polypyrrole hydrogels as efficient Fenton catalysts

Journal of Colloid and Interface Science 505 (2017) 130–138

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Regular Article

One-pot preparation of ternary reduced graphene oxide nanosheets/ Fe2O3/polypyrrole hydrogels as efficient Fenton catalysts Junshuai Zhang a, Tongjie Yao a,⇑, Chenchen Guan a, Nanxi Zhang c, Hui Zhang b, Xiao Zhang a, Jie Wu b,⇑ a

MIIT Key Lab of Critical Materials Technology for New Energy Conversion and Storage, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin, China Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education, School of Chemistry and Materials Science, Heilongjiang University, Harbin, China c School of Life Science and Technology, Harbin Institute of Technology, Harbin, China b

g r a p h i c a l a b s t r a c t Ternary reduced graphene oxide nanosheets/Fe2O3/polypyrrole hydrogels were prepared in one-step. They showed superior removal efficiency towards methylene blue via heterogeneous Fenton reaction.

a r t i c l e

i n f o

Article history: Received 26 January 2017 Revised 1 May 2017 Accepted 25 May 2017 Available online 26 May 2017 Keywords: Polypyrrole (PPy) Fe2O3 Reduced graphene oxide Hydrogels Heterogeneous Fenton reaction

a b s t r a c t Ternary reduced graphene oxide nanosheets (rGSs)/Fe2O3/polypyrrole (PPy) hydrogels with Fe2O3 nanoparticles (NPs) embedded between rGSs and PPy layer were prepared in one-pot. The ternary hydrogels exhibited an interconnected and porous three-dimensional network with co-existence of macropores and mesopores. Fe2O3 NPs uniformly dispersed on rGS surface with the diameter of 8.8 nm. Control experiments were carried out to investigate the roles of components in formation of ternary hydrogels. During heterogeneous Fenton degradation of methylene blue (MB) dyes, the ternary hydrogels exhibited much better removal efficiency than the reference samples, not only because rGSs and PPy layer altered the adsorption, dispersity and diameter of Fe2O3 NPs; but also owing to the structural merits of ternary hydrogels. The effects of operating conditions, such as initial MB concentrations, dosages of catalysts and H2O2, were carefully investigated. With the help of Fe2O3 NPs, ternary rGSs/Fe2O3/PPy hydrogels could be easily separated via a magnet. In recycling experiments, they showed superior reusability. Ó 2017 Elsevier Inc. All rights reserved.

1. Introduction In recent decades, the water pollution caused by persistent organic substances, such as fertilizers, pesticides and dyes, has ⇑ Corresponding authors. E-mail addresses: [email protected] (T. Yao), [email protected] (J. Wu). http://dx.doi.org/10.1016/j.jcis.2017.05.101 0021-9797/Ó 2017 Elsevier Inc. All rights reserved.

become a worldwide environmental problem, and its effective remediation is an ongoing subject. Advanced oxidation processes (AOPs) are powerful technologies, and accepted as promising alternative approaches for organic water contamination. Fenton reaction, as one of the most important AOPs, owns its unparalleled advantages, including benign process, superior removal efficiency, no selectivity and easy operation and maintaining [1–3].

