**1. Introduction**

The use of organic synthetic dyes has increased dramatically and uncontrollably in last few decades. Di fferent types of dyes are frequently employed in plastics, paper, cosmetics, leather, and textile industries for coloring purposes [1–3]. These dyes are released in water as e ffluents, which are of low biological oxygen demand (BOD) and high chemical oxygen demand (COD) [4]. Some of these dyes, such as azo-dyes, are toxic and carcinogenic in nature. Their addition into nearby streams and rivers contaminates water and greatly upsets the biological activities of aquatic life [5,6]. It is highly desirable to explore e fficient technologies for remediation and separation of these potential pollutants from e ffluents.

Various protocols and techniques, such as reverse osmosis, precipitation, coagulation, membrane filtration, chemical oxidation, electrochemical methods, ion exchange, and adsorption are used to remove these dyes and other hazardous materials from polluted water [7,8]. However, adsorption is the most frequently used technique to remove dyes from water, because this technique, in addition to easiness and low cost, causes low generation of residues and the adsorbent used may be regenerated and reused [9–11]. Several adsorbents, such as rice husk, sawdust, activated carbon, orange peel, and chitosan, have been used to remove dyes from aqueous environment [12–15]. However, the major drawback of the use of these materials is that they must be activated either physically or chemically before use. Physically these adsorbents are usually activated at very high temperature, which needs high energy. After removal of dyes, desorption must be carried out to regenerate the adsorbent, which is sometimes complicated and mostly generates secondary pollutants [16], while if thrown without treatment, they will cause water pollution. These complications make the use of these materials very expensive and time consuming, and threatening to the environment. Although activated carbon has been known as the most e fficient adsorbent owing to its high specific surface area, its use is also restricted due to the non-selectivity and regeneration issues. Therefore, there is a need for the development of an environment-friendly material that is easy to regenerate [17].

In recent years, some conducting polymers, such as polyaniline, polythiophene, polypyrrole, and their composites with other materials have attracted much interest because of their conducting behavior and fascinating physicochemical properties. Such materials have been successfully applied in solar cells, fuel cells, sensors, super-capacitors, and for corrosion protection in organic coating [18–20]. Polypyrrole/TiO2, polypyrrole/graphene oxide/Fe3O4, and polyaniline/magnetite have also been applied as adsorbents to remove dyes and heavy metals from aqueous environments [21–23]. Polyaniline, which exists in various oxidation states, is environmentally stable and a good conducting material with excellent electrochemical properties and can be easily prepared with less cost [24–26]. PANI and its composites with other materials, such as TiO2, MnO2, Fe2O3, SeO2, SiO2, Ag, Cd, and Zn, have been applied in sensors, biosensors, rechargeable batteries, fuel cells, and solar cells [27–31]. Some of these composites have also been used as adsorbents to remove heavy metals and dyes from aqueous environments [32,33]. Janaki et al. [34] removed Coomassie brilliant blue, congo red, and methylene blue from aqueous solution using polyanilline/chitosan composites. Sultana et al. [35] synthesized copper ferrite nanoparticles doped polyanilline for removal of direct yellow-27 from aqueous solution. Ayad and Al-Naser [1] applied polyanilline nanotube base as an adsorbent to remove methylene blue from an aqueous environment.

Magnetic materials such as Fe3O4 have attracted special attraction from scientists because of their numerous applications, such as in drug delivery systems [36], magnetic resonance imaging (MRI) [37,38], e fficient hyperthermia for removal of cancer [39], clinical diagnosis [40], and removal of heavy metals form aqueous solution [41,42]. Fe3O4 can be prepared by a number of methods, including hydrothermal method [43], chemical co-precipitation method [44], sol-gel [45], gas phase [46], liquid phase [47], and micro emulsion methods [48]. Polyaniline/magnetite(Fe3O4) composites have the advantage of being stable at high temperatures and can be synthesized easily from low cost materials, which make them superior over the other existing natural/synthetic and biodegradable polymers for the adsorption of dyes. They can be regenerated easily after adsorption and due to their conductive nature, electrochemical study of these materials after adsorption can be carried out. Several reports are available on the use of PANI/iron-oxide-based materials as adsorbents for dyes; a comparison of adsorption properties of these materials with the present work is made in Table S1 of Supplementary Information.

The present study is aimed at investigating the adsorption capacity of Fe3O4 and PANI by synthesizing PANI/Fe3O4 composites for the removal of Basic blue 3 dye from aqueous solution. For comparison, PANI and Fe3O4 were also synthesized and tested for dye removal e fficiency. Chemical co-precipitation protocol was adopted for the preparation of Fe3O4 in basic medium in the temperature range of 85–90 ◦C. PANI and PANI/Fe3O4 composites were synthesized by chemical oxidation method

using FeCl3 as an oxidant. The synthesized Fe3O4, PANI and composites were characterized with Fourier transforms infrared spectroscopy (FTIR), scanning electron microscopy (SEM), X-ray di ffraction (XRD), energy Dispersive X-Ray spectroscopy (EDX), and surface area measurements. Batch adsorption experiments were carried out to study the e ffect of pH, initial concentration of dye, contact time, and temperature on the adsorption phenomenon by using UV-Visible spectrophotometer. The resulted data were fitted into Friundlich, Langmuir, Tempkin, and The Dubinin-Radushkevitch (D-R) adsorption models. Kinetics and thermodynamic aspects of the adsorption of Basic blue 3 dye on these materials were also investigated. 
