2.2 Experimental Design
A schematic diagram of experimental setup was shown in Fig. 1, composed by a PVC pipe column with the inner dimension of 75 mm, and a direct current power source. At the bottom of the soil column, 8~10 mm grain diameter of gravel was loaded with a thickness of 15 cm to simulate a groundwater aquifer while ensuring uniform water retention at the bottom of the column. Above gravel, a volume-weight of 1.4 g/cm3 of soil were filled, and a piece of non-woven fabrics was set between gravel and soil to prevent the entering of soil into gravel layer. Multilayer electrodes in each column consist of four electrodes that were electrode A (EA), electrode B (EB) electrode C (EC) and electrode D (ED) as shown in Fig. 1 with the electrode spacing is 20 cm. EA and EC are connected to the positive pole of the power supply, while EB and ED are connected to the negative pole of the power supply. The multilayer electrodes were made by the titanium belt within two pieces carbon fiber. A water inlet was installed at the bottom of the column connected to saline water with rubber hose. The stable water level (fluctuated in ±1 cm) was maintained by a water level controller (HHY7G, JYB-714G, XinLing Electric Co., Ltd, China), which connected with a peristaltic pump and a water tank.
In each test, four columns were set, and each water inlet of soil column were connected to saline water with T-junction to ensure at the same water level. Voltages of 0, 4, 10, 20 V were provided to the four columns respectively. The initial water level was controlled at -90±1 cm, and then raised at a rate of 1-2 cm per day to -50 cm. The current of the soil during the water level rises was monitored continually. When the experiment finished, the soil was sampled in different layers (L1-L15) to measure the water content, electrical conductivity, pH and the concentration of anion and cation in soil.