## 3 Results and Discussion  
### Characterization of the Adsorbent  
The functional groups on the surface of the fresh chitosan, Mt-rich clay, and MCC adsorbents were evaluated by FT-IR analysis (Figure 1). For chitosan (Figure 1a), the band at a vibrational frequency range of 3200\(-\)3450 cm\({}^{-1}\) corresponded to the overlapped band between N-H and O-H stretching bands [16]; the bands at 2910 cm\({}^{-1}\) and 2850 cm\({}^{-1}\) were attributed to the aliphatic C-H stretching, the bands at 1659 cm\({}^{-1}\) and 1450 cm\({}^{-1}\) represented the N-H bending, and that at 1010 cm\({}^{-1}\) was ascribed to C-O stretching [17; 18]. The FT-IR spectrum of the Mt-rich clay (Figure 1c) exhibited a peak at a vibrational frequency of 3650 cm\({}^{-1}\), which was assigned to the free O-H vibration. The broad peak at 3200-3500 cm\({}^{-1}\) (center at 3435 cm\({}^{-1}\)) was attributed to the symmetric O-H stretching of H-bonded adsorbed water molecules and the O-H stretching of the clay. The band at 1630 cm\({}^{-1}\) corresponded to the H-O-H bending mode of adsorbed molecules of water on the clay. The bands of clay were observed at 1045 cm\({}^{-1}\) for the Si-O out-of-plane stretching [19; 20], 954 cm-1 for the Al-OH-Al bending, 829 cm\({}^{-1}\) for the (Al,Mg)-OH stretching, 529 cm\({}^{-1}\) for the Si-O-Al bending, and 477 cm\({}^{-1}\) for the Si-O-Mg bending [21]. These were skeleton of clay. The FT-IR spectrum of the MCC (Figure 1b) showed a broad peak at 3300-3600 cm\({}^{-1}\), which was assigned to the O-H stretching vibration. The vibrational peaks at 3200-3450 cm\({}^{-1}\), 2895-2950 cm\({}^{-1}\), and 1659 cm\({}^{-1}\) were attributed to N-H stretching, C-H stretching, and N-H bending vibrations, respectively. These peaks represented the main functional groups of chitosan [12]. The main functional  
Figure 1: Representative Fourier transform-infrared spectrometry (FT-IR) spectrum of (**a**) chitosan, (**b**) montmorillonite chitosan composite (MCC) and (**c**) montmorillonite (Mt)-rich clay. groups of clay were found at 1045, 954, 829, 529 and 477 cm\({}^{-1}\), respectively, as mentioned above. Thus, it was evident that the spectrum of the MCC expressed both the distinctive spectrum of chitosan and clay, and so the MCC was synthesized successfully. The textural properties of the Mt-rich clay, chitosan, and MCC are shown in Table 1. The N\({}_{2}\) adsorption/desorption isotherms and pore size distribution are plotted, as shown in Figure S1. The Mt-rich clay exhibited a markedly higher S\({}_{\rm BET}\) and V\({}_{\rm P}\) than those of chitosan (81 and 60.7-fold, respectively) or the MCC (21.4 and 18.2-fold, respectively). The S\({}_{\rm BET}\) and V\({}_{\rm P}\) of the MCC was much lower than the Mt-rich clay because chitosan molecules intercalated into the lamellar structure of the clay via ion exchange between NH\({}_{3}\)\({}^{+}\) ions of chitosan and Na\({}^{+}\) ions of clay [22; 23]. Some of the chitosan covered its surface. On the other hand, it was surprising that the D\({}_{\rm P}\) of the MCC was greater than that of chitosan (3.1-fold) or clay (1.3-fold). This possibly reflected that some chitosan intercalated into the lamellar structure of the clay, which was also supported by the XRD results (Figure 2). The crystalline structure of the adsorbents was evaluated by XRD analysis (Figure 2). For the clay (Figure 2a), the three groups of distinguishable peaks represented the groups of Mt, quartz (Q), and kyanite (Ky). The peaks at a 20 of 6.62\({}^{\circ}\), 18.31\({}^{\circ}\), 19.97\({}^{\circ}\), 22.10\({}^{\circ}\), 23.88\({}^{\circ}\), and 25.91\({}^{\circ}\) represent the (001), (002), (020), (021), (111), and (\(-\)112) planes of Mt with a monoclinic structure (JCPDS Card No. 13-0135), respectively. The peaks of Q with a hexagonal structure (JCPDS Card No. 76-0823) were observed at a 20 of 20.99\({}^{\circ}\), 26.66\({}^{\circ}\), 36.81\({}^{\circ}\), 39.77\({}^{\circ}\), 42.57\({}^{\circ}\), and 45.97\({}^{\circ}\).