## 3 Results and Discussion  
### Characterization of BCs  
The effects of pyrolysis temperature on the physicochemical properties (basic composition, structural properties and surface functional groups) of BCs were studied. The surface morphologies of BCs were observed via SEM (Figure 1A-G). Obviously, BC samples after grinding all show irregular morphologies. Irregular-shaped protrusions were observed on the surfaces of CPP200, CPP250, CPP300 and CPP350 that were pyrolyzed at lower temperatures. When the pyrolysis temperatures were higher, BCs' surfaces became smoother. The micropores of the samples after grinding were further detected using BET analysis. As shown in Figure 2A, the specific surface area (SSA) and the total pore volume (TPV) of CPP200 were calculated to be 1.1931 m\({}^{2}\)/g and 0.013629 cm\({}^{3}\)/g, respectively. Both the SSA and TPV of BCs increased as pyrolysis temperature increased, which were 1.6900 m\({}^{2}\)/g and 0.017045 cm\({}^{3}\)/g for CPP500, respectively. Obviously, the TPV became larger with an increase in pyrolysis temperature. The pore sizes and size distributions present in theBCs determined from low-temperature N\({}_{2}\) adsorption/desorption measurements were also affected, indicating that the thermal decomposition of more volatile components occurred under higher temperatures, and high levels of carbonization were implemented (Figure 2B,C). SEM images of CPP200 post-adsorption of MB are presented in Figure 1H. CPP200 seems to thicken as a result of MB adsorption onto the carbonaceous surface. The observed CPP200-MB with varied thickness might be related to an uneven distribution of polar functional groups, such as -COOH and -OH, onto the surface of CPP200. The elemental compositions of BCs were also analyzed (Table 1). The higher the pyrolysis temperature used, the more the volatile molecules that were released. That is, higher levels of carbonization occurred at higher pyrolysis temperatures. In the pyrolysis temperature range of 200-350 \({}^{\circ}\)C, the atomic ratio of carbon (C) in BCs remained stable at lower levels (61.09-71.14 at.%), which was much lower than those prepared at higher pyrolysis temperatures in the range of 400-500 \({}^{\circ}\)C. In contrast, the atomic ratios of oxygen (O) (25.54-37.35 at.%) of BCs that were prepared at lower temperatures were higher than those (11.30-15.63 at.%) prepared at higher temperatures. Due to the homogeneity of raw materials, the atomic ratio of sulfur (S) varied without regularity (Figure 3a-c). Alkali metal elements could be detected in BC samples (Figure 3d,e). The CEC value of CPP200 was determined by the sum of all cations to be 0.0218 cmol\({}_{\text{c}}\)/g, and the contents of alkali metal elements decreased significantly after the adsorption process, confirming that the water-soluble salts of BCs would be released [29]. The newly appeared nitrogen (N) content as well as the increased S content in the sample after adsorption of MB confirmed the successful adsorption of N- and S-containing MB. Figure 2: (**A**) N\({}_{2}\) adsorption/desorption isotherms for CPP200 and CPP500; (**B**) BJH pore volume distribution of CPP200; (**C**) BJH pore volume distribution of CPP500. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**BCs** & **C** & **O** & **S** & **Mg** & **K** & **Ca** & **N** \\ \hline CPP200 & & & & 0.260 & 0.510 & 0.280 & - \\ CPP250 & 61.1–71.1 & 25.5–37.4 & 0.200–0.320 & 0.0900 & 0.990 & 0.170 & - \\ CPP300 & & & 0.150 & 2.38 & 0.330 & - \\ CPP350 & & & 0.220 & 1.28 & 0.250 & - \\ CPP400 & & & 0.110 & 5.01 & 0.200 & - \\ CPP450 & 78.3–85.3 & 11.3–15.6 & 0.320–0.550 & 0.150 & 2.40 & 0.220 & - \\ CPP500 & & & 0.0400 & 2.72 & 0.0300 & - \\ CPP200-MB & 62.3 & 36.5 & 0.810 & 0.0400 & - & 0.0100 & 0.100 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Elemental compositions (at.%) of BC samples prepared by vacuum pyrolysis and CPP200 post-adsorption of MB. Figure 3: Elemental mapping of CPP200: (**a**) C; (**b**) O; (**c**) S; (**d**) Ca; and (**e**) Mg.