## 3 Results and Discussion
### Fractionation Changes  
The Cu form in contaminated soil is analyzed by the Tessier five-step extraction method. The Tessier method is a five-step continuous extraction method proposed by [PERSON] et al. This method divides the distribution of various binding forms of metal elements in detail. This method divides heavy metals into five binding forms: the metal exchangeable state (exchangeable state), carbonate binding state (carbonate state), iron (manganese) oxide binding state (iron/manganese state), organic matter and sulfide binding state (organic state), and residue lattice binding state (residue state). Figure 7 and Table 4 show the fractions of copper in the initial and 10-day remediation soils. The proportion of copper in water-soluble (WS), exchangeable (EXCH), reducible (RED), oxidizable (OX), and residual (RES) stages in the initial soil are 12.6%, 16.3%, 47.9%, 9.58%, and 13.6%, respectively. After 10-days of EK remediation, the removal efficiency of WS copper is the highest in all tests, and the removal of OX and RES copper is not satisfactory. Specifically, the removal efficiency for RES is almost not influenced by PRB and electrolytes. Copper is mainly concentrated in the middle of the soil samples, and the removal efficiency in the section close to the cathode is lower than that close to the anode, obviously because Cu is easy to precipitate at high pH conditions. Conversely, water-soluble, exchangeable, and reducible copper are easily desorbed under acidic conditions, so Cu in these forms is better removed near the anode.  
Comparison of the Cu forms after EK and EK-PRB remediation demonstrates that EK-PRB remediation can remove more RED Cu; because steel slag has played a role in improving the reducibility of the electrolyte. Partial iron atoms in the steel slag are oxidized as FeO, and there exists a small amount of elemental iron itself, which makes steel slag present a certain degree of reducibility. Then the reducibility of the electrolyte will be increased due to its providing electrons to the solution, which facilitates the removal of the reducible Cu.  
The EK-PRB coupling remediation method can effectively avoid the increase in soil pH; it can reduce the accumulation of heavy metals, and also reduce the accumulation range of heavy metals, and promote the removal of heavy metals in the soil; it has better energy utilization efficiency and unit energy consumption. Compared with the single EK test, the energy consumption reduction is 10.1-46.1%. More reducible copper can be removed; the removal rate is increased by 7-21%, while the removal rate of exchangeable copper is increased by 9.5-32.1%.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline
**Test** & **Initial** & **S1** & **S2** & **S3** & **S4** & **S5** \\ \hline
**Test 1** & 250.0 & 72.8 & 327.7 & 228.6 & 173.3 & 174.9 \\
**Test 2** & 250.0 & 84.3 & 186.4 & 172.8 & 164.6 & 142.1 \\
**Test 3** & 250.0 & 76.8 & 117.9 & 190.2 & 135.0 & 104.4 \\
**Test 4** & 250.0 & 96.2 & 201.1 & 172.6 & 164.2 & 153.9 \\
**Test 5** & 250.0 & 81.9 & 288.1 & 171.4 & 157.2 & 152.9 \\
**Test 6** & 250.0 & 87.1 & 109.4 & 143.3 & 143.3 & 131.7 \\
**Test 7** & 250.0 & 64.4 & 78.5 & 155.0 & 101.2 & 86.2 \\
**Test 8** & 250.0 & 98.4 & 170.0 & 153.0 & 145.8 & 140.8 \\ \hline \end{tabular}
\end{table}
Table 4: Results of fractionation change of Cu after EK tests.