## 3 Results and Discussion  
### Changes in Electric Current and Energy Consumption  
Figure 2 shows the change in electric current during the 10-day electrokinetic remediation. The currents of T1-T8 at the beginning of the tests are 187, 20, 32, 23, 150, 25, 50, and 31 mA, respectively. The changes in electric current in the tests are mainly attributed to variations in the conductive capacity and the ability of electrolytes to extract metal ions from contaminated soil [25]. The currents of T2-T8 increase initially and then decrease. The magnitude of the current is closely related to ion movement during electrokinetic remediation. At the initial stage of the test, the ions' desorption in the soil and the electrolysis of the electrolyte increase the electrical conductivity of the overall system, and then the action of electromigration, electroosmotic flow, and electrophoresis gradually decrease the flow of ions in the soil. After 10 days of electrokinetic remediation, except for T2 and T6, the  
Figure 1: A schematic diagram of the experimental reactor for electrokinetic remediation: (**1**) Soil chamber, (**2**) Anode chamber, (**3**) Cathode chamber, (**4**) Electrolyte solution reservoir, (**5**) Graphite electrode, (**6**) Plexiglass and filter paper, (**7**) DC power supply, (**8**) PRB (**9**) Peristaltic pump. currents of the other six groups of tests are all lower than the initial values. The currents change significantly in the T1 and T5 tests using 0.1 m KCl as the electrolyte because the KCl solution enhances the electrolytic reaction and produces a large number of K\({}^{+}\), Cl\({}^{-}\), H\({}^{+}\), and OH\({}^{-}\) ions, which can improve the conductivity of the solution. The comparison of Figure 2a,b can reflect the influence of steel slag PRB on the change of electric current during remediation. The initial current values of the tests with PRB except T5 are slightly higher, which is probably caused by the introduction of additional ions from steel slag. The reason that the initial current of T5 is less than that of T1 is that the steel slag surface has a negative charge and can absorb positive K\({}^{+}\) ions, thus reducing the number of free ions in the EK-PRB system. After 10 days of remediation, the currents of T1-T8 is 8, 27, 22, 15, 6, 26, 20, and 11 mA, respectively. The peak values of the currents are greater, and peak points occur earlier when PRB is applied under other conditions being the same, which indicates that better electromigration is obtained with PRB. Figure 3 shows the accumulated energy consumption in eight groups of EK tests within 10 days. The energy consumption is calculated by the following equation,  
\[W=\int\limits_{0}^{t}UIdt \tag{1}\]  
where \(W\) is the electric energy consumption of removing contaminant (kWh), \(U\) isapplied voltage (V), \(I\) is current intensity (mA), and \(t\) is treatment time (h). Figure 3: Accumulated energy consumption over 10 days. (**a**) EK Remediation; (**b**) EK-PRB Remediation. Figure 2: Electric current changes over 10 days. (**a**) EK Remediation; (**b**) EK-PRB Remediation. The Cu removal efficiency, \(\eta\), is expressed as follows,  
\[\eta=\frac{C_{0}-C_{t}}{C_{0}}\times 100\% \tag{2}\]  
where \(C_{t}\) represents the Cu concentration (mg\(\cdot\)kg\({}^{-1}\)) at time \(t\), and \(C_{0}\) is initial Cu concentration (mg\(\cdot\)kg\({}^{-1}\)). As can be seen from Figure 3a,b, the order of energy consumption is T1 \(>\) T5 \(>\) T7 \(>\) T3 \(>\) T6 \(>\) T2 \(>\) T8 \(>\) T4. Compared with the tests without PRB, the tests with PRB will produce greater energy consumption except for the T5 test. In addition to the energy required for the migration of toxic metals, the energy loss will also be caused by the migration of other ions in the soil, water electrolysis, and heating effects. Energy utilization efficiency is represented by the energy consumed per unit mass of the pollutant. The smaller value of \(W^{\prime}\) means higher energy utilization efficiency.