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Unfortunately, the applications of homogeneous Fenton system (Fe2+/H2O2) are greatly limited by expensive and complex process for disposal of iron sludge, as well as difficult reusability of catalysts. To overcome these drawbacks, heterogeneous Fenton catalysts have been developed to replace traditional homogeneous Fenton system. Among them, magnetic materials, such as Fe3O4, Fe2O3 and FeOOH, are deeply studied [4–7], not only because of being abundant and inexpensive, but also due to high activity. More importantly, they can be quickly separated by external magnetic field, which dramatically improves the reusability. In heterogeneous Fenton reaction, H2O2 converts to OH on the catalyst surface to initiate the decomposition of organic pollutants. Obviously, the surface area is a crucial factor to determine the reaction rate. Compared with bulk-phase materials, nanoparticles (NPs) process larger surface area and supply more active sites for conversion of H2O2; therefore, they are more suitable candidates for heterogeneous Fenton catalysts. Nevertheless, once the size decreases to nanoscale, magnetic NPs are prone to aggregate because of strong anisotropic dipolar interactions, which inevitably results in loss of high dispersity and activity. To improve their stability and activity, a support is usually required to load magnetic NPs [8,9]. Graphene, a two-dimensional (2D) nanosheet of sp2-hybridized carbon, has attracted much attention, owing to large surface area, high ratio of lateral size to thickness, impressive mechanical strength and rich electronic states. Some studies have explored using reduced graphene oxide nanosheets (rGSs) as support for magnetic NPs to perform heterogeneous Fenton reaction [10–12]. The enhanced catalytic activity and stability were attributed to the synergistic effects between rGSs and magnetic NPs. However, due to strong p-p attractions and hydrogen interactions, rGSs tended to restack, which led to an obvious reduction of accessible surface area and stability, and hence loss of the efficacy. At present, assembling 2D rGSs to three-dimensional (3D) hydrogels was a feasible method to solve this problem, since it not only effectively avoided the restacking, but also maximally preserved the outstanding intrinsic properties of rGSs. Additionally, the porous structure was usually generated in 3D hydrogels, which was beneficial for improving the mass diffusion and transport in Fenton reaction. Recently, many approaches have been established to construct graphene-based hydrogels, including hydrothermal method, chemical reduction and electrochemical method [13–15]. Some papers have reported preparation of magnetic NPs/graphene hydrogels. However, it usually required multiple steps to synthesize the target products. For example, Yang and co-workers firstly synthesized Fe3O4 NPs, and then assembled the rGSs to 3D hydrogels in the presence of Fe3O4 NPs [16]. Xiao and co-workers have prepared Fe2O3/rGSs via formation of FeOOH as precursor of Fe2O3 under high temperature [17]. Apparently, complex procedure resulted in low efficiency, time-wasting, and suffered from difficulty in scaling up. From this view, the synthesis of magnetic NPs/graphene hydrogels in a simple way, especially in one-pot, still represented a technologically crucial yet challenging problem. Based on above consideration and our previous work [18,19], herein, we have explored a feasible way to prepare rGSs/Fe2O3/ polypyrrole (PPy) hydrogels. In hydrogels, rGSs acted as skeletons and PPy acted as ‘‘cross-linker” to connect skeletons together, which led to an interconnected and porous structure. The merits of our work were: (i) Reduction and assembly of graphene oxide nanosheets (GSs), polymerization of pyrrole monomer and synthesis of Fe2O3 NPs were combined together; therefore, the preparation process was only required one-pot under ambient condition. (ii) 3D porous structure of rGSs/Fe2O3/PPy hydrogels not only avoided the restacking of rGSs, but also favored for mass transport and diffusion. (iii) Owing to the isolation of PPy molecular chains and electrostatic force between negatively charged GSs and Fe3+,

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the average size of highly dispersed Fe2O3 NPs was only 8.8 nm, which was beneficial for increasing their surface area; and hence, promoting the degradation efficiency. (iv) The detachment, leaching and aggregation were hindered via confining Fe2O3 NPs between rGSs and PPy layer, which improved their stability. The morphologies, structures and compositions of ternary hydrogels were characterized in detail, and the roles of components in the formation process were revealed. They were applied as heterogeneous Fenton catalysts towards the degradation of methylene blue (MB) dyes. The removal efficiency and reusability were carefully investigated.

2. Materials and methods 2.1. Materials The pyrrole monomer was purchased from Alfa-Aesar and was distilled under reduced pressure and stored at 4 °C prior to use. MB dyes, FeCl3
6H2O, NaBH4, KNO3, KMnO4, H2SO4, H2O2 (30 wt %), NH3
H2O (28 wt%) and graphite were purchased from Sinopharm Chemical Reagent Co., Ltd. All chemicals were analytical grade and used as received. The water used was purified through a Millipore system with a resistivity of 18.2 MX
cm 1. 2.2. Preparation of rGSs/Fe2O3/PPy hydrogels GSs were synthesized from graphite powder according to the Hummers’ method [20], and the concentration was adjusted to 1.7 mg/mL. Typically, 60 lL pyrrole monomer was added into 30 mL as-prepared GSs solution. Subsequently, 0.1 g FeCl3
6H2O was added into above mixture to initiate the oxidation polymerization. The solution color gradually changed from dark-brown (the color of GSs solution) to black (the color of PPy homopolymer). After 6.0 h, 1.0 mL NH3
H2O was injected into the system, and the reaction was allowed to proceed for another 1.0 h. Finally, the products were washed by water for 3 times, and then freezedried. 2.3. Heterogeneous Fenton degradation of MB dyes A series of experiments were carried out to measure the activity of samples towards degradation of MB dyes. All experiments were performed at room temperature and pH = 6.5. Typically, rGSs/ Fe2O3/PPy hydrogels were added into MB solution and mechanically stirred for 10 min to achieve uniform dispersion. Then, H2O2 was injected to initiate Fenton reaction, and this time was set to be the start time. At given time intervals, the reaction solution was sampled and removed hydrogels, then measured by UV–vis spectrophotometer. The degradation efficiency was analyzed by Ct/C0, (Ct and C0 was the MB concentrations at time t and 0, respectively, which was measured from the intensity of absorbance At/ A0). The reaction rate constant k was calculated by a linear plot of ln(Ct/C0) vs. reaction time t based on the pseudo-first-order kinetic equation. In recycling study, 100 mg hydrogels were added into 30 mL MB solution with a concentration of 50 mg/L. Then, 10 mL H2O2 was immediately injected into above reaction solution to initiate the Fenton reaction. After complete degradation, the hydrogels was fixed at the bottom of flask by a magnet. Finally, the supernatant liquid was carefully removed by a straw. In next cycle, 30 mL MB solution was added, and the hydrogels remained at the bottom of flask were re-dispersed under mechanically stirring. Subsequently, 10 mL H2O2 was injected to restart the Fenton reaction. The process was repeated for 4 times.

