PNNL

Abstract

The transport of polystyrene microspheres of two different diameters (1.1-µm and 5.7-µm) was examined in packed glass beads (-µm) under a variety of environmentally relevant ionic strength and flow conditions. Numerical models accounting for advection, dispersion, attachment, and limited detachment were capable of simulating the observed breakthrough-elution data, but were incapable of simulating the observed profile of numbers of attached microspheres versus distance from the column entrance, which was much steeper than expected based on a constant rate of attachment across the length of the column. The ubiquity of apparent decreases in attachment rate with distance of transport among microspheres, bacteria, and viruses leads to the conclusion that such effects reflect processes that are fundamental to filtration. Potential processes hypothesized to contribute to the apparent decrease in attachment rate with distance of transport include: distributed interaction potentials among the microsphere population; straining; and depletion of the colloid concentration in the pore water adjacent to grain surfaces. It is shown that all three mechanisms are viable contributors to apparent decreases in attachment rate with distance of transport, and that direct observations are needed to determine the conditions under which each of these potential contributors is relevant.