Comparison of microsphere deposition in porous media versus simple shear systems

Abstract

Experiments were conducted to compare deposition fluxes of 1.1 and 5.7 μm carboxylate-modified polystyrene latex microspheres in packed porous media and simple shear systems. A range of flow conditions were examined in the packed columns (2, 4, and 8 m day −1 average pore water velocities), and impinging jet cells (0.01, 0.03, and 0.05 ml min −1 jet discharge rates), and were scaled to yield equivalent near-surface velocities in the two systems. Two ionic strengths (0.006 and 0.02 M NaCl) were examined under electrostatically unfavorable attachment conditions, whereas electrostatically favorable conditions were examined at a single ionic strength (0.02 M). Deposition fluxes onto quartz and glass substrata were examined. Initial attachment fluxes were sometimes higher, sometimes lower, in the porous media relative to the simple shear systems under equivalent conditions. Deposition efficiencies (deposition flux normalized to deposition flux under favorable conditions) were consistently higher in the porous media relative to the impinging jet when the angular substrate (quartz) was examined, whereas deposition efficiency was lower in the glass bead porous media relative to the glass substratum in the impinging jet under the one condition examined. These results corroborate the hypothesis that deposition in quartz media is enhanced by the presence of rear stagnation points, whereas this phenomenon was not manifested in the smooth spherical porous media. Hydrodynamic drag was implicated as an important governor of the initial attachment flux on the basis of lack of deposition of 5.7 μm microspheres despite theoretical predictions, as well as an observed lack of increase in 1.1 μm microsphere attachment flux with increased flow rate under favorable conditions, and an observed decrease in 1.1 μm microsphere attachment flux with increased flow rate under unfavorable conditions. Experiments run with a mixture of the 1.1 and 5.7 mm microspheres indicated association of the different sized microspheres on the surface. Results from the packed porous media suggested indirect association (via the hydrodynamic field) at the grain surface, whereas observations in the impinging jet indicated direct association of the 1.1 and 5.7 mm microspheres.