# Revalorization of _Posidonia oceanica_ Waste for the Thermochemical Production of Biochar
## 1 Introduction  
Biochar is a stable carbon-rich by-product synthesized through thermochemical processes, mainly pyrolysis/carbonization of plant and animal-based biomass [1]. Pyrolysis requires an absence of oxidizing agent as well as moderate temperatures (400-600 \({}^{\circ}\)C) to obtain a high solid biochar yield [2]. Conversely, high temperatures are used to achieve high yields of liquid bio-oil and syngas [3; 4]. The yields of each pyrolysis product (bichcar, bio-oil, and syngas) also depend on the heating rate and the vapor residence time in the reactor. The production of biochar is favored by lower heating rates (0.1-10 \({}^{\circ}\)C/s) and higher vapor residence time [4]. In recent years, the role of biochar has been studied in the domain of environmental management. It promises to improve agriculture and the environment in several ways: (i) through soil remediation, due to its high organic carbon content, which implies high stability in soils and excellent nutrient-retention properties [5]; (ii) through waste management, because waste biomass is widely used for biochar production due to cost-effectiveness and food security advantages [6]; (iii) via climate change mitigation, because biochar can be carbon-negative and hence be used to remove carbon dioxide from the atmosphere [7]; and (iv), thanks to bioenergy production, based on the use of the gases generated in the pyrolysis processes [1; 8]. The elemental composition of biochar and its properties are determined by the biomass material employed and the features of the carbonization process [9]. Biomass waste includes forestry and agricultural residues suchas wood, leaves, trunks, peels or husks, and exceta from humans and animals. In the case of plant-based biomass, there are three widespread lignocellulosic components: cellulose (40-60 wt%), hemicellulose (15-30 wt%), and lignin (10-25 wt%). Their percentages depend on the biomass source [4]. _Posidonia oceanica_, one of the most abundant aquatic plants in the Mediterranean Sea and especially in coastal areas, is characterized by high contents of cellulose (38 wt%), hemicellulose (21 wt%), and lignin (27 wt%) [10]. _Posidonia oceanica_ is key to the preservation of coastal ecosystems because it produces large amounts of oxygen and represents a source of food, shelter, and protection for many fish species and other underwater animals [11]. In addition, it prevents coastal erosion by reducing wave energy. _Posidonia oceanica_ is composed of roots, stems, and long leaves and forms continuous meadows that are 0.5 m to 40 m deep. When the leaves die, they lose their photosynthetic capacity and are separated from the rest of the plant. In this way, a large amount of leaves accumulate on beaches, where they decompose [12]. This waste reaches \(5\cdot\times 10^{6}\) to \(5\times 10^{7}\) tonnes per year in Mediterranean areas [10]. In summer, _Posidonia oceanica_ wastes need to be removed from touristic areas because of unpleasant odours, mainly due to the formation of hydrogen sulphide, and the presence of insects, such as beach flies. The process of cleaning beaches entails a significant loss of sand, which translates into a high annual cost. These marine residues are usually disposed of in landfills, at a considerable cost for the local authorities [13]. In recent years, a number of studies have focused on the possible revalorization of the renewable _Posidonia oceanica_ waste. It has been used as a source of cellulose to reinforce conventional polymers such as polyethylene [14] and biodegradable packaging materials based on starch [13, 15], gluten protein [16] or polylactic acid [17, 18, 19]. _Posidonia oceanica_ fibres have also been studied as a low-cost and renewable adsorbent for removing dyes, such as methylene blue [20] or basic blue 41 [21] from aqueous solution, and for removing heavy metals [22, 23] and organic compounds [24].