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2.4. Characterization A SU8000 scanning electron microscope (SEM) with primary electron energy of 15 kV was employed to examine the surface morphologies of products. The structure and nanoparticle diameter were determined by a JEOL-2010 transmission electron microscope (TEM) operating at 200 kV. X-ray photoelectron spectra (XPS) were collected by using a VG ESCALABMKII spectrometer with Mg Ka excitation (1253.6 eV). Binding energy calibration was based on C 1 s at 284.6 eV. Fourier-transform infrared (FT-IR) spectra were measured in wavenumbers ranging from 400 to 4000 cm 1 using a Nicolet Avatar 360 FT-IR spectrophotometer. Inductively coupled plasma atomic spectroscopy (ICP) was performed on an Optima 7000 DV. Brunauer-Emmett-Teller (BET) measurements were performed by using a Micromeritics ASAP 2010 M analyzer on the dried sample which had been degassed at 120 °C under vacuum. By using the Barrett-Joyner-Halenda (BJH) model, the pore size distributions were derived from the adsorption branches of the isotherms. X-ray diffraction (XRD) data were collected on a Siemens D-5005 X-ray diffractometer with Cu Ka radiation (k = 1.5418 Å). The field magnetization dependence of the products was measured using a MPMS-7 superconducting quantum interference device (SQUID) magnetometer at magnetic fields up to 50 kOe. A Lambda 750 UV–vis spectrophotometer was employed to monitor MB degradation. 3. Results and discussion 3.1. Structure and composition analysis Fig. 1 shows the preparation procedure of rGSs/Fe2O3/PPy hydrogels, which begins with preparation of negatively charged GSs whose surface is modified with plenty of carboxyl and hydroxyl groups. After adding FeCl3
6H2O, the pyrrole monomer polymerized to PPy homopolymer. Meanwhile, GSs were reduced to rGSs by pyrrole monomer [21,22]. Because PPy homopolymer was positively charged, it easily covered onto the rGS surface due to the electrostatic force and p-p interactions. In our experiment, the oxidant FeCl3
6H2O was excess; therefore, the surplus FeCl3
6H2O was hydrolyzed, and then dehydrated to Fe2O3 NPs in the presence of NH3
H2O via a well-known co-precipitation process [23]. Because the reduction and assembly of GSs, polymerization of pyrrole monomer and synthesis of Fe2O3 NPs were integrated together, the ternary rGSs/Fe2O3/PPy hydrogels were obtained in one-pot, which could be confirmed by a tube inversion experiment (Fig. S1) [24]. Fig. 2 shows the FT-IR spectra of original GSs and rGSs/Fe2O3/ PPy hydrogels. In Fig. 2a, the characteristic OAH, C@O, C@C, CAOH and CAO groups on GS surface emerge at 3399, 1730, 1626, 1224 and 1051 cm 1 [25]. Fig. 2b reveals PPy homopolymer is successfully introduced into the hydrogels, because the bands at 3428 and 1557 cm 1 are attributed to the stretching modes of NAH

Fig. 2. FT-IR spectra of (a) GSs and (b) rGSs/Fe2O3/PPy hydrogels.

