## 3 Results and Discussion
### The Effect of Biomass Particle Size on Biochar Surface Properties  
Figure 2A shows that biochar from particles with 2 mm size had a pH of 1.1, which increased to 12.0 when increasing the average particle size to 7 mm. However, further increasing the particle size from 7 to 30 mm led to reduced pH from 12.0 to 2.3. Likewise, the reaction temperature was strongly positively correlated with the pH (R\({}^{2}\) = 0.91) (Figure 2B). Therefore, the variation in particle size and temperature not only contributed to the changing pH of the biochar, but they also affected its surface chemistry because of the extreme pH levels, as presented in Table 4. The carboxylic acidic functional groups of biochar decreased from 0.012 to 0 mmol g\({}^{-1}\) when the particle size increased from 2 to 7 mm, whereas further increasing the particle size from 7 to 30 mm caused an increase of carboxylic functional groups from 0 to 0.012 mmol g\({}^{-1}\). Opposite to this, the basic functional group was maximum at the particle size of 7 mm and decreased to 0.012 mmol g\({}^{-1}\) at smaller or larger particles. Acidic biochars exhibited carboxylic and basic functionalities which were correlated with the reaction temperature with R\({}^{2}\) of 0.88 and 0.73, respectively. This was different from the airflow and moisture content experiments that only had the basic functional groups. In addition, pH was found to be correlated with the carboxylic and basic functional groups with R\({}^{2}\) of 0.96 and 0.63, respectively. Variations in the carbonization temperature have been observed to play an important role in the surface chemistry of biochar, and these changes can be initially identified by measuring the pH of the carbonized materials. This effect has been widely reported in the literature [11; 37]. The increase in peak temperature from 657 to 840 \({}^{\circ}\)C when the particle size increased from 2 to 7 mm indicated an increase in the oxidation of the biomass (Figure 2B). This degree of oxidation can be observed in the decrease of the H/C atomic ratio from 0.35 to 0.27 that suggested a decrease in polar functional \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Moisture Content (\%)**} & **Basic Functional Groups** & \multicolumn{2}{c}{**BET Surface Area**} & \multicolumn{2}{c}{**Atomic Ratios**} \\ \cline{3-4}  & **(mmol g\({}^{-1}\))** & **(m\({}^{2}\) g\({}^{-1}\))** & **H/C** & **O/C** \\ \hline
10 & 0.115 \({}^{\circ}\) & 405.3 & 0.271 \({}^{\mathrm{a}}\) & 0.100 \({}^{\mathrm{a}}\) \\
14 & 0.135 \({}^{\mathrm{b}}\) & 380.3 & 0.200 \({}^{\mathrm{a}}\) & 0.068 \({}^{\mathrm{b}}\) \\
18 & 0.137 \({}^{\mathrm{a}}\)\({}^{\mathrm{b}}\) & 384.0 & 0.200 \({}^{\mathrm{a}}\) & 0.065 \({}^{\mathrm{b}}\) \\
22 & 0.150 \({}^{\mathrm{a}}\) & 352.8 & 0.186 \({}^{\mathrm{a}}\) & 0.075 \({}^{\mathrm{b}}\) \\ \hline \hline \end{tabular} Different superscripts represent significant differences in the order of a \(>\) b \(>\) c.  
\end{table} 
Table 3: Atomic ratios and basic functional groups of biochar from pine woodchips produced at varying biomass moisture contents. BET: Brunauer-Emmett-Teller. Figure 2: (**A**) pH of biochar produced from wood chips with varying particle sizes (airflow 20 L/min, moisture content 10%, and biomass compactness 0 kg), and (**B**) the correlation between pH and reaction temperature. Different letters represent significant difference level at 10%. groups (e.g., carboxylic) [1]. Likewise, it was evident that the acidic condition of the biochar was not achieved at 7 mm particle size (pH = 12.04). In the opposite way, the increase in particle size from 7 to 30 mm resulted in lower peak temperatures from 840 to 614 \({}^{\circ}\)C. Consequently, the carboxylic functional group increased, and the H/C ratio raised from 0.27 to 0.54 which was also correlated with the reaction temperature (R\({}^{2}\) = 0.72).