**1. Introduction**

The discovery of conducting polymers in 1977 initiated an interesting field of research. These polymers showcased fascinating physico-chemical properties which made them suitable for numerous applications [1]. Polyaniline, polythiophene, polypyrrole and their derivatives are the most studied conducting polymers [2–5] and show optical as well as conducting properties due to the presence of π conjugated electrons in their skeleton [6]. Polyaniline (PANI) has gained a lot interest among the conducting polymers family because it can be synthesized easily from low-cost materials. It is highly conductive and possesses good environmental stability [3,7,8].

A number of methods including chemical oxidation, electro-chemical oxidation, enzymatic, interfacial, self-assembling and seeding methods have been applied to synthesize PANI [9–13]. Chemical and electro-chemical oxidation methods are the most common methods which involve the polymerization of aniline in an acidic or basic medium. However, the conducting emeraldine form of PANI is usually synthesized in an acidic environment [14]. PANI has been e ffectively applied in corrosion protection, batteries, solar cells, supercapacitors and adsorption of heavy metals and dyes from an aqueous solution [3,15–18]. The suitability of PANI as an adsorbent to remove dyes from an aqueous environment is due to the presence of a large number of amine and imine functional groups which are expected to interact with dyes. The charge transfer induced by doping enables PANI to interact with ionic species through electrostatic interactions [19]. Although PANI has been used widely as an adsorbent for the removal of dyes from water, its performance is restricted due to two main challenges. Firstly, its particles aggregate due to intermolecular interactions, resulting in the decrease of surface area and hence the adsorption capacities [20]. Secondly, acid doped PANI is prone to de-doping due to the evaporation of the small acid molecules at room temperature. This causes a reduction in the surface charge of PANI which ultimately a ffect the electrostatic interaction between PANI and dye [21].

To overcome these challenges, considerable work has been done in recent years to synthesize composites of PANI with inorganic substances such as Ag, Cd, SiO2, TiO2, ZnO, MnO2 and magnetic oxide (Fe3O4) [22–25]. These composites exhibit characteristics electrical, optical, catalytic and mechanical properties that are better than single components in some cases. The composites of PANI and Fe3O4 have attracted much attention because of easy synthesis and numerous applications in areas such as in biosensors, sensors, solar cells and purification of water [26–29].

Just like PANI, magnetic oxide also finds applications in drug delivery systems [30], clinical diagnosis [31], e fficient hyperthermia for the removal of cancer [32], microwave devices, magnetic resonance imaging (MRI) [33,34] and the removal of heavy metals from an aqueous solution [35,36]. Electric explosion of wire, laser target evaporation and biomineralization are commonly used for controlled size and morphology of Fe3O4 [37], but the wet chemical methods, like the chemical co-precipitation method [38], sol-gel [39], hydrothermal method [40], gas phase [41], liquid phase [42] and two-phase methods such as microemulsion methods [43] are also used for the preparation of Fe3O4.

In general, composites of PANI and Fe3O4 have been synthesized either through in situ formations of magnetic oxide composites in the presence of PANI [44] or polymerization of aniline monomers in the presence of iron oxide. In comparison with the former, the latter strategy gives better results because of the magnetic properties of the resulting hybrid materials [45].

Bhaumik et al. [46] prepared nanofibers composites from metallic nanoparticles and PANI and applied these composites to remove arsenic (V), chromium (VI) and Congo red from an aqueous solution. Di fferent polymer salts (PANI–HNO3, PANI–H2SO4 and PANI–H3PO4) are reported to use as adsorbents to remove Direct Blue 78 (DB78) from water [47]. The dye uptake was in the order PANI–H3PO4 > PANI–H2SO4 > PANI–HNO3. The rate of adsorption was decreased as the concentration of DB78 and pH of dye solution increased. The adsorption followed pseudo-second-order kinetics. Cui and co-workers [48], studied the adsorption of Hg (II) onto polyaniline/attapulgite (PANI/ATP) composites. (PANI/ATP) composites were synthesized by the chemical oxidation method. The maximum amount of dye adsorbed was 800 mg/g when the pH of Hg (II) solution was 5.9 and followed pseudo-second-order kinetics.

In the present study, PANI/Fe3O4 is used as an adsorbent to remove Acid Blue 40 (AB40) from water. The adsorption behaviors of PANI/Fe3O4 were compared with PANI and Fe3O4 which were synthesized and tested according to our previous work [49]. The chemical oxidation method was used to synthesize PANI and PANI/Fe3O4 composites using FeCl3·6H2O as an oxidant in an acidic medium, while the chemical co-precipitation method was adopted to synthesize Fe3O4 materials in the basic medium at a temperature of 85–90 ◦C. All these synthesized materials were characterized through UV-Vis, SEM, FTIR, EDX and surface area measurements. Adsorption study was carried out to determine the effect of pH, initial concentration, temperature, contact time, adsorbent dosage and ionic strength on adsorption phenomenon using UV-Vis spectroscopy. Freundlich, Langmuir, D–R and Tempkin adsorption isotherm models were applied to analyze the adsorption data. The adsorption mechanism was determined on the basis of kinetic study. Thermodynamic aspects of adsorption of AB40 on these materials were also investigated.
