Retention Site Contribution Toward Silver Particle Immobilization in Porous Media

Abstract

AbstractThis work investigates the role that pore structure plays in colloid retention across scales with a novel methodology based on image analysis. Experiments were designed to quantify–with robust statistics–the contribution from commonly proposed retention sites toward colloid immobilization. Specific retention sites include solid‐water interface, air‐water interface, air‐water‐solid triple point, grain‐to‐grain contacts, and thin films. Variable conditions for pore‐water content, velocity, and chemistry were tested in a model glass bead porous medium with silver microspheres. Concentration signals from effluent breakthrough and spatial profiles of retained particles from micro X‐ray Computed Tomography were used to compute mass balances and enumerate pore‐scale regions of interest in three dimensions. At the Darcy‐scale, retained colloids follow non‐monotonic deposition profiles, which implicates effects from flow‐stagnation zones. The spatial distribution of immobilized colloids along the porous medium depth was analyzed by retention site, revealing depth‐independent partitioning of colloids. At the pore‐scale, dominance and overall saturation of all retention sites considered indicated that the solid‐water interface and wedge‐shaped regions associated with flow‐stagnation (grain‐to‐grain contacts in saturated and air‐water‐solid triple points in unsaturated conditions) are the greatest contributors toward retention under the tested conditions. At the interface‐scale, xDLVO energy profiles were in agreement with pore‐scale observations. Our calculations suggest favorable interactions for colloids and solid‐water interfaces and for weak flocculation (e.g., at flow‐stagnation zones), but unfavorable interactions between colloids and air‐water interfaces. Overall, we demonstrate that pore‐structure plays a critical role in colloid immobilization and that Darcy‐, pore‐ and interface‐scales are consistent when the pore structure is taken into account.