# Granular Activated Carbon from Grape Seeds Hydrothermal Char
## 3 Results and Discussion  
### Pore Volumes and Textural Characterization by Nitrogen Physisorption at 77 K  
Figure 1 shows selected adsorption nitrogen isotherms at 77 K of hydrocarbons and GSACs. Desorption isotherms have not been reported because they substantially overlap with adsorption isotherms. Hydrochars present gas physisorption isotherms of type II according to the IUPAC classification [38], typical of adsorbents with macro-pore dominance or non-porous structures. A small shoulder at low P/Po indicates a small portion of micro-pores, which accommodate monolayer adsorption. By chemical activation, micro-pores are progressively developed that transform Type II isotherms of precursors into reversible Type I, which are typical of highly microporous adsorbents. The GSAC samples show Type Ia isotherms, concave to the P/P\({}_{\mathrm{O}}\) axis, and the nitrogen amount adsorbed at 77 K approaches a limiting value. This limiting uptake is governed by the accessible micro-pore volume, rather than by the internal surface area. A steep uptake at very low P/P\({}_{0}\) is due to enhanced adsorbent-adsorptive interactions in narrow micro-pores (micro-pores of molecular dimensions), resulting in micro-pore filling at very low P/P\({}_{\mathrm{O}}\). These Type Ia isotherms are specifically given by microporous materials having mainly narrow micro-pores (of width \(<\) -1 nm). The sharp increase occurring in the high P/P\({}_{0}\) region represents unrestricted adsorption due to macro-pores. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Sample ID** & **HTC Temp. (\({}^{\circ}\)C)** & **KOH:Hydrochar Ratio** \\ \hline H180 & 180 & - \\ H180K025 & 180 & 0.25:1 \\ H180K050 & 180 & 0.50:1 \\ H180K100 & 180 & 1:1 \\ H220 & 220 & - \\ H220K025 & 220 & 0.25:1 \\ H220K050 & 220 & 0.50:1 \\ H220K100 & 220 & 1:1 \\ H250 & 250 & - \\ H250K025 & 250 & 0.25:1 \\ H250K050 & 250 & 0.50:1 \\ H250K100 & 250 & 1:1 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Identification codes of hydrochars and activated carbon samples. Such a sharp increase is still present in the GSAC samples, witnessing that macro-pores are not eliminated by the activation treatment. Table 2 shows, for each produced sample, the yields (Yact and Yovr, Equations (1) and (2)) and the textural characterization: apparent surface area S\({}_{\text{BET}}\), apparent micro-pore surface area S\({}_{\text{micro}}\), total pore volume V\({}_{\text{tot}}\), micro-pore volume V\({}_{\text{micro}}\) and its percentage in respect to total pore volume, the acid-basic character pH\({}_{\text{p2c}}\). When the sole HTC step is considered, a higher carbonization temperature resulted in a lower hydrochar yield. This result can be explained with the increased severity of the HTC process, as reported for grape marc, i.e., a mixture of grape seeds and skins [39; 40], and, more generally, agro-wastes [41; 42]. Conversely, when the activation step is also considered, the GSAC yields were quite similar and did not reflect the dependence of hydrochar yield on temperature. GSAC yields were comprised in the range 30-37% for all of the samples, but H250K025 that showed a higher yield (41%). Likely, a mild HTC treatment leaves significant amounts of soft biomass matter that is easily consumed by the activator, thus causing a mere loss of matter, instead of an increase in porosity. Conversely, severe HTC conditions are more effective in yielding a material that can be later successfully activated by developing (micro) porosity, as evidenced in Figure 2 below discussed in detail. Table 2 shows that hydrochars had a very low surface area, which was dramatically increased by the activation step. After activation, apparent BET surface area (S\({}_{\text{BET}}\)) increased up to three orders of magnitude, from a single digit to 1018 m\({}^{2}\)/g. In general, a higher HTC temperature and a higher amount of activator favored the development of surface area.