Reduction of Contaminants in Soil and Water By Direct Electric Current - Electrokinetics for Petroleum and Environmental Engineers - page 80

clay was used to mix a slurry at 35:65 solid:water ratio by mass. The mix-
ing water and the electrode chamber waters were made up of 30,000 ppm
electrolyte solution of instant ocean sea salt. Constant voltage, adjusted
between 3.5V and 3.0V was applied across the electrodes to maintain a
direct electric current of about 100mA. The clay slurry and the electrode
chamber waters were filled up to 40 cm from the bottom of the tank. The
water levels in the electrolyte chambers were maintained at the mud line to
alleviate any change in hydraulic potential. Five water pressure head (piezo-
tubes) and voltage (auxiliary electrodes) measurement probes were placed
inside the clay slurry. The electrical gradients and the pore water pressures
were measured across the dewatering/settling clay at time intervals.
The long-term experimental results (~ 7 days) showed that the settle-
ment or the dewatering of clay slurry was hindered due to spatially non-
linear and temporally oscillatory voltage gradients, which give way to
non-uniform distribution of stagnant pore water pressures within the clay
slurry. These results are presented in Figures 2.38 and 2.39 for randomly
selected time steps of the voltage gradient and pore-water pressure mea-
surements, respectively. The significance of these observations is that the
local voltage gradients appear to have direct influence on the development
of local pressure gradients within the electro-osmotic flow region of a high
salt content medium. Fluctuating pore water pressures induce competing
flow regimes resulting in stagnant pore pressures that can impede electro-
osmotic mass transport. In Figure 2.38 the voltage gradient distribution
between adjacent probe locations shows clearly zones of high and low gra-
dients, above and below the self-manifesting threshold value of 0.015 V/
cm. In final analysis, the nonlinear and oscillatory voltage gradients were

1.0

6 hrs
83 hrs
143 hrs
250 hrs

34 hrs
118 hrs
203 hrs
397 hrs

Initial gradient=0.034 V/cm

0.8

0.6

0.4

0.2 Higher rate Lower rate Higher rate Lower rate

EO Flow tendency

0.0
0.0

0.2 0.4 0.6
Normalized distance from anode end, x/L

0.8 1.0

Figure 2.38 Spatial and temporal distribution of voltage gradient across adjacent probes
at select times during EK (Muraoka et al., 2011)