## 1 Introduction  
Biomass is a widely available source of energy, particularly important in developing countries, but cannot be considered a technically ideal fuel due to physical and chemical properties, each often having fibrous nature, high moisture content, volatile components, alkali and alkaline earth metallic content and a relatively low bulk density and heating value [1; 2; 3]. Thus, pre-treatment of biomass is in many cases convenient for efficient use as energy source. A broad range of biological (mainly anaerobic digestion and fermentation) and thermochemical (torrefaction and pyrolysis) treatments are typically used to improve the fuel properties of raw biomass [2; 4; 5]. From those processes, liquid or gaseous biofuels and even some valuable products are derived. As an alternative to the classical thermochemical methods, hydrothermal carbonization (HTC), also referred to as wet torrefaction, is becoming an increasingly attractive way of biomass conversion. It operates in presence of water, at comparatively mild temperatures (180-300 \({}^{\circ}\)C) and a corresponding saturation pressure [6; 7; 8; 9; 10; 11]. The resulting solid product, usually called hydrochar (HC), is more stable and has higher carbon content than the starting substrate and improved (higher) heating value with respect to the char resulting from slow-pyrolysis or conventional carbonization at the same temperature [12; 13; 14]. In addition to the hydrochar, HTC gives rise to a high organic load aqueous stream [15; 16], and a gas consisting mainly of CO\({}_{2}\). Hydrochars have several industrial and environmental applications, such as for soil improvers [17] and solid fuel, whether upon direct combustion [18] or through gasification [12]. HTC chars may have also potential uses as sorbents for CO\({}_{2}\) sequestration, methane and hydrogen storage, in energy storage devices (Li/Na ion batteries, supercapacitors, fuel cells) and as precursors for activated carbon preparation [7; 18; 19; 20; 21; 22; 23]. In general, hydrochars show relatively low BET surface areas and pore volumes, which hinders their application as adsorbents [24]. Nevertheless, [PERSON] [23] reported the presence of ultramicroproes (narrow micropores) in several sugar-derived hydrocarbons, as determined by CO\({}_{2}\) adsorption. Low-cost adsorbents can be obtained from HC upon further activation with different agents (CO\({}_{2}\), steam, acids, bases, or salts), allowing substantial pore volume and surface area increases, and even changes of the morphological structures of HTC chars [24; 25; 23]. A number of works have reported on the application of activated hydrocarbons in aqueous-phase adsorption for the removal of heavy metals [24; 26; 27; 28], dyes [29; 24], phenolic compounds [30] and emerging pollutants [18; 31]. Gas-phase applications have been also studied, including, VOCs [32] and CO\({}_{2}\)[31; 33] adsorption. Hydrochars treated by H\({}_{2}\)O\({}_{2}\) oxidation have been used for lead removal from water [34]. Agricultural wastes are recognized as interesting feedstocks for inexpensive carbon materials [35]. Among them, grape seeds represent up to 15% of the solid wastes from the wine industry, where they are mostly burnt as fuel. Some papers have reported on the preparation of activated carbons by both physical and chemical activation from this precursor [36; 37; 38; 39; 40]. Activated carbon is widely available at low cost in many regions and its structural characteristics (granular morphology, size and preferential distribution of the lignocellulosic material in the periphery of the seed) make it particularly attractive for that purpose [36]. Essentially microporous activated carbons have been obtained by physical (CO\({}_{2}\)) and chemical (KOH and K\({}_{2}\)CO\({}_{3}\)) activation [36; 40]. Using phosphoric acid as the activating agent allowed obtaining also predominantly microporous carbons but with a broader porous texture with some significant contribution of mesoporosity [36]. Recently, [PERSON] et al.