and CAC in the pyrrole ring; the band at 914 cm 1 is assigned to ring deformation; the bands at 1184, 1037 and 790 cm 1 are related to the stretching, bending and wagging vibrations of CAH group [26]. In addition, the peaks at 668, 541 and 427 cm 1 are due to the stretching mode of FeAO, indicating formation of Fe2O3 NPs. Moreover, compared with Fig. 2a, the peaks belonged to GSs become weak (such as C@O, C@C and CAO groups), confirming the GSs are reduced by pyrrole monomer. To better acquire surface information of ternary rGSs/Fe2O3/PPy hydrogels, XPS were selected as characterizing tools. In survey spectrum (Fig. S2), the peaks emerged at 722, 708, 529, 397 and 282 eV are indexed to Fe2p1/2, Fe2p3/2, O1s, N1s and C1s, respectively. Fig. 3a and 3b present the core-level C1s spectra of GSs and hydrogels. The deconvoluted C1s spectrum of GSs is curvefitted into four peaks at 284.6, 286.7, 287.4 and 288.5 eV, corresponding to CAC/C@C, CAO, C@O and OAC@O groups, respectively [27]. In ternary hydrogels, the C1s spectrum can be fitted into five peaks, since an additional peak at 285.6 eV deriving from C-N group in pyrrole ring appears [28]. The percentage of oxidized carbon species significantly decreases from 58% (in GSs) to 36%, verifying the GSs are reduced to rGSs by pyrrole monomer. Similar to the previous reports [29], in Fig. 3c, the core-level N1s spectrum of hydrogels is divided into three peaks with binding energy of 398.4 (@NA), 399.8 (ANHA), and 401.6 eV (N+), confirming the existence of PPy in hydrogels. The core-level spectrum of Fe2p is shown in Fig. 3d, besides two distinct peaks located at 710.7 and 724.1 eV correspond to Fe2p3/2 and Fe2p1/2, a characteristic satellite peak centered at 718.9 eV can be clearly observed, which is assigned to Fe2p3/2 of Fe3+ in Fe2O3 NPs [30]. XRD pattern of rGSs/Fe2O3/PPy hydrogels is shown in Fig. 4. Six diffraction peaks at 2h = 30.6, 35.7, 43.3, 57.3, 63.0 and 71.6° are

Fig. 1. Preparation procedure of ternary rGSs/Fe2O3/PPy hydrogels.

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Fig. 3. (a) Core-level C1s spectrum of GSs; (b–d) Core-level spectra of C1s, N1s and Fe2p in rGSs/Fe2O3/PPy hydrogels.

Fig. 4. XRD pattern of ternary rGSs/Fe2O3/PPy hydrogels.

assigned to (2 0 0), (3 1 1), (4 0 0), (5 1 1), (4 4 0) and (6 2 0) planes of cubic c-Fe2O3 NPs (JCPDS No. 39-1346) [31]. These evidences have verified the formation of Fe2O3 NPs in ternary hydrogels. Fig. 5 shows the SEM images of as-prepared rGSs/Fe2O3/PPy hydrogels at different magnifications. These images exhibit an interconnected and porous 3D network with continuous macropores in the micrometer size range (Fig. 5a and b). As commonly known, the rGSs usually exhibited smooth and sheet-like morphol-

ogy, while PPy homopolymer displayed rough and cauliflower-like feature [32]. According to these apparent differences, it is easy to distinguish them in a magnified image. In Fig. 5c, rGSs act as skeletons and PPy homopolymer acts as physical cross-linker, since individual rGSs are adhered together by PPy homopolymer. Fig. 6 shows the corresponding TEM images of rGSs/Fe2O3/PPy hydrogels. Besides the wrinkled and folded texture of rGSs, plenty of Fe2O3 NPs deposit on rGS surface. The geometric confinement of Fe2O3 NPs within rGSs and PPy layer not only promotes stability and avoids detachment; but also enhances interface contact and suppresses agglomeration. The size distribution is estimated statistically via measuring 200 Fe2O3 diameters. From the histogram (Fig. S3), their sizes mainly range from 6.5 to 11.5 nm with an average size of 8.8 nm. Incorporation of Fe2O3 NPs endows the ternary hydrogels with superparamagnetic property, since their remnant magnetization (0.95 emu/g) and coercivity (26 Oe) are very small (top left of Fig. 7). The magnetic hysteresis loop shows their saturation magnetization is 12.2 emu/g (20.2 wt% Fe2O3 loading). This implies ternary hydrogels can be quickly separated via a magnet, which is favorable for recycling usage (bottom right of Fig. 7). BET data reveals the surface area of ternary hydrogels is about 63 m2/g (Fig. 8a). From pore size distribution in Fig. 8b, we can see the mesopores with average diameter of 24 nm appear. A comprehensive analysis of SEM images and BJH data indicates the coexistence of mesopores and macropores in ternary hydrogels. Macropores account for majority; while mesopores account for only a small part. The total pore volume is 0.1 cm3/g based on BJH data. Such special porous 3D structure was suitable for heterogeneous Fenton reaction, since it not only promoted the mass

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Fig. 5. SEM images of ternary rGSs/Fe2O3/PPy hydrogels at different magnifications.

