catalysts Article
Synthesis of Magnetic Carbon Supported Manganese Catalysts for Phenol Oxidation by Activation of Peroxymonosulfate Yuxian Wang 1, *, Yongbing Xie 2 , Chunmao Chen 1 , Xiaoguang Duan 3 , Hongqi Sun 4 and Shaobin Wang 3 1 2
3 4
*
State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing, 18 Fuxue Road, Beijing 102249, China;
[email protected] Beijing Engineering Research Center of Process Pollution Control, Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China;
[email protected] Department of Chemical Engineering, Curtin University, GPO Box U1987, Perth 6845, Australia;
[email protected] (X.D.);
[email protected] (S.W.) School of Engineering, Edith Cowan University, 270 Joondalup Drive, Joondalup 6027, Australia;
[email protected] Correspondence:
[email protected]; Tel.: +86-10-8973-9078
Academic Editor: Keith Hohn Received: 2 November 2016; Accepted: 21 December 2016; Published: 26 December 2016
Abstract: Magnetic core/shell nanospheres (MCS) were synthesized by a novel and facile one-step hydrothermal method. Supported manganese oxide nanoparticles (Fe3 O4 /C/Mn) were obtained from various methods (including redox, hydrothermal and impregnation) using MCS as the support material and potassium permanganate as the precursor of manganese oxide. The Mn/MCS catalysts were characterized by a variety of characterization techniques and the catalytic performances of Fe3 O4 /C/Mn nanoparticles were tested in activation of peroxymonosulfate to produce reactive radicals for phenol degradation in aqueous solutions. It was found that Fe3 O4 /C/Mn catalysts can be well dispersed and easily separated from the aqueous solutions by an external magnetic field. Kinetic analysis showed that phenol degradation on Fe3 O4 /C/Mn catalysts follows the first order kinetics. The peroxymonosulfate activation mechanism by Fe3 O4 /C/Mn catalysts for phenol degradation was then discussed. Keywords: magnetic separation; sulfate radicals; Phenol; manganese oxides; carbon spheres
1. Introduction Toxic and hazardous organic compounds, such as dyes and phenolic products, broadly exist in the wastewater generated from industrial processes and have caused severe problems to the environment [1,2]. Due to their strong toxicity even at low concentration [3], development of efficient treatment technologies is in urgent demand. Advanced oxidation processes (AOPs) involving Fenton reaction, photocatalysis, electrocatalysis and various chemical methods have attracted intensive research interests for decomposing the organic contaminants due to their high efficiency and complete degradation capability [4]. Previous investigations have demonstrated that Fenton/Fenton-like reactions utilizing hydroxyl radicals (OH·) could decompose organic contaminants efficiently [5,6]. However, these processes suffer from shortcomings like metal leaching, pH adjustment, production of large quantity of sludge and cost-intensive production [7]. As alternatives to hydroxyl radicals involved in Fenton reactions, sulfate radicals (SO4 − ) which can be generated by activation of peroxymonosulfate (Oxone, peroxymonosulfate (PMS)) have gain Catalysts 2017, 7, 3; doi:10.3390/catal7010003
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intensive attention [8–10]. Sulfate radicals have a higher redox potential than hydroxyl radicals (E0 = 3.1 V vs. E0 = 2.7 V) and enable them to be more desirable for persistent organic pollutants (POPs) degradation [11]. Cobalt oxides are proven to be the effective heterogeneous catalysts for PMS activation [12,13]. Nevertheless, due to the metal leaching, employment of cobalt based catalysts would result in the secondary pollution caused by the toxicity of cobalt ions [14,15]. Moreover, improper recycling of these nanosized particles would also bring contaminant to the environment. Utilizing magnetic separation for catalysts recycle has attracted considerable research attention. Compared with traditional separation technologies such as centrifugation and filtration, magnetic separation through an external magnetic field is more convenient and less cost-intensive [16]. Magnetite (Fe3 O4 ) nanoparticles are prevalently employed as magnetic cores owing to their outstanding magnetic and electrochemical properties [17–20]. However, their high surface area to volume ratio and strong dipole–dipole attraction make them prone to aggregation, and the limited functional groups circumvent their further applications [16]. To circumvent such drawbacks, barrier materials have been investigated to prevent self-aggregation and to isolate from attached functional components [16,21]. Recently, carbon coated magnetite nanospheres (Fe3 O4 /C) have been employed because of their low cytotoxicity and highly modifiable surface [22,23]. In the pioneer study, we synthesized such magnetic carbon nanospheres and loaded with cobalt oxides as catalysts [24]. It was found that the as-synthesized materials demonstrated excellent recyclability under external magnetic field. Compared to cobalt oxides, manganese oxides are less toxic and more abundant in earth. Due to the unique redox loop (Mn3+ /Mn4+ or Mn2+ /Mn3+ ) involving a single electron transfer and the superior chemical and physical properties, manganese oxides demonstrate great potentials as catalysts [14]. In our previous studies, manganese oxides at different chemical states showed excellent catalytic abilities for PMS activation [13]. Although fruitful researches have been so far conducted for utilizing various manganese based catalysts for activation of PMS for environmental remediation, few studies focused on the supported manganese oxides catalysis [25]. In this study, three types of supported manganese oxide catalysts were prepared from redox, impregnation and hydrothermal methods, respectively, and magnetic carbon nanospheres (Fe3 O4 /C) were utilized as the supporting media. To evaluate morphological and physicochemical properties, the as-prepared catalysts were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), N2 sorption, Fourier Transform infrared spectroscopy (FT-IR) and thermogravimetric analysis (TGA) techniques. The heterogeneous catalytic performance of these catalysts was investigated by activation of PMS for oxidation of phenol solution and the reaction kinetics were investigated. Moreover, to better evaluate the catalytic performance of the as-prepared materials and the influence of electron donating group (EDG) and electron withdrawing group (EWG) on the degradation efficiency, p-cresol which including a methyl group (EDG) and 4-nitrophenol which including a nitro group (EWG) were utilized as the target pollutants. 2. Results and Discussion 2.1. Characterization of Fe3 O4 /C/Mn Hybrids Figure 1 shows SEM images for investigation of the structure and the morphology of the samples. It can be found that all of the samples present a sphere-like morphology with a size range between 20 and 30 nm. Moreover, in these SEM images, the nanoparticles agglomerated to form bulk structures, which indicates the strong inter-molecular magnetic dipolar interaction induced by the magnetic core [22]. The Mn species were expected to homogeneously distribute on the spheres. Compared with Mn/MCS-H (Figure 1E), the morphology of Mn/MCS-R and Mn/MCS-I (Figure 1A,C) showed a more uniformed sphere-like structure, suggesting the calcination process in the two methods regulated the shape of hybrids.
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Figure 2 displays TEM images of the as-synthesized magnetic carbon supported manganese Catalysts 2017, 7, 3 3 of 17 catalysts. As seen, metal/carbon clusters were formed for all of these three samples. Fe 3 O4 (small bright dots) with a diameter of 10 nm and the amorphous carbon formed were found in bright dots) with a diameter of 10 nm and the amorphous carbon formed were found in the form of the formaggregated of highly aggregated composites. For Mn/MCS-R, Mn/MCS-I, large dart dotsrefer mightto refer highly composites. For Mn/MCS‐R, Mn/MCS‐I, the large the dart dots might the to the formed manganese oxide. For Mn/MCS-H, beads/tubes structure was observed ascribing to the formed manganese oxide. For Mn/MCS‐H, beads/tubes structure was observed ascribing to the hydrothermal process. hydrothermal process.
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Figure 1. Scanning electron microscopy (SEM) images of Fe 4/C/Mn hybrids: (A,B) from Fe3O4/C/Mn Figure 1. Scanning electron microscopy (SEM) images3Oof Fe3 O4 /C/Mn hybrids: (A,B) from hybrid by redox method (Mn/MCS‐R); (C,D) from Fe3O4/C/Mn hybrid by impregnation method Fe 3 O4 /C/Mn hybrid by redox method (Mn/MCS-R); (C,D) from Fe3 O4 /C/Mn hybrid by 3O4/C/Mn hybrid by hydrothermal method (Mn/MCS‐H). (Mn/MCS‐I); and (E,F) from Fe impregnation method (Mn/MCS-I); and (E,F) from Fe3 O4 /C/Mn hybrid by hydrothermal method (Mn/MCS-H).
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(C) Figure 2. Transmission electron microscopy (TEM) images of: Mn/MCS‐R (A); Mn/MCS‐I (B); and Figure 2. Transmission electron microscopy (TEM) images of: Mn/MCS-R (A); Mn/MCS-I (B); Mn/MCS‐H (C). and Mn/MCS-H (C).
