Submitted to: Journal of Environmental Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/21/2010
Publication Date: 3/1/2010
Publication URL: http://www.ars.usda.gov/SP2UserFiles/Place/53102000/pdf_pubs/P2295.pdf
Citation: Torkzaban, S., Kim, H.K., Simunek, J., Bradford, S.A. 2010. Hysteresis of Colloid Retention and Release in Saturated Porous Media During Transients in Solution Chemistry. Journal of Environmental Science and Technology. 44(5):1662-1669. Interpretive Summary: Natural soil and groundwater environments experience transients in solution chemistry as a result of rainfall, irrigation, and evapotranspiration. These transient conditions influence the chemical interactions between soil particles and colloids, such as pathogenic microorganisms, and colloid-associated contaminants. The objective of this work was to study colloid retention and release during transients in solution chemistry to gain insight on mechanisms controlling the fate of colloids in the environment. Results indicate the importance of chemical interactions to immobilize colloids. During transient chemical conditions the role of pore structure, soil roughness, colloid aggregation, colloid release and movement of colloids on the soil surface become important factors. This information will be of interest to scientists and engineers concerned to predicting the fate of colloids and microorganisms in the environment.
Technical Abstract: Saturated packed column and micromodel transport studies wereconducted to gain insightonmechanismsof colloid retention and release under unfavorable attachment conditions. The initial deposition of colloids in porous media was found to be a strongly coupled process that depended on solution chemistry and pore space geometry. During steady state chemical conditions, colloid deposition was not a readily reversible process, and micromodel photos indicated that colloids were immobilized in the presence of fluid drag.Uponstepwise reduction in eluting solution ionic strength (IS), a sharp release of colloids occurred in each step which indicates that colloid retention depends on a balance of applied (hydrodynamic) and resisting (adhesive) torques which varied with pore space geometry, surface roughness, and interaction energy. When the eluting fluid IS was reduced to deionized water, the final retentionlocationsoccurredneargrain-graincontacts,andcolloid aggregation was sometimes observed in micromodel experiments. Significant amounts of colloid retention hysteresis with IS were observed in the column experiments, and it depended on the porous medium (glass beads compared with sand), the colloid size (1.1 and 0.5 µm), and on the initial deposition IS. These observations were attributed to weak adhesive interactions thatdependedonthe double layer thickness (e.g., the depth of the secondary minimum and/or nanoscale heterogeneity), colloidmasstransfer on the solid phase to regions where the torque and force balances were favorable for retention, the number and extent of grain-grain contacts, and surface roughness.