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Title: Coupling of physical and chemical mechanisms of colloid straining in saturated porous media

item Bradford, Scott

Submitted to: Water Resources Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/20/2007
Publication Date: 5/2/2007
Citation: Bradford, S.A., Torkzaban2, S., Walker, S.L. 2007. Coupling of physical and chemical mechanisms of colloid straining in saturated porous media. Water Resources Research. Vol 41:3012-3024

Interpretive Summary: The transport of colloids, such as pathogenic microorganisms, in soil and groundwater environments has been reported to be strongly dependent on the chemical composition of the water. This observation has typically been attributed to chemical interactions between the colloids and soil particles, but this previous research has not fully considered the potential influence of the soil pore size and the water flow rate in this retention process. Experimental and computer modeling studies were undertaken to better understand the influence of small pores on the retention of colloids for various chemical compositions of the water, water flow rates, and sizes of the sand. Results indicate that retention of colloids in small pores is highly dependent on the water composition and flow rate, as well as on the size of the sand and colloid. An improved understanding of the role of physical and chemical factors on colloid removal in small pores is needed to predict the migration and fate of pathogenic microorganisms in many natural systems.

Technical Abstract: Filtration theory does not include the potential influence of pore structure on colloid removal by straining. Conversely, previous research on straining has not considered the potential influence of chemical interactions. Experimental and theoretical studies were therefore undertaken to explore the coupling of physical and chemical mechanisms of colloid straining under unfavorable attachment conditions (pH=10). Negatively charged latex microspheres (1.1 and 3 micron) and quartz sands (360, 240, and 150 micron) were used in packed column studies that encompassed a range in suspension ionic strengths (6-106 mM) and Darcy water velocities (0.1-0.45 cm per min). Derjaguin-Landau-Verwey-Overbeek (DLVO) calculations and batch experiments suggest that attachment of colloids to the solid-water interface was not a significant mechanism of deposition for the selected experimental conditions. Breakthrough curves and hyperexponential deposition profiles were strongly dependent on the solution chemistry, the system hydrodynamics, and the colloid and collector grain size, with increasing deposition occurring for increasing ionic strength, lower flow rates, and larger ratios of the colloid to the median grain diameter. For select systems, the ionic strength of the eluant solution was decreased to 6 mM following the recovery of the breakthrough curve. In this case, only a small portion of the deposited colloids was recovered in the effluent and the majority was still retained in the sand. These observations suggest that the extent of colloid removal by straining is strongly coupled to solution chemistry. Increasing the solution ionic strength is believed to increase the force and number of colloids in the secondary minimum of the DLVO interaction energy distribution. These weakly associated colloids can be funneled to small pores formed adjacent to grain-grain junctions by hydrodynamic forces. Colloid-colloid interactions may be enhanced by hydrodynamic forces and a higher frequency of colloid-colloid collisions in these locations.