Modeling Colloid Transport in Unsaturated Porous Media and Validation with Laboratory Column Data

Abstract

Colloidal particles can act as carriers to enhance the transport of contaminants in porous media by reducing retardation effects. When colloids are present, especially in the vadose zone, the system can be treated as consisting of four phases: an aqueous phase, the stationary soild matrix phase, a carrier (colloidal particles) phase, and a stagnant air phase. In the work reported a mathematical model based on mass balance equations was developed to describe the transport and fate of colloids in an unsaturated porous medium. Colloidal particles can sorb onto the air‐water interface as well as to the solid matrix surfaces. Colloid/matrix mass transfer rate is represented by first‐order kinetics, while the colloid/air‐water interface mass transfer mechanism is formulated by second‐order kinetics. The model was applied to simulate the migration of colloids through an unsaturated finite column. Numerical solutions were obtained to provide estimates of colloidal concentrations. A sensitivity analysis of the transport model was utilized to assess the effect of several parameters on model behavior. The aqueous phase colloid concentration is quite sensitive to changes in the rate coefficient of colloidal deposition on the solid matrix. However, the colloidal capture on the air‐water interface is affected by the fraction of air‐water interface available for particle deposition as well as by the rate coefficient of colloidal capture on the air‐water interface, k3. As k3 increases, the available sorption site reduces quickly. Furthermore, increasing colloidal sorption capacity significantly retards the colloidal migration. The model results match successfully with experimental data of Wan and Wilson [1994a] and Wan et al. [1994]. A comparison between the breakthrough curves of hydrophobic and hydrophilic colloids suggests that the hydrophobic colloids have less affinity for both solid matrix and the air‐water interface. Experimental data and model results show that the presence of the air‐water interface retards the colloid transport very significantly. Finally, the proposed model is extended to simulate the colloid transport under transient water flow conditions. Simulations show that aqueous phase colloid concentration varies along the column following a trend similar to the saturation profile for both hydrophobic and hydrophilic colloids. However, the migration profile of hydrophilic colloid suspension is faster than the hydrophobic one.