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An electro-Fenton system using magnetite coated metallic foams as cathode for dye degradation
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An electro-Fenton system using magnetite coated metallic foams as cathode for dye degradation Thi May Doa,b, Ji Young Byuna,b, Sang Hoon Kima,b, a b
⁎
Materials Architecturing Research Center, Korea Institute of Science and Technology, Seoul, 136-791, Republic of Korea Department of Nanomaterials Science and Engineering, University of Science and Technology, Daejeon, 305-350, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Keywords: Fenton process Metal foam Magnetite Methylene blue Advanced oxidation process
An electro-Fenton system with a magnetite washcoated metal foam as cathode and graphite as anode was successfully applied for the discoloration of methylene blue in aqueous. The effect of pH, applied voltage, supporting electrolyte, electrode inner space, and catalyst dosages were investigated and optimized. Using this cathode, methylene blue was removed with > 99.8% removal rate at 10 ppm after 60 min and with > 95.2% at 50 ppm after 120 min of reaction. Furthermore, those cathodes could be reused at least three times without performance degradation. Due to high degradation capability, simple recovery and high reusability, magnetite washcoated metal foams could be an effective cathode for electro-Fenton systems for removing dyes in wastewater.
1. Introduction Dyes released from many industries such as textiles, leather, cosmetic, printing, and plastics are major problems for environment. Most of the dyes are harmful to aquatic and human life because of their toxic, carcinogenic and mutagenic properties. For these reasons, their removal from the contaminated water is of high priority. Therefore, many methods, including chemical, physical and biological treatments, have been used for the discoloration of reactive dyes from the wastewater [1,2]. Among the various methods, Fenton reaction, consisting of ferrous ion and hydrogen peroxide, has been proven to be an effective method to degrade organic pollutants in wastewater [3]. Hydroxyl radicals generated from the reaction are capable of oxidizing pollutant molecules to less polluting molecules [4]. Hydroxyl radicals have powerful oxidation potential (2.8 V) only lower than fluorine (3 V) and higher than ozone (2.07 V) [5]. Fenton reaction offers numerous advantages, such as high efficiency, non-necessity of special equipment, simple, and mild operating conditions (pressure and temperature) [6]. However, in spite of above-mentioned advantageous properties, the cost of expensive H2O2 and the huge amount of ferrous iron sludge after Fenton process treatment are the main obstacles for large-scale applications of the method for wastewater treatment. Recently, electro-Fenton (EF) process is drawing considerable attention as an efficient method combining electrochemical methods with Fenton’s reagents, which could be a solution for the problems of the conventional Fenton
⁎
reaction mentioned above [7]. This hybrid process involves the continuous generation of in situ H2O2 via reduction of oxygen in the water (Eqs. (1) and (2)). The catalytic reaction is propagated by Fe2+ regeneration (Eqs. (3) and (4)), which can take place by reduction of Fe3+ with H2O2 in the water [8]. Then the active hydroxyl radicals can attach and oxidize pollutants (RH) to less toxic compounds, even turning into non-toxic chemicals such as CO2 and H2O (Eqs. (5)–(7)) [9]. EF process was shown to be successful for the removal of dyes [10–12]. O2 + 2H+ + 2e• → H2O2
(1)
Fe2+ + H2O2 → Fe3+ + OH• + %OH
(2)
%OH + H2O2 → H2O + HO2%
(3)
Fe3+ + HO2% → Fe2+ + O2 + H+
(4)
Fe2+ + HO2% → Fe3+ + HO2•
(5)
Fe2+ + %OH → Fe3+ + OH•
(6)
%OH + RH → R% + H2O
(7)
Until recently, a lot of iron oxide minerals including magnetite (Fe3O4), hematite (α-Fe2O3), and goethite (α-FeOOH) have been widely used in Fenton reaction [13–15]. Among these catalysts, magnetite
Corresponding author at: Materials Architecturing Research Center, Korea Institute of Science and Technology, Seoul, 136-791, Republic of Korea. E-mail address: [email protected] (S.H. Kim).
http://dx.doi.org/10.1016/j.cattod.2017.05.016 Received 6 December 2016; Received in revised form 29 March 2017; Accepted 4 May 2017 0920-5861/ © 2017 Elsevier B.V. All rights reserved.
Please cite this article as: Do, T.M., Catalysis Today (2017), http://dx.doi.org/10.1016/j.cattod.2017.05.016
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2.2. Fabrication of magnetite washcoated metal foam
nanoparticles have attracted most of increasing research interest. This is because those inverse spinel magnetite particles possess many advantages, such as facile preparation, high stability and the enhanced production of %OH by Fe2+ in its structure [16]. Moreover, transfer of electrons between ferrous and ferric ions in the octahedral sites can avoid substantial loss of Fe [17]. In some EF systems, Fe3O4 nanoparticles are added in the wastewater in order to increase the efficiency of decomposition of pollutant molecules in the system [18–21]. In those cases, separating magnetite particles from the treated water is still a cumbersome process, although magnetite particles can be collected by a magnet after treatment. In this paper, we integrated magnetite particles to the cathode of an EF system by washcoating metal foams with magnetite powder. In this novel electrode, magnetite particles were used as iron source and metal foams were employed as conducting and porous catalyst supports, maximizing the reaction efficiency by increasing the exposed surface area of the cathode and minimizing mass transport limitations. Furthermore, the foams could be easily lifted up from the treated water, leaving no sludge and no magnetite powder in the solution after wastewater treatment. Therefore, it can be suggested that the combination of magnetite particles with metal foams may offer synergistic and improved cathodes for EF systems. The EF system was applied to degrade methylene blue (MB) in water. The influence of several operating parameters, such as pH solution, voltage, catalyst loading, initial MB concentration, electrolyte on MB degradation and the reuse ability of the cathode were investigated.
