## 2 Materials and Methods
### Experimental Setup  
In order to analyze the electrokinetic remediation removal efficiency of a steel slag reaction wall under various electrolyte conditions, the four electrolytes of sodium chloride, acetic acid, citric acid, and PASP are selected for the comparison tests of steel slag PRB and no PRB, respectively. The test list is shown in Table 2.  
Figure 1 shows the schematic diagram of an electrokinetic remediation test reactor used in this study. The system consists of a perspex reactor containing contaminated soil (length \(\times\) width \(\times\) height = 30 \(\times\) 10 \(\times\) 10 cm), two electrode compartments (length \(\times\) width \(\times\) height = 10 \(\times\) 10 \(\times\) 10 cm), two high-purity graphite electrodes (length \(\times\) width \(\times\) height = 10 \(\times\) 0.5 \(\times\) 10 cm), two peristaltic pumps, two electrolyte reservoirs (1 L), and a power supply. Two pieces of cellulose filter paper were placed between the electrolysis chamber and the soil chamber to prevent the contaminated soil from entering the electrolysis chamber. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Property** & **Cu Concentration/(mg/kg)** & **Water Content/\%** & **pH** & **Conductivity/(mS/cm)** \\ \hline Value & 250 \(\pm\) 10 & 32 & 5.4 & 0.62 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Physical and chemical properties of the contaminated samples. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Test** & **Method** & **Electrolyte** & **Voltage/cm** & **Running Time/d** \\ \hline
**Test 1** & Electrolyte & 0.1 M KCl & 1.0 & 10 \\
**Test 2** & Electrolyte & 0.1 M acetic acid & 1.0 & 10 \\
**Test 3** & Electrolyte & 0.1 M citric acid & 1.0 & 10 \\
**Test 4** & Electrolyte & 100 mL/L PASP & 1.0 & 10 \\
**Test 5** & PRB + Electrolyte & 0.1 M KCl & 1.0 & 10 \\
**Test 6** & PRB + Electrolyte & 0.1 M acetic acid & 1.0 & 10 \\
**Test 7** & PRB + Electrolyte & 0.1 M citric acid & 1.0 & 10 \\
**Test 8** & PRB + Electrolyte & 100 mL/L PASP & 1.0 & 10 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Experimental conditions for electrokinetic remediation of Cu-contaminated soil. The following preparations are made before tests. First, prepare the electrolyte solution by mixing deionized water and the chemical substance in proportion, in which all chemicals used in the tests are of analytical grade. Secondly, mix the electrolyte solution with the air-dried contaminated soil using a mixer to prepare a saturated soil sample with a water content of 32%. Then, layer it into a perspex container and compact it by hand. The final height of the soil sample is 7 cm, and the weight is about 3.9 kg. The prepared soil samples are placed for 12 h before the repair tests. In the EK-PRB test, an additional 0.25 kg dry fine steel slag is placed near the cathode as PRB material. At the beginning of the tests, an electrolyte solution was added to the electrolytic chamber by a peristaltic pump (iCE 3300) and replenished to the anode chamber every 12 h to maintain the continuous operation of the tests. All EK tests were performed under a constant pressure gradient (1 v/cm) for 10 days. The soil chamber was evenly divided into five sections (S1-S5) from anode to cathode. During the tests, soil samples were taken from each section at regular intervals for pH and total copper concentration tests, and the copper forms of soil samples in each section were determined at the end of the test on day 10. In order to analyze the removal mechanism of electrokinetic remediation, the electrical conductivity, pH value, total copper content, and Cu forms of the initial contaminated soil and the soil after electrokinetic remediation were measured. Current changes can be recorded in real-time from the DC power supply. The pH values of the soil in five slices were measured with a pH meter (PHS-3E; INSTRUMENT, China) after extraction with deionized water at the ratio of 1:2.5 (soil:water).