One-Pot Synthesis of Magnetic Polypyrrole Nanotubes for Adsorption of Cr(VI) in Aqueous Solution

A novel and eﬃcient route is proposed to fabricate Fe 3 O 4 /polypyrrole (Fe 3 O 4 /PPy) nanotubes via a one-pot process. The one-pot strategy involves the synthesis of Fe 3 O 4 /PPy nanotubes by oxidative polymerization of pyrrole (Py) monomer using Fe 3+ as an oxidant in the presence of methyl orange (MO) and Fe 3+ used as iron source to form Fe 3 O 4 simultaneously in basic conditions without adding any additional iron source and oxidant. The eﬀects of Fe 3+ concentration on the morphology and adsorption capacity of the Fe 3 O 4 /PPy nanotubes were investigated. The Fe 3 O 4 /PPy nanotubes exhibit a tubular structure. Fe 3 O 4 nanoparticles are well dispersed among the PPy nanotubes. The Fe 3 O 4 /PPy nanotubes exhibit excellent magnetic property, which make them easy to separate from wastewater by magnetic separation. The diameter of the PPy nanotubes decreased with the increase of the Fe 3+ concentration. The Fe 3 O 4 /PPy nanotubes showed strong adsorption capability for Cr(VI) with the maximum adsorption capacity of about 451.45mg · g − 1 , which is signiﬁcantly higher than bare Fe 3 O 4 nanoparticles. Cr(VI) was adsorbed on Fe 3 O 4 /PPy nanotubes by ion exchange and chelation, where Cr(VI) was partially reduced to Cr(III) due to the existence of − NH + on the Fe 3 O 4 /PPy nanotubes. Furthermore, the Fe 3 O 4 /PPy nanotubes are recyclable, retaining 90% of the initial removal eﬃciency after 5 adsorption/desorption cycles.


