## 3 Results and Discussion
### Incubation Study
[28] reported an excellent efficiency of LDH-modified natural zeolite in removing Cd and Pb from artificially contaminated soil. They reported more than 70% removal of Cd and Pb at pH 7 in adsorption and incubation studies. In comparison with pristine zeolite samples, they reported a 1.5-1.6 times higher removal efficiency of LDH-modified zeolites. Electrostatic interaction, coprecipitation, and metal complexation with LDH surface functional groups were reported as the main mechanisms in the stabilization of Cd and Pb. Table 6 provides a comparison between the removal efficiency of Cd and Pb contaminants by different adsorbents of natural and modified zeolite based on the findings of this study and previous research in the literature. Figure 8: Efficiency of natural and modified zeolite adsorbents in removing Cd (**A**) and Pb (**B**) from incubated mining contaminated soil. NZ-N, natural zeolite with nanoparticle size; LDH-N, LDH-zeolite with nanoparticle size; HCl-N, HCl-zeolite with nanoparticle size. Our results indicate that similar mechanisms could be responsible for the strong sorption of Cd and Pb, as evident by our findings of concentration of the AA extraction in the incubation study. Moreover, HCl-N and ZN-N also showed credible ability in removing heavy metals, which could be due to the three-dimensional framework of alumina and silicate tetrahedral units in zeolite composition, containing interconnected channels and pores, which helps zeolite to hold and adsorb heavy metal contaminants [51]. In research work on contaminated soil, [PERSON] et al. [66] reported that NZ reduced the extractable concentration of Cd and Pb in soil and removed 90.7 and 80.4% of Cd and Pb, respectively. [PERSON] et al. [67] reported a significant drop in the exchangeable fraction of Pb in contaminated soil treated with NZ. Also, porous aluminosilicate structure and net negative surface charge on NZs indicated more affinity of NZ for cationic pollutants [27]. Overall, the incubation experiment revealed a trend of removing heavy metals similar to results obtained from the kinetics experiment, indicating quick and rapid removal initially, which later moved toward equilibrium. Similarly, AA extraction revealed that with increasing incubation period, the adsorption capacity of tested sediments for Pb and Cd enhanced, and little to no quantity of Pb and Cd extractable fraction was found in the later stages of incubation. In addition, the natural zeolite and modified zeolites as prepared in this study were very stable during the incubation experiment, and the positive characteristics of the modified zeolites (e.g., high surface area and increased surface functional groups) were not affected by the incorporation with the soil. This implies that modified zeolites will remain highly effective in the sorption of heavy metals even after the incorporation with the soil in the field for a long time. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Adsorbent** & **Metal** & **Removal Efficiency (\%)** & **Reference** \\ \hline NZ & Cd, Pb & 85, 34\% & [59] \\ NZ & Cd, Pb & \textgreater{}99\% & [55] \\ NZ & Cd, Pb & 91, 80\% & [64] \\ LDH-MZ & Cd, Pb & \textgreater{}70\% & [28] \\ NaCl-Z & Cd, Pb & 73, 56\% & [59] \\ LDH-N & Cd, Pb & \textgreater{}98\% & This study \\ NZ-N & Cd, Pb & 97, 96\% & This study \\ HCl-N & Cd, Pb & 96, 97\% & This study \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Comparison of removal efficiency of heavy metal contaminants by different adsorbents. Figure 9: Efficiency of natural and modified zeolite adsorbents in stabilizing extractable Cd (**A**) and Pb (**B**) from incubated mining contaminated soil. NZ-N, natural zeolite with nanoparticle size; LDH-N, LDH-zeolite with nanoparticle size; HCl-N, HCl-zeolite with nanoparticle size; CK, control soil. Zeolite sediments used in this study are natural deposits, and based on the performed chemical analysis, their characteristics (e.g., EC, pH, and CEC) are within acceptable limits for healthy soil (Table 3).