Geoscience Reference
In-Depth Information
Table 5.1
Chemical composition and charge characteristics of selected layer silicates (McBride
1994
)
Mineral
Chemical
structure
Structure
Charge per
half unit cell
Structural
charge
(cmol
c
/kg)
Tetrahedral
Octahedral
Montmorillonite
Ca
0.165
Si
4
(Al
1.67
Mg
0.33
)
O
10
(OH)
2
2:1 dioctahedral
0
-0.33
92
Beidelite
Ca
0.25
(Si
3.5
Al
0.5
)Al
2
O
10
(OH)
2
2:1 dioctahedral
-0.5
0
135
Talc
Si
4
Mg
3
O
10
(OH)
2
2:1 trioctahedral
0
0
0
Vermiculite
Mg
0.31
(Si
3.15
Al
0.85
)
(Mg
2.69
Fe
0.23
2:1 trioctahedral
-0.85
0.23
157
3+
Fe
0.08
2+
)O
10
(OH)
10
Kaolinite
Si
2
Al
2
O
5
(OH)
4
1:1 dioctahedral
0
0
0
Serpentine
Si
2
Mg
3
O
5
(OH)
4
1:1 trioctahedral
0
0
0
forces, which in turn stabilize colloidal suspensions. An opposite situation supports
interparticle association in negatively charged colloids and induces flocculation.
These behaviors validate the diffuse double-layer model, which assumes that the
layer-silicate surface can be treated as a structurally featureless plane with an
evenly distributed negative charge (van Olphen
1967
).
Oxides and hydroxides of Al, Fe, Mn, and Si may exist in the subsurface mainly
as a mixture (known also as a solid solution) rather than as pure mineral phases.
They are considered amphoteric materials, characterized by no permanent surface
charge. Their cation and anion exchange capacities reflect adsorption of potential-
determining ions such as H
+
and OH
-
. Different surfaces have a diverse affinity
for H
+
and OH
-
ions and thus exhibit various points of zero charge (PZC). Details
of various models for variable charge minerals may be found in the extensive
review of McBride (
1989
).
Humic substances, including both humic and fulvic acids, are the main sub-
surface organic components capable of adsorbing contaminants. The functional
groups on humic materials control the cation exchange capacity (CEC) and the
complexation of metals. In the case of humic substances, for example, the CEC
generally is calculated to be at least one electric charge (i.e., ionized group) per
square nanometer (Oades
1989
) although in some cases it may range from 0.3 to
1.3 (Greenland and Mott
1978
). Because humic substances are polydisperse and
characterized by diverse chemistry, it is difficult to obtain a well-defined under-
standing of their capacity for ion exchange and metal complexation.
Organo-mineral association in the subsurface is a natural process controlled by
a range of bonding mechanisms, and therefore, it is practically impossible to
separate one from other. The resulting organo-mineral complex has surface
properties different from the original components. For example, hydrophilic clay
surfaces may become hydrophobic.
