Basic Blue Dye Adsorption from Water Using Polyaniline/Magnetite (Fe3O4) Composites: Kinetic and Thermodynamic Aspects

Owing to its exciting physicochemical properties and doping–dedoping chemistry, polyaniline (PANI) has emerged as a potential adsorbent for removal of dyes and heavy metals from aqueous solution. Herein, we report on the synthesis of PANI composites with magnetic oxide (Fe3O4) for efficient removal of Basic Blue 3 (BB3) dye from aqueous solution. PANI, Fe3O4, and their composites were characterized with several techniques and subsequently applied for adsorption of BB3. Effect of contact time, initial concentration of dye, pH, and ionic strength on adsorption behavior were systematically investigated. The data obtained were fitted into Langmuir, Frundlich, Dubbanin-Rudiskavich (D-R), and Tempkin adsorption isotherm models for evaluation of adsorption parameters. Langmuir isotherm fits closely to the adsorption data with R2 values of 0.9788, 0.9849, and 0.9985 for Fe3O4, PANI, and PANI/Fe3O4 composites, respectively. The maximum amount of dye adsorbed was 7.474, 47.977, and 78.13 mg/g for Fe3O4, PANI, and PANI/Fe3O4 composites, respectively. The enhanced adsorption capability of the composites is attributed to increase in surface area and pore volume of the hybrid materials. The adsorption followed pseudo second order kinetics with R2 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, which indicate the spontaneous and exothermic nature of the adsorption process.


Introduction
The use of organic synthetic dyes has increased dramatically and uncontrollably in last few decades. Different types of dyes are frequently employed in plastics, paper, cosmetics, leather, and textile industries for coloring purposes [1][2][3]. These dyes are released in water as effluents, 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 efficient technologies for remediation and separation of these potential pollutants from effluents.

Synthesis of Fe 3 O 4
Chemical co-precipitation method was used to synthesize Fe 3 O 4 by mixing FeCl 3 ·6H 2 O and FeSO 4 ·7H 2 O in a molar ratio of 2:0.5. DBSA was used as the emulsifying agent. The reaction was performed in basic medium (pH 10) in the temperature range of 85-90 • C. Then, 5 M ammonia solution (60 mL) was added as precipitating agent, which turned the reaction mixture black. The mixture was continuously stirred for about 2 h. Then, it was washed with plenty of distilled water and ethanol until the filtrate became clear. The resulting black precipitate was dried in an oven at 80 • C for 10 h and finally annealed in a furnace at 600 • C for 5 h [49]. The schematic representation of the process is presented in Scheme 1.
Materials 2019, 12, x FOR PEER REVIEW 3 of 28 characterized with Fourier transforms infrared spectroscopy (FTIR), scanning electron microscopy (SEM), X-ray diffraction (XRD), energy Dispersive X-Ray spectroscopy (EDX), and surface area measurements. Batch adsorption experiments were carried out to study the effect 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.

Synthesis of PANI
PANI was synthesized via chemical oxidation method by adding 0.3 mol (0.82 mL) aniline in 30 mL double distilled water. Then, 0.02 mol (0.25 mL) Do-decylbenzene sulphonic acid (DBSA) prepared in 40 mL double distilled water was added as an emulsifying agent as well as a dopant. Afterwards, 0.01 M FeCl3. 6H2O solution (30 mL) was added dropwise to this mixture as an oxidant. The solution was stirred on a magnetic stirrer for about 12 h. Initially, the solution was a milky white color, but after an hour the solution turned light green and then dark green after 3 hours. Finally, the product was extensively washed with acetone and double distilled water till the filtrate became clear and dried in an oven at 60 °C for 24 h.

Synthesis of Fe3O4
Chemical co-precipitation method was used to synthesize Fe3O4 by mixing FeCl3·6H2O and FeSO4·7H2O in a molar ratio of 2:0.5. DBSA was used as the emulsifying agent. The reaction was performed in basic medium (pH 10) in the temperature range of 85-90 °C. Then, 5 M ammonia solution (60 mL) was added as precipitating agent, which turned the reaction mixture black. The mixture was continuously stirred for about 2 h. Then, it was washed with plenty of distilled water and ethanol until the filtrate became clear. The resulting black precipitate was dried in an oven at 80 °C for 10 h and finally annealed in a furnace at 600 °C for 5 h [49]. The schematic representation of the process is presented in Scheme 1.

Synthesis of PANI/Fe 3 O 4 Composites
Chemical oxidation method was used to synthesize PANI/Fe 3 O 4 composites. First, 0.2 g Fe 3 O 4 was mixed with 1.818 mL of aniline suspended in double distilled water (50 mL) and DBSA (0.5 mL). The mixture was stirred for about 30 min and followed by addition of 0.15 M FeCl 3 ·6H 2 O prepared in 40 mL double distilled water as oxidizing agent. Initially the reaction mixture was milky white due to DBSA but turned reddish brown after addition of Fe 3 O 4 particles. When oxidant was added a light green color appeared within 20 min, which changed into dark black after about 2 h. After 8 h continuous stirring, the synthesized product was washed with acetone and plenty of double distilled water. Finally, the clean precipitate was dried in an oven at 60 • C for 24 h. The schematic representation of the process in provided in Scheme 2.

Synthesis of PANI/Fe3O4 Composites
Chemical oxidation method was used to synthesize PANI/Fe3O4 composites. First, 0.2 g Fe3O4 was mixed with 1.818 mL of aniline suspended in double distilled water (50 mL) and DBSA (0.5 mL). The mixture was stirred for about 30 min and followed by addition of 0.15 M FeCl3·6H2O prepared in 40 mL double distilled water as oxidizing agent. Initially the reaction mixture was milky white due to DBSA but turned reddish brown after addition of Fe3O4 particles. When oxidant was added a light green color appeared within 20 min, which changed into dark black after about 2 h. After 8 h continuous stirring, the synthesized product was washed with acetone and plenty of double distilled water. Finally, the clean precipitate was dried in an oven at 60 °C for 24 h. The schematic representation of the process in provided in Scheme 2. Scheme 2. Synthesis of PANI/Fe3O4 Composite.

