## 3 Results and Discussion  
### Physico-Chemical Properties of the Prepared b-HdA  
The nitrogen adsorption-desorption isotherms of the p-HdA and b-HdA are presented in Figure 1a. Both the p-HdA and b-HdA exhibited type IV isotherms with type H\({}_{3}\) hysteresis loops, suggesting that the materials had mesoporous structures with the aggregates of particles forming slit-like pores [34; 35]. The BJH pore size distribution curves illustrated in Figure 1b confirmed that mesopores were predominant for both materials. It can also be seen from Figure 1b that the p-HdA had a broader pore size distribution compared to the HdA pellets.  
Figure 1: Typical nitrogen adsorption–desorption isotherms (a), pore size distribution (b), X-ray diffraction (XRD) patterns of the powdered HdA (p-HdA) and the porous bead form of HdA (b-HdA) (c), and pH\({}_{\text{pac}}\) of b-HdA (d).  
The mean pore diameters of p-HdA and b-HdA were 16.22 nm and 22.73 nm, respectively, as presented in Table 1. The BET specific surface area of the b-HdA was half that of the p-HdA, which could be foreseen due to the effect of particle size on the BET specific surface area [36].  
The XRD patterns of the p-HdA and b-HdA presented in Figure 1c matched well with the standard patterns of the hexagonal crystal hydroxyapatite compiled by the International Centre for Diffraction Data (ICDD) card no. 01-074-0565 and 01-082-2956, respectively. The sharp, narrow, and well-resolved diffraction peaks observed in Figure 1c corresponded to the Miler indices of 100, 002, 210, 211, 112, 300, 202, 130, 222, 132, 213, 321, and 004, respectively, confirming the crystallized structure of the materials [37; 38]. Furthermore, no additional peaks were found in the diffraction pattern of the b-HdA, indicating that hydroxyapatite was the only phase and PVB completely decomposed into volatile products at 550 \({}^{\circ}\)C [39]. Furthermore, a slightly higher intensity of the diffraction peaks was obtained for b-HdA compared to that of p-HdA, which might have resulted from the fact that the sintering process improved the crystallinity of the material [40].  
Figure 1d shows that the pHpzc of the b-HdA was 7.9, which was similar to the precursor-HdA powder and other previous studies [38; 41; 42]. However, it is noteworthy to mention that the pHpzc of the HdA can fall in a wide range (4.3-9.2) depending on the synthesized method, HdA precursor, stoichiometry and purity, experimental conditions, and techniques [21; 43]. It has also been reported that the pHpzc value of the HdA correlates proportionally with the quantity of Ca in the HdA surface [44].  
Figure 2a,b depicts the photographs of the p-HdA and b-HdA, respectively. The prepared b-HdA was typically spherical with an approximate diameter of 2.0 mm. The surface morphology of the p-HdA and b-HdA were recorded using SEM analysis, as shown in Figure 2c,d. The b-HdA was a rough and multiporous surface. Only the constitutive elements of HdA (O, Ca, and P) were detected according to the EDX results (data not shown), which were consistent with the XRD results obtained above.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Material** & **BET Surface Area (m\({}^{2}\)/g)** & **Pore Volume (cm\({}^{3}\)/g)** & **Mean Pore Size (nm)** & **Adsorption Capacity of Co(II) (mg/g)** \\ \hline p-HdA & 67.14 & 0.27 & 16.22 & 14.07 \\ b-HdA & 34.91 & 0.18 & 22.73 & 7.73 \\ \hline \hline \end{tabular}  
* Pore volume was calculated at P/\(P_{0}\) = 0.990; b Obtained with a 10 mg/L initial concentration of Co(II) and the temperature at 293 K. BET: Brunauer–Emmett–Teller.  
\end{table}
Table 1: Elemental compositions of the p-HdA and b-HdA.  
Figure 2: Microscopic and SEM images of p-HdA (**a**,**c**) and b-HdA (**b**,**d**).