# Revalorization of _Posidonia oceanica_ Waste for the Thermochemical Production of Biochar
## 1 Introduction
### Biochar Preparation and Characterization  
The biochar was prepared via pyrolysis of dried 1 mm particles of washed _Posidonia oceanica_ in a muffle furnace (Hobersal, Barcelona, Spain) at three temperatures: 300, 400, and 500 \({}^{\circ}\)C, which were selected based on a previous study by [PERSON] et al. [43]. The tests started with the heating of the muffle using a heat ramp for 45 min. It was then kept at a constant temperature for one hour. The biochar characterization was performed following the International Biochar Initiative [44], conducting an elemental analysis, an X-ray fluorescence analysis, the measurement of pH, electrical conductivity, percentage of CaCO\({}_{3}\), organic carbon, percentage of ash, bulk density and thermogravimetric analysis. An elemental analysis of the biochar was performed using a Thermo Finnigan Flash 1112 Series Elemental Analyzer (Thermo Fisher Scientific, Waltham, MA, USA). It was carried out in accordance with the standard test DIN 51732:2014 based on the European Biochar Certificate [37]. An X-ray fluorescence biochar analysis was executed in the same way as for the _Posidonia oceanica_ leaves in a PW2400 automatic sequential X-ray fluorescence spectrometer (Philips Co., Westborough, MA, USA). For the pH measurements, a Crison Basic 20 pH meter (Mettler Toledo, Columbus, USA) was used. This parameter was estimated according to the international norm ISO 10390:2005 and following the European Biochar Certificate [45]. This standard describes an instrumental method for routine pH determination using a glass electrode in a 1/5 (_v/v_) suspension of biochar sample in water. To summarize, 5 mL of biochar sample (approximately 2 g of dried sample) were added to 25 mL of distilled water and stirred for 1 h. The pH was then measured. The electrical conductivity of the biochar samples was measured following the norm DIN ISO 11265:1997-06 as recommended by the European Biochar Certificate [46]. A total of 20 g of biochar sample were added to 200 mL of distilled water and stirred for 1 h. The suspension was then filtered, and the conductivity of the filtered water was measured. The salt content was estimated using an empirical factor based on the conductivity of a 0.01 M KCl solution and expressed as mg KCl/L. Three measurements were taken for each biochar sample. The amount of total inorganic carbon was mainly due to CaCO\({}_{3}\), its determination in the samples was based on the reaction that takes place when biochar is attacked by hydrochloric acid:  
\[\text{CaCO}_{3}\left(\text{s}\right)+2\text{HCl}\left(\text{l}\right)\to \text{CO}_{2}\left(\text{g}\right)+\text{CaCl}_{2}\left(\text{s}\right)+\text {H}_{2}\text{O}.\]  
The CaCO\({}_{3}\) percentage was analyzed following the norm DIN 51726:2004-06 as published in the European Biochar Certificate [47]. A Bernard calcimeter was used, which consisted of attacking the carbonates with acid and quantifying the released CO\({}_{2}\). The organic carbon was calculated by determining the difference between the total carbon content obtained by the biochar's elemental analysis and the inorganic carbon determined from the CaCO\({}_{3}\) percentage. For their part, the ashes were analysed according to the DIN 51719: 1997 standard and the European Biochar Foundation and Certificate [48], as in the case of the _Posidonia oceanica_ samples. In essence, the method consisted of the combustion of the samples at 550 \({}^{\circ}\)C in a muffle oven (Hobersal, Barcelona, Spain) at a constant weight. The results were obtained by weighing the difference between the initial and final masses. Furthermore, the ash content values were also determined by thermogravimetric analysis (TGA) and derivative thermogravimetric analysis (DTG), which were equally used to determine the volatile matter and fixed carbon [41]. Once the temperature program was completed, the decomposition curves of the different biochar were obtained.