# Granular Activated Carbon from Grape Seeds Hydrothermal Char
## 1 Introduction
Therefore, it is not possible to find distinct optimal parameters for each process step (i.e., carbonization and activation), which could potentially result into more efficient operations. If, rather than a single-stage, a two-stage process is considered, the optimal conditions for the carbonization step would result in less usage of activator. This would translate into less harmful chemicals being involved in the process. Furthermore, moving to a two-stage process would imply a potentially better control of porosity development, with the possibility of tailoring the final product for the desired application. Therefore, despite the need for two reactors, a two-stage process could potentially give interesting and valuable advantages. In such a scheme, the carbonization of the precursor can be performed through different alternative technologies, such as torrefaction, pyrolysis, and hydrothermal carbonization (HTC), among which HTC can be considered as the most \"green\" process, as it minimizes the energy uptake [23]. Indeed, by applying HTC alone, hydrochar (i.e., the solid product from HTC) from lignocellulosic materials already gains ability as adsorbent due to its surface chemistry. Even though the porosity of hydrochar is very low, less than 10 m\({}^{2}\)/g, the hydrochar surface is rich in oxygenated functional groups (carboxylic, hydroxyl/phenolic, carbonyl), which have significant capabilities for the sorption of particular molecules in solution through cation exchange mechanisms [24]. The adsorption capacity of hydrochar can be further increased by developing its porosity using activation at an elevated temperature. The heat treatment with a chemical or physical activating agent is effective for pore development, but may suffer from surface functional group losses as a tradeoff [16]. Recently, HTC of biomass followed by physical or chemical activation to produce activated carbon has been extensively studied. [PERSON] and [PERSON] [25] tested sawdust in order to produce high surface area activated carbon (>1200 m\({}^{2}\)/g) by the two-stage method: HTC followed by chemical activation. They used a KOH/hydrochar ratio of 2, or, alternatively, 4, which allowed for the development of a large number of micro-pores suitable for CO\({}_{2}\) capture. [PERSON] et al. [26] performed HTC followed by CO\({}_{2}\) or H\({}_{3}\)PO\({}_{4}\) activation using four different wastes: grass cuttings, horse manure, beer waste, and bio-sludge. They produced activated carbons showing a wide range of surface areas (489-1073 m\({}^{2}\)/g) to be used for CO\({}_{2}\) adsorption. [PERSON] et al. [27] produced porous carbon spheres from HTC at 250 \({}^{\circ}\)C followed by KOH activation at 800 \({}^{\circ}\)C. Starting from cassava and tapioca flour, they obtained surface areas approaching 1000 m\({}^{2}\)/g. Larger pore size (meso-pores) activated carbon, suitable for wastewater treatment applications, was prepared form hard biomass (coconut shells and date stones) hydrochar by using NaOH as activating agent [28; 29]. Raw hydrochar and hydrochar activated with KOH was prepared and used for water treatment by [PERSON] et al. [30], starting from sewage sludge. Recently, [PERSON] et al. [31] produced hydrochar from the digestate of a biogas plant and activated it with KOH. The resulting activated product revealed to be very effective for gas treatment, with a good affinity towards CO\({}_{2}\). In the present study, grape seeds activated carbon was produced by means of a two-stage process. The first stage is represented by HTC: grape seeds were treated in liquid pressurized water at moderate temperatures in the range 180-250 \({}^{\circ}\)C. Through HTC, the seeds can be processed without prior drying, thus avoiding the considerable amount of energy that is required for water evaporation. This can be a crucial advantage when compared to other carbonization processes, like pyrolysis, which requires relatively dry substrates (maximum acceptable moisture content \(\approx\)30%) and a dry environment [32].