CONCENTRATION DEPENDENT TRANSPORT OF COLLOIDS IN SATURATED POROUS MEDIA

Authors: BETTAHAR, MEHDI - UC RIVERSIDE POST-DOC; Bradford, Scott

Submitted to: Journal of Contaminant Hydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/9/2005
Publication Date: 11/14/2005
Citation: Bettahar, M., Bradford, S.A. 2005. Concentration dependent transport of colloids in saturated porous media. Journal of Contaminant Hydrology. 82:99-117.

Interpretive Summary: Many environmental contaminants occur as colloid particles (pathogenic microorganisms) or are associated with colloids (heavy metals, radionuclides, pesticides, and pharmaceuticals). Knowledge of the processes and factors that control colloid transport and fate is therefore required to assess contaminant potential and extent, and to design efficient remediation strategies. This manuscript reports on studies examining the influence of colloid concentration on colloid migration. Results indicate that variations in colloid concentration can significantly influence transport behavior, with lower colloid concentrations producing less mobility and more retention by the soil. The size of the colloid particle and the soil also influenced this behavior, with increased retention occurring in finer textured soils and for larger colloids. Theory and a mathematical model used to predict and describe colloid transport was improved to characterize the observed data.

Technical Abstract: A series of soil column experiments was undertaken to explore the influence of colloid input concentration on the transport and fate of several colloid sizes in three soils. Stable mono-dispersed colloids and porous media that are negatively charged were employed in these studies. Decreasing the colloid input concentration and porous medium grain size, and increasing the colloid size, all resulted in increased mass retention at the column inlet and lower relative concentrations in the effluent. Conventional colloid attachment and/or blocking theory was not consistent with this behavior because it predicts much less retention at the soil inlet and increasing retention with increasing input concentration. Alternatively, the retention of colloids was hypothesized to be hindered at higher concentrations due to an increased frequency of colloid collisions. The trend of decreasing peak effluent concentration with decreasing grain size of the sand or increasing size of the colloid was attributed to straining, which increases in frequency and strength in finer textured media and for larger colloids. Higher colloid concentrations and coarser textured sands also produced a slightly earlier breakthrough time for colloids compared to that of bromide, presumably due to enhanced anion exclusion. The colloid transport data was described and analyzed using a numerical model that accounts for colloid attachment, detachment, blocking, straining, and a novel colloid liberation term. Simulations demonstrate that colloid attachment, blocking, and detachment had a relatively minor influence on the observed transport behavior. Consideration of colloid attachment, straining, and liberation significantly improved the description of the experimental data.