Preparation of Magnetic Composite Polyaniline/Fe3O4−Hydrotalcite and Performance in Removal of Methyl Orange

Magnetic composite fabricated from polyaniline and Fe3O4-hydrotalcite (Pan/MHT) was successfully applicated for removal of methyl orange (MO) from wastewater. The structure and properties of Pan/MHT were characterized by Fourier-transform infrared spectroscopy, scanning electron microscopy, X-ray diffraction, vibrating sample magnetometer, and Brunauer-Emmett-Teller adsorption isotherm. Adsorption kinetic results indicated that the adsorption process followed pseudosecond-order kinetic model ( R 2 = 0.999 ), MO adsorption onto Pan/MHT was well described by Freundlich isotherm ( R 2 = 0.994 ), and the MO adsorption capacity of 2 Pan/MHT obtained the highest with Q e = 156.25   mg / g . Batch adsorption experiments were carried out using magnetic composite with the effects of initial MO concentration, solution pH, and adsorbent dosage. The results revealed that the magnetic Pan/MHT exhibited efficient adsorption of MO in the aqueous solution as a result of the affinity for organic dyes, microporous structure, and suitable surface area for adsorption (15,460 m2/g). The superparamagnetic behavior of Pan/MHT (with H c = 18.56   Oe , M s = 23.38 × 10 − 3   emu / g , and M r = 0.91 × 10 − 3   emu / g ) helps that it could be separated from the solution and performs as an economical and alternative adsorbent to removal and degrade azo dye from wastewater. Pan/MHT was also investigated to reuse after desorption of MO in 0.1 M HCl, and the results show that 2 Pan/MHT can be reused for 4 cycles with Q e = 79.66   mg / g .


Introduction
Dyes are widely used in many fields, such as textile, cosmetic, pigment, paper industry, dyeing, or tanning industry. In particular, textile industry wastewater contains a lot of dyes with large molecule structure including the azo group which is sustainable and difficult to decompose [1]. The presence of organic dyes is a main cause of environmental pollution problems because they are nonbiodegradable; so, an effective method is necessary to remove them from wastewater sources. It is estimated that 10-15% lost in the effluent during the dyeing process [2]. If they are not promptly treated strictly before releasing to the environment, they can destroy the ecosystem of aquatic organisms and threaten the self-cleaning capacity of the receiving source and the human life.
Although both polyaniline (Pan) and HT have high adsorption capacity and have been widely used in water treatment. The most important problem is the ability to recover and reuse them. Filtration is an easy method and is widely used but it's not so effective and fast as using the magnetic field. When combined with a magnetic particle Fe 3 O 4 , the composite or hybrid from HT will have a magnetic property. As a result, the composite produces a constant suspension in an aqueous solution and is immediately separated from the external environment. On the other hand, modification of hydrotalcite including investigation of magnetic Fe 3 O 4 /HT nanocomposites (MHT) may have great potential for photocatalysis and environmental remediation [43].
As a conductive polymer, polyaniline (Pan) meets the requirements for environmental treatment applications as both an adsorbent and a photo catalyst according to the following characteristics: high adsorption area, flexible bandgap, and steady−state of activation [44]. Pan nanofiber is an effective adsorbent in studying the removal of methyl orange (MO) and black [45,46]. Pan is also fabricated as a nanotube and nanoparticle, and they all have high efficiency for removal of MO and crystal violet as well as methylene blue [38,[47][48][49][50][51]. Pan nanoparticles give the highest adsorption capacity due to their high surface area of 1083 m 2 /g. Besides, it was used as an efficient adsorbent to remove for crystal violet, and methyl orange with the maximum adsorption capacity reaching 245 and 220 mg/g, respectively. In addition, Pan acts as a photo catalyst thanks to its electrical conductivity to increase protonation. Oxidation reactions occur on the catalyst surface, decomposition of the pigment produces intermediate compounds, and the final products are CO 2 , SO 4 2− , NO 3 − , and NH 4 + . Therefore, the fast charge generation and slow charge recombination lead to enhanced photo catalytic activity.
