## 3 Results and Discussion
### Adsorption Isotherms  
The adsorption isotherms obtained for the removal of ammonium and phosphate by various substrates (CG-Z, FG-Z and CG-G) are shown in Figures 6 and 7 and Tables 3 and 4. The adsorption behavior was characterized by fitting the data to Langmuir and Freundlich isotherm models at different temperatures (10 \({}^{\circ}\)C and 25 \({}^{\circ}\)C) and initial concentrations.  
The Langmuir model fit all substrates better for ammonium adsorption (Figure 6, Table 3), as shown by higher R\({}^{2}\) values (0.81-0.98), than the Freundlich model (0.71-0.93). The CG-Z substrate exhibited the highest theoretical maximum adsorption efficiency (q\({}_{\rm m}\)) of 286.93 mg/kg (at 25 \({}^{\circ}\)C). This was followed by FG-Z, with an efficiency of 285.71 mg/kg (at 25 \({}^{\circ}\)C), and CG-G, with an efficiency of 54.79 mg/kg (at 10 \({}^{\circ}\)C) (Table 2). With an adsorption affinity of 0.548 L/mg at 10 \({}^{\circ}\)C, CG-Z had the greatest Langmuir constant (k\({}_{\rm L}\)), indicating increased ammonium binding to zeolite surfaces. CG-Z and FG-Z showed moderate increases in q\({}_{\rm m}\) with increasing temperature from 10 \({}^{\circ}\)C to 25 \({}^{\circ}\)C, while CG-G showed a decrease in q\({}_{\rm m}\) (Table 3). Many studies report that the Langmuir model best describes the adsorption of ammonium onto natural or modified zeolite [67, 68, 69], while others report that the Freundlich model has the best fit [55, 70, 71, 72].  
Regarding the phosphate adsorption isotherm, the Freundlich model showed higher R\({}^{2}\) values for all three tested substrates, indicating that this model fits the experimental data better than the Langmuir model (Table 4). In all cases, the 1/n value was less than 1, indicating that phosphate adsorption on the three substrates (i.e., CG-Z, FG-Z, and CG-G) is favorable. According to the literature, the optimal adsorption isotherm for phosphate adsorption on zeolite depends on its characteristics and may vary. Several studies report  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Substrate**} & \multirow{2}{*}{**T (\({}^{\circ}\)C)**} & \multicolumn{4}{c}{**Langmuir Isotherm**} & \multicolumn{4}{c}{**Freundlich Isotherm**} \\ \cline{3-8}  & & **q\({}_{\rm m}\) (mg/kg)** & **k\({}_{\rm L}\) (L/mg)** & **R\({}^{2}\)** & **1/n** & **k\({}_{\rm F}\) (mg/kg)** & **R\({}^{2}\)** \\ \hline \multirow{2}{*}{CG-Z} & 10 & 18.14 & 0.12 & 0.90 & 0.72 & 2.05 & 0.97 \\  & 25 & 23.71 & 0.14 & 0.90 & 0.63 & 3.34 & 0.96 \\ \hline \multirow{2}{*}{FG-Z} & 10 & 24.15 & 0.08 & 0.98 & 0.69 & 2.12 & 0.99 \\  & 25 & 16.80 & 0.36 & 0.72 & 0.47 & 4.71 & 0.87 \\ \hline \multirow{2}{*}{CG-G} & 10 & 12.81 & 0.43 & 0.87 & 0.40 & 4.20 & 0.96 \\  & 25 & 15.65 & 0.36 & 0.80 & 0.48 & 4.34 & 0.95 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Adsorption isotherm parameters of phosphate.  
Figure 7: Langmuir and Freundlich fitting plots for phosphorus adsorption on (**a**) coarse-grained zeolite (CG-Z); (**b**) fine-grained zeolite (FG-Z); and (**c**) coarse-grained gravel (CG-G).  
that the Freundlich model best describes phosphate adsorption on zeolite (natural or modified) [34; 73], while others report that the Langmuir model has the best fit [62; 74].  
According to [PERSON] et al. [75], smaller particles offer a greater specific surface area for microbial adhesion, and chemical interactions are supported by the particle size effect (fine-grained > coarse-grained zeolite). However, [PERSON] et al.'s [76] findings on thermally enhanced zeolites are in contrast to the moderate temperature sensitivity of phosphate adsorption on fine zeolites, indicating the limitations of natural zeolites in high-temperature applications.