Submitted to: Journal of Environmental Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/21/2003
Publication Date: 8/4/2003
Citation: BRADFORD, S.A., SIMUNEK, J., BETTAHAR, M., VAN GENUCHTEN, M.T., YATES, S.R. MODELING COLLOID ATTACHMENT, STRAINING, AND EXCLUSION IN SATURATED POROUS MEDIA. JOURNAL OF ENVIRONMENTAL SCIENCE AND TECHNOLOGY. 2003. v. 37. p. 2242-2250. Interpretive Summary: Knowledge of the processes that control colloid transport and fate is required to assess contamination potential and protect drinking from pathogenic microorganisms, and to quantify colloid-facilitated transport of many contaminants (heavy metals, radionuclides, pesticides, pharmaceuticals, and pathogens). The ability to accurately simulate colloid transport is hampered by limited knowledge of the processes controlling colloid migration. This work presents a conceptual model to describe colloid accessibility (exclusion) and interactions (attachment and straining) within the soil. This information should aid scientist and engineers concerned with pathogen transport and fate in soils, and in the development of efficient strategies to minimize groundwater contamination.
Technical Abstract: A conceptual model for colloid transport is developed that accounts for colloid attachment, straining, and exclusion. Colloid attachment and detachment is modeled using first-order rate expressions, whereas straining is described using an irreversible first-order straining term that is depth dependent. Exclusion is modeled by accounting for the colloid accessible pore space in transport parameters. Fitting attachment and detachment model parameters to colloid transport data provided a reasonable description of effluent concentration curves, but the spatial distribution of retained colloids at the column inlet was severely underestimated in systems that exhibited significant colloid mass removal. A more physically realistic description of the colloid transport data was obtained by simulating colloid attachment, detachment, and straining. Fitted straining coefficients were found to systematically increase with increasing colloid size and decreasing median grain size. A correlation was developed to predict the straining coefficient from colloid and porous medium information. The use of this correlation and predicted attachment coefficients captured the trends in the experimental data. Numerical experiments indicated that increasing the colloid excluded volume of the pore space resulted in earlier breakthrough and higher peak effluent concentrations as a result of higher pore water velocities and lower residence times, respectively. The velocity enhancement due to colloid exclusion was predicted to increase with increasing exclusion volume and increasing soil gradation.