Geoscience Reference
In-Depth Information
When cations of differing charge are involved, the exchange equation
(Eq.
2.44
) is modified. Gapon (
1933
) proposed that in the case of an exchange
between mono- and bivalent cations, the expression should be
0
@
1
A
M
þ
M
2
þ
M
þ
M
2
þ
¼ K
G
;
ð
2
:
45
Þ
2
1
=
2
ads
aq
where the solute concentration is measured in terms of activity and the adsorption
is measured in terms of equivalents, M denotes metal ion, and K
G
is the Gapon
coefficient.
The exchange properties of negatively charged surfaces do not affect different
cations that have the same valence. This is due to differences in size and polari-
zation among the cations themselves, the structural characteristics of the surfaces,
and the differences in the surface charge distribution. The preference of the
minerals in the subsurface for monovalent cations decreases according to the order
Cs [ Rb [ K [ NH
4
[ Na [ Li, which is known as the lyotropic series. In this
case, a greater attraction of the surfaces for less-hydrated cations may be observed.
This mechanism is relevant to minerals having isomorphic substitution in the
divalent alkaline earth cations and trivalent cations. In the case of organomineral
complexes, the pattern of cation selectivity is modified considerably (Greenland
and Hayes
1981
).
The selectivity of the exchange between two cations is specific to saline sub-
surface systems, described by the sodium adsorption ratio (SAR). Based on Na
+
content in soil solutions, the US Salinity Laboratory (US Salinity Laboratory
1954
)
derived the equation
½
Na
þ
Ca
2
þ
SAR ¼
q
ð
2
:
46
Þ
þ
Mg
2
þ
2
where the solution concentration is in lmol/L.
Metal cation adsorption processes include exchange, Coulombic, and site-
specific adsorption. Heavy metal cations exhibit exchange reactions with nega-
tively charged surfaces of clay minerals. Cationic adsorption is affected by the pH
and in an acid environment (pH \ 5.5), and some heavy metals do not compete
with Ca
2+
(a ubiquitous constituent in the subsurface) for mineral adsorption sites.
At
a
higher
pH,
heavy
metal
adsorption
increases
abruptly
and
becomes
irreversible.
Negative adsorption occurs when a charged solid surface faces an ion in an
aqueous suspension and the ion is repelled from the surface by Coulomb forces.
The Coulomb repulsion produces a region in the aqueous solution that is depleted
of the anion and an equivalent region far from the surface that is relatively
