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reduced graphene oxide
Journal of Water Process Engineering 5 (2015) 101–111
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Enhanced heterogeneous Fenton degradation of Methylene Blue by nanoscale zero valent iron (nZVI) assembled on magnetic Fe3 O4 /reduced graphene oxide Bo Yang, Zhang Tian, Li Zhang, Yaopeng Guo, Shiqiang Yan ∗ College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, PR China
a r t i c l e
i n f o
Article history: Received 5 September 2014 Received in revised form 30 January 2015 Accepted 30 January 2015 Keywords: Nanoscale zero valent iron Graphene Magnetite Heterogeneous Fenton oxidation Methylene Blue
a b s t r a c t Highly effective nanoscale zero valent iron (nZVI) immobilized on magnetic Fe3 O4 -reduced graphene oxide (Fe0 –Fe3 O4 –RGO) was successfully fabricated and firstly proposed as a heterogeneous Fenton catalyst for the removal of Methylene Blue (MB). The characterization of the hybrids revealed that Fe3 O4 were distributed on RGO nanosheets evenly, the nZVI were wrapped by Fe3 O4 assembled on the RGO nanosheets. The effects of pH value, initial concentration of MB, catalyst dosage, and hydrogen peroxide (H2 O2 ) concentration on the degradation of MB were systematically investigated. Typically, 98.0% removal of 50 mg/L MB could be achieved within 60 min with the initial pH value of 3.00, H2 O2 concentration 0.8 mmol/L, catalyst dose 0.10 g/L. The analysis of kinetics showed that the removal of MB followed the pseudo-second-order kinetics better than the pseudo-first-order kinetics. In addition to its high catalytic activity, the reusability and superparamagnetism make it a promising candidate as heterogeneous Fenton catalyst to remove organic contaminants in water. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Fenton technology has been extensively used as advanced oxidation processes (AOPs) for the treatment of various organic pollutants in water, especially for persistent and nonbiodegradable contaminants [1–3]. Fenton reaction involves reaction between metal catalyst and hydrogen peroxide to generate hydroxyl radicals, which can nonselective attack organic compounds to mineralize. Traditional homogeneous Fenton catalysts have many disadvantages of narrow pH requirement and the production of iron sludge [4,5]. To overcome these drawbacks, heterogeneous catalysts were brought up to replace the homogeneous Fenton catalysts. It has been reported that many kinds of iron oxide catalysts, such as magnetite (Fe3 O4 ), hematite (ɑ-Fe2 O3 ), goethite (ɑ-FeOOH), Fe0 /Fe2 O3 , Fe2 O3 /carbon and Fe0 /Fe3 O4 can be used as heterogeneous catalysts to activate H2 O2 to produce hydroxyl radicals, which can decompose organic pollutants to H2 O, CO2 , and inorganic salts [6–12]. However, all these fundamental catalysts have a relatively weak catalytic activity and a relative low degradation rate.
∗ Corresponding author. Tel.: +86 931 8912582; fax: +86 931 8912582. E-mail address: [email protected] (S. Yan). http://dx.doi.org/10.1016/j.jwpe.2015.01.006 2214-7144/© 2015 Elsevier Ltd. All rights reserved.
Graphene, a two dimension lamellar structure, has high surface area, fast electron transport speed and other unique properties, which make it a good candidate in various fields [13]. Graphene oxide is usually derived from exfoliation of graphite oxide, which has abundant oxygenated functional groups on the basal plane and can be used as supporter of the various metal and metal oxides. It can prevent the aggregation of Fe3 O4 and further can enhance the catalytic activity due to the synergistic effects between GO sheets and Fe3 O4 [14–16]. Nanoscale zero-valent iron (nZVI) is a promising reagent for environmental remediation of polluted water [17–21]. nZVI is highly active and thus can provide Fe3 O4 the electrons needed for the reduction of FeIII to FeII and accelerate the generation of hydroxyl radicals [22]. However, nZVI is also unstable in water and can be oxided to form a core shell structure where the core is zero valent iron and the shell is iron oxides or hydroxides [23]. To reinforce the activation of nZVI, nZVI was either assembled on the graphene sheet or wrapped by small Fe3 O4 anchored on the graphene sheets in our work. The application of the Fe0 –Fe3 O4 –RGO hybrid to degrade organic pollutants has never been reported before to the best of our knowledge. In this study, Fe0 –Fe3 O4 –RGO was synthesized by two steps: Fe3 O4 –RGO was synthesized through an easy one-step coprecipitation route. nZVI was assembled on them by insitu reduction of ferrous salts. The hybrids were then characterized
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by transmission electron microscopy (TEM), X-ray diffraction spectra (XRD), Fourier transform infrared spectra (FTIR), Brunauer–Emmett–Teller (BET), X-ray photoelectron spectroscopy (XPS), and vibrating sample magnetometer (VSM). The influence of different parameters, such as initial MB concentration, H2 O2 dosage, pH, catalysts dosage, and temperature were studied to optimize the reaction condition. The kinetic of the catalytic reaction and possible degradation mechanism of the hybrids were also evaluated and proposed. 2. Experimental 2.1. Materials Graphite flask was obtained from Nanyang BoXing mining Co., Ltd. (China). Ferric chloride hexahydrate (FeCl3 ·6H2 O), ferrous sulfate heptahydrate (FeSO4 ·7H2 O) and sodium borohydride (NaBH4 ) were purchased from the Sinopharm Group Chemical Reagent Co., Ltd. Methylene Blue was purchased from Northwest Geological Institute of Nonferrous Metals (Xi’an, China). All other chemicals were acquired from Kelun chemical company (Chengdu, China). All reagents were of analytical grade and used without further purification. The deionized water was used throughout the experiment. 2.2. Preparation of graphene oxide Graphene oxide was prepared by oxidation of graphite powder with a mixture of acids and KMnO4 [24]. In brief, a 9:1 mixture of concentrated H2 SO4 /H3 PO4 (120:13 mL) was added to a mixture of graphite powder (1.0 g) and KMnO4 (6.0 g), causing a slight exotherm to 35–40 ◦ C. The reaction was heated to 50 ◦ C and stirred for 12 h. Then the reaction mixture was cooled to room temperature and poured into a mixture of water and ice (130 mL) containing 30% H2 O2 (6 mL). The supernatant was decanted away after settling overnight and the remaining solid was washed in succession with deionized water, 30% HCl (60 mL), water and ethanol until the pH of solution was almost neutral. Finally, the solid was washed with absolute ethanol three times followed by centrifugation (4000 rpm) and the supernatant was decanted away. Graphene oxide powder was obtained after vacuum evaporation and dried in vacuum oven for 6 h at 60 ◦ C. 2.3. Preparation of Fe3 O4 –RGO The Fe3 O4 –RGO nanocomposite was prepared by coprecipitation method in the presence of graphene oxide [25]. Graphite oxide (68.5 mg) was dispersed in 250 mL deionized water and ultrasonicated for more than 2 h to get transparent and homogeneous graphene oxide solution. FeCl3 ·6H2 O (0.7296 g, 2.699 mmol) and FeSO4 ·7H2 O (0.3753 g, 1.350 mmol) dispersed separately in 25 mL deionized water were added to the GO suspension. The aqueous suspension was vigorously stirred with an insert of N2 for 30 min at 80 ◦ C. Ammonium hydroxide (NH3 ·H2 O) (5.0 mL) was quickly added into the reaction mixture and the mixture was stirred vigorously for 2 h. Subsequently, 6 mL hydrazine hydrate was added to the mixture at 90 ◦ C and the stirring was maintained for further 4 h to ensure complete reduction of GO. The result product was washed several times with deionized water and ethanol before vacuum dried at 60 ◦ C. 2.4. Preparation of Fe0 –Fe3 O4 –RGO hybrids The synthesis of Fe0 –Fe3 O4 –RGO via reduction of FeSO4 ·7H2 O by NaBH4 in the present of Fe3 O4 –RGO was similar to the method of Lv et al. [19], as described below: the as synthesized Fe3 O4 –RGO was redispered in 300 mL oxygen free water and ultrasonicated for
30 min. FeSO4 ·7H2 O (0.1190 g, 0.428 mmol) was firstly dissolved in 20 mL deionized water and then transferred to the solution. Under the protection of N2 , 100 mL 0.01 M NaBH4 alkaline aqueous was added slowly to the reaction to guarantee complete reduction of FeSO4 ·7H2 O (Eq. (1)). The final product was washed with ethanol and vacuum dried at 60 ◦ C before use. − 0 Fe(H2 O)2+ 6 + 2BH4 → Fe ↓ +2B(OH)3 + 7H2 ↑
(1)
2.5. Characterization of catalyst The crystallographic structure of the composites was analyzed with an X’Pert Pro Philips X-ray diffractometer with Cu K␣ radiation ( = 0.15406 nm), and the scan range (2) was from 5◦ to 80◦ . The morphologies and distribution of the catalysts were characterized by transmission electron microscopy (TEM) using a Tecnai G2 F30 instrument. Fourier transform infrared (FTIR) spectra were recorded on a Nicolet Nexus 670 PerkinElmer spectrometer to explore the chemical functional groups. The magnetic properties of the hybrids were measured with vibrating sample magnetometer (LDJ9600 Lakeshore cryotronics 730 vibrating sample magnetometer) at room temperature. XPS measurements were performed using a PHI-5702 multifunctional spectrometer using Al K␣ radiation to analyze the surface composition of the nanoparticles. The specific surface area, pore volume and pore size distribution of the composites were measured by TriStar II 3020 at 77 K. Prior to measurements, the samples were degassed at 80 ◦ C overnight. 2.6. Heterogeneous Fenton degradation of MB The degradation experiments were conducted in a 250 mL conical flask with a magnetic agitation. The reactions were carried out by adding a desired dosage of H2 O2 to a pH adjusted solution containing the catalysts and 100 mL different concentration MB. In the beginning, the nanocomposites were added into the MB solutions and ultrasonicated for 10 min to achieve the adsorption/desorption equilibrium between catalyst and contaminants before H2 O2 was added. Samples were withdrawn at predetermined time intervals and filtered through a 0.22 m filter. 10 L 1 M n-butanol was added into 1 mL sample to quench the reaction [6]. The concentration of MB was measured by a UV–visible spectrophotometer at 665 nm. The total organic carbon (TOC) of MB was measured for the mineralization experiment. For the successive recycle experiment, the used catalyst was separated by external magnetic field and washed with water and ethanol to wipe off the adsorbed MB. And then, the catalyst was dispersed in 100 mL solution containing same concentration of MB. The concentration of iron ions remaining in aqueous solution after reaction was measured using an inductively coupled plasma atomic emission spectrometer (ICP-AES). 3. Results and discussion 3.1. Characterization of nanoparticles The X-ray diffraction spectra of the nanoparticles were shown in Fig. 1. The peak at 2 = 10.5◦ indicating that the graphite has been successfully oxided to graphite oxide (Fig. 1(c)). The diffraction peaks (30.3◦ , 35.6◦ , 43.1◦ , 57.2◦ , 62.7◦ ) of Fe3 O4 –RGO hybrids corresponded to the (2 2 0), (3 1 1), (4 0 0), (5 1 1), (4 4 0) planar of face-centered cubic structure of Fe3 O4 (JCPDS NO.19-0629) (Fig. 1(b)) [26]. The average crystallite size calculated from the prominent peak (3 1 1) by using scherrer equation (Eq. (2)) was 15.4 nm, where Dh k l is the crystallite size perpendicular to the normal line of the (h k l) plane, K is a constant (0.89), ˇh k l is the full width at half maximum (fwhm) of the (hkl) diffraction peak, h k l is the Bragg angle of the (h l k) peak, and is the X-ray wave-
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Fig. 1. X-ray diffraction spectra of Fe0 –Fe3 O4 –RGO (a), Fe3 O4 –RGO (b), GO (c).
