# Granular Activated Carbon from Grape Seeds Hydrothermal Char
## 3 Results and Discussion
### Pore Volumes and Textural Characterization by Nitrogen Physisorption at 77 K

Table 2: Yields and textural characterization of the produced samples. Figure 2: Pore size distribution (PSD): (**a**) hydrochar sample H220; and, (**b**) GSAC samples H220K025, H220K050, H220K050, H220K100. into wider micro-pores (10 to 20 A) and meso-pores, as indicated by the increase of the wider size pore peaks. The activation procedure also caused important changes to the nature of the surface. Indeed, the pH point of zero charge (pH\({}_{\text{PZC}}\)) data, as reported in Table 2, testifies that the hydrochar has normally an acidic nature, with pH\({}_{\text{PZC}}\) values ranging between 5.6 and 6.4. After the activation treatment, the resulting GSACs tend to have a basic surface. However, the addition of increasing amounts of activator tends to reduce the basic nature of the GSACs. Indeed, with a KOH:hydrochar ratio of 0.25, pH\({}_{\text{PZC}}\) ranged between 9.5 and 10.7, while with a ratio of 1, pH\({}_{\text{PZC}}\) was in the range 7.8-8.1. The above discussion actually reflects the activation theory related to KOH, which foresees the following main reactions [43]:  
\[2\text{KOH}\rightarrow\text{K}_{2}\text{O}+\text{H}_{2}\text{O} \tag{3}\]  
\[\text{C}+\text{H}_{2}\text{O}\rightarrow\text{CO}+\text{H}_{2} \tag{4}\]  
\[\text{CO}+\text{H}_{2}\text{O}\rightarrow\text{CO}_{2}+\text{H}_{2} \tag{5}\]  
\[6\text{KOH}+2\text{C}\rightarrow2\text{K}+3\text{H}_{2}+2\text{K}_{2}\text{ CO}_{3} \tag{6}\]  
Reactions (4) and (6) are responsible for the decrease of yield by activation since carbon is consumed while creating cavities. It is generally accepted that the activation mechanism for pore development consists of three different paths, which can occur in sequence or in parallel: (1) opening inaccessible pores, (2) creating new pores, and, (3) widening existing pores [43]. Our data support that the first and second paths are predominant at relatively low KOH:hydrochar ratio, while the third path prevails at higher KOH:precursor ratio. The development of micro-porosity with low amounts of activator was also observed by [PERSON] et al. [22]. It can be inferred that, by increasing the amount of activator, such micro-pores enlarged, until they eventually collapsed and formed larger cavities. By this process, meso- and macro-porosity increased (Table 2). However, the HTC temperature played a crucial role. Increasing the KOH:hydrochar ratio from 0.25 to 0.50 was beneficial in terms of surface area and micro-porosity development for all of the samples, independently of the HTC temperature used. A different situation is obtained when further increasing the KOH:hydrochar ratio, namely from 0.50 to 1. Actually, the use of the highest HTC temperature and the highest amount of activator was counterproductive for surface area and micro-pore development (H250K100): Table 2. At such operating conditions, pore walls clearly collapsed and surface area reduced. Thus, it is definitely counterproductive carbonizing at 250 \({}^{\circ}\)C and activating at a KOH:hydrochar ratio equal to 1, while it is definitely positive to use a KOH:hydrochar of at least 0.50. Given this, it is not obvious to state which are the best operating conditions for the two-stage process analyzed. A techno-economic assessment would help in their selection, although such considerations are outside the scope of this work.