## 2 Results and Discussion
### Physiochemical Properties of Biochar
#### 2.2.2 Electron Microscopy Analysis of Biochar  
With a scanning electron microscope, we can observe the real, high-resolution, three-dimensional (3D) surface of biochar [23]. From Figure 1, we can see that the number of surface pores gradually increased with pyrolysis temperature, and a mesh structure appeared before transforming into a bar-like structure. Finally, the structure collapsed and fragmented. The temperature at which large amounts of micropores appeared in the rice-husk biochar was 400-500 \({}^{\circ}\)C. At 600 \({}^{\circ}\)C, the bar-like structure was evident, and the structure finally fragmented at 700 \({}^{\circ}\)C. Microporce development was not that evident on the surface of the cotton-straw biochar at 300-500 \({}^{\circ}\)C. A large number of micropores appeared instead at 600 \({}^{\circ}\)C, followed by severe fragmentation at 700 \({}^{\circ}\)C. A gas-adsorption method was used to measure the specific surface area (Brunauer-Emmett-Teller (BET) surface area) and the pore size of the various biochar samples (Table 3). The results show that the BET area for rice-husk biochar increased the most at 400-500 \({}^{\circ}\)C, with a value of 191.30 m\({}^{2}\cdot\)g\({}^{-1}\), while that of cotton-straw biochar exhibited the greatest increase at 500-600 \({}^{\circ}\)C, with values reaching 200 m\({}^{2}\cdot\)g\({}^{-1}\). The mean pore size also slightly increased with temperature. The difference between the maximum and minimum pore size was 0.123 nm for rice husk, and 0.579 nm for cotton straw. The micropore volume of rice-husk biochar gradually increased with pyrolysis temperature. On the other hand, the micropore volume did not show large changes at 300-600 \({}^{\circ}\)C for cotton-straw biochar, although it increased three-fold at 700 \({}^{\circ}\)C. Such increases in the number of micropores and the specific surface area with increasing pyrolysis temperature [24] were already described in previous studies; however, there are large differences in the exact values. This may be directly associated with the material source, pyrolysis temperature, and pyrolysis duration [25].  
\begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \multirow{2}{*}{**Biochar**} & \multirow{2}{*}{**pH**} & \multicolumn{6}{c}{**Element Content (\%)**} & \multicolumn{6}{c}{**Atomic Ratio**} \\ \cline{3-10}  & & **N** & **C** & **H** & **O** & **S** & **H/C** & **O/C** & **(O + N)/C** & **C/N** \\ \hline RH300 & 5.31 & 0.67 & 45.99 & 2.48 & 26.55 & 0.01 & 0.05 & 0.58 & 0.59 & 68.64 \\ RH400 & 6.49 & 0.57 & 44.23 & 2.42 & 20.46 & 0.01 & 0.05 & 0.46 & 0.48 & 77.60 \\ RH500 & 8.15 & 0.54 & 24.63 & 2.00 & 14.87 & 0.10 & 0.05 & 0.35 & 0.36 & 78.94 \\ RH600 & 9.20 & 0.39 & 38.84 & 1.40 & 10.03 & 0.03 & 0.04 & 0.26 & 0.27 & 99.59 \\ RH700 & 9.42 & 0.31 & 28.44 & 1.00 & 7.62 & 0.10 & 0.04 & 0.27 & 0.28 & 91.74 \\ CS300 & 6.61 & 1.23 & 64.04 & 3.81 & 29.36 & 0.06 & 0.06 & 0.46 & 0.48 & 52.07 \\ CS400 & 8.51 & 1.34 & 70.23 & 3.34 & 22.00 & 0.06 & 0.05 & 0.31 & 0.33 & 52.41 \\ CS500 & 9.72 & 1.01 & 74.24 & 3.18 & 17.99 & 0.30 & 0.04 & 0.24 & 0.26 & 73.50 \\ CS600 & 10.24 & 0.94 & 79.15 & 2.41 & 14.46 & 0.23 & 0.03 & 0.18 & 0.19 & 84.20 \\ CS700 & 10.42 & 1.03 & 77.13 & 2.13 & 16.39 & 0.57 & 0.03 & 0.21 & 0.23 & 74.88 \\ \hline \end{tabular}
\end{table}
Table 2: Biochar pH and elemental composition of the biochar materials obtained at various pyrolysis temperatures.  
Figure 1: Scanning electron microscopy (SEM) images of the biochar materials obtained from rice husk (RH) and cotton straw (CS) at various pyrolysis temperatures (300, 400, 500, 600, and 700 \({}^{\circ}\)C).