Introduction
With the rapid development of industry, water pollution is increasingly becoming a ubiquitous environmental problem. e pollution of toxic dyes and heavy metal ions exists widely because they are commonly used in or generated by a number of industrial processes. Among them, the heavy metal ion Cr(VI) was considered to be a major pollutant because it is widely applied in chromium plating, textile industries, photography, and printing inks [1,2]. In addition, Cr(VI) has the high toxicity and tends to bioaccumulate in the human body through the food chain, causing great damage to humans and other organisms. erefore, the recommended maximum allowable concentrations of Cr(VI) in domestic water supply and inland surface water are, respectively, 0.05 mg/L and 0.1 mg/L [3,4]. At present, many methods have been used to reduce/remove Cr(VI) from aqueous solution, such as adsorption method, precipitation method, and membrane separation method [5]. Among them, the adsorption technology has been extensively studied due to its economic feasibility and high removal ability of toxic heavy metals and other pollutants [6].
Magnetic nanoparticles (MNPs) have attracted much attention to remove harmful heavy metal ions from wastewater because they have high adsorption capacity, high specific surface area, easy separation and regeneration, and surface functional group modification [7][8][9]. Furthermore, MNPs have outstanding magnetic properties and can be magnetically separated from solution using a magnet [10]. However, MNPs tend to agglomerate and have poor thermodynamic stability due to their high surface energy [11,12]. ese reduced reactivity sites and the specific surface area of MNPs, which will further influence the removal of heavy metal ions by MNPs. At present, coating MNPs using functional polymers can improve properties of MNPs, in particular, conductive polymers [13] (e.g., polypyrrole, polythiophene, and polyaniline). Of conducting polymers, PPy was the most studied conductive polymer due to the excellent stability in the environment [14] and good removal capacity of heavy metal [15,16]. e coating of PPy can avoid agglomeration and oxidation of MNPs. Furthermore, PPy has good adsorption properties for Cr(VI) from wastewater owing to their rich functional groups and environmental stability [17][18][19].
At present, preparation and application of magnetic PPy composites in removal of heavy metal have been reported. For example, Chávez-Guajardo et al. [20] first prepared the maghemite nanoparticles using chemical coprecipitation methods. Subsequently, they synthesized the PPy/c-Fe 2 O 3 (MNCs) through emulsion polymerization of pyrrole. MNCs were used as active agents for removing Cr(VI) and Cu(II), which showed adsorption capacity of 209 mg·g −1 for Cr(VI). Other researchers have prepared Fe 3 O 4 /PPy microspheres by similar methods, the Fe 3 O 4 particles played the role of "seeds" in composite microspheres, and the PPy was used as the "pulp and peel" to form "core-shell" structure. When used as an absorbent of heavy ions, Fe 3 O 4 / PPy microspheres exhibit the maximum adsorption capacity for Ag(I) of 143.3 mg·g −1 [21] and Cr(VI) removal of 238.1 mg·g −1 [22]. e adsorption capacity needs to be further improved. erefore, in order to further improve the adsorption capacity of magnetic PPy composites, Wang et al. [23] have been synthesized uniform orange-like Fe 3 O 4 /PPy composite microspheres, the as-obtained Fe 3 O 4 /PPy microspheres showed strong adsorption capability with an adsorption capacity of about 209.2 mg·g −1 , and the adsorption capacity of Cr(VI) was not improved due to the lack of effective active sites. Moreover, Tuo et al. [24] synthesized Fe 3 O 4 /PPy composite nanofibers via in situ chemical polymerization, and Fe 3 O 4 /PPy nanofibers were employed to remove Cr(VI) and showed the strong adsorption capacity for Cr(VI) (312 mg·g −1 ). ese Fe 3 O 4 /PPy composites not only avoided the agglomeration of MNPs, but also enhanced the mechanical properties of PPy and exhibited high magnetic properties and low separation cost.
However, there are three challenges that limit the industrial application of magnetic PPy composites. e first is related to the low-adsorption capacity for Cr(VI), which will lead to the use of a large number of adsorbents in the treatment of wastewater containing heavy metal ions. e other is tedious synthesis processes of magnetic PPy composites. e last is loading amount of Fe 3 O 4 on the adsorption capacity that has not been studied. erefore, it is necessary to develop available technologies to prepare the magnetic PPy composites with excellent adsorption capacity.
us, we use a simple method for synthesis of the Fe 3 O 4 / PPy nanotubes using Py monomer as carbon and nitrogen source. MO was used as template, and Fe 3+ initiated polymerization of Py monomer to form PPy nanotubes; subsequently, the remaining Fe 3+ as iron source formed Fe 3 O 4 in basic conditions. e effect of Fe 3+ on the adsorption capacity was studied. en, the structure and morphology of the synthesized Fe 3 O 4 /PPy nanotubes were characterized. Adsorption kinetics and isothermal adsorption of Fe 3 O 4 / PPy nanotubes will be studied. Meanwhile, the adsorption mechanism of Fe 3 O 4 /PPy nanotubes will be investigated to explore the high removal capability of Cr(VI) and easy separation of Fe 3 O 4 /PPy nanotubes.  (Figure 1). In a typical polymerization process, 0.98 g MO and a certain amount of FeCl 3 •6H 2 O were dissolved in 480 mL ultrapure water. Afterwards, 0.70 mL of Py was poured into the above mixture and slowly stirred for reaction 20 h at 25°C. 10 mL NH 3 •H 2 O were added, which reacts under the N 2 atmosphere for 4 h. Finally, the obtained green precipitates were washed repeatedly using ethanol and ultrapure water and then were dried at 60°C for 24 h. In order to study the effect of FeCl 3 •6H 2 O on morphology and property of Fe 3 O 4 /PPy nanotubes, Fe 3 O 4 /PPy nanotubes were synthesized by the mass of FeCl 3 •6H 2 O ranging between 4.50 g, 5.40 g, 6.75 g, 9.00 g, 13.50 g, and 27.00 g; the molar ratios of Py to Fe 3+ (n (Py: Fe 3+ )) were 0.6, 0.5, 0.4, 0.3, 0.2, and 0.1; therefore the obtained products were named as S1, S2, S3, S4, S5, and S6, respectively.

