## 3 Results and Discussion
### Chemical Properties of the Surface Samples  
For the commercial-activated carbon, 2.00% (\(\pm\)0.17) of the ash content was observed. For the oxidized materials by chemical modification at varying times, the AC-1h showed 1.40% (\(\pm\)0.16), and for the AC-2h, it was 1.00% (\(\pm\)0.14) (Figure 1). The decrease in the ash percentage was due to the treatment with HNO\({}_{3}\), which eliminated part of the ash in the unoxidized carbon, as mentioned by [PERSON] et al. in 2022 [17]. Moreover, the samples with 21 days of incubation in the absence of CaSiO\({}_{3}\) showed a slight increase in the ash percentage attributed to precipitates on the surface from the ions present in the SBF solution, with an ash content of 3.00 (\(\pm\)0.23) for AC-21, 3.21 (\(\pm\)0.25) for AC-1h21, and 3.41% (\(\pm\)0.32) for AC-2h21. Finally, the samples with CaSiO\({}_{3}\) (AC-HAp, AC-1 hHAp, and AC-2 hHAp) presented a very significant increase in the ash percentage value at 4.72% (\(\pm\)0.45), 7.15 (\(\pm\)0.45), and 11.53% (\(\pm\)0.42), respectively. The increased values depended on the formation of hydroxyapatite deposits on the surface of the materials. The inorganic materials remained in the crucible at the end of the analysis since the boiling point was higher than the temperature used for the ash determination [18; 19; 20]. To identify the presence of HAp in the samples, X-ray fluorescence was used. In Table 1, the elemental composition of the samples is presented, and the differences between the Ca and P percentages are analyzed. \begin{table}
\begin{tabular}{c c c c c c} \hline \multirow{2}{*}{**Sample**} & \multicolumn{5}{c}{**Composition**} \\ \cline{2-6}  & **Si** & **P** & **K** & **Ca** & **Fe** \\ \hline AC & 0.58\% & 0.13\% & 0.75\% & 1.09\% & 0.81\% \\ \hline AC-21 & 0.46\% & 0.35\% & 0.47\% & 2.78\% & 1.05\% \\ \hline AC-HAp & 0.60\% & 0.46\% & 0.35\% & 4.77\% & 0.55\% \\ \hline \end{tabular}
\end{table} 
Table 1: Compositions of carbon-based materials. The commercial AC presented 1.09% Ca and 0.13% P due to the pyrolysis of the carbon precursor and activating agent (H\({}_{3}\)PO\({}_{4}\)). The materials incubated for 21 days, the AC-21, AC-1h21, and AC-2h21, showed an increase in the Ca and P percentages, with 2.78, 3.98, and 4.01% of the calcium and 0.35, 0.34, and 0.37% of the phosphorus, respectively. A higher increase in the percentage of Ca and P incubated with CaSiO\({}_{3}\) was observed, with 4.77, 5.37, and 6.25% for the Ca and 0.46, 0.35, and 0.40% for the P, corresponding to the AC-HAp, AC-1 hHAp, and AC-2 hHAp. The increased Ca and \(p\) values in these samples were due to the ion deposition in the incubation with the SBF solution [21; 22; 23], and the results demonstrated that, depending on the presence of CaSiO\({}_{3}\), an increase of Ca and P was observed. For the materials that were only oxidized, a reduction in the values of the Ca and P was observed due to the contact time with HNO\({}_{3}\) in the oxidation process, which removed the ash contained in the AC. FTIR analysis identified the functional groups present in the adsorbent materials. In Figure 2, the IR spectra showed that all the adsorbent materials studied presented adsorption bands very similar to each other. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Sample**} & \multicolumn{5}{c}{**Composition**} \\ \cline{2-6}  & **Si** & **P** & **K** & **Ca** & **Fe** \\ \hline AC-1h & 0.55\% & 0.12\% & 0.25\% & 0.98\% & 0.39\% \\ \hline AC-1h21 & 0.68\% & 0.34\% & 0.50\% & 3.98\% & 0.37\% \\ \hline AC-1 hHAp & 0.52\% & 0.35\% & 0.69\% & 5.37\% & 0.29\% \\ \hline AC-2h & 0.44\% & 0.13\% & 0.16\% & 0.89\% & 0.31\% \\ \hline AC-2h21 & 0.49\% & 0.37\% & 0.38\% & 4.01\% & 0.32\% \\ \hline AC-2 hHAp & 0.55\% & 0.40\% & 0.89\% & 6.25\% & 0.45\% \\ \hline \hline \end{tabular}
\end{table} 
Table 1: _Cont._  
Figure 1: Ash content in percent of the different carbons tested (AC: activated carbon; 21: 21 days incubated; Xh: oxidized time; HAp: incubated for 21 days with CaSiO\({}_{3}\)).