## 3 Results and Discussions  
### Physicochemical Properties of BCs  
To analyze the morphology, SEM was conducted on it and the results are shown in Figure 1. The 700 HSBC and 700 PSBC exhibit a rich pore structure while the 350 HSBC and 350 PSBC have a limited pore structure, suggesting that the higher pyrolysis temperature is attributable to the formation of pore structures in biochar [24]. The relationship between temperature and the development of biochar pores is such that higher pyrolysis temperatures contribute to the creation of more intricate and well-defined pore structures in biochars. This leads to an increase in specific surface area and pore volume, ultimately enhancing the adoption capacity of the biochars. Besides, the higher pyrolysis biochars have a rougher surface than the lower pyrolysis biochars, which could enhance the adsorption capacity of biochars. Therefore, elevated temperatures play a crucial role in shaping biochar pore structures, which in turn impacts their adsorption performance. Figure 2 shows the Raman spectrum of BCs, which can be observed in two peaks. The disorder degree of biochar was corresponding to the D-band (1348 cm\({}^{-1}\)) while the G-band (1590 cm\({}^{-1}\)) was reflecting the graphitization degree [25,26]. The intensity of D and G peaks of the higher pyrolysis biochars is significantly stronger than that of the lower pyrolysis biochars, while the peak area ratio between D and G bands (ID/IG) of BC are similar. \begin{table}
\begin{tabular}{c c c c c c c c} \hline  & \(\mathbf{S_{BET}}\) & \(\mathbf{V_{t}}\) & \(\mathbf{D_{p}}\) (nm) & \(\mathbf{A_{1}}\) & \(\mathbf{D_{N1}=3+A_{1}}\) & \(\mathbf{A_{2}}\) & \(\mathbf{D_{N2}=3+A_{2}}\) \\ \hline
350 HSBC & 143.1 & 0.008714 & 3.14 & \(-\)0.215 & 2.785 & \(-\)0.016 & 2.984 \\
700 HSBC & 683.4 & 0.184537 & 3.56 & \(-\)0.349 & 2.651 & \(-\)0.043 & 2.957 \\
350 PSBC & 75.12 & 0.007810 & 7.4 & \(-\)0.119 & 2.881 & \(-\)0.038 & 2.962 \\
700 PSBC & 691.1 & 0.104923 & 3.21 & \(-\)0.436 & 2.564 & \(-\)0.027 & 2.973 \\ \hline \end{tabular}
\end{table} 
Table 2: Pore characteristics of the 700 HSBC, 700 PSBC, 350 HSBC and 350 PSBC, respectively. Figure 1: SEM image of (**a**) 700 HSBC, (**b**) 700 PSBC, (**c**) 350 HSBC and (**d**) 350 PSBC with 5 \(\upmu\)m magnification. The results reveal that the high pyrolysis temperature could affect both the graphitization degree and disorder degree of biochar. The identification of four distinct functional groups--Basic groups, Acidic groups, Phenolic hydroxyl groups, and Carboxyl groups--on the surface of BCs was established using the Boehm method, as illustrated in Figure 3. The analyses indicate that basic groups dominate as the most abundant groups across all four biochar materials. Subsequent high pyrolysis condition leads to a decrease in the proportion of alkaline groups, with acidic groups assuming the primary fraction, accompanied by an increase in the content of phenolic hydroxyl groups and carboxyl groups. Generally, at higher pyrolysis temperatures, there is a decrease in alkaline groups and an increase in acidic groups, phenolic hydroxyl groups, and carboxyl groups. This indicates that the pyrolysis temperature influences the transformation of functional groups in the biochars, leading to alterations in their chemical composition. Furthermore, the content of functional groups can influence the zeta potential of the biochars. Owing to the low content of acidic groups, all the biochars exhibited a negative zeta potential, even under acidic conditions (Figure 4). When materials are produced at high temperatures such as 700 degrees, fluctuations in their zeta potentials can occur due to various factors. Firstly, the high temperature can lead to changes in the material's structure, affecting its surface charge distribution and causing fluctuations in the zeta potential. Secondly, the properties of the material, including surface morphology and solubility, may change at high temperatures, which can also influence the zeta potential.