Page 109 - Partition & Adsorption of Organic Contaminants in Environmental Systems

100 ADSORPTION OF VAPORS ON MINERALS AND OTHER SOLIDS

6.5 ADSORPTION OF WATER AND ORGANIC VAPORS

In the N 2 uptake by solids, the vapor is adsorbed mainly via the induced dipole
(London) forces between the vapor and the solid, since N 2 has zero dipole
moment, although its small quadrupole moment may contribute to a weak
polar interaction with the solid. By contrast, water has a large dipole moment
and an exceptional H-bonding power; thus the adsorption of water vapor on
a solid is expected to depend critically on the surface property of the solid. If
water vapor penetrates a solid as well as adsorbing on the solid surface, the
result will be more complicated. In general, a polar vapor should adsorb much
more efficiently onto a polar solid surface than onto a nonpolar surface
because of powerful polar and H-bonding interactions between adsorbate and
adsorbent (solid). The adsorption of a relatively nonpolar vapor onto various
solids should be largely independent of the surface polarity, since the adsorp-
tion derives primarily from the dispersion forces between adsorbate and solid
surfaces. The first situation is especially merited for the adsorption of water
vapor onto various natural solids. In sharp contrast to water, benzene is largely
apolar, having only a very weak H-bonding character (Barton, 1975). Thus, the
adsorption of benzene vapor on a solid should be insensitive to the solid polar-
ity, and the adsorption data should be largely representative of the adsorptive
behavior of other nonpolar or weakly polar organic vapors. The purpose for
comparing the adsorption data of water with the data of benzene on organic-
matter-free solids is to give the reader a simple and clear picture of the rela-
tive adsorptive powers of water and uncharged organic compounds on the
various minerals and natural solids. This information helps explain the com-
petitive adsorption of water against nonionic organic solutes onto the polar
soil mineral component.
The adsorption isotherms of water and benzene vapors on a range of solid
samples, where the adsorbed mass per unit weight of the solid, Q (mg/g), is
plotted against the relative pressure (P/P°), are illustrated in Figures 6.7 to
6.13. In comparing the adsorbed masses of different vapors on a given solid,
it is necessary to take into account the difference in density of the condensed
adsorbates, since an adsorbate with a higher density will invariably lead to a
greater adsorbed mass when a given solid surface area or a given pore space
is occupied. For vapors condensed onto a smooth (open) surface area,the mass
2/3
1/3
of a condensed liquid (adsorbate) is related approximately to its d l M value,
as mentioned earlier. When vapors condensed into a pore space, as with acti-
vated carbon or charcoal, the adsorbed mass per unit filled volume is approx-
imately proportional to d l of the adsorbate. Thus, if the adhesive force between
adsorbate and solid is greater than the adsorbate’s cohesive force and if the
adhesive force involved is primarily of London force (Manes, 1998), one would
expect benzene vapor to exhibit a somewhat greater mass uptake than water
vapor on a smooth surface; the reverse would apply for vapor adsorption onto
a pore space. We will subsequently examine the adsorptive behavior of water
with some minerals and other solids.