## 4 Conclusions  
In this study, the combined remediation of cadmium- and arsenic-contaminated soil by phytoremediation and stabilization was studied, and the main conclusions were as follows:  
(1) With a focus on the characteristics of heavy metal pollution in the soil of the impact area of the Zhongshan Huilong arsenic smelting plant in Guangxi, research was conducted on the reduction in bioavailability of As and Cd in the soil. Local materials (steel slag, pyrolusite and FeSO4) and remediation plants (_Cynodon dactylon, Bidens pilosa_, and _Bohemeria nivea_) were utilized. Steel slag, pyrolusite, and ferrous sulfate were identified as the optimal remediation materials, and Bidens pilosa was recognized as the best remediation plant. The optimal material ratio of 2:1:10 for steel slag, pyrolusite, and ferrous sulfate, with a dosage of 9% by weight of the contaminated soil, resulted in a 90.7% reduction in arsenic bioavailability and an 89.1% reduction in cadmium bioavailability in the soil. Field experiments at the Zhongshan Huilong arsenic smelting plant in Guangxi confirmed the stability and effectiveness of this remediation strategy, achieving a reduction of over 88.5% in bioavailability for both metals. (2) Based on the experimental results and related mechanism studies of the combined remediation of soil heavy metals in the impact area of the Zhongshan Huilong arsenic smelting plant in Guangxi, this study proposed a mechanism for the combined remediation of soil heavy metals by the synergy of material and plant. The introduction of remediation \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Remediation Scheme** & **Remediation Cost (S/Ha.)** & **Remediation Effect** & **Reference** \\ \hline Single material remediation & 4631.3 & 31.0\% As, 44.3\% Cd, and 96.8\% Pb & [83] \\ Single material remediation & 3909.1 & 85.4\% As & [84] \\ Single material remediation & 6460.4 & 30.2\% As and 28.6\% Pb & [85] \\ Single phytoremediation & 5811.9 & 26.7\% As, 11.4\% Cd, and 23.3\% Pb & [85] \\ Material–plant combined remediation & 3100.9 & 90.7\% As and 89.1\% Cd & This work \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Cost-effectiveness and treatment effect analysis of the remediation. materials into the soil increased the content of Fe(III) and Mn(IV), oxidizing highly active As(III) to less active As(V), thereby reducing the bioavailability of As in the soil. During the growth process of the remediation plants, organic acids released by plant roots formed complexes with Cd in the soil through chelation, increasing the stable, organically bound form of Cd after remediation, thereby reducing the bioavailability of Cd in the soil. After combined remediation, the abundance of microbial genera with high tolerance to heavy metals increased in the soil. These genera reduced the bioavailability of As and Cd in the soil through adsorption, chelation, or biological enrichment. Additionally, certain genera also promoted plant growth, facilitating vegetation restoration in the contaminated area and enhancing the effectiveness of combined remediation. (3) Through a cost-benefit analysis of the remediation, the combined remediation technology in this study demonstrated favorable technical, economic, and commercial feasibility. Conceptualization, J.C. and Y.T. ; methodology, J.C. and C.Z. ; formal analysis, C.Z. and Y.T. ; writing--original draft preparation, C.Z. and J.C.; writing--review and editing, C.Z. and J.C. supervision, J.C. funding acquisition, J.C. All authors have read and agreed to the published version of the manuscript. The National Key Research and development program [2019 YFC1803500]; the Science and Technology Innovation Program of Hunan Province [2022 RC1039]; Research Foundation of the Department of Natural Resources of Hunan Province [HBZ20240148]; International and Regional Science and Technology Cooperation and Exchange Program of Hunan Association for Science and Technology [2024 SKX-KJ-06]. Not applicable. This study does not involve research on humans or animals. Not applicable.