Geoscience Reference
In-Depth Information
Adsorption removes a compound from the bulk phase and thus affects its
behavior in the subsurface environment. Due to some hysteresis effects, sometimes
reflected in formation of bound residues, the release of compounds from the solid
phase to the liquid or gaseous phase does not always reach the amount of adsorbate
retained on solid surfaces.
When measured adsorption data are plotted against the concentration value of
the adsorbate at equilibrium, the resulting graph is called an adsorption isotherm.
The mathematical description of isotherms invariably involves adsorption models
described by Langmuir, Freundlich, or Brauner, Emmet and Teller (known as the
BET model). Discussion of these models is given in Part III, as conditions relevant
to chemical-subsurface interactions are examined.
2.3.1 Adsorption of Charged Ionic Compounds
Adsorption of charged ionic compounds on the surface of a solid phase is subject to a
combination of chemical binding forces and electric fields. The solid phase has a net
charge that, in contact with liquid or gaseous phases, is faced by one or more layers
of counter- or co-ions having a net charge equal to and separated from the surface
charge. Electrical neutrality on the colloidal surface requires that an equal amount of
charge of the opposite sign must accumulate in the liquid phase near the charged
surface. For a negatively charged surface, this means that positively charged cations
are electrostatically attracted to the charged surface. Anions are repelled by such a
surface with diffusion forces acting in an opposite direction, such that there is a
deficit of anions near the surface. The overall pattern, known as a diffuse double
layer (DDL), is described by the Gouy-Chapman theory (see
Sect. 5.4
). This theory
assumes that the exchangeable cations exist as point charges, the colloid surfaces are
planar and infinite in extent, and the surface charge is distributed uniformly over the
entire colloid surface. Stern (
1924
) and Grahame (
1947
) refined this theory, showing
that the counter-ions are unlikely to approach the surface more closely than the ionic
radii of anions and the hydrated radii of cations. Detailed, critical presentations of
diffuse double-layer theory applied to earth materials can be found in Sposito (
1981
,
1989
), Sparks (
1988
), and Bolt et al. (
1991
).
Cation exchange and selectivity are processes involving the cationic concen-
tration in solution, the cation dimensions, and the configuration of the exchange
sites. For cationic molecules, the retention properties follow the relation
A
aq
B
aq
¼ K
k
A
ads
B
ads
;
ð
2
:
44
Þ
where K
k
is a selectivity coefficient that expresses the inequality of the activity
ratios of the cationic molecules, A and B, in solution (aq) and adsorbed (ads). Kerr
(
1928
) was the first to propose this equation, and K
k
is therefore called the Kerr
coefficient.