Fig. 6. TEM images of ternary rGSs/Fe2O3/PPy hydrogels at different magnifications.

Fig. 7. Magnetic hysteresis loop of ternary rGSs/Fe2O3/PPy hydrogels at 295 K. Insets show the magnetic hysteresis loop at a low field (top left), and the magnetic separation of hydrogels by a magnet (bottom right).

transport and diffusion, but also directly exposed the Fe2O3 NPs to reactants.

3.2. Formation mechanism To reveal the roles of different components in ternary hydrogel, three reference rGSs/PPy, rGSs/Fe2O3 and Fe2O3/PPy composites were prepared. Their morphologies and structures are shown in Fig. 9. In the absence of Fe2O3 NPs, the rGSs/PPy composites can form porous 3D structure (Fig. 9a), indicating rGSs and PPy are main factors in construction of hydrogels; while Fe2O3 NPs have little influence on this process. Of course, the binary rGSs/PPy hydrogels could not be recycled by magnet.

No hydrogel structure is formed in rGSs/Fe2O3 composites (Fig. 9b), Fe2O3 NPs randomly deposit on rGS surface and serious aggregation can be easily observed. The phenomenon suggested: first, individual rGS could not be connected together without PPy at ambient condition, further confirming PPy acted as linkedbridge in generation of 3D hydrogels. Second, the aggregation of Fe2O3 NPs was hindered with the aid of PPy via isolation of molecular chains. In Fig. 9c, Fe2O3/PPy composites exhibit a spherical morphology with a typical core/shell structure, since thin PPy shell covers on Fe2O3 surface (inset of Fig. 9c). According to Fig. 9c, we reasonably inferred PPy homopolymer also coated on Fe2O3 surface in rGSs/ Fe2O3/PPy hydrogels. Such coverage dramatically hindered the detachment, leaching and aggregations of Fe2O3 NPs; and hence, promoting the stability of ternary hydrogels. The average Fe2O3 size is measured to be 11.6 and 10.5 nm in Fe2O3/PPy and rGSs/ Fe2O3 composites, respectively. Both of them were larger than that of rGSs/Fe2O3/PPy hydrogels (8.8 nm). Therefore, we thought the small size of Fe2O3 NPs in ternary hydrogels was due to the synergistic effect generated between electrostatic force (supplied by negatively charged GSs and Fe3+) and isolation of PPy molecular chains, since both of them could reduce Fe2O3 diameter and improve the dispersity [33]. It was necessary to mention that freeze-dried technique was crucial for preservation of monolithic 3D architecture. In a control experiment, the rGSs/Fe2O3/PPy hydrogels were dried in oven at 60 °C under ambient pressure for a day. In this case, no interconnected and porous 3D framework appears; instead, only the ‘‘soil block-like” structure can be seen in low-resolution SEM image (Fig. S4a). The magnified images reveal their wrinkled morphology (Fig. S4b and S4c), indicating serious aggregation occurs. Such phenomenon can be further confirmed by the side view, as layer by layer restacking of rGSs can be directly observed (Fig. S4d). During heat-dried process at ambient pressure, a capillary tension in pore walls was generated at solid-liquid-vapor interface by the pore liquid surface tension, which resulted in structural collapse and dimensional shrinkage in heat-dried composites [34].

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Fig. 8. (a) Nitrogen adsorption-desorption isotherms of rGSs/Fe2O3/PPy hydrogels; (b) the corresponding BJH pore size distributions.

Fig. 9. (a) SEM image of binary rGSs/PPy hydrogels; (b) TEM image of rGSs/Fe2O3 composites; (c) TEM image of Fe2O3/PPy composites. Inset shows the magnified image.

