Geoscience Reference
In-Depth Information
similar to hydrophobic sorption. The second mechanism of interest comprises the
specific interactions of a dissociated (ionic) species with various functional groups
on the sorbent surface. Several models developed for predicting the ion exchange
of inorganic ions may be used for predicting this type of sorption. A third sorption
mechanism, molecular ion pairing, involves transfer of organic ions from the
aqueous phase to the organic surface phase. A fourth mechanism covers transfer of
organic ions from the aqueous phase to the organic surface, while the counter-ions
remain in the electric double layer of the aqueous phase. The relative contribution
of each of these mechanisms depends on (1) the extent of compound dissociation
as a function of the acid dissociation constant, pK
a
, and solution pH; (2) the ionic
charge status of the solid interface as a function of the pH and of the point of zero
charge; and (3) the ionic strength and composition of the water phase.
Often, contaminants reach the subsurface as complex mixtures, and therefore,
an understanding of the adsorption process under these more complicated condi-
tions is required. Under a waste disposal site, where organic or organo-metal
complexes are involved, for example, sorption may involve multiphase (water and
organic) solvent interactions. To deal with this combination of parameters, one can
use the theoretical approach of Rao et al. (
1985
). This approach is based on the
predominance of solvophobic interactions for predicting sorption of hydrophobic
organic chemicals from mixed solvents. With increasing volume fractions of a
completely miscible organic solvent in a binary mixed solvent, the hydrophobic
organic solvent sorption coefficient decreases exponentially because the solubility
and sorption coefficient are inversely related.
Further in-depth discussions of nonionic pollutant adsorption on subsurface
components can be found in the classical review of Mortland (
1970
) or in the
reviews of Calvet (
1989
), Hassett and Banwart (
1989
), Hayes and Mingelgrin
(
1991
), Delle Site (
2001
), as well as in a number of topics (such as those by Theng
1974
; Greenland and Hayes
1981
; Saltzman and Yaron
1986
; Yaron et al.
1996
;
and Schwarzenbach et al.
2003
).
5.6 Other Factors Affecting Adsorption
Independent of the molecular properties of contaminants, the subsurface solid-
phase constituents are a major factor that control the adsorption process. Both the
mineral and organic components of the solid phases interact differentially with
ionic and nonionic pollutants, and in all cases, environmental factors, such as
temperature, subsurface water content, and chemistry, affect the mechanism,
extent, and rate of contaminant adsorption.
The structural properties of the subsurface clay fraction are a controlling factor
in defining the rate and extent of the ion exchange process. In the case of kaolinite,
for example, the tetrahedral layers of adjacent clay sheets are held tightly by H
bonds and only planar external surface sites are available for exchange. In contrast,
under adequate hydration conditions, smectites are able to swell, allowing a rapid
