**4. Conclusions**

PANI, Fe3O4 and their composite can effectively be utilized for the removal of AB40 dye from aqueous environment. The comparison of adsorption behavior of the three materials for the uptake of AB40 reveals that the dye interaction with PANI was higher than both Fe3O4 and composites materials. This enhancement in adsorption on PANI can be attributed to the electrostatic interaction between oppositely charged sites of PANI and AB40. Greater number of active sites leading to physical forces also enhanced the adsorption of dye on PANI. In case of PANI/Fe3O4 composites the lone pair electrons present on the oxygen causes repulsive interaction with the negatively charged dye and reduces the adsorption. This fact was confirmed in the effect of ionic strength on adsorption where PANI/Fe3O4 composites showed higher adsorption than pristine PANI. The maximum amount of dye adsorbed on PANI, Fe3O4 and PANI/Fe3O4 composites were 264.9, 130.5 and 216.9 mg g<sup>−</sup>1, respectively. The enhancement of adsorption on PANI was also supported by its smaller value of activation energy than Fe3O4 and PANI/Fe3O4 composites. Freundlich adsorption isotherm model fitted more closely with the adsorption data. The adsorption was high in acidic conditions and followed pseudo-second-order kinetics. The negative sign of the values of enthalpy changes, entropy changes and Gibbs free energy changes confirmed spontaneous and exothermic nature of adsorption.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/1996-1944/12/18/2854/s1, Figure S1: UV-Visible calibration curve of AB40.

**Author Contributions:** A.M. wrote the original draft and executed all the experiments. A.u.H.A.S. supervised. A.u.H.A.S. and S.B. contributed to writing and corrected and edited the manuscript.

**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. wants to thank the Alexander von Humboldt Foundation Germany for support.

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