Fig. 2. FTIR spectra of Fe0 –Fe3 O4 –RGO (a), Fe3 O4 –RGO (b), GO (c).
length. No significant peaks corresponding to reduced graphene oxide were found in the Fe3 O4 –RGO XRD pattern, indicating the reduction and more disorders of reduced graphene oxide in the Fe3 O4 –RGO hybrids. The typical peak of nZVI at 2 = 44.5◦ has not been observed in the spectrum of Fe0 –Fe3 O4 –RGO due to the Fe0 content in the hybrids is so low that exceeds the detection limitation of the machine (Fig. 1(a)) [19]. Dhkl =
K ˇhkl coshkl
(2)
The FTIR spectra of GO, Fe3 O4 –RGO, Fe0 –Fe3 O4 –RGO were shown in Fig. 2. The FTIR spectra of GO confirmed the presence
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of the oxygen containing functional moieties in carbon frameworks. The bands at 3419, 1737, 1222, 1056 cm−1 corresponded to the stretch vibration of OH and C O in carboxylic acid moieties, the stretch vibration of C O C, and the C O stretching vibration of epoxide, respectively [27]. The band at 1620 cm−1 could be ascribed to the skeletal vibration of graphitic domains. The decline of the intensity of the peak at 3419 cm−1 , the shift of peak from 1737 cm−1 to 1564 cm−1 in the spectrum of Fe3 O4 –RGO (formation of COO when coating with Fe3 O4 ) and the new peak at 578 cm−1 (Fe O vibration) confirmed that Fe3 O4 has been loaded on the reduced graphene oxide sheets through metal-carbonyl coordination [28]. The FTIR spectrum of Fe0 –Fe3 O4 –RGO is similar to that of Fe3 O4 –RGO indicating that no new functional groups were formed in the Fe0 –Fe3 O4 –RGO hybrids. The nanocomposite was further analyzed using XPS to investigate the valence state of Fe and the interaction between Fe0 , Fe3 O4 , and RGO as shown in Fig. 3(a–d). The wide scan of the hybrids showed photoelectron lines at binding energy of 284, 528, and 711 eV corresponding to the C1s, O1s, and Fe2p, respectively. The deconvolution of the C1s peak was consisted of three peaks at 284.4 eV, 285.6 eV, and 288.3 eV, which were ascribed to the C C, C O, and C(O)O in graphene oxide sheets. The O1s spectrum of the nanocomposite could be deconvoluted into three peaks corresponding to Fe O (529.9 eV), C O (531.1 eV), and C O (533.0 eV), respectively. The binding energy of Fe2p3/2 of the hybrid was located at 711.5 eV. While the peak of Fe2p1/2 appeared at 724.3 eV. The satellite peak at 716.3 eV was also observed indicating the successfully formation of Fe3 O4 [21,29,30]. No typical peak at 706.9 eV was found due to that the nanoscale Fe0 was wrapped by Fe3 O4 that beyond the XPS measurement scope (1–10 nm below surface of the material) [9]. TEM image of GO, Fe3 O4 –RGO, and Fe0 –Fe3 O4 –RGO with different magnification were shown in Fig. 4. TEM image of GO as shown in Fig. 4(a1) and (a2) with different magnification exhibited a thin and large film with some wrinkles on the planar indicating that graphite oxide has been successfully exfoliated to few layers graphene oxide which was beneficial for the loading of Fe3 O4 and Fe0 . The image of Fe3 O4 –RGO was shown in Fig. 4(b1) and (b2) with different magnification. It was observed from Fig. 4(b1) that Fe3 O4 were anchored on RGO sheets and distributed evenly. It could be observed from Fig. 4(b2) that the size of the Fe3 O4 on GO was around 15–20 nm with a relatively narrow size distribution and a certain extent aggregation, which was in accordance with the result of XRD measurement. The image showed that the loading of Fe3 O4 on the graphene oxide sheets prohibited the aggregation of Fe3 O4 in the certain degree. The TEM image of Fe0 –Fe3 O4 –RGO as shown in Fig. 4(c1) and (c2) displayed that much larger spherical Fe0 about 100 nm wrapped by Fe3 O4 anchored on reduced graphene oxide sheets, which indicates Fe0 has been introduced to the hybrids successfully. The existence of relatively small size Fe3 O4 anchored on the RGO sheets impelled nZVI to form isolated spheres rather than to become the ordinary necklace chain-like structure in the absence of Fe3 O4 –RGO. N2 adsorption–desorption isotherms were performed to investigate the porous structure and surface area of the nanocomposites. Fig. 5(a–c) showed the nitrogen adsorption–desorption isotherms of GO, Fe3 O4 –RGO, and Fe0 –Fe3 O4 –RGO, respectively. The nitrogen isotherms of GO resulted in type IV shape with H2 hysteresis in the range of 0.4–0.99. This indicated that the hybrids were mesoporous structure. The pore size distribution calculated by BJH desorption model showed that GO has an average pore size of about 3.60 nm. In contrast to GO, the hysteresis loop for Fe3 O4 –RGO was not saturated at high P/P0, and could be ascribed to H3 type hysteresis. The pore size distribution of Fe3 O4 –RGO was in the range from 1 to 50 nm and the average pore size was calculated to be 8.54 nm by BJH desorption method. Such vicissitudes in hysteresis and pore
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Fig. 3. XPS photoelectron spectroscopy of wide scan (a), C1s (b), O1s (c), and Fe2p (d) of Fe0 –Fe3 O4 –RGO.
size distribution might be ascribed to the aggregates of Fe3 O4 on RGO nanosheets forming slit shape pores. The hysteresis loops for Fe0 –Fe3 O4 –RGO followed the H3 IV type. The pore size distribution of Fe0 –Fe3 O4 –RGO was broader than that of Fe3 O4 –RGO and the average pore width was calculated to be 10.10 nm by BJH method. The BET surface area and the pore volume of the hybrids increased in the sequence of GO, Fe0 –Fe3 O4 –RGO, Fe3 O4 –RGO; However, pore width obeyed the order of GO, Fe3 O4 –RGO, and Fe0 –Fe3 O4 –RGO according to the data provided in Table 1. Overall it can be drawn that the hybrids has relatively high surface area and big pore width, which was beneficial for the improvement of catalyst activity [31,32]. The magnetization curves of Fe3 O4 –RGO and Fe0 –Fe3 O4 –RGO were shown in Fig. 6. The magnetic hysteresis loops were Slike curves. The results showed no remanence or coercivity in the hybrids suggesting that there was no remaining magnetization when external magnetic field was removed and the magnetic
composites were superparamagnetic. The specific saturation magnetization Ms of Fe3 O4 –RGO and Fe0 –Fe3 O4 –RGO was 48.86 emu/g and 65.77 emu/g, respectively, which guaranteed magnetic separation of catalysts [33].
3.2. Comparison of different catalysts on the degradation of MB Fig. 7 demonstrated the performance of H2 O2 alone, nZVI alone, Fe0 –Fe3 O4 –RGO alone, Fe3 O4 with H2 O2 , Fe3 O4 –RGO with H2 O2 , nZVI with H2 O2 , and Fe0 –Fe3 O4 –RGO with H2 O2 on the degradation of MB under the condition of pH 3.00, H2 O2 dosage 0.8 mmol/L, and catalyst dosage 0.1 g/L. It was found that MB could hardly be degraded with only H2 O2 alone due to the weak oxidation ability of H2 O2 . Only 5% removal of MB was achieved via the adsorption of nZVI compared with 9% removal of MB by the Fe0 –Fe3 O4 –RGO after 240 min reaction, which was due to that the higher surface area and more abundant functional groups of Fe0 –Fe3 O4 –RGO compared to
Table 1 Surface area, pore volume, and pore size data of GO, Fe3 O4 –RGO, Fe0 –Fe3 O4 –RGO. Hybrids
BET (m2 g−1 )
Single point adsorption total pore volume of pores (cm3 g−1 )
BJH desorption average pore width (nm)
GO Fe3 O4 –RGO Fe0 –Fe3 O4 –RGO
49.98 185.83 139.82
0.033 0.394 0.331
3.602 8.540 10.101
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Fig. 4. TEM image of GO (a1, a2), Fe3 O4 –RGO (b1, b2), and Fe0 –Fe3 O4 –RGO (c1, c2).
