Colloid Transport in Environmental Granular Porous Media

Introduction. Colloid transport in porous media is relevant to environmental and engineered contexts ranging from granular filtration for drinking water treatment to design of porous media for separation processes. It is useful here to briefly highlight particular environmental contexts. One such context is pathogen transport in groundwater, as driven by concern for disease outbreaks and the corresponding need to improve pathogen transport prediction.

The concern for an increasing presence of engineered nanoparticles in the environment drives the need to better understand and predict their transport in porous media. An ongoing challenge over the past several decades concerns delivery of engineered nanoparticles or bacteria with novel metabolic properties to remediate sites contaminated with nonaqueous phase organic liquids. Additionally, colloid-facilitated contaminant transport (e.g. radionuclides or other sorbing contaminants) drives interest in prediction of colloid transport in porous media.

Our goal in this article is to give the reader a conceptual understanding of the fundamental processes governing colloid transport in porous media. We purposefully use a conversational tone since this is our distillation of the accumulated literature. We hope that we can provide a rapid understanding of the complex processes that might otherwise be difficult to cull from the accumulated literature. Readers interested in a deeper-than-conceptual understanding may consult the many references provided herein, including recent reviews. We hope that nonexperts and experts alike will find our conceptual approach helpful. We do not intend this to be a complete review of the field, and we apologize that many deserving papers are not cited.

What Filtration Is Not: Straining. You may wonder what is so hard about predicting the transport of pathogens and other particulate contaminants in groundwater. To appreciate the complexity of this problem, you first need to understand that filtration in sand and other porous media, as performed in many natural and engineered contexts, is not due to straining. Straining is the capture of particles too large to pass through the pore throats between sand grains (e.g. rinsing seeds in a strainer). If straining were the removal mechanism in filtration, our natural and engineered sand filters would rapidly clog, rendering them useless. Instead, to be effective, sand filtration must allow nano- and micro-sized particles (herein called colloids) to penetrate into the porous media and attach on grain surfaces even though the colloids are much smaller than the pore throats.

Unlike molecules (which are tens to hundreds of times smaller than nanoparticles), colloids do not diffuse rapidly enough to necessarily find surfaces when moving in the pore space. They move like blimps relative to mosquitos. So, unlike molecules, colloids may or may not reach a surface. Hence, colloids do not achieve equilibrium with sediment, wherein transfer toward and away from surfaces eventually equalizes. Equilibrium allows transport of molecules to/from surfaces to be predicted with a simple experimentally determined partition constant, not so for colloids. If colloids do reach a surface, their relatively large size produces relatively strong van der Waals attraction that holds them onto the surface.

They detach only if some sudden shift in solution conditions (e.g. pressure) forces their detachment. While many studies have inferred straining as the primary process of colloid retention in packed columns, weaknesses in these inferences have been reviewed. For our purposes, straining is not interesting, since beyond a given threshold, there is no dependence on colloid size, fluid velocity, and colloid-collector interaction, as described in detail in Johnson et al.