Page 54 - gas transport in porous media

CHAPTER 4

SOLID/GAS PARTITIONING

SAY-KEE ONG
Dept. of Civil, Construction and Environmental Engineering, Iowa State University, Ames, IA, USA

Some of the earliest environmentally related studies on sorption of vapors onto soils
involved pesticides, herbicides and fumigants. In these studies, the objective was
to assess the extent and effects of adsorption on the performance of the pesticides
(Hance, 1965). Over the last two decades, information on adsorption of pesticides
onto soils is being complimented with information on adsorption of volatile organic
compounds (VOCs) from poor management of waste disposal sites, landfills and
leaking underground storage tanks. Soils in the unsaturated zone are generally coated
with a film of water as the air pores of the unsaturated zone is saturated with water at a
relative humidity greater than 99%. The amount of water retained by soil is a function
of soil grain size, pore sizes, pore volume distribution, and the total surface area. Soils
with very low or dry surfaces are generally found only on the surface of the ground (1–
2 cm from the surface) and soils in arid regions. Moisture content in the unsaturated
zone may be assumed to be approximately close to field capacity.
Vapor partitioning may be visualized as follows; under moist conditions, VOCs
from the vapor phase distribute into soil matrices by (i) sorption at the gas-water
vapor interface, (ii) dissolution into the aqueous phase, (iii) adsorption onto the
mineral surface from the aqueous phase, (iv) partitioning into the organic matter
from the aqueous phase, and (v) condensation of VOCs into pores. Under very low
moisture conditions, typically of surface soils, direct sorption onto the mineral and
direct partitioning into organic matter from the gas phase may occur in competition
with water molecules or sorption may occur on top of the sorbed water molecules.
Figure 4.1 illustrates the various mechanisms.
Sorption of VOCs onto oven-dried soils (typically surface soils) has been found
to be greatly affected by the external surface area of the soil (Call, 1957; Jurinak,
1957; Jurinak and Volman, 1957; Chiou and Shoup, 1985; Rhue et al., 1988; Ong and
Lion, 1991a; Ong and Lion, 1991b). For low concentrations typically encountered
in ambient conditions, partitioning into solid phase may be characterized by a linear
partition coefficient. However for high concentrations or vapor pressures that are
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C. Ho and S. Webb (eds.), Gas Transport in Porous Media, 47–54.
© 2006 Springer.