## 3 Results
### Analysis of Copper Concentration  
Figure 9 describes the distribution of copper ion concentration in the cathode and anode, and the initial copper concentration in the contaminated soil is 1640 mg/kg. After treatment, the copper concentration at the anode of T4-T6 groups decreases to 1040, 1241, and 900 mg/kg, and the removal ratios are 36.6%, 24.3%, and 45.1%, respectively. Copper concentration at the cathode decreases to 1520, 1180, and 820 mg/kg, and the removal ratios are 7.3%, 28.1%, and 50.0%, respectively. The order of treatment effect at the anode: T6 > T4 > T5. It indicates that electro-osmosis method contributes more to the treatment of Cu-contaminated soil compared to the vacuum method. It is mainly because, under the function of the electric field, the directional flow of water carries part of the copper ions dissolved in water and part of the soil particles adsorbed with copper to move together At the same time, the copper ions with positive charges will also be driven by electro-migration, moving from the anode to the cathode directionally. For the vacuum method, there is no applied electric field, and the soil drains and consolidates under the negative vacuum pressure while carrying copper ions for transport. In comparison to electro-osmosis, it lacks an external driving force. As for the treatment effect at the cathode, due to the function of the applied electric field, water and copper are mainly transported from the anode to the cathode, accumulating at the cathode, resulting in the highest concentration of copper at the cathode by electro-osmosis.  
Moreover, under the function of the electric field, the water is electrolyzed and a large amount of hydroxide ions are released from the cathode, which generates the precipitation of copper hydroxide flocculant when it meets copper ions. In turn, this leads to the obstruction of cathode drainage and the accumulation of contaminants. Thus, the copper concentration at the cathode is higher for electro-osmosis.  
The reaction equation is shown as follows:  
\[2\mathrm{H_{2}O}-4\mathrm{e^{-}}\rightarrow\mathrm{O_{2}(\uparrow)}+4\mathrm{ H^{+}} \tag{1}\]  
\[4\mathrm{H_{2}O}+4\mathrm{e^{-}}\rightarrow2\mathrm{H_{2}(\uparrow)}+4 \mathrm{OH^{-}} \tag{2}\]  
\[\mathrm{Cu^{2+}}+2\mathrm{OH^{-}}\rightarrow\mathrm{Cu(OH)_{2}(\downarrow)} \tag{3}\]  
Figure 8: Distribution law of soil cracks.  
For the vacuum method using the cathode direct discharge system, the cathode acts as a channel for the direct action of the vacuum and the cathode drainage is smoother. The contaminants are discharged from the anode with the water flow to the cathode, and the difference in the concentration of contaminants between the two levels is small. The vacuum electro-osmosis method has the advantages of both, with the best decontamination effect, and the concentration of contaminants at the cathode and anode is relatively close, which means the treatment effect is more uniform. Therefore, it is concluded that the adoption of the electro-osmosis method is capable of achieving the directional transport of contaminants and the partial discharge of contaminants combined with the vacuum method, which proves the feasibility of the integration of reduction and remediation by vacuum electro-osmosis method.