Geology Reference
In-Depth Information
Al
3
þ
Al(OH)
2(aq)
þ
2H
(aq)
*b
2
¼
*K
1
*K
2
¼
10
10.55
(aq)
þ
2H
2
O
(1)
"
(3.33)
Al
3
þ
Al(OH)
3(aq)
þ
3H
(aq)
*b
3
¼
*K
1
*K
2
*K
3
¼
10
17.25
(aq)
þ
3H
2
O
(1)
"
(3.34)
Al
3
þ
Al(OH)
4(aq)
þ
4H
(aq)
*b
4
¼
*K
1
*K
2
*K
3
*K
4
¼
10
22.85
(aq)
þ
4H
2
O
(1)
"
(3.35)
Clearly, these hydrolysis reactions are dependent on hydrogen ion
activity. The relationship between speciation and pH, which is influ-
enced by the characteristics of the metal ion, will be discussed further in
Section 3.2.4.1.
3.2.3.3 Inorganic Complexes. The main inorganic ligands in oxyge-
nated freshwaters, in addition to OH
, are HCO
3
,CO
3
2
,Cl
,SO
4
2
,
and F
, and, under anoxic conditions, HS
and S
2
(see Section 3.2.4.4).
The stability of complexes formed between metal ions and inorganic
ligands depends on the nature of the metal ion as well as the properties of
the ligand. Table 2 shows some of the major complexed metal species
involving inorganic ligands in oxygenated freshwaters at pH 8.
3.2.3.4 Surface Complex Formation. Metal ions form both outer and
inner sphere complexes with solid surfaces, e.g. hydrous oxides of iron,
manganese, and aluminium. In addition, metal ions, attracted to charged
surfaces, may be held in a diffuse layer, which, depending upon ionic
strength, extends several nanometres from the surface into solution.
Table 2 Major species in freshwaters
[M
x1
(aq)
]/[M
TOT (aq)
]
Metal ion
Major species
Mg
II
Mg
21
0.94
Ca
II
Ca
21
0.94
Al
III
Al(OH)
3
0
, Al(OH)
2
1
, Al(OH)
4
1
10
9
Mn
IV
MnO
2
0
-
Fe
III
Fe(OH)
3
0
, Fe(OH)
2
1
, Fe(OH)
4
2
10
11
Ni
II
Ni
21
, NiCO
3
0
0.4
Cu
II
CuCO
3
0
, Cu(OH)
2
0
0.01
Zn
II
Zn
21
, ZnCO
3
0
0.4
Pb
II
PbCO
3
0
0.05
Source: Adapted from Sigg and Xue,
33
and reproduced with permission from W. Stumm and J.J.
Morgan, Aquatic Chemistry 3rd Edn,
r
Wiley, 1996.
