## 3 Experimental Results and Analysis
### Scanning Electron Microscopy Test and Analysis of Results  
Secanning electron microscopy (SEM) uses a fine beam of electron probe, irradiates the surface of a sample, and accepts the reflected secondary electrons, which are transformed through a series of processing into a picture of the microstructure of the sample. It is a common method used for studying microstructure, and the SEM process is much easier to perform due to the ease of the sample preparation. SEM has a higher resolution and has been widely used in major disciplines. For the mercury pressure test, we used the same specimens, and liquid nitrogen was used to freeze-dry the test soil. Samples were directly taken around the mercury pressure soil samples. The more flat and uniform small pieces of soil were used for the electron microscope scanning test. The sample is illustrated in Figure 3.  
Figure 3 depicts the microstructure map of Pb\({}^{2+}\)-contaminated soil under the action of different concentrations for 5 days using electron microscopy, when the soil was magnified to 500 times. Soil is a complex three-phase soil (solid-liquid-gas); the solids in the soil are different; the size of different substances and their different nature result in the conductivity of the soil also varying, so it was necessary to spray high-conductivity substances on the soil. The surface of the soil contained some potholes and inequalities. These factors affected the final imaging effect of the microscopic result map. As seen in Figure 3, the corresponding black areas were produced in the corners of the image, which were also caused by the  
Figure 2: Pore size–cumulative mercury input relationship curve of Guilin red clay under the effect of different concentrations of heavy metal ions. aforementioned factors. Even though it was not possible to binarize the image of the electron microscope at 500 times magnification, in general, the phenomenon of fissures in Guilin red clay increased with the increase in heavy metal Pb\({}^{2+}\) concentration when the image was magnified to 500 times, which was caused by the erosion of the cementing material in the soil caused by the entry of Pb\({}^{2+}\) into the interior of the soil. This is consistent with the results of the previous mercury pressure test described in Equation (1). When the soil sample was magnified 20,000 times with an electron microscope, it resulted in the original and binarized processed images depicted in Figure 4.  
Figure 4: _Cont._  
Figure 3: Microstructure of soil after 5 days of Pb\({}^{2+}\) contaminations at different concentrations (500\(\times\)). Figure 4 shows that when the electron microscope was magnified 20,000 times, the concentration of lead increased, the pores on the clay surface gradually developed and arranged in a point-to-point manner, and the soil particles were loosely arranged. The unit presented in the figure is at the particle level, which represents the spacing between clay particles and the tightness of the arrangement. The microstructure of the remodeled red clay soil was relatively complex. Because the main mineral components of Guilin red clay are kaolinite, trilbite, goethite, and quartz, the bulk, strip, and scale shapes could be seen in the electron microscope scanning. Using MATLAB to select different gray levels of  
Figure 4: Microstructure of soil after 5 days of contamination with different concentrations of lead ions (20,000\(\times\)). the original image in Figure 4a,c,e,g, binarization processing for the image Figure 4b,d,f,h, and two functions Figure 4b,d,f,h, we concluded that the porosity was the black and white pixels in the binary images. The number calculated according to the proportion of its porosity is shown in Table 2.  
As seen in Table 2, with the increase in heavy metal ion concentration, the binary porosity data of Guilin remolded red clay at 20,000 times magnification obtained by SEM represented the spacing relationship between the clay minerals.