Batch Adsorption Study for Removal of BB3 Dye
Basic blue 3 dye solution of desired concentrations ranging 0.01-110 (mg/L) were prepared in 20 mL volume by dilution method from the respective stock solution. To these solutions, Fe3O4 was added and shacked in a shaker at a speed of 150 rpm for 90 min. These solutions were then filtered and the concentration of dye was determined using a carry-50 UV-Visible spectrophotometer. The amount of dye adsorbed was determined by using the following equation [50].
where qe (mg/g) is the amount of dye adsorbed at equilibrium, Ci and Ce are the initial concentration and the concentration of dye present at equilibrium, respectively, m (g) is the amount of adsorbent added, and V (L) is the volume of solution. The effects of contact time, pH, initial concentration of dye, temperature, and ionic strength on the adsorption process were studied. The data obtained were used to calculate the kinetics and thermodynamic parameters. The same procedure was adopted for studying adsorption of Basic blue 3 dye on PANI and PANI/Fe3O4 composites. After adsorption of BB3 dye on PANI/Fe3O4 composite, it was collected in filter paper with plenty of double distilled water to run out the adsorbed dye. After removal of BB3, the PANI/Fe3O4 composite was washed with 0.1 M HCl, to remove the remaining dye from the surface. In this way composites were regenerated and could be reused

Batch Adsorption Study for Removal of BB3 Dye
Basic blue 3 dye solution of desired concentrations ranging 0.01-110 (mg/L) were prepared in 20 mL volume by dilution method from the respective stock solution. To these solutions, Fe 3 O 4 was added and shacked in a shaker at a speed of 150 rpm for 90 min. These solutions were then filtered and the concentration of dye was determined using a carry-50 UV-Visible spectrophotometer. The amount of dye adsorbed was determined by using the following equation [50].
where q e (mg/g) is the amount of dye adsorbed at equilibrium, Ci and Ce are the initial concentration and the concentration of dye present at equilibrium, respectively, m (g) is the amount of adsorbent added, and V (L) is the volume of solution. The effects of contact time, pH, initial concentration of dye, temperature, and ionic strength on the adsorption process were studied. The data obtained were used to calculate the kinetics and thermodynamic parameters. The same procedure was adopted for studying adsorption of Basic blue 3 dye on PANI and PANI/Fe 3 O 4 composites . After adsorption of BB3 dye on PANI/Fe 3 O 4 composite, it was collected in filter paper with plenty of double distilled water to run out the adsorbed dye. After removal of BB3, the PANI/Fe 3 O 4 composite was washed with 0.1 M HCl, to remove the remaining dye from the surface. In this way composites were regenerated and could be reused

Characterization
The surface morphologies of Fe 3 O 4 , PANI, and PANI/Fe 3 O 4 composites were studied with scanning electron microscopy (SEM) using a JSM-6490 (JEOL, Tokyo, Japan) electron microscope. FTIR spectra of the Fe 3 O 4 , PANI, and Fe 3 O 4 /PANI composites were recorded with IRAffinity-1S Shimadzu Fourier Transform Infrared Spectrophotometer (Shimadzu, Tokyo, Japan) in the spectral range of 400 to 4000 cm −1 . X-ray diffraction (XRD) were recorded with by using Cu Kα radiations (λ = 1.5405 Å) through JEOL JDX-3532 (JEOL, Tokyo, Japan). UV-Visible spectrophotometer (Perkin Elmer, Buckinghamshire, UK) was used to find out the concentration of dye in the solution and to check the amount of dye adsorbed on the composite. Energy-dispersive X-ray (EDX) spectrophotometer model (Oxford, UK) Inca 200 was used for determination of elemental composition. BET surface areas of PANI, Fe 3 O 4 , and composite before and after adsorption were determined in N 2 atmosphere by adsorption-desorption method with surface area analyzer model 2200 e Quanta Chrome (Boynton Beach, FL, USA).

Characterization
After synthesis, different techniques were used in order to know about the structural and morphological features and to get insights into the formation of composites and their adsorption properties. For comparison purposes, the same studies were carried out in parallel for Fe 3 O 4 and PANI alone.

SEM Study
The surface morphology of Fe 3 O 4 , PANI, and PANI/Fe 3 O 4 composites were studied with scanning electron microscopy. The SEM image (Figure 1a) shows that Fe 3 O 4 consists of finite spherical shape with average particle size of 0.25 µm, which tends to form aggregates. It is somewhat porous in texture and becomes rough after adsorption of BB3 (Figure 1b). The adsorption of dye on the surface of Fe 3 O 4 decreases its porosity, as reported elsewhere [51]. Shreepathi and Holze reported fibrous morphology of PANI prepared in different concentrations of DBSA [52]. The SEM image of PANI synthesized in this work shows cauliflower-like surface morphology, which after adsorption of dye changes into clusters of small ball-like structures, shown in Figure 1c

UV-Vis Spectroscopic Study
UV-Vis spectroscopy is widely used for studying optical properties of materials. UV-Visible spectra of Fe 3 O 4 , PANI, and PANI/Fe 3 O 4 composites were recorded in ethanol and chloroform. Figure 2A shows the UV-Vis spectra of Fe 3 O 4 , PANI, and PANI/Fe 3 O 4 composites before adsorption of BB3. In Fe 3 O 4 spectrum, the band at 441.9 is due to the surface plasmon resonance effect (SPR). The surface plasmon resonance phenomenon occurs due to interactions between incident radiations and valence electrons of the metal atom in Fe 3 O 4 and causes the valence electron of the metal to oscillate with the frequency of the electromagnetic source [57]. The other band at 570.7 nm arises due to the presence of DBSA moieties in the synthesized magnetic oxide particles, as reported earlier [58].

