Experimental Confirmation of the Interception History Paradigm for Colloid (Micro and Nanoparticle) Transport in Porous Media

For pathogens, engineered nanomaterials, micro- and nanoplastics, and other colloids, variance from expectations of Colloid Filtration Theory is well-demonstrated under unfavorable conditions where a repulsive barrier exists in colloid-surface interactions. Specifically, their retention profiles (RPs) are nonexponential. We present experiments demonstrating that nonexponential RPs arise from variations in interception history among attached colloids wherein the fraction of the colloid population that attaches after multiple interceptions is negligible under favorable conditions and is significant to dominant under unfavorable conditions. We show that RPs were exponential only for colloids that attached under favorable conditions, whereas RPs were nonmonotonic for colloids that attached under unfavorable conditions, with RPs for multiple intercepting attachers assuming γ distributions having maxima at transport distances that increased with interception order. We show that in our experiments the value of attachment efficiency (α) was greater for multiple than single interception attachers, and we speculate on the origin of this change in α. We emphasize that such variance from overall exponential RP reflects a fundamental aspect of colloid transport under unfavorable conditions, as they arise without significant variations in colloid size, surface properties, and density, and without straining and detachment.