*2.6. Characterization*

FTIR spectra of the synthesized materials were registered in the spectral range of 400 to 4000 cm<sup>−</sup><sup>1</sup> through a Fourier transmission infrared spectrophotometer (Shimadzu, Tokyo, Japan). X-ray di ffraction (XRD) patterns were recorded by using Cu K α radiations of wavelength 1.5405 Å with the help of a JEOL JDX-3532 (JEOL, Tokyo, Japan). The concentration of dye in the solution and its adsorbed amount onto the synthesized materials were checked through UV-visible spectrophotometer (Perkin Elmer, Buckinghamshire, UK). An energy-dispersive X-ray (EDX) spectrophotometer (Inca 200, Oxford, UK) was utilized to determine the percentage of di fferent elements. Brunauer–Emmett–Teller (BET) surface areas of the composite as well as PANI and Fe3O4, were determined in the N2 atmosphere through adsorption–desorption method with a surface area analyzer model 2200 e Quanta Chrome (Quanta Chrome, Boynton Beach, FL, USA). The surface morphologies were studied through scanning electron microscopy (SEM) (JSM-6490, JEOL, Tokyo, Japan).

### **3. Results and Discussion**

### *3.1. Scanning Electron Microscopy (SEM)*

SEM images provide interesting information about surface morphology and size of adsorbent materials under investigation. Figure 1a,b shows SEM images of Fe3O4 particles before and after adsorption of AB40 dye. The Fe3O4 particles are round in shape with an average size of 0.15 μm. After adsorption of AB40, the porosity decreases in the agglomerated surface of Fe3O4 but the average particles size increases to 0.23 μm (Figure 1b). Keyhanian et al. [50] have reported the agglomeration of magnetic particles of Fe3O4 after adhering of methyl violet dye from an aqueous solution.

Rods- or wires-like porous structure can be seen in the SEM image of PANI with 0.21 μm average diameter of the rods (Figure 1c). These rods are aggregated to each other like fibers. After the adsorption of AB40 (Figure 1d), the morphology of PANI changes to a cauliflower shape with some needle-like structures present on the surface. Such a change in morphology was also reported during the adsorption of anionic dyes on PANI doped with Potash Alum [51]. Figure 1e shows an SEM image of PANI/Fe3O4 composites. It shows a porous morphology where Fe3O4 particles have adhered with PANI interconnected rods. Similar morphology was depicted by nanocomposites of PANI/ Fe3O4 coated on MnFe2O4 [52]. Just like PANI, morphology of PANI/Fe3O4 composites also changes after the adsorption of AB40. The dye distributes homogeneously over the surface of composite imparting a broccoli-like appearance to it as shown in Figure 1f.

**Figure 1.** SEM images of magnetic oxide (Fe3O4), polyaniline (PANI) and PANI/Fe3O4 composite before (**<sup>a</sup>**,**c**,**<sup>e</sup>**) and after (**b**,**d**,**f**) adsorption of acid blue 40 (AB40) dye.
