# Granular Activated Carbon from Grape Seeds Hydrothermal Char
## 2 Materials and Methods  
Grape seeds were collected from an alcohol producer company located in the Trentino province, in the North-East of Italy. The company receives grape marc from local winemaking producers. Grape seeds are separated from grape skins by wet sieving. They are then dried and normally sold for grape seed oil extraction. Thus, grape seeds were collected dry, with a residual moisture content of about 5 wt %, as measured in our laboratory. In a hypothetical GSAC industrial production process, the drying step could be avoided and the moist grape seeds could be directly fed to the HTC reactor. The production of GSAC was carried out through two stages: HTC followed by KOH activation. Importantly, grape seeds were not ground in order to preserve the granular nature of the seeds. Such choice was done in order to obtain a granular material that can be directly utilized as an adsorbent, without prior granulation operation, or, eventually, size reduction. HTC was carried out in a batch autoclave. The experimental setup included a 50 mL cylindrical stainless steel reactor whose walls were surrounded by an electrical band heater. A thermocouple and a pressure meter allowed for the measurement of process temperature and pressure. The reactor temperature was kept constant by means of a temperature control loop. Further details about such experimental apparatus can be found elsewhere [33; 34]. HTC experiments were carried out at three different temperatures: 180, 220, and 250 \({}^{\circ}\)C. For each run, 6.0 g of grape seeds were loaded in the reactor, along with 20.0 g of distilled water: HTC runs were performed at a dry biomass to water ratio of about 0.3. The use of such a high ratio is motivated by the fact that a HTC industrial plant should work at high biomass loads to minimize operating and investment costs [35]. The reactor was then heated up to reaction temperature; the heating phase lasted about 15-20 min, depending on the desired reaction temperature. Once the set-point temperature had been reached, it was held constant for 1 h. The reactor was then cooled down by placing a cold (\(-24\)\({}^{\circ}\)C) massive metal disk at the reactor base and blowing compressed air at the reactor walls, in order to ensure a relatively fast cooling (15 min or lower). After carbonization, the solid product, i.e., hydrochar, was separated from the aqueous-phase by filtration and then dried overnight at 105 \({}^{\circ}\)C in a ventilated oven. Such a drying operation was executed in order to determine the hydrochar yield. Indeed, this would not be strictly necessary in an actual industrial process, since the subsequent impregnation operations have to be conducted in a wet environment. The dried hydrochar was then subjected to chemical impregnation using potassium hydroxide (KOH, Merck, Darmstadt, Germany). The KOH:hydrochar weight ratio was varied between 0.25 and 1, in order to test the effect of a reduced amount of chemical as activating agent. Indeed, the proposed two-step approach is able to greatly reduce the amount of chemical needed. When only KOH activation is utilized, it is common to use a KOH amount that is even more than three times the char weight. Impregnation was done by manually mixing hydrochar with a KOH concentrated aqueous solution at room temperature, and then drying in an oven at 100 \({}^{\circ}\)C overnight. The impregnated hydrochar was then weighed, supported inside a ceramic pipe, and subjected to the heat treatment at 600 \({}^{\circ}\)C under N\({}_{2}\) flow inside a horizontal furnace. After reaching the set-point temperature, it was held for 1 h before cooling down to room temperature. The so activated hydrochar, i.e., GSAC, was then removed from the pipe and washed using 0.5 N HCl solution several times, followed by hot distilled water, until a constant pH of the filtrate was reached.