3.1 The Variation of Current and Calculated Resistance
The EK process involves the construction of an electrical field within the salty soil and leads to the generation of electrical potential, which caused the movement and the migration of ionic species and induces the changing of electrical current (Kim et al., 2011). Meanwhile, soil resistivity is a comprehensive characterization of soil physical properties, which is correlated with soil salinity, moisture, porosity, saturation, permeability coefficient, etc. (Beck et al., 2011; Chang & Cheng, 2007). Therefore, the resistance of soil columns (R) were calculated in this study by Ohm ’s Law (R=U/I) to reflect the changing of soil salinity.
The calculated resistances across the salty soil column as a function of the duration of the treatment under different applied voltages as the water level raised gradually are shown in Fig. 2. The variations of the electrical current across the column are shown in Fig. S1. Results indicated that the electrical current in all columns were increased to the maximum value of 27 mA, 68 mA and 114 mA in the first few days with the voltage at 4 V, 10 V and 20 V, respectively. Then declined sharply and kept at the range of 8-26 mA, 20-34 mA, 13-38 mA, respectively. The final electrical current in the soil with the voltage at 4V, 10V and 20V were 14 mA, 20 mA and 16 mA, respectively, and showed in the order of 10 V > 20 V >4 V. Similarly, the calculated resistances of soil were reduced to the minimum value at 137 Ω, 135 Ω and 164 Ω after few days under the application of voltage at 4 V, 10 V and 20 V, respectively, then showed a slow increasing trend. The final calculated resistances of soil with the voltage at 4 V, 10 V and 20 V were 266 Ω, 487 Ω and 1272 Ω, respectively, and showed in the order of 20V> 10V> 4V.
The similar trends have been observed in previous studies (Klouche et al., 2020; Bessaim et al., 2019). As capillary water gradually raised, the electric gradient was established. Simultaneously, desorption and mobilization of ions from soil generate the liberation of free charged ionic species, result reduction of the salty soil column resistance and therefore raising the current intensity (Cameselle and Reddy 2012; Wu et al. 2015). Moreover, the generations of hydrogen and hydroxide ions during the pore fluid water by electrolysis may also another reason for the above phenomenon in the first few days (Bahemmat et al. 2015). The specific process included the charged ions in the soil media moved towards the electrode of opposite charge by electromigration and the pore water moved towards the cathode by electro-osmosis (Bahemmat et al., 2015; Li et al., 2014). Since water is the necessary medium for the movement of ions, soil moisture plays a very important role in V-EK process, a low degree of soil moisture increases the electrical resistance of the soil. The final soil moisture at different conditions were shown in Fig. S2. High resistance of soil reduces the efficiency of the process drastically, thus reduces the EK effects and most of the charged energy will be lost as heat (Fahadani et al., 2018). The resistance of soil with voltage at 20 V is about three times than that of soil with voltage at 10 V. It proves that why maximum electrical current was observed in soil with voltage at 10 V.