3.3.2 Anions
Fig.5 and Fig.S5 illustrates the
distributions of Cl- (exogenous added ion),
SO42-,
NO3- concentrations in different soil
layers after V-EK treatment. For control column (applied voltage = 0 V),
the Cl-, SO42- and
NO3- concentrations increases from the
bottom up and accumulates in the surface layer owning to the evaporation
especially for Cl- and
NO3-. However, there was no
accumulation of SO42- observed at the
top surface. The reason for this was considered that sulfate tends to
form insoluble CaSO4, which may decrease its migration
(Kim et al., 2012).
For V-EK tests, Cl- decreased >97% compared with control
column at the voltage of 10 V and 20 V, and mainly accumulated around
EA (electrode located at -80 cm). The maximum
concentration values of Cl- around EAwere reached 8.4mg/g, 5.8mg/g and 2.7mg/g at the voltages of 4V, 10V and
20V, respectively. The concentrations of Cl- were
detected with a range of 0.01-0.02 mg/g and 0.02-0.06 mg/g at the
surface soil with the voltage at 20V and 10V respectively, while the
concentrations of Cl were 0.84-2.70 mg/g in control test. Results
indicated that Cl- upward migration from shallow
groundwater were effective mitigated and prevented at the applied
voltage of 10V and 20V. Moreover, Cl- displayed the most obvious
difference between control and V-EK treatment. The increasements of Cl-
were range from 1095% to 3901% at control column. While the maximum
increment of Cl- were 11887%, 8233%, 3724% occurred around anode at
the applied voltage of 4V, 10V and 20V respectively. The result showed
that Cl- could better mitigated by V-EK treatment test as applied
voltage increase.
For NO3-, ions migrated to anode and
settled around it (Choi et al., 2009), which was mostly influenced by
the developed pH gradient due to electrolysis (Edi et al., 2000). The
concentration at the surface after V-EK process were much lower (0.02
mg/g at 20V and 0.04 mg/g at 10V) than the control column (0.54 mg/g).
Additionally, SO42- were also well
mitigated and prevented above -40 at the applied voltage of 10 and 20V.
With voltage at 20 V, the decrement rate of
SO42- were at the range of 57%-75%.
As can be seen from the above, the salts upward migration from shallow
groundwater were effective mitigated and prevented at the applied
voltage of 10 V and 20 V, but less effective at 4 V.