UV-Vis Spectroscopic Study
UV-Vis spectroscopy is widely used for studying optical properties of materials. UV-Visible spectra of Fe3O4, PANI, and PANI/Fe3O4 composites were recorded in ethanol and chloroform. Figure  2A shows the UV-Vis spectra of Fe3O4, PANI, and PANI/Fe3O4 composites before adsorption of BB3. In Fe3O4 spectrum, the band at 441.9 is due to the surface plasmon resonance effect (SPR). The surface plasmon resonance phenomenon occurs due to interactions between incident radiations and valence electrons of the metal atom in Fe3O4 and causes the valence electron of the metal to oscillate with the frequency of the electromagnetic source [57]. The other band at 570.7 nm arises due to the presence of DBSA moieties in the synthesized magnetic oxide particles, as reported earlier [58].
In the spectrum of PANI, the band at 325-338 nm is due to π-π* transitions of the benzenoid ring and the band at 660-680 nm is attributed to excitation of the quinoid ring [59]. The spectrum of PANI/Fe3O4 composites shows a small band at 441 nm due to doping of benzenoid amine with Fe3O4 particles, while the band at 770 nm is due to the change from polaron to bipolaron state, suggesting interactions between PANI and Fe3O4 materials, which is in close resemblance to the already reported results [60,61]. In the spectrum of PANI, the band at 325-338 nm is due to π-π* transitions of the benzenoid ring and the band at 660-680 nm is attributed to excitation of the quinoid ring [59]. The spectrum of PANI/Fe 3 O 4 composites shows a small band at 441 nm due to doping of benzenoid amine with Fe 3 O 4 particles, while the band at 770 nm is due to the change from polaron to bipolaron state, suggesting interactions between PANI and Fe 3 O 4 materials, which is in close resemblance to the already reported results [60,61]. Figure 2B shows the UV-Vis spectra of Fe 3 O 4 , PANI, and PANI/Fe 3 O 4 composites after adsorption of BB3, respectively. The appearance of absorption band at 647-651nm in all the spectra clearly indicates the adsorption of BB3 on Fe 3 O 4 , PANI, and PANI/Fe 3 O 4 composites. As reported previously, BB3 gives a strong absorption band at 654 nm [62]. This absorption band is more intense in the spectrum of the composites as compared to the spectra of PANI and Fe 3 O 4 . The enhancement in the intensity of the absorption band of the composite around 650 nm shows strong interactions and adsorption capability of PANI/Fe 3 O 4 composites towards BB3 as compared to pristine PANI and Fe 3 O 4 . indicates the adsorption of BB3 on Fe3O4, PANI, and PANI/Fe3O4 composites. As reported previously, BB3 gives a strong absorption band at 654 nm [62]. This absorption band is more intense in the spectrum of the composites as compared to the spectra of PANI and Fe3O4. The enhancement in the intensity of the absorption band of the composite around 650 nm shows strong interactions and adsorption capability of PANI/Fe3O4 composites towards BB3 as compared to pristine PANI and Fe3O4.

FTIR Spectroscopy
FTIR spectroscopy is used to study and identify organic, polymeric, and in some cases inorganic materials. Figure 3A shows FTIR spectra of Fe3O4, PANI, and PANI/Fe3O4 composites before adsorption of BB3. The details of FTIR signals associated with different types of vibrations are summarized in Table S2 of the supplementary information.

FTIR Spectroscopy
FTIR spectroscopy is used to study and identify organic, polymeric, and in some cases inorganic materials. Figure 3A shows FTIR spectra of Fe 3 O 4 , PANI, and PANI/Fe 3 O 4 composites before adsorption of BB3. The details of FTIR signals associated with different types of vibrations are summarized in Table S2 of the Supplementary information. A characteristic absorption band is observed at 554.8 cm −1 due to the stretching vibration of Fe-O bonds in the Fe 3 O 4 spectrum. In an early study, stretching vibrations of Fe-O bonds were reported at 560 cm −1 [63]. This shift in the Fe-O band towards lower frequency in the present study may be due to the presence of DBSA in the Fe 3 O 4 particles. Peaks at 1133.6 and 1534.6 cm −1 correspond to CH 2 bending modes of DBSA. Similarly, a weak peak at 3494.3 cm −1 is because of -OH stretching attached to the Fe 3 O 4 surface and shows close resemblance to the already reported work [64]. Another weak band at 1734.7 cm −1 is assigned to residual NH 4 OH, as already reported elsewhere [65]. The peak at 554.8 cm −1 is due to stretching vibrations of Fe-O disappearing and a new peak at 539.5 cm −1 appearing, showing BB3 dye adsorbtion onto Fe 3 O 4 , as shown in Figure 3B. This is because of the interaction of oxygen present in the dye structure with Fe of Fe 3 O 4 . The appearance of more intense peaks at 1224.6 and 1365.7 in Figure 3B is also attributed to the adsorption of BB3 [66].
FTIR spectrum of PANI is also shown in Figure 3A. Peaks at 1568 cm −1 and 1466 cm −1 are due to C=C and C=N stretching vibrations of benzoinoid and quinoid rings, respectively. Phang and Kuramoto have reported the C=C and C=N stretching vibrations of PANI at 1572 and 1497 cm −1 , respectively [54]. The bands at 1307.6 cm −1 are due to C-N•+ stretching of secondary aromatic amine of PANI doped with protic acid. The peak at 670.1 cm −1 shows out-of-plane bending vibrations of the C-H bond. The peak at 1017.9 cm −1 is assigned to -SO 3 H group of DBSA bonded to nitrogen of PANI. The bands at 1133.7 and 829.2 cm −1 are assigned to the aromatic C-H bending in-plane and out-of-plane deformation of C-H. The peaks at 2844.6, 2931.6, and 3249.9 cm −1 are assigned to N-H stretching vibrations of secondary amines. In the early research, such peaks appeared in the range of 3000-3500 cm −1 [67]. The shifting towards the low frequency range in the present work may be due to the presence of DBSA. After adsorption of BB3 dye, all these peaks shift towards high frequency, with a decrease in the intensity of peaks at 2844.6 and 2931.6 cm −1 , as shown in the Figure 3B  A characteristic absorption band is observed at 554.8 cm −1 due to the stretching vibration of Fe-O bonds in the Fe3O4 spectrum. In an early study, stretching vibrations of Fe-O bonds were reported at 560 cm −1 [63]. This shift in the Fe-O band towards lower frequency in the present study may be due to the presence of DBSA in the Fe3O4 particles. Peaks at 1133.6 and 1534.6 cm −1 correspond to CH2 bending modes of DBSA. Similarly, a weak peak at 3494.3 cm −1 is because of -OH stretching attached to the Fe3O4 surface and shows close resemblance to the already reported work [64]. Another weak band at 1734.7 cm −1 is assigned to residual NH4OH, as already reported elsewhere [65]. The peak at 554.8 cm −1 is due to stretching vibrations of Fe-O disappearing and a new peak at 539.5 cm −1 appearing, showing BB3 dye adsorbtion onto Fe3O4, as shown in Figure 3B. This is because of the interaction of oxygen present in the dye structure with Fe of Fe3O4. The appearance of more intense peaks at 1224.6 and 1365.7 in Figure 3B is also attributed to the adsorption of BB3 [66].
FTIR spectrum of PANI is also shown in Figure 3A. Peaks at 1568 cm −1 and 1466 cm −1 are due to C=C and C=N stretching vibrations of benzoinoid and quinoid rings, respectively. Phang and Kuramoto have reported the C=C and C=N stretching vibrations of PANI at 1572 and 1497 cm −1 , respectively [54]. The bands at 1307.6 cm −1 are due to C-N•+ stretching of secondary aromatic amine of PANI doped with protic acid. The peak at 670.1 cm −1 shows out-of-plane bending vibrations of the C-H bond. The peak at 1017.9 cm −1 is assigned to -SO3H group of DBSA bonded to nitrogen of PANI. The bands at 1133.7 and 829.2 cm −1 are assigned to the aromatic C-H bending in-plane and out-ofplane deformation of C-H. The peaks at 2844.6, 2931.6, and 3249.9 cm −1 are assigned to N-H stretching vibrations of secondary amines. In the early research, such peaks appeared in the range of 3000-3500 cm −1 [67]. The shifting towards the low frequency range in the present work may be due All these peaks appeared in the FTIR spectra of PANI/Fe 3 O 4 composites, with a slight shift towards low frequency, as shown in Figure 3A. The shifting of absorption bands towards low frequency shows the existence of physical forces between PANI and Fe 3 O 4 . The band at 3249.9 cm −1 in the FTIR spectrum of PANI is replaced by a broad absorption plateau in the FTIR spectrum of PANI/Fe 3 O 4 composites. The appearance of a very small peak at 539.5 cm −1 , due to Fe-O bond stretching, shows the presence of Fe 3 O 4 in the composite [68]. The absorption bands in the FTIR spectrum of PANI/Fe 3 O 4 shift towards low frequency after adsorption of BB3, as was also observed in the spectra of PANI and Fe 3 O 4 , but the peaks are more intense in the former case, as shown in Figure 3B.