that of nZVI. The removal efficiency of Fe3 O4 –RGO was 86.3% higher than that of bare Fe3 O4 (77.0%) in the presence of H2 O2 . This is mainly due to the synergistic effect between the RGO and the Fe3 O4 . The degradation of MB by nZVI with H2 O2 reached equilibrium (85.9%) within 30 min demonstrating that nZVI was highly effective in nZVI/H2 O2 system. Most importantly, the degradation efficiency of Fe0 –Fe3 O4 –RGO reached 98% discoloration within 60 min, which showed shorter equilibrium time needed and higher removal efficiency than that of the other catalysts described above. It has been proven that the zero valent iron can form numerous small batteries when coupling with Fe3 O4 or reduced graphene oxide, which will accelerate the electron transform from Fe0 to Fe3 O4 and then provide more reactive sites. The more active sites on the catalyst surface can catalyze H2 O2 to produce more HO• to decolorize MB in the Fe0 –Fe3 O4 –RGO/H2 O2 system. The synergistic effects of Fe0 , Fe3 O4 , graphene, and H2 O2 all contribute to the high efficient degradation of MB.
iron species, and the stability of hydrogen peroxide. The experiments were carried out at three different pH values 2.00, 3.00, and 4.00 with H2 O2 concentration 0.8 mmol/L, catalyst dosage 0.10 g/L, MB concentration 50 mg/L. The optimum pH was found to be 3.00, where the rate of the reaction is the fastest. When the pH was increased above 4.00, the reaction rate decreased sharply possibly can be ascribed to the formation relatively inactive ferryl ion (FeO2+ ) (Eq. (3)), the lower oxidation potential of hydroxyl radicals, the decomposition of H2 O2 , and the deactivation of catalyst with the formation of ferric hydroxide complex at high pH values [20]. At pH value of 2.00, the reaction efficiency also decreased compared to that in the pH 3.00, which was caused mainly by two aspects factor: on the one hand, at low pH, ferrous ions can form iron complex species [Fe(H2 O)6 ]2+ ,which produced hydroxyl radicals slowly, on the other hand, in the presence of high concentration of H+ , hydrogen peroxide could be solvated with H+ to form stable oxonium ion [H3 O2 ]+ , which also eased the production of hydroxyl radicals [34]. Fe2+ + H2 O2 → FeO2+ + H2 O
(3)
3.3. Effect of operational parameters on the degradation of MB It can be seen from Fig. 8a that pH has a decisive influence on the degradation of MB due to its role in controlling the catalytic activity, the activity of the oxidant and the substrate, the dominant
The effect of initial MB concentration on the rate of degradation was investigated with pH 3.00, H2 O2 concentration 0.8 mmol/L, catalysts concentration 0.10 g/L as shown in Fig. 8b. The initial MB concentration showed a negative effect on its discoloration. The
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Fig. 5. Nitrogen adsorption–desorption isotherm and BJH pore size distribution (inset) of GO (a), Fe3 O4 –RGO (b), Fe0 –Fe3 O4 –RGO (c).
Fig. 6. Magnetization curve of Fe3 O4 –RGO (a) and Fe0 –Fe3 O4 –RGO (b).
Fig. 7. MB degradation under different conditions: H2 O2 (0.8 mmol/L) alone (a), nZVI (0.10 g/L) alone (b), Fe0 –Fe3 O4 –RGO (0.10 g/L) alone (c), Fe3 O4 (0.10 g/L) and H2 O2 (0.8 mmol/L) (d), Fe3 O4 –RGO (0.10 g/L) and H2 O2 (0.8 mmol/L) (e), nZVI (0.10 g/L) and H2 O2 (0.8 mmol/L) (f) Fe0 –Fe3 O4 –RGO (0.10 g/L) and H2 O2 (0.8 mmol/L) (g) (reaction conditions: pH 3.00, [MB] = 50 mg/L, T = 25 ◦ C)
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Fig. 8. Effect of operating parameters on MB degradation in the Fe0 –Fe3 O4 –RGO/H2 O2 heterogeneous system: (a) initial pH value, (b) initial MB concentration, (c) H2 O2 dosage, (d) catalyst dose. Except for the investigated parameter, other parameters fixed on pH 3.00, [MB] = 50 mg/L, [catalyst] = 0.10 g/L, [H2 O2 ] = 0.8 mmol/L, and T = 25 ◦ C.
initial reaction rate decreased when the initial concentration of MB increased from 20 to 40 mg/L. The negative effect might be caused by two aspects reasons: first, the molar ratio oxidant/MB was higher for the lower MB solution, for the reason that the initial concentration of oxidant was the same for different initial concentration MB solution. The higher oxidant/MB ratio was beneficial for the degradation of MB. Second, it was likely that more MB molecules might be adsorbed onto the surface of catalysts occupying the active sites when increasing the concentration of initial MB concentration, resulted in forming less hydroxyl radicals at the catalyst’s surface [20,35]. The effect of H2 O2 concentration was investigated as shown in Fig. 8c. Experiments were conducted at different H2 O2 concentrations with catalyst 0.10 g/L, pH 3.00, MB concentration 50 mg/L. The concentration of H2 O2 was varied from 0.6 mmol/L to 1.0 mmol/L. With 0.8 mmol/L H2 O2 , the second-order kinetic rate was 0.0071 L/(mg min), which was higher than that of 0.6 mmol/L H2 O2 (0.0059 L/(mg min) and nearly equaled to the k2 of 1.0 mmol/L H2 O2 (0.0070 L/(mg min). Therefore, 0.8 mmol/L was the optimum H2 O2 concentration for the degradation of MB. At low H2 O2 concentration range (<0.8 mmol/L) the degradation rate increased with an increase in the dosage of hydrogen peroxide. However, when the H2 O2 concentration was greater than 0.8 mmol/L, the degradation efficiency was not enhanced with an increase of the concentration
of H2 O2 . This phenomenon could be explained as follows: at low H2 O2 concentration, the increase of H2 O2 will produce more hydroxyl radicals. While at high H2 O2 concentration, excess H2 O2 played a role as a scavenger of the highly potential hydroxyl radicals (Eq. (4)), generating perhydroxyl radicals, which has much lower oxidation capabilities and resulted in the decrease of efficiency of degradation [36]. HO • + H2 O2 → HOO • + H2 O
(4)
The influence of catalyst dosage on the degradation of MB by Fe0 –Fe3 O4 –RGO was illustrated in Fig. 8d. The increase of Fe0 –Fe3 O4 –RGO addition from 0.04 g/L to 0.10 g/L accelerated the kinetic rate of degradation, because the increase in the amount of the Fe0 –Fe3 O4 –RGO catalyst increased the presence of active sites on the catalyst surface as well as the promoted free hydroxyl radical generation. Nevertheless, the degradation degree was not enhanced when the catalyst further increased, which was attributed to the scavenging of hydroxyl radicals through undesirable reactions (Eq. (5)). Thus, the optimum catalyst dose was 0.10 g/L in the experiment. ≡ FeII + HO• →≡ FeIII + HO−
(5)
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3.4. Kinetics analysis As the MB degradation kinetics was significantly influenced by different initial parameters, initial MB concentration, H2 O2 dosage, and catalyst addition. Comparative studying based on two stage first-order and second-order reaction kinetics were used to study the decolorization kinetics of MB in Fenton oxidation process [21]. The individual expression was presented as below:Pseudo-firstorder reaction kinetics: dCt = −k1 (Ct − Ce ) dt
(6)
Pseudo-second-order reaction kinetics: dCt = −k2 (Ct − Ce )2 dt
(7)
By integrating the Eqs. (6) and (7) under the boundary condition Ct = C0 at t = 0 and Ct = Ct at t = t, the following equations could be obtained: Ct − Ce ln = −k1 t C0 − Ce
(8)
Fig. 9. Effect of temperature on MB degradation catalyzed by Fe0 –Fe3 O4 –RGO (experimental conditions: [MB] = 50 mg/L, [catalyst] = 0.10 g/L, [H2 O2 ] = 0.8 mmol/L, pH 3.00).
1 1 = + k2 t Ct − Ce C0 − Ce
(9)
3.5. Effect of temperature
where Ct is the concentration of MB at time t, C0 is the initial concentration of MB, and Ce is the concentration of MB at equilibrium. k1 , k2 is the apparent kinetic rate constants of first-order and second-order reaction kinetics. The study of the kinetics of a Fenton process is complicated because it includes multiple step reactions. The two stage pseudo-first-order and the pseudo-second-order reaction kinetics were employed to describe the reaction process. For the pseudofirst-order-kinetics, the calculation of the kinetic constants was subdivided into two stages. The initial stage (0–10 min) and the second stage (10–60 min) of the degradation were linearly fitted separately based on Eq. (8). The pseudo-second-order kinetics could be used to describe the overall Fenton oxidation process (0–60 min) (Eq. (9)). The two different models were applied to describe the kinetic pattern for that the reaction rate of first 10 min is very high, while the second stage proceeds sluggishly. Previous studies have revealed that the removal of organic compounds in the nZVI/H2 O2 system usually complies with the first order reaction kinetics or two stage pseudo-first-order reaction kinetics models [37]. However, these kinetic models were not suitable for describing the experimental data in this experiment. It has been confirmed that the chemical oxidation of MB is a surface mediated process [38]. The mass transport of H2 O2 in aqueous to the surface of the catalyst is not a rate-limiting procedure [39]. In addition, the amount of MB molecule in the initial stage was far more excessive in aqueous solution compared with the relatively low concentration of H2 O2 and hydroxyl radicals in solution. Thus in the initial stage of reaction the controlling factor was the availability of ≡ FeII sites on the surface of the catalyst and transfer rate of H2 O2 to the catalyst surface. The pseudo first order reaction rate model can be used to describe the kinetics well for that the HO• concentration remains steady in the first stage reaction (0–10 min). In the second stage of reaction (10–60 min), the decrease of concentration of H2 O2 and more MB molecules and intermediates adsorbed on the surface of the catalyst occupying the active sites both contribute to the decrease of the reaction rate [40]. Complete removal of MB needs a relatively long hysteretic second stage, which can’t be described well by pseudo first-order reaction kinetics model. Taking into account the overall Fenton process from 0 to 60 min of the reaction, it is obvious that the pseudo second order kinetics describe the reaction in a more accurate way [41].