In summary, both Pan and MHT have high efficiency in the removal of azo dye. However, studies on the adsorption applications of Pan are limited because Pan has a low surface area resulting in low adsorption efficiency. Therefore, many researches are carried out to composing Pan particles with other materials to enhance the adsorption capacity by various synthesis methods. Another way, many reports have shown that MHT material has been suitable for an adsorbent. So, the combination of Pan and MHT to create a Pan/MHT hybrid or composite material has great potential, and it is still not widely exploited for adsorption, especially in wastewater treatment to adsorp dyes (MO, Congo red, ...).
The structure of the hydroxide porous layer and the ability to exchange ions in MHT combined with Pan will give good capacitive properties and high environmental sustainability, because of the special structure that induces ion exchange, restructuration, and the large surface area, so that Pan/MHT can "trap" the MO dye molecules into its structure, recover MO molecules, and remove them from wastewater. In addition, by the restructuring ability, the Pan/ MHT material can be desorped for reuse, easily recovered by magnets, saving costs.
For the above advantages of Pan/MHT composite, in this paper, we synthesized Pan/MHT composite with various mass ratios of m Pan /m MHT = 0:5, 1, and 2. Then, we investigate the adsorption kinetic and isotherm. We also studied the effect of contact time, pH, and adsorbent dose onto the adsorption capacity of MO in water.  (Figure 1). An accurate amount of 0.1 g Fe 3 O 4 was dispersed in distilled water (10 mL). A mixture of Zn(NO 3 ) 2 .4H 2 O and Al(NO 3 ) 3 .9H 2 O with Zn 2+ /Al 3+ molar ratio of 2.0 was dissolved in 100 mL distilled water and then coprecipitated into the Fe 3 O 4 dispersion. The pH of the reaction solution was kept at around 9:5 ± 0:1. Then, the coprecipitated mixture was heated for aging at 80°C for 24 hours. The MHT was neutralized with distilled water and dried at 60°C for 6 hours obtain brown MHT powder.

Preparation of Polyaniline/MHT (Pan/MHT).
Pan/ MHTs were fabricated using in situ oxidative polymerization ( Figure 2). An accurate amount of 0.2 g MHT was dispersed in a distilled water (40 mL) and alcohol (10 mL) mixture. 0.5 mL of Tween 80 5% was added to the mixture and then sonicated in 30 minutes to acquire a well−dispersed colloidal suspension. The desired amount of aniline dissolved in 25 mL of HCl 0.1 M. The mixture was stirred for 1 hour for complete mixing at 4°C. An accurate amount of APS (n APS /n aniline = 1/5) was dissolved in the rest of 25 mL HCl and precooled at 4°C for 15 min. APS solution was dropped into the reaction mixture and when completed, the reaction was kept at 4°C with mechanical stirring at 500 rpm. The polymerization was finished after 8 hours, then washed the suspension with a mixture of distilled water:ethanol (70 : 30) for 5 times, and dried in a vacuum dryer at 50°C for 5 hours to obtain a dark green composite Pan/MHT.  Adsorption Science & Technology Powder X−ray diffraction patterns of Pan, MHT, 0.5 Pan/ MHT, 1 Pan/MHT, and 2 Pan/MHT composites were obtained using a Bruker D8 Advance diffractometer with CuKα radiation (1,5406 Å) at room temperature in the atmospheric pressure. Data were collected in the range of 2 theta from 5 to 70°with a step of 0.02°and a scanning rate of 1°/min.
The structure and morphology of MHT, Pan, and Pan/ MHT composites were observed by scanning electron microscopy (SEM). SEM observations were carried out using a Thermo Scientific E−SEM instrument.
The magnetic property of MHT and series of Pan/MHT were analyzed with a vibration sample magneparameter (VSM) method using a Lake Shore Crytotronics device.
The specific surface area and average pore size of Pan/ MHT and MHT were determined by nitrogen adsorption t 76 K. The Brunauer−Emmett−Teller (BET) method was employed for the corresponding calculation.

