## 3 Results and Discussion
### Adsorption Isotherms
However, the reduction in MB removal efficiency was slow for samples 190 HC and 220 HC reaching 68% and 48%, respectively, for C\({}_{0}\) equal to 300 mg L\({}^{-1}\), while sample 250 HC showed a quite quick reduction in the MB removal efficiency accounting for 48% already in correspondence of a C\({}_{0}\) value of 200 mg L\({}^{-1}\) and further, to 35% for a C\({}_{0}\) equal to 300 mg L\({}^{-1}\). Considering the KOH modified hydrocarbons (Figure 5b), results show an increasing MB removal efficiency for initial concentrations of the MB solutions ranging between 10 and 100 mg L\({}^{-1}\) accounting for 88%, 95% and 97% for sample  
Figure 4: Methylene blue (MB) adsorption isotherms on (**a**) raw hydrocarbons and (**b**) modified hydrochars and activated carbons, AC. 190 MHC, 220 MHC and 250 MHC, respectively, at C\({}_{0}\) equal to 100 mg L\({}^{-1}\). Further, these removal efficiencies remained stable for C\({}_{0}\) values up to 300 mg L\({}^{-1}\) confirming that the treatment with KOH caused a significant change in the structure of the hydrocarbons produced at all the HTC temperatures investigated and enhanced the adsorption capacity for removing MB from the aqueous solutions. As expected, results of the MB adsorption isotherms onto activated carbon revealed a high MB removal efficiency (\(\geq\)99%) for all the initial concentrations of MB investigated. The equilibrium adsorption data were analyzed using Langmuir, Freundlich and Tempkin adsorption isotherm models. The linear MB adsorption plots for raw and modified hydrocarbons by the three models are shown in Figure A4 in Appendix A. Table 2 summarizes the parameters of Langmuir, Freundlich and Tempkin isotherms and the correlation coefficients; the parameters were obtained from the intercepts and slopes of the straight lines resulting from the linear regression of the data points. For raw hydrocarbons, results show that the correlation coefficients for Langmuir isotherms are significantly higher than those for Freundlich and Tempkin isotherms. The Langmuir isotherms describe closely the MB adsorption by raw hydrocarbons, suggesting that adsorption is localized on a monolayer and all the adsorption sites are homogeneous [50]. The maximum value of adsorption capacity is 70.51 mg g\({}^{-1}\), related to the sample subjected to 190 \({}^{\circ}\)C HTC process. After the treatment with KOH of this sample, the adsorption capacity increased to 247.06 mg g\({}^{-1}\) according to the linear regression. However, after KOH treatment the correlation coefficient decreases strongly for the 190 \({}^{\circ}\)C samples (from 0.9923 to 0.1237), and this behavior is also observed for the 220 and 250 \({}^{\circ}\)C samples, meaning that the modified hydrocarbons do not follow Langmuir isotherms. On the contrary, Tempkin isotherms exhibit higher correlation coefficients for MB sorption by the modified \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Samples**} & \multicolumn{3}{c}{**Langmuir Equation**} & \multicolumn{3}{c}{**Freundlich Equation**} & \multicolumn{3}{c}{**Tempkin Equation**} \\ \cline{2-10}  & **qm** & **K\({}_{\text{L}}\)** & **R\({}^{2}\)** & **K\({}_{\text{F}}\)** & **R\({}^{2}\)** & **R\({}^{2}\)** & **R\({}^{2}\)** & **R\({}^{2}\)** \\  & **(mg g\({}^{-1}\))** & **(L mg\({}^{-1}\))** & **R\({}^{2}\)** & **(L mg\({}^{-1}\))** & **n** & **R\({}^{2}\)** & **B** & **K\({}^{2}\)** & **(L g\({}^{-1}\))** & **R\({}^{2}\)** \\ \hline
190 HC & 70.51 & 0.05 & 0.9923 & 5.78 & 5.81 & 0.9151 & 12.49 & 1.20 & 0.9589 \\
220 HC & 54.29 & 0.38 & 0.9676 & 9.74 & 2.44 & 0.7733 & 8.78 & 6.12 & 0.7122 \\
250 HC & 37.64 & 0.14 & 0.9845 & 5.83 & 2.52 & 0.7659 & 6.43 & 2.57 & 0.7801 \\
190 MHC & 247.06 & 0.01 & 0.1237 & 3.60 & 1.24 & 0.8000 & 34.51 & 0.17 & 0.8201 \\
220 MHC & 140.13 & 0.06 & 0.8694 & 10.74 & 1.45 & 0.9412 & 29.35 & 0.70 & 0.9525 \\
250 MHC & 203.16 & 0.07 & 0.4208 & 13.89 & 1.21 & 0.8156 & 36.36 & 0.98 & 0.8823 \\ \hline \hline \end{tabular}
\end{table}