Environmental Engineering Reference
In-Depth Information
O
2
(g)+4H
+
+4e
-
= 2H
2
O
1.0
Cr
2
O
7
-2
HCrO
4
-
0.6
CrO
4
-2
Cr
+3
0.2
-0.2
Cr(OH)
4
-
-0.6
H
2
(g)+4OH
-
= 4e
-
+4H
2
O
-1.0
0
2
4
6
8
10
12
14
pH
0.6[Fe]/[Cr]
3[Fe]/[Cr]
5[Fe]/[Cr]
11[Fe]/[Cr]
Figure 2.34
Post-EK distribution of Cr species in soil depicted on Pourbaix diagram
(Weeks and Pamukcu, 2001)
clay injected with Fe(II) showed that an externally applied electric field
caused an additional “cathodic current” that drove forth the reduction of
Cr(VI) in clay (Pamukcu et al., 2004). The results in these experiments
showed that the system ORP (oxidation-reduction potential) increased by
a positive shift from the standard solution ORP in the presence of the clay
and the induced electrical field, as was shown earlier in Figure 2.12.
More evidence for electrokinetically assisted surface transformations in
kaolinite clay was found when a clean specimen of the clay was permeated
with polymer coated dispersed nano-iron particles of positive surface
charge (Sun et al., 2006) under an electrical gradient of 0.1 V/cm (Pamukcu
et al., 2008). When there is no large concentration of a dominant ion in
the clay matrix (i.e. contaminating heavy metals), the primary electron
receptors are water and residual dissolved oxygen. Hence, the chemical
reactions, which oxidize the nano-iron are:
Fe
0
()
s
+
2
H O aq
()
→
Fe
+
2
()
aq
+
H
()
g
+
2
OH
−
()
aq
(2.13)
2
2
2
Fe
0
()
s
+
4
H
+
()
aq
+
O
()
aq
→ +
2
Fe
+
2
()
aq
2
H O l
()
(2.14)
2
2
Equation 2.13 is the dominant redox process in presence of overwhelm-
ing concentration of water as the solvent. As hydrogen ions are utilized in
Equation 2.14, the pH would increase at corrosion locations resulting in
simultaneous decline in the solution ORP. The transient ORP measured
across a 20cm long, 2 mm thick bed of clay (see Figure 2.11) injected
with nanoiron (nZVI) using EK, are presented in Figure 2.35. The ORP
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