## 2 Materials and Methods
### Test Devices  
In this study, a detachable test system is developed for vacuum electro-osmosis experiments, as shown in Figure 2.  
From the left to the right, there is a monitoring system, electro-osmosis system, sample tank, and vacuum system. For a single vacuum test, only the electro-osmosis system in the scheme needs to be turned off. For a single electro-osmosis test, only the bottom of the internal model tank is perforated to discharge water, while the vacuum control system is turned off.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameters** & **Pollution-Free** & **Contaminated** \\ \hline Specific gravity (\(G_{\mathrm{s}}\)) & 2.74 & 2.75 \\ Initial moisture content & 75 & 75 \\ (\(\omega_{\mathrm{0}}\)), \% & 28 & 1640 \\ Copper content, mg/kg & 28 & 1245 \\ Conductivity (\(\sigma\)), \(\mu\)S/cm & 281 & 8.8 \\ pH & 5.5 & 8.8 \\ Cation exchange capacity, & 38.0 & 20.5 \\ cmol (+) /kg & & \\ Hydraulic permeability & 7.741 \(\times 10^{-9}\) & \(5.218\times 10^{-9}\) \\ coefficient (\(k_{\mathrm{h}}\)), m/s & \(1.894\times 10^{-9}\) & \(2.485\times 10^{-9}\) \\ Electric permeability & \(1.894\times 10^{-9}\) & \(2.485\times 10^{-9}\) \\ coefficient (\(k_{\mathrm{e}}\)), m\({}^{2}\cdot\mathrm{V}^{-1}\cdot\mathrm{s}^{-1}\) & & \\ \hline \hline \end{tabular}
\end{table}
Table 1: Basic physical and chemical characteristics of test sediment.  
Figure 1: Location of dredged sediment from Tai Lake.  
The monitoring system includes a SIN-R5000C paperless recorder, a sensor set, and monitoring software to monitor the soil potential distribution and current intensity in real time. It also includes a soil conductivity detector, pH detector, vane shear strength, and other devices to collect post-test parameters. The bottle of water collection serves as a water separation device for the vacuum pump, on the one hand, and collects the drained pore water for quantitative analysis of drainage, on the other hand.  
The electro-osmosis system includes MS3010-DS direct current power supply, wire, electrokinetic geo-synthetics (EKG) drainage board, etc. The DC power supply has a coarse and fine adjustment function, which is convenient to adjust and control the voltage. EKG drainage board is made of geo-synthetic material and metal wire, similar in shape to ordinary plastic drainage board, which can also serve as a conductive electrode, and is durable and corrosion resistant.  
The sample tank includes two components, the internal plexiglass sample tank is designed for filling the sediment sample. The internal size is 300 mm \(\times\) 200 mm \(\times\) 200 mm. The external plexiglass sample tank is designed for forming a gap with the internal sample tank to function as a sealing ditch. The sealing ditch is available to prevent the edge of the vacuum membrane from squeezing the small size specimen in the vacuum process. The internal size of the external sample tank is 350 mm \(\times\) 250 mm \(\times\) 250 mm. This study utilizes the vacuum membrane to apply vacuum pressure to the sample and inserts the vacuum membrane into the gap between the internal and external sample tank. Holes are created in the upper part of the vacuum membrane to allow for the passage of monitoring wires, and the air-tightness of the system is ensured by means of a mud seal.  
The vacuum system includes Jiabaoli-550D oil-free vacuum pump, vacuum controller, vacuum connecting pipe, vacuum membrane, etc. When the vacuum pump starts working to the upper threshold, it will stop running. When the vacuum degree decreases below the lower threshold with the influence of the incomplete air-tightness of the environment, the vacuum pump will start running again. It cannot only save energy, but also control the vacuum degree to be always stable at the design vacuum degree to meet the test requirements.