For washcoating magnetite particles on metal foams, we followed basically the same procedure we applied for washcoating γ-alumina particles on FeCrAl foams [22]. Magnetite slurry was made by vigorous stirring of the suspension in distilled water for 30 min. Desired solid content in the slurry was monitored by controlling the ratio of the amount of magnetite to that of distilled water. The cleaned metallic supports were washcoated with magnetite particles by dipping the supports into the slurry for 30 s and pulling out. After pulling out, they were dried by air blowing to remove residual slurry. The washcoated supports were then dried thoroughly in an oven in argon environment for 1 h at 120 °C [23]. 2.3. Electro-Fenton experiment A diagram of the experimental set up is presented in Fig. 2. Batch electrolytic experiments for 10 mg/l of methylene blue (MB) (10 ppm) solution were carried out in a 100 ml cylindrical beaker. 100 ml of the solution was chosen as the working volume for the electrolysis experiments. The pH of solution was controlled by using 0.2 M H2SO4 and 0.1 M NaOH. Graphite plate with effective area of 3 × 4 cm was used as anode. When carbonaceous materials are used as anode in electrochemical setups, there is a possibility of anodic decomposition of the material. Actually, nanostructured carbons such as carbon nanofibers are very unstable and oxidized in a short time when used as anode. Therefore, in such an electrochemical setup, dimensionally stable anodes (DSA) made of precious metals and metal oxide are typically used as anode for their stability. However, for practical applications, DSAs are too expensive. Carbonaceous materials can be stable as anode depending on their structures. For example, Boron-doped-diamond (BDD) is an excellent anode. Still, BDD is also expensive. Graphite is in the middle in terms of price and stability. Therefore, for practical applications, regular change of graphite anode is more practical. As our study is aiming practical applications of our electro Fenton system, we chose graphite as anode. Fe3O4 washcoated FeCrAl foam with 2 × 2 cm in size was used as cathode. The electrodes were placed inside the beaker vertically and adjusted to required inner electrode spacing. The solution was stirred thoroughly by using a magnetic bar at 240 rpm. Na2SO4 was used as the supporting electrolyte in the initial stage of
2. Experimental 2.1. Chemicals and materials The metal foams we used in the study were FeCrAl alloy foam and they were received from Alantum [22,23]. Their surface is corrugated so that washcoating oxide particles on the surface is facilitated (Fig. 1). Magnetite nanoparticles (Fe3O4), Na2SO4, Methylene Blue (MB), NaOH, H2SO4, acetone, and ethanol were purchased from SIGMA ALDRICH. They were used without further purification.
Fig. 1. Optical microscope (left) and SEM (right) images of FeCrAl foam.
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Fig. 2. Schematic diagram of the experimental set up.
factors. Actually, pH is one of main parameters that determine the efficiency of (electro) Fenton systems in general. For Fenton process, acidic conditions are always preferred because of the stability of ferrous ions. However, if the solution is too acidic (pH < 3), H2O2 forms oxonium ions (H3O2+) by reacting with proton (H+) [9]. Therefore, pH 3–4 is usually the optimum pH values for Fenton reactions. At pH 3, the rate of H2O2 production is high and decrease as pH values increase [9]. With increasing pH values, H2O2 decompose to H2O and O2 molecules. Also, fewer ferrous ions are available for activating H2O2 to generate OH radicals with higher pH values as iron ions form hydroxide complexes [25]. With less stable H2O2 and fewer ferrous ions available with higher pH, MB removal efficiency was observed to be lower at higher pH values (Fig. 3). We chose the pH value of 3 as the optimal condition for our EF process and applied the pH condition for further experiments. This result is in good agreement with those in previous researches [26].
experiments. During the electrolysis, samples were collected at various time intervals and remaining concentration of MB was analyzed using UV–vis (Varian Cary 100 Conc). The percentage degradation of MB was calculated using the following equation:
%MB degradation (%) = 100 ×
Co − C t Co
where Co, Ct is the initial and final MB concentrations after EF reaction, respectively Leaching of Fe ions from magnetite was determined during the electrolysis. The aqueous phase was properly diluted and the Fe content was measured using an atomic absorption spectrophotometer (PerkinElmer AAnalyst-400). After initial electro-Fenton reaction, the foam was pulled out from the solution, water-washed for the removal of absorbed MB, and dried at 120 °C for 1 h. The materials were subsequently used in another oxidation cycle under the same conditions described above.
3.2.2. Effect of voltage In an electro-Fenton system, H2O2 is generated in situ by O2 reduction on cathode, which is dependent on the applied voltage. The MB removal kinetics of the EF process at different voltages is shown in Fig. 4. An increase in MB removal efficiency of the system was observed with the increase of voltage from 1 to 4 V, which is mainly due to the increase in hydroxyl radical production rate. Because the degradation efficiency did not increase significantly at applied voltage higher than 2 V, 2 V was selected as the optimal voltage for further study.
3. Results and discussion 3.1. Metal foam and magnetite coating Fig. 1 shows optical and SEM images of FeCrAl foam used in our study. Average pore size was about 800 μm. Their porosity was 90%. Detailed characteristics of the magnetite coating on FeCrAl foam was studied in our previous study [23]. In short, up to 10 wt% of magnetite powder could be coated on FeCrAl foam with good adhesion strength between coated magnetite particles and metal foam surface. This magnetite coated FeCrAl foam was used as cathode.
