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.