# Revalorization of _Posidonia oceanica_ Waste for the Thermochemical Production of Biochar
## 1 Introduction
Orthophosphate, which contributes to water quality deterioration and eutrophication in lakes, rivers, and coastal areas, was also successfully adsorbed into _Posidonia oceanica_ fibres [25]. They have also been shown to be a suitable source of lignocellulosic fibres to produce pulp and paper [26]. In addition, a crystal absorber constituted of _Posidonia oceanica_ fibrous spheres can be made to develop acoustic materials [27]. Recently, a study on the benefits of _Posidonia oceanica_ leaf extracts to treat diseases revealed anti-inflammatory properties [28] and the inhibition of human cancer cell migration [29]. However, the potential use of _Posidonia oceanica_ waste to produce biochar via thermochemical processes has hitherto been scarcely investigated. [PERSON] et al. studied the feasibility of producing bio-oil and biochar from _Posidonia oceanica_ by pyrolysis. They found stability and alkalinity characteristics of interest to apply in soils [3]. [PERSON] et al. synthesized activated biochar from the pyrolysis of dead _Posidonia oceanica_ leaves [9]. It was tested as an adsorbent material for toxic metal ions [30]. In this way, the use of thermally treated _Posidonia oceanica_ residues has been shown to be a low-cost adsorbent material with high selectivity toward phosphate from real wastewater [30]. It is very important to characterise biochar when it is generated because the content of multivalent metal elements can improve its efficiency. [PERSON] et al. [31] studied the role of a biochar's calcium content in removing/recovering phosphorus. In addition, it has been shown that the CaCO\({}_{3}\) present in biochar improves its stability and CO\({}_{2}\) sequestration efficiency [32]. Additionally, it has been shown that CaO can favour the formation of H\({}_{2}\) and CH\({}_{4}\) in pyrolysis processes [33]. Other authors have also studied how particle size affects pH levels, micronutrients, and macronutrients [34]. For all these reasons, when the applicability of biochar- that has been generated from an insufficiently studied biomass-needs to be verified, it is important to conduct an exhaustive characterisation. This work studies the main characteristics of biochar produced at different temperatures from washed _Posidonia oceanica_. Water consumption should be reduced for washing, because water is too precious of a resource and should not be used to clean waste in such a considerable amount. For this reason, it is proposed that this flushing be carried out with water from sewage treatment plants. The physicochemical characteristics of the reclaimed water are within the limits permitted for this type of water. The permitted uses include environmental use, to which the present study can be assimilated since the activity consists of washing to dissolve the salinity of the Posidonia oceanica. The aim was to optimise the production of biochar from this biomass, thus avoiding the displacement of tonnes of _Posidonia oceanica_ to landfills every year and giving the waste a second life by transforming it into biochar. The production of biochar from biomass has an average energy cost of 1.5 MJ/kg [35], which, if translated into money, would have a production cost of 0.05 \(\  
The _Posidonia oceanica_ sample was also characterized in terms of its composition by thermochemical analysis, such as combustion and pyrolysis. As is known, combustion takes place under oxidant conditions with air (21% oxygen, 79% nitrogen), while pyrolysis occurs in an inert atmosphere (nitrogen only).