## 3 Results and Discussion
### Hydrochar Solid Fraction
#### 3.1.2 Effect of Initial pH and Solid to Water Ratio  
Although the more relevant literature results identify reaction temperature and residence time as the main process parameters affecting the amount and chemical composition of the hydrochar produced, it is worth mentioning that both the solid to water ratio and the initial pH could play an important role, given that one of the goals of the HTC process is to break down the unbending structure of the starting biomass material into small and lower molecular weight chains. In fact, it is well known that addition of sulfuric acid or acetic acid in the reaction mixture can positively influence the HTC process by catalyzing hydrolysis reactions of cellulose and hemicellulose formed in the experimental conditions usually adopted. Regarding the effect of the initial pH, results reported in Table 3 show that the initial pH impacts the hydrochar yield, increasing as the higher acid concentration increases, with the highest yield of 30.12 wt% attained for SA-HC1, almost 50% higher than that observed in the reference experiment (180 \({}^{\circ}\)C, 60 min). However, these change slightly as the acetic acid concentration increases. Incidentally, we observe that CH3 COOH is a weak acid whereas H2 SO4 is a strong acid when the first dissociation constant is concerned. Results also suggest that rather than the pH itself, it is the type of additive used that affects the yield. Indeed, the change in hydrochar yields could be explained considering that during the sulfuric acid catalyzed hydrolysis, insoluble humins indistinguishable from HC products are formed, with a subsequent enhancement of the number of solid products obtained. On the contrary, the acetic acid is a natural by-product of HTC of lignocellulosic feedstocks, due to the hydrolysis and dehydration of cellulose and hemicellulose in the presence of subcritical water into short-chain organic acids: primarily acetic, formic, and lactic acid [38,39]. Therefore, a further addition of acetic acid in the reaction media, favoring the degradation of biomass components, leads to a higher heat production and a lower hydrochar yield. Results obtained in this study (see Table S1 in ESI) also indicate that the role of the biomass/water ratio on hydrochar yield is insignificant for all values investigated. This finding is in agreement with the results reported by [PERSON] et al. [40] for the HTC \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Observed Peak** & **Possible Functional Groups** & **Sample** & **B.E.T. S.A.** & **Pore Volume** & **Pore Radius** \\
**Intensity** & & & **(m\({}^{2}\)/g)** & **(cc/g)** & **Dv(r) (Å)** \\ \hline
3300 cm\({}^{-1}\) & O–H (alcohols, phenols, carboxylic acid) & S-HC\({}_{180-60}\) & 4.9 & 0.009 & 17.9 \\
3000 cm\({}^{-1}\) & C–H (aliphatic methyl) & S-HC\({}_{210-60}\) & 5.5 & 0.008 & 20.4 \\
1701 cm\({}^{-1}\) & C–O (ketone, aldehydes, amides) & S-HC\({}_{240-60}\) & 7.7 & 0.010 & 17.6 \\
1608 cm\({}^{-1}\) & C–C (aromatic rings, carbonyl, quinone, ester or carboxyl groups) & S-HC\({}_{270-60}\) & 9.1 & 0.011 & 17.8 \\
1527 cm\({}^{-1}\) & N–O (nitro) & & & & \\
1431 cm\({}^{-1}\) & COO– (carboxylate) & S-HC\({}_{300-60}\) & 18.4 & 0.029 & 22.4 \\
1120 cm\({}^{-1}\) & C–O of lignocellulosic & & & & \\
1026 cm\({}^{-1}\) & C–O (carboxylic acid, esters) & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: FTIR assignments and structural properties of the hydrochar samples at different reaction temperatures and a fixed reaction time of 60 min. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sample** & **pH 1** & **pH 2** & **Hydrochar Yield (wt\%)** \\ \hline S-H\({}_{180-60}\) & - & 3.60 & 18.74 \\ SA-HC1 & 1 & 1.61 & 30.62 \\ SA-HC2 & 2 & 1.89 & 27.36 \\ AA-HC1 & 1 & 1.20 & 21.51 \\ AA-HC2 & 2 & 1.80 & 19.39 \\ AA-HC3 & 3 & 2.40 & 15.24 \\ \hline \hline \end{tabular} \({}^{\dagger}\) theoretically calculated; 2 experimentally measured.  
\end{table} 
Table 3: Hydrochar yields as a function of initial pH.