## 3 Results and Discussion
### Study on Combined Remediation Mechanism
According to the independent \(t\)-tests on different forms of arsenic, the equivalence test of variance showed no significant differences across the five forms (P1 > 0.05), which provided a basis for conducting equivalence tests on the means. The results indicate significant differences (P2 < 0.05) for all chemical forms except for exchangeable and water-soluble arsenic, confirming that the changes in all other chemical forms were statistically significant. This demonstrated that the remediation materials effectively transformed highly reactive arsenic into more stable residual forms, thereby promoting the immobilization of arsenic in the soil and reducing its bioavailability. As illustrated in Figure 6B, during the remediation process, the percentage of highly reactive acid-extractable cadmium decreased by 3.2 percentage points, and the percentage of reducible forms decreased by 2 percentage points, while the percentage of oxidizable forms increased by 3.3 percentage points. Furthermore, the percentage of less reactive residual cadmium increased by 1.9 percentage points. The independent \(t\)-test results, including the test of variance equivalence which showed P1 > 0.05, allowed for mean equivalence testing, revealing significant changes in the metal forms of cadmium after remediation (P2 < 0.05). This indicates significant differences in the migration of cadmium forms before and after remediation. These results suggested that the remediation materials effectively transformed acid-extractable cadmium into more stable oxidizable forms and reducible forms into more stable residual forms, thereby enhancing the immobilization of cadmium in the soil and reducing its bioavailability. In order to investigate the changes in heavy metals and other important elements in the soil before and after remediation, XPS spectra analysis was conducted on the corresponding soil samples. The results are shown in Figures 7 and 8. Figure 6: The speciation distribution of As (**A**) and Cd (**B**) in contaminated soil (a and b represent significant differences between each other, indicating the comparison results of different groups. \(p\) < 0.05). Figure 7A,B represented XPS full spectra of the soil before and after remediation, respectively. Upon comparing the two figures, it was observed that the peak intensities of Si2p at 103.11 eV and Al2p at 74.93 eV decreased, indicating the involvement of these elements in the stabilization process of heavy metals. The mechanism was that SiO\({}_{2}\) and Al\({}_{2}\)O\({}_{3}\) participated in hydration reactions [15], as illustrated by reaction Equations (5) and (6):  
\[\text{SiO}_{2}+\text{Ca(OH)}_{2}+\text{nH}_{2}\text{O}\text{-}\text{CaO}\text{ -SiO}_{2}\text{-}\text{(n+1)}_{2}\text{O} \tag{5}\]  
\[\text{Al}_{2}\text{O}_{3}+\text{Ca(OH)}_{2}+\text{nH}_{2}\text{O}\text{-}\text{CaO} \text{-Al}_{2}\text{O}\text{-}\text{(n+1)}_{2}\text{O} \tag{6}\]  
The produced calcium aluminum hydrate and calcium silicate hydrate improved the solidification and stabilization ability of remediation material towards Cd through physical encapsulation and chemical reactions (involving Ca\({}^{2+}\) undergoing ion exchange with Cd\({}^{2+}\)), thereby diminishing the bioavailability of Cd in the soil [58; 59]. Figure 8: XPS fine spectrogram of contaminated soil before and after remediation ((**A**) Cd 3d, (**B**) As 3d, and (**C**) Fe 2p). Figure 7: XPS full spectrogram ((**A**) the soil before remediation; (**B**) the soil after remediation). By comparing the fine spectra of each element before and after remediation, notable changes were observed in Cd, As, and Fe. The detailed XPS spectra results for Cd, As, and Fe are presented in Figure 8.  
Figure 8A displays the XPS spectra for Cd 3d in the soil both before and after remediation. Post-remediation, the Cd 3d peak position shifted positively, although the peak area remained largely unchanged.