## 3 Results and Discussion
### Results of Material-Plant Combined Remediation Experiment  
In a combined material-plant remediation experiment, _Bidens pilosa_ was used alongside a formulated mixture of steel slag, pyrolusite, and FeSO4, which were applied to contaminated soil. The composition was tested at a fixed 11% material usage, varying the mass ratios to determine the optimal combination. According to the findings presented in Figure 4, the most effective ratio for soil remediation was 2:1:10 for steel slag, pyrolusite, and ferrous sulfate, respectively [54]. Figure 4 illustrates how the adjustments in the mass ratios of steel slag (SS), manganese ore (Py), and ferrous sulfate (FS) influenced the bioavailability of arsenic (As) and cadmium (Cd). Figure 4A,B showed the impact of varying steel slag ratios. A lower ratio enhanced arsenic stabilization, increasing the bioavailability reduction from under 80% to over 90%. Conversely, reducing steel slag detrimentally affected cadmium stabilization, decreasing the bioavailability reduction from over 90% to under 80%. In Figure 4C, ferrous sulfate was added to the binary system of steel slag and manganese ore, and the variation in the mass ratio of ferrous sulfate (in five different mass ratios) was investigated for the combined remediation of materials and plants. When the mass ratio of ferrous sulfate in the ternary system increased from 6 to 10 (in the five formulations), the overall reduction rate of arsenic bioavailability increased from below 80% to above 85%, thereby promoting the solidification and stabilization of arsenic in the soil. Conversely, a higher mass ratio of ferrous sulfate in the ternary system resulted in a poorer reduction effect on the bioavailability of Cd (Figure 4D). As the mass ratio of ferrous sulfate in the ternary system increased from 6 to 10, the overall reduction rate of Cd bioavailability after combined remediation decreased from above 88% to below 80%, hindering the solidification and stabilization of Cd in the soil. The results from Figure 4C,D indicated that an increased mass ratio of ferrous sulfate in the ternary system promoted the solidification and stabilization of As in the soil, while a decreased mass ratio hindered the solidification and stabilization of Cd. The overall data from Figure 4 indicated that optimizing the ternary system's mass ratios could concurrently stabilize As and Cd in the soil of the Zhongshan Hui Long arsenic smelter. Under the optimal formulation (2:1:10 ratio of steel slag, manganese ore, and ferrous sulfate), the remediation reduced the bioavailability of As by 90.2% and Cd by 89.3%. From the experimental results above, the combined material-plant remediation potted experiments were further conducted, with the material dosage in the soil as a variable, to determine the optimal material dosage. Figure 5 illustrates the dynamic relationship between material dosage and the reduction in bioavailability for arsenic (As) and cadmium (Cd) in contaminated soil. The graph demonstrates that the reduction rate of As bioavailability increased with higher material dosages up to 9%, beyond which it stabilized. Specifically, the bioavailability of As decreased significantly from 53.1% to 90.7% as the material dosage rose from 1% to 9%. Conversely, while the reduction rate for Cd initially rose from 60.5% to 89.1% within the same dosage range, it declined to 76.3% when the material dosage was increased further to 19%. The trends observed could be attributed to the interaction between the remediation materials and the soil contaminants. The formation of stable complexes between the introduced iron ions and arsenic oxides reduced the mobility of arsenic, thereby enhancing its immobilization in the soil [55]. In contrast, the introduction of sulfate ions likely led to the formation of soluble cadmium sulfate complexes, increasing the mobility of Cd, especially at higher material dosages [56].