Batch Studies.
The adsorption experiments were performed using the batch equilibrium technique in aqueous solutions at room temperature. Duplicate 20 mg Pan/MHT samples were equilibrated by shaking with 25 mL of MO solution with initial concentrations (C 0 ) ranging between 5 mg/L and 20 mg/L and the effect of pH of MO solution ranging between 3 and 10, and the effect of the adsorbent dose was also studied. The effect of contact time was studied at 15−720 minutes, and the equilibrium was reached at 120 minutes. The MO concentration in the supernatant was measured by a UV − VIS spectrophotometer at a wavelength corresponding to the maximum absorbance for MO (460 nm) and determined using a linear regression equation obtained by plotting a calibration curve for MO over a range of concentrations. All of the adsorptions of MO in batch studies were carried out using a Jasco V630 UV − VIS spectrophotometer.
The adsorption of MO on Pan/MHT could be calculated from the difference between the initial concentration (C 0 ) and the equilibrium one (C e ). The adsorption was expressed in terms of adsorption percentage (%) which was calculated from the following equations: where V ðLÞ and m ðgÞ represent the mass of Pan/MHTs and the volume of the suspension, respectively. Here, C s is the capacity of MO sorption on Pan/MHT after adsorption equilibrium (mg/g).

Desorption and Reusability of Pan/MHT.
For the reusability study, 0.1 g of Pan/MHT was used repeatedly for the adsorption of MO. An adsorption process was performed by adding 0.1 g Pan/MHT in 150 mL of MO at 20 mg/L and shaking for 24 hours to equilibrium adsorption. After that, the Pan/MHT was separated from the MO solution by using a magnetic bar, and the MO solution was analyzed by UV-VIS spectrophotometry to determine the MO adsorption capacity.
The desorption process was performed by dispersing Pan/MHT previously used in the adsorption process in 100 mL of HCl 1.0 M which promotes MO desorption. The desorption was performed in about 1 hour and then separating Pan/MHT by using a magnetic bar. The MO concentration in HCl was also analyzed by UV-VIS spectrophotometry. The cycles of reuse of the same Pan/ MHT sample were carried out for a total of 4 cycles. The   Pan characterized with the strong absorptions at 3439, 2941, 1569, 1495, 1301, 1130, and 875 cm −1 , which was the evidence of the emeraldine form of polyaniline [53][54][55]. The vibration at about 3220 cm −1 is assigned for protonated radical cation N − H + ·, and the vibration at about 3443 cm −1 is assigned for the N−H stretching. The characteristic band at 1515 cm −1 was assigned to both C=N and C−N stretching of quinoid ring structure, and the band at 1481 cm −1 was attributed to both C=C and C−N of benzenoid ring structure. All of these absorption peaks were characterized for the inplane bending vibration. The band at around 1301 cm −1 indicated the vibration of the C − N stretching of benzenoid ring, and the one at around 875 cm −1 was the C − H out of plane bending vibration parade-substituted benzene indicating polyaniline polymer formation. In addition, the strong band at 1130 cm −1 was assigned for the C − N=C group attached on the quinoid ring.     Adsorption Science & Technology MHT material, the 34.6°peak that appeared in HT is divided into two subpeaks. The first peak at a smaller diffraction angle shows the peak of the hydrotalcite, and the second one at a slightly larger diffraction angle shows the magnetite core deposited on the surface of the hydrotalcite corresponding to the magnetite particle combined on the surface of HT. XRD diffraction spectra of MHT material are consistent with that of Sulistyaningsih et al. [52].

Results and Discussion
The results from the figure show the characteristic diffraction peak of Pan at 2θ = 13°, 2θ = 21°, and 2θ = 25°. Typical diffraction peaks of MHT at 2θ = 11:7°, 2θ = 11:8°, and 2θ = 11:6°. With different from ratios of m Pan /m MHT , there is also different typical diffraction peak of MHT at 2θ = 11:6°and 2θ = 12°. It is because of the bond between Pan and MHT in the material, and the interaction between Pan and MHT can be influenced by hydroxides groups on the MHT surface, affecting the structure and material properties.
