Page 60 - gas transport in porous media

Chapter 4: Solid/Gas Partitioning
53
behaves independently and does not affect the other (Baehr and Corapcioglu, 1987;
Nye et al., 1994). This assumption which may be true for aqueous phase sorption of
hydrophobic VOCs (Chiou et al., 1988) but may not be true for vapor phase sorption
under dry conditions. For soils with low moisture conditions, VOCs may compete
with each other for adsorption sites while on the other hand, sorption of one class
of VOC may be enhanced by another class of VOC. If a condensed VOC phase is
formed, this phase, in turn, may act as a medium in which other hydrophobic VOCs
may partition into. Lindner (1990) showed that certain types of VOCs would compete
for sorption sites onto moist minerals while, in some cases, sorption of some types
of VOCs were enhanced. The work done by Lindner was for low concentrations
(P/P o < 2%) and the VOCs used were chlorinated solvents and components of fuel.
Nye et al. (1994) investigated the sorption of m-xylene and n-dodecane vapors on
air-dried and oven-dried soil. They found that on air-dried soil the addition of a
second component increased the sorption of the first for all relative vapor pressures.
However, work done by Amali et al. (1994) showed that the ternary system of TCE,
toluene and water did not indicate any increase in the sorption of one of the VOCs in
the presence of the other compounds.

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