The effect of temperature on the degradation of MB was conducted at four different temperatures from 293 K to 323 K as shown in Fig. 9. It was observed that the oxidation rate accelerated with an increase of the temperature, which was due to the more radicals generated to attack MB molecules. The apparent rate constants increased when the temperature increased from 293 K to 323 K, which led to the raise of the reaction rate. However, when taking the practical improvement of efficiency and convenience of operation into consideration, room temperature was chosen for other experiment condition. The pseudo-second-order reaction kinetics was applied to simulate the rate of the degradation of the MB as shown in Table 2 and Fig. 10. The apparent kinetic constant k of different temperature was obtained from the regression analysis based on the pseudo-secondorder reaction kinetics at temperature from 293 K to 323 K. It can be seen from the R2 that the second order kinetics equations could fit the data very well suggesting that the reaction rate obeyed the pseudo-second-order kinetic pattern. The Arrhenius equation was used to calculate the apparent activation energy as expressed in Eq. (10).
E a
k = A exp −
(10)
RT
Where A is the pre-exponential factor, Ea is the apparent activation energy (J mol−1 ), R is the ideal gas constant (8.314 J mol−1 K−1 ), and T is absolute temperature (K). The logarithmic form is as follows: − ln k = − ln A +
Ea RT
(11)
The Arrhenius plot of –lnk2 versus 1/T was shown in Fig. 11. The −lnk2 versus 1/T followed a good linear relationship (R2 = 0.981). The apparent activation energy Ea was calculated to be 58.57 kJ/mol Table 2 The kinetic rate constants for the degradation of MB at different temperature. No.
T (◦ C)
pH
CMB (mg/L)
CH2 O2 (mmol/L)
Ccat (g/L)
k2 (L/mg min)
R2
1 2 3 4
20 30 40 50
3.00 3.00 3.00 3.00
50 50 50 50
0.8 0.8 0.8 0.8
0.10 0.10 0.10 0.10
0.0051 0.0093 0.0228 0.0416
0.979 0.982 0.937 0.879
B. Yang et al. / Journal of Water Process Engineering 5 (2015) 101–111
Fig. 10. Pseudo-second-order reaction kinetics for the degradation of MB catalyzed by Fe0 –Fe3 O4 –RGO (experimental conditions: [MB] = 50 mg/L, [catalyst] = 0.10 g/L, [H2 O2 ] = 0.8 mmol/L, pH 3.00).
109
Fig. 12. Discoloration (a) and TOC removal (b) during the degradation of MB (experimental conditions: pH 3.00, [MB] = 50 mg/L, [catalyst] = 0.10 g/L, [H2 O2 ] = 0.8 mmol/L, and T = 25 ◦ C).
3.7. Comparison of degradation of MB Some conducted researches on the degradation of MB by Fenton-like reaction are shown in Table 3. As can be seen, the Fe0 –Fe3 O4 –RGO/H2 O2 presented higher removal efficiency than homogeneous Fe2+ /H2 O2 , Fe3 O4 /rGO/H2 O2 , and GT-Fe/H2 O2 heterogeneous catalyst system. However, the removal efficiency of MB by Fe/Al2 O3 -MCM-41 was higher than our work. Thus, the comparison suggests that Fe0 –Fe3 O4 –RGO has great potential as heterogeneous catalyst to degrade organic pollutants in aqueous solution. 3.8. Mechanism discussion It is likely that the reaction followed the pathway illustrated in Scheme 1. As is known, the heterogeneous oxidation of the MB occurred after the adsorption and decomposition of hydrogen per-
Fig. 11. The Arrhenius plot of –lnk2 versus 1/T of the degradation of MB catalyzed by Fe0 –Fe3 O4 –RGO (experimental conditions: [MB] = 50 mg/L, [catalyst] = 0.10 g/L, [H2 O2 ] = 0.8 mmol/L, pH 3.00).
[42,43], indicating that the reaction required moderate energy for the contribution of high reactivity of the catalyst. 3.6. Mineralization of MB in the Fe0 –Fe3 O4 –RGO/H2 O2 system The mineralization level of MB in Fe0 –Fe3 O4 –RGO/H2 O2 system was evaluated through the degree of TOC removal as shown in Fig. 12. The total organic carbon (TOC) removal efficiency was set as a general index to study the mineralization effect in this system. Normalized TOC concentration (%TOCremoval ) was used to quantitatively characterize the total MB mineralization. This parameter is defined as in Eq. (12), where TOCt and TOC0 are the TOC values at reaction time t and 0. It could be observed that 93.5% discoloration and 46.8% of TOC removal was reached within 60 min. The residual TOC may associate with some small molecular organics generated from the catalytic reaction [44,45]. %TOCremoval =
1−
TOCt TOC0
× 100
(12) Scheme 1. The possible Fenton catalytic oxidation process.
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Table 3 Comparison of degradation of MB by Fenton and Fenton-like methods by different catalysts. Catalyst
[Catalyst]
Fe2+ Fe3 O4 /rGO GT-Fe Fe/Al2 O3 –MCM-41 Fe0 –Fe3 O4 –RGO
3.58 × 10−5 mol/dm3 0.30 g/L 1.0 g/L 1.0 g/L 0.10 g/L
[H2 O2 ] 60 mM 5.0 mL 10.0% 1.0 × 10−6 mol/L 0.8 mmol/L
[MB]
Removal efficiency
Refs.
10 mg/L 50.0 mg/L 100 mg/L 50 mg/L
90% in 10 min 100% in 2 h 98.6% in 120 min 80% in 5 min100% in 200 min 100% in 5 min 98.0% in 60 min
[36] [46] [21] [47] This work
oxide on the catalyst’s surface. The adsorption of H2 O2 on the ≡ FeII (Eq. (13)) site of the catalyst was fast and could be described by the Langmuir model (Eq. (14)) [39]. The equilibrium constant for the adsorption of hydrogen peroxide on the catalyst’s surface is defined by Eq. (15). And the total concentration of iron sites is calculated by Eq. (16). ≡ FeII + H2 O2 →≡ FeII H2 O2
(13)
Ka [H2 O2 ] [≡ FeII H2 O2 ] = [≡ FeII ]T 1 + Ka [H2 O2 ]
(14)
Ka =
[≡ FeII H2 O2 ]
(15)
[≡ FeII ][H2 O2 ]
[≡ FeII ]T = [≡ FeII ] + [≡ FeII H2 O2 ]
(16)
After the adsorption of H2 O2 on the surface of the catalyst, the reactive site ≡ FeII H2 O2 on the surface of magnetite would then convert to ≡ FeIII and produce hydroxyl radicals simultaneously (Eq. (17)). ≡ FeII H2 O2 →≡ FeIII + HO • + HO−
(17)
The formed hydroxyl radicals can then react by three different and competitive pathways: (i) the oxidation of organics (Eq. (18)). (ii) The reaction with H2 O2 leading to the formation of less oxidative radicals HOO• (Eq. (4)). (iii) The hydroxyl radicals transfer one electron to ≡ FeII species to convert to the ≡ FeII I sites (Eq. (19)). The reduction of ≡ FeII I sites was accompanied with the generation of less oxidative HO2 • simultaneously (Eq. (20)). HO • + MB → products
(18)
≡ FeII + HO • →≡ FeIII + OH−
(19)
≡ FeIII + H2 O2 →≡ FeII + • OOH + H+
(20)
Then the electron would transform from Fe0 to the active site ≡ FeIII located at the octahedral site of Fe3 O4 to regenerate the active ≡ FeII site of the Fe3 O4 (Eq. (21)). It was a possible route for that Fe3 O4 had a octahedral site that can accommodate both Fe2+ and Fe3+ [48]. This process was thermodynamically favorable. The interface between Fe0 and Fe3 O4 was metal oxide, which has been proven to be metal-semiconductor with very low electron transfer resistance. The possibility of the electron transfer from Fe0 to Fe3 O4 makes the reduction of ≡ FeIII and regenerated the ≡ FeII site feasible [10]. 0
Fe0 + 2 ≡ FeIII → 3 ≡ FeII E = 1.21V
(21)
The supporter reduced graphene oxide has also played important role in this process: (i) the introduction of graphene not only increased the surface area of the catalyst but also promote the electron transfer due to good electron conductivity of graphene. (ii) Cationic MB molecules could be easily adsorbed to the surface of GO by two approaches: electrostatic attraction as well as – conjugation based on the negatively charged structure and part conjugation system of GO. Such adsorption increases the effective concentration of MB molecules significantly near the surfaces of the Fe0 –Fe3 O4 –RGO composite. (iii) The hydrophilic nature of GO leads to the well dispersion of Fe0 –Fe3 O4 –RGO composite in water, thus improve the wettability of the catalyst in aqueous solution [49]. The increase of the efficiency of transformation of ≡ FeIII to ≡ FeII
Fig. 13. Reuse of the catalyst (experiment conditions: [H2 O2 ] = 0.8 mmol/L, [catalyst] = 0.10 g/L, pH 3.00, T = 25 ◦ C).
