## 3 Results and Discussion
### Chloride Adsorption Behavior of LDH
\[Q_{\text{e}}=\frac{K_{\text{L}}Q_{\text{m}}C_{\text{e}}}{1+K_{\text{L}}C_{ \text{e}}} \tag{2}\]  
where \(K_{\text{L}}\) (L mol\({}^{-1}\)) is related to the binding energy, and \(Q_{\text{m}}\) (mg g\({}^{-1}\)) means the maximum amount of chloride adsorption per unit mass of LDH for monolayer coverage. \(Q_{\text{e}}\) is the amount of chloride adsorption at the equilibrium (mg g\({}^{-1}\)), and \(C_{\text{e}}\) is the chloride concentration at equilibrium (mol/L). The fitting result of C-high is shown in Figure 12.  
was 36.16 L mol\({}^{-1}\), and \(Q_{\rm m}\) was 155.88 mg g\({}^{-1}\). The value of \(Q_{\rm m}\) was much higher than that reported in most of the literature [22; 23; 24]. The comparison with the literature in terms of \(Q_{\rm m}\) is shown in Table 2.  
The LDH after anion exchange reaction in different NaCl solutions was characterized by XRD in order to investigate the detailed chloride adsorption process and to provide an explanation for the different chloride adsorption capacities of C-high and C-low shown above. As can be seen from Figure 13a, the peak of the (003) plane appeared at 10.04\({}^{\circ}\), 10.11\({}^{\circ}\), 10.13\({}^{\circ}\), 10.73\({}^{\circ}\), 11.52\({}^{\circ}\), and 11.58\({}^{\circ}\) when the NaCl solution was 0.002, 0.005, 0.01, 0.02, 0.04, and 0.08 mol/L, respectively. With the right shift of 2\(\theta\) degrees, the interlayer anions changed from nitrates to chlorides gradually, indicating that more chlorides were loaded in the LDH interlayer when the NaCl concentration increased. Therefore, the chloride \begin{table}
\begin{tabular}{l l l} \hline \hline
**LDH** & \(Q_{\rm m}\) (mg g\({}^{-1}\)) & **Literature** \\ \hline MgAl-LDH-NO\({}_{3}\)\({}^{-}\) & 155.88 & Our work \\ MgAl-LDH-pAB & 33.54 & 22 \\ MgAl-LDH-NO\({}_{3}\)\({}^{-}\) & 107.00 & 23 \\ MgAl-LDH-NO\({}_{2}\)\({}^{-}\) & 88.75 & 23 \\ MgAl-LDH-NO\({}_{3}\)\({}^{-}\) & 111.29 & 24 \\ CaAl-LDH-NO\({}_{3}\)\({}^{-}\) & 115.55 & 24 \\ ZnAl-LDH-NO\({}_{3}\)\({}^{-}\) & 148.28 & 24 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The comparison with the literature in terms of \(Q_{\rm m}\). Figure 11: (**a**) The chloride concentration after chloride adsorption for 24 h and chloride reduction percentage of sample C-high and C-low in different NaCl solutions; (**b**) The amount of chloride adsorption at the equilibrium (mg g\({}^{-1}\)) in different NaCl solutions. Figure 12: Langmuir isothermal curve for chloride adsorption reaction of C-high. adsorption amount increased as shown in Figure 11 and Table 2. The XRD result of C-low after immersion in different NaCl solutions is shown in Figure 13b. It can be seen that the (003) plane occurred at around 11.20\({}^{\circ}\), mainly corresponding to the intercalated carbonates in the coprecipitation process. This verifies the explanation for the weak chloride adsorption property of C-low stated above. It should be noted that the adsorption of chlorides by LDH is mainly dependent on the anion exchange reaction between the intercalated anion in the LDH gallery and the aggressive chlorides in the environment, thus the propensity of the anion exchange reaction has a dominant influence on the adsorption ability of chlorides. Based on our results, more carbonates were intercalated in the LDH gallery of C-low than that of C-high, and they are difficult to replace by chlorides due to their strong affinity with the interlayer; therefore, C-low presented a much lower adsorption ability in comparison with C-high. The FTIR characterization of C-high and C-low after immersion in various NaCl solutions for 24 h was also carried out. The results are shown in Figure 14. In Figure 14a, the two peaks corresponding to carbonates and nitrates appeared in the FTIR spectrum of C-high.