## 3 Results and Discussion
### Chemical Properties of the Surface Samples
The differences between the samples were the intensity of the signals. Initially, at 3584 cm\({}^{-1}\), a small and sharp peak was observed and corresponded to an O-H tension due to the hydroxyl that was present in the material since phenols were formed in the activation process [24]. The spectral bands around 2924-2853 cm\({}^{-1}\) belonged to the C-H stretching. Moreover, certain bands were appreciated between around 2400 and 2350 cm\({}^{-1}\), according to the C=O bond tension [25] and the overtones of the carboxylic groups, due to the oxidation process. Small signals at 1712 cm\({}^{-1}\) attributed to a band belonging to carbonyl stretching. In addition, all the materials had a very sharp band at about 1219 cm\({}^{-1}\) that was interpreted as a C-O strain, which increased considerably in the modified materials. Finally, at 775 cm\({}^{-1}\), they corresponded to an aromatic substitution of the ortho type by the formation of the compounds with C present in the aromatic group and another element or a bond with an active site. Small signals of the phosphate group were observed at 1116 and 1085 cm\({}^{-1}\), corresponding to HAp. An increase in the intensity of the AC-1h21 for the phosphate group signal can be attributed to the initial formation of HAp on the surface. Moreover, a morphology analysis was carried out to corroborate Hap's existence. The morphology of the commercial-activated carbon (Figure 3) was a typical porous and irregular microstructure [24; 26], with a particle size of 12-40 mm, presenting meso and macropores with an average pore diameter (p\({}_{\rm d}\)) of 34.00 nm and 1.38 \(\upmu\)m, respectively. Figure 3: SEM images of the AC; (**a**) 300\(\times\) and (**b**) 3000\(\times\) magnification. Figure 2: IR spectra of the carbon samples with different modifications. Figure 4 shows the micrographs of the oxidized materials. The 1 h (AC-1h) and the 2 h (AC-2h) oxidized carbon presented a change in the morphology, with a more irregular surface and an increase in the pore diameter; likewise, a skeletal morphology [27] in the material with a 2 h oxidation is observed. The change in morphology is directly related to the chemical oxidation with HNO\({}_{3}\)[28], as the acid in contact with the material modified the textural properties, increasing the surface porosity due to the carbon bonds' degradation. For the samples incubated for 21 days, shown in Figure 5, typical HAp morphology was not observed; however, a dispersion of the ions and an initial growth of HAp were observed in a slight shining on the SEM images of the composite surface. The reason is that the Ca\({}^{2+}\) ions' concentration was insufficient for the spontaneous precipitation process to take place [22; 23]. Figure 4: SEM images of the oxidized carbon samples; (**a**) AC-1h and (**b**) AC-2h. Figure 5: SEM images of the carbon samples incubated for 21 days; (**a**) AC-21, (**b**) AC-1h21, and (**c**) AC-2h21. Furthermore, the samples incubated with CaSiO\({}_{3}\), as shown in Figure 6, showed a growth on the surface of the agglomerated deposits of the particles with a sphere-like morphology, attributed to the formation of the hydroxyapatite, with an average particle size of \(\sim\)779.00 \(\upmu\)m. The particle size varies due to the agglomeration, as particles of smaller than average size were visible (Figure 6c). The addition of CaSiO\({}_{3}\) during the incubation was a determinant factor for the formation of HAp on the substrate surface [29; 30; 31]. Calcium silicate powders transferred the ions to the SBF solution and adding Ca\({}^{2+}\) oversaturated the solution and successfully achieved the precipitation of hydroxyapatite on the surface [22]. In addition, there was a relationship between the oxidation degree and the growth of the hydroxyapatite particles on the carbon surface, explained by the fact that a higher concentration of the oxygenated functional groups on the carbon surface, derived from the modification with HNO\({}_{3}\), interacted with the calcium ions, and the precipitation of HAp was achieved.