Based on above analysis, the formation mechanism of rGSs/ Fe2O3/PPy hydrogels could be expressed as follow: when FeCl3
6H2O was added into the mixture of pyrrole monomer and GSs, on the one hand, pyrrole monomer polymerized to PPy between individual GS; on the other hand, GSs were reduced to rGSs. Instead of the original p-p interaction and hydrogen band between different GSs, intercalating PPy layer developed a strong adhesive force, and acted as the link-bridge to connect neighboring rGSs together (Fig. 1). Therefore, under the guidance of PPy, rGSs as building block blossomed into 3D hydrogel architecture. After addition of NH3
H2O, Fe2O3 NPs were synthesized on rGSs, and PPy covered on their surface, since the polymerization of pyrrole monomer still proceeded at this time. With the help of freezingdried technique, the porous 3D structure of ternary hydrogels was well preserved without collapse and shrinkage.

3.3. Heterogeneous Fenton degradation of MB dyes The catalytic performance of ternary rGSs/Fe2O3/PPy hydrogels in heterogeneous Fenton reaction was evaluated by using the model degradation of MB dyes in the presence of H2O2. Ternary hydrogels were magnetically stirred in MB solution for 10 min to achieve uniform dispersion, during which some MB dyes were removed by adsorption. After addition of H2O2, Fenton reaction was initiated and played a leading role in degradation. Fig. 10a shows typical UV–vis spectrum of MB decolorization at given time intervals. The intensity of maximum absorption wavelength gradually decreases with increasing the time. After 80 min, the peak at 665 nm completely vanishes, suggesting the MB dyes are totally decomposed. The rate constant k is determined by a linear plot of ln(Ct/C0) and reaction time t. All plots match the pseudo-first-

order reaction equation very well, and k is calculated to be 3.14  10 2 min 1 (Fig. 10b). The removal efficiencies of pure Fe2O3 NPs, binary rGSs/PPy hydrogels, physically mixed Fe2O3 NPs and binary rGSs/PPy hydrogels, and ternary rGSs/Fe2O3/PPy hydrogels with the same mass were compared in detail (Fig. 11). For pure Fe2O3 NPs (Fig. 11a), their degradation efficiency is only 17% after 120 min. In case of binary rGSs/PPy hydrogels, their removal efficiency is 85% after 120 min (Fig. 11c). Among these heterogeneous Fenton catalysts, rGSs/Fe2O3/PPy hydrogels display the highest degradation rate, as their removal efficiency reaches 100% after 80 min (Fig. 11e). To distinguish the contributions of adsorption and heterogeneous Fenton reaction in removal process, we have measured adsorption capacity in the absence of H2O2. Fig. 11b shows the removal efficiency is about 33% by adsorption itself after 80 min, suggesting the heterogeneous Fenton reaction plays a leading role and its degradation efficiency is about 67%. To verify OH was the main active species in heterogeneous Fenton process, tert-butyl alcohol (t-BuOH), a well-known scavenger for OH, was used in a control experiment [35]. In Fig. S5, the decomposition efficiency of MB apparently decreases 45% in the presence of 0.1 mM t-BuOH. Considering the degradation efficiency of 33% is caused by adsorption; obviously, OH played a key role in Fenton degradation of MB dyes. Based on foregoing data, we thought the quick decolorization rate of rGSs/Fe2O3/PPy hydrogels was owing to the synergistic effects between adsorption and heterogeneous Fenton reaction. MB dye was a planar molecule with aromatic backbone, which could form strong p-p interaction with both PPy and rGSs [36]. The interconnected and porous structure of ternary hydrogels was favorable for quick adsorption, which led to much higher MB and H2O2 concentrations inside of ternary hydrogels. As described above, Fe2O3 NPs with diameter of 8.8 nm dispersed between PPy

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Fig. 10. (a) Time-dependent UV–vis spectra of the mixture of MB and H2O2 in the presence of rGSs/Fe2O3/PPy hydrogels; (b) reaction rate constant k estimated by the slope of ln(Ct/C0) vs. reaction time.

Compared with ternary rGSs/Fe2O3/PPy hydrogels, the mixture of binary rGSs/PPy hydrogels and Fe2O3 NPs exhibits relatively low removal efficiency (93% at 80 min), indicating Fe2O3 NPs can effectively enhance the decomposition of MB dyes only when they are loaded in hydrogels. 3.4. Influence factors on MB degradation

Fig. 11. Removal efficiencies of MB dyes by adsorption or Fenton reaction in the presence of various samples: (a) Fenton reaction by using pure Fe2O3 NPs; (b) adsorption by using rGSs/Fe2O3/PPy hydrogels; (c) Fenton reaction by using rGSs/ PPy hydrogels; (d) Fenton reaction by using mixture of rGSs/PPy hydrogels and Fe2O3 NPs; (e) Fenton reaction by using rGSs/Fe2O3/PPy hydrogels. Reaction condition: 30 mL [MB]0 = 80 mg/L, H2O2 = 10 mL, samples = 15 mg.

layer and rGSs. They could directly contact with adsorbed MB to perform Fenton reaction; and hence, the degradation rate was largely accelerated. This explanation could be confirmed by Fig. 11d.

