## 3 Results and Discussion
### Removal Efficiency of Copper  
Figure 5 shows the concentration of contaminants in the S1-S5 section for eight groups of tests during the 10-day electrokinetic remediation. The contaminants accumulate in the S1 section at the beginning of the repair. Electrolytes in soil react with heavy metals and form an electronegative metal complex, which will migrate to the anode when it is under an acidic condition. Then, the metal complex disintegrates and releases the positive metal ion. These metal ions will migrate to the cathode, and some of them form a metal complex in the soil during migration, which moves to the anode again. It is the above cyclic process that produces the highest concentration of Cu in S1 in the first two days of the tests. As remediation time goes on, the maximum concentration of Cu ions occurs in the S2-S4 sections because the soil near the anode is gradually acidified, causing the Cu accumulation region to migrate inward gradually. The accumulation range and amount of Cu are affected by the electrolyte solution. Cu accumulations for the cases of CA and PASP are less than those of KCl and HAc by comparing the residual amount of Cu during the remediation period. This indicates that the removal efficiencies of Cu are better when CA and PASP are used as electrolytes. This is because CA and PASP, as ideal metal complex agents, can be applied to a wide range of pH values, and the chemical properties of the metal complexes formed by them are stable. Figure 4: Changes of soil pH in different sections after electrokinetic remediation. (**a**) EK Remediation; (**b**) EK-PRB Remediation. Figure 5: Changes in concentration of contaminants during electrokinetic remediation. (**a**) T1; (**b**) T2; (**c**) T3; (**d**) T4; (**e**) T5; (**f**) T6; (**g**) T7; (**h**) T8. The removal rates, \(\eta\), of Cu in the soil after the 10-day remediation of the EK and EK-PRB tests of the four electrolytes are plotted in Figure 6, which is more distinctly observed than the removal efficiencies of Cu under the conditions of various electrolytes and the addition of PRB. The application of PRB is more beneficial to the removal of heavy metals in the soil compared to EK alone. Both the amount and range of Cu accumulation reduce. The removal efficiencies for the four electrolytes increase by 5%, 40%, 22%, and 36%, respectively. In the process of EK remediation alone, copper ions and soluble salts in the soil move directionally and attach to the electrode surface under the action of the electric field, increasing the resistance of the electrode and reducing the electrical conductivity, which influences the efficiency of electrokinetic remediation. Conversely, steel slag PRB has a high adsorption capacity for Cu ions and soluble salts, which can avoid polluting electrolytes. In addition, steel slag can absorb hydroxide ions generated by the cathode, reduce the soil pH value, and avoid the formation of Cu(OH)\({}_{2}\) that block the soil pores. Compared with the EK tests (T1-T4), the removal efficiency increased significantly after using PRB (T5-T8). It is commonly recognized that the KCl solution has good conductivity and can enhance the current and electroosmotic flow during electrokinetic remediation because it contains a large amount of Cl\({}^{-}\) and combines with Cu to form Cu-Cl, a complex that will prevent soil from re-adsorbing Cu and thus improve the desorption rate of Cu. However, the 10-day remediation results indicate that the removal efficiency of heavy metals for 0.1 m KCl solution is not ideal. The average removal efficiency of T1 and T5 is 22.32% and 34.87%, respectively, and the removal efficiency of various sections is quite different. The removal efficiency of the S1 section is the highest, reaching 75%, while Cu accumulation in the S2 section even exceeds the initial value.