## 3 Results and Discussion
### Methylene Blue Adsorption
#### 3.2.3 Adsorption Kinetics  
Figure 3a presents the removal percentage of methylene blue with H-180 against the time. The adsorption increases rapidly in the first 3 h, reaching almost 60% of removal. From 13 h onwards, around 80% of methylene blue was removed and this removal percentage remained practically constant until 24 h. According to other studies that also evaluate methylene blue adsorption by hydrochar, a significant number of adsorption sites were occupied by MB molecules as the adsorption time increased, resulting in a gradual slowing down of the subsequent mass transfer and adsorption process until equilibrium was reached [46; 47; 52]. In general, it is possible to identify three stages: 10-60 min, 90-480 min, and 720-1440 min. [PERSON] and colleagues suggest that, in the first stage, the presence of numerous vacant active sites on the surface of the adsorbent facilitates the interaction between methylene blue molecules and active sites, thereby achieving a high adsorption rate. In the second stage, the rate of adsorption declines due to a reduction in the number of available active sites and the driving force, which is attributable to a decrease in the concentration of methylene blue. In the final stage, the reduction in the number of vacant  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Isotherm** & **Parameter** & **H-180** & **H-215** \\
**Model** & **Parameter** & **H-180** & **H-215** \\ \hline \multirow{4}{*}{Langmuir} & \(q_{max}\) (mg\(\cdot\)g\({}^{-1}\)) & 69.01\(\pm\) 2.93 & 67.78 \(\pm\) 2.83 \\  & \(q_{max}\) (mg\(\cdot\)g\({}^{-1}\)) & (56.40–81.61) * & (55.60–79.96) * \\  & K\({}_{\mathrm{L}}\) & 0.048 \(\pm\) 0.012 & 0.068 \(\pm\) 0.018 \\  & \(R^{2}\) & 0.9669 & 0.9636 \\  & \(MRD\) (\%) & 11.56 & 9.89 \\ \hline \multirow{4}{*}{Freundlich} & \(K_{\mathrm{F}}\) & 13.92 \(\pm\) 3.08 & 14.87 \(\pm\) 3.09 \\  & \(n\) & 3.79 \(\pm\) 0.57 & 3.92 \(\pm\) 0.58 \\  & \(R^{2}\) & 0.9258 & 0.9289 \\  & \(MRD\) (\%) & 25.54 & 27.68 \\ \hline \multirow{4}{*}{Sips} & \(q_{max}\) (mg\(\cdot\)g\({}^{-1}\)) & 80.35 \(\pm\) 7.18 & 79.96 \(\pm\) 7.23 \\  & (49.45–111.25) * & (48.85–111.07) * \\ \cline{1-1}  & K\({}_{\mathrm{S}}\) & 0.090 \(\pm\) 0.020 & 0.112 \(\pm\) 0.022 \\ \cline{1-1}  & n & 0.672 \(\pm\) 0.100 & 0.647 \(\pm\) 0.099 \\ \cline{1-1}  & \(R^{2}\) & 0.9852 & 0.9850 \\ \cline{1-1}  & \(MRD\) (\%) & 9.26 & 11.96 \\ \hline \hline \end{tabular}
* 95% confidence interval.  
\end{table}
Table 3: Values of parameters related to Langmuir, Freundlich, and Sips isotherm models.  
active sites and dye concentration, coupled with the rise in electrostatic repulsion forces resulting from the adsorption of cationic molecules on the surface, might be the causes of the observed slow and steady adsorption rate, indicating that equilibrium was reached [53].  
In Figure 3b, kinetic models (PFO, PSO, and Elovich) were adjusted to the experimental data. As shown in Table 4, the Elovich kinetic model fits better than the others, considering the \(R^{2}\) and MRD values. The suitability of the Elovich model compared with the PFO and PSO models was also reported in another study of methylene blue adsorption by hydrochar [30]. The Elovich equation is suitable for describing chemisorption in systems with heterogeneous adsorbing surfaces [30; 54; 55; 56], suggesting therefore that this type of adsorption occurs between H-180 and methylene blue.