Page 56 - gas transport in porous media

Chapter 4: Solid/Gas Partitioning
49
Table 4.1. Examples of solid–gas sorption coefficients onto dry soils
Solid–gas partition Solid–water partition
Organic carbon Surface area coefficients, coefficient,
2
3
3

Soil type content (%) (m /g) K (cm /g) K d (cm /g)
d
Rhineback a 3 82.5 1,377 1.41
Harford a 1.6 44.0 1,669 0.96
Gila a 0.4 98.5 1,831 0.3
a
Montmorillonite 0.02 733 23,300 0.31
Iron Oxides b N/A 273 15,840 N/A
Kaolinite b N/A 10 440 N/A
Aquifer Soil c 0.1 18.7 1,464 0.1
Yolo c 1.1 80.6 3,401 0.58
a
Ong and Lion (1991a)
b
Shimizu et al. (1992)
c
Peterson et al. (1995)
N/A – not available
properties of the sorbate and sorbent. Most of these equations relate the adsorption
coefficients to the enthalpies of vaporization, enthalpies of adsorption, and saturation
vapor pressures (e.g., Goss and Schwarzenbach, 1998; Goss and Schwarzenbach,
1999). However, these equations do not consider the effects of moisture or humid-
ity on the sorption of VOCs. Typical solid–vapor partitioning coefficients, K ,

d
for trichloroethylene (TCE) on oven-dried soils are presented in Table 4.1. As a
comparison, the aqueous solid–water partitioning coefficients, K d , are also presented.
In a moist environment, the mass of VOC sorbed is less than the mass of VOC
sorbed on oven-dried surfaces due to competition for sorption sites between water
molecules and VOCs (Call, 1957; Chiou and Shoup, 1985; Rhue et al., 1989; Ong
and Lion, 1991b; Thibaud et al., 1992; Goss, 1993; Storey et al., 1995). The charged
surfaces of minerals and soils have a higher affinity or preference for water molecules
than the hydrophobic vapor compounds. Figure 4.2 shows the impact of different
relative humidities on the sorption of TCE onto alumina.
Assuming that the mass of VOC sorbed for each process is additive, the mass of
VOC sorbed can be written as:

Total mass sorbed = Mass sorbed at the solid–water interface
+ Mass dissolved in the liquid phase
+ Mass sorbed at the gas–water interface
+ Mass sorbed at the solid–gas interface
+ Condensation into pores

If sufficient moisture is present on the surface of the soil, the mass sorbed at the
solid–gas interface can be neglected. Ignoring condensation, the vapor partitioning