3.2.3. Parametric study of MB concentration The effect of initial MB concentration in the range of 10–50 mg/l (10–50 ppm) was investigated with 0.12 g/l Fe3O4 loaded on FeCrAl foam. As expected, the percentage degradation of MB was decreased with increasing initial MB concentration (Fig. 5(a)). The increase in dye concentration resulted in the larger amount of dye molecule for a given amount of OH radical in the solution. For example, the percentage of MB removal gradually decreased from 99.88% to 85.49% with the increase of MB concentration from 10 to 50 mg/l (in 60 min). However, it should be noted that although the degradation rate decreased at higher MB concentrations, the absolute amount of removed MB was significantly increased. For instance, when the MB concentration was increased from 10 to 50 mg/l, the amount of removed MB increased from 9.99 to 47.58 mg/l after 60 min electrolysis. This observation suggests that under our experimental conditions, there were enough OH radicals generated from H2O2 by O2 reduction on cathode and active
3.2. Electro-Fenton reaction 3.2.1. Effect of pH solution The pH value of MB solution affects not only the iron solubility and complexation but also the redox recycling of +2 and +3 states of iron [24]. Thus, the effect of six different initial pH values (3.0, 3.5, 4.0, 5.0, 7.0, and 11.0) on our electro-Fenton system was tested. As can be seen in Fig. 3, MB degradation was more effective under acidic conditions, and less effective under basic conditions. The MB degradation efficiency of EF process after 20 min of electrolysis were 90.98%, 81.98%, 45.83%, 24.43%, 23.66% and 17.84% at pH 3.0, 3.5, 4.0, 5.0, 7.0, and 11.0, respectively. There are many factors that affect production rate and stability of H2O2, and pH value of the solution is one of the 3
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Fig. 3. Effect of pH of solution on MB degradation. 0.12 g/l catalyst, 2 V, 10 mg/l MB, 240 rpm, 0.05 M Na2SO4, RT.
system. The kinetics of MB degradation as a function of inner electrode gap was described in Fig. 5(c). With the decrease of space from 6.5 cm to 4.5 cm, the MB removal efficiency increased. Other parameters were 0.12 g/l of magnetite dosage, 0.05 M Na2SO4. Low removal efficiency at 6.5 cm is mainly due to the increase in ohmic drop and decrease in the mass transfer rate of Fe3+. When the spacing decreased from 3.5 to 2.5 cm, a slight decrease in degradation efficiency was observed. This might be due to the oxidation of electro regenerated Fe2+ to Fe3+ at anode as the cathode is too close to the anode. Therefore, we chose 3.5 cm as the optimal electrode spacing for this EF reaction. Supporting electrolyte enhances the conductivity and accelerates the electron transfer in electro Fenton systems [26]. The effect of Na2SO4 concentration on MB degradation was investigated at various concentrations from 5 to 50 mM. Other parameters were 10 mg/l MB and inner space 3.5 cm. It was found that the efficiency of the system significantly increased with increasing Na2SO4 concentration from 5 mM to 50 mM (Fig. 5(d)). Therefore, the best Na2SO4 concentration for MB degradation was found to be 50 mM.
Fig. 4. Effect of applied voltage on MB degradation. 0.12 g/l catalyst, pH 3, 2 V, 10 mg/l MB, 240 rpm, 0.05 M Na2SO4, RT.
3.2.4. Fe leaching study The variation of iron ions during the electrolysis reaction is described in Fig. 6. The sudden increase in iron concentrations at the initial stages of the electrolysis is thought to be due to the leaching of Fe ions from magnetite particles in acidic conditions [27,28], not from electrochemical oxidation of sacrificial anode [29]. Note that magnetite was coated on the cathode in our system. As the measured amount of Fe ions was the total amount of Fe2+ and Fe3+ combined, experimentally it was not possible to tell how much Fe2+ ions were available at each measured amount of iron ions. Still, considering that the total concentration of iron ions increased as time went on, we assumed that more and more Fe2+ ions were available for hydroxyl radical production.
sites on iron oxide surface to decompose H2O2 to OH radicals. The dosage of loaded Fe3O4 particles is an important parameter affecting the production of hydroxyl radical, controlling the efficiency of EF process. It is known that if the concentration of magnetite particles is either too low or too high, it will negatively affect the efficiency of EF system. In order to optimize the magnetite concentration of EF reaction, the removal efficiency of MB was measured at five initial concentrations of 0.08 g/l, 0.12 g/l, 0.2 g/l, 0.3 g/l, and 0.4 g/l of magnetite slurry washcoated on FeCrAl foams for 10 ppm MB solution (Fig. 5(b)). An effective MB removal was observed for 0.12 g/l of magnetite with optimal dye removal efficiency of 97.05% after 30 min of treatment. The magnetite concentrations of 0.12 g/l, 0.2 g/l, and 0.3 g/l show the same behavior for MB removal efficiency. The less efficiency of the system at 0.08 g/l of magnetite is mainly due to the insufficient production of hydroxyl radical in the presence of lesser iron species. In contrast to this, lesser MB removal at higher magnetite concentration (0.4 g/l) is thought to be mainly due to the scavenging reaction of excess iron species such as the production of hydroperoxyl radicals and the reaction of ferrous ions with hydroxyls radical. Therefore, 0.12 g/l of magnetite was chosen as optimum for this EF reaction. The inner electrode spacing is also an important parameter for EF
3.2.5. Comparison of magnetite coated FeCrAl foam with conventional cathodes After optimizing our system, it was compared with a conventional EF system with 0.712 mM FeCl2 solution as catalyst which was equivalent to 40 mg/l of iron ions corresponding to the maximum leached Fe ion concentration in Fig. 6. For comparison, stainless steel and graphite were used as the cathode. As can be seen in Fig. 7(a), MB degradation rate was consistently higher for our system with magnetite coated FeCrAl foams (FeCrAl@Fe3O4 in Fig. 7) than that of the Fe2+catalyzed conventional systems, although total amount of iron ions in 4
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Fig. 5. Effect of various parameters on MB degradation. (a) initial MB concentration, (b) amount of Fe3O4 loaded on FeCrAl, (c) inner electrode space, (d) concentration of supporing electrolyte Na2SO4. pH 3, 2 V, 240 rpm, RT.