The crystalline structures of the samples were identified by XRD patterns, as shown in Figure 3. The crystalline structures of the samples were identified by XRD patterns, as shown in Figure 3. As seen, XRD patterns of the three magnetic carbon spheres (MCS) related samples had the same As seen, XRD patterns of the three magnetic carbon spheres (MCS) related samples had the same characteristic diffraction peaks at 30.1°, 35.4°, 43.1°, 56.9° and 62.5° and were well agreement with ◦ , 35.4◦ , 43.1◦ , 56.9◦ and 62.5◦ and were well agreement with characteristic diffraction 30.1lattice inverse spinal structure peaks Fe3O4 at with constants of α 8.397 Å (JCPDS No. 65‐3107) [26]. As inverse spinal structure Fe3 O4 with lattice constants of α = 8.397 Å (JCPDS No. 65-3107) [26]. illustrated in the figure, the characteristic peaks corresponded to crystal planes of (2 2 0), (3 1 1), (4 0 As illustrated in the figure,3Othe characteristic peaks corresponded to crystal planes of (2 2 0), (3 1 1), 0), (4 2 2) and (4 4 0) of Fe 4, respectively. Moreover, in all of these XRD patterns, no obvious sharp (4diffraction peaks (27° and 55°) corresponding to the graphite or graphite oxide can be observed [14], 0 0), (4 2 2) and (4 4 0) of Fe3 O4 , respectively. Moreover, in all of these XRD patterns, no obvious sharp diffraction peaks (27◦ and 55◦ ) corresponding to the graphite or graphite oxide can be observed [14], indicating that most of the carbon prepared with these methods was amorphous. For XRD patterns indicating that most of the carbon withpeaks these methods was amorphous. For XRD of Mn/MCS‐R and Mn/MCS‐I, no prepared characteristic were identified for MnOx due to the patterns heavy ofinterference of Fe Mn/MCS-R and Mn/MCS-I, no characteristic peaks were identified for MnO due to the heavy 3O4 peaks and low manganese loading rate. However, for Mn/MCS‐H, characteristic x interference of Fe3 O4 peaks and low manganese loading rate. However, for Mn/MCS-H, characteristic diffraction peaks for α‐MnO 2 phase (JCPDS No. 44‐0141, tetragonal, I4/m, a = b = 9.78 Å, c = 2.86 Å) diffraction peaks for α-MnO2 phase (JCPDS No. 44-0141, tetragonal, I4/m, a = b = 9.78 Å, c = 2.86 Å) were observed [27]. were observed Figure 4 [27]. shows N2 sorption isotherms and the pore size distributions of three Fe3O4/C/Mn Figure 4 shows N2 sorption isotherms and the pore size distributions of three Fe3 O4 /C/Mn hybrids. As seen, all the three samples presented a type IV isotherm with a type H3 hysteresis loop, indicating a typical mesoporous structure [24]. The hysteresis loops and pore size distributions (inset hybrids. As seen, all the three samples presented a type IV isotherm with a type H3 hysteresis figure) for all of the samples are also quite similar: narrow hysteresis loops at a relative pressure P/P 0 loop, indicating a typical mesoporous structure [24]. The hysteresis loops and pore size distributions range of 0.5 for to all 0.95 be observed modal pore diameter distribution centered at (inset figure) of could the samples are alsowith quitesingle similar: narrow hysteresis loops at a relative pressure around 2.5 nm. The BET surface area and the pore volume for each sample are summarized in Table 1. P/P 0 range of 0.5 to 0.95 could be observed with single modal pore diameter distribution centered at Mn/MCS‐I showed the highest specific surface area which is almost three times higher than that of around 2.5 nm. The BET surface area and the pore volume for each sample are summarized in Table 1. 2/g). Correspondingly, the pore volumes follow the order of Mn/MCS‐I > Mn/MCS‐H (74.6 vs. 26.6 m Mn/MCS-I showed the highest specific surface area which is almost three times higher than that of 2 Mn/MCS‐R > Mn/MCS‐H. The high surface area and pore volumes volume of sample might be > Mn/MCS-H (74.6 vs. 26.6 m /g). Correspondingly, the pore follow the Mn/MCS‐I order of Mn/MCS-I ascribed to the calcination involved in the sample synthesis processes. Mn/MCS-R > Mn/MCS-H. The high surface area and pore volume of sample Mn/MCS-I might be ascribed to the calcination involved in the sample synthesis processes.
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Figure 3. X‐ray diffraction (XRD) patterns of various Fe 3O4/C/Mn /C/Mn hybrids. Figure 3. X-ray diffraction (XRD) patterns of various Fe3 O hybrids. 4 Figure 3. X‐ray diffraction (XRD) patterns of various Fe3O4/C/Mn hybrids. 0.006 0.006 0.005
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Surface Area 2 m2/g) Catalyst Surface Area(S (SBET BET m /g) Mn/MCS‐R 53.8 Mn/MCS-R 53.8 74.6 Mn/MCS‐I Mn/MCS-I 74.6 Mn/MCS‐H 26.6 Mn/MCS-H 26.6 Catalyst
Pore Volume First‐Order Rate R2 First-Order Rate 3/g) −1) 3 (cm Constant (min Pore Volume (cm /g) Constant (min−1 ) 0.11 0.074 0.996 0.11 0.14 0.052 0.074 0.989 0.14 0.052 0.058 0.056 0.056 0.991 0.058
R2 0.996 0.989 0.991
FT‐IR was employed to investigate the surface functional groups of the magnetic carbon sphere FT-IR was employed to investigate the surface functional groups of the magnetic carbon sphere −1 was supported manganese catalysts (Figure 5). As seen, a broad band between 3600 and 3000 cm supported manganese catalysts (Figure 5). As seen, a broad band between 3600 and 3000 cm−1 was observed for all of the samples, which were resulted from the stretching vibration of the hydroxyl observed for all of the samples, which were resulted from the stretching vibration of the hydroxyl −1 in the spectra groups [28]. Besides hydroxyl groups, the intense peaks observed at 1699 and 1381 cm −1 in the spectra groups [28]. Besides hydroxyl groups, the intense peaks observed at 1699 and 1381 cm were attributed to the C=O and C–C vibrations, respectively [29]. The occurrence of these C=O and were attributedthe to the C=O and C–C vibrations, respectively [29]. occurrence these and C–C indicates carbonization of glucose [30]. In addition, the The IR band found ofat 579 C=O cm−1 was assigned to the characteristic peaks of Fe–O vibration of Fe3O4 [31], which confirms the formation of
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carbonization of glucose [30]. In addition, the IR band found at 579 cm−17 of 17 was assigned to the characteristic peaks of Fe–O vibration of Fe3 O4 [31], which confirms the formation −1 for of magnetic core. However, compared with Mn/MCS-H, a new peak was created at 2150 −1 cm magnetic core. However, compared with Mn/MCS‐H, a new peak was created at 2150 cm for the the FT-IR spectra of annealed Mn/MCS samples (Mn/MCS-R and Mn/MCS-I), which is assigned FT‐IR spectra of annealed Mn/MCS samples (Mn/MCS‐R and Mn/MCS‐I), which is assigned to the to the creation of on C≡the C on the carbon surface during annealing process [32]. Moreover, compared creation of C≡C carbon surface during annealing process [32]. Moreover, compared with − 1 −1 with Mn/MCS-R and Mn/MCS-H, peak intensity of C–C vibration at 1381 cm increase significantly Mn/MCS‐R and Mn/MCS‐H, peak intensity of C–C vibration at 1381 cm increase significantly for for Mn/MCS-I, suggesting surface feature of MCS was reconstructedand andmore morecarbon carbon atoms atoms were were Mn/MCS‐I, suggesting surface feature of MCS was reconstructed 3 3 connected by sp hybridization after heat treatment. hybridization after heat treatment. connected by sp C–C indicates the Catalysts 2017, 7, 3
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Figure 5. Fourier transform infrared spectroscopy (FT‐IR) spectra of three Fe O4/C/Mn /C/Mn hybrids. Figure 5. Fourier transform infrared spectroscopy (FT-IR) spectra of three Fe33O hybrids. 4
