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United States Department of Agriculture

Agricultural Research Service

Research Project: Protection of Food and Water Supplies from Pathogen Contamination Title: A theoretical analysis of colloid attachment and straining in chemically heterogeneous porous media

Authors
item Bradford, Scott
item Torkzaban, Saeed -
item Shapiro, Alexander -

Submitted to: Langmuir
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 20, 2013
Publication Date: June 11, 2013
Repository URL: http://www.ars.usda.gov/SP2UserFiles/Place/53102000/pdf_pubs/P2431.pdf
Citation: Bradford, S.A., Torkzaban, S., Shapiro, A. 2013. A theoretical analysis of colloid attachment and straining in chemically heterogeneous porous media. Langmuir. 29:6944-6952.

Interpretive Summary: A fundamental understanding of processes that control the attachment and straining of colloids (such as microorganisms) in soils is needed to assess and mitigate the risk of a variety of contaminants to water supplies. In this work we use colloids as a surrogate for microbes, and calculate the fraction of the soil surface area that contributes to colloid attachment and straining. Simulation results demonstrate that attachment is more important for higher solution ionic strengths, for more flexible colloids, for greater variations in charge, and for smaller colloids and water velocities. Conversely, straining at roughness locations and grain-grain contacts will be more important for the opposite conditions. Findings from this study will be of interest to scientists and engineers concerned with developing models to predict the fate of colloids, microbes, and nanoparticles in the environments.

Technical Abstract: A balance of applied hydrodynamic (TH) and resisting adhesive (TA) torques was conducted over a chemically heterogeneous porous medium that contained random roughness of height hr to determine the fraction of the solid surface area that contributes to colloid immobilization (Sf*) under unfavorable attachment conditions. This model considers resistance due to deformation and the horizontal component of the adhesive force (FAT), spatial variations in the pore scale velocity distribution, and the influence of hr on lever arms for TH and TA. Values of Sf* were calculated for a wide range of physicochemical properties to gain insight into mechanisms and factors influencing colloid immobilization. Colloid attachment processes were demonstrated to depend on solution ionic strength (IS), the colloid radius (rc), the Young’s modulus (K), the amount of chemical heterogeneity (P+), and the Darcy velocity (q). Colloid immobilization was also demonstrated to occur on a rough surface in the absence of attachment. In this case, Sf* depended on IS, rc, the roughness fraction (f 0), hr, and q. Roughness tended to enhance TA and diminish TH. Consequently, the effect of IS on Sf* was enhanced by hr relative to attachment. In contrast, the effects of rc and q on Sf* were diminished by hr in comparison to attachment. Colloid immobilization adjacent to macroscopic roughness locations shares many similarities to grain-grain contact points and may be viewed as a type of straining process. In general, attachment was more important for higher IS and variance in the secondary minimum, and for smaller rc, q, and K, but diffusion decreased these values. Conversely, straining was dominant for the opposite conditions. Discrepancies in the literature on mechanisms of colloid retention are likely due to a lack of consideration of all of these factors.

Last Modified: 12/18/2014