can be described by the following first-order reaction kinetics by hydroxyl radicals [31]:
ln
Ct = −k app × t, Co
where Co and Ct are the MB concentrations at initial and at the given time t, respectively, and kapp = k[( OH,MB). The plot of −ln(Ct/Co) against t for the above equation is shown in Fig. 7(b). The coefficients of determination R2 for the first-order rate of our system (FeCrAl@Fe3O4 in Fig. 7) and conventional system with FeCl2 were 0.997 and 0.993, respectively. Thus, the first-order rate expressions satisfactorily fit the data. Furthermore, the apparent pseudo-first-order constant for our system (kapp = 0.1103) was found to be about 3.7 times higher than that for the conventional FeCl2 system (kapp = 0.0293). This result indicates that our EF system with magnetite coated FeCrAl as cathode exhibits higher efficiency on MB degradation than the conventional system with FeCl2. Fig. 6. Fe concentration change during EF reaction. 3.5 cm, 2 V, 10 mg/l MB, 240 rpm, 0.05 M Na2SO4. RT.
3.2.6. Detection of hydroxyl radical In order to investigate the reactive oxidizing species mediated in the process, the influence of different scavengers on the degradation of MB was investigated. Hydroquinone (H2Q) [32] and L-Ascorbic acid (Ac) [33] were added to the solution to inhibit the hydroxyl radical production in the system. As it can be seen in Fig. 8, the degradation of MB is strongly inhibited in the presence of H2Q and Ac. At 30 min of reaction time, MB degradation decreased from 97.19% to 59.82% and 66.96% in the presence of H2Q 3.10−4 M and Ac 10−3 M, respectively. In addition, by increasing the concentrations of H2Q and Ac, MB
our system was less than those of the conventional systems. We attribute this superior performance of our system to the fact that the source of our iron ions is the cathode itself and H2O2 was generated at the cathode by the reduction of oxygen in the solution [30]. As the generated iron ions are concentrated around the cathode, it should have been more efficient for decomposing generated H2O2. Kinetics of the three systems in Fig. 7(a) was further analyzed as follows. Assuming that the electro-Fenton oxidation for MB degradation 5
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Fig. 7. (a) Comparison of Fe2+ catalyzed EF and FeCrAl@Fe3O4-EF, (b) Kinetic models for the comparison. 3.5 cm, 2 V, 10 mg/l MB, 240 rpm, 0.05 M Na2SO4.
Fig. 8. Effect of radical scavenger (a) H2Q and (b) L-Ascorbic (Ac) acid on MB degradation. 3.5 cm, 2 V, 10 mg/l MB, 240 rpm, 0.05 M Na2SO4. RT.
Fig. 9. MB removal mechanism of the EF process using FeCrAl@Fe3O4 as a cathode.
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Fig. 10. Reuse of catalyst. 2 V, 10 mg/l MB, 240 rpm, 3.5 cm, 0.05 M Na2SO4, 60 min of EF reaction.
10 ppm methylene blue was removed with > 99.8% removal rate after 60 min and with > 95.2% at 50 ppm after 120 min of reaction. Furthermore, those cathodes could be reused at least three times without performance degradation. Based on the experimental results, possible reaction pathways were suggested. Due to high degradation capability, simple recovery and high reusability, magnetite washcoated metal foams could be an effective cathode for electro-Fenton systems for removing dyes in wastewater.
removal efficiency was decreased. These results indicate that MB is mainly decomposed by the attack of %OH. 3.2.7. Degradation pathways On the basis of the results so far, we proposed a possible pathway for the MB degradation in our EF process with magnetite coated FeCrAl foam cathode (Fig. 9). Firstly, in acidic conditions, iron ions were leached from the Fe3O4 surface on the cathode into the solution. At the same time, oxygen in the solution adsorbed on the surface of the cathode metallic foam, and the oxygen was combined with proton to produce H2O2 in situ. Because leached iron ions and produced H2O2 are concentrated around the cathode, generation of hydroxyl radicals is facilitated. MB is a cationic dye [34] and is attracted towards cathode and reacts with hydroxyl radicals to be decomposed. There can be some possibility of direct anodic oxidation of the pollutant molecules. However, as MB is a cationic dye and MB molecules will be attracted towards cathode, anodic oxidation cannot be the main decomposition pathway. Decomposition of MB mainly occurs on cathode surface. One indication supporting our assumption is that the degradation efficiency varies significantly depending on the type of cathode (Fig. 7). If anodic oxidation of the dye were the main decomposition pathway, influence of the type of the cathode would have been negligible.
