## 3 Results
### SEM Representation  
The three SEM tests were from the repeat test in a single factor experiment (pyrolysis temperature 450 \({}^{\circ}\)C, heating rate 1 \({}^{\circ}\)C/s, temperature holding time 10 min, without additive); metal-carbonate-added test in a single factor experiment (pyrolysis temperature 450 \({}^{\circ}\)C, heating rate 1 \({}^{\circ}\)C /s, temperature holding time 10 min, Na\({}_{2}\)CO\({}_{3}\) additive) and the optimal condition test (pyrolysis temperature 550 \({}^{\circ}\)C, heating rate 2 \({}^{\circ}\)C/s, temperature holding time 15 min, without additive). For the SEM, the amplification factor is 10,000, the scale length is 8 \(\upmu\)m, and images are shown in Figure 7.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Test No.** & **Adsorbing Capacity mg/g** & **AVG (Average) mg/g** & **SD (Standard Deviation) mg/g** \\ \hline
1\({}^{\ast}\) & 140.564 & & \\
2 & 142.172 & 140.038 & 2.440 \\
3 & 137.378 & & \\ \hline \hline \end{tabular} \({}^{\ast}\): The result was picked from the orthogonal experiment.  
\end{table}
Table 6: The verification test results.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Variance** & **Sum Of** & **DOF (Degree** & **MS (Mean** & & \\
**Squares** & **Of Freedom)** & **Square)** & **F-Value** & **Significance** \\ \hline A & 4591.9187 & 3 & 1530.64 & 1.599 & \begin{tabular}{c} Insignificant \\ Insignificant \\ Extremely significant \\ \end{tabular} \\ B & 3833.0697 & 3 & 1277.69 & 1.335 & \
\begin{tabular}{c} Insignificant \\ Insignificant \\ Extremely significant \\ \end{tabular} \\ C & 759.8648 & 3 & 253.288 & 0.265 & Extremely significant \\ D & 5580.7002 & 3 & 1860.23 & 1.943 &
\begin{tabular}{c} Insignificant \\ \end{tabular} \\ Deviation & 2871.7577 & 3 & 957.253 & & \\ \hline \hline \end{tabular} Note: F\({}_{0.01}\)(3,3) = 29.46, F\({}_{0.05}\) (3.3) = 9.28  
\end{table}
Table 5: The orthogonal experiment results analysis of variance table.  
Figure 7: SEM (scanning electron microscope) images of biochar: (**a**) repeat test group in single factor experiment; (**b**) metal-carbonate-added test group; (**c**) the optimal condition test.  
Figure 7a shows the images of the repeat test group in a single factor experiment with an adsorbing capacity of 33.911 mg/g (pyrolysis temperature 450 \({}^{\circ}\)C, heating rate 1 \({}^{\circ}\)C/s, temperature holding time 10 min, without additive). As can be seen in Figure 7a, the surface pores of the biochar are quite small, the diameters of those pores are less than 1 \(\upmu\)m and have a non-uniform distribution.  
Figure 7b represents the result of the metal-carbonate-added test group in a single factor experiment with an adsorbing capacity of 43.748 mg/g (pyrolysis temperature 450 \({}^{\circ}\)C, heating rate 1 \({}^{\circ}\)C/s, temperature holding time 10 min, Na\({}_{2}\)CO\({}_{3}\) additive). As can be seen in Figure 7b, the number of biochar surface pores is larger than that in Figure 7a and the surface pores are more structured and evenly distributed although the surface pores are shallow.  
Figure 7c shows an image of the optimal condition test with an adsorbing capacity of 140.564 mg/g (pyrolysis temperature 550 \({}^{\circ}\)C, heating rate 2 \({}^{\circ}\)C/s, temperature holding time 15 min, without additive). The image shows that in this test, biochar has the largest number of surface pores, the biggest pore diameter, the deepest surface pores, and the most even distribution.