Quantifying the effects of size exclusion on colloidal particle transport in porous media at a pore scale and column scale

Abstract

Abstract Colloid‐facilitated transport has been widely acknowledged as a significant contributor to surface water and groundwater pollution. Compared to water‐dissolved solutes, colloids are subject to a size exclusion effect (SEE) due to their inherent size when transported through porous media. However, only a few studies have quantitatively assessed SEE on colloid transport in porous media. In this study, we conducted dynamic observation experiments (in a quartz sand micro‐model) using a high‐speed camera and directly observed the size exclusion phenomenon of colloid transport in pores. Moreover, we further performed column experiments to quantitatively characterize the breakthrough curves of silica colloids with diameters ranging from 0.1 to 5 µm through different‐size sands. Silica colloids displayed a distinct SEE, as demonstrated by their earlier breakthroughs compared to the NaNO3 tracer. This observation served as a quantitative indicator of the SEE. The breakthrough curves were fitted using the advection‐dispersion equation to quantify this phenomenon, allowing for calculating the water saturation that is inaccessible to mobile colloids (γ). It was determined that γ exhibited a significant negative linear correlation (p < 0.01) with the average grain size (d50) of porous media. Additionally, power–law correlations were identified between γ and the colloid diameter (dp) and dp/d50. When the grain size of porous media remained below 1031 µm, an increase in the colloid diameter (from 0.1 to 5 µm) resulted in a more noticeable SEE, leading to the proportion of inaccessible pore regions to all pores increasing from 0.017 to 0.098, with this change following a power–law trend. These findings provide valuable insights into quantifying the SEE on silica colloids during transport in porous media. Furthermore, they provide theoretical support for future quantitative research on the mechanisms underlying colloid‐facilitated transport.