## 2 Results and Discussion
### Physiochemical Properties of Biochar
#### 2.2.3 Infrared Spectra of Biochar  
FTIR spectroscopy offers great advantages for the determination of functional groups and structural changes in materials [26]. Figure 2 shows the results of the FTIR analysis (wavenumber range of 4000-450 cm\({}^{-1}\)) of the biochar samples obtained from both materials at various pyrolysis temperatures. According to IR data tables [27] and previous studies [28; 29], the bands at 3300-3500 cm\({}^{-1}\) correspond to the vibration of hydroxyl groups, with that at 3468 cm\({}^{-1}\) mainly due to the vibration of phenolic hydroxyl groups. The bands at 2918-2850 cm\({}^{-1}\) correspond to the symmetrical or asymmetrical stretching vibration of the -CH\({}_{3}\) and -CH\({}_{2}\) groups of aliphatic or naphthenic hydrocarbons, and the bands at 1730-1600 cm\({}^{-1}\) correspond to the stretching vibration of carboxyl C=O groups or C=C and C=O groups in aromatic rings. The bands at 1300-1000 cm\({}^{-1}\) are attributed to the stretching vibration of C-O moieties often present in phenols or hydroxyl groups. However, some researchers believe that this band corresponds to the vibration of the C-O-C pyran ring skeleton. The bands at 797 and 466 cm\({}^{-1}\) correspond to the vibration absorption of Si-O-Si. From Figure 2, we can see the bands for hydroxyl and phenolic hydroxyl groups (-OH), -CH\({}_{3}\) and -CH\({}_{2}\) groups of aliphatic or naphthenic hydrocarbons, carboxyl C=O groups, and the C=C and C=O groups of aromatic rings in the biochar samples from rice husk and cotton straw. However, the Si-O-Si vibration absorption band present in the rice-husk biochar was not present in the cotton-straw samples. In the rice-husk biochar, the intensity of the -OH, -CH\({}_{3}\), -CH\({}_{2}\), C=C, and C=O absorption bands decreased with increasing pyrolysis temperature. This indicates that the presence of these groups in the samples gradually decreased. The changes in the absorption-band intensity for the C-O and Si-O-Si groups were not significant. In the cotton-straw biochar, the absorption-band intensity of the -OH, -CH\({}_{3}\), -CH\({}_{2}\), C=C, and C=O groups firstly increased before decreasing. It can be concluded that these fingerprint regions were relatively complex.  
\begin{table}
\begin{tabular}{c c c c} \hline
**Biochar** & **Brunauer-Emmett-Teller (BET)** & **Mean Pore Size (nm)** & **Micropore Volume** \\  & **Area (m\({}^{2}\)-g\({}^{-1}\))** & **(cm\({}^{3}\)-g\({}^{-1}\))** \\ \hline RH300 & 78.401 & 3.697 & 0.009 \\ RH400 & 101.295 & 3.670 & 0.008 \\ RH500 & 292.595 & 3.772 & 0.015 \\ RH600 & 377.717 & 3.815 & 0.032 \\ RH700 & 406.244 & 3.820 & 0.049 \\ CS300 & 35.258 & 3.417 & 0.010 \\ CS400 & 117.536 & 3.065 & 0.011 \\ CS500 & 300.103 & 4.096 & 0.013 \\ CS600 & 500.807 & 3.807 & 0.014 \\ CS700 & 553.709 & 3.785 & 0.052 \\ \hline \end{tabular}
\end{table}
Table 3: Biochar specific surface area, mean pore size, and micropore volume for the materials obtained at various pyrolysis temperatures.  
Figure 2: Fourier-transform infrared (FTIR) spectra of the biochar samples obtained at various pyrolysis temperatures. Notes: RH300, RH400, RE500, RH600, and RH700 indicate that rice husk were pyrolyzed at 300, 400, 500, 600 and 700 \({}^{\circ}\)C, respectively; CS300, CS400, CS500, CS600, and CS700 indicate that cotton straw were pyrolyzed at 300, 400, 500, 600 and 700 \({}^{\circ}\)C, respectively.