EDX Spectroscopy
EDX study is very important to analyze elemental composition of materials. Figure 4 shows the EDX spectra of  3.1.5. XRD Study X-ray diffraction is an important technique used to determine the structure and composition of synthesized materials. Figure 5A shows XRD patterns of Fe3O4, PANI, and PANI/Fe3O4 composites before adsorption of BB3. The characteristic diffraction peaks appeared at 2θ = 24.04°, 33.06°, 35.6°, 49.3°, 53.9°, and 62.7° in the XRD spectrum of Fe3O4 , which indicates spinel cubic crystals of Fe3O4. The formation of a strong peak at 33.06° indicates the formation of Fe3O4. These peaks were matched with the standard cards on powder diffraction files-2 (PDF 89-598) and have close agreement [71]. After adsorption of BB3, the intensities of diffraction peaks decrease due to interactions between dye and Fe3O4 ( Figure 5B) [72].
XRD spectrum ( Figure 5A) of PANI shows its amorphous nature. No apparent change is observed in the spectrum of PANI after adsorption of BB3 ( Figure 5B). Deshpande et al. [73] have reported a PANI film with amorphous shape. One can observe the presence of Fe3O4 in the PANI matrix due to diffraction peaks in the XRD spectrum of PANI/Fe3O4, but the intensities of these peaks are smaller than those in the spectrum of pure Fe3O4 particles, showing interaction between Fe3O4 and PANI. Obviously, the crystanality in the composites arises due to the presence of Fe3O4 particles. After adsorption of BB3 the peaks in the XRD spectrum of the composites simply disappeared. These In the EDX spectrum of the composite, the contents of both C and O increase after interaction with BB3 (Figure 4f), which suggests the adsorption of BB3 on the composite [70]. These observations support the results obtained through UV-Vis and FTIR spectroscopies. The formation of a strong peak at 33.06 • indicates the formation of Fe 3 O 4 . These peaks were matched with the standard cards on powder diffraction files-2 (PDF 89-598) and have close agreement [71]. After adsorption of BB3, the intensities of diffraction peaks decrease due to interactions between dye and Fe 3 O 4 ( Figure 5B) [72]. observations indicate the strong overlaying layer of the dye on the surface of composites, thereby blunting the XRD peaks that were observed before adsorption of the dye [74].

Surface Area Analysis
Surface area analysis has a major role in the adsorption phenomenon. The surface areas of Fe3O4, PANI, and PANI/Fe3O4 composites before and after adsorption of BB3 were determined by adsorption-desorption of nitrogen gas through Brunauer-Emmett-Teller (BET) method ( Figure 6) [75]. The obtained results are summarized in Table 1, which show that the surface areas of Fe3O4, PANI, and PANI/Fe3O4 composites before adsorption of BB3 are 65.818, 70.263, and 99.759 m 2 /g, respectively ( Figure 6A). After adsorption of BB3, the surface areas of Fe3O4, PANI, and PANI/Fe3O4 composites decreased to 46.608, 46.698, and 53.196 m 2 /g, respectively ( Figure 6B). The decrease in surface areas of Fe3O4, PANI, and PANI/Fe3O4 composites after adsorption of dye confirms that PANI/Fe3O4 composites can adsorb comparatively more dye than Fe3O4 and PANI. These results correlate to those obtained through SEM, XRD, EDX, and FTIR. XRD spectrum ( Figure 5A) of PANI shows its amorphous nature. No apparent change is observed in the spectrum of PANI after adsorption of BB3 ( Figure 5B). Deshpande et al. [73] have reported a PANI film with amorphous shape. One can observe the presence of Fe 3 O 4 in the PANI matrix due to diffraction peaks in the XRD spectrum of PANI/Fe 3 O 4 , but the intensities of these peaks are smaller than those in the spectrum of pure Fe 3 O 4 particles, showing interaction between Fe 3 O 4 and PANI. Obviously, the crystanality in the composites arises due to the presence of Fe 3 O 4 particles. After adsorption of BB3 the peaks in the XRD spectrum of the composites simply disappeared. These observations indicate the strong overlaying layer of the dye on the surface of composites, thereby blunting the XRD peaks that were observed before adsorption of the dye [74].

