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Title: Pore-scale simulations to determine the applied hydrodynamic torque and colloid immobilization

Author
item Bradford, Scott
item TORKZABAN, SAEED - Lawrence Berkeley National Laboratory
item WIEGMANN, ANDREAS - Fraunhofer Institute

Submitted to: Vadose Zone Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/1/2010
Publication Date: 2/1/2011
Citation: Bradford, S.A., Torkzaban, S., Wiegmann, A. 2011. Pore-scale simulations to determine the applied hydrodynamic torque and colloid immobilization. Vadose Zone Journal. 10:252-261.

Interpretive Summary: Recent research indicates that microorganism retention in soil is a complex process that depends on forces associated with water flow and chemical properties. The objective of this study was to quantify and predict the forces due to water flow that act on microorganisms adjacent to soil surfaces. This was accomplished through detailed pore-scale simulations of water flow in sphere packs, in conjunction with newly developed theory. This information was subsequently used to determine the fraction of the solid surface area that can contribute to retention and the corresponding maximum solid phase concentration of attached microorganisms for given water flow and chemical conditions. This information will be of interest to scientists and engineers concerned with predicting the fate of microorganisms in the environment.

Technical Abstract: The importance of adhesive and diffusion forces on colloid retention is well established, and theory has been developed in the literature to predict these factors. Conversely, the role of hydrodynamic forces and torques on colloid retention has received considerably less attention. Recent research has indicated that the applied hydrodynamic torque (Tapplied) can play a significant role in colloid retention under unfavorable attachment conditions that are typical of most natural environments. The value of Tapplied varies spatially in porous media over several orders of magnitude due to the pore space geometry, and is also a function of the colloid size and water velocity. Information on the spatial variability of Tapplied in various sized sphere packs (25 spheres) was obtained from pore-scale water flow simulations that were conducted over a wide range of Darcy velocities, grain sizes and distributions, and porosities. This information was used in conjunction with linear interpolation and scaling techniques to predict the log-normal cumulative density function (CDF) of Tapplied for these simulation conditions and for a wide range of colloid sizes. This information was subsequently used to predict the fraction of the solid surface that contributes to colloid retention (Sf) for a given value of the resisting adhesive torque (i.e, interaction energy), and locations on the solid surface where colloid retention are theoretically possible. The predicted values of Sf in this work provides valuable information to predict the combined influence of adhesive and hydrodynamic forces and torques on the colloid sticking efficiency and the maximum solid phase colloid concentration (time and concentration dependence of colloid retention).