Thermal stability of the three Fe3O4/C/Mn catalysts was investigated by TG‐DTA under air flow Thermal stability of the three Fe3 O4 /C/Mn catalysts was investigated by TG-DTA under air flow (Figure 6). For TGA profiles of Mn/MCS‐R and Mn/MCS‐H (Figure 6A,C), three obvious weight (Figure 6). For TGA profiles of Mn/MCS-R and Mn/MCS-H (Figure 6A,C), three obvious weight processes were observed. A slight weight loss between 30 and 110 °C can be assigned to the removal processes were observed. A slight weight loss between 30 and 110 ◦ C can be assigned to the removal of physically adsorbed water molecules. Further elevation of temperature caused a characteristic of physically adsorbed water molecules. Further elevation of temperature caused a characteristic step/peak in the range from 250 to 400 °C which also reflects in a sharp peak on the heat flow, step/peak in the range from 250 to 400 ◦ C which also reflects in a sharp peak on the heat flow, indicating indicating the decomposition of carbon skeleton for the carbon coated on the Fe3O4 [33]. When the decomposition of carbon skeleton for the carbon coated on the Fe O4 [33]. When temperature temperature ◦reached 500 °C, another slight weight loss was induced, 3 indicating the oxidation of reached 500 C, another slight weight loss was induced, indicating the oxidation of magnetic Fe3 O4 magnetic Fe3O4 core to Fe2O3. However, as seen in TGA profiles for Mn/MCS‐I (Figure 6B), a three‐ core to Fe2 O3 . However, as seen in TGA profiles for Mn/MCS-I (Figure 6B), a three-stage weight loss stage weight loss process was observed. The first weight loss occurred before 300 °C, which could be process was observed. The first weight loss occurred before 300 ◦ C, which could be attributed to the attributed to the desorption of the surface adsorbed water. The second weight loss occurred between desorption of the surface adsorbed water. The second weight loss occurred between 400 and 600 ◦ C, 400 and 600 °C, which is attributed to loss of lattice oxygen and the transforming of the oxidizing which is attributed to loss of lattice oxygen and the transforming of the oxidizing states. The final states. The final weight loss occurred between 750 and 850 °C, which could be ascribed to the further weight loss occurred between 750 and 850 ◦ C, which could be ascribed to the further loss of the lattice loss of the lattice oxygen and reduction to the Mn3O4, the most stable state of manganese oxide. The oxygen and reduction to the Mn3 O4 , the most stable state of manganese oxide. The TGA pattern of TGA pattern of Mn/MCS‐I was quite similar as our previously reported α‐MnO 2 [14]. Therefore, we Mn/MCS-I was quite similar as our previously reported α-MnO2 [14]. Therefore, we presumed that presumed that the crystal form of Mn in Mn/MCS‐I was α‐MnO2. In addition, the total weight loss the crystal form of Mn in Mn/MCS-I was α-MnO2 . In addition, the total weight loss was much less was much less than those of Mn/MCS‐R and Mn/MCS‐H, suggesting that some of the carbon had than those of Mn/MCS-R and Mn/MCS-H, suggesting that some of the carbon had been consumed been consumed during annealing in muffle furnace within sample synthesis processes. From the TG‐ during annealing in muffle furnace within sample synthesis processes. From the TG-DTA analysis, DTA analysis, it can be deduced that manganese oxide in different oxidizing states has been formed it can be deduced that manganese oxide in different oxidizing states has been formed for each of for each of Fe3O4/C/Mn hybrid synthesized via different methods. Figure 6D shows the TGA curve of Fe3 O4 /C/Mn hybrid synthesized via different methods. Figure 6D shows the TGA curve of the fresh the fresh MCS. According to mass losses in the three Fe 3O4/C/Mn hybrids, it could be calculated that MCS. According to mass losses in the three Fe3 O4 /C/Mn hybrids, it could be calculated that the mass the mass fractions of manganese oxide included in Mn/MCS‐R, Mn/MCS‐I and Mn/MCS‐H was about fractions of manganese oxide included in Mn/MCS-R, Mn/MCS-I and Mn/MCS-H was about 7.3%, 7.3%, 5.7% and 5.4%, respectively. 5.7% and 5.4%, respectively.
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Figure 6. Thermogravimetric 4/C/Mn hybrid: (A) (A) Mn/MCS‐R; (B) Figure 6. Thermogravimetricanalysis analysis(TGA) (TGA)curves curvesof ofFe Fe3O hybrid: Mn/MCS-R; 3O 4 /C/Mn (B) Mn/MCS-I; and (C) Mn/MCS-H (D) TGA curve of MCS. Mn/MCS‐I; and (C) Mn/MCS‐H (D) TGA curve of MCS.
2.2. Catalytic Oxidation of Phenol 2.2. Catalytic Oxidation of Phenol Figure 7A describes the adsorption and phenol degradation on the three Fe 3O/C/Mn Figure 7A describes the adsorption and phenol degradation on the three Fe3 O hybrids. 4 4/C/Mn hybrids. In order to investigate the catalytic activity of the samples, control experiments were carried out to In order to investigate the catalytic activity of the samples, control experiments were carried out evaluate phenol removal caused ambient to evaluate phenol removal causedby bycatalyst catalystadsorption adsorptionand and PMS PMS self‐activation self-activation at at ambient environment. Without any catalyst, PMS induced negligible phenol degradation by itself. Shown in environment. Without any catalyst, PMS induced negligible phenol degradation by itself. Shown in the figure, less than 10% of phenol was removed after 180 min, revealing that PMS itself could not be the figure, less than 10% of phenol was removed after 180 min, revealing that PMS itself could not be effectively activated by the ambient environment to produce reactive species for significant phenol effectively activated by the ambient environment to produce reactive species for significant phenol degradation. For the adsorption tests carried out with the presence of catalysts only, three catalysts degradation. For the adsorption tests carried out with the presence of catalysts only, three catalysts showed similar phenol removal profiles, in which phenol removal fluctuated for the first 60 min to showed similar phenol removal profiles, in which phenol removal fluctuated for the first 60 min to achieve adsorption/desorption equilibrium. However, subsequently these catalysts did not show any achieve adsorption/desorption equilibrium. However, subsequently these catalysts did not show significant phenol degradation. For phenol removal profiles of catalytic reactions, commercial MnO any significant phenol degradation. For phenol removal profiles of catalytic reactions, commercial2 presented phenol removal at 20%. Mn/MCS‐R with PMS provided the best phenol degradation rate MnO2 presented phenol removal at 20%. Mn/MCS-R with PMS provided the best phenol degradation and 100% removal of phenol was achieved within 180 both rate and 100% removal of phenol was achieved within 180min. min.While Whilephenol phenoldegradation degradation for for both Mn/MCS‐I and Mn/MCS‐H with PMS were similar and around 90% of the initial phenol was removed Mn/MCS-I and Mn/MCS-H with PMS were similar and around 90% of the initial phenol was removed after 180 min. after 180 min. Control experiments were carried out to investigate the catalytic activities of the carbon sphere, Control experiments were carried out to investigate the catalytic activities of the carbon sphere, Fe Fe33O O44, MCS and MnO , MCS and MnOxx (Figure 7B). As seen, carbon sphere demonstrated significant capability for (Figure 7B). As seen, carbon sphere demonstrated significant capability for phenol adsorption due to its its highly highly porous porous structure. structure. However, catalytic degradation degradation profile phenol adsorption due to However, its its catalytic profile almost overlapped the adsorption profile, indicating the negligible catalytic activity. Fe O44 displayed almost overlapped the adsorption profile, indicating the negligible catalytic activity. Fe33O displayed insignificant phenol adsorption capability and catalytic activity since less than 10% of phenol was insignificant phenol adsorption capability and catalytic activity since less than 10% of phenol was removed 180 min min for for both both conditions. conditions. MCS still removed after after 180 MCS showed showed some some catalytic catalytic activity, activity, however, however, still incomparable with those of Mn/MCSs’. MnO incomparable with those of Mn/MCSs’. MnOxx was obtained by calcination of MnSO was obtained by calcination of MnSO44·∙H H22O at 200 °C O at 200 ◦ C for 4 h. As observed, around 85% of the phenol was decomposed when MnO for 4 h. As observed, around 85% of the phenol was decomposed when MnOxx was utilized as the was utilized as the catalyst for PMS activation. catalyst for PMS activation.
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Figure 7. Residual phenol amount using MCS related catalysts (A) and other control factors (B) (C/C 0) Figure 7. Residual phenol amount using MCS related catalysts (A) and other control factors (B) (C/C high performance liquid chromatography (HPLC) (270 nm wavelength). as revealed by high by performance liquid chromatography (HPLC) (270 nm wavelength). Reaction 0 ) as revealed Reaction condition:0 = 20 mg/L, catalyst loading = 0.2 g/L, Oxone loading = 2.0 g/L, Temperature: 25 °C. [phenol]0 = 20 mg/L, catalyst loading = 0.2 g/L, Oxone loading = 2.0 g/L, condition: [phenol] Temperature: 25 ◦ C.
