Fig. 4 Variations of cations at different voltage compared to the initial value
Na+ control for planting soils is critical because Na+ has a dispersive effect on soil particles and prevents root penetration (Amini et al., 2016). The highest Na+ decrements (>99%) in soil column were found between EB (electrode located at -60 cm) and EC (electrode located at -40 cm) after V-EK treatment, which reduce the source ions that would rise to the surface. Moreover, the concentration of Na+ ions after V-EK treatment were significantly lower than the control column. For the soil with voltage at 20 V, the lower concentration of Na+ was detected with a range of 0.06-0.08 mg/g at the surface soil compared to the control column (0.21-0.60 mg/g). This is due to the combined process of the high ionic mobility of Na+ under the electrical field and soil ion exchange (Klouche et al., 2020; Lee et al., 2012).
Variations of K+, Ca2+ and Mg2+ were calculated compared with the control column under different voltage system (4, 10, 20V). There was no obvious change of K+ in Fig.4 and Fig.S4, which may be due to its low level. And the other two ions in the surface soil (>-15 cm) reached the decreased percentage of 70-96% and sustained at low concentration. Moreover, between EB and ED (-20 - -60 cm), it observed that ions were also significantly declined, except Ca2+ under the condition of 4 V. Further, Ca2+ and Mg2+ had significant increase tendency below -60cm, reached max percentages of 3163% and 1018%. The OH-produced by the cathode (EB) migrated to EA and formed alkaline environment, leads to the precipitation of ionic compounds, thus reducing their ionic mobility and accumulated around EA (anode at -80cm, Klouche et al., 2020).
From the figure, Ca2+ shows the most obvious variation compared with the other three ions after V-EK treatment, the maximum increment of Ca2+ were reached 8035%, 8568% and 3154% at applied voltage of 4V, 10V and 20V respectively. Since Ca2+ was an endogenous ion, which conclusively that the increase come from the soil alone. And the result proved that V-EK treatment increases the content of Ca2+, which was believed from solubilization of minerals that inside the soil due to the acid environment produced by the anode. Because the increasement of Ca2+ at different voltage was positive correlation to the change in pH (Fig. S3), which also related to the intensity of the current.