Effects of Collector Surface Roughness On Mechanisms and Morphology of Colloid Deposition in Porous Media Under Unfavorable Conditions.

A thorough understanding of colloid deposition mechanisms is essential for protecting groundwater from being contaminated by biocolloids (e.g., viruses, bacteria, and protozoa) and by those contaminants whose transport can be facilitated by mobile particles. Predictions of colloid retention in porous media under unfavorable conditions by the classic filtration theory (CFT) have been found to deviate considerably from experimental observations. The discrepancies have, in part, been attributed to the fact that CFT does not consider the influences of collector surface roughness, which commonly exists in natural porous media. Our study examined the morphology of colloid deposition onto rough collector surfaces under unfavorable conditions via laboratory experiments and theoretical analyses. Saturated column experiments were conducted with bacteria- and virus-sized latex microspheres and various collectors (sand and glass beads) at different solution ionic strengths and flow rates. Confocal microscope and micromodels were used to visualize deposition of the particles in sand and glass bead systems. Results from this study indicate: 1) the location of colloid deposition on rough surfaces depends on colloid retention mechanism, i.e., via primary- or secondary-minimum; 2) surface roughness favors colloid deposition not only in primary-energy minimum via decreasing energy barrier but also by increasing secondary-minimum deposition by increasing the dominance of adhesive over hydrodynamic drag torque; and 3) depending on the deposition location of the colloids, their release responds differently to changes in solution chemistry and hydrodynamics. In this presentation, we will discuss the interplay among solution chemistry, flow rate, and surface roughness and their combined effects on colloid retention in porous media.