Based on these phenol degradation profiles, a first order kinetic mode (Equation (1)) was applied to evaluate the catalytic reaction kinetics. Based on these phenol degradation profiles, a first order kinetic mode (Equation (1)) was applied to evaluate the catalytic reaction kinetics.
(1) C ln = −kt (1) C0 0 is the phenol concentration at initial time. K is the where C is phenol concentration at time (t) and C
first order reaction rate constant. Figure 8 shows that phenol degradation curve was well fitted (R2 > where C is phenol concentration at time (t) and C0 is the phenol concentration at initial time. K is 0.98) with the first‐order kinetics. the first order reaction rate constant. Figure 8 shows that phenol degradation curve was well fitted The catalytic activities of these as‐synthesized supported manganese catalysts does not seem to (R2 > 0.98) with the first-order kinetics. be correlated with their surface areas. In fact, the highest BET specific surface area of Mn/MCS‐I (74.6 The catalytic activities of these as-synthesized supported manganese catalysts does not seem m2/g) did not lead to the fastest reaction rate, as well as this materials showed a similar catalytic to be correlated with their surface areas. In fact, the highest BET specific surface area of Mn/MCS-I activity as that of Mn/MCS‐H, although its BET surface area was around 3 times less than Mn/MCS‐ (74.6 m2 /g) did not lead to the fastest reaction rate, as well as this materials showed a similar catalytic I (26 m2/g vs. 75 m2/g). Remarkably, Mn/MCS‐R, which had a surface area of 53.8 m2/g, displayed the activity as that of Mn/MCS-H, although its BET surface area was around 3 times less than Mn/MCS-I highest catalytic activity and the reaction rate constant. (26 m2 /g vs. 75 m2 /g). Remarkably, Mn/MCS-R, which had a surface area of 53.8 m2 /g, displayed Several studies also revealed that the catalytic activities of supported metal oxides were the highest catalytic activity and the reaction rate constant. independent on the surface area [34,35]. Cobalt oxides loaded on Al2O3, SiO2 and TiO2 were prepared by Sun et al. and their activities were tested for phenol degradation [34]. It was found that, even though Co/TiO2 possessed the least surface area, it demonstrated the highest catalytic activity for
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Several studies also revealed that the catalytic activities of supported metal oxides were independent on the surface area [34,35]. Cobalt oxides loaded on Al2 O3 , SiO2 and TiO2 were prepared by Sun et al. and their activities were tested for phenol degradation [34]. It was found that, even Catalysts 2017, 7, 3 10 of 17 though Co/TiO2 possessed the least surface area, it demonstrated the highest catalytic activity for decomposition of phenol. Wang et al. synthesized MnO /ZnFe O magnetic catalysts with different 4 decomposition of phenol. Wang et al. synthesized MnO22/ZnFe2O2 4 magnetic catalysts with different shapes for environmental remediation purposes [36]. Compared with the sea-urchin shaped catalyst, shapes for environmental remediation purposes [36]. Compared with the sea‐urchin shaped catalyst, the corolla shaped catalysts showed lower BET specific surface area but superior catalytic activities the corolla shaped catalysts showed lower BET specific surface area but superior catalytic activities for activation of PMS for phenol degradation. In this study, TGA results suggested that the order of for activation of PMS for phenol degradation. In this study, TGA results suggested that the order of catalytic activity on these catalysts are well in agreement with their corresponding manganese oxide catalytic activity on these catalysts are well in agreement with their corresponding manganese oxide loading amount. Mn/MCS‐R, obtaining the greatest manganese oxide loading amount (7.3%), loading amount. Mn/MCS-R, obtaining the greatest manganese oxide loading amount (7.3%), showed showed the highest catalytic activity. the highest catalytic activity. 0
ln(C/C0)
-1
-2
2
Mn/MCS-R R =0.981 2 Mn/MCS-I R =0.997 2 Mn/MCS-H R =0.991 First order kinetics fitting curve
-3
-4
0
20
40
60
80
100 120 140 160 180
Time (min)
Figure 8. First order kinetic model of reactions. Y axis values are referred to the residual phenol. Figure 8. First order kinetic model of reactions. Y axis values are referred to the residual phenol.
In the past few years, investigations have been carried out for both homogeneous and
In the past few years, investigations have been carried out for both homogeneous and heterogeneous catalytic reactions using Mn based catalysts. Anipsitakis and Dionysiou [37] used heterogeneous catalytic reactions using Mn for based catalysts. Anipsitakis Dionysiou Mn2+ for homogeneous activation of PMS 2,4‐dichlorophenol (2,4‐DCP) and degradation. In [37] their used − Mn2+studies, 24% of initial amount of 2,4‐DCP was removed within 4 h when HSO for homogeneous activation of PMS for 2,4-dichlorophenol (2,4-DCP) degradation. In their 5 concentration was 50 − concentration was studies, 24% of initial amount of 2,4-DCP was removed within 4 h when HSO ppm. Watts et al. [6] investigated oxidative and reductive pathways in manganese‐catalyzed Fenton 5 reactions and reducing species were and generated by crystalline MnO2‐ 50 ppm. Watts etfound al. [6]that investigated oxidative reductive pathwaysand in amorphous manganese-catalyzed catalysed decomposition of H 2O2reducing . Edy et al. [38] synthesized α‐Mn 2O3 in different shapes using a one‐ Fenton reactions and found that species were generated by crystalline and amorphous MnOstep hydrothermal method and investigated their catalytic activities for activation of PMS to degrade 2 -catalysed decomposition of H2 O2 . Edy et al. [38] synthesized α-Mn2 O3 in different shapes 2O3 cubic possessed the best catalytic activity and for 25 ppm phenol, usingphenol. They found that α‐Mn a one-step hydrothermal method and investigated their catalytic activities for activation of PMS 100% decomposition achieved within 60 min when catalyst usage of 0.4 g/L. Liang et al. [15] reported to degrade phenol. They found that α-Mn2 O3 cubic possessed the best catalytic activity and for 25 ppm mesoporous MnO2 supported Co3O4 nanoparticles synthesized by impregnation and their phenol, 100% decomposition achieved within 60 min when catalyst usage of 0.4 g/L. Liang et al. [15] performances in heterogeneous activation of peroxymonosulfate (PMS) for phenol degradation. They reported mesoporous MnO synthesized by impregnation and their 2 supported Co3 O4 nanoparticles found that when Co 3O4 loading rate is 3 wt %, MnO 2 supported Co3O4 catalysts could achieve 100% performances in heterogeneous activation of peroxymonosulfate phenol removal at phenol concentration of 25 ppm within 120 min. (PMS) for phenol degradation. They found that when Co3 O4 loading rate is 3 wt %, MnO2 supported Co3 O4 catalysts could achieve It has been proposed that PMS activation process was induced by redox reactions [14], while the 4+/Mn3+ provides the redox potential. Thus, the mechanism of catalytic oxidation of 100%transition of Mn phenol removal at phenol concentration of 25 ppm within 120 min. phenol on Fe 3 O 4 /C/Mn hybrids with PMS could be proposed as follows. It has been proposed that PMS activation process was induced by redox reactions [14], while the transition of Mn4+ /MnS3+ provides mechanism of (2) Mn IV the HSOredox → Spotential. Mn III Thus, SO the OH S solidof catalytic oxidation phenol on Fe3 O4 /C/Mn hybrids with PMS could be proposed as follows. S
Mn III
HSO
→ S
Mn IV
SO
H
S
solid
(3)
SO5 − →H SO−→Mn SO(III) +OH S − Mn(IV) + HSO SO4 −H+ OH− (S − solid) SO
OH
C H OH → serveral steps → CO
H O
SO
S − Mn(III) + HSO5 − → S − Mn(IV) + SO5 − + H+ (S − solid)
(4) (5)
(2) (3)
Figure 9 shows photographs of Fe3O4/C/Mn catalysts when they were dispersed in water and SO4 − + H2 O → SO4 2− + OH· + H+ (4) were separated by an external magnetic field. As seen, all of the catalysts could be well dispersed to form stable brown suspension after a 1 min long sonication (Figure 9A). When a magnet was SO4 − + OH· + C6 H5 OH → serveral steps → CO2 + H2 O + SO4 2− (5) approached, catalysts in each of the glass vials accumulated to the side near the magnet quickly. The
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Figure 9 shows photographs of Fe3 O4 /C/Mn catalysts when they were dispersed in water and were separated by an external magnetic field. As seen, all of the catalysts could be well dispersed to form stable brown suspension after a 1 min long sonication (Figure 9A). When a magnet was approached, catalysts in each of the glass vials accumulated to the side near the magnet quickly. Catalysts 2017, 7, 3 11 of 17 The solution became clear within 2 min (Figure 9B). After the magnetic was removed and as-mentioned solution became clear within 2 min (Figure 9B). After the magnetic was removed and as‐mentioned sonicating procedure was repeated, all three samples re-dispersed in water and formed the stable sonicating procedure was repeated, all three samples re‐dispersed in water and formed the stable suspension again. Therefore, since attraction and dispersion processes can be readily altered by suspension again. Therefore, since attraction and dispersion processes can be readily altered by simply approaching or removing an external magnetic field, the as-synthesized Fe3 O4 /C/Mn catalysts simply approaching or removing an external magnetic field, the as‐synthesized Fe3O4/C/Mn catalysts demonstrated excellent water dispersion and magnetic separation ability for effective separation. demonstrated excellent water dispersion and magnetic separation ability for effective separation.
