## 3 Results and Discussion
### Effects of Single Dye and Dye Mixtures on Adsorption Efficiency at the Chosen Conditions  
Predicted values of various variables for optimum adsorption efficiency of MB and CR presented in Figure 2. In the case of MB, pH 9.4, pyrolysis temperature 673 \({}^{\circ}\)C, and 40 \({}^{\circ}\)C of adsorption temperature were predicted as the best conditions for MB adsorption, implying 45% of adsorption efficiency and 90 mg MB/g BC of adsorption capacity (Figure 2A). In contrast, Figure 1B,D shows the influence of solution pH between the adsorption of CR and biochar. The adsorption of CR was higher at lower pH, indicating the BCs are protonated when the solution pH (4-6) is lower than the pH,pzc of the BCs (6.3, 9.2, and 9.3). The electrostatic attraction between the positively charged BCs and the negatively charged CR as an anionic dye will be strengthened with pH decrease, resulting in the increase of CR adsorption. Figure 2: Predicted values of different variables (pH, pyrolysis temperature (PT, \({}^{\circ}\)C), and adsorption temperature (AT, \({}^{\circ}\)C)) for optimum adsorption efficiency. (**A**): MB, (**B**): CR). Symbol (\(\downarrow\)) is the chosen conditions for the adsorption of mixtures of MB and CR. The MB adsorption capacity of maple-derived BCs used in this investigation was higher than that of anaerobic digestion residue-derived BCs (9.5 mg MB/g BC), sewage and tea waste-derived BCs (12.5 mg MB/g BC), pulp and paper sludge-derived BCs (33 mg MB/g BC), and sewage sludge-derived BCs (14-45 mg MB/g BC) but lower than that of vermicompost-derived BCs (174 mg MB/g BC), orange peel (526 mg/g BC), and banana biomass (426 mg/g BC) BCs [2; 7; 17; 24; 25]. These results indicate that the maple-derived BCs possessed significant ability (e.g., functional groups, metals, and electrostatic attraction) with regard to MB adsorption. As shown in Figure 2B, the CR adsorption of maple-derived BCs was great at pH 4, 550 \({}^{\circ}\)C of pyrolysis temperature, and 30 \({}^{\circ}\)C of adsorption temperature. At these conditions, the CR adsorption efficiency and capacity were 99% and 198 mg CR/g BC, respectively. According to previous studies, the CR adsorption capacity by rice straw, vermicompost, wood chip-derived BCs was 190, 31, and 110 mg CR/g BC, respectively, and for activated carbon it was 769 mg CR/g BC [2; 4]. The CR adsorption mechanisms by these BCs often contained electrostatic interaction and oxygen-containing functional groups, indicating the chemical sorption between BCs and CR [2]. The chosen conditions (i.e., solution pH 7, 30 \({}^{\circ}\)C of adsorption temperature, and 550 \({}^{\circ}\)C of pyrolysis temperature) were selected to evaluate the effect of single dye and dye mixture on adsorption efficiency according to the RSM results (Figure 3). In particular, among them, the initial pH of the dye solution was set at pH 7 to avoid the additional use of chemicals for the adjustment of pH and to reduce the bias to acid and base. Moreover, the adsorption and pyrolysis temperatures were chosen for the experiment, taking into account the cost. At these chosen conditions, the adsorption efficiencies of both MB and CR by single reaction were 25% (adsorption capacity: 49 mg MB/g BC) and 97% (adsorption capacity: 195 mg CR/g BC), respectively, as shown in Figure 3. On the other hand, the adsorption efficiency for each dye in a dye mixture reaction appeared as 68% for MB and 74% for CR, indicating the increase of MB adsorption efficiency and the decrease of CR adsorption efficiency compared to those of a single reaction. These results suggest that the competition and the reaction time for the adsorption of each dye on the active sites of the BC surface may be possible causes, although it could not be fully understood. Based on these finding, this biochar-based adsorptive approach for the treatment of dye mixtures can be applied, depending on the characteristics of dye wastewater such as pH, initial concentration of each dye, and sorts of dye.