4.1.2 Non-adsorptive Retention
Trapping
The entrapment of solid particles and large dissolved molecules in the pore space of
the soil forms one of the major mechanisms in the retention of pollutants in the soil. This type
of retention occurs following the three mechanisms
1. Caking. This may occur physically when the pollutant particles are larger than the
soil pores. In this case, the entrapped particles form a layer (cake) on the surface where the
pore sizes become too small.
2. Straining. Straining occurs when pollutant particles are about the size of the soil
pores. They move down the pores until they are entrapped at the entrance to a pore which is
too small to pass through.
3. Physical-chemical trapping. The limitation of the flow, through the clogging of the
pore space, may occur because of physical or chemical transformation, such as the
production, by chemical reactions, of new products having molecular sizes that exceed that of
the soil pores.