Figure 9. Photographs of the separation and dispersion processes: (A) without external magnetic field; Figure 9. Photographs of the separation and dispersion processes: (A) without external magnetic field; and (B) with external magnetic field.
and (B) with external magnetic field.
In order to evaluate the influence of reaction conditions on catalytic phenol degradation, further studies were carried out using the most effective catalyst, Mn/MCS‐R. Figure 10 illustrates phenol In order to evaluate the influence of reaction conditions on catalytic phenol degradation, further degradation on Mn/MCS‐R as a function of different temperatures (25, 35, and 45 °C). As shown in studies were carried out using the most effective catalyst, Mn/MCS-R. Figure 10 illustrates phenol the figure, phenol degradation rate was enhanced when reaction temperature was elevated, degradation on Mn/MCS-R as a function of different temperatures (25, 35, and 45 ◦ C). As shown in indicating the endothermic property of this catalytic reaction. Higher temperature induced the the figure, phenol degradation rate was enhanced when reaction temperature was elevated, indicating production of more reactive species. The time intervals necessary to guarantee 100% phenol the endothermic property of this catalytic reaction. Higher temperature induced the production of degradation were 120 and 60 min at the temperatures of 35 and 45 °C, respectively. Based on the first‐ moreorder kinetics, reaction rate constants at varying temperatures were obtained and they were related reactive species. The time intervals necessary to guarantee 100% phenol degradation were 120 and 60 min at the temperatures of 35 and 45 ◦ C, with respectively. Based on the first-order reaction with temperature by the Arrhenius equation high regression coefficients, shown kinetics, in the inset rate constants at varying temperatures were obtained and they were related with temperature by the figure. The activation energy was then calculated to be 32.1 kJ/mol. very high few regression investigations have been shown reported on inset supported catalysts on ArrheniusPreviously, equation with coefficients, in the figure.MnOx The activation energy activation of PMS for phenol degradation. However, in recent years, investigations were carried out was then calculated to be 32.1 kJ/mol. for heterogeneous of PMS using supported cobalt oxide catalysts. Very recently, we on Previously, very activation few investigations have been reported onas supported MnOx catalysts investigated the catalytic activity of magnetic cobalt/carbon sphere/Fe3O4 composites (Co/MCS) for activation of PMS for phenol degradation. However, in recent years, investigations were carried catalytic oxidation of phenol, and found the activation energy at 49.1 kJ/mol [24]. For comparison, out for heterogeneous activation of PMS using supported cobalt oxide as catalysts. Very recently, we Table 2 summarizes the activation energies obtained from these researches on PMS activation using investigated the catalytic activity of magnetic cobalt/carbon sphere/Fe3 O4 composites (Co/MCS) for supported cobalt oxide as catalysts. As seen, activation energies of supported cobalt oxide catalysts catalytic oxidation of phenol, and found the activation energy at 49.1 kJ/mol [24]. For comparison, are within the range of 40–70 kJ/mol and thus the as‐synthesized supported manganese oxide catalyst Table(Mn/MCS‐R) presents much lower activation energy. 2 summarizes the activation energies obtained from these researches on PMS activation using supported cobalt oxide as catalysts. As seen, activation energies of supported cobalt oxide catalysts are within the range of 40–70 kJ/mol and thus the as-synthesized supported manganese oxide catalyst (Mn/MCS-R) presents much lower activation energy.
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1.0 1.0
-2.8
o
25 C oo 25 35 C C oo 35 45 C C o 45 C
0.8 0.8
-2.8 -3.0 -3.0 -3.2
ln(K)ln(K)
Residual Phenol Amount (C/C ) ) Residual Phenol Amount (C/C 0 0
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0.6 0.6 0.4 0.4
-3.2 -3.4
2
R =0.991
-3.6 -3.8 -3.8
0.2 0.2 0.0 0.0 0 0
2
R =0.991
-3.4 -3.6
20 20
40 40
60 60
0.00315
0.00320
0.00315
0.00320
80
0.00325
0.00330
0.00335
0.00325
0.00330
0.00335
-1
1/T(K ) -1
1/T(K )
100 120 140 160 180
80 (min) 100 120 140 160 180 Time Time (min) Figure 10. Phenol degradation on Mn/MCS‐R at different reaction temperature. Reaction conditions: Figure 10. Phenol degradation on Mn/MCS-R at different reaction temperature. Reaction conditions: Figure 10. Phenol degradation on Mn/MCS‐R at different reaction temperature. Reaction conditions: [phenol] [phenol] 20 mg/L, catalyst loading = 0.2 g/L, Oxone loading = 2.0 g/L. 0 =0 = 20 mg/L, catalyst loading = 0.2 g/L, Oxone loading = 2.0 g/L.
[phenol]0 = 20 mg/L, catalyst loading = 0.2 g/L, Oxone loading = 2.0 g/L. Table 2. Activation energies of representative Mn‐ and Co‐based catalysts. Table 2. Activation energies of representative Mn- and Co-based catalysts. Table 2. Activation energies of representative Mn‐ and Co‐based catalysts.
Catalyst Catalyst Co/MCS Catalyst Co/MCS Co/carbon xerogel Co/MCS Co/carbon xerogel Co/MnO 2 Co/carbon xerogel Co/MnO Co/activated carbon Co/MnO2 2 Co/activated carbon Co/activated carbon Co/red mud Co/red mud Co/red mud Co/SBA‐15 Co/SBA-15 Fe3OCo/SBA‐15 4/C/Mn hybrids Fe3 O4 /C/Mn hybrids Fe3O4/C/Mn hybrids
Activation Energy (kJ/mol) Activation Energy (kJ/mol) 49.1 (kJ/mol) Activation Energy 49.1 48.3 49.1 48.3 42.5 48.3 42.5 59.7 42.5 59.7 59.7 47.2 47.2 47.2 67.4 67.4 67.4 32.1 32.1 32.1
Reference Reference [24] Reference [24] [39] [24] [39] [15] [39] [15] [40] [15] [40] [40] [41] [41] [41] [42] [42] [42] This study This study This study
Residual Phenol Amount (C/C ) 0) Residual Phenol Amount (C/C 0
Figure 11 presents the catalytic activity of regenerated Mn/MCS‐R by simple water washing in Figure 11 presents the catalytic activity of regenerated Mn/MCS‐R by simple water washing in Figure 11 presents the catalytic activity of regenerated Mn/MCS-R by simple water washing in phenol degradation. In the second run, the significant decrease in catalytic activity was observed and phenol degradation. In the second run, the significant decrease in catalytic activity was observed and phenol degradation. In the second run, the significant decrease in catalytic activity was observed and 80% of initial phenol was degraded after 180 min, which indicates the deactivation of the catalyst. For 80% of initial phenol was degraded after 180 min, which indicates the deactivation of the catalyst. For the third run, catalytic activity further decreased and around 40% of initial phenol remained after the 80% of initial phenol was degraded after 180 min, which indicates the deactivation of the catalyst. reaction. The decrease of catalytic activity can be assigned to the attachment of reaction intermediates Forthe third run, catalytic activity further decreased and around 40% of initial phenol remained after the the third run, catalytic activity further decreased and around 40% of initial phenol remained reaction. The decrease of catalytic activity can be assigned to the attachment of reaction intermediates on the the reaction. catalyst surface with active sites. Moreover, due be to assigned the strong Waals force, these after The decrease of catalytic activity can tovan the de attachment of reaction on the catalyst surface with active sites. Moreover, due to the strong van de Waals force, these intermediates cannot be fully removed by simple water washing. In addition, the passivation of the intermediates on the catalyst surface with active sites. Moreover, due to the strong van de Waals force, intermediates cannot be fully removed by simple water washing. In addition, the passivation of the catalyst might be resulted from the composition change of the Mn valence states. these intermediates cannot be fully removed by simple water washing. In addition, the passivation of catalyst might be resulted from the composition change of the Mn valence states. the catalyst might be resulted from the composition change of the Mn valence states. 1.0 First run Second First run run Third runrun Second Third run
