3.1.1. Optical Studies

Figure 2A represents the UV-Vis spectra of the synthesized materials before adsorption of the dye. A weak band in the region of 450 nm arises due to the interaction of electromagnetic radiations with the valence electrons of iron in the Fe3O4. As a result, the valence electron of the metal atom starts to oscillate with the frequency of the electromagnetic source [53]. This phenomenon is known as surface plasmon resonance (SPR). Another band at 485.85 nm is due to the presence of DBSA moiety with Fe3O4 and closely resembles already reported work [54]. The two characteristic bands of PANI can be observed in its spectrum at 333.91 and 633.42 nm. The band at 633.42 nm is due to charge transfer from the benzenoid ring to the quinoid ring and the band at 333.91 nm is attributed to <sup>ᴫ</sup>-ᴫ\* transitions of the benzenoid ring [55]. In the spectrum of PANI/Fe3O4 composites, the band at 333.91 nm shows a redshift due to the doping of the benzenoid amine with Fe3O4 particles. Moreover, the bipolaron band at 633.42 nm is shifted to 773.14 nm suggesting that some physical interactions between PANI and Fe3O4 particles may exist [56].

Figure 2B represent UV-visible spectra of Fe3O4, PANI and composite of Fe3O4 with PANI after adsorption of AB40. One can observe a band in the region of 618–620 nm in all the spectra of Fe3O4, PANI and composite of Fe3O4 and PANI which indicates the adsorption of AB40. This band has been demonstrated that AB40 shows strong absorption at 620 nm [57]. The intensity of this band is higher for PANI, which is different from our previous work where more intense peaks, due to adsorption of Basic Blue 3 dye, was observed in the spectrum of PANI/Fe3O4 composite [49]. The reason can be explained by the fact that in the PANI/Fe3O4 composite, the positively charged active sites of PANI are covered by Fe3O4. Moreover, the oxygen of Fe3O4 behave as negatively charged sites, which may cause repulsion to the negative charge of the anionic dye and hence reduces its adsorption.

**Figure 2.** UV-visible spectrum of Fe3O4, PANI and PANI/ Fe3O4 composite (**A**) before and (**B**) after adsorption of AB40.

### 3.1.2. Energy Dispersive X-ray (EDX) Study

Figure 3 shows the EDX analysis of PANI, Fe3O4 and PANI/Fe3O4 composites before and after adsorption of AB40. The weight percent of Fe and O in Fe3O4 is 68.74 and 29.15, respectively. After the adsorption of AB40, the weight percent of Fe decreases from 68.74 to 62.09, while the percent weight of O and C increases due to the presence of oxygen and carbon in the AB40 texture. Similarly, the appearance of nitrogen and Sulphur in spectrum 3b is more evidence of the adsorption of AB40 onto Fe3O4, as these elements are present in the dye texture [58]. Figure 3c shows the EDX spectrum of PANI before adsorption of AB40. One can observe a 9.54 percent nitrogen and 68.06 percent carbon by weight in this spectrum. The presence of sulfur and oxygen may be due to the presence of DBSA while Fe and Cl may be due to the presence of FeCl3·H2O which was used as oxidant. After adsorption of AB40, although weight percent of carbon decreases but weight percent of nitrogen and oxygen increases which shows that AB40 adsorb on PANI (Figure 3d) [59]. In the EDX spectrum of PANI/Fe3O4 composite, peaks for nitrogen, oxygen, carbon and iron can clearly be observed in Figure 3e, confirming the formation of composites. The sulfur percent by weight is 2.66 and is due to the presence of some moiety of DBSA. After the adsorption of AB40, the weight percent of carbon, nitrogen and sulfur is increased (Figure 3f) [60].

**Figure 3.** EDX spectra of Fe3O4, PANI and PANI/Fe3O4 composites before (**<sup>a</sup>**,**c**,**<sup>e</sup>**) and after (**b**,**d**,**f**) adsorption of AB40.
