Geology Reference
In-Depth Information
mechanism relates to the acid-base behaviour of oxides in suspension.
Metal oxides (most commonly Fe, Mn) and clay minerals have frayed
edges on account of broken metal-oxygen bonds. The surfaces can be
hydrolysed and exhibit amphoteric behaviour
X
O
ð
s
Þ
þ
H
aq
Ð
X
OH
ð
s
Þ
X
O
ð
s
Þ
þ
H
aq
Ð
X
OH
2
ð
s
Þ
The hydroxide surface exhibits a different charge depending upon the
pH. Cations other than H
1
can act as the potential determining ion. The
point of zero charge (PZC) is the negative log of the activity at which the
surface exhibits no net surface charge. At the PZC
½
X
O
ð
s
Þ
¼½
X
OH
2
ð
s
Þ
The PZC for some mineral solids found in natural waters are shown in
Table 8. Clearly, the extent to which such surfaces can adsorb metal
cations will be dependent upon the pH of the solution. At the pH typical
of seawater, most of the surfaces indicated in Table 8 would be nega-
tively charged and would readily adsorb metal cations.
4.3.2.2 Adsorption Processes. Physical or non-specific adsorption in-
volves relatively weak attractive forces, such as electrostatic attraction
and van der Waals forces. Adsorbed species retain their co-ordinated
Table 8 The PZC for some mineral phases
Mineral
pH
PZC
a-Al
2
O
3
9.1
a-Al
2
(OH)
3
5.0
g-AlOOH
8.2
CuO
9.5
Fe
3
O
4
6.5
a-FeOOH
7.8
g-Fe
2
O
3
6.7
'Fe(OH)
3
' (amorphous)
8.5
MgO
12.4
d-MnO
2
2.8
b-MnO
2
7.2
SiO
2
2.0
ZrSiO
4
5
Feldspars
2-2.4
Kaolinite
4.6
Montmorillonite
2.5
Albite
2.0
Chrysotile
412
Source: adapted from Stumm and Morgan, 1996.
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