## 3 Results and Discussion
### Chloride Adsorption Behavior of LDH  
Chloride is the most common aggressive anion in a corrosive medium, and the concentration of chloride in the solution has a significant influence on the corrosion of metals; therefore, in this manuscript, the chloride adsorption property of LDH was investigated. In order to characterize the chloride adsorption behavior of LDH and obtain the amount of chloride adsorption at equilibrium (mg/g), 1 g LDH was added to 100 mL of a different NaCl solution and subjected to rigorous magnetic stirring for 24 h. The pH of the NaCl solution before the addition of LDH was measured to be 7.2, while the pH of NaCl solutions with a concentration of 0.02 mol/L rose to 8.8 and 8.9 after 24 stirring with C-high and C-low, respectively. This slight increase in pH can be attributed to the release of OH\({}^{-}\) anions from the LDH gallery by the anion exchange reaction between OH\({}^{-}\) and chlorides. The remaining chloride concentration was measured, and the chloride reduction percentage was calculated, which is shown in Figure 11a. As can be seen from this figure, the reduction percentage increased from 40% to 55% gradually when the NaCl concentration was increased from 0.002 to 0.04 mol/L, and it decreased to 34% when the NaCl concentration was further increased to 0.08 mol/L. For the sample C-low, the reduction percentage was 25% when the NaCl solution was 0.002 mol/L, and it decreased gradually to about 1% when the NaCl concentration increased to 0.01 mol/L. With the further increase in NaCl concentration, the reduction percentage was around 0%, which was probably caused by the relatively lower sensitivity of the Ag/AgCl probe at a higher chloride concentration. This result indicates the poor chloride adsorption ability of C-low. This result may be probably caused by the existence of a large number of carbonates in its gallery and the low adsorption capacity of LDH itself. According to the XRD results in Figure 6, the amount of CO3\({}^{2-}\) in the gallery of C-low was remarkably higher than that of C-high. As carbonates have a strong affinity with the interlayer, it is hard for the chlorides in the solution to replace carbonates in the LDH gallery by an anion exchange reaction. As a result, the chloride adsorption ability of C-low was found to be much lower than that of C-high. The chloride uptake amount of LDH was calculated according to Equation (1) [22]:  
\[Q_{\text{e}}=\frac{1000(C_{0}-C_{\text{e}})VM}{m} \tag{1}\]  
where \(Q_{\text{e}}\) is the amount of chloride adsorption at the equilibrium (mg g\({}^{-1}\)), and \(C_{0}\) and \(C_{\text{e}}\) are the initial and equilibrium chloride concentration (mol/L), respectively. \(V\) is the solution volume (L); \(M\) is the molar mass of Cl element with a value of 35.5 mol/g, and m is the mass of LDH (g). The \(Q_{\text{e}}\) of LDH in different NaCl solutions is shown in Figure 11b. It can be seen that \(Q_{\text{e}}\) of C-high increased remarkably from 2.84 to 97.63 mg g\({}^{-1}\) when the NaCl concentration was increased from 0.002 mol/L to 0.08 mol/L. In the case of C-low, \(Q_{\text{e}}\) remained at 1.78 mg g\({}^{-1}\) when the NaCl solution increased from 0.002 to 0.005 mol/L, demonstrating that the chloride adsorption ability was maximized at a low concentration at 0.002 mol/L. As explained above, the \(Q_{\text{e}}\) stayed around 0 mg g\({}^{-1}\) when the NaCl concentration further increased due to the lower sensitivity of the Ag/AgCl probe at a higher NaCl concentration. Based on the above analysis, it can be concluded that the sample C-high possessed superior chloride adsorption ability in comparison with the sample of C-low. According to the literature, the Langmuir isotherm can provide better fitting for the chloride adsorption by LDH; therefore, the Langmuir isotherm (Equation (2)) was used for fitting in this work [22].