## 3 Results and Discussion
### Effect of Initial Cl\({}^{-}\) Concentration  
To examine the effect of initial Cl\({}^{-}\) concentration on the chloride ion removal by CAL, an adsorption isothermal experiment was conducted at room temperature (25 \({}^{\circ}\)C). Figure 5. The effect of initial Cl\({}^{-}\) concentration on Cl\({}^{-}\) adsorption on CAL and nonlinear fits achieved using Freundlich isotherm and Langmuir isotherm ((a) CAL-60; (b) CAL-70; (c) CAL-80; (d) CAL-100; (e) CAL-120) exhibits the adsorption isotherms of CAL. Langmuir and Freundlich equations were used for nonlinear fitting of data as expressed in the following:  
\[\mathrm{Q_{e}=\frac{Q_{m}k_{L}C_{e}}{1+k_{L}C_{e}}} \tag{4}\]  
\[\mathrm{Q_{e}=k_{F}C_{e}^{1/n}} \tag{5}\]  
where \(\mathrm{C_{e}}\) (mmol\(\cdot\)L\({}^{-1}\)) and \(\mathrm{Q_{e}}\) (mg\(\cdot\)g\({}^{-1}\)) are the equilibrium concentration of remain chloride and adsorption capacity of chloride ion by CAL at equilibrium, respectively. \(\mathrm{Q_{m}}\) (mg\(\cdot\)g\({}^{-1}\)) represents the maximum adsorption amount of Cl\({}^{-}\) on CAL calculated by Langmuir isotherm, \(k_{L}\) (L\(\cdot\)mg\({}^{-1}\)) is Langmuir constant, which affects the adsorption ability and binding energy. \(k_{F}\) ((mg/g) (L/mg)\({}^{1/n}\)) and \(1/n\) are Freundlich constants, which affect the adsorption capacity and strength of adsorbent, respectively.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \multirow{2}{*}{**Samples**} & \multirow{2}{*}{
\begin{tabular}{c} **Qe,ex** \\ **(mg\(\cdot\)g\({}^{-1}\))** \\ \end{tabular} } & \multicolumn{2}{c}{**Pseudo-First-Order**} & \multicolumn{3}{c}{**Pseudo-Second-Order**} \\ \cline{3-8}  & & **Qe,ca** & **k1** & **R2** & **Qe,ca** & **k1** & **R2** \\  & & **(mg\(\cdot\)g\({}^{-1}\))** & **(1\(\cdot\)min\({}^{-1}\))** & **(mg\(\cdot\)g\({}^{-1}\))** & **(g\(\cdot\)mg\({}^{-1}\))** & **R2** \\ \hline
**CAL-60** & 93.548 & 88.417 & 0.064 & 0.906 & 91.844 & 0.00119 & 0.948 \\ \hline \end{tabular}
\end{table}
Table 1: Experimental data of Cl\({}^{-}\) uptake on CAL-60 and adsorption kinetics parameters obtained from two adsorption kinetics models.  
Figure 5: _Cont._The fitting results using the above two isotherm models are exhibited in Figure 5. The effect of initial Cl\({}^{-}\) concentration on Cl\({}^{-}\) adsorption on CAL and nonlinear fits achieved using Freundlich isotherm and Langmuir isotherm ((a) CAL-60; (b) CAL-70; (c) CAL-80; (d) CAL-100; (e) CAL-120) and Table 2. Langmuir isotherm fitting showed a higher correlation coefficient (R\({}^{2}\)) as compared with Freundlich isotherm (Table 2), suggesting that the Langmuir isotherm was more consistent with the experimental results than the Freundlich isotherm. The 1/n value of Freundlich isotherm ranged from 0 to 1 and suggested a favorable adsorption [33; 34]. According to the results shown in Table 2, the 1/n values for all CAL samples (0.526-0.574) indicated that the adsorption of Cl\({}^{-}\) on synthesized CAL was favorable. The theoretical maximum absorbance of chloride ion by CAL samples calculated from Langmuir equation was 211.324 mg/g, 182.117 mg/g, 174.903 mg/g, 160.914 mg/g and 155.960 mg/g, respectively, suggesting that enhancing aging temperature can improve the maximum adsorption capacity. Similar to other LDH-like materials, the particle size of CAL exhibited a significant effect on the adsorption performance. On the one hand, the particle size reduction can increase the specific area of CAL, which will offer more adsorption sites for Cl\({}^{-}\) resulting in a higher adsorption capacity [35]. Additionally, more coordination sites with less coordination number for anion exchange will be formed (like the edge area of LDHs) [29; 36] due to the reduction of particle size, which can enhance the removal efficiency of CAL.