Reduction of Contaminants in Soil and Water By Direct Electric Current - Electrokinetics for Petroleum and Environmental Engineers - page 78

The ORP versus pH obtained from earlier experiments of similar con-
figuration as above (Pamukcu et al., 2008) are plotted in Figure 2.36. The
diagram depicts approximate regions of corrosion, passivity and immunity
for iron (Davis, 2000). The cluster of data that fell within the immunity
zone corresponded to the first 6 hours of electrokinetic transport where
the ORP at the cathode end was below -1.0V . This indicated that the nano-
iron particles had already been transported to the cathode side of the clay
bed by the end of 6 hours of treatment or earlier. The evidence that cor-
roborated with the visual observations of color change in clay (i.e., indica-
tive of oxidation of iron) was the cluster of data that fell within the passivity
zone. This data corresponded to measurements made later, from 6 to 46
hours. By the end of 46 hours, the corrosion products with higher ORP
appeared at the cathode end of the clay bed, as was evidenced by the brown
tone color change in that region of the test bed.
The average voltage distributions across the clay bed with and with-
out nZVI are plotted in Figure 2.37 (Pamukcu et al., 2008). As observed,
while the EK only test displayed a constant electrical gradient of 0.12 V/cm
(i.e., equation of the linearly fit line) for the 48-hour duration of the test,
the electrical gradient fluctuated between adjacent auxiliary electrodes
inserted in soil bed for the nZVI case. The magnitude of these oscillations
ceased attaining a constant profile in time. Variation of the electric gradi-
ent influences the velocity of the migrating particles from one station to
the next, causing accumulation in some locations and evacuation in oth-
ers. Theoretically, the particle velocity is expected to decrease at regions of

5

Average voltage distribution
with nano-iron

ANODE

CATHODE

4

3

2

Average voltage distribution
w/o nano-iron:
[V = -0.12x + 5]

1

0 0

5 10

Distance from anode electrode [cm]

15 20 25 30 35

Figure 2.36 Average distribution of voltage in clay with and without nZVI transport over
48 hours of EK (Pamukcu et al., 2008)