## 1 Introduction  
Citrus fruits are among the most cultivated and processed fruits worldwide, with an annual production of about 152 million tons [1]. The citrus processing industry generates huge amounts of residues mainly in the form of pulp and peels, with the latter accounting for almost 50% of the wet fruit mass [2]. With more than 50 million metric tons in 2020 [3], the juice industry alone generates a huge volume of orange peel waste (OPW) that requires suitable management, taking into consideration the high OPW biodegradability that causes its fast fermentation, which is often uncontrolled. Therefore, the direct disposal of this secondary product without previous proper processing, raises serious environmental issues and economic loss for the citrus industry since traditional disposal strategies, such as incineration or landfilling, are expensive and insufficient in terms of environmental protection and energy efficiency [4, 5]. To date, several technological innovations have been proposed to manage the OPW, mainly aimed at converting the bio-waste into a valuable resource for the extraction of essential oil and the sustainable production of biogas, bioethanol and other biobased chemicals such as pectin, bioethanol and acids [6; 7; 8; 9]. Moreover, innovative approaches, such as the catalytic upgrading and the extraction of bioactive compounds, leading to an ecofriendly production of active ingredients having applications in different sectors, have been proposed in the last years. Even if the valorization of agro-industrial wastes by the preparation of bioactive materials can be considered a sustainable methodology, other factors also need to be taken into account, including the extraction method, the employed reagents and solvents, general expense and employing the use of toxic chemicals with long time/energy-demanding procedures, poor selectivity, and large volumes of solvents [10]. Furthermore, considering that citrus processing wastes contain high amounts of moisture, there is a great interest in their use as feedstock for thermochemical processes as well [11]. The hydrothermal carbonization (HTC) is simply a thermochemical conversion process carried out in a water medium under autogeneous pressure at a relatively mild temperature (180-300 \({}^{\circ}\)C) and represents a promising treatment technique for the wet lignocellulosic biomass waste since it permits overcoming the drawbacks of conventional thermochemical processes that require the use of dry feedstocks [11]. During the hydrothermal carbonization, biomass is dehydrated in-situ and processed into solid, liquid and non-condensable gaseous products. The distribution of products depends on the process conditions and on the feedstock used, but in general 50-80% of the original biomass is present in the solid product, 5-20% in the aqueous phase which contains inorganic and organic matter, and 2-5% in the gas phase which is mainly composed of CO\({}_{2}\)[12]. Hydrothermal methods are largely used in petroleum-based refineries and are getting more and more attention from both scientific and industrial researchers since their process parameters can be easily translated into modern biorefineries aimed at the upgrading of lignocellulosic residues and wastes [13]. With respect to other (bio)refinery processes, HT protocols present several advantages in terms of sustainability since they generally adopt mild reaction conditions (e.g., temperature and pressure) without any homogeneous or heterogeneous catalysts and in the presence of water as a green reaction solvent (used as such or in combination with simple aliphatic alcohols) [13]. The carbonaceous residue obtained by the HTC of the citrus processing waste is rich in oxygenated functional groups, making it a promising material in a wide range of applications, including pollutants adsorption [14], soil amendment [15], as fuel in energy applications [11] and as a low-cost material for capacitor and sensing applications, as recently reported by some of the authors [16; 17; 18; 19].