Acknowledgements This work was supported by a grant from the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2015R1A2A2A04004411) and the future R & D Program (2E26960) of KIST. References [1] F.L.Y. Lam, X. Hu, Ind. Eng. Chem. Res. 52 (2013) 6639–6646. [2] A. Rodríguez, G. Ovejero, J.L. Sotelo, M. Mestanza, J. García, Ind. Eng. Chem. Res. 49 (2009) 498–505. [3] M. Hartmann, S. Kullmann, H. Keller, J. Mater. Chem. 20 (2010) 9002–9017. [4] Z. Ai, T. Mei, J. Liu, J. Li, F. Jia, L. Zhang, J. Qiu, J. Phys. Chem. C 111 (2007) 14799–14803. [5] M.S. Lucas, J.A. Peres, Dyes Pigm. 71 (2006) 236–244. [6] J.H. Ramirez, F.J. Maldonado-Hódar, A.F. Pérez-Cadenas, C. Moreno-Castilla, C.A. Costa, L.M. Madeira, Appl. Catal. B: Environ. 75 (2007) 312–323. [7] S. Wang, Dyes Pigm. 76 (2008) 714–720. [8] E. Rosales, M. Pazos, M.A. Sanromán, Chem. Eng. Technol. 35 (2012) 609–617. [9] M. Zhou, Q. Yu, L. Lei, G. Barton, Sep. Purif. Technol. 57 (2007) 380–387. [10] E. Rosales, M. Pazos, M.A. Longo, M.A. Sanromán, Chem. Eng. J. 155 (2009) 62–67. [11] N. Daneshvar, S. Aber, V. Vatanpour, M.H. Rasoulifard, J. Electroanal. Chem. 615 (2008) 165–174. [12] A. Özcan, Y. Şahin, A.S. Koparal, M.A. Oturan, J. Hazard. Mater. 153 (2008) 718–727. [13] J. Zhang, J. Zhuang, L. Gao, Y. Zhang, N. Gu, J. Feng, D. Yang, J. Zhu, X. Yan, Chemosphere 73 (2008) 1524–1528. [14] L. Guo, F. Chen, X. Fan, W. Cai, J. Zhang, Appl. Catal. B: Environ. 96 (2010) 162–168. [15] J.S. Shen, J. Zhu, Y. Kong, T. Li, Z. Chen, Water Sci. Technol. 68 (2013) 1614–1621. [16] J. Chun, H. Lee, S.-H. Lee, S.-W. Hong, J. Lee, C. Lee, J. Lee, Chemosphere 89 (2012) 1230–1237. [17] S. Sun, H. Zeng, J. Am. Chem. Soc. 124 (2002) 8204–8205. [18] M. Luo, S. Yuan, M. Tong, P. Liao, W. Xie, X. Xu, Water Res. 48 (2014) 190–199. [19] P.V. Nidheesh, R. Gandhimathi, S. Velmathi, N.S. Sanjini, RSC Adv. 4 (2014) 5698–5708. [20] Z. He, C. Gao, M. Qian, Y. Shi, J. Chen, S. Song, Ind. Eng. Chem. Res. 53 (2014) 3435–3447. [21] P.V. Nidheesh, R. Gandhimathi, N.S. Sanjini, Sep. Purif. Technol. 132 (2014) 568–576. [22] D.H. Kim, B.Y. Yu, P.R. Cha, W.Y. Yoon, J.Y. Byun, S.H. Kim, Surf. Coat. Technol. 209 (2012) 169–176. [23] T.M. Do, J.Y. Byun, S.H. Kim, Res. Chem. Intermed. (2016), http://dx.doi.org/10. 1007/s11164-016-2440-z online published. [24] E. Brillas, I. Sirés, M.A. Oturan, Chem. Rev. 109 (2009) 6570–6631.
3.2.8. Reuse of catalyst In order to evaluate the reusability of FeCrAl@Fe3O4 cathode, reuse test was carried out under the optimal condition obtained so far. Fig. 10 shows the degradation efficiency of MB for repeated runs. It was found that the removal efficiency was maintained up to three cycles after which there was a slight decrease in the degradation rate (Fig. 10(b)), showing reasonable stability and reusability of our magnetite coated FeCrAl foam cathode. After 5–6 repeated decomposition studies, graphite anode seems to be slightly oxidized with part of anode surface slightly turned to gray. In such cases, decomposition performance recovered when we replace the anode. As it is neither difficult nor expensive to replace graphite anode, we believe that this degradation of graphite anode is not a significant problem for practical application of our system. 4. Conclusions An electro-Fenton system with magnetite washcoated metal foam as cathode and graphite as anode was successfully applied for the discoloration of methylene blue in aqueous solution. There was no need for addition of H2O2 and Fe2+ ions to the system. The effect of pH, applied voltage, supporting electrolyte, electrode inner space, and catalyst dosages were investigated and optimized. Using this cathode, 7
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An electro-Fenton system using magnetite coated metallic foams as cathode for dye degradation Thi May Doa,b, Ji Young Byuna,b, Sang Hoon Kima,b, a b
⁎
Materials Architecturing Research Center, Korea Institute of Science and Technology, Seoul, 136-791, Republic of Korea Department of Nanomaterials Science and Engineering, University of Science and Technology, Daejeon, 305-350, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Keywords: Fenton process Metal foam Magnetite Methylene blue Advanced oxidation process
An electro-Fenton system with a magnetite washcoated metal foam as cathode and graphite as anode was successfully applied for the discoloration of methylene blue in aqueous. The effect of pH, applied voltage, supporting electrolyte, electrode inner space, and catalyst dosages were investigated and optimized. Using this cathode, methylene blue was removed with > 99.8% removal rate at 10 ppm after 60 min and with > 95.2% at 50 ppm after 120 min of reaction. Furthermore, those cathodes could be reused at least three times without performance degradation. Due to high degradation capability, simple recovery and high reusability, magnetite washcoated metal foams could be an effective cathode for electro-Fenton systems for removing dyes in wastewater.
