Environmental Engineering Reference
In-Depth Information
4.3.2 A
DSORPTION OF
M
ETAL
I
ONS ON
S
OILS AND
S
EDIMENTS
Figure 4.9a represents a mineral surface (silica) where an electron-deficient silicon
atom and groups of electron-rich oxygen atoms are presented to the aqueous phase.
The surface hydroxyl groups are similar to water molecules in that both can form
hydrogen bonds. The adsorption energy for water on a silica surface is
500 mJ/m
2
.
Theenergydecreasesasthesurfacewatermoleculesareprogressivelyreplaced.Water
is therefore more favored by most mineral surfaces. There are two possible orienta-
tionsofwatermoleculesonamineralsurfaceasdepictedinFigure4.9b.Aninteresting
consequence of these orientations is that water molecules in direct contact with the
metal oxide can adopt specific, favorable conformations and can simultaneously par-
ticipate in the tetrahedral three-dimensional network that gives it the special features
noted earlier.
The strongly polar mineral surface possesses a surface charge density and a double
layer near it. The surface charge distribution can be obtained from the application of
the Poisson-Boltzmann equation. It is given by
−
2
1
/
2
sinh
zF
,
RTI
π
ε
ψ
2
RT
σ
e
=
(4.59)
F
2
where
z
is the valence of ions in the electrolyte (e.g., for KCl,
z
=
1),
ε
is the dielectric
10
−
10
C/V/m at 298 K),
I
is the ionic strength (mol/m
3
)
,
R
is the gas constant (8.3 J/K/mol),
F
is the Faraday constant (96,485 C/mol), and
constant of water
(
=
7.2
×
ψ
is the surface potential (J/C/V). The surface charge density is determined by the pH of
the adjacent solution. Therefore, both H
+
and OH
−
are called
potential determining
ions
. Based on the reactivities of these ions with the surface hydroxyl groups, the
(a)
O
Si
H
H
O
O
SiO
2
Si
H
H
Water
(b)
Oxygen in the 2nd layer
(Water)
Oxygen in 1st layer
(Metal oxide)
FIGURE 4.9
A typical mineral surface (silica) (a) where an electron-deficient silicon atom
and groups of electron-rich oxygen atoms are presented to the aqueous phase (b).
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