Page 114 - Partition & Adsorption of Organic Contaminants in Environmental Systems

ADSORPTION OF WATER AND ORGANIC VAPORS 105

tion of cations on surfaces may also reduce the surface sites otherwise avail-
able for adsorption.) For K-SAz-1, the relatively weak and linear water uptake
+
appears to reflect a gradual increase in K hydration with increasing humidity
2+
(P/P°); for Ca-SAz-1, the strong Ca hydration leads to a sharp and nonlin-
ear water uptake (Chiou et al., 1997). On kaolinite, the water uptake is only
moderately higher than the benzene uptake, as the solid has a siloxane plane
on one side and a presumably more hydrophilic gibbsite plane on the other.
The water uptake by kaolinite may also involve a small hydration with a resid-
ual amount of cations situated on clay edges. Thus, except possibly for mont-
morillonite with weak solvating cations, water is adsorbed generally more
strongly than benzene.
On activated carbon, the very weak water uptake relative to an organic
vapor uptake has been ascribed to the small polarizability per unit (liquid)
volume of water and to the exceptionally large water cohesive energy density
(Manes, 1998), that is, the cohesive energy of water is much greater than the
adhesive energy between water and nonpolar activated-carbon surfaces. The
observed small water uptake at low relative humidity (P/P°) probably results
from adsorption onto the ash (mineral) of activated carbon and from the polar
and H-bonding interactions of water with trace oxygenated impurities on the
carbon surface. At high P/P°, the condensation of water vapor into the carbon
pore space is promoted by the lower energy needed to concentrate the vapor
to saturation for adsorptive condensation. The similarity in maximum
adsorbed (liquid) volume between benzene and water (ca. 0.64mL/g for
benzene and 0.57mL/g for water) is characteristic of the adsorbate pore-filling
process with a highly porous adsorbent.
The strong water adsorption on most minerals has a direct consequence on
the efficiency of mineral adsorption of organic solutes (contaminants) from
water solution. The adsorption of nonpolar and weakly polar solutes from
water on most minerals is expected to be greatly suppressed by water (because
of their weak adsorptive competition against water). As to montmorillonite
clays, or such clays in soils or sediments, it is also reasonable to expect that
their adsorption of nonpolar solutes from water will probably not be signifi-
cant. In rare cases where soils have a very high montmorillonite content that
is saturated primarily with poor solvating cations, their adsorption of highly
polar solutes from water may then become significant. The adsorption of
organic solutes from water by activated carbon, or possibly by charcoal-like
natural substance, should not be strongly suppressed by water. In the next
chapter, we will be concerned with the uptake of organic compounds from
water and other media to soils (or sediments), the latter comprising both
organic and mineral matter as their basic constituents. The preceding account
on adsorption of water vapor against an organic vapor (benzene) provides a
good background for elucidating the roles of mineral and organic matter in
the soil uptake.