson et al, 1996; Li et al, 2017; Molnar et al, 2015; Pazmino et al, 2014; Shen et al, 2013) have shown that the presence of surface charge heterogeneity can increase colloid deposition in primary minima by locally reducing the repulsive energy barrier and increasing primary minimum depths and thereby increasing the adhesive forces. Furthermore, the scanning electron microscopy image in Supplemental Fig.…”

A proximity‐based image‐processing algorithm for colloid assignment in segmented multiphase flow datasets

“…Understanding the mechanisms associated with these phenomena can improve risk assessment after a contaminant spill or subsurface pathogen outbreak. Recently published review articles covering both saturated (Molnar et al ., 2015) and unsaturated (Flury & Aramrak, 2017) colloid transport describe modelling and experimental efforts aimed at more accurate prediction of the fate of colloids in the environment. However, these models and calculations are constructed under highly idealized conditions, therefore additional experimental evidence will be required for further model validation and verification.…”