## 3 Results and Discussion
### Adsorption  
Zn\({}^{2+}\) is a common heavy metal ion, which is widely distributed in the wastewater from smelting, electroplating, and steel rolling industries. Thus, the adsorption performance of red mud and the prepared adsorbents on Zn\({}^{2+}\) were investigated. As shown in Figure 6, the adsorption of Zn\({}^{2+}\) was ranked in the following order: red mud \(<\) P-120 \(<\) P-270 \(<\) P-200. The adsorption data was fitted with the non-linear Langmuir and Freundlich models, respectively, and the results showed that the regression coefficient values (\(R^{2}\)) of Langmuir model were approximately 0.99 for red mud and the three adsorbents (Table 1), which exceeded those of the Freundlich model. This finding indicated that the red mud and the prepared adsorbents have an energetically homogeneous surface [34] for the adsorption of Zn\({}^{2+}\). P-200 showed the desirable adsorption capacity for Zn\({}^{2+}\) (89.6 mg/g), higher than that of red mud, P-120, P-270, and other reported magnetic adsorbents, such as graphene oxide (73 mg/g) [35], magnetic nano-silicon materials (51.2 mg/g) [36], and dry algae (60 mg/g) [37]. Therefore, P-200 has great potential for Zn\({}^{2+}\) adsorption. Figure 5: Scanning electron microscopy (SEM) images of (**A**) red mud, (**B**) P-120, (**C**) P-200, and (**D**) P-270. Figure 6: Adsorption isotherm of Zn\({}^{2+}\) on red mud, P-120, P-200, and P-270. Experimental conditions: equilibrium time: 12 h, P-200 dosage: 0.01 g, initial pH: 4. The adsorption capacity of Zn\({}^{2+}\) was in agreement with the number of surface coordination sites on red mud and the three adsorbents, which could be determined by \(H_{\rm s}\) with the Gran method. As shown in Figure 7 and Table 1, \(H_{\rm s}\) could be listed in the following order: red mud \(<\) P-120 \(<\) P-270 \(<\) P-200, which was consistent with the adsorption capacities of Zn\({}^{2+}\) on these adsorbents. P-200 showed the optimal \(H_{\rm s}\) of 4.35 mmol/g, indicating that it had sufficient surface coordination sites for Zn\({}^{2+}\) adsorption. The distribution coefficient (\(K_{\rm d}\)) was a useful index for comparing the adsorption capacities of red mud and the obtained adsorbents for Zn\({}^{2+}\) under the same experimental condition [38; 39], and calculated according to the following equation:  
\[K_{\rm d}=\frac{q_{\rm e}}{C_{\rm e}}, \tag{12}\]  
where \(q_{\rm e}\) is the amount of Zn\({}^{2+}\) adsorbed on red mud or the obtained adsorbents (mg/g), and \(C_{\rm e}\) is the equilibrium concentration of Zn\({}^{2+}\) in the solution. As shown in Figure 8A, \(K_{\rm d}\) decreased with the increase of the initial concentration of Zn\({}^{2+}\). Generally, at a high initial concentration of Zn\({}^{2+}\), the surface sites of P-200 became saturated with Zn\({}^{2+}\)[39; 40], which caused large number of unabsorbed Zn\({}^{2+}\) remaining in the liquid fraction and apparently exhibited low \(K_{\rm d}\) (Figure 8A and Table 2). However, at a low initial concentration of Zn\({}^{2+}\), P-200 showed a high value of \(K_{\rm d}\) in comparison with P-120, P-270 and commercial adsorbents (Table 2), such as graphene oxide [36] and Tunisian smectite [41], demonstrating that P-200 had desirable performance for sorption of Zn\({}^{2+}\). Other adsorbents, such as magnetic core-silica shell particles [36], dry algae [37], and magnetic modified chitosan [42], showed very low \(K_{\rm d}\) (Table 2), indicating that they were fairly ineffective for Zn\({}^{2+}\) adsorption.