3.1.3. SEM Images. The morphology of the material was observed by SEM images in Figure 6. Pan/MHT morphology was observed, in which the distribution of Pan and MHT differed markedly in the proportions. The structural morphology of MHT is a porous material consisting of multilayers, and Pan has the structure of a long carbon chain. The distribution of MHT for Pan is different when changing the mass ratio of Pan. This result is completely consistent with the results of structural analysis by significantly reduced X−ray diffraction.
3.1.4. Vibrating Sample Magnetometer (VSM) Analysis. The magnetic properties of MHT and series of Pan/MHT were investigated using a vibrating sample magnetometer at room temperature, and the magnetization curves of MHT and series of Pan/MHT were shown in Figure 7. The roughly coincident hysteresis curves show that the grain size of the MHT metal phase is very small, nanometers in size and is a superparamagnetic material. Because MHT is the hybrid material from HT with only 1 wt% Fe 3 O 4 particle, the superparamagnetic characteristic of MHT is the low value of Ms and Mr that are 2:73 × 10 −3 emu/g and 0:10 × 10 −3 emu/g, and H c is 20.06 Oe. Pan is a conductive polymer, and it tends to help increasing the ferromagnetic behavior for MHT. However, Pan is diamagnetic, so that when the mass ratio of m Pan / m MHT increased, the values of M s and M r decreased as the results in Table 1. And this result is consistent with the announcement of Umare et al. and Jokar et al. [56,57]. These magnetic properties are high enough that Pan/MHT materials can be separated from the solution by an external magnet.
The low M r value indicates that the Pan/MHT can be easily separated from the solution by an external magnetic field and redistributed into the solution after the magnetic field is disconnected. This will be an advantage for the separation, recovery, and reuse of the adsorbent as well as the (e)   Figure 9 shows the effect of contact time of 0.5 Pan/MHT, 1 Pan/MHT, and 2 Pan/ MHT with 100 mL of 20 mg/L MO solution. The adsorption amounts of MO increased rapidly in the initial 30 minutes for all samples. Then, the adsorption amounts increased slighter in the next 30 minutes. It becomes slow to attained equilibrium time which is 120 minutes for 0.5 Pan/MHT, 1 Pan/MHT, and 240 minutes for 2 Pan/MHT in the same conditions. The difference of equilibrium time for MO adsorption obtained by various adsorbents can be explained by the MO anion adsorption on the adsorbents by surface and anion exchange mechanism. The adsorbent with the higher mass ratio of Pan/MHT, the more MO can be adsorbed. This result is due to the complex adsorption mechanism of Pan such as surface adsorption, anion −exchange, …, which is consistent with the research of Monika et al. and Wang et al. [58,59].
Kinetic modeling of the adsorption process provides a prediction of adsorption rates and allows the determination of suitable rate expressions characteristic of the possible reaction mechanisms. In this study, the most frequently used models, the pseudofirst−order, the pseudosecond−order, and the intraparticle diffusion model were tested.
Pseudofirst−order kinetics: the linearized from of the pseudofirst−order linearized equation is given as where t and Q e (mg/g) are the MO removal at time t and at equilibrium. k 1 (min −1 ) is the equilibrium rate constant of the pseudofirst-order equation. The plots of log ðQ e − Q t Þ vs. t are shown in Figure 10(a), and k 1 is predicted Q e can be determined from the slope and intercept of the plot, respectively. Pseudo−secondorder kinetics: the linearized form of the pseudosecond-order equation is given as where k 2 [g/(mg min)] is the equilibrium rate constant of the pseudosecond−order equation. The plot of t/Q t vs. t is shown in Figure 10(b), and k 2 and Q e can be determined from the slope and intercept of the plot, respectively. Intraparticle diffusion model: the intraparticle diffusion model assumes that film diffusion is negligible, and the only rate-controlling step is intraparticle diffusion in the adsorption process [60]. Hence, the adsorbed amount at time t should be directly proportional to t 0:5 rather than t. The model is given as where k id is the intraparticle diffusion rate constant (mg/g min 0.5 ), which can be calculated from the slope of the linear plot of Q t vs. t 0:5 and can be determined from the slope of the plot shown in Figure 10(c). The kinetic parameters obtained from the application of different models are shown in Table 2. The pseudosecond

Effect of pH on the MO Adsorption.
