Complementary colloid and collector nanoscale heterogeneity explains microparticle retention under unfavorable conditions

Abstract

Nano- and micro-scale particle (colloid) retention in environmental granular media is greatly reduced when the profile of colloid–grain surface interactions as a function of their separation distance includes a repulsive barrier. Under such unfavorable attachment conditions, typical of environmental settings, simulation of colloid retention predicts zero retention unless nanoscale heterogeneity is included to locally reduce or eliminate the repulsive barrier and allow attachment to the grain (collector). Simulations previously incorporated nanoscale heterogeneity on either the collector or the colloid, whereas complementary contributions of collector and colloid nanoscale heterogeneity have not been previously examined to the knowledge of the authors. The sizes and surface coverages of nanoscale heterogeneity on colloid (carboxylate modified polystyrene latex, CML) and collector (silica) surfaces that act complementarily to explain experimentally-observed retention in impinging jet experiments was herein examined for colloid sizes ranging from 0.11 to 6.8 μm at pH ranging 6.7 to 8.0 and ionic strength (IS) ranging 6.0 to 20.0 mM. We demonstrate that complementary contributions of power law size-distributed nanoscale heterogeneity; 25 to 90 nm radii on the collector; 5 to 60 nm radii on the colloids, captured the observed retention across the entire colloid size range (including previously uncaptured retention of >2.0 μm CML) for all pH and IS conditions. This approach greatly reduced the required maximum size of heterodomains (nanoscale attractive zones) from 320 nm radii required when heterogeneity was incorporated solely on either the collector or colloid surface and constrains the scales of spectroscopically-observed surface heterogeneity relevant to colloid retention.