## 3 Results and Discussion
### Characterization
#### 3.1.4 FTIR Study  
A FTIR study was conducted to detect the characteristic groups of PANI and its composites with Fe\({}_{3}\)O\({}_{4}\) before and after removal of Cr(VI) and Ni(II) as shown in Figure 4a-i. Significant peaks observed at 536.6 and 855.9 cm\({}^{-1}\) corresponded to the Fe\(-\)O bond stretching vibrations in the spectrum of Fe\({}_{3}\)O\({}_{4}\), thus confirming its formation. Similarly, a peak at 3466.4 cm\({}^{-1}\) was attributed to the stretching vibrations due to the -OH group attached to the surface of Fe\({}_{3}\)O\({}_{4}\). The presence of DBSA in the vicinity of Fe\({}_{3}\)O\({}_{4}\) was also confirmed by the peak at 1649.6 cm\({}^{-1}\) which showed the stretching vibration of the  
Figure 3: UV-visible spectra of Fe\({}_{3}\)O\({}_{4}\), PANI, and PANI/Fe\({}_{3}\)O\({}_{4}\) (**a–c**) before adsorption, (**d–f**) after adsorption of Cr(VI), and (**g–i**) after adsorption of Ni(II). \(-\)CH\({}_{2}\)- group present in the texture of DBSA. [PERSON] et al. have observed Fe-O bond stretching at 550 cm\({}^{-1}\) in SiO\({}_{2}\) coated Fe\({}_{3}\)O\({}_{4}\) particles [52]. A FTIR spectrum of pristine PANI before adsorption is shown in Figure 4d. A characteristic peak showing the C-N-C bending mode in the aromatic ring appears at 574.7 cm\({}^{-1}\). Another peak at 667.9 cm\({}^{-1}\) can be assigned to bending mode vibrations due to the C-C bond in the aromatic ring. Peaks at 819.6 and 1121.3 cm\({}^{-1}\) were attributed to the bending vibrations due to out plane and in plane C-H bonds in the benzenoid ring [53]. Similarly, a peak appearing at 1309.5 cm\({}^{-1}\) shows stretching vibrations due to \(-\)C-N\({}^{+}\) in the secondary aromatic amine. [PERSON] et al. have synthesized polypyrrole/polyaniline composite through the interfacial polymerization method and observed stretching vibrations due to \(-\)C-N\({}^{+}\) of the secondary aromatic amine in the pristine PANI at 1297 cm\({}^{-1}\)[54]. Peaks at 1461.4 and 1592.9 cm\({}^{-1}\) can be assigned to C=N and C=C stretching vibrations of the quinoid and benzenoid rings of the PANI texture. The presence of DBSA moiety in the PANI backbone was also confirmed by the peak appearing at 1005.8 cm\({}^{-1}\), which was attributed to the stretching vibration due to S=O bond of -SO\({}_{3}\)H group [55]. Symmetric and asymmetric stretching vibrations due to the C-H bonds in PANI were represented respectively by two peaks at 2840.3 and 2933.3 cm\({}^{-1}\). Formation of PANI/Fe\({}_{3}\)O\({}_{4}\) composites were confirmed by a weak peak at 511.2 cm\({}^{-1}\) which corresponded to the stretching vibration due to the Fe-O bond in Fe\({}_{3}\)O\({}_{4}\) as shown in Figure 4g. Moreover, the intensity of peaks at 2813.1 and 2926.5 cm\({}^{-1}\) decreased, showing some interactions between the active sites present on the PANI surface and Fe\({}_{3}\)O\({}_{4}\) particles. Interactions between PANI and Fe\({}_{3}\)O\({}_{4}\) particles in composite materials were also observed in the slight shift in % transmittance towards longer wavelength regions as reported elsewhere [56]. After adsorption of Cr(VI), a slight shift towards low frequency range was observed in all peaks in the spectra of Fe\({}_{3}\)O\({}_{4}\), PANI, and composite materials thus indicating some coating of chromium on adsorbents. Moreover, in the spectrum of Fe\({}_{3}\)O\({}_{4}\) after adsorption of Cr(VI), peaks at 3404.9 and 2969.5 cm\({}^{-1}\) were attributed to the stretching and bending vibrations due to the O-H bond of water molecules [43]. A peak present at 1724.5 cm\({}^{-1}\) was associated to hydroxyl group deformation vibrations. Similarly a peak appeared at 1044.2 cm\({}^{-1}\) which indicated intrinsic vibrations of the Cr-O bond, thus  
Figure 4: FTIR spectra of Fe\({}_{3}\)O\({}_{4}\), PANI, and PANI/Fe\({}_{3}\)O\({}_{4}\) composites (**a**,**d**,**g**) before adsorption, (**b**,**e**,**h**) after Cr(VI) adsorption, and (**c**,**f**,**i**) after Ni(II) adsorption. confirming the adherence of Cr(VI) on to Fe\({}_{3}\)O\({}_{4}\).