## 3 Results  
### The Effect of Pyrolysis Temperature  
The effect of different pyrolysis temperatures (450 \({}^{\circ}\)C, 500 \({}^{\circ}\)C, 550 \({}^{\circ}\)C, 600 \({}^{\circ}\)C) on the biochar adsorption is shown in Figure 3. These experiments were conducted at a heating rate of 0.5 \({}^{\circ}\)C/s, a temperature holding time of 10 min and without additive.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**No.**} & \multicolumn{3}{c}{**Factor**} \\ \cline{2-4}  & **Factor** & **Level** & **Condition** \\ \hline
1–4 & A (pyrolysis temperature \({}^{\circ}\)C) & 450, 500, 550, 600 & B = 0.5 \({}^{\circ}\)C/s, C = 10 min, D = None \\
5–8 & B (heating rate \({}^{\circ}\)C/s) & 0.5, 1, 1.5, 2 & A = 450 \({}^{\circ}\)C, C = 10 min, D = None \\
9–12 & C (temperature holding time min) & 10, 15, 20, 25 & A = 450 \({}^{\circ}\)C, B = 0.5 \({}^{\circ}\)C/s, D = None \\
13–16 & D (additive) & None, Char, ZnCl\({}_{2}\), Na\({}_{2}\)CO\({}_{3}\) & A = 450 \({}^{\circ}\)C, B = 0.5 \({}^{\circ}\)C/s, C = 10 min \\ \hline \hline \end{tabular}
\end{table}
Table 2: The single factor experiment conditions.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**No.**} & \multicolumn{2}{c}{**A: Pyrolysis Temperature**} & \multicolumn{2}{c}{**B: Heating Rates (\({}^{\circ}\)C/s)**} & \multicolumn{2}{c}{**C: Temperature**} & \multirow{2}{*}{**D: Additive**} & \multirow{2}{*}{**Biochar Yield**} & \multicolumn{2}{c}{**Adsorbing Capacity**} \\  & \multicolumn{2}{c}{(\({}^{\circ}\)C)} & & & & & \\ \hline
1 & 450 & 0.5 & 10 & None & 5.00\% & 6.989 \\
2 & 450 & 1 & 15 & Char & 3.66\% & 7.507 \\
3 & 450 & 1.5 & 20 & ZnCl\({}_{2}\) & 3.19\% & 17.603 \\
4 & 450 & 2 & 25 & Na\({}_{2}\)CO\({}_{3}\) & 2.86\% & 13.979 \\
5 & 500 & 0.5 & 15 & ZnCl\({}_{2}\) & 2.55\% & 27.958 \\
6 & 500 & 1 & 10 & Na\({}_{2}\)CO\({}_{3}\) & 2.39\% & 41.160 \\
7 & 500 & 1.5 & 25 & None & 2.72\% & 41.936 \\
8 & 500 & 2 & 20 & Char & 2.68\% & 15.791 \\
9 & 550 & 0.5 & 20 & Na\({}_{2}\)CO\({}_{3}\) & 2.88\% & 45.158 \\
10 & 550 & 1 & 25 & ZnCl\({}_{2}\) & 2.97\% & 39.348 \\
11 & 550 & 1.5 & 10 & Char & -1.42\% & 10.096 \\
12 & 550 & 2 & 15 & None & 2.82\% & 140.564 \\
13 & 600 & 0.5 & 25 & Char & 1.72\% & 16.567 \\
14 & 600 & 1 & 20 & None & 5.00\% & 62.128 \\
15 & 600 & 1.5 & 15 & Na\({}_{2}\)CO\({}_{3}\) & 3.66\% & 9.060 \\
16 & 600 & 2 & 10 & ZnCl\({}_{2}\) & 3.19\% & 67.305 \\ \hline \hline \end{tabular}
\end{table}
Table 3: The orthogonal experiment schedule and result.  
As shown in Figure 3, with the increasing pyrolysis temperature, the biochar yield decreases, but the adsorbability of the biochar increases. The adsorbability of biochar reached a peak at a pyrolysis temperature of 600 \({}^{\circ}\)C, and the highest adsorbing capacity was 45.302 mg/g. It is worth mentioning that the moisture content of the sample was about 98.16%. After pyrolysis, the maximum biochar yield was 5%, and the actual conversion rate was 63.8%, which was relatively high. High temperature can promote the secondary reaction of biochar with other products and vapor in the reactor, thus the biochar yield decreases as the pyrolysis temperature increases. The influence of the pyrolysis temperature on the adsorbability of biochar was not obvious, but from 550 \({}^{\circ}\)C to 600 \({}^{\circ}\)C, the adsorbability of biochar increased significantly. It may suggest that a high pyrolysis temperature can promote a secondary reaction of biochar with the gas and liquid products [13,18], thus the biochar structure was optimized and the adsorbability of the biochar rose significantly. Combined with the result of carbon yield, the variation trend of the biochar yield reduced after 550 \({}^{\circ}\)C, which means that the secondary reaction was roughly completed and this also proved that a secondary reaction could improve the adsorbability of biochar to some extent.