## 3 Results and Discussion  
### Characterization of Biochar  
The physicochemical properties of the BCs produced at three different pyrolysis temperatures (i.e., 350, 550, and 750 \({}^{\circ}\)C) have been already discussed in detail in our previous report [9]. In brief, H, O, and N contents decreased with the increase of pyrolysis temperature. C contents also decreased in the current investigation due to the loss of volatile compounds [2]. Nevertheless, O/C and H/C values indicating hydrophobicity and aromaticity decreased with the increase of pyrolysis temperature [9]. In addition, the BET surface area of BCs gradually increased (2-191 m\({}^{2}\)/g) with higher pyrolysis temperature, indicating cracking of feedstock and development of the pore structures. Similarly, results have also been observed and reported for various BCs [2; 10]. The pH,pzc of BC350, BC550, and BC750 was 6.3, 9.2, and 9.3, respectively. These results imply that BCs have a different surface charge at thechosen solution pH [10; 20]. In addition, it is thought that the higher pH,pzc of BC550 and BC750 causes concentrated alkali salts (e.g., Ca, Mg, Na, K, etc.) by the increase of pyrolysis temperature [10; 20]. The XRD patterns of BCs were also analyzed and reported in our previous work [9]. The 2-theta diffraction peaks at 15, 24, 26, and 30 for MgCO\({}_{3}\) and 29, 35, 39, 43, 47, and 48 for CaCO\({}_{3}\) were detected [7; 9]. BC550 and BC750 showed higher CaCO\({}_{3}\) crystallinity, while BC350 showed higher MgCO\({}_{3}\) crystallinity. These results may imply that CaCO\({}_{3}\) leads to a remarkable dye adsorption. Please see the detailed discussion of adsorption mechanisms in Section 3.4. Effects of pH, Pyrolysis Temperature, and Adsorption Temperature on Adsorption Efficiency from RSM Results  
In the present study, the different operating conditions including pH (4, 7, and 10), adsorption temperature (20, 30, and 40 \({}^{\circ}\)C), and pyrolysis temperature (350, 550, and 750 \({}^{\circ}\)C) were evaluated using a Box-Behnken design to study their influence on dye adsorption efficiency. The response (adsorption efficiency, %) is presented as summarized in Table 3. As shown in Figure 1A-F, the two-dimensional (2D) contour plots describe the effect of various variables (pH, adsorption temperature, and pyrolysis temperature) on single dye adsorption efficiency. As shown in Figure 1A,C, the adsorption of MB was facilitated at the high solution pH of 9-10. These results are consistent with previous research using vermicompost-derived BCs and vegetal fiber activated carbons [2; 21]. When the solution pH is higher than pH,pzc (BCs: 6.3, 9.2, and 9.3), the surface of biochar becomes negatively charged, while it has a positive charge at a pH lower than pH,pzc. Therefore, the adsorption of MB could increase owing to the increase in the electrostatic attraction between negatively charged BCs and positively charged MB as a cationic dye at higher pH [17; 22; 23]. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Run**} & \multicolumn{3}{c}{**Independent Variables**} & \multicolumn{2}{c}{**Response (Adsorption Efficiency, \%)**} \\ \cline{2-6}  & **X1** & **X2 (\({}^{\circ}\)C)** & **X3 (\({}^{\circ}\)C)** & **Methylene Blue** & **Congo Red** \\ \cline{2-6}  & **pH** & \ \begin{tabular}{c} **Pyrolysis** \\ **Temperature** \\ \end{tabular}  & \begin{tabular}{c} **Adsorption** \\ **Temperature** \\ \end{tabular}  & **Experimental** & **Experimental** \\ \hline
1 & 4 & 350 & 30 & 11.38 \(\pm\) 0.54 & 52.61 \(\pm\) 3.37 \\
2 & 10 & 350 & 30 & 20.54 \(\pm\) 0.75 & 15.01 \(\pm\) 0.01 \\
3 & 4 & 750 & 30 & 13.84 \(\pm\) 1.64 & 98.70 \(\pm\) 2.40 \\
4 & 10 & 750 & 30 & 23.88 \(\pm\)These results were similar to the phenomena (the CR adsorption onto vermicompost-derived BCs) from the previous literature, but there was no effect of solution pH on the CR adsorption by rice straw, wood chip, and Korean cabbage-derived BCs [2,4].