1.0 0.8 0.8 0.6 0.6 0.4 0.4 0.2 0.2 0.0 0.0 0 0
20
40
60
20
40
60
80
100
120 140 160 180
80 (min) 100 120 140 160 180 Time Time (min) Figure 11. Phenol degradation on Mn/MCS‐R at different runs after recycling. Reaction conditions: Figure 11. 0Phenol degradation on Mn/MCS-R at different runs after recycling. Reaction conditions: Figure 11. Phenol degradation on Mn/MCS‐R at different runs after recycling. Reaction conditions: [phenol] = 20 mg/L, catalyst loading = 0.2 g/L, Oxone loading = 2.0 g/L, and T = 25 °C. [phenol] 20 mg/L, catalyst loading = 0.2 g/L, Oxone loading = 2.0 g/L, and T = 25 ◦ C. [phenol] 0 = 20 mg/L, catalyst loading = 0.2 g/L, Oxone loading = 2.0 g/L, and T = 25 °C. 0=
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Removal pollutants amount (C/C0)
To better better evaluate evaluate the the versatility versatility of of the the as-prepared as‐prepared materials, materials, catalytic catalytic degradation degradation tests tests To employing p‐cresol and p‐nitrophenol as the target pollutants were carried out (Figure 12) For p‐cresol employing p-cresol and p-nitrophenol as the target pollutants were carried out (Figure 12) For p-cresol and p‐nitrophenol, since the para‐position of the phenol was substituted by a methyl group and a and p-nitrophenol, since the para-position of the phenol was substituted by a methyl group and a nitro group, respectively, the influence of electron donating group (EDG)‐methyl group, and electron nitro group, respectively, the influence of electron donating group (EDG)-methyl group, and electron withdrawing group (EWG)‐nitro group, on the degradation efficiency was evaluated. As seen, both withdrawing group (EWG)-nitro group, on the degradation efficiency was evaluated. As seen, both p‐cresol and p‐nitrophenol demonstrated the inertness for PMS attacking, which was similar as the p-cresol and p-nitrophenol demonstrated the inertness for PMS attacking, which was similar as the phenol. This recalcitrance suggested that with the involvement of EDG or EWG, PMS is not favorable phenol. This recalcitrance suggested that with the involvement of EDG or EWG, PMS is not favorable for neither electrophilic attack attack nor nor nucleophilic nucleophilic attack attack on on the the aromatic aromatic ring. ring. However, with the the for neither electrophilic However, with presence of catalysts, the degradation efficiency for the target pollutants differed. Compared with presence of catalysts, the degradation efficiency for the target pollutants differed. Compared with phenol, p‐nitrophenol displayed some recalcitrance for catalytic PMS degradation and around 15% phenol, p-nitrophenol displayed some recalcitrance for catalytic PMS degradation and around 15% of of which was still remained in the reaction solution at the end of 180 min. P‐cresol demonstrated a which was still remained in the reaction solution at the end of 180 min. P-cresol demonstrated a higher higher reaction rate than phenol and complete degradation occurred at 90 min. As discussed, sulfate reaction rate than phenol and complete degradation occurred at 90 min. As discussed, sulfate and and hydroxyl radicals, are electrophilic, were main active species generated PMS hydroxyl radicals, whichwhich are electrophilic, were the mainthe active species generated from PMSfrom activation. activation. The involvement of methyl group (EWG) on the aromatic ring could be beneficial for The involvement of methyl group (EWG) on the aromatic ring could be beneficial for attracting these attracting these electrophilic reactive species since the electrons from the methyl group improved the electrophilic reactive species since the electrons from the methyl group improved the nucleophilicity nucleophilicity of the aromatic ring. In contrast, the substituted EWG groups on the aromatic ring of the aromatic ring. In contrast, the substituted EWG groups on the aromatic ring hindered this hindered this nucleophilicity of the aromatic ring and creating the resistance for the attacking from nucleophilicity of the aromatic ring and creating the resistance for the attacking from sulfate and sulfate and hydroxyl radicals. hydroxyl radicals. 1.0 p-nitrophenol: PMS only p-nitrophenol: PMS with Mn/MCS-R p-cresol: PMS only p-cresol: PMS with Mn/MCS-R phenol: PMS only phenol: PMS with Mn/MCS-R
0.8 0.6 0.4 0.2 0.0 0
20
40
60
80
100
120
140
160
Time (min)
180
Figure 12. group (EWG) and electron donating group (EDG) on Figure 12. Effects Effectsof ofelectron electronwithdrawing withdrawing group (EWG) and electron donating group (EDG) 0 = 20 mg/L, [phenol] 0 = 50 ppm, [p‐cresol] 0 degradation efficiency. Reaction conditions: [p‐nitrophenol] on degradation efficiency. Reaction conditions: [p-nitrophenol] = 20 mg/L, [phenol] = 50 ppm, 0 0 0 = 50 ppm, catalyst loading = 0.2 g/L, PMS loading: 2 g/L, temperature: = 20 ppm, [p‐chlorophenol] [p-cresol] 0 = 20 ppm, [p-chlorophenol] 0 = 50 ppm, catalyst loading = 0.2 g/L, PMS loading: 2 g/L, 25 °C. temperature: 25 ◦ C.
3. Materials and Methods 3. Materials and Methods 3.1. Materials 3.1. Materials Iron Iron (II) (II) chloride chloride tetrahydrate tetrahydrate (99.9%), (99.9%), iron iron (III) (III) hexahydrate hexahydrate (99.9%), (99.9%), manganese manganese sulfate sulfate monohydrate (99.8%) and potassium permanganate (99.8%) were purchased from Sigma Aldrich. monohydrate (99.8%) and potassium permanganate (99.8%) were purchased from Sigma Aldrich. D‐ D -glucose(99.9%) (99.9%)was wasobtained obtainedfrom fromFluka. Fluka. Ammonia Ammoniasolution solution(28%) (28%) was was obtained obtained from from Ajax glucose Ajax Finechem. High purity nitrogen gas (99%) was obtained from BOC (WA, Australia). All chemicals Finechem. High purity nitrogen gas (99%) was obtained from BOC (WA, Australia). All chemicals were used as received without any further purification. were used as received without any further purification. 3.2. Synthesis of Manganese Loaded Magnetic Carbon Spheres (Fe33O O44/C/Mn) /C/Mn) 3.2. Synthesis of Manganese Loaded Magnetic Carbon Spheres (Fe Magnetic carbon carbon nanospheres nanosphereswere weresynthesized synthesized a modified hydrothermal method, Magnetic via via a modified hydrothermal method, and and detailed procedures can be found in our recent publication [24]. The obtained sample was labeled detailed procedures can be found in our recent publication.[24] The obtained sample was labeled as as MCS. a typical synthesis redoxmethod, method,1.5 1.5g gof ofMCS MCSwere wereput putinto into a a quartz quartz boat boat and MCS. In In a typical synthesis of of redox and then then
transferred to a muffle furnace for annealing in air at 200 °C for 1 h. Then, 0.5 g annealed MCS was