Characterization of Fe 3 O 4 /PPy Nanotubes.
e morphology of Fe 3 O 4 /PPy nanotubes was examined using scanning electron microscope (SEM), selected area electron diffraction (SAED), and transmission electron microscope (TEM). e surface functional group of Fe 3 O 4 /PPy nanotubes was characterized with Fourier transform infrared (FT-IR) spectrophotometer. e crystal structure of Fe 3 O 4 / PPy nanotubes was characterized by X-ray diffraction (XRD). e magnetization curves of Fe 3 O 4 /PPy nanotubes were measured with Vibration Sample Magnetometer (VSM). e thermal stability was performed by ermal Analysis (TGA) in the range of 50°C to 800°C at N 2 atmosphere. e adsorption mechanism of the adsorbent was characterized with X-ray photoelectron spectroscopy (XPS). e adsorption experiment was carried out by keeping in contact 0.01 g of different adsorbents with 50 mL of 160 mg·L −1 Cr(VI) standard solution, and it was shaking in the oscillator at 25°C and 150 rpm for 24 h. In addition, the absorbance of Cr(VI) solution was determined at 540 nm with UV-visible spectrophotometer. e removal efficiency (R) and adsorption capacity (Q e ) of Cr(VI) were determined using the following equations:

Removal of Cr(VI
where C o and C e represent the initial concentration of Cr(VI) and residual concentration of Cr(VI), respectively, V represents the initial volume of Cr(VI) solution, and m represents the mass of Fe 3 O 4 /PPy nanotubes. e influence of reaction times on the adsorption property of Cr(VI) with Fe 3 O 4 /PPy nanotubes was discussed by varying the time from 0 to 24 h when the initial concentration of Cr(VI) was 160 mg·L −1 . Q t was determined using where Q t and C t are the adsorption capacity and the concentration of Cr(VI) at t, respectively.

