## 3 Results and Discussion
### Characterization
All hydrochars present gas physisorption isotherms of type II b according to the IUPAC classification [39], typical of materials with slit macro-pores with an average diameter of around 100 nm and attributable to the formation of aggregates of discoidal particles. Macro-pores were present in small amount in all samples, thus confirming the low values of BET surface area observed. Adsorption/desorption isotherms are very close, indicating a narrow and homogenous pore distribution. Moreover, the presence of the hysteresis loop confirms the prevalent presence of macro-pores, except for the 190 MHC sample. Furthermore, the shape of the hysteresis loop is of type H3, located at the high relative pressure range, displaying two steep, parallel branches, without a plateau. This shape indicates the presence of flat particles and slit pores. After the KOH treatment, hysteresis loops are wider, especially for the sample 220 MHC, due to an amplification of the pore distribution curve. Moreover, in the case of sample 220 MHC the shape of the hysteresis loop can be considered as intermediate between type H3 and H4, indicating a higher reduction in the number of pores compared to the other modified hydrochars. As expected, specific surface area values are much lower in comparison with reported ones for activated carbon materials, obtained from different organic sources [40; 41]. However, the presence of various functional groups on the sample surfaces, the alkaline treatment and the presence of metal ions could modify in a more efficient way the interactions between adsorbent and MB adsorbate, as already reported for low-temperature treated organic-based adsorbents [42]. SEM micrographs (Figure A2 in Appendix A) show a smooth surface without pores in the micrometer size range for all the samples investigated. The FTIR analysis results of raw and modified hydrochars are illustrated in Figure 2. This analysis allows determining of the main functional groups present on the hydrochars' structures, which could attribute to them different adsorption capacities. The FTIR spectra of raw (Figure 2a) and modified (Figure 2b) hydrochars are quite similar in terms of their functional groups, since peaks fall in the same wavenumber interval. The bands within the range of 3600-3000 cm\({}^{-1}\) correspond to O-H groups such as alcohols, phenols and carboxylic acids. These groups can provide hydrochars with good cation exchange capacity [6; 43], just like MB, that is positively charged. Moreover, the peak has a fairly wide width meaning that there is a complex interaction among the same surrounding functional groups. The bands at 2920-2850 cm\({}^{-1}\) are attributable to aliphatic C-H stretching bonds, as due to -CH\({}_{2}\) (methylene) or -CH\({}_{3}\) (methyl) groups, without aromatic components. The bands within the range of 1650-1530 cm\({}^{-1}\) could correspond to C-O groups of amide groups and carboxylates. From a qualitative point  
Figure 1: Nitrogen adsorption/desorption isotherms at 77 K of (**a**) raw and (**b**) modified hydrochars. of view, samples obtained at 190 \({}^{\circ}\)C do not show enhanced structural changes before and after the basic treatment, unless a decrease in intensity of the band located at about 1630 cm\({}^{-1}\), maybe due to a decrease in the adsorbed water content. For the samples produced at higher temperatures, a general decrease in intensity for the signals related to oxygen-containing functional groups was observed with the alkaline treatment, as a result of the partial removal of these groups from the sample surfaces, leading to a more homogeneous outer structure, as also evidenced by SEM analysis, confirming data from the elemental analysis. The HTC treatment has a significant effect on the amount of heavy metals in the sewage sludge hydrochars [44]. The total contents of heavy metals, alkali earth metals and nutrient elements in the various samples is presented in Figure 3. In Table 1 in Appendix A the concentration values are reported.