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transferred to a muffle furnace for annealing in air at 200 ◦ C for 1 h. Then, 0.5 g annealed MCS was mixed with 0.11 g (0.07 mol) KMnO4 in a beaker. The mixture was dispersed in 50 mL of ultrapure water under sonication for 10 min. Then the beaker was immerged in a water bath at 70 ◦ C and kept stirring for 4 h. The resulting brownish precipitate was filtered and washed with ultrapure water for three times and dried in an oven at 80 ◦ C overnight. The sample was marked as Mn/MCS-R. In the synthesis of impregnation method, 0.5 g MCS was dispersed in 50 mL of ethanol under sonication for 10 min. Then, 0.12 g (0.07 mol) MnSO4 ·H2 O was added in the as-dispersed MCS solution. The mixed solution was stirred overnight to ensure the fully evaporation of ethanol. The resulted mixture solid was then transferred to a muffle furnace to anneal in air at 200 ◦ C for 4 h. After that the annealed mixture was washed with ultrapure water for three times and dried in an oven at 80 ◦ C overnight. The sample was denoted as Mn/MCS-I. For hydrothermal method, 0.5 g MCS was dispersed in 80 mL of ultrapure water under sonication for 10 min. Then, 0.12 g (0.07 mol) MnSO4 ·H2 O was added in the as-dispersed MCS solution. The mixed solution was stirred for at least 30 min to ensure the homogeneous dispersion. Then the mixed solution was transferred into a Teflon-lined stainless steel autoclave with the capacity of 120 mL. The autoclave was then sealed and maintained at 140 ◦ C for 12 h and was then cooled down to room temperature naturally. The products were harvested by vacuum filtration and washed with ultrapure water for 3 times before being dried at 80 ◦ C overnight. The obtained samples were denoted as Mn/MCS-H. To compare, carbon microspheres, Fe3 O4 and MnOx were synthesized. Carbon microspheres were synthesized by hydrothermal pyrolysis of glucose at 180 ◦ C for 18 h. For preparation of Fe3 O4 nanoparticles, FeCl3 and FeCl2 at the molar ratio of 2:1 were dissolved in the ultrapure water. After the mixture solution was bubbled with nitrogen flow for 10 min, 28% ammonia solution was added dropwisely to make solution pH 10. After stirring for 1 h, Fe3 O4 nanoparticles were harvested by filtering the mixture solution and washed by ultrapure water/ethanol for 3 times. Manganese oxide was obtained by calcination of MnSO4 ·H2 O at 200 ◦ C for 4 h and labeled ad MnOx . 3.3. Characterization of Materials The morphology and chemical compositions of the catalysts were observed on a ZEISS NEON 40EsB Field Emission Scanning Electron Microscope (FESEM, ZEISS, Germany). XRD patterns were obtained on a Bruker D8 (Bruker-AXS, Karlsruhe, Germany) diffractometer using a filtered CuKa radiation (λ = 1.5418 Å) with an accelerating voltage of 40 kV and current of 30 mA. N2 adsorption/desorption was measured using a Micromeritics Tristar 3000 (Micromeritics, GA USA) to obtain pore volume and the Brunauer-Emmett-Teller (BET) specific surface area. Prior to measurement, the samples were degassed at 120 ◦ C for 5 h under vacuum condition. Fourier transform infrared (FT-IR) spectra were obtained from a Perkin-Elmer Spectroscopy (Perkin-Elmer, UK) 100 with a resolution of 4 cm−1 in transmission mode at room temperature. The Mn content and thermal stability of Mn/MCS and reference materials were investigated using thermal gravimetric analysis-differential scanning calorimetry (TGA-DSC, Mettler-Toledo, Switzerland) in air on a Mettler-Toledo Stare system. The air flow rate was 100 mL/min and the heating rate was 10 ◦ C/min. 3.4. Catalytic Activity Tests The catalytic oxidation of phenol was carried out in a 500 mL reactor containing 20 ppm of phenol solution with a constant stirring of 400 rpm. The reactor was attached to a stand and dipped in a water bath with a temperature controller. Unless specifically stated, the reaction temperature was 25 ◦ C. In a typical test, firstly, 0.1 g catalyst was added into the phenol solution and stirred for 30 min to achieve adsorption-desorption equilibrium, then Oxone® (2KHSO5 ·KHSO4 ·K2 SO4 , PMS, obtained from Aldrich) was added into the solution at 2 g/L. At certain time, 1 mL sample was withdrawn by a syringe and filtered into a HPLC vial, which was added of 0.5 mL methanol to quench the reaction. The concentration of phenol was analyzed by a Varian HPLC with a UV detector and a wavelength set
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at 270 nm. A C-18 column was used to separate the organics while the mobile phase with a flow rate of 1 mL/min was made of 30% CH3 CN and 70% water. 4. Conclusions Fe3 O4 /C/Mn catalysts were synthesized using redox, impregnation and hydrothermal methods, respectively. The catalytic activities of these magnetically separable Mn based catalysts (Mn/MCS) were tested for activation of PMS in producing oxidative radicals for degradation of phenol. It was found that Fe3 O4 /C/Mn hybrids synthesized by redox method showed the best catalytic activity. Moreover, all of the as-prepared Mn catalysts presented a higher activity than commercial MnO2 catalysts. Phenol catalytic degradation on these supported Mn catalysts followed the first order reaction kinetics and the activation energy of phenol degradation on Fe3 O4 /C/Mn hybrids synthesized by the redox method was 32.1 kJ/mol. This study provided a feasible approach for removal of organic pollutants by magnetically separable catalysts. Acknowledgments: The authors greatly appreciate the financial supports from the National Natural Science Youth Foundation of China (No. 21606253) and Science Foundation of China University of Petroleum, Beijing (No. 2462016YJRC013). The authors also acknowledge the use of equipment as well as scientific and technical assistance of the Curtin University Electron Microscope Facility and Centre for Microscopy Characterization, which have been partially funded by the University, State and Commonwealth Governments. Author Contributions: Y.W. and H.S. conceived and designed the experiments; Y.W. and X.D. performed the experiments; Y.W. and C.C. analyzed the data; S.W. and Y.X. contributed reagents/materials/analysis tools; Y.W. wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.
References 1. 2. 3. 4.
5. 6. 7. 8.
9. 10.
11.
Gupta, V.K.; Ali, I.; Saleh, T.A.; Nayak, A.; Agarwal, S. Chemical treatment technologies for waste-water recycling—An overview. RSC Adv. 2012, 2, 6380–6388. [CrossRef] Gupta, V.K.; Srivastava, S.K.; Tyagi, R. Design parameters for the treatment of phenolic wastes by carbon columns (obtained from fertilizer waste material). Water Res. 2000, 34, 1543–1550. [CrossRef] Heng, S.; Yeung, K.L.; Julbe, A.; Ayral, A.; Schrotter, J.-C. Preparation of composite zeolite membrane separator/contactor for ozone water treatment. Microporous Mesoporous Mater. 2008, 115, 137–146. [CrossRef] Tušar, N.N.; Mauˇcec, D.; Rangus, M.; Arˇcon, I.; Mazaj, M.; Cotman, M.; Pintar, A.; Kauˇciˇc, V. Manganese functionalized silicate nanoparticles as a fenton-type catalyst for water purification by advanced oxidation processes (AOP). Adv. Funct. Mater. 2012, 22, 820–826. [CrossRef] Wang, S. A comparative study of fenton and fenton-like reaction kinetics in decolourisation of wastewater. Dyes Pigm. 2008, 76, 714–720. [CrossRef] Watts, R.; Sarasa, J.; Loge, F.; Teel, A. Oxidative and reductive pathways in manganese-catalyzed fenton’s reactions. J. Environ. Eng. 2005, 131, 158–164. [CrossRef] Hu, L.; Yang, X.; Dang, S. An easily recyclable Co/SBA-15 catalyst: Heterogeneous activation of peroxymonosulfate for the degradation of phenol in water. Appl. Catal. B 2011, 102, 19–26. [CrossRef] Anipsitakis, G.P.; Dionysiou, D.D. Degradation of organic contaminants in water with sulfate radicals generated by the conjunction of peroxymonosulfate with cobalt. Environ. Sci. Technol. 2003, 37, 4790–4797. [CrossRef] [PubMed] Rastogi, A.; Al-Abed, S.R.; Dionysiou, D.D. Sulfate radical-based ferrous-peroxymonosulfate oxidative system for PCBs degradation in aqueous and sediment systems. Appl. Catal. B 2009, 85, 171–179. [CrossRef] Anipsitakis, G.P.; Dionysiou, D.D.; Gonzalez, M.A. Cobalt-mediated activation of peroxymonosulfate and sulfate radical attack on phenolic compounds. Implications of chloride ions. Environ. Sci. Technol. 2005, 40, 1000–1007. [CrossRef] Tsitonaki, A.; Petri, B.; Crimi, M.; MosbÆK, H.; Siegrist, R.L.; Bjerg, P.L. In situ chemical oxidation of contaminated soil and groundwater using persulfate: A review. Crit. Rev. Env. Sci. Technol. 2010, 40, 55–91. [CrossRef]
Catalysts 2017, 7, 3
12. 13.
14.
15. 16. 17.
18.
19. 20. 21. 22. 23. 24. 25.
26. 27. 28. 29. 30. 31. 32.
33.
34.
