## 2 Results and Discussion
### Physiochemical Properties of Biochar  
#### 2.2.1 Biochar pH and Elemental Composition  
Table 2 shows the biochar pH and elemental composition of both materials obtained at various pyrolysis temperatures. Between 300 \({}^{\circ}\)C and 700 \({}^{\circ}\)C, the pH of the rice-husk biochar increased from acidic (pH = 5.31) to strongly basic (pH = 9.42), which was consistent with previous studies (pH = 5.37-9.82) [16], while the pH of the cotton-straw biochar increased from neutral (pH = 6.61) to strongly basic (pH = 10.42). A previous study found that the pH of biochar typically ranges from 4 to 12 [17], and the results of the present study fell within that range. A possible reason behind the pH increase with temperature is that the hemicellulose, cellulose, lignin, and other components of the raw materials undergo dehydration, dehydroylation, and other chemical reactions to produce acidic substances. At low temperatures, the amount of acidic substances that volatilises is relatively low, and thus, the remaining content in the biochar residues is relatively high. As the temperature increases, the volatilisation of highly acidic substances increases, and the pH correspondingly increases [18]. The relative content of acidic or basic substances in a sample is reflected in its pH value. The pyrolysis temperature range at which significant changes in the content of acidic and basic substances were observed was 300-500 \({}^{\circ}\)C.  
With regards to the elemental content in biochar produced at various pyrolysis temperatures, the highest content corresponded to carbon, followed by oxygen, hydrogen, and nitrogen, while the sulphur content was almost negligible. This was consistent with the results of many previous studies [19; 20]. With increasing pyrolysis temperature, the carbon content gradually decreased in the rice-husk biochar, but continuously increased in the cotton-straw biochar. The H/C ratio was indicative of the aromaticity of biochar: the smaller the H/C value, the stronger the aromaticity, and the stronger the biochar structure. The O/C and (O + N)/C ratios indicated the hydrophilicity and polarity of biochar. Higher O/C and (O + N)/C values mean stronger hydrophilicity and polarity [21]. From Table 2, it can be seen that, at higher pyrolysis temperatures, the H/C ratio of the two materials decreased, as well as the O/C and (O + N)/C ratios. Therefore, at higher temperatures, the aromaticity of the biochar obtained from these two materials increased, and its structure became more stable. On the other hand, the hydrophilicity and polarity decreased. The changes between 300 \({}^{\circ}\)C and 600 \({}^{\circ}\)C were relatively larger. The C/N ratio is an indicator of the speed of microbial degradation of organic matter. The higher the C/N value, the more difficult microbial degradation is, i.e., the stronger the anti-degradation properties of the material [22]. This value reached the maximum for both rice-husk and cotton-straw biochar samples at 600 \({}^{\circ}\)C.  
\begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline
**Biochar** & **RH300** & **RH400** & **RH500** & **RH600** & **RH700** & **CS300** & **CS400** & **CS500** & **CS600** & **CS700** \\ \hline Yield & 37.52 & 28.74 & 27.6 & 24.85 & 23.32 & 51.64 & 33.61 & 22.24 & 20.91 & 19.92 \\ (\%) & \(\pm\)0.01 & \(\pm\)0.01 & \(\pm\)0.01 & \(\pm\)0.00 & \(\pm\)0.00 & \(\pm\)0.03 & \(\pm\)0.02 & \(\pm\)0.02 & \(\pm\)0.00 & \(\pm\)0.01 \\ Ash content & 64.19 & 66.06 & 66.56 & 75.35 & 76.2 & 5.54 & 8.52 & 11.45 & 12.29 & 12.15 \\ (\%) & \(\pm\)0.01 & \(\pm\)0.00 & \(\pm\)0.01 & \(\pm\)0.01 & \(\pm\)0.01 & \(\pm\)0.00 & \(\pm\)0.00 & \(\pm\)0.02 & \(\pm\)0.01 & \(\pm\)0.02 \\ \hline \end{tabular}
\end{table}
Table 1: Biochar yield and ash content of the biochar samples obtained from rice husk (RH) and cotton straw (CS) at various pyrolysis temperatures (300, 400, 500, 600, and 700 \({}^{\circ}\)C).