Self-encapsulation of organic contaminants

The evaporation of organic contaminants of different volatility from a porous sandfill was investigated in laboratory experiments. A significant decrease of the evaporation rate was observed under conditions of compacted fill, at high concentrations of the low-volatile component. This can be explained by the formation of a layer with a low mass fraction of the solvent near the gas-phase boundary of the contaminated zone. The term ''self-encapsulation'' is suggested for this phenomenon. Theoretical considerations show that this effect can occur if a volatile component evaporates from a mixture with a substance of low volatility and if the total mass transfer resistance is mainly within the contaminant phase. The formation of a solid layer at the surface can enhance this encapsulating effect for the remaining volatile material. A similar behaviour can be expected for contaminants consisting of components of different solubility which get in contact with flowing water. (orig.)