Colloid Deposition in Environmental Porous Media: Deviation from Existing Theory is the Norm; Not the Exception

Existing theories describing the process of colloid filtration in natural and engineered contexts assume that the rate of deposition onto porous media is spatially invariant. This article demonstrates that colloid deposition rate coefficients vary ubiquitously with transport distance under environmental conditions (conditions involving repulsion between colloids and porous media surfaces) for both biological and non biological colloids. This is demonstrated via laboratory experiments in packed beds of quartz sand and glass beads using polystyrene latex microspheres and a bacterial strain isolated from soil. The form of spatial variation of the deposition rate coefficient depends sensitively on solution conditions and ranges from monotonic decrease with distance to non-monotonic increase-then-decrease with distance. These findings require re-examination of water treatment and protection protocols, as well as colloid transport models, that assume a spatially invariant deposition rate coefficient.