# Removal of Nitrate Ions Using Thermally and Chemically Modified Bioadsorbents
## 3 Results and Discussion
### Adsorption Tests  
From the adsorption tests using MYP, it was found that the increase in NO\({}_{3}\) adsorption capacity is proportional to the increase in the initial pH of the solution (Figure 4); from pH 4 on, the effect of this variable is not very significant in the process, suggesting that the number of positive surface charges remains the same. From pH 8, the concentration of OH ions increases and, therefore, competition from OH for the adsorption of nitrates may arise, then nitrates are adsorbed less [46]. However, the higher removal rate of NO\({}_{3}\)\({}^{-}\) at pH 12 could be due to its conversion to the gaseous species NH\({}_{3}\) and NH\({}_{4}\)\({}^{+}\), which can volatilize during the removal process or be retained in the active centers of the adsorbent [47]. H\({}^{+}\) ions would strongly compete against the cationic species at lower pHs for active adsorption sites on the biochar surface, thus inhibiting adsorption capacity and removal efficiency. In addition, the protonation of negatively charged functional groups on the biochar surface hinders the interaction between the species and the biochar surface [48]. The highest adsorption capacity was obtained at 0.5 mm, which is due to the fact that a small particle has a larger surface area thanks to the number of pores per unit of mass, which favors the number of active binding sites and the available contact surface, resulting in a higher mass transfer and fast adsorption [49]. The highest adsorption capacity of the anion under study was obtained at pH 12 and 0.5 mm of particle size. Figure 4: Effect of pH and particle size on the adsorption capacity of NO\({}_{3}\) on MYP. Figure 3: Scheme of interactions between NO\({}_{3}\) and biochar adsorbent. Table 1 shows the results obtained from the analysis of variance with a confidence level of 95%, finding that pH is the significantly influential effect on nitrate adsorption capacity on MYB, with an absolute error of 2.86. Likewise, 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 of Equation (10) reports a high R\({}^{2}\) value of 96.23%, which implies a high correlation between the observed values and predicted values [50]. \[q_{t}=20.5+0.679A-12B+0.023A^{2}-0.326 AB+7.02B^{2} \tag{10}\]  
where \(q_{t}\) (m/g) is the adsorption capacity of the adsorbent, \(A\) is the pH of the solution and \(B\) (mm) is the adsorbent particle size. The positive and negative signs before the terms indicate the synergistic and antagonistic effect of the respective variables [51; 52]. When using carbon from yam skins (CYP) and commercial carbon, it is observed that the increase in pH has an inversely proportional effect on the increase in adsorption capacity. The adsorption capacity is observed to reach its maximum at pH 2 and decreases with increasing pH (Figure 5). This could be due to the fact that, when the pH of the solution increases, the surface of the adsorbents is negatively charged and the adsorption capacity of nitrate decreases, because the negatively charged surface sites on the adsorbent did not favor nitrate due to electrostatic repulsion [53; 54]. The behavior shown by CYP and CC was similar to that reported when clay was used in the removal of nitrate, obtaining the highest adsorption capacity of 3.25 mg/L at \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Source** & **Sum of Squares** & **F-Ratio** & \(p\)**-Value** \\ \hline A: pH & 128.0 & 7.36 & 0.0188 \\ \hline B: Particle size & 28.6 & 1.64 & 0.2243 \\ \hline AA & 0.959 & 0.05 & 0.8186 \\ \hline AB & 1.54 & 0.09 & 0.7712 \\ \hline BB & 2.13 & 0.12 & 0.7328 \\ \hline Total error & 209.0 & & \\ \hline Total (corr.) & 371.0 & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Analysis of variance for NO\({}_{3}\) on MYB. Figure 5: pH effect on NO\({}_{3}\) adsorption capacity on CYP and CC. pH 2 [54].