# Removal of Nitrate Ions Using Thermally and Chemically Modified Bioadsorbents
## 3 Results and Discussion
### Adsorption Kinetics  
Kinetics was evaluated to analyze the effect of time on the adsorption process and determine the adsorbent's service time. [PERSON]'s models, pseudo-first and pseudo-second order, were used to adjust the experimental data and identify the possible mechanisms involved in the adsorption [39]. The adsorption kinetics are presented in Figure 6, and the parameters of adjustment to the models are summarized in Table 5.  
It is observed that rapid adsorption occurs in the first minutes of the process, due to the availability of active sites, and that equilibrium is reached in the first 30 min in all cases. The fast adsorption in the initial minutes of the process is due to the influence of diffusive phenomena from the solution to the surface of the adsorbent and later to the interior of the pores [5]. In the kinetics of nitrate adsorption on MYP, a decrease in the amount of adsorbed solute is observed due to the biomass's surface saturation by NO3 ions [60]. In a prior study, a similar behavior to that found when using MYP was observed when using comercial Cloisite(r) 10A; the reduction in the number of adsorbent anions over time did not have a clear explanation [70] as well as when using an organolay prepared using montmorillonite and hexadecylpyridinium chloride [71] and onion peels [72]. This suggests that during the adsorption process, desorption episodes occur, especially when the time is prolonged. In addition, this would provide valuable information for the determination of the service life of the adsorbent. The adsorption of NO3 on the evaluated adsorbents was adjusted to the three tested models, with R\({}^{2}\) > 0.95 in all cases. This suggests that the speed-limiting step of adsorption is chemisorption due to physicochemical interactions between the two phases [73], and the rate of adsorption depends on a mechanism operating at active sites on the surface of the biomass [39]. Although the kinetic constants of the MYP were the highest, it was the adsorbent that presented less adsorption capacity, obtaining \(q_{e}\) of the order CYP > CC > MYP.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Model** & **Parameter** & **MYP** & **CYP** & **CC** \\ \hline \multirow{3}{*}{Pseudo-first order} & \(q_{e}\) (mg/g) & 21.389 & 26.669 & 24.621 \\ \cline{2-5}  & \(k_{1}\) (min\({}^{-1}\)) & 26.955 & 1.175 & 1.357 \\ \cline{2-5}  & R\({}^{2}\) & 0.984 & 0.999 & 0.999 \\ \hline \multirow{3}{*}{Pseudo-second order} & \(q_{e}\) (mg/g) & 21.389 & 24.701 & 24.636 \\ \cline{2-5}  & \(k_{2}\) (g/mg\(\cdot\)min) & 1.483 \(\times\) 10\({}^{16}\) & 1.329 & 2.957 \\ \cline{2-5}  & R\({}^{2}\) & 0.984 & 0.999 & 0.999 \\ \hline \multirow{3}{*}{Elovich} & \(\alpha\) (mg/g\(\cdot\)min) & 5.005 & 4.403 & 4.400 \\ \cline{2-5}  & \(\beta\) (g/mg) & 1.242 \(\times\) 10\({}^{44}\) & 7.015 \(\times\) 10\({}^{44}\) & 5.374 \(\times\) 10\({}^{44}\) \\ \cline{1-1} \cline{2-5}  & R\({}^{2}\) & 0.971 & 0.998 & 0.998 \\ \hline \hline \end{tabular}
\end{table}
Table 5: NO3 adsorption kinetics parameters.  
Figure 6: Adjustment of NO\({}_{3}\) adsorption kinetics over (**a**) MYP, (**b**) CYP and (**c**) CC.