Environmental Engineering Reference
In-Depth Information
X
Diffusion layer
ζ
ζ
ζ
ψ
0
ψ
0
ψ
0
Shear Plane
Shear Plane
Gouy Plane
Stern Plane
Bulk
Solution
Low salt concentration
High salt concentration
Shear Plane
Figure 2.8 (a)
The distribution of charges against charged clay surface in DDL formation;
(b)
The distribution of charges at low salt concentration;
(c)
The concept of electrolyte
ions adsorbing on the surface and reduce the zeta potential without altering the surface
potential or charge at high salt concentration. (illustrations courtesy of PERC,
http://perc.
ufl.edu/)
)
of the bulk solution. The surface charge or electokinetic potential of most
clay sharply varies with pH. For example, the PZC for most types of
kaolinite clays have been reported to coincide between pH 2.0 and 4.0,
depending on the electrolyte concentration of the pore solution (Bard and
Faulkner, 1980). Accordingly, as the DDL compresses or expands, Faradaic
currents (cathodic or anodic) can generate across Outer Helmholtz Plane
(i.e, frequently assumed where the electrokinetic potential and shear sur-
face develops), whereby the concentration gradients are overcome and the
electrical equilibrium restored.
Recently Brosky and Pamukcu (2013) proposed that a nonspontaneous
change in the oxidation-reduction potential (i.e., E
h
) of a clay-electrolyte
mixture represents the charge discharge due to the compression of the DDL.
They demonstrated a shift in E
h
values with increasing clay concentration
of an aqueous slurry during electrically enhanced reduction of Cu(II) by
Fe(II), as shown in Figure 2.9. The increased clay concentrations promoted
electrochemical reduction of Cu
2+
to more environmentally benign Cu
+
or
Cu
0
in the system. This is highly relevant to bottom waters and sediments
of surface water bodies, where many heavy metal contaminants are found.
Electrically treated clay can switch its surface potential sign in time as pH
lowers below an 'apparent' PZC. It can be seen from Figure 2.10 that the
PZC
is achieved for the kaolinite-Cu(II)-Fe(II) test slurries at around pH
2.4. The PZC signifies that the charged layers surrounding the clay particles
become saturated with charges from ions in solution as well as electrons
from current flow. At this state, the DDL capacitor is storing maximum
charge and therefore has reached its minimum capacitance and maximum
redox potential. Zeta potential data shows that electrically treated clay can
switch in zeta potential sign in time as the pH decreases below the 'appar-
ent PZC' of pH 2.4. Observing Figure 2.9, it appears that the clay systems
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