1. Introduction Dyes released from many industries such as textiles, leather, cosmetic, printing, and plastics are major problems for environment. Most of the dyes are harmful to aquatic and human life because of their toxic, carcinogenic and mutagenic properties. For these reasons, their removal from the contaminated water is of high priority. Therefore, many methods, including chemical, physical and biological treatments, have been used for the discoloration of reactive dyes from the wastewater [1,2]. Among the various methods, Fenton reaction, consisting of ferrous ion and hydrogen peroxide, has been proven to be an effective method to degrade organic pollutants in wastewater [3]. Hydroxyl radicals generated from the reaction are capable of oxidizing pollutant molecules to less polluting molecules [4]. Hydroxyl radicals have powerful oxidation potential (2.8 V) only lower than fluorine (3 V) and higher than ozone (2.07 V) [5]. Fenton reaction offers numerous advantages, such as high efficiency, non-necessity of special equipment, simple, and mild operating conditions (pressure and temperature) [6]. However, in spite of above-mentioned advantageous properties, the cost of expensive H2O2 and the huge amount of ferrous iron sludge after Fenton process treatment are the main obstacles for large-scale applications of the method for wastewater treatment. Recently, electro-Fenton (EF) process is drawing considerable attention as an efficient method combining electrochemical methods with Fenton’s reagents, which could be a solution for the problems of the conventional Fenton
⁎
reaction mentioned above [7]. This hybrid process involves the continuous generation of in situ H2O2 via reduction of oxygen in the water (Eqs. (1) and (2)). The catalytic reaction is propagated by Fe2+ regeneration (Eqs. (3) and (4)), which can take place by reduction of Fe3+ with H2O2 in the water [8]. Then the active hydroxyl radicals can attach and oxidize pollutants (RH) to less toxic compounds, even turning into non-toxic chemicals such as CO2 and H2O (Eqs. (5)–(7)) [9]. EF process was shown to be successful for the removal of dyes [10–12]. O2 + 2H+ + 2e• → H2O2
(1)
Fe2+ + H2O2 → Fe3+ + OH• + %OH
(2)
%OH + H2O2 → H2O + HO2%
(3)
Fe3+ + HO2% → Fe2+ + O2 + H+
(4)
Fe2+ + HO2% → Fe3+ + HO2•
(5)
Fe2+ + %OH → Fe3+ + OH•
(6)
%OH + RH → R% + H2O
(7)
Until recently, a lot of iron oxide minerals including magnetite (Fe3O4), hematite (α-Fe2O3), and goethite (α-FeOOH) have been widely used in Fenton reaction [13–15]. Among these catalysts, magnetite
Corresponding author at: Materials Architecturing Research Center, Korea Institute of Science and Technology, Seoul, 136-791, Republic of Korea. E-mail address: [email protected] (S.H. Kim).
http://dx.doi.org/10.1016/j.cattod.2017.05.016 Received 6 December 2016; Received in revised form 29 March 2017; Accepted 4 May 2017 0920-5861/ © 2017 Elsevier B.V. All rights reserved.
Please cite this article as: Do, T.M., Catalysis Today (2017), http://dx.doi.org/10.1016/j.cattod.2017.05.016
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2.2. Fabrication of magnetite washcoated metal foam
nanoparticles have attracted most of increasing research interest. This is because those inverse spinel magnetite particles possess many advantages, such as facile preparation, high stability and the enhanced production of %OH by Fe2+ in its structure [16]. Moreover, transfer of electrons between ferrous and ferric ions in the octahedral sites can avoid substantial loss of Fe [17]. In some EF systems, Fe3O4 nanoparticles are added in the wastewater in order to increase the efficiency of decomposition of pollutant molecules in the system [18–21]. In those cases, separating magnetite particles from the treated water is still a cumbersome process, although magnetite particles can be collected by a magnet after treatment. In this paper, we integrated magnetite particles to the cathode of an EF system by washcoating metal foams with magnetite powder. In this novel electrode, magnetite particles were used as iron source and metal foams were employed as conducting and porous catalyst supports, maximizing the reaction efficiency by increasing the exposed surface area of the cathode and minimizing mass transport limitations. Furthermore, the foams could be easily lifted up from the treated water, leaving no sludge and no magnetite powder in the solution after wastewater treatment. Therefore, it can be suggested that the combination of magnetite particles with metal foams may offer synergistic and improved cathodes for EF systems. The EF system was applied to degrade methylene blue (MB) in water. The influence of several operating parameters, such as pH solution, voltage, catalyst loading, initial MB concentration, electrolyte on MB degradation and the reuse ability of the cathode were investigated.
For washcoating magnetite particles on metal foams, we followed basically the same procedure we applied for washcoating γ-alumina particles on FeCrAl foams [22]. Magnetite slurry was made by vigorous stirring of the suspension in distilled water for 30 min. Desired solid content in the slurry was monitored by controlling the ratio of the amount of magnetite to that of distilled water. The cleaned metallic supports were washcoated with magnetite particles by dipping the supports into the slurry for 30 s and pulling out. After pulling out, they were dried by air blowing to remove residual slurry. The washcoated supports were then dried thoroughly in an oven in argon environment for 1 h at 120 °C [23]. 2.3. Electro-Fenton experiment A diagram of the experimental set up is presented in Fig. 2. Batch electrolytic experiments for 10 mg/l of methylene blue (MB) (10 ppm) solution were carried out in a 100 ml cylindrical beaker. 100 ml of the solution was chosen as the working volume for the electrolysis experiments. The pH of solution was controlled by using 0.2 M H2SO4 and 0.1 M NaOH. Graphite plate with effective area of 3 × 4 cm was used as anode. When carbonaceous materials are used as anode in electrochemical setups, there is a possibility of anodic decomposition of the material. Actually, nanostructured carbons such as carbon nanofibers are very unstable and oxidized in a short time when used as anode. Therefore, in such an electrochemical setup, dimensionally stable anodes (DSA) made of precious metals and metal oxide are typically used as anode for their stability. However, for practical applications, DSAs are too expensive. Carbonaceous materials can be stable as anode depending on their structures. For example, Boron-doped-diamond (BDD) is an excellent anode. Still, BDD is also expensive. Graphite is in the middle in terms of price and stability. Therefore, for practical applications, regular change of graphite anode is more practical. As our study is aiming practical applications of our electro Fenton system, we chose graphite as anode. Fe3O4 washcoated FeCrAl foam with 2 × 2 cm in size was used as cathode. The electrodes were placed inside the beaker vertically and adjusted to required inner electrode spacing. The solution was stirred thoroughly by using a magnetic bar at 240 rpm. Na2SO4 was used as the supporting electrolyte in the initial stage of
2. Experimental 2.1. Chemicals and materials The metal foams we used in the study were FeCrAl alloy foam and they were received from Alantum [22,23]. Their surface is corrugated so that washcoating oxide particles on the surface is facilitated (Fig. 1). Magnetite nanoparticles (Fe3O4), Na2SO4, Methylene Blue (MB), NaOH, H2SO4, acetone, and ethanol were purchased from SIGMA ALDRICH. They were used without further purification.
