## 3 Results and Discussion
### Physical Properties of the Biochars
[28] reported lower SSA values, from 18.6 to 36.4 m\({}^{2}\)/g, in unwashed biochars made of cocoa shell and corncob, using a kil to produce biochar at 350 \({}^{\circ}\)C for 3.5 h. [PERSON] [15] pointed out that SSA usually increases with biochar production temperature along with the number of micropores (<2 nm diameter), which \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & \multicolumn{2}{c}{**Specific Surface Area**} & \multicolumn{2}{c}{**Pore Volume**} & \multicolumn{2}{c}{**Mean Pore**} & \multicolumn{2}{c}{**Skeletal**} \\  & \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & **Size** & **Density** \\ \hline \multirow{2}{*}{**Sample**} & **A-BET** & **Micropore** & \multirow{2}{*}{**BJH (cm\({}^{3}\)/g)**} & **Micropores** & **Dp,BJH-D** & **(g/cm\({}^{3}\))** \\  & **(m\({}^{2}\)/g)** & **(m\({}^{2}\)/g)** & & **(cm\({}^{3}\)/g)** & **(nm)** & **(g/cm\({}^{3}\))** \\ \hline BB1 & \(144\pm 7\) & \(108\pm 5\) & 0.009 & 0.050 & 4.4 & \(1.764\pm 0.010\) \\ BB2 & \(81\pm 4\) & \(50\pm 2\) & 0.007 & 0.023 & 4.8 & \(1.690\pm 0.013\) \\ BB3 & \(54\pm 3\) & \(31\pm 2\) & 0.005 & 0.014 & 3.4 & \(1.656\pm 0.013\) \\ WB1 & \(267\pm 13\) & \(159\pm 8\) & 0.021 & 0.074 & 4.0 & \(1.591\pm 0.013\) \\ WB2 & \(264\pm 13\) & \(199\pm 10\) & 0.024 & 0.092 & 4.0 & \(1.609\pm 0.015\) \\ WB3 & \(234\pm 12\) & \(169\pm 8\) & 0.027 & 0.078 & 4.9 & \(1.537\pm 0.012\) \\ \hline \hline \end{tabular}  
* A-BET: specific surface area by Brunauer–Emmett–Teller method, BJH specific pore volume by the Barrett-Joyner-Halenda method, Dp,BJH-D: mean pore diameter by the Barrett-Joyner-Halenda method from the desorption curve data.  
\end{table} 
Table 3: Porosity and specific surface area of bamboo biochar (BB) and pinewood biochar (WB) with three replicates each (1, 2 and 3). Figure 2: Comparison of the SEM micrograph magnifications at 300\(\times\) and 800\(\times\) of BB1 and WB1 as examples of bamboo biochar (**a,b**) and pinewood biochar (**c,d**), respectively. Scanning electron microscope image of sample BB1 at (**a**) 300 \(\upmu\)m and (**b**) 100 \(\upmu\)m resolution and sample WB1 at (**c**) 300 \(\upmu\)m and (**d**) 100 \(\upmu\)m resolution. are more likely to significantly contribute to SSA, according to [PERSON] and [PERSON] [21]. Biochar skeletal densities exhibit values from 1.537 to 1.764 g/cm\({}^{3}\), consistent with results of [PERSON] [17], who reported 1.68 g/cm\({}^{3}\) for cellulose biochar produced at 700 \({}^{\circ}\)C. According to [PERSON] et al. [5], raising the pyrolysis temperature led to raised total pore volume but lowered average pore size in biochars produced from switchgrass, water oak and biosolids. Hydrophobicity of the biochars differed significantly. Figure 3 compares the time dependent contact angle of smooth planar surfaces covered with fine dust of the biochars, performed by the sessile drop method [50], as an approach to measure persistence of the surface repellency to a small drop of water. The longer the persistence, the higher hydrophobicity of the material. The 90-degree line defines a magnitude corresponding to a critical water repellency, so a material is hydrophilic when angles are below 90\({}^{\circ}\), and hydrophobic if above (e.g., 110\({}^{\circ}\)). Complete wettability of a surface produced a null degree angle. Overall, the initial contact angles (\(\alpha_{i}\)) were near 100\({}^{\circ}\) for the bamboo chars, and slightly above that for the pinewood chars. The bamboo biochars rapidly became hydrophilic (\(\alpha<\) 90\({}^{\circ}\)), before 100 s, whereas the pinewood biochars became hydrophilic after 500-800 s. The pinewood biochars (WB) were clearly more hydrophobic (i.e., \(>\)90\({}^{\circ}\) up to 600 s) than the bamboo biochars (BB), and differences within the groups were smaller. BB3 was the least and BB1 the most hydrophobic of the bamboo  
Figure 3: Hydrophobicity analysis by sessile drop method (SDM) of (**a**) bamboo biochars, BB and (**b**) pinewood biochars, WB.