16 of 17
Shukla, P.; Sun, H.; Wang, S.; Ang, H.M.; Tadé, M.O. Nanosized Co3 O4 /SiO2 for heterogeneous oxidation of phenolic contaminants in waste water. Sep. Purif. Technol. 2011, 77, 230–236. [CrossRef] Saputra, E.; Muhammad, S.; Sun, H.; Ang, H.-M.; Tadé, M.O.; Wang, S. A comparative study of spinel structured Mn3 O4 , Co3 O4 and Fe3 O4 nanoparticles in catalytic oxidation of phenolic contaminants in aqueous solutions. J. Colloid Interface Sci. 2013, 407, 467–473. [CrossRef] [PubMed] Saputra, E.; Muhammad, S.; Sun, H.Q.; Ang, H.M.; Tadé, M.O.; Wang, S.B. Different crystallographic one-dimensional MnO2 nanomaterials and their superior performance in catalytic phenol degradation. Environ. Sci. Technol. 2013, 47, 5882–5887. [CrossRef] [PubMed] Sun, H.; Liu, S.; Zhou, G.; Ang, H.M.; Tadé, M.O.; Wang, S. Reduced graphene oxide for catalytic oxidation of aqueous organic pollutants. ACS Appl. Mater. Interfaces 2012, 4, 5466–5471. [CrossRef] [PubMed] Yao, Y.; Yang, Z.; Sun, H.; Wang, S. Hydrothermal synthesis of Co3 O4 -graphene for heterogeneous activation of peroxymonosulfate for decomposition of phenol. Ind. Eng. Chem. Res. 2012, 51, 14958–14965. [CrossRef] Gong, J.-L.; Wang, B.; Zeng, G.-M.; Yang, C.-P.; Niu, C.-G.; Niu, Q.-Y.; Zhou, W.-J.; Liang, Y. Removal of cationic dyes from aqueous solution using magnetic multi-wall carbon nanotube nanocomposite as adsorbent. J. Hazard. Mater. 2009, 164, 1517–1522. [CrossRef] [PubMed] Zhang, P.; Zhan, Y.; Cai, B.; Hao, C.; Wang, J.; Liu, C.; Meng, Z.; Yin, Z.; Chen, Q. Shape-controlled synthesis of Mn3 O4 nanocrystals and their catalysis of the degradation of methylene blue. Nano Res. 2010, 3, 235–243. [CrossRef] Kaminski, M.D.; Nuñez, L. Extractant-coated magnetic particles for cobalt and nickel recovery from acidic solution. J. Magn. Magn. Mater. 1999, 194, 31–36. [CrossRef] Chen, C.; Hu, J.; Shao, D.; Li, J.; Wang, X. Adsorption behavior of multiwall carbon nanotube/iron oxide magnetic composites for Ni(II) and Sr(II). J. Hazard. Mater. 2009, 164, 923–928. [CrossRef] [PubMed] Yao, Y.J.; Miao, S.D.; Liu, S.Z.; Ma, L.P.; Sun, H.Q.; Wang, S.B. Synthesis, characterization, and adsorption properties of magnetic Fe3 O4 @graphene nanocomposite. Chem. Eng. J. 2012, 184, 326–332. [CrossRef] Lu, Z.; Dai, J.; Song, X.; Wang, G.; Yang, W. Facile synthesis of Fe3 O4 /SiO2 composite nanoparticles from primary silica particles. Colloids Interface A 2008, 317, 450–456. [CrossRef] Cheng, Y.; Tan, R.; Wang, W.; Guo, Y.; Cui, P.; Song, W. Controllable synthesis and magnetic properties of Fe3 O4 and Fe3 O4 @SiO2 microspheres. J. Mater. Sci 2010, 45, 5347–5352. [CrossRef] Wang, Y.; Sun, H.; Ang, H.M.; Tadé, M.O.; Wang, S. Magnetic Fe3 O4 /carbon sphere/cobalt composites for catalytic oxidation of phenol solutions with sulfate radicals. Chem. Eng. J. 2014, 245, 1–9. [CrossRef] Wang, Y.; Sun, H.; Ang, H.M.; Tadé, M.O.; Wang, S. Synthesis of magnetic core/shell carbon nanosphere supported manganese catalysts for oxidation of organics in water by peroxymonosulfate. J. Colloid Interface Sci. 2014, 433, 68–75. [CrossRef] [PubMed] Dey, R.; Mukherjee, N.; Ahammed, S.; Ranu, B.C. Highly selective reduction of nitroarenes by iron(0) nanoparticles in water. Chem. Commun. 2012, 48, 7982–7984. [CrossRef] [PubMed] Xiao, W.; Wang, D.; Lou, X.W. Shape-controlled synthesis of MnO2 nanostructures with enhanced electrocatalytic activity for oxygen reduction. J. Phys. Chem. C 2009, 114, 1694–1700. [CrossRef] Xu, J.; Chu, W.; Luo, S. Synthesis and characterization of mesoporous V-MCM-41 molecular sieves with good hydrothermal and thermal stability. J. Mol. Catal. A: Chem. 2006, 256, 48–56. [CrossRef] Titirici, M.M.; Thomas, A.; Antonietti, M. Replication and coating of silica templates by hydrothermal carbonization. Adv. Funct. Mater. 2007, 17, 1010–1018. [CrossRef] Sevilla, M.; Fuertes, A.B. The production of carbon materials by hydrothermal carbonization of cellulose. Carbon 2009, 47, 2281–2289. [CrossRef] Ai, Z.; Deng, K.; Wan, Q.; Zhang, L.; Lee, S. Facile microwave-assisted synthesis and magnetic and gas sensing properties of Fe3 O4 nanoroses. J. Phys. Chem. C 2010, 114, 6237–6242. [CrossRef] Huang, Y.; Dong, Z.; Jia, D.; Guo, Z.; Cho, W.I. Preparation and characterization of core-shell structure Fe3 O4 /C nanoparticles with unique stability and high electrochemical performance for lithium-ion battery anode material. Electrochim. Acta 2011, 56, 9233–9239. [CrossRef] Wang, J.; Zheng, S.; Shao, Y.; Liu, J.; Xu, Z.; Zhu, D. Amino-functionalized Fe3 O4 @SiO2 core-shell magnetic nanomaterial as a novel adsorbent for aqueous heavy metals removal. J. Colloid Interface Sci. 2010, 349, 293–299. [CrossRef] [PubMed] Sun, H.; Liang, H.; Zhou, G.; Wang, S. Supported cobalt catalysts by one-pot aqueous combustion synthesis for catalytic phenol degradation. J. Colloid Interface Sci. 2013, 394, 394–400. [CrossRef] [PubMed]
Catalysts 2017, 7, 3
35.
36.
37. 38.
39. 40. 41. 42.
17 of 17
Huang, Z.; Bao, H.; Yao, Y.; Lu, W.; Chen, W. Novel green activation processes and mechanism of peroxymonosulfate based on supported cobalt phthalocyanine catalyst. Appl. Catal. B 2014, 154–155, 36–43. [CrossRef] Wang, Y.; Sun, H.; Ang, H.M.; Tadé, M.O.; Wang, S. Facile synthesis of hierarchically structured magnetic MnO2 /ZnFe2 O4 hybrid materials and their performance in heterogeneous activation of peroxymonosulfate. ACS Appl. Mater. Interfaces 2014, 6, 19914–19923. [CrossRef] [PubMed] Anipsitakis, G.P.; Dionysiou, D.D. Radical generation by the interaction of transition metals with common oxidants. Environ. Sci. Technol. 2004, 38, 3705–3712. [CrossRef] [PubMed] Saputra, E.; Muhammad, S.; Sun, H.; Ang, H.-M.; Tadé, M.O.; Wang, S. Shape-controlled activation of peroxymonosulfate by single crystal α-Mn2 O3 for catalytic phenol degradation in aqueous solution. Appl. Catal. B 2014, 154–155, 246–251. [CrossRef] Sun, H.; Tian, H.; Hardjono, Y.; Buckley, C.E.; Wang, S. Preparation of cobalt/carbon-xerogel for heterogeneous oxidation of phenol. Catal. Today 2012, 186, 63–68. [CrossRef] Shukla, P.R.; Wang, S.; Sun, H.; Ang, H.M.; Tadé, M. Activated carbon supported cobalt catalysts for advanced oxidation of organic contaminants in aqueous solution. Appl. Catal. B 2010, 100, 529–534. [CrossRef] Saputra, E.; Muhammad, S.; Sun, H.; Ang, H.M.; Tadé, M.O.; Wang, S. Red mud and fly ash supported Co catalysts for phenol oxidation. Catal. Today 2012, 190, 68–72. [CrossRef] Shukla, P.; Sun, H.; Wang, S.; Ang, H.M.; Tadé, M.O. Co-SBA-15 for heterogeneous oxidation of phenol with sulfate radical for wastewater treatment. Catal. Today 2011, 175, 380–385. [CrossRef] © 2016 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC-BY) license (http://creativecommons.org/licenses/by/4.0/).
