## 3 Results and Discussion
### Effect of Coexistence Anions and Adsorptive Selectivity of CAL  
In general, there are some anions such as OH\({}^{-}\), SO\({}_{4}\)\({}^{2-}\) and CO\({}_{3}\)\({}^{2-}\) existing simultaneously in the pore solution of cement-based materials, which might hinder the removal of chloride ions by the synthesized CAL adsorbents. Therefore, the influence of foreign anions on the chloride uptake of CAL sample was investigated in which the initial concentration of Cl\({}^{-}\) was 80 mmol/L, the concentration of coexistence anions was kept at 40 mmol/L and 80 mmol/L and the other parameters remained unchanged. As shown in Figure 8, the removal efficiency of Cl\({}^{-}\) by CAL-60 decreased with increasing the concentration of coexistence anions. For hydroxyl ions, CAL-60 performed a slight enhancement in the adsorption of chloride ion at 40 mmol/L of OH\({}^{-}\), which may be due to the inhibition of CAL dissolution by high alkalinity as described above. While the concentration of OH\({}^{-}\) was increased to 80 mmol/L, the chloride uptake reduced, which was related to the excessive OH\({}^{-}\) and Cl\({}^{-}\) competing for the interlayer adsorption sites of CAL. For the oxyanion such as CO\({}_{3}\)\({}^{2-}\) and SO\({}_{4}\)\({}^{2-}\), their presence can significantly reduce the chloride ion removal efficiency of CAL-60. When their concentration was 80 mmol/L, Cl\({}^{-}\) uptake of CAL-60 showed a significant reduction from 95.5 mg\(\cdot\)g\({}^{-1}\) to 0.89 mg\(\cdot\)g\({}^{-1}\) and 1.59 mg\(\cdot\)g\({}^{-1}\), respectively. Meanwhile, when the three anions appeared in the adsorption system simultaneously, the high concentration of co-existing anions also significantly reduced the removal rate of CAL-60. To evaluate the interaction mechanism of CAL-60 and Cl\({}^{-}\) in the solution containing different interference anions, XRD was used to characterize the composition of samples collected before and after the chloride adsorption of CAL-60. As shown in Figure 9, it was observed that three coexistence anions had different effects on the phase change of CAL-60. In the presence of hydroxyl ions (40 mmol/L or 80 mmol/L), CAL-60 performed a phase transformation from CaAl-NO\({}_{3}\)\({}^{-}\) LDHs to CaAl-Cl\({}^{-}\) LDHs (hydrocalumite phase) and a reduction of \(\rm{d}_{002}\) value from 0.873 to 0.786 nm (see Figure 9a), indicating that the adsorption of Cl\({}^{-}\) on CAL occurred through anion exchange reaction between NO\({}_{3}\)\({}^{-}\) and Cl\({}^{-}\). In contrast, the characteristic diffraction peaks of CAL utterly disappeared after 24 h of mixing with a solution containing Cl\({}^{-}\) and SO\({}_{4}\)\({}^{2-}\). Meanwhile, there were only some calcium sulfate and sodium chloride residues with poor crystallinity in the samples collected after mixing (see Figure 9b). This could suggest that the layered structure of CAL-60 was completely destroyed due to its dissolution. Consequently, Cl\({}^{-}\) cannot be adsorbed by CAL through anion exchange reactions, while calcium sulfate is formed by the precipitation between Ca\({}^{2+}\) released by CAL and SO\({}_{4}\)\({}^{2-}\). Due to the above reasons, the  
Figure 7: XRD patterns of CAL-60 and CAL-60 samples after adsorption of Cl\({}^{-}\) under different pH conditions (N refers for CaAl-NO\({}_{3}\)\({}^{-}\) type LDH; H refers for Hydrocalumite). CAL sample completely lost its chlorine adsorption ability at higher concentrations of SO4\({}^{2-}\), which was consistent with those results shown in Figure 8.  
Moreover, the carbonate ions with higher concentration also deposited with free calcium ions to form calcium carbonate precipitate, and finally completely destroyed the layered structure of CAL (see Figure 9c,d), which was similar to SO4\({}^{2-}\). However, when the concentration of CO3\({}^{2-}\) was 40 mmol/L, CAL-60 sample retained the layered structure to adsorb Cl\({}^{-}\) by anion exchange (d\({}_{002}\) from 0.873 to 0.783 nm) due to its incomplete dissolution.