## 2 Materials and Methods
### Test Methods and Apparatus  
Soil-mixture specimens with a 7.50% water content and different zeolite contents were prepared by mixing the zeolite, bentonite, stone chips, and DIW. The mixture was stirred evenly and allowed to settle for 24 h. The mixture for each specimen was divided into five equally weighted parts that were separately filled into a mold (70 mm in diameter and 140 mm high) and compacted layer by layer using a compactor, with a compaction process using standard light compaction instrument, a 2.5 kg hammer weight, a hammer bottom diameter of 51 mm, drop distance of 305 mm, and unit body compaction work of about 592.20 KJ/m\({}^{3}\). The resulting soil column was vacuum-impregnated with compaction of 100% and a dry density of 1.99 g/cm\({}^{3}\) based on the liquid-plastic limit and compaction tests. The synthesized heavy metal-containing aqueous solution is made using the selected heavy metal (Zn\({}^{2+}\)). All solutions were made up of deionized water and diluted according to the desired concentrations. As shown in Figure 3a, permeability tests were conducted using a fully automatic soil permeability tester (GDS, UK), which mainly consisted of a pressure cell, a volume controller, and an air-pressure system. GDS uses the principle of normal head infiltration test to calculate the infiltration velocity and hydraulic gradient by measuring the seepage volume, the head height at different points, and thus the infiltration coefficient by substituting into Equation (1):  
\[k=\frac{VL}{A\Delta ht} \tag{1}\]  
where \(\Delta ht\) is the poor water head (m), \(k\) is the coefficient of permeability (cm/s), \(A\) is the water flowing through the cross-sectional area (cm\({}^{2}\)), \(V\) is the amount of water flowing through the specimen after t time (cm\({}^{3}\)), \(L\) is the length of saturated specimen (cm), and \(t\) is the test duration(s). The principle of instrumental determination of the permeability coefficient of mixed soil is based on a variable head experiment. The water pressures upstream and downstream of the adsorbent were controlled through the pressure-volume controller. The confining pressure was applied using the air-pressure system. The permeability coefficient measurement range was 10\({}^{-5}\)-10\({}^{-9}\) cm/s. The volume controller and air-pressure system had a pressure-configuration range of 0-1 MPa. Permeability tests were conducted at a confining pressure of 300 kPa, upstream water pressure of 200 kPa, and downstream water pressure  
Figure 3: Permeation experiment-related instruments. (**a**) GDS; (**b**) XRD; (**c**) SEM. of 100 kPa; the leachate was simulated using ZnCl\({}_{2}\) solutions of different concentrations (DIW, 0.01, 0.03, and 0.10 mol/L). In order to investigate the changes in the permeability coefficient of mixed soil from a microscopic point of view, we applied the X-ray diffraction (XRD) technique and the scanning electron microscope (SEM) technique to examine the mixed soil at the end of the permeability experiment. The XRD device was used to test the mineral composition of bentonite and zeolite with the radiation, as shown in Figure 3b. The surface morphology of bentonite after the adsorption experiment was characterized using the SEM (Zeiss Sigma 300) operated at an accelerating voltage of 5 kV to examine and analyze the surface morphology and provide evidence of the effects of heavy metal cations after adsorption, as shown in Figure 3c. Sorption of Zn\({}^{2+}\) cations was performed using the batch experiment technique of single solute reactions. The experiments were performed in 100 mL glass columns with stoppers containing ionic solution aqueous solutions of Zn\({}^{2+}\) with 1 stone chip-bentoite-zeolite mixture. The adsorbent and aqueous solution were shaken at 200 rpm to reach equilibrium concentration, while the columns were kept sealed to minimize losses to the atmosphere.