Colloid Transport and Retention in Unsaturated Porous Media: A Review of Interface‐, Collector‐, and Pore‐Scale Processes and Models

Abstract

A: AWI, air-water interface; DLVO, Derjaguin-Landau-Verwey-Overbeek; SWI, solid-water interface.S S : V Z M Our ability to accurately simulate the transport and reten on of colloids in the vadose zone is currently limited by our lack of basic understanding of colloid reten on processes that occur at the pore scale. This review discusses our current knowledge of physical and chemical mechanisms, factors, and models of colloid transport and reten on at the interface, collector, and pore scales. The interface scale is well suited for studying the interac on energy and hydrodynamic forces and torques that act on colloids near interfaces. Solid surface roughness is reported to have a signifi cant infl uence on both adhesive and applied hydrodynamic forces and torques, whereas non-Derjaguin-Landau-Verwey-Overbeek (DLVO) forces such as hydrophobic and capillary forces are likely to play a signifi cant role in colloid interac ons with the air-water interface. The fl ow fi eld can be solved and mass transfer processes can be quan fi ed at the collector scale. Here the potenal for colloid a achment in the presence of hydrodynamic forces is determined from a balance of applied and adhesive torques. The frac on of the collector surface that contributes to a achment has been demonstrated to depend on both physical and chemical condi ons. Processes of colloid mass transfer and reten on can also be calculated at the pore scale. Diff erences in collector-and pore-scale studies occur as a result of the presence of small pore spaces that are associated with mul ple interfaces and zones of rela ve fl ow stagna on. Here a variety of straining processes may occur in saturated and unsaturated systems, as well as colloid size exclusion. Our current knowledge of straining processes is s ll incomplete, but recent research indicates a strong coupling of hydrodynamics, solu on chemistry, and colloid concentra on on these processes, as well as a dependency on the size of the colloid, the solid grain, and the water content.

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