# Regeneration of Activated Carbons Spent by Waste Water Treatment Using KOH Chemical Activation
## 3 Results and Discussion  
N\({}_{2}\) adsorption and desorption isotherms of all the curves for S-AC, H-AC, C-AC-W, and C-AC-A at 77 K followed the type IV isotherms (Figure 2a) [23]. In addition, the empty symbols, which depict desorption curves (Figure 2a), had a slightly different pattern with adsorption. As shown in Figure 2b, the pore size distribution of ACs micropore clearly mixed the micropore and mesopores [24]. Figure 1: Schematic of experimental setup used for performing adsorption test. The quantity absorbed in the y-axis was dependent on the specific surface area. The N\({}_{2}\) adsorption quantity of H-AC was slightly higher than that of S-AC, which means the surface area rarely increased, even when thermally treated. The volatile compounds adsorbed on the carbon surface could be removed by the thermal treatment, but the surface area slightly increased. This is because the volatiles within the pores evaporated, while the pores simultaneously shrunk (Figure 2b). With the exception of H-AC, the pores of the ACs existed between 1-3 nm. The pores of H-ACs were distributed between 0.4-0.5 nm and 1-2 nm. This suggests that the pores of H-ACs shrunk, leading to a lower surface area. Both chemically activated S-ACs had an absorbed quantity higher than that of S-AC and H-AC, indicating that the both C-ACs have larger surface area compared to others. The surface area could be increased by the broken particles through the chemical activation, leading to the large surface areas. However, the increase of surface area in this study is mainly caused by the increase of total pore volume as shown in Table 1. When the N\({}_{2}\) adsorption of C-AC-W and -A was compared, the C-AC-A adsorption was higher than that of C-AC-W. This indicates that the acid treatment of chemically activated ACs effectively removed the generated ashes and the remained chemical agents used in the chemical activation method. The acid treatment removed the ashes and clear the pores, leading to the increase of surface area and pore volume increase simultaneously (Table 1). The C 1 s XPS spectrum, along with the de-convoluted peaks of prepared ACs included C-H, C-C, C-O, C=O, and O-C=O, and \(\pi\)-\(\pi\) bonding at 283.6-284.2, 284.6-284.8, 285.5-285.6, 286.7-286.8, 288.8-289.0, and 290.6-291.1 eV, respectively (Figure 3) [25,26]. When compared with S- and H-AC (Figure 3 a,b), the peaks of H-AC rarely changed, even when they were thermally treated. However, the peaks of chemically activated ACs were significantly different (Figure 3c,d). The concentration of the C-H peak in C-AC-W increased due to the non-graphitic carbonaceous species generated during the chemical activation. Consequently, the carbonaceous species were removed by the acid treatment, resulting in the disappearance of the peaks (Figure 3d) [27]. C-O and COOH peaks in XPS curves were observed to decrease and increase, respectively, after KOH chemical activation of ACs [28]. In this study, the peaks related to the C-O and O-C=O also decreased and increased, respectively, after the chemical activation (Figure 3c,d; Table 2). \begin{table}
\begin{tabular}{c c c c c} \hline  & **S-AC** & **H-AC** & **C-AC-W** & **C-AC-A** \\ \hline
**Specific surface area (m\({}^{2}\)/g)** & 681 \(\pm\) 20 & 709 \(\pm\) 18 & 1383 \(\pm\) 16 & 1612 \(\pm\) 9 \\
**Total pore volume (cm\({}^{3}\)/g)** & 0.38 \(\pm\) 0.06 & 0.40 \(\pm\) 0.06 & 0.78 \(\pm\) 0.04 & 0.85 \(\pm\) 0.03 \\ \hline \end{tabular}
\end{table} 
Table 1: Textural properties of the S-AC, H-AC, C-AC-W, and C-AC-A. Figure 2: Nitrogen adsorption and desorption isotherms (**a**) and pore size distribution (**b**) of S-AC, H-AC, C-AC-W, and C-AC-A. Solid and empty symbols represent nitrogen adsorption and desorption, respectively. After thermal treatment, the oxygen content of S-AC sharply decreased. The oxygen containing group was eliminated during the heat treatment. However, the ACs subjected to the chemical activation increased oxygen content due to the carbon oxidization.