3.1.3. FTIR Study

Figure 4A,B represent, respectively, FTIR spectra of Fe3O4, PANI and PANI/Fe3O4 composite before and after adsorption of AB40. The peak located at 543.1 cm<sup>−</sup><sup>1</sup> is due to the stretching vibration of the Fe–O bond in the Fe3O4 spectrum [61]. A wide peak at 3427.34 cm<sup>−</sup><sup>1</sup> shows stretching vibrations of –OH group attached to Fe3O4 surface [62]. The shifting of all peaks towards a lower frequency and the appearance of a very small peak at 2343.2 cm<sup>−</sup><sup>1</sup> in Figure 4B indicates that the AB40 dye comes in contact with Fe3O4 after adsorption [9,63].

FTIR spectrum of PANI shows –N–H group of secondary amine at 3231.5 cm<sup>−</sup>1. Similarly, the peaks at 2842.8 and 2932.8 cm<sup>−</sup><sup>1</sup> can be attributed to the symmetric and asymmetric stretching vibrations of the C–H bond, respectively. Vivekanandan et al. have reported such asymmetric and symmetric C–H stretching vibrations at 2923.62 and 2825.55 cm<sup>−</sup>1, respectively [9]. Peaks at 1602.8 and 1469.3 cm<sup>−</sup><sup>1</sup> attribute to C=C and C=N stretching vibrations of the benzenoid and quinoid rings. The band at 1304.2 cm<sup>−</sup><sup>1</sup> corresponds to the –C–N+ stretching vibrations of the secondary aromatic amine. Similarly, the peaks at 1140.3 and 826.5 cm<sup>−</sup><sup>1</sup> represent the bending vibrations of the aromatic C–H bond in plane and out of plane deformation [64]. The peak at 1020.4 cm<sup>−</sup><sup>1</sup> shows the S=O stretching vibrations of the –SO3H group, confirming the presence of DBSA moiety in the PANI texture [65,66]. The peak at 677.2 cm<sup>−</sup><sup>1</sup> shows the out of plane bending vibrations of the C–H bond.

In the spectrum of PANI/Fe3O4 composites, all peaks are shifted to the low-frequency range in comparison with PANI, indicating a presence of some physical forces between PANI and Fe3O4 particles. The appearance of the small peak at 542.7 cm<sup>−</sup><sup>1</sup> shows Fe-O stretching, which confirms the formation of PANI/Fe3O4 composites [67]. After the adsorption of AB40, there is a slight shift of peaks towards a low frequency, both in the spectrum of PANI and PANI/Fe3O4 composites. Moreover, the appearance of the peak at 2356.7 cm<sup>−</sup><sup>1</sup> shows the adsorption of AB40 dye on PANI and PANI/Fe3O4 composites [68]. This peak is more intense in the spectrum of AB40 adsorbed on PANI as compared to the PANI/Fe3O4 composite which is in agreemen<sup>t</sup> with the UV-visible study.

**Figure 4.** FTIR spectra of Fe3O4, PANI and PANI/ Fe3O4 composites before (**A**) and after (**B**) the adsorption of AB40.

### 3.1.4. Surface Area Study

The surface area of adsorbent plays a unique role in the adsorption study. The Brunauer–Emmett– Teller (BET) technique was employed to determine the average pore size radius, pore volume and specific surface area of PANI, Fe3O4 and PANI/Fe3O4 composite via nitrogen adsorption–desorption analysis (Figure 5). The results obtained are summarized in Table 1. The data shows that specific surface area of PANI/Fe3O4 composite is greater than PANI and Fe3O4 particles [69]. After the adsorption of AB40, the surface area of Fe3O4, PANI and PANI/Fe3O4 composite decreases [70]. However, the extent of reduction is more for PANI as compared to Fe3O4 and PANI/Fe3O4 composites, showing a greater adsorption of the dye on PANI.

**Figure 5.** Brunauer–Emmett–Teller (BET) surface area of Fe3O4, PANI and PANI/Fe3O4 composites before (**A**) and after (**B**) the adsorption of AB40.


**Table 1.** Surface area, average pore volume and pore radius of PANI, Fe3O4 and PANI/Fe3O4 composites.
