## 3 Results and Discussion
### Methylene Blue Adsorption
#### 3.2.2 Adsorption Isotherms  
Figure 2a,b shows the isotherm of methylene blue adsorption by H-180 and H-215, respectively. The \(q_{max}\) for both hydrocarbons was around 70 mg\(\cdot\)g\({}^{-1}\). Table 2 compares \(q_{max}\) for methylene blue of other hydrocarbons with that obtained in this study. The value obtained herein was similar to another hydrochar produced at 180 \({}^{\circ}\)C from citrus waste when the methylene blue adsorption was conducted at 4 \({}^{\circ}\)C, where \(q_{max}\) was 66 mg\(\cdot\)g\({}^{-1}\); however, when the adsorption temperature was increased to 20 and 36 \({}^{\circ}\)C, \(q_{max}\) reduced to 51 and 31 mg\(\cdot\)g\({}^{-1}\), respectively. In the same study, another hydrochar from winery waste, also obtained at 180 \({}^{\circ}\)C, presented \(q_{max}\) values between 29 and 37 mg\(\cdot\)g\({}^{-1}\) for adsorptions at 4, 20, and 36 \({}^{\circ}\)C, which are quite low compared with the results of this study. Additionally, it is worth noting that the BET surface area of citrus waste and winery waste hydrocarbons were 46 and 34 m\({}^{2}\cdot\)g\({}^{-1}\), respectively, while the surface area of H-180 was 14.8 m\({}^{2}\cdot\)g\({}^{-1}\)[42]. Even with a lower surface area, the hydrochar from Co-HTC of sawdust and sewage sludge at 180 \({}^{\circ}\)C produced herein had a higher adsorption capacity for methylene blue. Another study used only sewage sludge for hydrochar production at 190, 220, and 250 \({}^{\circ}\)C and showed \(q_{max}\) of 71, 54, and 38 mg\(\cdot\)g\({}^{-1}\), respectively, for methylene blue adsorption [43]. It is noteworthy that H-180 exhibits a comparable \(q_{max}\) value to the hydrochar derived from the 190 \({}^{\circ}\)C treatment in the aforementioned study. Additionally, the H-215 displays a higher \(q_{max}\) value than the hydrochar obtained from the 220 \({}^{\circ}\)C treatment in the aforementioned study. Therefore, it demonstrates that combining wastes, such as lignocellulosic materials and sewage sludge, in a Co-HTC process can improve the adsorption capacity of hydrochar. Despite the \(q_{max}\) obtained for H-180 and H-215, one can mention hydrochar activation to increase the adsorption capacity, as performed in some works [30; 44; 45; 46]. However, it is important to consider that an activation step would result in increasing the costs of adsorbent production, in addition to the generation of effluents when using wet routes. Furthermore, some studies that performed hydrochar activation demonstrated lower \(q_{max}\) values than those achieved for H-180 and H-215. For example, [PERSON] and colleagues synthesized hydrochar from corn straw (HC), which was also N-doped with urea (UHC), melamine (MHC), and NH\({}_{4}\)Cl (AHC) to increase methylene blue adsorption. Nevertheless, the \(q_{max}\) values of all hydrochars obtained were lower than those of H-180 and H-215 (HC: around 10 mg\(\cdot\)g\({}^{-1}\); UHC: <20 mg\(\cdot\)g\({}^{-1}\); MHC: <40 mg\(\cdot\)g\({}^{-1}\); AHC: <60 mg\(\cdot\)g\({}^{-1}\)) [47]. In another study, an oxide-hydrochar composite produced from pomeranate peels was activated with ZnCl\({}_{2}\), but the \(q_{max}\) value of the activated material was lowered to 50 mg\(\cdot\)g\({}^{-1}\)[48]. The \(q_{max}\) of a carbon-derived adsorbent for methylene blue can vary significantly depending on the feedstock and production conditions utilized. This phenomenon has been extensively discussed in several review articles. For instance, [PERSON] et al. reported a range of \(q_{max}\) values from 1.33 to 300 mg\(\cdot\)g\({}^{-1}\). Intriguingly, some activated carbons exhibited \(q_{max}\) values below 9 mg\(\cdot\)g\({}^{-1}\), notably lower than the values observed for H-180 and H-215 [22]. In another study, \(q_{max}\) values of carbon-based adsorbents--activated carbon, graphite porous carbon, and carbon nanotubes--ranged from 0.003 to 2936 mg\(\cdot\)g\({}^{-1}\), highlighting the variability in outcomes [49].