Nonadsorptive Retention of Contaminants

Nonadsorptive (physical) retention of chemicals in the subsurface has received less attention, despite the fact that significant quantities of contaminants can be retained by processes other than purely adsorptive ones. 

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Nonadsorptive retention of contaminants may occur when chemicals reach the subsurface as a separate liquid phase or are adsorbed on suspended particles or orgaiuc residues. Contaminated suspended particles originating from sludge disposal or polluted runoff, for example, can represent a substantial hazard to the subsurface environment. 

Nonadsorptive retention of contaminants can also be beneficial. For example, oil droplets in the subsurface are effective in developing a reactive layer or decreasing the permeability of a sandy porous medium. Coulibaly and Borden (2004) describe laboratory and field studies where edible oils were successfully injected into the subsurface, as part of an in-situ permeable reactive barrier. The oil used in the experiment was injected in the subsurface either as a nonaqueous phase liquid (NAPL) or as an oil-in-water emulsion. The oil-in-water emulsion can be distributed through sands without excessive pressure buildup, contrary to NAPL injection, which requires introduction to the subsurface by high pressure. 

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