## 3 Results and Discussion  
### Characteristics of Natural and Modified Zeolite  
A comparison of the instrumental analysis results (XRD, SEM/EDX, TGA/DTA, and N\({}_{2}\)-BET) of natural and modified zeolite is shown below. We present the mineralogical composition of the natural zeolite in Table 1. We could not identify some phases of the sample analysis by their interatomic distances in the X-ray diffraction analysis, and we have described them as unknown phases in Table 1. We determined the content of the unknown phases of the natural zeolite to be 7.9%. Natural zeolite minerals are complicated systems that contain many phases. The main minerals in the natural zeolite sample can be determined by X-ray diffraction analysis. The diffraction peaks with low intensity are observed by the side of the dominant diffraction peaks in the X-ray pattern. It is difficult to indicate the minerals corresponding to the diffraction peaks with low intensity in the original sample. Therefore, we noted the unknown minerals as x-phases. According to the XRD pattern in Figure 1, the diffraction peaks of the zeolite from the Tsagaantsav deposit appear in a range of 20 angle values: 10.13 (100), 11.43 (200), 15.38 (220), 17.49 (111), 19.36 (131), 22.68 (400), 26.27 (222), 32.17 (530), 36.06 (351), and 37.24 (441). According to these diffraction peaks, the zeolite from the Tsagaantsav deposit is dominated by clinoptilolite-K-type zeolites [37; 38; 39; 40; 41]. According to these results, the crystal structure of the natural zeolite may have changed due to the APTES molecules interacting with the zeolite surface during the modification. After the modification of the raw zeolite, the diffraction peak corresponding to clinoptilolite-K (d = 3.91 A) shifted slightly (d = 3.90 A) and the intensities of the diffraction peaks on the X-ray pattern were decreased. We present the SEM images of the natural and modified zeolite in Figure 2a,b. As can be seen in Figure 1, the intensity of the diffraction peaks decreased after the modification of the natural zeolite. This means that the degree of crystallization of the crystal structure of the sample is decreasing. Consequently, these changes may be due to the irregular appearance of the surface of the modified zeolite compared to the natural zeolite, Figure 2. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Mineral** & **Formula** & **Quantity (wt.\%)** \\ \hline Clinoptilolite-K & Ca\({}_{1.16}\)Na\({}_{1.8}\)Mg\({}_{0.25}\)Al\({}_{6.33}\)Si\({}_{29.81}\)O\({}_{72}\cdot\)20.1H\({}_{2}\)O & 60.3 \\ Quartz & SiO\({}_{2}\) & 2.7 \\ Goethite & FeO(OH) & 4.7 \\ Anorthoclase & (Na, K)[AlSi\({}_{3}\)O\({}_{8}\)] & 1.4 \\ Albite & Na[AlSi\({}_{3}\)O\({}_{8}\)] & 15.2 \\ Hematite & Fe\({}_{2}\)O\({}_{3}\) & 4.2 \\ Anorthite & CaAl\({}_{2}\)Si\({}_{2}\)O\({}_{8}\) & 2.9 \\ Unknown phases & - & 7.9 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The mineralogical composition of natural zeolite. We present the results of the elemental analysis in Table 2. According to the ratio of silicon and aluminum (Si/Al = 5.9 mol/mol) and chemical composition (9.1% of potassium), the zeolite from the Tsagaantsav deposit is dominated by clinoptiolite-K-type minerals. \begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Elements**} & \multicolumn{2}{c}{**Quantity (wt.\%)**} \\ \cline{2-3}  & **Natural Zeolite** & **Modified Zeolite** \\ \hline Al & 10.8 & 11.0 \\ Si & 66.2 & 73.8 \\ \hline K & 9.1 & 6.4 \\ Ca & 3.3 & - \\ Fe & 10.6 & 8.8 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Elemental analysis results. Figure 1: XRD patterns of natural and modified zeolite. Figure 2: SEM images of (**a**) natural and (**b**) modified zeolite. After the modification, the increase in the Si/Al ratio (by 0.6) and silicon content (by 7.3%) indicated that we could graft the groups of APTES onto the natural zeolite.