## 3 Results and Discussion
### Adsorption
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \multirow{2}{*}{**adsorbent**} & \multicolumn{3}{c}{**Langmuir**} & \multicolumn{3}{c}{**Freundlich**} & \multicolumn{2}{c}{\(H_{\rm s}\)} \\ \cline{2-9}  & \(q_{\rm m}\)**/(mg/g)** & \(K_{\rm L}\)**/(Li/mg)** & \(R^{2}\) & \(K_{\rm F}\)**/(mg/g)** & \(1\)**/\(n\)** & \(R^{2}\) & **mmol/g** \\ \hline Red mud & 11.43 & 0.14 & 0.998 & 4.36 & 0.2 & 0.846 & 0.51 \\ P-120 & 21.91 & 0.37 & 0.998 & 11.57 & 0.15 & 0.941 & 1.19 \\ P-200 & 89.61 & 0.44 & 0.998 & 49.26 & 0.11 & 0.715 & 4.35 \\ P-270 & 56.79 & 0.42 & 0.997 & 33.98 & 0.12 & 0.762 & 3.29 \\ \hline \end{tabular}
\end{table} 
Table 1: Parameters of the isotherm models and the surface sites concentrations of red mud and the obtained adsorbents. Figure 7: Gran plots of red mud, P-120, P-200, and P-270. Figure 8: (**A**) \(K_{\text{d}}\) and (**B**) \(K_{\text{d}}\) medium values of Zn2+ on the red mud, P-120, P-200, and P-270. The plots of adsorption capacity versus equilibrium concentration in solution did not yield a straight line (Figure 8A). Thus, an average \(K_{\mathrm{d}}\) value (\(K_{\mathrm{d\ medium}}\)) was calculated according to the method of Lu and Xu [43] and used to compare the adsorption performance of red mud and the obtained adsorbents for Zn\({}^{2+}\) adsorption. As shown in Figure 8B, \(K_{\mathrm{d\ medium}}\) of Zn\({}^{2+}\) on red mud and the obtained adsorbents were in the following order: P-200 \(>\) P-270 \(>\) P-120 \(>\) red mud, which was in agreement with the order of the surface site concentrations and the maximum adsorption capacities of red mud and the obtained adsorbents. As shown in Table 2, \(K_{\mathrm{d}}\) of raw red mud and the CO\({}_{2}\) neutralized red mud [9] were similar at the low initial concentration of Zn\({}^{2+}\), demonstrating that the method of CO\({}_{2}\) neutralization for red mud treatment was inefficient. However, \(K_{\mathrm{d}}\) of red mud dramatically increased from 0.71 L/g to 42.3 L/g after hydrothermal treatment at 200 \({}^{\circ}\)C. The improvement of \(K_{\mathrm{d}}\) value indicated enhancement of adsorption affinity and adsorption capacity of P-200 [44]. Thus, the hydrothermal method was better than that of CO\({}_{2}\) neutralization for improving the adsorption performance of red mud. XPS experiments were conducted to characterize P-200 before and after Zn\({}^{2+}\) adsorption. As shown in Figure 9A, the spectrum of P-200 contained the typical peaks of Al, Si, Ca, O, Fe, and Na. After Zn\({}^{2+}\) adsorption, the peak of Na 1s at 1072 eV disappeared (Figure 9A), and new peaks at the binding energies of 1020.7 eV and 1043.8 eV (Figure 9B) belonged to Zn\({}^{2+}\)[45]. This demonstrated that Zn\({}^{2+}\) was adsorbed on P-200, and that Na\({}^{+}\) was released from P-200. During the adsorption, the amount of Na\({}^{+}\) desorbed from P-200 showed a positive correlation with the amount of adsorbed Zn\({}^{2+}\) on P-200 (Figure 9C). This confirmed that the adsorption of Zn\({}^{2+}\) on P-200 was dominated by cationic exchange. Figure 9: **(A)** Full range and **(B)** Zn 2p XPS spectra of P-200 before and after Zn\({}^{2+}\) adsorption, and **(C)** amount of Na\({}^{+}\) desorbed as affected by Zn\({}^{2+}\) adsorption on P-200. Experimental conditions: equilibrium time: 12 h, P-200 dosage: 0.01 g, initial pH: 4. P-200 was a mixture of magnetite, hematite and Si/Al compounds, and exhibited functional sites \(\equiv\)SO\({}^{-}\) on its surface (S representing Fe, Si, or Al). In P-200 synthesis, Na\({}^{+}\) attached to the surface sites to form \(\equiv\)SO\({}^{-}\)Na\({}^{+}\) with the addition of NaOH, which was replaced by H\({}^{+}\) when pH decreased (Equation (3)). Therefore, both Na\({}^{+}\) and H\({}^{+}\) attached on P-200 surface in Zn\({}^{2+}\) adsorption. Zn\({}^{2+}\)was divalent-charged, which showed higher affinity to \(\equiv\)SO\({}^{-}\) than monovalent Na\({}^{+}\) and H\({}^{+}\)[46], and the surface-bound Na\({}^{+}\) or H\({}^{+}\) were subsequently replaced (Equations (13) and (14)).