## 3 Results and Discussion
### Study on Combined Remediation Mechanism  
To assess the impact of remediation materials on soil, leaching toxicity experiments were conducted on the remediation materials, and the results are shown in Table 6.  
Based on the data presented in Table 6, the leached concentrations of the eight heavy metals from the material were markedly lower than the upper limits specified in the Comprehensive Wastewater Discharge Standards (GB 8978-1996) [57]. Consequently, the remediation material demonstrated excellent safety and environmental compatibility. To assess the impact of the remediation material on the physicochemical properties of the soil, measurements of soil pH value, redox potential, and conductivity were taken before and after remediation, and the results are detailed in Table 7.  
As shown in Table 7, the influence of ferrous sulfate on soil properties was evaluated. Before remediation, the soil had a pH of 6.72, a redox potential of 461 mV, and an electrical conductivity of 67.9 \(\upmu\)S/cm. After applying ferrous sulfate, the pH decreased by 0.13 to 6.59, remaining within the acceptable range of \(\pm\)0.2, which indicated minor acidification that was still within safe limits for biological activity. The redox potential decreased by 35 mV to 426 mV, influenced by the reducing conditions induced by ferrous sulfate. However, this level was within the normal range for upland soils (200-750 mV), suggesting suitable conditions for most plant and microbial life. Furthermore, the soil's electrical conductivity increased by 15.6 \(\upmu\)S/cm to 83.5 \(\upmu\)S/cm, attributed to the higher ion concentration from the ferrous sulfate. Despite this increase, the conductivity stayed below 200 \(\upmu\)S/cm, ensuring the soil did not reach saline-alkali conditions. Overall, the remediation using ferrous sulfate was effective, adjusting soil properties within environmentally safe limits, and thus supporting its use in similar remediation efforts. To gain a deeper understanding of the changes in heavy metal properties in the soil of the contaminated area after material-plant combined remediation, an analysis of the chemical forms of As and Cd in the soil before and after remediation was conducted. The results are shown in Figure 6. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Items** & **Cd** & **As** \\ \hline Leaching concentrations of & & \\ heavy metals & 0.03 \(\pm\) 0.01 & 0.02 \(\pm\) 0.01 \\ Wastewater discharge & & \\ standards & 0.1 & 0.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Leaching toxicity analysis (mg/L). \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **pH Value** & **Eh (mV)** & **EC (\(\upmu\)S/cm)** \\ \hline The soil before & & & \\ remediation & & & \\ The soil after & & & \\ remediation & & & \\ \hline The table displayed the significant differences in the physicochemical properties (pH, Eh, and EC) of the soil before and after treatment of the contaminated soil, based on the results of a one-way ANOVA. Each set of data had a sample size of n = 3. The letters “a” and “b” indicate significant differences among the means, with the same letter indicating no significant difference. The significance level is denoted by \(p\), with \(p\) \(<\) 0.05 indicating a significant difference.  
\begin{tabular}{c c c} \hline \hline  & **pH Value** & **Eh (mV)** & **EC (\(\upmu\)S/cm)** \\ \hline The soil before & & & \\ remediation & & & \\ The soil after & & & \\ remediation & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Physicochemical properties of soil before and after remediation (\(p\) < 0.05). Based on the observations from Figure 6A, during the joint remediation process, the percentage of highly reactive exchangeable arsenic decreased by 0.6 percentage points, while the percentages of bound iron-manganese and organically bound arsenic each decreased by 3.1 percentage points. Additionally, the percentage of less reactive residual arsenic significantly increased by 4.7 percentage points.