# Potassium Recovery from Potassium Solution and Seawater Using Different Adsorbents
## 3 Results and Discussion
### K Recovery from Seawater  
K adsorption experiments using seawater were carried out to verify the effects of different K concentrations and pH conditions on K recovery from seawater using natural zeolite, ammonium acetate-treated zeolite, and manganese nodule. The K recovery on natural zeolite generally increased with increasing initial K concentration at pH 8.33 (Figure 5a). The K recovery from seawater by natural zeolite ranged 4-14% (Figure 5a). The ammonium acetate-treated zeolite also showed increasing K recovery with increasing K concentration from 23 mg L\({}^{-1}\) to 50 mg L\({}^{-1}\) at pH 8.33; however, further increase in the K concentration did not increase the K recovery (Figure 5b). The ammonium acetate-treated zeolite showed approximately 5-12% of K recovery. The manganese nodule showed a different trend with other adsorbents and had the highest recovery at the lowest concentration used at pH 8.33 (i.e., 23 mg L\({}^{-1}\)) (Figure 5c). The manganese nodule had the K recovery of 0-22%, and the maximum recovery was observed with the diluted seawater sample (i.e., initial K concentration of 23 mg L\({}^{-1}\)) at pH 8.33. Except for this condition (i.e., initial K concentration of 23 mg L\({}^{-1}\) at pH 8.33), the zeolite-based adsorbents showed higher K recovery than the manganese nodule, and the K recovery was not affected by the pH conditions. The results observed are similar to that observed with the KCl solutions (see Section 3.1.2). With the KCl solution, the zeolite-based adsorbent adsorbed more K than the manganese nodule at acidic pH condition, while the manganese nodule adsorbed more K at neutral pH condition (Figure 3a). With the seawater, regardless of the initial K concentrations, more K was recovered using the zeolite-based adsorbents at acidic pH condition (Figure 5). \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**adsorbent**} & \multicolumn{3}{c}{**Pseudo-First-Order**} & \multicolumn{3}{c}{**Pseudo-Second-Order**} & \\ \cline{2-7}  & \(k_{I}\) & \(q_{e}\) & \(k_{2}\) & \(q_{e}\) & \(h\) & \(R^{2}\) \\  & (\(h^{-1}\)) & (mg g\({}^{-1}\)) & \(R^{2}\) & (g mg\({}^{-1}\) h\({}^{-1}\)) & (mg g\({}^{-1}\)) & (mg g\({}^{-1}\) h\({}^{-1}\)) & \(R^{2}\) \\ \hline Natural zeolite At alkaline pH condition, the zeolite-based adsorbents were better for K recovery than the manganese nodule, except at the initial K concentration of 23 mg L\({}^{-1}\) (Figure 5). Figure 6 compares the K sorption ability of three adsorbents in the KCl solutions and seawater. At low K concentrations (i.e., 23 mg L\({}^{-1}\)), the K sorption was the greatest with the ammonium acetate-treated zeolite in the KCl solution at pH 4.57 (Figure 6a). However, with the KCl solution at pH 6.99 and seawater at pH 8.33, the manganese nodules generally  
Figure 5: Recovery rate of potassium (K) by (**a**) natural zeolite (NZ), (**b**) ammonium acetate–treated zeolite (TZ), and (**c**) manganese nodule (MN) at various initial K concentrations (i.e., diluted seawater (23 mg L\({}^{-1}\)), raw seawater (50 mg L\({}^{-1}\)), and concentrated seawater (120 mg L\({}^{-1}\))) at different pH conditions (i.e., pH 5.08 \(\pm\) 0.04 and pH 8.33 \(\pm\) 0.15). showed higher sorption capacities than the others (Figure 6a). At K concentrations of 50 and 120 mg L\({}^{-1}\), the zeolite-based absorbents were more effective in K recovery from the acidic KCl solutions and seawater, while the manganese nodules were more effective for the neutral KCl solutions (Figure 6b,c). Except for the K recovery from the acidic KCl solution, the manganese nodules can be a comparable or better K adsorbent. Figure 6: Sorption of potassium (K) on different adsorbents (i.e., natural zeolite (NZ), ammonium acetate-treated zeolite (ATZ), and manganese nodule (MN)) at average K concentration of (**a**) 23 mg L\({}^{-1}\), (**b**) 50 mg L\({}^{-1}\), and (**c**) 120 mg L\({}^{-1}\) at KCl pH 4.57, KCl pH 6.99, and seawater pH 8.33. Note that the absence of bar graphs in NZ and MN implies that there was no adsorption to those absorbents.