The result from Figure 11 shows that the adsorption efficiency of all three materials decreases gradually according to the pH value of the solution, the efficiency reaches the highest and most stable value at pH from 3 − 4, to pH 5, and the efficiency productivity started to decrease gradually. This is related to the surface area of the material, the presence of hydroxyl groups The Q e values of 2 Pan/MHT are always higher than ones of 0.5 Pan/MHT and 1 Pan/MHT. This can be due to the structure of 2 Pan/MHT with high surface area as the result analyzed from BET and SEM methods above. For a solid-liquid system, the equilibrium of adsorption is one of the important physic-chemical aspects in the description of adsorption behavior. In this work, the models of Langmuir [61] and Freundlich [62] isotherm are evaluated. The results obtained from the different models provide the behavior of the adsorption mechanisms and the surface properties and affinities of the adsorbent. The Langmuir isotherm is used to describe the equilibrium between the surface of the solid and the solution and is valid for monolayer adsorption onto a surface with a finite number of identical sites. The Langmuir model equation is given as where Q m (mg/g) is the maximum monolayer adsorption    where k f (mg/g) and n are the Freundlich constants related to adsorption capacity and adsorption intensity, respectively. The plots of log C e vs. log Q e were shown on Figure 13(b), and the values of 1/n and k f are calculated from the slope and intercept of the plots, respectively. All the correlation coefficients, R 2 values, and the constants calculated from Langmuir and Freundlich models are listed in Table 3. The R 2 of both Langmuir and Freundlich models of all three adsorbents is higher than 0.91; so, the adsorption of MO onto Pan/MHT composite appropriate with either monolayer of dyeing molecules covers the Pan/MHT surface or interaction between adsorbents and MO molecules. All three R 2 values of the Freundlich model of Pan/MHT composites are higher than R 2 of the Langmuir model. In particularly, R 2 = 0:9506, 0:9472, and 0:9912 corresponding to 0.5 Pan/MHT, 1 Pan/MHT, and 2 Pan/ MHT. In addition, the n values of three composites are higher than 2 (n = 2:0973, 2:119, and 2:1872 for 0.5 Pan/ MHT, 1 Pan/MHT, and 2 Pan/MHT, respectively) which indicates that the MO adsorption on Pan/MHT composite is not only monolayer surface adsorption. The mechanism of MO adsorption onto Pan/MHT composite is either of the surface adsorption or other types of interactions, such as electrostatic interaction, ion exchange, hydrogen bonding, and van der Waals forces. In all three composites, 2 Pan/ MHT is the most suitable for the Freundlich model and the Langmuir model too. The MO adsorption capacity of 2 Pan/MHT is also the highest with Q e = 156:25 mg/g.
Dubinin-Radushkevich (D-R) isotherm is generally used to describe the sorption isotherms of a single solute system (Figure 13(c)). The D-R isotherm can also help to confirm that the adsorption process was chemisorption or physical adsorption [63]. D-R equation is represented as follows: where Q e is equilibrium adsorbent-phase concentration of adsorbate (mg/g), and Q m is theoretical saturation capacity (mg/g). The plots of ε 2 vs. ln Q e were shown in Figure 10(c), and the values of β and ln Q m are calculated from the slope and intercept of the plots, respectively. β is the activity coefficient related to the mean free energy of adsorption (mol 2 /kJ 2 ), and ε is the polanyipotential (kJ/mol).
where R is the universal gas constant (8.314 J/mol.K), T is the Kelvin temperature (K), and C e is the equilibrium aqueous phase concentration of adsorbate (mg/L).
