## 3 Results and Discussion
### Adsorption Isotherms  
The adsorption parameters that were attained by fitting the equilibrium adsorption data with the four selected isotherm models are summarized in Table 2. The regression coefficient (\(R^{2}\)) values of the four adsorption isotherms in Table 2 show that the Sips, Freundlich, and Langmuir models provided better fits than the Tempkin model. Among these models, the Sips model was adapted to explain the uptake behavior of Co(II) onto the b-HdA. Figure 3 presents the equilibrium adsorption data fitted using the Sips adsorption model at different experimental temperatures. The maximum adsorption capacity (\(q_{max}\)) of the b-HdA that was calculated from the Sips isotherm for Co(II) ions at 293 K was 7.73 mg/g, which was about half that for p-HdA (14.07 mg/g) due to b-HdA's smaller BET specific surface area. The Sips model was developed from the basis of the Freundlich and Langmuir models, which are described by the dimensionless heterogeneity factor \(n_{s}\). At values of \(n_{s}\) close to (or exactly) 1, the Sips isotherm approaches the Langmuir isotherm, which suggests homogeneous adsorption. From Table 2, it was found that the value of \(n_{s}\) was much lower than 1 for all investigated temperatures, indicating that the b-HdA had an energetically heterogeneous surface [9]. It is believed that this heterogeneity was caused by the distinct binding sites present on the HdA surface, i.e., positively charged calcium ions and negatively charged hydroxyl, together with phosphate groups [1; 20; 44]. Consequently, different possible adsorption mechanisms can take place during the adsorption process, i.e., ion exchange and dissolution-precipitation according to Equations (2)-(4) [6; 20]. \[\text{Ca}_{10}(\text{PO}_{4})_{6}(\text{OH})_{2}+2\text{Co}^{2+}\to\text{Ca}_{ 8}\text{Co}_{2}(\text{PO}_{4})_{6}(\text{OH})_{2}+2\text{Ca}^{2+}, \tag{2}\]  
\[\text{Ca}_{10}(\text{PO}_{4})_{6}(\text{OH})_{2}+14\text{H}^{+}\to 10\text{Ca}^{2+}+6 \text{H}_{2}\text{PO}_{4}^{-}+2\text{H}_{2}\text{O}, \tag{3}\]  
\[10\text{Co}^{2+}+6\text{H}_{2}\text{PO}_{4}^{-}+2\text{H}_{2}\text{O}\to \text{Co}_{10}(\text{PO}_{4})_{6}(\text{OH})_{2}+14\text{H}^{+}. \tag{4}\]  
It is noteworthy that the ion exchange with Ca(II) ions in the HdA lattice was rapid and more favorably took place over the dissolution-precipitation process when the adsorbates were divalent ions [12; 43]. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Isotherms**} & \multirow{2}{*}{**Parameters**} & \multicolumn{4}{c}{**Temperature (K)**} \\ \cline{3-5}  & & **293** & **303** & **313** \\ \hline Langmuir & \(q_{max}\) & (mg/g) & 4.91 & 6.10 & 6.41 \\  & \(K\) & (L/mg) & 0.40 & 0.44 & 1.12 \\  & \(R^{2}\) & & 0.99 & 0.99 & 0.98 \\ \hline Freundlich & \(K_{F}\) & (mg/g, (L/mg)\({}^{1/n}\)) & 1.35 & 1.76 & 2.84 \\  & \(1/n\) & & 0.51 & 0.50 & 0.41 \\  & \(R^{2}\) & & 0.99 & 0.99 & 0.99 \\ \hline Sips & \(q_{max}\) & (mg/g) & 7.73 & 10.38 & 11.35 \\  & \(K_{s}\) & (1/mg) & 0.20 & 0.19 & 0.42 \\  & \(n_{s}\) & & 0.74 & 0.68 & 0.61 \\  & \(R^{2}\) & & 0.99 & 0.99 & 0.99 \\ \hline Tempkin & \(b_{T}\) & (kJ/mol) & 3.69 & 3.11 & 2.89 \\  & \(A_{T}\) & (L/mg) & 16.20 & 18.88 & 48.53 \\  & \(R^{2}\) & & 0.88 & 0.88 & 0.92 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Summary of the adsorption model parameters for Co(II) onto the b-HdA at various temperatures. The comparison of adsorption capacities of Co(II) using various adsorbents is presented in Table 3. It can be seen that the adsorption capacity of Co(II) by the b-HdA fabricated in this study was lower than that of other HdA-based adsorbents due to its larger size. Nevertheless, the b-HdA exhibited higher adsorption capacity than other granular adsorbents, such as activated carbon [45] and clinoptiolite [46]. This finding suggested that the b-HdA can serve as a competent adsorbent for practical applications in the removal of Co(II) from contaminated water.