3.1. Physicochemical Characteristics of the Samples
To validate the chemical structures of the PAni, Mt, CTAB-Mt and PAni@CTAB-Mt, UV-vis, FTIR, XRD and XPS analysis were also conducted. The UV-vis spectrum (
Figure 1a) of the PAni shows two obvious adsorptions at 323 and 610 nm, which are attributed to the π-π* transition in the benzenoid rings and π-polaron transition in the PAni chain [
12], respectively. These characteristic bands of PAni are slightly red-shifted in the UV-vis spectrum of the PAni@CTAB-Mt, suggesting the interaction between PAni chain and CTAB-Mt (323 to 329 nm and 610 to 624 nm). In the FTIR spectrum of the PAni (
Figure 2b), the bands located at 1592 and 1497 cm
−1 should be attributed to the C=C stretching vibration of the quinonoid and benzene rings, respectively. The contraction peak at 1309 cm
−1 corresponds to the C-N bond, while the stretching vibration of the -NH
+= group and the bending vibration of the C-H bond are located at 1114 cm
−1 and 749 cm
−1, respectively. These results prove the chemical structures of typical PAni products [
16]. The FTIR spectrum of the Mt shows that the band at 3623 cm
−1 is related to the Al-O-H inter-octahedral [
13]. The peaks observed at 1630 cm
−1 suggest the possibility of water hydration in the Mt sample (H-O-H stretching) [
14]. The band at 1007 cm
−1 is a characteristic of layered silicate minerals and is attributed to the triple degenerate Si-O stretching ν3 (in-plane) vibration. In addition, the band located at 919 cm
−1 is attributed to the deformation vibration of Al-OH-Al or Al-Al-OH [
12]; and the band at 793 cm
−1 is attributed to free silica or quartz admixtures, always present in natural Mt [
12,
13]. Furthermore, the band at 523 cm
−1 is attributed to the deformation mode of the Al-O-Si function. Moreover, the comparison of the FTIR spectra of the Mt sample after modification by CTAB shows the appearance of new bands in the spectral range between 1300 cm
−1 and 1575 cm
−1. As for the PAni@CTAB-Mt sample, the 1007 cm
−1 band (Si-O) of Mt is found to be shifted to 1013 cm
−1, and the shift of the characteristic absorption bands of PAni is also observed.
The chemical composition of the studied samples was evaluated by XRF. From the data reported in
Table 1, the predominance of aluminum can be observed, which is coherent with the trioctahedral character of the Mt minerals. The content of Fe, Na and K must be related to the presence of exchangeable cations. Moreover, the modification of the Mt with the CTAB and/or PAni shows higher organic matter content (CTAB-Mt 10.80 wt % and PAni@CTAB-Mt 14.06 wt %). The high organic matter values could be attributed to the formation of hybrid materials, which can lead to an increase in the adsorption capacity.
Figure 2a shows the X-ray diffraction patterns of Mt, CTAB-Mt, PAni@CTAB-Mt and PAni. Differences in crystallinity and mineralogical content of the starting samples can be detected between them. The pattern of the Mt sample showed sharp peaks, confirming the results of the montmorillonite clay analysis. The broad and intense peak at 2θ = 6.61° for the Mt sample corresponding to the (001) reflection in Mt modified by CTAB and PAni@CTAB shifts towards larger (d) spaces from 14.33 Å to 15.22 Å to 17.87 Å, respectively. The pattern of the PAni was characterized by lack of crystallinity. Since the conducting polymer material is in the mesoporous form, the formation of infinite non-uniform pores on the surface of the polymer was attributed to the amorphous nature of the product [
16].
