## 3 Results and Discussion
### Methylene Blue Adsorption  
#### 3.2.1 pH Effect  
Figure 1 presents the removal efficiency of methylene blue with H-180, H-215, and H-250. The results from each pH were submitted to ANOVA and Tukey tests at a 95% confidence level to ascertain whether there were statistically significant differences in the removal efficiencies of H-180, H-215, and H-250. The superior performance of H-180 and H-215 compared with H-250 is clearly noticeable for all pHs. At pH 2, H-215 outperformed H-180 and H-250 (\(p_{\rm{value}}\) < 0.05). Conversely, at pH 4, the removal efficiency of H-180 and H-250 were equal (\(p_{\rm{value}}\) > 0.05), whereas H-250 presented the lowest removal (9.4%) of all assays. For pH values of 6, 8, and 10, all hydrochars promoted different removal efficiencies (\(p_{\rm{value}}\) < 0.05), but H-250 remained the least effective adsorbent, irrespective of pH. The removal efficiency was found to exceed 60% at pH values of 6, 8, and 10 for both H-180 and H-215. However, H-250 exhibited a significantly lower adsorption capacity, with an efficiency of approximately 20% across all hydrochars at those pH values. Interestingly, at pH 10, the methylene blue removal was slightly higher with H-180 than with H-215 (\(p_{\rm{value}}\) < 0.05). It is a different trend in comparison with lower pH values, in which H-215, except for pH 4, presented superior removal efficiencies than H-180 (\(p_{\rm{value}}\) < 0.05).  
Figure 1: Influence of pH on methylene blue (100 mg\(\cdot\)L\({}^{-1}\)) adsorption (24 h at 30 \({}^{\circ}\)C and 100 rpm) by hydrochars (3 g\(\cdot\)L\({}^{-1}\)) from co-hydrothermal carbonization (Co-HTC) of sawdust and non-dewatered sewage sludge.  
In aqueous solutions, methylene blue can exist as a cationic (MB\({}^{+}\)) and as an undiscociated molecule (MB\({}^{0}\)). MB\({}^{0}\) predominates at pHs lower than 3.8, MB\({}^{0}\) (50%) and MB\({}^{+}\) (50%) coexisted equally at pH = 3.8, and MB\({}^{+}\) is the only species at pH > 6.0 [37]. Therefore, considering electrostatic interactions, one could explain the better adsorption performance of hydrocarbons at pH 6, 8, and 10 due to the hydrocarbons' pH\({}_{\text{PZC}}\) and the pH-dependent methylene blue speciation. Even for H-250, it is possible to note a slight increase in the removal efficiency at pH 6, 8, and 10 compared with pH 2 and 4. Similarly, [PERSON] and colleagues demonstrated that methylene blue removal was enhanced at basic pH due to the negative zeta potential of hydrochar, which was associated with elevated concentrations of OH\({}^{-}\) ions and deprotonation of functional groups. This resulted in enhanced electrostatic attraction and hydrogen bond interactions between the hydrochar and dye [38].  
The difference in removal efficiency of H-250 compared with H-180 and H-215 can be related to the surface functional groups of hydrochar. Hydrochars obtained from lignocellulosic materials can present oxygenated functional groups, such as phenolic, carboxylic, and lactonic, that can help with adsorption [39; 40; 41]. At low temperatures, Co-HTC can preserve more oxygenated functional groups in the hydrochar surface [12], which could explain why H-180 and H-250 outperformed H-250. [PERSON] and colleagues demonstrated that methylene blue adsorption was more effective for hydrochar derived from Co-HTC of bamboo and polyvinyl chloride than for hydrochar produced from a single feedstock. This finding was attributed to the presence of a greater number of hydroxyl and carboxyl groups on the surface of the Co-HTC-derived hydrochar, which improved the adsorption of methylene blue [11]. Therefore, given their better performance than H-250, only H-180 and H-215 were considered for the following tests, which were carried out at pH 10.