Surface Area Analysis
Surface area analysis has a major role in the adsorption phenomenon. The surface areas of Fe 3 O 4 , PANI, and PANI/Fe 3 O 4 composites before and after adsorption of BB3 were determined by adsorption-desorption of nitrogen gas through Brunauer-Emmett-Teller (BET) method ( Figure 6) [75]. The obtained results are summarized in Table 1 Beside surface area, BET calculation can also be applied to determine the pore volume and average pore diameter, as shown in Table 1.

Equilibrium Study
An equilibrium study is very valuable for understanding the interaction of BB3 with Fe3O4, PANI, and PANI/Fe3O4 composites. The adsorption data are shown in Table 2, which shows that the adsorption capacity of the dye on these materials increases as the concentration of dye increases. BB3 is a cationic dye and gets adsorbed on Fe3O4, PANI, and PANI/Fe3O4 composites from aqueous solution due to interactions with negative sites on the surface of the adsorbent. In the literature it has been explained that these binding sites are present (electron pair) on oxygen of Fe3O4 and nitrogen of amine and imine PANI and PANI/Fe3O4, which are capable of interacting with oppositely charged ions present in the dye [76]. The data obtained from the equilibrium study were fitted into Freundlich, Langmuir, Tempkin, and D-R adsorption isotherms for estimation of various adsorption parameters.
Freundlich adsorption equation is expressed by the following equation.  Beside surface area, BET calculation can also be applied to determine the pore volume and average pore diameter, as shown in Table 1.

Equilibrium Study
An equilibrium study is very valuable for understanding the interaction of BB3 with Fe 3 O 4 , PANI, and PANI/Fe 3 O 4 composites. The adsorption data are shown in Table 2, which shows that the adsorption capacity of the dye on these materials increases as the concentration of dye increases. BB3 is a cationic dye and gets adsorbed on Fe 3 O 4 , PANI, and PANI/Fe 3 O 4 composites from aqueous solution due to interactions with negative sites on the surface of the adsorbent. In the literature it has been explained that these binding sites are present (electron pair) on oxygen of Fe 3 O 4 and nitrogen of amine and imine PANI and PANI/Fe 3 O 4 , which are capable of interacting with oppositely charged ions present in the dye [76]. The data obtained from the equilibrium study were fitted into Freundlich, Langmuir, Tempkin, and D-R adsorption isotherms for estimation of various adsorption parameters.
where q e (mg/g) is the amount of dye adsorbed per gram of adsorbent, C e (mg/L) is the concentration of dye at equilibrium, K f is Freundlich isotherm constant, and n is the intensity of adsorbent. A plot of lnq e vs. lnC e is shown in Figure 7a.
where qe (mg/g) is the amount of dye adsorbed per gram of adsorbent, Ce (mg/L) is the concentration of dye at equilibrium, Kf is Freundlich isotherm constant, and n is the intensity of adsorbent. A plot of lnqe vs. lnCe is shown in Figure 7a. From the value of the slope obtained from the Freundlich adsorption isotherm, it can be demonstrated whether adsorption is favorable or unfavorable, reversible or irreversible. It also explains whether the system is heterogeneous or not [77]. If 1/n > 1, adsorption is unfavorable at low concentration but favorable at high concentration; if 1/n < 1, adsorption is favorable over the entire From the value of the slope obtained from the Freundlich adsorption isotherm, it can be demonstrated whether adsorption is favorable or unfavorable, reversible or irreversible. It also explains whether the system is heterogeneous or not [77]. If 1/n > 1, adsorption is unfavorable at low concentration but favorable at high concentration; if 1/n < 1, adsorption is favorable over the entire range of concentrations and the system is heterogeneous. However, if 1/n = 1, then the system is homogenous [78]. The values of 1/n obtained from the Freundlich adsorption isotherm in the present study are 0.9593, 0.8673, and 0.9112 for adsorption of BB3 on Fe 3 O 4 , PANI, and PANI/Fe 3 O 4 composites, respectively, as shown in Table 2. These values are in close resemblance with the literature showing that adsorption is favorable and heterogeneous. R 2 values show that the Freundlich adsorption isotherm fits to the adsorption data for Fe 3 O 4 , PANI, and PANI/Fe 3 O 4 composites.
The adsorption data were also analyzed through the Langmuir adsorption isotherm, which is expressed in the following equation.
where q max is the max adsorption capacity (mg/g), q e is the amount of dye adsorbed at equilibrium (mg/g), Ce is the equilibrium adsorption concentration (mg/L), and K L is the constant related to energy (Langmuir constant). From the Langmuir isotherm, R L (dimensionless separating factor) is calculated by the following equation.
From R L value it can be demonstrated whether adsorption is favorable, unfavorable, reversible, or irreversible. If R L value is less than one but more than zero (0 < R L < 1) adsorption is favorable, but if 1 < R L adsorption is unfavorable. If R L = 0 adsorption is irreversible and R L = 1 indicates that adsorption is reversible [79]. The adsorption data obtained through the Langmuir isotherm are given in Table 2, which show that the maximum adsorption capacities (q max ) are 7.474, 47.977, and 78.13 mg/g for Fe 3 O 4 , PANI and PANI/Fe 3 O 4 composites, respectively. The values of Langmuir constant (K L ) and dimensionless separating constant (R L ) for all the three types of adsorbents shows that adsorption of BB3 on Fe 3 O 4 , PANI, and PANI/Fe 3 O 4 composites is monolayer and favorable. R 2 values show that the Langmuir adsorption isotherm fits more closely to the adsorption data than the other isotherms.
Tempkin adsorption isotherm, shown in the Equation (5), was also applied to explain the adsorption data. q e = βlnK T +βlnC e R 2 values show that Tempkin isotherm does not fit very well to adsorption data as compared to Freundlich and Langmuir isotherms, but is still helpful in explaining the binding forces between adsorbents and adsorbate. K T is the binding constant at equilibrium and corresponds to maximum binding energy [80]. Its values calculated from the intercept of plot q e vs. lnC e (Figure 7c)  The Dubinin-Radushkevitch (D-R) adsorption equation has also been successfully applied to the data obtained by plotting lnq e vs. ε 2 , and is shown in Figure 7d. A linearized form of D.R adsorption equation is given below lnq e = lnq s −Bε 2 (7) where q s is the theoretical monolayer saturation capacity (mg/g), B is the constant, called D-R model constant, and ε 2 is the Polanyi potential, which is calculated by the Equation (8) where R is the general gas constant and T is the absolute temperature. From the D-R model, energy of adsorption was calculated by Equation (9) E ads = 1 In the literature it has been explained that for physical adsorption, the value of adsorption energy should be less than 40 kJ/mol [81]. Its value also tells about the route of adsorption through ion exchange process. In the early literature it has been explained that for ion exchange process the value of adsorption energy should be in the range of 8-16 kJ/mol. The values of q s calculated from the linear plot of D-R isotherm are 0.888, 9.183, and 20.54 mg/g for Fe 3 O 4 , PANI, and PANI/Fe 3 O 4 composites, respectively, showing that adsorption is physical. Similarly, values of (E ads ), shown in Table 2, demonstrate that adsorption does not follow ion exchange process [82]. A comparison of the adsorption efficiency of the synthesized materials with those reported earlier is also provided in Table 3.