The influence factors of heterogeneous Fenton reaction were investigated. Firstly, we have studied the dosage of catalysts on removal rate. As shown in Fig. 12a, 20 mg hydrogels have the highest MB degradation of 100% after 60 min. The removal efficiencies at 60 min of 15, 10 and 5.0 mg hydrogels gradually reduce from 86%, 56% to 26%. With increasing hydrogel usage, the amount of Fe2O3 NPs in hydrogels increases which leads to an enhanced Fenton reaction rate. Therefore, the more samples used, the better degradation efficiencies were. It is necessary to mention that nearly no MB dyes are degraded in the absence of ternary hydrogels (0.0 mg) after 120 min, further suggesting the degradation is catalyzed by Fe2O3 NPs. The effects of H2O2 dosage on MB decolorization are investigated and shown in Fig. 12b. The data indicates an enhanced degradation activity with increasing H2O2 usage, but exhibits saturation when the dosage increases from 7.5 to 10 mL, since the reaction rate only slightly improves from 2.87  10 2 to 3.14  10 2 min 1. Overdose of H2O2 could not further improve the degradation rate, because excessive H2O2 acted as scavenger of OH via reacting with OH to generate OOH [37,38]. Apparently,

Fig. 12. Experimental parameters on degradation of MB dyes: (a) usage of hydrogels; (b) usage of H2O2; (c) initial MB solution concentrations. Reaction condition: 30 mL [MB]0 = 80 mg/L, H2O2 = 5.0 mL, hydrogels = 15 mg. The corresponding parameters are changed in figures.

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this process resulted in the reduction of OH and removal efficiency. The initial concentration of MB solution was another important parameter in Fenton reaction. As shown in Fig. 12c, the MB dyes are completely decomposed within 60 and 100 min when their concentration is 50 and 80 mg/L. If the initial concentration further increases to 100 mg/L, the degradation efficiency decreases to 92.3% even after 120 min. With increasing the MB solution concentration from 50 to 80 mg/L and finally be 100 mg/L, the corresponding k value decreases from 6.26  10 2 to 2.82  10 2 and to 1.34  10 2 min 1. This was because more and more MB molecules adsorbed on the hydrogel surface and occupied a larger number of Fe2O3 active sites, which became unavailable for H2O2 and resulted in less OH generation [39].

3.5. Stability and reusability Besides catalytic activity, the stability and reusability were another two important properties for heterogeneous Fenton catalysts. Herein, the leaching of Fe2O3 NPs was tested by ICP measurements. With the help of geometric protection by rGSs and PPy layer, ICP data indicated their leaching percentage was very low (only 2.2 wt%), confirming good chemical stability of Fe2O3 NPs in ternary hydrogels. To evaluate the contribution of homogeneous Fenton reaction catalyzed by leaching Fe3+, the similar degradation experiment was carried out except the ternary hydrogels were replaced by FeCl3
6H2O. In Fig. S6, the homogeneous Fenton process exhibits a limited decomposition of MB dyes, which elucidates the degradation of MB dyes is mainly caused by the heterogeneous Fenton reaction. The reusability of rGSs/Fe2O3/PPy hydrogels was also studied. To avoid the loss of catalysts, the hydrogels were not taken out from the reaction flask; instead, they were fixed at the bottom of flask by a magnet, and the supernatant liquid was carefully removed by a straw. In next cycle, MB solution was added, and the hydrogels remained at the bottom of flask were re-dispersed under mechanically stirring. Subsequently, 10 mL H2O2 was injected to restart the Fenton reaction. As shown in Fig. 13, the MB decolorization ratio remained at a level of near 98.2% within 30 min after 4 consecutive cycles, indicating the catalysts showed an excellent reusability without significant loss in catalytic activity.

Fig. 13. Reusability of MB dyes in 4 successive cycles. Reaction condition: 30 mL [MB]0 = 50 mg/L, H2O2 = 10.0 mL, catalysts = 100 mg.