The textural characteristics of the prepared adsorbents were analyzed through N
2 adsorption–desorption isotherm tests (
Figure 2b). All the isotherms showed a type II isotherm with a distinctive type H3 hysteresis loop, suggesting the presence of a small macroporous component [
12,
17]. The pore size data width of the adsorbents is summarized in
Table 2. The specific surface area (S
BET) of Mt (91 m
2·g
−1) decreases after the modified surface and consists of 73 m
2·g
−1 for CTAB-Mt. Such reduction is determined by the practically complete filling of the micropores with CTAB molecules, resulting in the blocking of the access of nitrogen molecules to these pores. The PAni@CTAB-Mt adsorbent has an increased S
BET of 121 m
2·g
−1. This is due to the formation of a two-dimensional porous structure in the interlayer space of the PAni matrix.
XPS analysis was carried out to determine the surface elemental components (
Figure 3 and
Table 3). The atomic concentration of surface carbon increases with Mt modification by PAni and/or CTAB. Compared to Mt, the relative composition of aliphatic/aromatic carbon groups (C-C/C-H groups, ~284 eV) from CTAB-Mt and PAni@CTAB-Mt increases to 62.22% and 68.19%, respectively. Regarding the N1s spectrum (
Figure 4), the fitted XPS spectra of CTAB-Mt showed two distinct peaks at 399.42 and 400.75 eV, corresponding to –NH– and =NH– groups, respectively. In addition, PAni@CTAB-Mt shows that the relative composition of –NH–/=NH– has become 56.89% and 43.11%, respectively.
In order to compare the differences in the textural structures of Mt and modified Mt and to see the effectiveness of the modification made, TEM images of the samples were taken and are shown in
Figure 5. The TEM image in
Figure 5a demonstrates the typical structure of the Mt image. It can be noticed from this image that there is a homogeneous distribution of the clay flakes. The TEM images of the CTAB-Mt in
Figure 5b show a homogenous loading of the CTAB inside the Mt material. Comparison of the TEM micrographs in
Figure 5a,b confirmed the formation of the CTAB-Mt sample. Moreover, the TEM image in
Figure 5c depicts the distribution of the formed PAni assembled on CTAB within the interlayer spaces of Mt, which is compatible with the XRD and FTIR results in this investigation and confirms the formation of the Mt modified by PAni and/or CTAB.
Figure 6a shows the thermogravimetric analysis of the samples. Three distinct weight loss processes led to the thermal degradation of both PAni, CTAB-Mt and PAni@CTAB-Mt. The first weight loss below 130 °C is caused by the evaporation of the retained volatiles of the system and the adsorbed moisture. The elimination of oligomers took place in the second stage, which ran from 220 to 430 °C [
12]. The second thermos, that leads to a variety of degradation products, is what causes the third stage to occur after 430 °C [
13]. An increase in thermal stability was observed in the case of the CTAB-Mt (weight loss of 18.05%), which may be related to the effective integration of CTAB into the Mt structure. Compared to PAni@CTAB-Mt, it displayed a higher weight loss of 21.01% at 900 °C, which may be due to the fact that the CTAB-Mt contains PAni as a matrix in its interlayer. Moreover, the TGA curve data showed that above 220 °C the decomposition process of Mt starts and it exhibits a weight loss of 6.59% at 900 °C, whereas the weight loss of PAni is mainly concentrated between 290 °C and 530 °C, which is also related to the degradation of the PAni molecular chains [
15].
Figure 6b shows cyclic voltammograms (CVs) of samples with potentials ranging from 0.05 V to 1.0 V and scanning rates of 50 mv·s
−1. The results show that the CV curve of PAni@CTAB-Mt is clearly dissimilar from that of the tCTAB-Mt sample as a consequence of formation of PAni into the interlayer space. According to this result, the PAni matrix significantly affects the electrochemical behavior of PAni@CTAB-Mt, proving that it is more electrochemically active compared to the CTAB-Mt sample [
18]. Conversely, the results for CTAB-Mt show that the CV curve was relatively comparable to the CV of the Mt sample. However, it displayed a sustained peak current, indicating that CTAB and Mt were fully electrochemically durable.