# Removal of Nitrate Ions Using Thermally and Chemically Modified Bioadsorbents
## 3 Results and Discussion  
### Bioadsorbent Characterization  
According to the SEM micrograph, YP exhibits an irregular surface and mesopores (Figure 1a) [39]. It was evidenced that carbon, calcium and oxygen are present in both materials, with YP being the one with the most diverse structure, with traces of multiple elements such as potassium, silicon, aluminum, phosphorus, iron and copper [40]. These elements are related to the functional groups present in lignocellulosic materials [41] and their ability to capture contaminants due to electrostatic forces. They also presented agglomerates of particles from pectin, cellulose, lignin and wax [42]. After the NO3 adsorption, a porous surface in the form of a mesh and the appearance of folds in the biomaterial are observed (Figure 1b) [43]. There is evidence of the appearance of nitrogen in the high-intensity peak of 0.065 keV, which indicates that removal of the anion was successful [41]. The disappearance of Cu, P and S is also observed, and the decreasing in weight % of O, Al, Ca, Fe, Si and K.  
Figure 1: SEM micrographs and EDS spectrum of MYP (**a**) before and (**b**) after NO3 adsorption.  
Biochar synthesized from yam skins and modified with NH\({}_{4}\)Cl exhibited a heterogeneous surface with folds and pores (Figure 2a). The pores are in the macro- and mesopore range and allow access to the microporous structure. The presence of agglutinated networks in the exposed area is shown, which is attributed to the degree of aromaticity; the order of the graphitized structure was affected by the volatilization of the oxygen and hydrogen present in the YP structure, thus forming new aromatic bonds [44]. The morphology of the bioadsorbent was agglutinated after removing the anion, with the disappearance of the folds and the nets; likewise, it is observed that the surface is smoother. As with the MYP (Figure 1b), the EDS after adsorption showed the presence of nitrogen in the spectrum (Figure 2b), suggesting that the removal of the anion was successful [41].  
Figure 3 shows the interactions between the bioadsorbents impregnated with NH\({}_{4}\)Cl and the NO\({}_{3}\)\({}^{-}\) anion. The charge of the bioadsorbent surface groups enhances the electrostatic interactions with the contaminant. It allows the capture of the nitrogen nutrient, which is evidenced in Figures 1b and 2b, with a strong N peak in the EDS. Previously, [PERSON] et al. [42] found that the adsorption of nitrate on coconut shell functionalized with 3-aminopropyltriethoxysilane occurs through different steps, namely: coating of the adsorbent, diffusion of the contaminant to the outer surface of the adsorbent, diffusion at the effective internal adsorption site and capture of the contaminant on the surface of the modified adsorbent. Considering that the adsorption process involves physisorption and chemisorption on the bioadsorbents under study, it can be postulated that adsorption takes place by protonation of the active centers due to the introduction of NH\({}_{3}\)\({}^{+}\) groups  
Figure 2: SEM micrographs and EDS spectrum of biochar (**a**) before and (**b**) after NO\({}_{3}\) adsorption.  
during the impregnation of the yam peels and the surface becomes positively charged, thus favoring the adsorption of nitrate [45].