Fig. 1. Optical microscope (left) and SEM (right) images of FeCrAl foam.
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Fig. 2. Schematic diagram of the experimental set up.
factors. Actually, pH is one of main parameters that determine the efficiency of (electro) Fenton systems in general. For Fenton process, acidic conditions are always preferred because of the stability of ferrous ions. However, if the solution is too acidic (pH < 3), H2O2 forms oxonium ions (H3O2+) by reacting with proton (H+) [9]. Therefore, pH 3–4 is usually the optimum pH values for Fenton reactions. At pH 3, the rate of H2O2 production is high and decrease as pH values increase [9]. With increasing pH values, H2O2 decompose to H2O and O2 molecules. Also, fewer ferrous ions are available for activating H2O2 to generate OH radicals with higher pH values as iron ions form hydroxide complexes [25]. With less stable H2O2 and fewer ferrous ions available with higher pH, MB removal efficiency was observed to be lower at higher pH values (Fig. 3). We chose the pH value of 3 as the optimal condition for our EF process and applied the pH condition for further experiments. This result is in good agreement with those in previous researches [26].
experiments. During the electrolysis, samples were collected at various time intervals and remaining concentration of MB was analyzed using UV–vis (Varian Cary 100 Conc). The percentage degradation of MB was calculated using the following equation:
%MB degradation (%) = 100 ×
Co − C t Co
where Co, Ct is the initial and final MB concentrations after EF reaction, respectively Leaching of Fe ions from magnetite was determined during the electrolysis. The aqueous phase was properly diluted and the Fe content was measured using an atomic absorption spectrophotometer (PerkinElmer AAnalyst-400). After initial electro-Fenton reaction, the foam was pulled out from the solution, water-washed for the removal of absorbed MB, and dried at 120 °C for 1 h. The materials were subsequently used in another oxidation cycle under the same conditions described above.
3.2.2. Effect of voltage In an electro-Fenton system, H2O2 is generated in situ by O2 reduction on cathode, which is dependent on the applied voltage. The MB removal kinetics of the EF process at different voltages is shown in Fig. 4. An increase in MB removal efficiency of the system was observed with the increase of voltage from 1 to 4 V, which is mainly due to the increase in hydroxyl radical production rate. Because the degradation efficiency did not increase significantly at applied voltage higher than 2 V, 2 V was selected as the optimal voltage for further study.
3. Results and discussion 3.1. Metal foam and magnetite coating Fig. 1 shows optical and SEM images of FeCrAl foam used in our study. Average pore size was about 800 μm. Their porosity was 90%. Detailed characteristics of the magnetite coating on FeCrAl foam was studied in our previous study [23]. In short, up to 10 wt% of magnetite powder could be coated on FeCrAl foam with good adhesion strength between coated magnetite particles and metal foam surface. This magnetite coated FeCrAl foam was used as cathode.
3.2.3. Parametric study of MB concentration The effect of initial MB concentration in the range of 10–50 mg/l (10–50 ppm) was investigated with 0.12 g/l Fe3O4 loaded on FeCrAl foam. As expected, the percentage degradation of MB was decreased with increasing initial MB concentration (Fig. 5(a)). The increase in dye concentration resulted in the larger amount of dye molecule for a given amount of OH radical in the solution. For example, the percentage of MB removal gradually decreased from 99.88% to 85.49% with the increase of MB concentration from 10 to 50 mg/l (in 60 min). However, it should be noted that although the degradation rate decreased at higher MB concentrations, the absolute amount of removed MB was significantly increased. For instance, when the MB concentration was increased from 10 to 50 mg/l, the amount of removed MB increased from 9.99 to 47.58 mg/l after 60 min electrolysis. This observation suggests that under our experimental conditions, there were enough OH radicals generated from H2O2 by O2 reduction on cathode and active
3.2. Electro-Fenton reaction 3.2.1. Effect of pH solution The pH value of MB solution affects not only the iron solubility and complexation but also the redox recycling of +2 and +3 states of iron [24]. Thus, the effect of six different initial pH values (3.0, 3.5, 4.0, 5.0, 7.0, and 11.0) on our electro-Fenton system was tested. As can be seen in Fig. 3, MB degradation was more effective under acidic conditions, and less effective under basic conditions. The MB degradation efficiency of EF process after 20 min of electrolysis were 90.98%, 81.98%, 45.83%, 24.43%, 23.66% and 17.84% at pH 3.0, 3.5, 4.0, 5.0, 7.0, and 11.0, respectively. There are many factors that affect production rate and stability of H2O2, and pH value of the solution is one of the 3
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Fig. 3. Effect of pH of solution on MB degradation. 0.12 g/l catalyst, 2 V, 10 mg/l MB, 240 rpm, 0.05 M Na2SO4, RT.
system. The kinetics of MB degradation as a function of inner electrode gap was described in Fig. 5(c). With the decrease of space from 6.5 cm to 4.5 cm, the MB removal efficiency increased. Other parameters were 0.12 g/l of magnetite dosage, 0.05 M Na2SO4. Low removal efficiency at 6.5 cm is mainly due to the increase in ohmic drop and decrease in the mass transfer rate of Fe3+. When the spacing decreased from 3.5 to 2.5 cm, a slight decrease in degradation efficiency was observed. This might be due to the oxidation of electro regenerated Fe2+ to Fe3+ at anode as the cathode is too close to the anode. Therefore, we chose 3.5 cm as the optimal electrode spacing for this EF reaction. Supporting electrolyte enhances the conductivity and accelerates the electron transfer in electro Fenton systems [26]. The effect of Na2SO4 concentration on MB degradation was investigated at various concentrations from 5 to 50 mM. Other parameters were 10 mg/l MB and inner space 3.5 cm. It was found that the efficiency of the system significantly increased with increasing Na2SO4 concentration from 5 mM to 50 mM (Fig. 5(d)). Therefore, the best Na2SO4 concentration for MB degradation was found to be 50 mM.