Effect of Ionic Strength
Electrostatic interactions, such as ionic strength, greatly affect the surface properties of the adsorbent [91]. The effect of ionic strength on adsorption of BB3 (dye concentration 80 mg/L in 20 mL volume) on Fe 3 O 4 , PANI, and PANI/Fe 3 O 4 was observed by adding sodium sulphate solution in the range of 0.01-0.3 mol dm −3 . The obtained results (Figure 8) [71,92]. This competition is related to the interactions between hydrated ions and active sites of the adsorbent. Cations with a smaller hydrated radius occupy more active sites on the adsorbent, leading to stronger interaction with the adsorbent [93].
Fe3O4, PANI, and PANI/Fe3O4 composites decrease as the concentration of salt (ionic strength) increases. The minimum dye adsorption on Fe3O4, PANI, and PANI/Fe3O4 was observed at 0.25, 0.21, and 0.25 ionic strengths, respectively. The competition of Na + or SO4 2− ions with BB3 dye for active sites present on the surface of Fe3O4, PANI, and PANI/Fe3O4 might be a reason for the decrease in adsorption capability [71,92]. This competition is related to the interactions between hydrated ions and active sites of the adsorbent. Cations with a smaller hydrated radius occupy more active sites on the adsorbent, leading to stronger interaction with the adsorbent [93].

Effect of pH
The pH of the solution plays a major role in the removal of adsorbates from aqueous solutions. Figure 9 shows the effect of pH on the adsorption of BB3 on Fe3O4, PANI, and PANI/Fe3O4. As BB3 is a cationic dye, at low pH the H + ions compete with dye for active sites present on the surface of the adsorbent and protonate them. These active sites are Fe-O and -C-N groups. Similarly, the nitrogen and oxygen in the dye are also protonated. This causes electrostatic repulsion between dye and adsorbent, hence reducing adsorption [94]. As the pH of dye solutions increases, the adsorption increases and reaches a maximum for Fe3O4, PANI, and PANI/Fe3O4 composites when the pH of the dye solution is 12, 8, and 10, respectively. At high pH de-protonation of Fe-OH and -C-N-H groups occurs, resulting in negatively charged sites, such as Fe-O − and -C-N − , which have stronger interactions with dye. Figure 9 also indicates that after optimum pH, adsorption once again decreases. This may be due to hydroxylation of active sites of adsorbents [95].

Effect of pH
The pH of the solution plays a major role in the removal of adsorbates from aqueous solutions. Figure 9 shows the effect of pH on the adsorption of BB3 on Fe 3 O 4 , PANI, and PANI/Fe 3 O 4 . As BB3 is a cationic dye, at low pH the H + ions compete with dye for active sites present on the surface of the adsorbent and protonate them. These active sites are Fe-O and -C-N groups. Similarly, the nitrogen and oxygen in the dye are also protonated. This causes electrostatic repulsion between dye and adsorbent, hence reducing adsorption [94]. As the pH of dye solutions increases, the adsorption increases and reaches a maximum for Fe 3 O 4 , PANI, and PANI/Fe 3 O 4 composites when the pH of the dye solution is 12, 8, and 10, respectively. At high pH de-protonation of Fe-OH and -C-N-H groups occurs, resulting in negatively charged sites, such as Fe-O − and -C-N − , which have stronger interactions with dye. Figure 9 also indicates that after optimum pH, adsorption once again decreases. This may be due to hydroxylation of active sites of adsorbents [95].

Effect of Contact Time and Temperature
Contact time and temperature are also important parameters for explaining the adsorption phenomenon. The adsorption of BB3 on Fe3O4, PANI, and PANI/Fe3O4 composites as a function of time is shown in Figure 10a, which shows that the adsorption increases with the passage of time. This figure also shows that initially adsorption is fast and contributes significantly to the equilibrium, but as the time passes, the adsorption slows down and its contribution to equilibrium decreases. This is

Effect of Contact Time and Temperature
Contact time and temperature are also important parameters for explaining the adsorption phenomenon. The adsorption of BB3 on Fe 3 O 4 , PANI, and PANI/Fe 3 O 4 composites as a function of time is shown in Figure 10a, which shows that the adsorption increases with the passage of time. This figure also shows that initially adsorption is fast and contributes significantly to the equilibrium, but as the time passes, the adsorption slows down and its contribution to equilibrium decreases. This is due to filling of active sites on the surface of the adsorbent by the molecules of dye, and gradually adsorption becomes less effective. At this time, a dynamic equilibrium is established between the amount of dye adsorbed and desorbed from the adsorbent. This time is termed "equilibrium time" and the dye adsorbed at the equilibrium time is referred to as the maximum adsorption capacity of the adsorbent. It is evident from Figure 10a that the equilibrium time of adsorption is reached for Fe 3 O 4 , PANI, and PANI/Fe 3 O 4 composites within 50 to 60 min [96]. Figure 10b shows that adsorption of BB3 on PANI and PANI/Fe 3 O 4 composites is maximal at 30 • C and decreases beyond this temperature, indicating exothermic behavior.

