## 3 Results and Discussion
### Kinetics Experiment  
The kinetics study of Cd and Pb adsorption revealed an initial quick adsorption, which then moved toward gradual equilibrium with specific contact time. The kinetics study was conducted at time intervals of 0.5, 1, 4, 8, 16, and 24 h. A quick adsorption in the beginning attained equilibrium at 8 h, and after that, a steady increase in adsorption was observed (Figure 7). Among all examined adsorbents, LDH-N displayed the highest adsorption capacity for Cd and Pb, followed by NZ-N and HCl-N. Initially, quick adsorption illustrated more available active sites, which were filled gradually to attain steady equilibrium sorption. Similarly, higher removal of Cd and Pb was found, which moved toward steady equilibrium with time. The experimental data were further interpreted with first-order and pseudo-second-order kinetic models to further explore metal adsorption mechanisms. Both applied kinetics models fitted well to experimental data (R\({}^{2}\) = 0.94-0.99) and described that the adsorption was both surface and chemisorption (Table 5). The suitability of the pseudo-second-order kinetics model indicates chemisorption among adsorbent and adsorbate. In our study, we found ion exchange and metal complexation mechanisms for Cd and Pb adsorption, which could be due to chemical interaction between metal ions and surface functional groups. During the ion exchange mechanism, metal ions (Cd and Pb) move through the porous surface of zeolite and are exchanged with an abundance of cations, e.g., Ca and Mg, while in surface complexation, metal ions make complex ions with reactive ions of surface functional groups such as metal oxides. The applied kinetics models explained that surface electrostatic interaction between metal ions and adsorbent surfaces was not the only mechanism behind adsorption, and chemical interactions between metal ions and surface functional groups present on adsorbent surfaces showed that adsorption was also governed chemically [64]. A higher constant rate for Cd and Pb was found with LDH-N in applied first-order (0.07 and 0.08) and pseudo-second-order (0.172 and 0.874) models, respectively. In addition, higher obtained sorption capacity (1.599 and 0.382 mg g\({}^{-1}\)) indicated better performance and higher affinity of LDH-N for Cd and Pb, respectively. [PERSON] et al. [65] found similar sorption and removal efficiency of zeolite composite for Pb, which confirmed chemisorption by the best fitness of the pseudo-second-order kinetics model.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multicolumn{1}{c}{} & \multicolumn{6}{c}{**Cd**} \\ \hline
**Adsorbents** & \multicolumn{3}{c}{**First Order**} & \multicolumn{6}{c}{**Pseudo-Second Order**} \\ \hline  & **r\({}^{2}\)** & **k1** & **SEE** & **r\({}^{2}\)** & **SEE** & **k2\({}^{\prime}\)** & **qe (mg g\({}^{-1}\))** & **h** \\ \hline NZ-N & 0.95 & 0.06 & 0.0003 & 0.97 & 0.0007 & 0.171 & 1.529 & 0.438 \\ LDH-N & 0.96 & 0.07 & 0.0003 & 0.99 & 0.0009 & 0.172 & 1.599 & 0.399 \\ HCl-N & 0.93 & 0.06 & 0.0002 & 0.99 & 0.0007 & 0.170 & 1.556 & 0.411 \\ \hline  & \multicolumn{6}{c}{**Pb**} \\ \hline  & **r\({}^{2}\)** & **k1** & **SEE** & **r\({}^{2}\)** & **SEE** & **k2\({}^{\prime}\)** & **qe (mg g\({}^{-1}\))** & **h** \\ \hline NZ-N & 0.95 & 0.04 & 0.0003 & 0.99 & 0.0002 & 0.634 & 0.374 & 0.104 \\ LDH-N & 0.96 & 0.08 & 0.0009 & 0.99 & 0.0014 & 0.874 & 0.382 & 0.089 \\ HCl-N & 0.94 & 0.06 & 0.0004 & 0.98 & 0.0006 & 0.713 & 0.334 & 0.097 \\ \hline  & \multicolumn{6}{c}{NZ-N, natural zeolite with nanoparticle size; LDH-N, LDH- zeolite with nanoparticle size; HCl-N, HCl- zeolite with nanoparticle size.} \\ \end{tabular}
\end{table}
Table 5: Derived parameters of the kinetics of Cd and Pb adsorption and removal by natural and modified zeolite adsorbents.