# Potassium Recovery from Potassium Solution and Seawater Using Different Adsorbents
## 3 Results and Discussion  
### K Sorption by Different Adsorbents  
#### 3.1.1 K Sorption Isotherms Using KCl Solutions  
Figure 1 shows the sorption isotherms of K on three different adsorbents under acidic conditions (i.e., pH = 4.57). The K sorption capacities, indicated by \(q_{e}\), increased with increasing initial K concentrations (Figure 1a). The sorption data were fitted to the linearized forms of the Langmuir and Freundlich isotherm models (Figure 1b,c). Table 1 shows the parameters for the Langmuir and Freundlich isotherms derived from the experimental data using Equations (1) and (2) and the regression coefficients (R\({}^{2}\)). The K sorption on the adsorbents tested can be described by the L-type sorption isotherm, which suggests no strong competition between solvent and sorbate for the sorption sites [3]. Considering the R\({}^{2}\) values, the Freundlich isotherm model (i.e., R\({}^{2}\) = 0.92) could fit the K adsorption on the natural zeolite better than the Langmuir isotherm model (i.e., R\({}^{2}\) = 0.77), while the Langmuir isotherm model could fit the K adsorption on the ammonium acetate-treated zeolite and, in particular, the manganese nodule, better than the Freundlich isotherm model (Table 1). The \(q_{m}\) values, related to the maximum K sorption capacity, derived from the Langmuir isotherm model show that the ammonium acetate-treated zeolite has the greatest K adsorption capacity among the adsorbents studied, followed by the natural zeolite and manganese nodule (Table 1). However, the \(K_{d}\) value indicating the affinity of the binding sites of the manganese nodule was higher than that of the ammonium acetate-treated zeolite (Table 1). These results suggest that the manganese nodule has the higher affinity for K sorption than the ammonium acetate-treated zeolite, but there may be less binding sites available for K sorption on the manganese nodule than the ammonium acetate-treated zeolite. The Freundlich isotherm model parameters showed similar results. For example, the \(K_{F}\) value related to the sorption capacity of the ammonium acetate-treated zeolite was higher than the other \(K_{F}\) values (Table 1). The n value related to the sorption driving force of the manganese nodule was also higher than the n values of the other adsorbents (Table 1). \begin{table}
\begin{tabular}{c c c c c c c} \hline \multirow{2}{*}{**pH**} & \multirow{2}{*}{**Adsorbent**} & \multicolumn{3}{c}{**Langmuir Isotherm**} & \multicolumn{3}{c}{**Freundlich Isotherm**} \\ \cline{3-6}  & & \(q_{m}\) (mg g\({}^{-1}\)) & \(K_{d}\) & \(\mathbf{R^{2}}\) & \(K_{F}\) & \(n\) & \(\mathbf{R^{2}}\) \\ \hline \multirow{4}{*}{4.57} & Natural zeolite & 40 & 0.011 & 0.77 & 0.80 & 1.4 & 0.92 \\  & Ammonium acetate-treated zeolite & 42 & 0.057 & 0.93 & 2.9 & 1.8 & 0.90 \\  & Manganese nodule & 2.0 & 0.17 & 0.80 & 0.84 & 5.2 & 0.14 \\ \hline \multirow{4}{*}{6.99} & Natural zeolite & 5.8 & 0.045 & 0.54 & 0.55 & 2.0 & 0.77 \\  & Ammonium acetate-treated zeolite & 8.1 & 0.037 & 0.66 & 2.2 & 4.9 & 0.30 \\  & Manganese nodule & 9.7 & 0.018 & 0.11 & 2.1 & 3.8 & 0.14 \\ \hline \end{tabular}
\end{table} 
Table 1: Langmuir and Freundlich isotherm parameters for potassium sorption at pH 4.57 and 6.99. When the natural zeolite and the ammonium acetate-treated zeolite are compared, the K sorption capacities indicated by the \(q_{m}\) values and \(K_{F}\) values were greater for the ammonium acetate-treated zeolite (Table 1). This agrees with previous studies where the ammonium acetate-treated zeolite showed an improved Na sorption capacity over the untreated natural zeolite [10]. The greater K sorption capacity of the ammonium acetate-treated zeolite than the natural zeolite is also supported by the higher specific surface area and pore volume.