## 3 Results and Discussion
### Adsorption Isotherm  
We obtained the adsorption isotherm for the Cu(II) on the modified zeolite at various weights of the modified zeolite (from 0.05 to 1 g) while maintaining all the other parameters (i.e., agitation speed (\(n\) = 100 rpm), pH (pH = 5), contact time (\(t\) = 8 h), initial concentration of Cu(II) (\(C_{0}\) = 100 mg/L), and temperature (\(T\) = 25 \({}^{\circ}\)C)). We employed the Langmuir and [PERSON] isotherm models to describe the equilibrium metal adsorption. The linearized form of these models is expressed by Equations (7) and (9), respectively. \[C_{e}/q_{e}=(1/Qb)+(C_{e}/Q) \tag{7}\]  
where \(Q\) (mg/g) is the monolayer adsorption maximum capacity of the adsorbent, \(q_{e}\) (mg/g) is the adsorption capacity at equilibrium, \(C_{e}\) (mg/L) is the equilibrium concentration of Cu(II), and \(b\) (L/mg) is the adsorption constant related to the energy of adsorption. \(Q\) and \(b\) were found from the slope and intercept of the plot of \(1/q_{e}\) versus \(1/C_{e}\). The Langmuir model's dimensionless constant separation factor (\(K_{L}\)) is defined as:  
\[K_{L}=1/(1+bC_{0}) \tag{8}\]  
where \(K_{L}\) indicates the shape of the isotherms: unfavorable (\(K_{L}\) > 1), linear (\(K_{L}\) = 1), favorable (\(0<K_{L}<1\)), or irreversible (\(K_{L}\) = 0). \[logq_{e}=logK_{f}+nlogC_{e} \tag{9}\]  
where \(K_{f}\) (L/g) and \(n\) are empirical constants of the Freundlich isotherm. We calculated the isotherm model constants from the slope and intercept of the [PERSON] and [PERSON] isotherm model plots. The values of the isotherm constants of these models and correlation coefficients are shown in Table 6.  
Fecundlich-model adsorption-intensity (\(n\)) values between 1 and 10 indicate beneficial adsorption. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Isotherm Model** & \multicolumn{2}{c}{**Isotherm Constant**} \\ \hline  & \(Q\) (mg/g) & 6.176 \\ [PERSON] & \(b\) (L/mg) & 0.064 \\  & \(K_{L}\) & 0.144 \\  & \(R^{2}\) & 0.8946 \\ \hline  & \(K_{f}\) (L/g) & 1.372 \\ [PERSON] & \(n\) & 2.764 \\  & \(R^{2}\) & 0.9216 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Isotherm constants. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \(T\) **(K)** & \(C_{0}\) **(mg/L)** & \(\Delta G^{\circ}\) **(kJ/mol)** & \(\Delta H^{\circ}\) **(kJ/mol)** & \(\Delta S^{\circ}\) **(J/mol\(\cdot\)K)** \\ \hline
298 & & \(-\)16.976 & & \\
308 & & \(-\)18.674 & & \\
318 & 100 & \(-\)21.894 & & \\
328 & & \(-\)22.259 & & \\ \hline \hline \end{tabular}
\end{table} 
Table 5: The thermodynamic parameters of the adsorption. Based on the isotherm-model constant values (\(K_{L}\) = 0.144, \(n\) = 2.764) and the correlation coefficients (\(R^{2}\) = 0.8946, \(R^{2}\) = 0.9216), we concluded that Cu(II) adsorption onto the modified zeolite could follow the Freundlich isotherm model rather than the Langmuir isotherm model. As shown in Table 7, a comparative result shows that the Cu(II) adsorption capacities of modified synthesized nanocomposite adsorbents [45,46] are higher than modified natural zeolites and alumosilicate [25,33]. 3.4-Dihydroxyphenethylamine (dopamine) and silicon organic compounds such as 4-(3-triethoxysilypropy) thiosemicarbazide, \(N\),\(N^{\prime}\)-bis(3-triethoxysilypropy)|thiocarbamide, and 3-aminopropyltriethoxysilane (APTES) are used as a modifier agent in these studies. [PERSON] et al. [33] determined the surface characteristics of Russian zeolite modified with 4-(3-triethoxysilypropy) thiosemicarbazide, which is more similar to the surface characteristics of Mongolian zeolite modified with 3-aminopropyltriethoxysilane (APTES). In addition, they concluded that chelate complexes formed in the Cu(II) ad-sorption system. As can be seen in Table 7, the adsorption capacity of modified Mongolian zeolite was not necessarily high compared with other adsorbents. However, it could be regarded as a potential adsorbent for the removal of Cu(II) from wastewater for practical use; the modification method of the adsorbent is relatively simple, and it can be available for the removal of Cr(VI) [32].