Geology Reference
In-Depth Information
Table 5 The size distribution of trace metal species in natural waters
Size range Metal species
Examples
Phase state
Mc
21
,Cd
21
o
1 nm
Free metal ions
Soluble
NiCl
1
1-10 nm
Inorganic ion pairs
Soluble
HgCl
2
4
Inorganic complexes
Low molecular mass organic
complexes
Zn-fulvates
10-100 nm High molecular mass organic
complexes
Pb-humates
Colloidal
100-1000
nm
Metal species adsorbed onto
inorganic colloids
Co-MnO
2
Particulate
Metals associated with detritus Pb-Fe(OH)
3
4 1000 nm Metals absorbed into living
cells
Cu-clays
Particulate
Metals adsorbed onto or
incorporated into mineral
solids and precipitates
PbCO
3(s)
Source: adapted from de Mora and Harrison, 1984.
21
the stability constant is defined as
f
NaSO
4
g
f
Na
þ
gf
SO
2
ð
4
:
15
Þ
K
¼
g
4
Ion pair formation is important for Ca
21
,Mg
21
,SO
4
2
and HCO
3
.
If the attraction is sufficiently great, a dehydration reaction can occur
leading to covalent bonding. A complex consists of a central metal ion
sharing a pair of electrons donated by another constituent, termed a
ligand, acting as a Lewis base. The metal ion and ligand share a single
water envelope. Ligands can be neutral (e.g., H
2
O) or anionic (e.g., Cl
,
HCO
3
) species. A metal ion can co-ordinate with one or more ligands,
which need not be the same chemical entity. Alternatively, the cation can
share more than one electron pair with a given ligand thereby forming a
ring structure. This type of complex, known as a chelate, exhibits
enhanced stability largely due to the entropy effect of releasing large
numbers of molecules from the co-ordinated water envelopes.
Complex formation is an equilibrium process. Ignoring charges for the
general case of a metal, M and ligand, L, complex formation occurs as
M
þ
L
ML
for which the formation constant, K
1
, is given by
"
f
ML
g
f
M
gf
L
g
K
1
¼
ð
4
:
16
Þ