[MB] = 50 mg/L,
was one of the possible important reason why reaction rate in our experiment improved compared with previous report. 3.9. Stability and reuse of the catalyst To evaluate the reusability of the catalyst, successive experiments were performed as exhibited in Fig. 13. In each test, the catalyst was separated from the reaction solution by a magnet, washed with ethanol, and vacuum dried at 60 ◦ C. It could be seen that the catalyst remain 68.8% MB removal efficiency after five consecutive runs under the same reaction condition. The decrease of catalytic activity may due to two reasons: first, the surface of the catalyst was partial covered by MB and other byproducts even though it was washed with ethanol several times. Second, the iron leaching may be another important factor that could cause the loss of activity of the catalyst [50,51]. In each process, the concentration of leached iron in the after reaction solution was determined to be about 2.0 mg/L. Even though the activity of the catalyst dropped in some extent, the catalyst still exhibited some reusability value for the practical use for its convenient magnetic separation advantages. 4. Conclusions In our research, Fe0 –Fe3 O4 –RGO was successfully prepared and used as heterogeneous Fenton catalysts. Characterization proves that Fe3 O4 was attached to the RGO sheet through metal-carbonyl coordination and Fe0 was loaded on the RGO sheet surrounded by Fe3 O4 . The catalyst shows superparamagnetic properties and has high BET surface area. The different influential parameters on the degradation of MB were investigated systematically. It was found that under the optimum condition (pH 3.00, H2 O2 dosage 0.8 mmol/L, catalyst dosage 0.10 g/L), the catalyst showed very excellent discoloration efficiency, very fast reaction kinetic rate 0.0072 L/(mg min). The kinetics of discoloration of MB fit the pseudo-second-order kinetics. The apparent activation energy
B. Yang et al. / Journal of Water Process Engineering 5 (2015) 101–111
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Enhanced heterogeneous Fenton degradation of Methylene Blue by nanoscale zero valent iron (nZVI) assembled on magnetic Fe3 O4 /reduced graphene oxide Bo Yang, Zhang Tian, Li Zhang, Yaopeng Guo, Shiqiang Yan ∗ College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, PR China
a r t i c l e
i n f o
Article history: Received 5 September 2014 Received in revised form 30 January 2015 Accepted 30 January 2015 Keywords: Nanoscale zero valent iron Graphene Magnetite Heterogeneous Fenton oxidation Methylene Blue
a b s t r a c t Highly effective nanoscale zero valent iron (nZVI) immobilized on magnetic Fe3 O4 -reduced graphene oxide (Fe0 –Fe3 O4 –RGO) was successfully fabricated and firstly proposed as a heterogeneous Fenton catalyst for the removal of Methylene Blue (MB). The characterization of the hybrids revealed that Fe3 O4 were distributed on RGO nanosheets evenly, the nZVI were wrapped by Fe3 O4 assembled on the RGO nanosheets. The effects of pH value, initial concentration of MB, catalyst dosage, and hydrogen peroxide (H2 O2 ) concentration on the degradation of MB were systematically investigated. Typically, 98.0% removal of 50 mg/L MB could be achieved within 60 min with the initial pH value of 3.00, H2 O2 concentration 0.8 mmol/L, catalyst dose 0.10 g/L. The analysis of kinetics showed that the removal of MB followed the pseudo-second-order kinetics better than the pseudo-first-order kinetics. In addition to its high catalytic activity, the reusability and superparamagnetism make it a promising candidate as heterogeneous Fenton catalyst to remove organic contaminants in water. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Fenton technology has been extensively used as advanced oxidation processes (AOPs) for the treatment of various organic pollutants in water, especially for persistent and nonbiodegradable contaminants [1–3]. Fenton reaction involves reaction between metal catalyst and hydrogen peroxide to generate hydroxyl radicals, which can nonselective attack organic compounds to mineralize. Traditional homogeneous Fenton catalysts have many disadvantages of narrow pH requirement and the production of iron sludge [4,5]. To overcome these drawbacks, heterogeneous catalysts were brought up to replace the homogeneous Fenton catalysts. It has been reported that many kinds of iron oxide catalysts, such as magnetite (Fe3 O4 ), hematite (ɑ-Fe2 O3 ), goethite (ɑ-FeOOH), Fe0 /Fe2 O3 , Fe2 O3 /carbon and Fe0 /Fe3 O4 can be used as heterogeneous catalysts to activate H2 O2 to produce hydroxyl radicals, which can decompose organic pollutants to H2 O, CO2 , and inorganic salts [6–12]. However, all these fundamental catalysts have a relatively weak catalytic activity and a relative low degradation rate.
∗ Corresponding author. Tel.: +86 931 8912582; fax: +86 931 8912582. E-mail address: [email protected] (S. Yan). http://dx.doi.org/10.1016/j.jwpe.2015.01.006 2214-7144/© 2015 Elsevier Ltd. All rights reserved.
Graphene, a two dimension lamellar structure, has high surface area, fast electron transport speed and other unique properties, which make it a good candidate in various fields [13]. Graphene oxide is usually derived from exfoliation of graphite oxide, which has abundant oxygenated functional groups on the basal plane and can be used as supporter of the various metal and metal oxides. It can prevent the aggregation of Fe3 O4 and further can enhance the catalytic activity due to the synergistic effects between GO sheets and Fe3 O4 [14–16]. Nanoscale zero-valent iron (nZVI) is a promising reagent for environmental remediation of polluted water [17–21]. nZVI is highly active and thus can provide Fe3 O4 the electrons needed for the reduction of FeIII to FeII and accelerate the generation of hydroxyl radicals [22]. However, nZVI is also unstable in water and can be oxided to form a core shell structure where the core is zero valent iron and the shell is iron oxides or hydroxides [23]. To reinforce the activation of nZVI, nZVI was either assembled on the graphene sheet or wrapped by small Fe3 O4 anchored on the graphene sheets in our work. The application of the Fe0 –Fe3 O4 –RGO hybrid to degrade organic pollutants has never been reported before to the best of our knowledge. In this study, Fe0 –Fe3 O4 –RGO was synthesized by two steps: Fe3 O4 –RGO was synthesized through an easy one-step coprecipitation route. nZVI was assembled on them by insitu reduction of ferrous salts. The hybrids were then characterized
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by transmission electron microscopy (TEM), X-ray diffraction spectra (XRD), Fourier transform infrared spectra (FTIR), Brunauer–Emmett–Teller (BET), X-ray photoelectron spectroscopy (XPS), and vibrating sample magnetometer (VSM). The influence of different parameters, such as initial MB concentration, H2 O2 dosage, pH, catalysts dosage, and temperature were studied to optimize the reaction condition. The kinetic of the catalytic reaction and possible degradation mechanism of the hybrids were also evaluated and proposed. 2. Experimental 2.1. Materials Graphite flask was obtained from Nanyang BoXing mining Co., Ltd. (China). Ferric chloride hexahydrate (FeCl3 ·6H2 O), ferrous sulfate heptahydrate (FeSO4 ·7H2 O) and sodium borohydride (NaBH4 ) were purchased from the Sinopharm Group Chemical Reagent Co., Ltd. Methylene Blue was purchased from Northwest Geological Institute of Nonferrous Metals (Xi’an, China). All other chemicals were acquired from Kelun chemical company (Chengdu, China). All reagents were of analytical grade and used without further purification. The deionized water was used throughout the experiment. 2.2. Preparation of graphene oxide Graphene oxide was prepared by oxidation of graphite powder with a mixture of acids and KMnO4 [24]. In brief, a 9:1 mixture of concentrated H2 SO4 /H3 PO4 (120:13 mL) was added to a mixture of graphite powder (1.0 g) and KMnO4 (6.0 g), causing a slight exotherm to 35–40 ◦ C. The reaction was heated to 50 ◦ C and stirred for 12 h. Then the reaction mixture was cooled to room temperature and poured into a mixture of water and ice (130 mL) containing 30% H2 O2 (6 mL). The supernatant was decanted away after settling overnight and the remaining solid was washed in succession with deionized water, 30% HCl (60 mL), water and ethanol until the pH of solution was almost neutral. Finally, the solid was washed with absolute ethanol three times followed by centrifugation (4000 rpm) and the supernatant was decanted away. Graphene oxide powder was obtained after vacuum evaporation and dried in vacuum oven for 6 h at 60 ◦ C. 2.3. Preparation of Fe3 O4 –RGO The Fe3 O4 –RGO nanocomposite was prepared by coprecipitation method in the presence of graphene oxide [25]. Graphite oxide (68.5 mg) was dispersed in 250 mL deionized water and ultrasonicated for more than 2 h to get transparent and homogeneous graphene oxide solution. FeCl3 ·6H2 O (0.7296 g, 2.699 mmol) and FeSO4 ·7H2 O (0.3753 g, 1.350 mmol) dispersed separately in 25 mL deionized water were added to the GO suspension. The aqueous suspension was vigorously stirred with an insert of N2 for 30 min at 80 ◦ C. Ammonium hydroxide (NH3 ·H2 O) (5.0 mL) was quickly added into the reaction mixture and the mixture was stirred vigorously for 2 h. Subsequently, 6 mL hydrazine hydrate was added to the mixture at 90 ◦ C and the stirring was maintained for further 4 h to ensure complete reduction of GO. The result product was washed several times with deionized water and ethanol before vacuum dried at 60 ◦ C. 2.4. Preparation of Fe0 –Fe3 O4 –RGO hybrids The synthesis of Fe0 –Fe3 O4 –RGO via reduction of FeSO4 ·7H2 O by NaBH4 in the present of Fe3 O4 –RGO was similar to the method of Lv et al. [19], as described below: the as synthesized Fe3 O4 –RGO was redispered in 300 mL oxygen free water and ultrasonicated for
30 min. FeSO4 ·7H2 O (0.1190 g, 0.428 mmol) was firstly dissolved in 20 mL deionized water and then transferred to the solution. Under the protection of N2 , 100 mL 0.01 M NaBH4 alkaline aqueous was added slowly to the reaction to guarantee complete reduction of FeSO4 ·7H2 O (Eq. (1)). The final product was washed with ethanol and vacuum dried at 60 ◦ C before use. − 0 Fe(H2 O)2+ 6 + 2BH4 → Fe ↓ +2B(OH)3 + 7H2 ↑
(1)
2.5. Characterization of catalyst The crystallographic structure of the composites was analyzed with an X’Pert Pro Philips X-ray diffractometer with Cu K␣ radiation ( = 0.15406 nm), and the scan range (2) was from 5◦ to 80◦ . The morphologies and distribution of the catalysts were characterized by transmission electron microscopy (TEM) using a Tecnai G2 F30 instrument. Fourier transform infrared (FTIR) spectra were recorded on a Nicolet Nexus 670 PerkinElmer spectrometer to explore the chemical functional groups. The magnetic properties of the hybrids were measured with vibrating sample magnetometer (LDJ9600 Lakeshore cryotronics 730 vibrating sample magnetometer) at room temperature. XPS measurements were performed using a PHI-5702 multifunctional spectrometer using Al K␣ radiation to analyze the surface composition of the nanoparticles. The specific surface area, pore volume and pore size distribution of the composites were measured by TriStar II 3020 at 77 K. Prior to measurements, the samples were degassed at 80 ◦ C overnight. 2.6. Heterogeneous Fenton degradation of MB The degradation experiments were conducted in a 250 mL conical flask with a magnetic agitation. The reactions were carried out by adding a desired dosage of H2 O2 to a pH adjusted solution containing the catalysts and 100 mL different concentration MB. In the beginning, the nanocomposites were added into the MB solutions and ultrasonicated for 10 min to achieve the adsorption/desorption equilibrium between catalyst and contaminants before H2 O2 was added. Samples were withdrawn at predetermined time intervals and filtered through a 0.22 m filter. 10 L 1 M n-butanol was added into 1 mL sample to quench the reaction [6]. The concentration of MB was measured by a UV–visible spectrophotometer at 665 nm. The total organic carbon (TOC) of MB was measured for the mineralization experiment. For the successive recycle experiment, the used catalyst was separated by external magnetic field and washed with water and ethanol to wipe off the adsorbed MB. And then, the catalyst was dispersed in 100 mL solution containing same concentration of MB. The concentration of iron ions remaining in aqueous solution after reaction was measured using an inductively coupled plasma atomic emission spectrometer (ICP-AES). 3. Results and discussion 3.1. Characterization of nanoparticles The X-ray diffraction spectra of the nanoparticles were shown in Fig. 1. The peak at 2 = 10.5◦ indicating that the graphite has been successfully oxided to graphite oxide (Fig. 1(c)). The diffraction peaks (30.3◦ , 35.6◦ , 43.1◦ , 57.2◦ , 62.7◦ ) of Fe3 O4 –RGO hybrids corresponded to the (2 2 0), (3 1 1), (4 0 0), (5 1 1), (4 4 0) planar of face-centered cubic structure of Fe3 O4 (JCPDS NO.19-0629) (Fig. 1(b)) [26]. The average crystallite size calculated from the prominent peak (3 1 1) by using scherrer equation (Eq. (2)) was 15.4 nm, where Dh k l is the crystallite size perpendicular to the normal line of the (h k l) plane, K is a constant (0.89), ˇh k l is the full width at half maximum (fwhm) of the (hkl) diffraction peak, h k l is the Bragg angle of the (h l k) peak, and is the X-ray wave-
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Fig. 1. X-ray diffraction spectra of Fe0 –Fe3 O4 –RGO (a), Fe3 O4 –RGO (b), GO (c).