Desorption Experiment of Fe 3 O 4 /PPy Nanotubes.
0.01 g of the S2 was added in a conical flask containing 50 mL of a 160 mg·L −1 Cr(VI) solution and allowed the interaction to proceed during 24 h. en, the S2 was magnetically separated by an external magnetic field, whereas the residual solution was collected for analysis using UV-Vis spectrophotometer at 540 nm. For the desorption experiment, Cr(VI) loaded adsorbent (S2) was exposed to 50 mL, 0.50 M NaOH solution in constant-temperature oscillator for 3 h at 150 rpm. en, the S2 was washed by ultrapure water to neutralize and used in other adsorption/desorption cycles [20]. -2(f )); this peak is mainly (002) diffraction peak for amorphous carbon, and the peak height decreases with the increasing of the Fe 3+ concentration. For S1, other diffraction peaks except for (002) diffraction peaks are not observed in Figure 2(a). is is because Fe 3 O 4 is difficult to form under a low concentration of Fe 3+ and reduced Fe 2+ in the reaction system, which has been calculated by chemical titration method. As the amount of Fe 3+ increases, the six resolved different diffraction peaks appeared in the XRD of various Fe 3 O 4 /PPy nanotubes, correspond (220), (311),  Figure S1, and TEM images of various Fe 3 O 4 /PPy nanotubes, the SEM images, SAED pattern, and EDS profile of S2 are investigated as shown in Figure 3. e tubular structure has been formed when n(Py: Fe 3+ ) is 0.6, and particles agglomerated on the nanotubes are observed in Figure S1 (supporting information). PPy particles were formed before PPy nanotubes were formed, which is due to mechanism of coating growth for PPy nanotubes as the lack of oxidants. As the concentration of Fe 3+ increases, integrated and smooth nanotubes were observed and the diameters of PPy nanotubes were decreased from 480 to 270 nm. e higher concentration of FeCl 3 leads to the diameter of MO-FeCl 3 micelle template decreases, which causes a smaller diameter  of PPy nanotubes [26]. Also, the polymerization rate of Py monomer was quadratic with the concentration of FeCl 3 , and the rapid polymerization rate of PPy may affect the formation of tubular morphology. In the presence of  [ [28][29][30], and the stretching vibration of N-H (3413 cm −1 Figure 6). e TGA curves of the PPy nanotubes exhibit three steps in weight loss [31]. e weight loss processes of S1 and S2 were similar to PPy nanotubes because the content of Fe 3 O 4 in the sample is shallow (5.21% and 7.91%). e first step of weight loss below 100°C is mainly due to the loss of bound water and residual moisture. e next step starts around 100°C to 290°C; the weight loss may correspond to the decomposition of MO and small organic molecules. e weight loss between 300°C and 800°C mainly relates to the decomposition and carbonization of PPy nanotubes. e S4, S5, and S6 show similar weight loss trends. A weight loss of 6% in the range of 50°C to 150°C is observed, which is attributed to desorption of gases and evaporation of residual moisture. As the temperature exceeds 150°C, the weight loss is related to the decomposition of MO and small organic molecules, and the weight loss is enhanced with the increase of temperatures. A significant weight loss can be observed when the temperature exceeds 600°C, which is associated with the carbonization of PPy. From the thermograms, it has been seen that the rate of weight loss was much slower and residual quantity is much more with the increase in the concentration of Fe 3+ in the Fe 3 O 4 /PPy nanotubes [32]. is indicated that the loading amounts of Fe 3 O 4 in these PPy/Fe 3 O 4 nanotubes increase (5.21 %-23.52%) and verify the XRD results.  Figure 7, with the increase in concentration of Fe 3+ , the saturation magnetizations (M s ) of Fe 3 O 4 /PPy nanotubes increased from 7.41 to 40.45 emu·g −1 , which may be due to the increase of Fe 3 O 4 content during the reaction [33]. As shown in Table 1, the superparamagnetic behaviors of various Fe 3 O 4 /PPy nanotubes were confirmed by low M r and coercivity H c . Except for S1, Fe 3 O 4 /PPy nanotubes possess excellent magnetic properties,   Figure 8(b) shows the Q e of various Fe 3 O 4 /PPy nanotubes for Cr(VI) from solution. Q e increased with the increase in n(Py: Fe 3+ ) because the diameter of the Fe 3 O 4 /PPy nanotubes increases with the increase of n(Py: Fe 3+ ), and the more active sites were exposed, which cause enhanced the adsorption capacity.

