Excess Colloid Retention in Porous Media as a Function of Colloid Size, Fluid Velocity, and Grain Angularity

Abstract

The deposition and re-entrainment behaviors of five sizes of carboxylate-modified microspheres (ranging from 0.1 to 2.0 microm) were examined both in porous media and impinging jet systems under a variety of environmentally relevant pore fluid velocities (2-8 m day-'), and in both the absence and the presence of an energy barrier to deposition. The magnitudes of the deposition efficiencies were compared among the porous media and impinging jet systems under equivalent fluid velocities, solution chemistries, and surface chemistries. The observed deposition efficiencies were factors of about 5 to 50 greater in the porous media relative to the impinging jet across the entire size range of microspheres examined, demonstrating that this excess deposition in porous media is relevant to a wide range of colloid sizes. The magnitude of excess deposition increased with increasing fluid velocity, and was greatest for the smallest colloids (0.1 microm). A range between 15% and 40% of the excess retained colloids were released upon introduction of low ionic strength solution, indicating that they were retained via secondary energy minima without direct contact with the grain surfaces. The observations indicate that pore geometry is a critical governor of colloid deposition in the presence of an energy barrier, even in porous media composed of spherical collectors. A portion of this excess deposition results from retention in flow stagnation zones.

Importance of reversible attachment in predicting E. coli transport in saturated aquifers from column experiments

“…Classic colloid filtration theory, valid for uniform-sized particles travelling through uniform-size spherical beads, predicts a positive linear relationship between k i and pore velocity 46,47 . This relationship is qualitatively the same in natural heterogeneously-sized sand since empirical researchers find a linear relationship between k a , k d , k i and pore velocity 14,15,48,49 . This means that when pore velocity stagnates, k a , k d and k i , decrease and pore concentrations would remain more stable than in the single attachment mode models presented here.…”

“…Attachment and detachment rates vary dynamically with pore velocity 14,15,48,49 . In the present 2-D model, however, these rates are assumed to be static.…”

Solid mechanics in colloidal and bacterial filtration

“…Flow velocity is usually observed to be negatively correlated to filtration efficiency (Bergendahl & Grasso, 2000;Shen, Li, Huang, & Jin, 2007;Tong & Johnson, 2006). However when particles were deposited in strong mode the impact of flow velocity is limited (Shen et al, 2007).…”

“…Column test results typically indicated a negative correlation between flow velocity and filtration efficiency (Bergendahl & Grasso, 2000;Hendry et al, 1999;X. Li et al, 2005;Shen et al, 2010;Tong & Johnson, 2006;Torkzaban et al, 2015;Torkzaban et al, 2007). There could be, but not limited to, the following two explanations: (i) higher flow rate break through the clogging caused by colloidal/bacterial aggregation (Bergendahl & Grasso, 2000;X.…”

“…There could be, but not limited to, the following two explanations: (i) higher flow rate break through the clogging caused by colloidal/bacterial aggregation (Bergendahl & Grasso, 2000;X. Li et al, 2005;Shen et al, 2010;Torkzaban et al, 2015;Torkzaban et al, 2007) (ii) colloid/bacteria adhered to the collector surface are detached by the hydrodynamic drag force (Tong & Johnson, 2006). The column test does not provide detailed information on which explanation is more likely or exclusively correct.…”