Fig. 2. FTIR spectra of Fe0 –Fe3 O4 –RGO (a), Fe3 O4 –RGO (b), GO (c).
length. No significant peaks corresponding to reduced graphene oxide were found in the Fe3 O4 –RGO XRD pattern, indicating the reduction and more disorders of reduced graphene oxide in the Fe3 O4 –RGO hybrids. The typical peak of nZVI at 2 = 44.5◦ has not been observed in the spectrum of Fe0 –Fe3 O4 –RGO due to the Fe0 content in the hybrids is so low that exceeds the detection limitation of the machine (Fig. 1(a)) [19]. Dhkl =
K ˇhkl coshkl
(2)
The FTIR spectra of GO, Fe3 O4 –RGO, Fe0 –Fe3 O4 –RGO were shown in Fig. 2. The FTIR spectra of GO confirmed the presence
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of the oxygen containing functional moieties in carbon frameworks. The bands at 3419, 1737, 1222, 1056 cm−1 corresponded to the stretch vibration of OH and C O in carboxylic acid moieties, the stretch vibration of C O C, and the C O stretching vibration of epoxide, respectively [27]. The band at 1620 cm−1 could be ascribed to the skeletal vibration of graphitic domains. The decline of the intensity of the peak at 3419 cm−1 , the shift of peak from 1737 cm−1 to 1564 cm−1 in the spectrum of Fe3 O4 –RGO (formation of COO when coating with Fe3 O4 ) and the new peak at 578 cm−1 (Fe O vibration) confirmed that Fe3 O4 has been loaded on the reduced graphene oxide sheets through metal-carbonyl coordination [28]. The FTIR spectrum of Fe0 –Fe3 O4 –RGO is similar to that of Fe3 O4 –RGO indicating that no new functional groups were formed in the Fe0 –Fe3 O4 –RGO hybrids. The nanocomposite was further analyzed using XPS to investigate the valence state of Fe and the interaction between Fe0 , Fe3 O4 , and RGO as shown in Fig. 3(a–d). The wide scan of the hybrids showed photoelectron lines at binding energy of 284, 528, and 711 eV corresponding to the C1s, O1s, and Fe2p, respectively. The deconvolution of the C1s peak was consisted of three peaks at 284.4 eV, 285.6 eV, and 288.3 eV, which were ascribed to the C C, C O, and C(O)O in graphene oxide sheets. The O1s spectrum of the nanocomposite could be deconvoluted into three peaks corresponding to Fe O (529.9 eV), C O (531.1 eV), and C O (533.0 eV), respectively. The binding energy of Fe2p3/2 of the hybrid was located at 711.5 eV. While the peak of Fe2p1/2 appeared at 724.3 eV. The satellite peak at 716.3 eV was also observed indicating the successfully formation of Fe3 O4 [21,29,30]. No typical peak at 706.9 eV was found due to that the nanoscale Fe0 was wrapped by Fe3 O4 that beyond the XPS measurement scope (1–10 nm below surface of the material) [9]. TEM image of GO, Fe3 O4 –RGO, and Fe0 –Fe3 O4 –RGO with different magnification were shown in Fig. 4. TEM image of GO as shown in Fig. 4(a1) and (a2) with different magnification exhibited a thin and large film with some wrinkles on the planar indicating that graphite oxide has been successfully exfoliated to few layers graphene oxide which was beneficial for the loading of Fe3 O4 and Fe0 . The image of Fe3 O4 –RGO was shown in Fig. 4(b1) and (b2) with different magnification. It was observed from Fig. 4(b1) that Fe3 O4 were anchored on RGO sheets and distributed evenly. It could be observed from Fig. 4(b2) that the size of the Fe3 O4 on GO was around 15–20 nm with a relatively narrow size distribution and a certain extent aggregation, which was in accordance with the result of XRD measurement. The image showed that the loading of Fe3 O4 on the graphene oxide sheets prohibited the aggregation of Fe3 O4 in the certain degree. The TEM image of Fe0 –Fe3 O4 –RGO as shown in Fig. 4(c1) and (c2) displayed that much larger spherical Fe0 about 100 nm wrapped by Fe3 O4 anchored on reduced graphene oxide sheets, which indicates Fe0 has been introduced to the hybrids successfully. The existence of relatively small size Fe3 O4 anchored on the RGO sheets impelled nZVI to form isolated spheres rather than to become the ordinary necklace chain-like structure in the absence of Fe3 O4 –RGO. N2 adsorption–desorption isotherms were performed to investigate the porous structure and surface area of the nanocomposites. Fig. 5(a–c) showed the nitrogen adsorption–desorption isotherms of GO, Fe3 O4 –RGO, and Fe0 –Fe3 O4 –RGO, respectively. The nitrogen isotherms of GO resulted in type IV shape with H2 hysteresis in the range of 0.4–0.99. This indicated that the hybrids were mesoporous structure. The pore size distribution calculated by BJH desorption model showed that GO has an average pore size of about 3.60 nm. In contrast to GO, the hysteresis loop for Fe3 O4 –RGO was not saturated at high P/P0, and could be ascribed to H3 type hysteresis. The pore size distribution of Fe3 O4 –RGO was in the range from 1 to 50 nm and the average pore size was calculated to be 8.54 nm by BJH desorption method. Such vicissitudes in hysteresis and pore
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Fig. 3. XPS photoelectron spectroscopy of wide scan (a), C1s (b), O1s (c), and Fe2p (d) of Fe0 –Fe3 O4 –RGO.
size distribution might be ascribed to the aggregates of Fe3 O4 on RGO nanosheets forming slit shape pores. The hysteresis loops for Fe0 –Fe3 O4 –RGO followed the H3 IV type. The pore size distribution of Fe0 –Fe3 O4 –RGO was broader than that of Fe3 O4 –RGO and the average pore width was calculated to be 10.10 nm by BJH method. The BET surface area and the pore volume of the hybrids increased in the sequence of GO, Fe0 –Fe3 O4 –RGO, Fe3 O4 –RGO; However, pore width obeyed the order of GO, Fe3 O4 –RGO, and Fe0 –Fe3 O4 –RGO according to the data provided in Table 1. Overall it can be drawn that the hybrids has relatively high surface area and big pore width, which was beneficial for the improvement of catalyst activity [31,32]. The magnetization curves of Fe3 O4 –RGO and Fe0 –Fe3 O4 –RGO were shown in Fig. 6. The magnetic hysteresis loops were Slike curves. The results showed no remanence or coercivity in the hybrids suggesting that there was no remaining magnetization when external magnetic field was removed and the magnetic
composites were superparamagnetic. The specific saturation magnetization Ms of Fe3 O4 –RGO and Fe0 –Fe3 O4 –RGO was 48.86 emu/g and 65.77 emu/g, respectively, which guaranteed magnetic separation of catalysts [33].
3.2. Comparison of different catalysts on the degradation of MB Fig. 7 demonstrated the performance of H2 O2 alone, nZVI alone, Fe0 –Fe3 O4 –RGO alone, Fe3 O4 with H2 O2 , Fe3 O4 –RGO with H2 O2 , nZVI with H2 O2 , and Fe0 –Fe3 O4 –RGO with H2 O2 on the degradation of MB under the condition of pH 3.00, H2 O2 dosage 0.8 mmol/L, and catalyst dosage 0.1 g/L. It was found that MB could hardly be degraded with only H2 O2 alone due to the weak oxidation ability of H2 O2 . Only 5% removal of MB was achieved via the adsorption of nZVI compared with 9% removal of MB by the Fe0 –Fe3 O4 –RGO after 240 min reaction, which was due to that the higher surface area and more abundant functional groups of Fe0 –Fe3 O4 –RGO compared to
Table 1 Surface area, pore volume, and pore size data of GO, Fe3 O4 –RGO, Fe0 –Fe3 O4 –RGO. Hybrids
BET (m2 g−1 )
Single point adsorption total pore volume of pores (cm3 g−1 )
BJH desorption average pore width (nm)
GO Fe3 O4 –RGO Fe0 –Fe3 O4 –RGO
49.98 185.83 139.82
0.033 0.394 0.331
3.602 8.540 10.101
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Fig. 4. TEM image of GO (a1, a2), Fe3 O4 –RGO (b1, b2), and Fe0 –Fe3 O4 –RGO (c1, c2).