Magnetic
e influence of adsorption times on the Q t of various Fe 3 O 4 /PPy nanotubes was investigated, the initial concentration of Cr(VI) was 160 mg·L −1 at 25°C, and the adsorption time was varied from 0 h to 24 h (Figure 9). Q t sharply increased up to an adsorption time of 2 h. Subsequently, the increase became relatively slow, and the adsorption equilibrium was reached at about 6 h. is adsorption behavior could be explained as follows. Generous active sites on Fe 3 O 4 /PPy nanotubes at the initial stage caused rapid adsorption of Cr(VI). However, the remaining active sites on the vacant surface sites were not readily occupied because of the increase in the inactivation of active sites and the repulsive force between the solute ions in the adsorbent and those in the liquid phase. e effect of initial solution pH on removal of Cr(VI) by the PPy/Fe 3 O 4 nanocomposite is shown in Figure 10. e concentration of Cr(VI) was 160 mg·L −1 at 25°C, and the solution pH was varied from 1 to 9. It is evident from Figure 10 that both the adsorption amount and removal rate of Cr(VI) decrease with the increase in the pH of the solution. is can be associated with the larger amount of OH − then present in the solution, which will compete with the CrO 2− 4 for the same adsorption sites on the PPy surface. e effect of the initial concentration of Cr(VI) from 20 mg/L to 200 mg/L on the adsorption properties was investigated at 25°C for 24 h. Figure 11 shows that the adsorption capacity (Q e ) increases rapidly with the increase in initial concentration when the initial concentration of Cr(VI) is lower than 120 mg/L. e increase of Q e slows down gradually when the initial concentration of Cr(VI) is higher than 120 mg/L, and when the initial Cr(VI) concentration is increased to 160 mg/L, the adsorption equilibrium can be reached. e adsorption capacity is very low when the Cr(VI) concentration is low. is is because only a small amount of Cr(VI) reacts with the adsorption active site of adsorbent when the initial concentration is low. e increase of the initial concentration of Cr(VI) shows sharply increase of adsorption capacity. is increase in the adsorption capacity of Cr(VI) is the driving force for mass transfer becomes higher. e adsorption equilibrium process is eventually reached due to the adsorption active site of the adsorbent being occupied and steric hindrance. Moreover, it can be seen from Figure 11 that the maximum adsorption capacity of the Cr(VI) was 435.05 mg/g. e removal rate decreased as the initial concentration of Cr(VI) increased, and the maximum removal rate could reach 94.62% when the initial concentration of Cr(VI) was 20 mg/L.

Adsorption Kinetics.
To investigate the adsorption mechanism and describe the adsorption process of Cr(VI) on the surface of Fe 3 O 4 /PPy nanotubes, the ion adsorption

Advances in Materials Science and Engineering
isotherms obtained in the adsorption experiments were fitted by pseudo-first-order kinetic model [34], pseudosecond-order kinetic model [35], and intraparticle diffusion kinetic model [36,37]. is study on the adsorption kinetics of Cr(VI) can provide the basic data, theoretical reference, and technical support for Cr(VI) separation in the wastewater to use Fe 3 O 4 /PPy nanotubes, which has practical implications: pseudo-second-order model : where K 1 and K 2 are the equilibrium constants for kinetic models, Q e1 and Q e2 are the adsorption capacity of Cr(VI) on Fe 3 O 4 /PPy nanotubes at equilibrium, and K i and C are the rate constant and the intercept of the intraparticle diffusion model. e kinetic curves obtained by fitting the adsorption isotherms with kinetic models are shown in Figures 12(a)  and 12(b), respectively. e corresponding rate constants of adsorption kinetic models and R 2 make a list in Table 2. It can be seen from Table 2, compared with the quasi-firstorder kinetic model, the pseudo-second-order kinetic model well described the adsorption process of Cr(VI) on Fe 3 O 4 / PPy nanotubes (R 2 1 < R 2 2 ), and Q e calculated using the model was close to the experimental value. ese results indicate that the rate control of Cr(VI) adsorption by Fe 3 O 4 /PPy nanotubes was controlled by a chemisorption mechanism, which mainly involved electron sharing and electron transfer between Fe 3 O 4 /PPy nanotubes and Cr ions. e adsorption process was analyzed to confirm the mainly rate-controlling step with the intraparticle diffusion model. For Figure 12( [38], and it involves physical adsorption and chemical adsorption. e adsorption parameters were obtained according to the slope and intercept as shown in Table 3. e results show that intraparticle diffusion was the main rate-limiting step in the integrated adsorption process of Cr(VI).

