Colloid population heterogeneity drives hyperexponential deviation from classic filtration theory

The deposition behaviors of carboxylate-modified polystyrene latex microspheres (six sizes ranging from 0.1 to 2.0 mu m) in packed porous media (soda-lime glass beads) were examined under a variety of environmentally relevant pore fluid velocities (4-8 m-day(-1)) in the presence of an energy barrier to deposition. Hyperexponential profiles of retained colloids were observed for all microsphere sizes (0.1-2.0 mu m) at two fluid velocities (4 and 8 m-day(-1)). Experiments with columns in series demonstrated for three distinct sizes of microspheres that colloid population heterogeneity drove hyperexponential deviation from filtration theory. A significant portion of retained colloids was released upon introduction of low ionic strength solution, indicating that the majority of colloids were retained via secondary energy minima. However, there was no preferential re-entrainment of secondary minimum-associated colloids near the column inlet, indicating that the hyperexponential deviation from classic filtration theory was not due to deposition in the secondary energy minimum.