Effect of Adsorbent Dose
The effect of adsorbent dose on adsorption of BB3 (50 mg/L) is studied with different amounts (0.02 g, 0.06 g, and 0.1 g) of Fe3O4, PANI, and PANI/Fe3O4 composites, respectively. The results are shown in the Figure 11, which shows that amount of adsorption of BB3 increases as the amount of adsorbent increases. This shows that as the amount of adsorbent increases, more active sites are available for the adsorption of dye, which results in more interactions between dye and adsorbent. The figure shows that the adsorption capacity of PANI/Fe3O4 composites is more than Fe3O4 and PANI.

Effect of Adsorbent Dose
The effect of adsorbent dose on adsorption of BB3 (50 mg/L) is studied with different amounts (0.02 g, 0.06 g, and 0.1 g) of Fe 3 O 4 , PANI, and PANI/Fe 3 O 4 composites, respectively. The results are shown in the Figure 11, which shows that amount of adsorption of BB3 increases as the amount of adsorbent increases. This shows that as the amount of adsorbent increases, more active sites are available for the adsorption of dye, which results in more interactions between dye and adsorbent. The

Kinetic Study
Kinetic study is very important to explain the adsorption phenomenon. The data obtained from adsorption of BB3 dye were analyzed through Lagergren's pseudo first order, Ho and McKay's pseudo second order, and Weber and Morris's intra particle diffusion models by using Equations (10)- (12).
where qe and qt are the amount of dye adsorbed (mg g -1 ) at equilibrium and at time t, K1, K2, and Kd are rate constant of pseudo first order (min −1 ), pseudo second order (g mg −1 min −1 ), and intra-particle diffusion models (g mg −1 min −1/2 ), respectively. C (mg g -1 ) is the constant and t is the time in minutes. Figure 12a-c shows the fitted curves of pseudo first order, pseudo second order, and intra-particle diffusion models for BB3 adsorbed on Fe3O4, PANI, and PANI/Fe3O4 composites, respectively. The kinetics data obtained are shown in Table 4. The correlation factor (R 2 ) indicates that the pseudo second order kinetic model fits more closely to data as compared to the pseudo first order and intraparticle diffusion models. Values of rate constant indicate that as the temperature increases, rate of adsorption decreases [77,78].

Kinetic Study
Kinetic study is very important to explain the adsorption phenomenon. The data obtained from adsorption of BB3 dye were analyzed through Lagergren's pseudo first order, Ho and McKay's pseudo second order, and Weber and Morris's intra particle diffusion models by using Equations (10)- (12). log(q e −q t ) = logq e − K 1 t 2.303 (10) where q e and q t are the amount of dye adsorbed (mg g -1 ) at equilibrium and at time t, K 1 , K 2 , and K d are rate constant of pseudo first order (min −1 ), pseudo second order (g mg −1 min −1 ), and intra-particle diffusion models (g mg −1 min −1/2 ), respectively. C (mg g -1 ) is the constant and t is the time in minutes. Figure 12a-c shows the fitted curves of pseudo first order, pseudo second order, and intra-particle diffusion models for BB3 adsorbed on Fe 3 O 4 , PANI, and PANI/Fe 3 O 4 composites, respectively. The kinetics data obtained are shown in Table 4. The correlation factor (R 2 ) indicates that the pseudo second order kinetic model fits more closely to data as compared to the pseudo first order and intra-particle diffusion models. Values of rate constant indicate that as the temperature increases, rate of adsorption decreases [77,78].

Mechanism of Adsorption
Actually, many factors, such as structure and charge on dye, surface of adsorbent, hydrophilic, and hydrophobic properties, electrostatic interaction, and physical forces, such as hydrogen bonding and dipole-dipole interaction, affect the adsorption of BB3 on PANI/Fe3O4 composites. Therefore, different mechanisms can be proposed for the adsorption of BB3 on Fe3O4, PANI, and PANI/ Fe3O4 composites. When BB3 is added to water, it dissociates in a positively-charged complex cation and negatively charged chloride ion as shown below (Scheme 3).

Mechanism of Adsorption
Actually, many factors, such as structure and charge on dye, surface of adsorbent, hydrophilic, and hydrophobic properties, electrostatic interaction, and physical forces, such as hydrogen bonding and dipole-dipole interaction, affect the adsorption of BB3 on PANI/Fe 3 O 4 composites. Therefore, different mechanisms can be proposed for the adsorption of BB3 on Fe 3 O 4 , PANI, and PANI/ Fe 3 O 4 composites. When BB3 is added to water, it dissociates in a positively-charged complex cation and negatively charged chloride ion as shown below (Scheme 3). There is a possibility of H-bonding between amine and imine groups of PANI/Fe3O4 with nitrogen and oxygen present in the BB3 structure. Similarly, the surface hydroxyl groups of Fe3O4 may also form H-bonds with dye molecules [97].
There may exist Vander Waal's interaction between hydrophobic parts of the dye and hydrophobic parts of the PANI/Fe3O4 composite, because the nonpolar groups have a tendency to There is a possibility of H-bonding between amine and imine groups of PANI/Fe 3 O 4 with nitrogen and oxygen present in the BB3 structure. Similarly, the surface hydroxyl groups of Fe 3 O 4 may also form H-bonds with dye molecules [97].
There may exist Vander Waal's interaction between hydrophobic parts of the dye and hydrophobic parts of the PANI/Fe 3 O 4 composite, because the nonpolar groups have a tendency to associate in aqueous solution. Another possibility is the existence of electrostatic interaction between positively-charged nitrogen present in the dye structure and a lone pair present on the nitrogen of amine and imine group of PANI and PANI/Fe 3 O 4 [98]. The adsorption behavior of BB3 on PANI/Fe 3 O 4 in basic medium is shown as the following (Scheme 4). There is a possibility of H-bonding between amine and imine groups of PANI/Fe3O4 with nitrogen and oxygen present in the BB3 structure. Similarly, the surface hydroxyl groups of Fe3O4 may also form H-bonds with dye molecules [97].

