# Potassium Recovery from Potassium Solution and Seawater Using Different Adsorbents
## 3 Results and Discussion
### K Sorption Kinetics  
The K sorption kinetics were studied in order to find out information about the rate of K adsorption on the natural zeolite and the ammonium acetate-treated zeolite, since they were better adsorbent than the manganese nodule. With the natural zeolite, the equilibrium  
Figure 3: (**a**) Maximum potassium (K) sorption capacity (\(q_{m}\)) and (**b**) the affinity of the binding sites (\(K_{d}\)) of the natural zeolite (NZ), ammonium acetate-treated zeolite (ATZ), and manganese nodule (MN) at pH 4.57 and 6.99 derived from the Langmuir isotherm model.  
reached approximately after 24 h (indicated with the dashed line in Figure 4a). With the ammonium acetate-treated zeolite, the equilibrium almost reached within 2 h (indicated with the dashed line in Figure 4a).  
Figure 4: (**a**) Adsorbed amounts of potassium (K) on zeolite\(-\)based adsorbents with respect to time, (dashed line: the moments where each system reaches equilibrium); (**b**) the pseudo\(-\)first\(-\)order kinetics equation fitted to the 4 h K sorption data (solid line: ammonium acetate-treated zeolite, dashed line: natural zeolite); and (**c**) the pseudo\(-\)second\(-\)order kinetics equation fitted to the 72 h K sorption data (solid line: ammonium acetate\(-\)treated zeolite, dashed line: natural zeolite).  
The K sorption was fitted to the pseudo-first-order kinetics equation (Equation (3)) and the pseudo-second-order kinetics equation (Equation (4)). The pseudo-first-order kinetics equation, written by [PERSON] and modified by [PERSON] and [PERSON], can demonstrate the relationship between adsorption amount at equilibrium and the reaction rate of adsorbents and adsorbate, in the aqueous solution [20; 21]. The pseudo-second-order kinetics equation, written by [PERSON] and [PERSON] and modified by [PERSON] and park, can represent surface adsorption including chemisorption [21; 22; 23]. The \(q_{e}\) values obtained in 3.1.1 were used in the kinetics equation.  
For both the natural zeolite and the ammonium acetate-treated zeolite, not a single straight line could fit all the K sorption data over 48 h with the pseudo-first-order kinetics, since no further sorption was observed after 2 h. Thus, only the initial three data were fitted to the pseudo-first-order kinetics to derive the parameters (Figure 4b). This means that the pseudo-first-order kinetics are not suitable for predicting the K sorption on zeolite over the entire sorption period (i.e., 48 h). The pseudo-second-order kinetics equation could fit a straight line to the entire K sorption data over 48 h (Figure 4c). Moreover, the R\({}^{2}\) values indicate that pseudo-second-order kinetics are more suitable than pseudo-first-order kinetics for K sorption with both the natural zeolite and ammonium acetate treated-zeolite (Table 2). This suggests that the rate-controlling step during the K sorption reaction on zeolite was the chemical sorption [3]. Both the pseudo-first-order rates and the pseudo-second-order rates show that the K sorption on the ammonium acetate treated-zeolite was faster than that on the natural zeolite (Table 2).