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Colloid Movement in Unsaturated Porous Media

Recent Advances and Future Directions

^a Department of Civil, Environmental, and Architectural Engineering, University of Colorado at Boulder, 428 UCB, Boulder, CO 80309-0428
^b School of Forestry and Environmental Studies, Yale Univ., Sage Hall, 205 Prospect Street, New Haven, CT 06511

View larger version (69K): [in a new window]   Fig. 1. Processes affecting colloid movement in unsaturated porous media. Colloid deposition mechanisms include attachment to grains by physicochemical filtration, attachment to immobile air–water interfaces (water flow is around bubble trapped in a pore), attachment by straining in water-saturated pores, and entrapment in thinning water films during draining. Colloid mobilization mechanisms include colloid dispersion by chemical perturbation, expansion of water films during imbibition, air–water interface scouring during imbibition and drainage, and shear mobilization (soil profile from Tarbuck and Lutgens, 1997).

View larger version (124K): [in a new window]   Fig. 2. Negatively charged latex colloids (0.95 µm) deposited preferentially onto an air bubble trapped in a pore body of a porous-medium micromodel (from Wan and Wilson, 1994a).

View larger version (39K): [in a new window]   Fig. 3. Model-computed results and those measured in duplicate experiments on silica-colloid mobilization from columns of quartz sand: (A,B) measured specific discharge at column boundaries, (C,D) measured moisture content (symbols) and modeled moisture content (lines) for three positions along the 32-cm-long columns (z = 0 at column top), and (E,F) colloid breakthrough pulses generated by successive increases in flow rate (from Saiers and Lenhart, 2003b).

View larger version (18K): [in a new window]   Fig. 4. Colloid mass flux (filled circles) and porewater flow rates (solid lines) measured during two ponded infiltration experiments. Colloid concentrations peak during the passage of both wetting and drying fronts (from El-Farhan et al., 2000).

View larger version (30K): [in a new window]   Fig. 5. Cumulative colloid mobilization as a function of the square root of time during leaching through intact macroporous soil cores. The linear relationship between these variables suggests that the kinetics of colloid mobilization were controlled by diffusion (from Jacobsen et al., 1997).

View larger version (110K): [in a new window]   Fig. 6. Representation of a single equivalent macropore with colloid mass transfer between three phases: mobile water, immobile water, and crust. The horizontal arrows indicate colloid diffusion between phases, and the vertical arrows indicate advective colloid transport (from Schelde et al., 2002).