Environmental Engineering Reference
In-Depth Information
The tendency for NMs in water to remain in suspended and stable (i.e., do not
aggregate) depends primarily upon their surface chemistry. Surface chemistry gives rise
to charge densities on the surface, which manifests as measurable zeta potentials in
water. The point of zero charge (i.e., isoelectric point, pH
IEP
) for several NMs is
available (Table 16.2). At pH levels above the pH
IEP
the zeta potential will be negative,
and positive when pH < pH
IEP
.
Table 16.2
Representative pH
IEP
values for several types of NMs
Material
pH
IEP
Natural NMs (Stumm, 1992)
Silica dioxide
Kaoline
Alpha Manganese oxide
Calcite
Gamma aluminum oxide
Magnesium Oxide
Algae
Aerobacter aerogenes
Bacteriophage
2.0
4.2
5.0
8.5
9.6
12.3
2.8
< 2
3.5
Engineered NMs (Westerhoff et al., in-press; Zhang et
al., 2008a-b)
Solublized fullerene (nC
60
)
Titanium dioxide
Zinc oxide
Nickel oxide
Silica
Iron oxide (Fe
2
O
3
)
Hematite
CdTe quantum dot (thioglycolate capping ligand)
< 2
5.5
9.8
9.5
< 2
6.8
8.5
< 2
16.5.1 Factors Affecting NM Interactions
The stability of particles in water can be assessed using Derjaguin-Landau-
Verwey-Overbeek (DLVO) theory (Hiemenz and Rajagopalan, 1997). Briefly, DLVO
theory accounts for two forces between the particles, van der Waals (vdW) attraction
(
vdW
) and electrical double layer (EDL) repulsion (
EDL
). The sum of these two forces
determines whether the net interaction between particles is either repulsive or attractive
(Eq. 16.1)
ze
ψ
exp(
)
−
1
⎡
2
2
⎛
2
⎞
⎤
64
π
nrKT
A
2
r
2
r
H
+
4
rH
2
KT
2
Φ=Φ+Φ=
⋅
exp(
−⋅
κ
H
) (
)
−
+
+
ln
⎢
⎜
⎟
⎥
Total
EDL
vdW
2
ze
ψ
2
2
2
2
2
κ
6
H
+
4
rH
H
+
4
rH
+
4
r
H
+
4
rH
+
4
r
⎣
⎝
⎠
⎦
exp(
)
+
1
2
KT
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