**4. Conclusions**

PANI/Fe3O4 composites, whose syntheses were confirmed through various spectroscopic techniques, such as SEM, FTIR, EDX, UV, and XRD, can effectively be utilized as adsorbents for removal of BB3 (cationic dye) from aqueous solution. It was envisaged that the synergy between PANI and magnetite would impart promising properties onto the composite material, as a high amount of dye (78.13 mg/g) was adsorbed on PANI/Fe3O4 composites in comparison to that adsorbed for Fe3O4 (7.474 mg/g) and PANI (47.977). The enhanced adsorption capability of the composites is attributed to the increase in surface area and pore volume of the hybrid materials. The adsorption followed pseudo second order kinetics, with R<sup>2</sup> values of 0.873, 0.979, and 0.999 for Fe3O4, PANI, and PANI/Fe3O4 composites, respectively. The activation energy, enthalpy, Gibbs free energy changes, and entropy changes were found to be 11.14, −32.84, −04.05, and −0.095 kJ/mol for Fe3O4, 11.97, −62.93, −07.78, and −0.18 kJ/mol for PANI, and 09.94, −74.26, −10.63, and −0.210 kJ/mol for PANI/Fe3O4, respectively, indicating the spontaneous and exothermic nature of the adsorption process. The Langmuir adsorption isotherm model fitted more closely to the data. The adsorption was greater in basic medium than in acidic medium. The adsorption was well-described by the pseudo second order kinetic model. Thermodynamically, adsorption is proven to be exothermic and spontaneous.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/1996-1944/12/11/1764/s1, Table S1: Comparison of different synthesis methods for PANI/iron oxide and their use as adsorbent for removal of various dyes, Table S2: Summery of FTIR absorption bands.

**Author Contributions:** A.M. performed experimental work, formal analysis, and writing of main draft. A.-u.-H.A.S. supervised and contributed in writing and editing. S.B. and G.R. contributed to writing and formal analysis.

**Funding:** This research was funded by the Higher Education Commission Pakistan (project No. 20-1647 and 20–111/NRPU/R&D/HEC). The APC was funded by the German Research Foundation and the Open Access Publication Funds of the Technische Universität Braunschweig.

**Acknowledgments:** We acknowledge support from the German Research Foundation and the Open Access Publication Funds of the Technische Universität Braunschweig. S.B. acknowledges support from the Alexander von Humboldt Foundation Germany.

**Conflicts of Interest:** The authors declare no conflict of interest.