The D-R model is mainly used to estimate the average free energy of adsorption (kJ/mol) when E < 8 kJ/mol, the adsorption is physical adsorption and when E > 8 kJ/mol, the adsorption is chemisorption [64].    Figure 15(a). MO is characterized by the peak at 2896 cm -1 for asymmetric CH 3 stretching vibration; the peaks at 1520 cm -1 and 1421 cm -1 are assigned for C=C-H in plane CH bend; peak at 817 cm -1 is characteristic of the benzene ring. The peak at 1608.32 cm -1 is assigned for -N=N-, and the peaks at 1201 cm -1 and 1121 cm -1 are assigned for -C-N. The peaks at 697 cm -1 , 623 cm -1 , and 574 cm -1 are assigned for the stretching vibration -C-S-, and the peak at 1367 cm -1 is assigned for the stretching vibration S=O. These characteristics also exist on Pan/MHT materials after adsorption, demonstrating the presence of MO dye on the material, and this result is consistent with the announcement of Harikumar et al. [65].  13 Adsorption Science & Technology result is the surface of the material more homogeneous than the original. It is mean that the material samples after adsorption have almost filled gaps.
3.4. Desorption of MO and Reusability of Pan/MHT. Another parameter that defines the functionality of adsorbent material is its capacity to be regenerated and reused in several adsorption and desorption cycles. For this reason, MO adsorption and desorption experiments were performed with Pan/MHT by using a 0.1 M HCl solution as an eluent. This eluent was chosen because in acid conditions, Pan is in emeraldine salt and is doped with H+ ions, and MO is in its cationic form [66,67]. Therefore, due to electrostatic repulsions, the MO molecules would be desorbed from the composite structure. Figure 16 shows the adsorption and desorption efficiency of MO after 4 cycles. The result show that the MO adsorption decreases to 12.86%, 14.28%, and 9.16% according to using 0.5 Pan/MHT, 1 Pan/MHT, and 2 Pan/MHT. The MO desorption from 0.5 Pan/MHT, 1 Pan/MHT, and 2 Pan/MHT also decreases to 21.51%, 19.7%, and 13.57%, respectively. This can be due to the increase of Pan in a composite structure, which enhances both adsorption and desorption of MO on the adsorbent. Thus, Pan/MHT sur-face can be regenerated without a problem. Although MO is completely desorbed, the adsorption percentage decreases, and this behavior is because of the loss of Pan due to stirring and material management. Therefore, this material can be reused in multiple cycles if it wants to be coupled to a recirculation system.
The results show that at a low pH 3 − 4, the MO adsorption capacity (H %) obtained over 80% after 3 cycles of reuse. When the solution pH > 5 − 7, the MO adsorption decreased and obtained 60− 73% efficiency of MO removal after 3 cycles of reuse. This shows that the MO adsorption and desorption are suitable for performance in acidic environments at pH 3 − 4.

Mechanism of Adsorption.
With all the results above, the process of adsorption, recovery, and desorption of MO on Pan/MHT was described as Figure 17(a). The adsorption and desorption mechanism of MO on Pan/MHT are the combination of chemisorption and physical adsorption, which can be illustrated in Figures 17(b)  14 Adsorption Science & Technology a conducting polymer with NH + group on full backbone that will interact with M-SO3 -. The electrostatic interactions are preferably involved in MO adsorption onto Pan particles. Besides, the adsorption experiment studied on Pan/MHT was found to have followed the pseudosecond-order model which indicates the chemisorption mechanism of Pan/MHT (ii) The presence of MHT gives the good adsorption of MO because of the anion exchangeable. For this mechanism, the anion associated with the interlayer Pan/MHT is exchanged with MO anionic molecules in solution. It also means that the MO anion will be trapped into the interlayer structure, and it indicates the physical adsorption mechanical of Pan/MHT

Conclusion
The MHT material has been synthesized with 0.

Data Availability
The data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest
The authors declare that there is no conflict of interest regarding the publication of this paper.