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4. Conclusions In this manuscript, we have designed a one-pot method to prepare ternary rGSs/Fe2O3/PPy hydrogels by combining the reduction and assembly of GSs, polymerization of pyrrole monomer and synthesis of Fe2O3 NPs together. The as-prepared hydrogels displayed an interconnected and porous structure, where rGSs and PPy homopolymer acted as skeletons and cross-linker, respectively. Owing to isolation of PPy molecular chains and electrostatic force between Fe3+ and negatively charged GSs, the diameter of highly dispersed Fe2O3 NPs was only 8.8 nm. With the help of structural merits and synergistic effect between adsorption and heterogeneous Fenton reaction, rGSs/Fe2O3/PPy hydrogels exhibited much better removal efficiency towards MB dyes than reference samples. Their degradation rate was improved with the increase of H2O2 dosage and hydrogel usage, but reduced following the increase of initial MB concentrations. Due to the protection of rGSs and PPy layers, Fe2O3 NPs showed good stability with leaching amount of only 2.2 wt% in Fenton reactions. During the recycling tests, the ternary hydrogels showed good reusability, since no obvious loss of catalytic activity could be found even after 4 successive runs. Acknowledgements This work was supported by the National Nature Science Foundation of China (no. 21674028 and 21404035). Natural Science Foundation of Heilongjiang Province of China (E2015005). The Open Project of State Key Laboratory of Supramolecular Structure and Materials (no. sklssm 201725). The Open Project of Key Laboratory of Functional Inorganic Material Chemistry (Heilongjiang University), Ministry of Education. University Nursing Program for Young Scholars with Creative Talents in Heilongjiang Province. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2017.05.101. References [1] M. Munoz, Z.M. de Pedro, J.A. Casas, J.J. Rodriguez, Preparation of magnetitebased catalysts and their application in heterogeneous Fenton oxidation-a review, Appl. Catal. B: Environ. 176 (2015) 249–265. [2] A. Dhakshinamoorthy, S. Navalon, M. Alvaro, H. Garcia, Metal nanoparticles as heterogeneous Fenton catalysts, ChemSusChem 5 (2012) 46–64. [3] J.L. Wang, L.J. Xu, Advanced oxidation processes for wastewater treatment: formation of hydroxyl radical and application, Crit. Rev. Environ. Sci. Technol. 42 (2012) 251–325. [4] M.S. Yalfania, A. Georgi, S. Contreras, F. Medina, F.D. Kopinke, Chlorophenol degradation using a one-pot reduction-oxidation process, Appl. Catal. B: Environ. 104 (2011) 161–168. [5] M. Munoz, Z.M. de Pedro, J.A. Casas, J.J. Rodriguez, Chlorophenols breakdown by a sequential hydrodechlorination-oxidation treatment with a magnetic PdFe/c-Al2O3 catalyst, Water Res. 47 (2013) 3070–3080. [6] T. Zeng, X.L. Zhang, S.H. Wang, Y.R. Ma, H.Y. Niu, Y.Q. Cai, Assembly of a nanoreactor system with confined magnetite core and shell for enhanced Fenton-like catalysis, Chem. Eur. J. 20 (2014) 6474–6481. [7] Z.M. Cui, Z. Chen, C.Y. Cao, L. Jiang, W.G. Song, A yolk-shell structured Fe2O3@mesoporous SiO2 nanoreactor for enhanced activity as a Fenton catalyst in total oxidation of dyes, Chem. Commun. 49 (2013) 2332–2334. [8] H.Y. Ji, X.C. Jing, Y.G. Xu, J. Yan, H.P. Li, T.P. Li, L.Y. Huang, Q. Zhang, H. Xu, H.M. Li, Magnetic g-C3N4/NiFe2O4 hybrids with enhanced photocatalytic activity, RSC Adv. 5 (2015) 57960–57967. [9] X.F. Li, X. Liu, L.L. Xu, Y.Z. Wen, J.Q. Ma, Z.C. Wu, Highly dispersed Pd/PdO/Fe2O3 nanoparticles in SBA-15 for Fenton-like processes: confinement and synergistic effects, Appl. Catal. B: Environ. 165 (2015) 79–86. [10] N.A. Zubir, C. Yacou, J. Motuzas, X. Zhang, X.S. Zhao, J.C.D. Costa, The sacrificial role of graphene oxide in stabilising a Fenton-like catalyst GO-Fe3O4, Chem. Commun. 51 (2015) 9291–9294. [11] P.H. Shao, J.Y. Tian, B.R. Liu, W.X. Shi, S.S. Gao, Y.L. Song, M. Ling, F.Y. Cui, Morphology tunable ultrafine metal oxide nanostructures uniformly grown on graphene and their applications in the photo-Fenton system, Nanoscale 7 (2015) 14254–14263.

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