# Removal of Nitrate Ions Using Thermally and Chemically Modified Bioadsorbents
## 3 Results and Discussion
### Adsorption Tests
The same trend was observed when using biochar prepared from quaternized sugar cane bagasse, achieving an adsorption capacity of 28.3 mg/g at pH 2 [55]. It is clear that as the pH increases, the adsorption of nitrate decreases. This is due to increased competition between nitrate and hydroxide ions for the same sites on the adsorbent surface. Additionally, at acidic pH, the surface of the bioadsorbent is positively charged thanks to protonation, which increases the electrostatic attraction between the sorbate surface and the negatively charged nitrate ions. Table 2 shows that when evaluating CYP and CC, the most influential parameter is pH with an absolute error of 1.85; it was found that the decrease in this factor favors the adsorption capacity of nitrate on the evaluated carbons due to the protonation of the active centers located on the surface of the adsorbent, which allows the anion to be electrostatically attracted to the active sites and captured there (Figure 3). From the application of the RMS, the second-order equation describing the adsorption process was obtained, revealing that the regression is statistically significant, and based on the ANOVA results, the regression model in Equation (11) reports a high R\({}^{2}\) value of 94.7%, which implies a high correlation between the observed values and predicted values [50]. \[q_{t}=38.7-1.43A-2.41B+0.671 AB+0.0041B^{2} \tag{11}\]  
where \(q_{t}\) (m/g) is the adsorption capacity of the adsorbent, \(C\) (mg/L) is the initial concentration of the contaminant, \(T\) (\({}^{\circ}\)C) is the temperature and \(D\) (g) and the adsorbent dose. The positive and negative signs before the terms indicate the synergistic and antagonistic effect of the respective variables [51; 52]. a monolayer on the surface of the adsorbent with a homogeneous affinity for the active centers [57]. \begin{table}
\begin{tabular}{c c c} \hline \hline
**adsorbent** & \(q_{\text{max}}\) **(mg/g)** & **Reference** \\ \hline Functionalized cellulose grafted epichlorohydrin & 232.56 & [59] \\ \hline Anionic biographene nanosheet & 182.506 & [60] \\ \hline Cheat straw crosslinked with epichlorohydrin and dimethylamine & 128.97 & [61] \\ \hline Crosslinked chitosan beads & 104.0 & [62] \\ \hline Magnetic amine-crosslinked biopolymer based on corn stalk & 102.04 & [63] \\ \hline Rice hull & 74.41 & \\ \hline Amberlite IRA-900 & 74.41 & \\ \hline Pine bark & 65.73 & [64] \\ \hline Moringa husk & 63.25 & \\ \hline Sugarcane bagasse & 63.25 & \\ \hline Coconut shell & 55.18 & [64] \\ \hline Wheat stalk resin & 50.24 & [65] \\ \hline Persimmon tealeaf & 48.36 & [64] \\ \hline Lauan sawdust & 47.74 & \\ \hline _Arundo donax_ L. reed-based anion exchange resin & 44.61 & [66] \\ \hline Cotton stalk resin & 39.15 & [65] \\ \hline Chitosan microspheres & 32.15 & [4] \\ \hline Polyurethane/sepiolite cellular nanocomposites & 23.30 & [3] \\ \hline Carbon from yam peel modified with NH4 Cl & 10.834 & Present study \\ \hline Activated carbon modified with cetyl trimethyl ammonium chloride & 10.80 & [67] \\ \hline Granular Fe–Zr–chitosan complex & 10.60 & [68] \\ \hline Zeolite modified with cetylpyridinium bromide & 9.68 & [69] \\ \hline Commercial carbon & 8.612 & Present study \\ \hline Organosilica & 7.75 & [70] \\ \hline Yam peel modified with NH4 Cl & 7.675 & Present study \\ \hline Chitosan doped with Fe3+ & 5.00 & [58] \\ \hline \hline \end{tabular}
\end{table} 
Table 4: \(q_{\text{max}}\) values using different adsorbents.