that of nZVI. The removal efficiency of Fe3 O4 –RGO was 86.3% higher than that of bare Fe3 O4 (77.0%) in the presence of H2 O2 . This is mainly due to the synergistic effect between the RGO and the Fe3 O4 . The degradation of MB by nZVI with H2 O2 reached equilibrium (85.9%) within 30 min demonstrating that nZVI was highly effective in nZVI/H2 O2 system. Most importantly, the degradation efficiency of Fe0 –Fe3 O4 –RGO reached 98% discoloration within 60 min, which showed shorter equilibrium time needed and higher removal efficiency than that of the other catalysts described above. It has been proven that the zero valent iron can form numerous small batteries when coupling with Fe3 O4 or reduced graphene oxide, which will accelerate the electron transform from Fe0 to Fe3 O4 and then provide more reactive sites. The more active sites on the catalyst surface can catalyze H2 O2 to produce more HO• to decolorize MB in the Fe0 –Fe3 O4 –RGO/H2 O2 system. The synergistic effects of Fe0 , Fe3 O4 , graphene, and H2 O2 all contribute to the high efficient degradation of MB.
iron species, and the stability of hydrogen peroxide. The experiments were carried out at three different pH values 2.00, 3.00, and 4.00 with H2 O2 concentration 0.8 mmol/L, catalyst dosage 0.10 g/L, MB concentration 50 mg/L. The optimum pH was found to be 3.00, where the rate of the reaction is the fastest. When the pH was increased above 4.00, the reaction rate decreased sharply possibly can be ascribed to the formation relatively inactive ferryl ion (FeO2+ ) (Eq. (3)), the lower oxidation potential of hydroxyl radicals, the decomposition of H2 O2 , and the deactivation of catalyst with the formation of ferric hydroxide complex at high pH values [20]. At pH value of 2.00, the reaction efficiency also decreased compared to that in the pH 3.00, which was caused mainly by two aspects factor: on the one hand, at low pH, ferrous ions can form iron complex species [Fe(H2 O)6 ]2+ ,which produced hydroxyl radicals slowly, on the other hand, in the presence of high concentration of H+ , hydrogen peroxide could be solvated with H+ to form stable oxonium ion [H3 O2 ]+ , which also eased the production of hydroxyl radicals [34]. Fe2+ + H2 O2 → FeO2+ + H2 O
(3)
3.3. Effect of operational parameters on the degradation of MB It can be seen from Fig. 8a that pH has a decisive influence on the degradation of MB due to its role in controlling the catalytic activity, the activity of the oxidant and the substrate, the dominant
The effect of initial MB concentration on the rate of degradation was investigated with pH 3.00, H2 O2 concentration 0.8 mmol/L, catalysts concentration 0.10 g/L as shown in Fig. 8b. The initial MB concentration showed a negative effect on its discoloration. The
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Fig. 5. Nitrogen adsorption–desorption isotherm and BJH pore size distribution (inset) of GO (a), Fe3 O4 –RGO (b), Fe0 –Fe3 O4 –RGO (c).
Fig. 6. Magnetization curve of Fe3 O4 –RGO (a) and Fe0 –Fe3 O4 –RGO (b).
Fig. 7. MB degradation under different conditions: H2 O2 (0.8 mmol/L) alone (a), nZVI (0.10 g/L) alone (b), Fe0 –Fe3 O4 –RGO (0.10 g/L) alone (c), Fe3 O4 (0.10 g/L) and H2 O2 (0.8 mmol/L) (d), Fe3 O4 –RGO (0.10 g/L) and H2 O2 (0.8 mmol/L) (e), nZVI (0.10 g/L) and H2 O2 (0.8 mmol/L) (f) Fe0 –Fe3 O4 –RGO (0.10 g/L) and H2 O2 (0.8 mmol/L) (g) (reaction conditions: pH 3.00, [MB] = 50 mg/L, T = 25 ◦ C)
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Fig. 8. Effect of operating parameters on MB degradation in the Fe0 –Fe3 O4 –RGO/H2 O2 heterogeneous system: (a) initial pH value, (b) initial MB concentration, (c) H2 O2 dosage, (d) catalyst dose. Except for the investigated parameter, other parameters fixed on pH 3.00, [MB] = 50 mg/L, [catalyst] = 0.10 g/L, [H2 O2 ] = 0.8 mmol/L, and T = 25 ◦ C.
initial reaction rate decreased when the initial concentration of MB increased from 20 to 40 mg/L. The negative effect might be caused by two aspects reasons: first, the molar ratio oxidant/MB was higher for the lower MB solution, for the reason that the initial concentration of oxidant was the same for different initial concentration MB solution. The higher oxidant/MB ratio was beneficial for the degradation of MB. Second, it was likely that more MB molecules might be adsorbed onto the surface of catalysts occupying the active sites when increasing the concentration of initial MB concentration, resulted in forming less hydroxyl radicals at the catalyst’s surface [20,35]. The effect of H2 O2 concentration was investigated as shown in Fig. 8c. Experiments were conducted at different H2 O2 concentrations with catalyst 0.10 g/L, pH 3.00, MB concentration 50 mg/L. The concentration of H2 O2 was varied from 0.6 mmol/L to 1.0 mmol/L. With 0.8 mmol/L H2 O2 , the second-order kinetic rate was 0.0071 L/(mg min), which was higher than that of 0.6 mmol/L H2 O2 (0.0059 L/(mg min) and nearly equaled to the k2 of 1.0 mmol/L H2 O2 (0.0070 L/(mg min). Therefore, 0.8 mmol/L was the optimum H2 O2 concentration for the degradation of MB. At low H2 O2 concentration range (<0.8 mmol/L) the degradation rate increased with an increase in the dosage of hydrogen peroxide. However, when the H2 O2 concentration was greater than 0.8 mmol/L, the degradation efficiency was not enhanced with an increase of the concentration
of H2 O2 . This phenomenon could be explained as follows: at low H2 O2 concentration, the increase of H2 O2 will produce more hydroxyl radicals. While at high H2 O2 concentration, excess H2 O2 played a role as a scavenger of the highly potential hydroxyl radicals (Eq. (4)), generating perhydroxyl radicals, which has much lower oxidation capabilities and resulted in the decrease of efficiency of degradation [36]. HO • + H2 O2 → HOO • + H2 O
(4)
The influence of catalyst dosage on the degradation of MB by Fe0 –Fe3 O4 –RGO was illustrated in Fig. 8d. The increase of Fe0 –Fe3 O4 –RGO addition from 0.04 g/L to 0.10 g/L accelerated the kinetic rate of degradation, because the increase in the amount of the Fe0 –Fe3 O4 –RGO catalyst increased the presence of active sites on the catalyst surface as well as the promoted free hydroxyl radical generation. Nevertheless, the degradation degree was not enhanced when the catalyst further increased, which was attributed to the scavenging of hydroxyl radicals through undesirable reactions (Eq. (5)). Thus, the optimum catalyst dose was 0.10 g/L in the experiment. ≡ FeII + HO• →≡ FeIII + HO−
(5)
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3.4. Kinetics analysis As the MB degradation kinetics was significantly influenced by different initial parameters, initial MB concentration, H2 O2 dosage, and catalyst addition. Comparative studying based on two stage first-order and second-order reaction kinetics were used to study the decolorization kinetics of MB in Fenton oxidation process [21]. The individual expression was presented as below:Pseudo-firstorder reaction kinetics: dCt = −k1 (Ct − Ce ) dt
(6)
Pseudo-second-order reaction kinetics: dCt = −k2 (Ct − Ce )2 dt
(7)
By integrating the Eqs. (6) and (7) under the boundary condition Ct = C0 at t = 0 and Ct = Ct at t = t, the following equations could be obtained: Ct − Ce ln = −k1 t C0 − Ce
(8)
Fig. 9. Effect of temperature on MB degradation catalyzed by Fe0 –Fe3 O4 –RGO (experimental conditions: [MB] = 50 mg/L, [catalyst] = 0.10 g/L, [H2 O2 ] = 0.8 mmol/L, pH 3.00).
1 1 = + k2 t Ct − Ce C0 − Ce
(9)
3.5. Effect of temperature
where Ct is the concentration of MB at time t, C0 is the initial concentration of MB, and Ce is the concentration of MB at equilibrium. k1 , k2 is the apparent kinetic rate constants of first-order and second-order reaction kinetics. The study of the kinetics of a Fenton process is complicated because it includes multiple step reactions. The two stage pseudo-first-order and the pseudo-second-order reaction kinetics were employed to describe the reaction process. For the pseudofirst-order-kinetics, the calculation of the kinetic constants was subdivided into two stages. The initial stage (0–10 min) and the second stage (10–60 min) of the degradation were linearly fitted separately based on Eq. (8). The pseudo-second-order kinetics could be used to describe the overall Fenton oxidation process (0–60 min) (Eq. (9)). The two different models were applied to describe the kinetic pattern for that the reaction rate of first 10 min is very high, while the second stage proceeds sluggishly. Previous studies have revealed that the removal of organic compounds in the nZVI/H2 O2 system usually complies with the first order reaction kinetics or two stage pseudo-first-order reaction kinetics models [37]. However, these kinetic models were not suitable for describing the experimental data in this experiment. It has been confirmed that the chemical oxidation of MB is a surface mediated process [38]. The mass transport of H2 O2 in aqueous to the surface of the catalyst is not a rate-limiting procedure [39]. In addition, the amount of MB molecule in the initial stage was far more excessive in aqueous solution compared with the relatively low concentration of H2 O2 and hydroxyl radicals in solution. Thus in the initial stage of reaction the controlling factor was the availability of ≡ FeII sites on the surface of the catalyst and transfer rate of H2 O2 to the catalyst surface. The pseudo first order reaction rate model can be used to describe the kinetics well for that the HO• concentration remains steady in the first stage reaction (0–10 min). In the second stage of reaction (10–60 min), the decrease of concentration of H2 O2 and more MB molecules and intermediates adsorbed on the surface of the catalyst occupying the active sites both contribute to the decrease of the reaction rate [40]. Complete removal of MB needs a relatively long hysteretic second stage, which can’t be described well by pseudo first-order reaction kinetics model. Taking into account the overall Fenton process from 0 to 60 min of the reaction, it is obvious that the pseudo second order kinetics describe the reaction in a more accurate way [41].