O N N Cl
There may exist Vander Waal's interaction between hydrophobic parts of the dye and hydrophobic parts of the PANI/Fe3O4 composite, because the nonpolar groups have a tendency to associate in aqueous solution. Another possibility is the existence of electrostatic interaction between positively-charged nitrogen present in the dye structure and a lone pair present on the nitrogen of amine and imine group of PANI and PANI/Fe3O4 [98]. The adsorption behavior of BB3 on PANI/Fe3O4 in basic medium is shown as the following (Scheme 4). where R represents the non-polar part of the PANI/Fe3O4 with =NH,-NH2 of PANI, and -OH group of Fe3O4.
During the adsorption process the amount of energy released compensates for the entropy change of adsorbed molecules and depends upon the forces between adsorbent and adsorbate molecules; the stronger the force, the more energy will be released. The energy released during the adsorption process for H-bond is (2-40 kJ/mol), dipole-dipole interaction is (2-29 kJ/mol), Vander Waals forces is (4-10 kJ/mol), and is about 5 kJ/mol for hydrophobic forces, and more than 60 kJ/mol for electrostatic interaction [99]. In the present study the enthalpy change are −32.84, −62.93, and −74.26 kJ/mol when BB3 adsorbs on Fe3O4, PANI, and PANI/Fe3O4, respectively.

Calculation of Thermodynamic Parameters
Thermodynamic parameters, such as activation energy, Gibb's free energy change, enthalpy change, and entropy change, are helpful to explain the nature of adsorption. Activation energy is calculated by Arrhenius equation, shown below. k = Aexp (−Ea/RT) (13) where Ea is the activation energy, T is the absolute temperature, A is the Arrhenius constant, and k is the rate constant. Gibb's Free energy change is calculated by the following equation.  During the adsorption process the amount of energy released compensates for the entropy change of adsorbed molecules and depends upon the forces between adsorbent and adsorbate molecules; the stronger the force, the more energy will be released. The energy released during the adsorption process for H-bond is (2-40 kJ/mol), dipole-dipole interaction is (2-29 kJ/mol), Vander Waals forces is (4-10 kJ/mol), and is about 5 kJ/mol for hydrophobic forces, and more than 60 kJ/mol for electrostatic interaction [99]. In the present study the enthalpy change are −32.84, −62.93, and −74.26 kJ/mol when BB3 adsorbs on Fe 3 O 4, PANI, and PANI/Fe 3 O 4 , respectively.

Calculation of Thermodynamic Parameters
Thermodynamic parameters, such as activation energy, Gibb's free energy change, enthalpy change, and entropy change, are helpful to explain the nature of adsorption. Activation energy is calculated by Arrhenius equation, shown below. k = Aexp (−Ea/RT) (13) where Ea is the activation energy, T is the absolute temperature, A is the Arrhenius constant, and k is the rate constant. Gibb's Free energy change is calculated by the following equation.
Enthalpy change and entropy change are calculated by Van't Hoff equation by plotting the lnq e /C e vs. 1/T, as given below.
where ∆H is the enthalpy change and ∆S is the change in entropy, and T is the absolute temperature. Figure 13a shows the Arrhenius plot, obtained by plotting lnK 2 vs. 1/T after adsorption of BB3. From the slope the activation energy values of adsorption of BB3 were to found to be 11.14, 11.97, and 09.94 kJ/mol, respectively, which indicate that adsorption is physical and is a diffusion control process (Table 5) [100]. The value of enthalpy change is also helpful in explaining the adsorption phenomenon. It was reported that enthalpy change in the range of 84-420 kJ/mol suggests chemical interaction between dye and adsorbent (chemisorption), while its value below 84 kJ/mol indicates physical adsorption [95]. The values of enthalpy change in the present work, as shown in Table 5 composite, respectively, thereby confirming the physical process. The negative sign of ∆H indicates that adsorption is exothermic. The ∆G value is also helpful in explaining the adsorption phenomenon, it explains the spontaneity and non-spontaneity of adsorption. The negative sign for ∆G shown in Table 5 indicates that adsorption is exothermic and spontaneous. The ∆G values in the range of −20 to 0 kJ/mol show physiosorption, and from −400 to −80 kJ/mol show chemisorption [101,102].
The ∆G values for Fe 3 O 4, PANI, and PANI/Fe 3 O 4 composite used as adsorbents are −04.05, −07.78, and −10.63 kJ/mol, respectively, which suggest that adsorption of BB3 dye on all the three adsorbents is physical, exothermic, and spontaneous [103]. These observations strongly correlate with the data presented in Section 3.5 for temperature effect on the absorption phenomenon.
the slope the activation energy values of adsorption of BB3 were to found to be 11.14, 11.97, and 09.94 kJ/mol, respectively, which indicate that adsorption is physical and is a diffusion control process (Table 5) [100]. The value of enthalpy change is also helpful in explaining the adsorption phenomenon. It was reported that enthalpy change in the range of 84-420 kJ/mol suggests chemical interaction between dye and adsorbent (chemisorption), while its value below 84 kJ/mol indicates physical adsorption [95]. The values of enthalpy change in the present work, as shown in Table 5, are −32.84, −62.93, and −74.26 kJ/mol for the adsorption of BB3 on Fe3O4, PANI, and PANI/Fe3O4 composite, respectively, thereby confirming the physical process. The negative sign of ΔH indicates that adsorption is exothermic. The ΔG value is also helpful in explaining the adsorption phenomenon, it explains the spontaneity and non-spontaneity of adsorption. The negative sign for ΔG shown in Table 5 indicates that adsorption is exothermic and spontaneous. The ΔG values in the range of −20 to 0 kJ/mol show physiosorption, and from −400 to −80 kJ/mol show chemisorption [101,102]. The ΔG values for Fe3O4, PANI, and PANI/Fe3O4 composite used as adsorbents are −04.05, −07.78, and −10.63 kJ/mol, respectively, which suggest that adsorption of BB3 dye on all the three adsorbents is physical, exothermic, and spontaneous [103]. These observations strongly correlate with the data presented in Section 3.5 for temperature effect on the absorption phenomenon.

Conclusions
PANI/Fe 3 O 4 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/Fe 3 O 4 composites in comparison to that adsorbed for Fe 3 O 4 (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 2 values of 0.873, 0.979, and 0.999 for Fe 3 O 4 , PANI, and PANI/Fe 3 O 4 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 Fe 3 O 4 , 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/Fe 3 O 4 , 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.