Geology Reference
In-Depth Information
exchange capacity (CEC) for a given phase is a measure of the number
of exchange sites present per 100 g of material. This is operationally
defined by the uptake of ammonium ions from 1 mol L
1
ammonium
acetate at pH 7. The specific surface area and CEC are given in Table 9
for several sorption-active materials.
There are several factors that influence the affinity of cations towards
a given surface. Firstly, the surface coverage will increase as a function
of the cation concentration. Secondly, the affinity for the exchange site is
enhanced as the oxidation state increases. Finally, the higher the charge
density of the hydrated cation, the greater will be its affinity for the
exchange site. In order of increasing charge density, the group I and II
cations are
Ba
o
Sr
o
Ca
o
Mg
o
Cs
o
Rb
o
K
o
Na
o
Li
4.3.2.4 Role of Surface Chemistry in Biogeochemical Cycling. Reac-
tions at the aqueous-particle interface have several consequences for
material in the marine environment, from estuaries to the deep-sea
sediment-water boundary. Within estuarine waters, suspended particles
experience a dramatic change in the composition and concentration of
dissolved salts. A number of halmyrolysates can be formed. Clay
minerals undergo cation exchange as Mg
21
and Na
1
replace Ca
21
and K
1
. Alternatively, montmorillonite may take up K
1
becoming
transformed to illite. Hydrogenous components, in particular Mn and
Fe oxides, may be precipitated onto the surfaces of suspended particles.
The particulate material generally accumulates organic coatings within
estuaries, which together with an increase in the ionic strength of the
surrounding solution, leads to the formation of stable colloids. Both the
oxide and organic coatings can subsequently scavenge other elements in
the estuary.
Table 9 The specific surface area and cation-exchange capacities of several
sorption-active materials
Specific surface area
(mg
2
g
1
)
Cation-exchange capacity
(meq/100 g)
Material
Calcite (
o
2 mm)
12.5
—
Kaolinite
10-50
3-15
Illite
30-80
10-40
Chlorite
—
20-50
Montmorillonite
50-150
80-120
Freshly precipitated Fe(OH)
3
300
10-25
Amorphous silicic acid
—
11-34
Humic acids from soils
1900
170-590
Source: adapted from Fo¨ rstner and Wittman, 1981.
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