## 3 Results and Discussion
### Adsorption Isotherms  
The equilibrium adsorption isotherm is studied in detail, since it can provide information about the surface properties of hydrocarbons and their adsorption behavior. Adsorption equilibrium is a dynamic concept, i.e., it is achieved when the rate of (dye) adsorption is equal to the rate of (dye) desorption [49]. Figure 4 depicts the MB adsorption isotherms onto raw hydrochars, modified hydrocarbons and activated carbons. Considering raw hydrocarbons (Figure 4a), isotherms are of the same type; they are characterized by a large increase in the amount adsorbed at low concentrations, while the amount adsorbed stabilizes around a limit value, different for the various hydrocarbons, when the concentration is higher. For all the treatment temperatures the q\({}_{\rm e}\) increased quickly for C\({}_{\rm e}\) lower than 30 mg L\({}^{-1}\), while the increase in q  
Figure 3: Heavy metals, alkali metals and nutrient content in mixture sludge, raw and modified hydrochars. slowed down for C\({}_{\rm e}\) values higher than 50 mg L\({}^{-1}\). The amount of adsorbed MB increased when the HTC treatment temperature increased from 190 to 220 \({}^{\circ}\)C; on the contrary, the amount of adsorbed MB decreased for the hydrocarbon obtained at 250 \({}^{\circ}\)C. However, the hydrochar that revealed the best adsorption capacity is the 190 HC because it shows the lower C\({}_{\rm e}\) concentration (95 mg L\({}^{-1}\)) compared to samples 220 HC (150 mg L\({}^{-1}\)) and 250 HC (190 mg L\({}^{-1}\)), when the equilibrium is reached using the most concentrated initial MB solution (295 mg L\({}^{-1}\)). These findings, which may appear to be contradictory, are explained by the fact that the quantity of hydrochar used in the various tests was slightly variable (Section 2.4 and Tables A2 and A3 in Appendix A). The q\({}_{\rm e}\) increased quickly for low values of C\({}_{\rm e}\) for modified hydrocarbons (Figure 4b), too. However, the isotherm trends are quite different from those of raw hydrocarbons. Thus, it is clear that the KOH treatment produced strong changes in the adsorption characteristics of the hydrocarbons. Actually, for all the modified hydrocarbons the C\({}_{\rm e}\) values, measured after 48 h of test, are much lower (maximum C\({}_{\rm e}\) value: 30 mg L\({}^{-1}\)) than those of raw hydrocarbons (maximum C\({}_{\rm e}\) value: 190 mg L\({}^{-1}\)) and this corresponds to a higher amount of MB adsorbed. Considering all the hydrocarbons, the sample that shows the best adsorption capacity is 250 MHC, i.e., that produced by HTC of sewage sludge at 250 \({}^{\circ}\)C for 3 h and post-treatment with KOH. In Figure 4b the adsorption isotherm trend for activated carbons is depicted, too. As expected, the q\({}_{\rm e}\) increased very quickly, reaching a value of 113 mg g\({}^{-1}\) at the highest C\({}_{0}\) concentration, and C\({}_{\rm e}\) values were very reduced and always lower than 0.65 mg L\({}^{-1}\). The pH values of the MB solutions were evaluated. The pH value slightly increased after the addition of MB to the deionized water, passing from a value of 5.5, referring to the deionized water, to values in the range 6.2-6.6. The trend of the pH variation corresponding to a MB concentration ranging between 10 and 300 mg L\({}^{-1}\) has been reported in Figure A3 in Appendix A. The percentages of MB removed by hydrocarbons, referring to different C\({}_{0}\) of the MB solutions, are reported in Figure 5.  
Figure 5a shows the MB removal of raw hydrocarbons. Results reveal a high MB removal efficiency for initial concentrations of MB ranging between 10 and 100 mg L\({}^{-1}\) accounting for an average value of 90%, 95% and 87% for samples 190 HC, 220 HC and 250 HC, respectively. The removal efficiency decreased below 87% (190 HC), 89% (220 HC) and 65% (250 HC) for C\({}_{0}\) values higher than 150 mg L\({}^{-1}\).