## 3 Results and Discussion
### Chemical Composition  
The first approximation of the chemical composition of the different powders was obtained from energy dispersive spectroscopy performed during the scanning microscopy. As can be seen in Figure 4, corresponding to _Scorpaena scrofa_ treated at 950 \({}^{\circ}\)C, the main components of the powders were phosphorus, calcium, magnesium, and sodium. Figure 3: SEM micrographs of **(a)**_Scorpaena scrofa_, **(b)**_Trachurus trachurus_, and **(c)**_Salmo salar_ bones calcined at 750 \({}^{\circ}\)C, as well as **(d)**_Scorpaena scrofa_, **(e)**_Trachurus trachurus_, and **(f)**_Salmo salar_ bones calcined at 950 \({}^{\circ}\)C.  
Figure 2: Crushed fish bones before and after thermal treatment. EDS gives just a qualitative evaluation of the chemical composition, therefore inductively coupled plasma optical emission spectroscopy (ICP-OES) was carried out in order to perform a quantitative evaluation of the chemical compositions of the powders obtained. This analysis allowed us to determine the amount of heavy metals present in the fish samples, as well as the Ca/P molar ratio. The elementary composition of the given species for the different temperatures and fish powder samples is listed in Table 3. As already pointed out by the EDS analyses, calcium and phosphorus were the two main components in all samples, showing a similar amount of both elements with an average Ca/P molar ratio of around 1.81 \(\pm\) 0.2. This value is slightly higher than that of stoichiometric hydroxyapatite (1.67), which is due to the fact that what we have here is a type B hydroxyapatite as a result of the substitution of (PO4)\({}^{-3}\) groups by (CO3)\({}^{-2}\). This type of hydroxyapatite has a higher degree of biocompatibility than that of type A, where the hydroxyl groups (OH)\({}^{-}\) are substituted by (CO3)\({}^{-2}\) groups. Hence, type B hydroxyapatite obtained from natural sources will have a better acceptance from a human body perspective [24; 25; 26]. In general terms, the results of the EDS analysis agree with those from the ICP-OES. Moreover, EDS and ICP-OES analyses show the important presence of ions, such as Na, Mg, Sr, and K. These elements usually replace Ca ions in biological HA obtained from natural sources while promoting higher biological activity and thus enhance a faster growth of bone structures and cell adhesion [27]. HA presents in a hexagonal structure and features two calcium sites (CaI and CaII). Due to significant differences in ionic radius, some ions may be incorporated into the hydroxyapatite lattice structure. Cations smaller than Ca (radius = 0.99 A), e.g., Na or Mg, tend to occupy the CaI site, while larger cations (Sr and K) usually replace calcium at the CaII site [28]. Sodium and magnesium play a significant role in bone metabolism and osteoporosis, while strontium reduces bone resorption and  
Figure 4: Energy dispersive spectroscopy spectrum obtained at several points of the sample _Scorpuean scrofa_ at 950 \({}^{\circ}\)C. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Samples** & **Calcination Temperature (\({}^{\circ}\)C)** & **Ca** & **P** & **Mg** & **K** & **Sr** & **Ca/P (Molar)** \\ \hline _Scorp_ & 750 & 35.88 & 20.23 & 1.04 & 0.22 & 0.19 & 1.77 \(\pm\) 0.02 \\ \hline _Trach_ & 750 & 37.81 & 20.37 & 1.22 & 0.24 & 0.23 & 1.86 \(\pm\) 0.02 \\ \hline _Salmo_ & 750 & 41.14 & 22.74 & 0.97 & 0.28 & 0.18 & 1.81 \(\pm\) 0.02 \\ \hline _Scorp_ & 950 & 23.65 & 13.53 & 1.20 & 0.17 & 0.24 & 1.75 \(\pm\) 0.02 \\ \hline _Trach_ & 950 & 39.26 & 21.61 & 1.25 & 0.20 & 0.24 & 1.82 \(\pm\) 0.02 \\ \hline _Salmo_ & 950 & 31.20 & 18.43 & 1.02 & 0.23 & 0.19 & 1.69 \(\pm\) 0.02 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Elemental composition of the samples at 750 and 950 \({}^{\circ}\)C and their corresponding Ca/P ratio. increases the formation of new bone structures, which may lead to increased prevention of fractures [29; 30; 31].