Adsorption Isotherms.
We try to use the Langmuir [39], Freundlich [40], Temkin [41], and Dubinin and Radushkevich [42] isotherm adsorption models to describe the observed behavior of Cr(VI) adsorption towards S2. e related expressions are as follows: Freundlich : ln Q e � ln K F + 1 n ln C e , where C e and Q m are equilibrium concentration and equilibrium adsorption capacity, K L and K F are Freundlich and Langmuir equilibrium constant, respectively, the value of R is 8.314, T is the reaction temperature, and K T is the equilibrium binding constant. e curves obtained by fitting the experimental data with isothermal adsorption models are shown in Figure 13. e relative parameters calculated from the four models  Table 4. From the R 2 value, we can see that the Langmuir isotherm model can better describe our adsorption data, with R 2 values of 0.9918, and Q max calculated with this model was close to the actual value.
ese results also show that the adsorption of Cr(VI) is  Advances in Materials Science and Engineering achieved by forming a monolayer on a uniform surface by a limited number of identical adsorption sites. In addition, the R L values determined for the initial concentrations of Cr(VI) (20-200 mg·L −1 ) were between 0 and 1, indicating that the system was favorable for adsorption.
A comparison of our work with other sorbents reported in literature has been registered, and the Q m , adsorption conditions for removal of Cr(VI) and separation methods of different sorbents [10,22,[43][44][45][46][47][48][49] are shown in Table 5. e results indicated that Fe 3 O 4 /PPy nanotubes prepared in this   To investigate the adsorption mechanism of Cr(VI) onto Fe 3 O 4 /PPy nanotubes, the XPS spectra before and after Fe 3 O 4 /PPy nanotubes adsorption are shown in Figure 14. As shown in Figure 14(a), after the adsorption of Cr(VI), the Cr2p peak was found on the nanotubes, which confirmed the existence of elemental Cr on the nanotubes. e high resolution spectrum of Cr2p is shown in Figure 14(b); there are two energy bands at 576.88 eV and 586.78 eV, which correspond to the Cr(2p3/2) and Cr(2p1/2) orbitals, respectively [10,50]; the binding energies of 582.64 eV (2p3/2) and 590.36 eV (2p1/2) correspond to Cr(VI), and that of 588.82 eV (2p1/2) correspond to Cr(III). e binding energies of these peaks are similar to those reported by Bhaumik et al. [51]. After adsorption, the Cr(VI) and Cr(III) amounts are 34.53% and 65.47% in nanotubes, respectively, indicating that partial Cr(VI) was reduced by redox adsorption of S2. Figures 14(c) .88 eV, which can be assigned to quinoid imine (�N-), imino group (-NH-), and positively charged nitrogen (-NH + ). After adsorption, the area of the peak at 399.64 eV and 402.88 eV decreases. e ion exchange could happen between Cr(VI) and the quinoid imine (�N-), and redox adsorption can happen between Cr(VI) and nitrogen with positive charge (-NH + ) [56]; the partly Cr(VI) was reduced to Cr(III).

Reusability of Fe 3 O 4 /PPy
Nanotubes. e reusability and regeneration ability of adsorbents are very important in practical applications. In this work, the solvent regeneration method was used to recover the adsorbent, which had the advantages of simple operation, high efficiency, no secondary pollution, and low cost [26]. Cr(VI) was removed from chromium loaded S2 adsorbent (0.01 g) which was conducted using 50 mL of 0.5 M NaOH. Figure 14(i) shows the cycle stability of S2 adsorption of Cr(VI). e results show that about 90% removal efficiency of Cr(VI) could be retained after five cycles. is indicates that the Fe 3 O 4 /PPy nanotubes have good reusability.    e adsorption kinetics data could be well fitted with the pseudosecond-order kinetic model, indicating that the adsorption process was controlled by a physical and chemical adsorption mechanism. e adsorption isotherm of Cr(VI) on Fe 3 O 4 /PPy nanotubes was highly consistent with the Langmuir isotherm adsorption model, suggesting that Cr(VI) adsorption was single-layer adsorption and a favorable process. e XPS study also showed that the ion exchange and reduction were the adsorption mechanism for removal of Cr(VI) by the Fe 3 O 4 /PPy nanotubes. e Fe 3 O 4 /PPy nanotubes could be recyclable, retaining the removal efficiency at about 90% after five cycles, indicating the potential of the adsorbent to remove Cr(VI) from wastewater.

Data Availability
e data used to support the findings of this study are included within the article.