# Revalorization of _Posidonia oceanica_ Waste for the Thermochemical Production of Biochar
## 3 Results and Discussion
### Biochar Characterization  
The biochar characterization was carried out following the International Biochar Initiative Product Testing Guidelines for Biochar That is Used in Soil [44]. The results of the different analyses summarised (Table 4). As is observable, the values obtained for all elements decreased as the temperature increased. Also worthy of note is the H/C ratio: the higher a fuel's H/C ratio, the higher its CO2 emissions when compared to those of water, as its carbon content will be lower than its hydrogen content. Therefore, it is possible to conclude that the higher the temperature of biochar production, the lower its CO2 emissions. The International Biochar Initiative classifies biochar into three categories depending on its organic carbon content, setting a lower limit that must be exceeded for any biochar to be used. The lower limit is 10% organic carbon. A biochar with more than 60% would belong to class 1, with more than 30% it would belong to class 2, and below 30%, it would belong to class 3. The analysis allows concluding that the biochar produced at 300 \({}^{\circ}\)C belongs to class 2. In contrast, those produced at higher temperatures belong to class 3. The comparison between biochar at 400 \({}^{\circ}\)C and biochar at 500 \({}^{\circ}\)C shows that the carbon content continues to decline, but the ash content decreases (Table 4). This is probably due to the formation and emission of polycyclic aromatic compounds (PAHs), as these tend to form under conditions of incomplete organic matter combustion at temperatures above 400 \({}^{\circ}\)C.  
Regarding the results obtained for pH, conductivity, salt content, and bulk density, no large variations were found for the parameters studied. One exception was pH, which \begin{table}
\begin{tabular}{c c c c c c c c} \hline  & & \multicolumn{4}{c}{\(C_{o}\) (Hg/g)} & & \\ \hline  & \(\mathbf{F}^{-}\) & \(\mathbf{CI}^{-}\) & \(\mathbf{NO^{2-}}\) & \(\mathbf{NO^{3-}}\) & \(\mathbf{Br}^{*}\) & \(\mathbf{PO_{4}}^{3-}\) & \(\mathbf{SO_{4}}^{2-}\) \\ \hline Before washing & 26.8 & 34,461.3 & 458.5 & 7020.4 & 204.7 & 25.1 & 12,170.2 \\ After washing & 26.6 & 1863.7 & 1152.3 & 13,975.6 & 185.4 & 23.2 & 7468.6 \\ \hline \end{tabular}
\end{table} 
Table 3: Values of \(C_{o}\) for each halogen emitted during combustion. varied by two units between two samples (biochar 300 \({}^{\circ}\)C and biochar 400 \({}^{\circ}\)C). These parameters were compared to those in the literature for biochar made from plant residues and were found to be within the normal range for application to soils. In fact, [PERSON] et al. [54] performed a germination test on a plant waste biochar with very similar characteristics to that found for _Posidonia oceanica_ biochar. The author obtained germination results of more than 85%. Biochar obtained from _Posidonia oceanica_ can therefore be assumed to be suitable for soil remediation. In addition, since biochar has relatively high conductivity and pH, it can be considered as feasible to apply it as a remediation for acidic soils. Such feasibility has, in fact, already been an object of study, and positive results were obtained [55]. Finally, the results obtained from the thermogravimetric analysis of the different biochars produced at 300, 400, and 500 \({}^{\circ}\)C are presented (Figure 2). As the biochar comes from a plant (_Posidonia oceanica_) pyrolysis, the decomposition of three main components was expected: hemicellulose, which has a decomposition temperature between 220 and 315 \({}^{\circ}\)C; cellulose, whose decomposition occurs from 315 \({}^{\circ}\)C and up to about 400 \({}^{\circ}\)C; and, finally, lignin, whose decomposition is somewhat more difficult, in some cases requiring maximum temperatures of 900 \({}^{\circ}\)C to decompose [56]. As presumed, three peaks of matter decomposition were observed in the first biochar (300 \({}^{\circ}\)C) and in the third (500 \({}^{\circ}\)C). These peaks occurred at the expected temperature ranges, corresponding to the decomposition of hemicellulose, cellulose, and lignin.