How nanoscale surface heterogeneity impacts transport of nano- to micro-particles on surfaces under unfavorable attachment conditions

Abstract

The impact of nanoscale surface heterogeneity on retention of nano-to-micro-scale particles (colloids) on surfaces governs colloid transport in the environment where unfavorable conditions (repulsive barrier present) are prevalent. Applications include water resource protection and contaminant remediation, and colloid sizes range from viruses and engineered nanomaterials (e.g., ∼50 nm) to protozoa and activated carbon (e.g., ∼5 μm). Prediction and colloid delivery require understanding how nanoscale heterogeneity impacts size dependence of colloid retention under unfavorable relative to favorable conditions. This dependence has not been previously investigated. We report experiments on soda lime glass (silica) with carboxylate-modified polystyrene latex colloids (0.1, 0.25, 1.1, 2.0, 4.4, 6.8 μm) under varied ionic strengths (0.006 and 0.02 M) and pH (6.7 and 8.0) in an impinging jet system representing upstream sides of porous media grains. These experiments demonstrate dramatically reduced attachment efficiencies (α) for n–μ transition colloids (0.2 to 2 μm) relative to smaller (e.g., <0.2 μm) or larger (e.g., >2 μm) sizes with equivalent surface properties. We demonstrate via mechanistic trajectory simulations incorporating discrete representative nanoscale heterogeneity (DRNH) that for n–μ transition colloids, their least combined diffusion and fluid drag in the near-surface fluid domain increased their residence times prior to encountering nanoscale heterogeneity, both phenomena thereby reducing the likelihood of colloid attachment under unfavorable conditions. The generality of this phenomenon was examined using silica colloids, and by compiling reported colloid retention in porous media. We discuss how this new understanding may guide strategies for targeted delivery in porous media.