The effect of temperature on the degradation of MB was conducted at four different temperatures from 293 K to 323 K as shown in Fig. 9. It was observed that the oxidation rate accelerated with an increase of the temperature, which was due to the more radicals generated to attack MB molecules. The apparent rate constants increased when the temperature increased from 293 K to 323 K, which led to the raise of the reaction rate. However, when taking the practical improvement of efficiency and convenience of operation into consideration, room temperature was chosen for other experiment condition. The pseudo-second-order reaction kinetics was applied to simulate the rate of the degradation of the MB as shown in Table 2 and Fig. 10. The apparent kinetic constant k of different temperature was obtained from the regression analysis based on the pseudo-secondorder reaction kinetics at temperature from 293 K to 323 K. It can be seen from the R2 that the second order kinetics equations could fit the data very well suggesting that the reaction rate obeyed the pseudo-second-order kinetic pattern. The Arrhenius equation was used to calculate the apparent activation energy as expressed in Eq. (10).
E a
k = A exp −
(10)
RT
Where A is the pre-exponential factor, Ea is the apparent activation energy (J mol−1 ), R is the ideal gas constant (8.314 J mol−1 K−1 ), and T is absolute temperature (K). The logarithmic form is as follows: − ln k = − ln A +
Ea RT
(11)
The Arrhenius plot of –lnk2 versus 1/T was shown in Fig. 11. The −lnk2 versus 1/T followed a good linear relationship (R2 = 0.981). The apparent activation energy Ea was calculated to be 58.57 kJ/mol Table 2 The kinetic rate constants for the degradation of MB at different temperature. No.
T (◦ C)
pH
CMB (mg/L)
CH2 O2 (mmol/L)
Ccat (g/L)
k2 (L/mg min)
R2
1 2 3 4
20 30 40 50
3.00 3.00 3.00 3.00
50 50 50 50
0.8 0.8 0.8 0.8
0.10 0.10 0.10 0.10
0.0051 0.0093 0.0228 0.0416
0.979 0.982 0.937 0.879
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Fig. 10. Pseudo-second-order reaction kinetics for the degradation of MB catalyzed by Fe0 –Fe3 O4 –RGO (experimental conditions: [MB] = 50 mg/L, [catalyst] = 0.10 g/L, [H2 O2 ] = 0.8 mmol/L, pH 3.00).
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Fig. 12. Discoloration (a) and TOC removal (b) during the degradation of MB (experimental conditions: pH 3.00, [MB] = 50 mg/L, [catalyst] = 0.10 g/L, [H2 O2 ] = 0.8 mmol/L, and T = 25 ◦ C).
3.7. Comparison of degradation of MB Some conducted researches on the degradation of MB by Fenton-like reaction are shown in Table 3. As can be seen, the Fe0 –Fe3 O4 –RGO/H2 O2 presented higher removal efficiency than homogeneous Fe2+ /H2 O2 , Fe3 O4 /rGO/H2 O2 , and GT-Fe/H2 O2 heterogeneous catalyst system. However, the removal efficiency of MB by Fe/Al2 O3 -MCM-41 was higher than our work. Thus, the comparison suggests that Fe0 –Fe3 O4 –RGO has great potential as heterogeneous catalyst to degrade organic pollutants in aqueous solution. 3.8. Mechanism discussion It is likely that the reaction followed the pathway illustrated in Scheme 1. As is known, the heterogeneous oxidation of the MB occurred after the adsorption and decomposition of hydrogen per-
Fig. 11. The Arrhenius plot of –lnk2 versus 1/T of the degradation of MB catalyzed by Fe0 –Fe3 O4 –RGO (experimental conditions: [MB] = 50 mg/L, [catalyst] = 0.10 g/L, [H2 O2 ] = 0.8 mmol/L, pH 3.00).
[42,43], indicating that the reaction required moderate energy for the contribution of high reactivity of the catalyst. 3.6. Mineralization of MB in the Fe0 –Fe3 O4 –RGO/H2 O2 system The mineralization level of MB in Fe0 –Fe3 O4 –RGO/H2 O2 system was evaluated through the degree of TOC removal as shown in Fig. 12. The total organic carbon (TOC) removal efficiency was set as a general index to study the mineralization effect in this system. Normalized TOC concentration (%TOCremoval ) was used to quantitatively characterize the total MB mineralization. This parameter is defined as in Eq. (12), where TOCt and TOC0 are the TOC values at reaction time t and 0. It could be observed that 93.5% discoloration and 46.8% of TOC removal was reached within 60 min. The residual TOC may associate with some small molecular organics generated from the catalytic reaction [44,45]. %TOCremoval =
1−
TOCt TOC0
× 100
(12) Scheme 1. The possible Fenton catalytic oxidation process.
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Table 3 Comparison of degradation of MB by Fenton and Fenton-like methods by different catalysts. Catalyst
[Catalyst]
Fe2+ Fe3 O4 /rGO GT-Fe Fe/Al2 O3 –MCM-41 Fe0 –Fe3 O4 –RGO
3.58 × 10−5 mol/dm3 0.30 g/L 1.0 g/L 1.0 g/L 0.10 g/L
[H2 O2 ] 60 mM 5.0 mL 10.0% 1.0 × 10−6 mol/L 0.8 mmol/L
[MB]
Removal efficiency
Refs.
10 mg/L 50.0 mg/L 100 mg/L 50 mg/L
90% in 10 min 100% in 2 h 98.6% in 120 min 80% in 5 min100% in 200 min 100% in 5 min 98.0% in 60 min
[36] [46] [21] [47] This work
oxide on the catalyst’s surface. The adsorption of H2 O2 on the ≡ FeII (Eq. (13)) site of the catalyst was fast and could be described by the Langmuir model (Eq. (14)) [39]. The equilibrium constant for the adsorption of hydrogen peroxide on the catalyst’s surface is defined by Eq. (15). And the total concentration of iron sites is calculated by Eq. (16). ≡ FeII + H2 O2 →≡ FeII H2 O2
(13)
Ka [H2 O2 ] [≡ FeII H2 O2 ] = [≡ FeII ]T 1 + Ka [H2 O2 ]
(14)
Ka =
[≡ FeII H2 O2 ]
(15)
[≡ FeII ][H2 O2 ]
[≡ FeII ]T = [≡ FeII ] + [≡ FeII H2 O2 ]
(16)
After the adsorption of H2 O2 on the surface of the catalyst, the reactive site ≡ FeII H2 O2 on the surface of magnetite would then convert to ≡ FeIII and produce hydroxyl radicals simultaneously (Eq. (17)). ≡ FeII H2 O2 →≡ FeIII + HO • + HO−
(17)
The formed hydroxyl radicals can then react by three different and competitive pathways: (i) the oxidation of organics (Eq. (18)). (ii) The reaction with H2 O2 leading to the formation of less oxidative radicals HOO• (Eq. (4)). (iii) The hydroxyl radicals transfer one electron to ≡ FeII species to convert to the ≡ FeII I sites (Eq. (19)). The reduction of ≡ FeII I sites was accompanied with the generation of less oxidative HO2 • simultaneously (Eq. (20)). HO • + MB → products
(18)
≡ FeII + HO • →≡ FeIII + OH−
(19)
≡ FeIII + H2 O2 →≡ FeII + • OOH + H+
(20)
Then the electron would transform from Fe0 to the active site ≡ FeIII located at the octahedral site of Fe3 O4 to regenerate the active ≡ FeII site of the Fe3 O4 (Eq. (21)). It was a possible route for that Fe3 O4 had a octahedral site that can accommodate both Fe2+ and Fe3+ [48]. This process was thermodynamically favorable. The interface between Fe0 and Fe3 O4 was metal oxide, which has been proven to be metal-semiconductor with very low electron transfer resistance. The possibility of the electron transfer from Fe0 to Fe3 O4 makes the reduction of ≡ FeIII and regenerated the ≡ FeII site feasible [10]. 0
Fe0 + 2 ≡ FeIII → 3 ≡ FeII E = 1.21V
(21)
The supporter reduced graphene oxide has also played important role in this process: (i) the introduction of graphene not only increased the surface area of the catalyst but also promote the electron transfer due to good electron conductivity of graphene. (ii) Cationic MB molecules could be easily adsorbed to the surface of GO by two approaches: electrostatic attraction as well as – conjugation based on the negatively charged structure and part conjugation system of GO. Such adsorption increases the effective concentration of MB molecules significantly near the surfaces of the Fe0 –Fe3 O4 –RGO composite. (iii) The hydrophilic nature of GO leads to the well dispersion of Fe0 –Fe3 O4 –RGO composite in water, thus improve the wettability of the catalyst in aqueous solution [49]. The increase of the efficiency of transformation of ≡ FeIII to ≡ FeII
Fig. 13. Reuse of the catalyst (experiment conditions: [H2 O2 ] = 0.8 mmol/L, [catalyst] = 0.10 g/L, pH 3.00, T = 25 ◦ C).
[MB] = 50 mg/L,
was one of the possible important reason why reaction rate in our experiment improved compared with previous report. 3.9. Stability and reuse of the catalyst To evaluate the reusability of the catalyst, successive experiments were performed as exhibited in Fig. 13. In each test, the catalyst was separated from the reaction solution by a magnet, washed with ethanol, and vacuum dried at 60 ◦ C. It could be seen that the catalyst remain 68.8% MB removal efficiency after five consecutive runs under the same reaction condition. The decrease of catalytic activity may due to two reasons: first, the surface of the catalyst was partial covered by MB and other byproducts even though it was washed with ethanol several times. Second, the iron leaching may be another important factor that could cause the loss of activity of the catalyst [50,51]. In each process, the concentration of leached iron in the after reaction solution was determined to be about 2.0 mg/L. Even though the activity of the catalyst dropped in some extent, the catalyst still exhibited some reusability value for the practical use for its convenient magnetic separation advantages. 4. Conclusions In our research, Fe0 –Fe3 O4 –RGO was successfully prepared and used as heterogeneous Fenton catalysts. Characterization proves that Fe3 O4 was attached to the RGO sheet through metal-carbonyl coordination and Fe0 was loaded on the RGO sheet surrounded by Fe3 O4 . The catalyst shows superparamagnetic properties and has high BET surface area. The different influential parameters on the degradation of MB were investigated systematically. It was found that under the optimum condition (pH 3.00, H2 O2 dosage 0.8 mmol/L, catalyst dosage 0.10 g/L), the catalyst showed very excellent discoloration efficiency, very fast reaction kinetic rate 0.0072 L/(mg min). The kinetics of discoloration of MB fit the pseudo-second-order kinetics. The apparent activation energy
B. Yang et al. / Journal of Water Process Engineering 5 (2015) 101–111
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