Fig. 4. Effect of applied voltage on MB degradation. 0.12 g/l catalyst, pH 3, 2 V, 10 mg/l MB, 240 rpm, 0.05 M Na2SO4, RT.
3.2.4. Fe leaching study The variation of iron ions during the electrolysis reaction is described in Fig. 6. The sudden increase in iron concentrations at the initial stages of the electrolysis is thought to be due to the leaching of Fe ions from magnetite particles in acidic conditions [27,28], not from electrochemical oxidation of sacrificial anode [29]. Note that magnetite was coated on the cathode in our system. As the measured amount of Fe ions was the total amount of Fe2+ and Fe3+ combined, experimentally it was not possible to tell how much Fe2+ ions were available at each measured amount of iron ions. Still, considering that the total concentration of iron ions increased as time went on, we assumed that more and more Fe2+ ions were available for hydroxyl radical production.
sites on iron oxide surface to decompose H2O2 to OH radicals. The dosage of loaded Fe3O4 particles is an important parameter affecting the production of hydroxyl radical, controlling the efficiency of EF process. It is known that if the concentration of magnetite particles is either too low or too high, it will negatively affect the efficiency of EF system. In order to optimize the magnetite concentration of EF reaction, the removal efficiency of MB was measured at five initial concentrations of 0.08 g/l, 0.12 g/l, 0.2 g/l, 0.3 g/l, and 0.4 g/l of magnetite slurry washcoated on FeCrAl foams for 10 ppm MB solution (Fig. 5(b)). An effective MB removal was observed for 0.12 g/l of magnetite with optimal dye removal efficiency of 97.05% after 30 min of treatment. The magnetite concentrations of 0.12 g/l, 0.2 g/l, and 0.3 g/l show the same behavior for MB removal efficiency. The less efficiency of the system at 0.08 g/l of magnetite is mainly due to the insufficient production of hydroxyl radical in the presence of lesser iron species. In contrast to this, lesser MB removal at higher magnetite concentration (0.4 g/l) is thought to be mainly due to the scavenging reaction of excess iron species such as the production of hydroperoxyl radicals and the reaction of ferrous ions with hydroxyls radical. Therefore, 0.12 g/l of magnetite was chosen as optimum for this EF reaction. The inner electrode spacing is also an important parameter for EF
3.2.5. Comparison of magnetite coated FeCrAl foam with conventional cathodes After optimizing our system, it was compared with a conventional EF system with 0.712 mM FeCl2 solution as catalyst which was equivalent to 40 mg/l of iron ions corresponding to the maximum leached Fe ion concentration in Fig. 6. For comparison, stainless steel and graphite were used as the cathode. As can be seen in Fig. 7(a), MB degradation rate was consistently higher for our system with magnetite coated FeCrAl foams (FeCrAl@Fe3O4 in Fig. 7) than that of the Fe2+catalyzed conventional systems, although total amount of iron ions in 4
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Fig. 5. Effect of various parameters on MB degradation. (a) initial MB concentration, (b) amount of Fe3O4 loaded on FeCrAl, (c) inner electrode space, (d) concentration of supporing electrolyte Na2SO4. pH 3, 2 V, 240 rpm, RT.
can be described by the following first-order reaction kinetics by hydroxyl radicals [31]:
ln
Ct = −k app × t, Co
where Co and Ct are the MB concentrations at initial and at the given time t, respectively, and kapp = k[( OH,MB). The plot of −ln(Ct/Co) against t for the above equation is shown in Fig. 7(b). The coefficients of determination R2 for the first-order rate of our system (FeCrAl@Fe3O4 in Fig. 7) and conventional system with FeCl2 were 0.997 and 0.993, respectively. Thus, the first-order rate expressions satisfactorily fit the data. Furthermore, the apparent pseudo-first-order constant for our system (kapp = 0.1103) was found to be about 3.7 times higher than that for the conventional FeCl2 system (kapp = 0.0293). This result indicates that our EF system with magnetite coated FeCrAl as cathode exhibits higher efficiency on MB degradation than the conventional system with FeCl2. Fig. 6. Fe concentration change during EF reaction. 3.5 cm, 2 V, 10 mg/l MB, 240 rpm, 0.05 M Na2SO4. RT.
3.2.6. Detection of hydroxyl radical In order to investigate the reactive oxidizing species mediated in the process, the influence of different scavengers on the degradation of MB was investigated. Hydroquinone (H2Q) [32] and L-Ascorbic acid (Ac) [33] were added to the solution to inhibit the hydroxyl radical production in the system. As it can be seen in Fig. 8, the degradation of MB is strongly inhibited in the presence of H2Q and Ac. At 30 min of reaction time, MB degradation decreased from 97.19% to 59.82% and 66.96% in the presence of H2Q 3.10−4 M and Ac 10−3 M, respectively. In addition, by increasing the concentrations of H2Q and Ac, MB
our system was less than those of the conventional systems. We attribute this superior performance of our system to the fact that the source of our iron ions is the cathode itself and H2O2 was generated at the cathode by the reduction of oxygen in the solution [30]. As the generated iron ions are concentrated around the cathode, it should have been more efficient for decomposing generated H2O2. Kinetics of the three systems in Fig. 7(a) was further analyzed as follows. Assuming that the electro-Fenton oxidation for MB degradation 5
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Fig. 7. (a) Comparison of Fe2+ catalyzed EF and FeCrAl@Fe3O4-EF, (b) Kinetic models for the comparison. 3.5 cm, 2 V, 10 mg/l MB, 240 rpm, 0.05 M Na2SO4.
Fig. 8. Effect of radical scavenger (a) H2Q and (b) L-Ascorbic (Ac) acid on MB degradation. 3.5 cm, 2 V, 10 mg/l MB, 240 rpm, 0.05 M Na2SO4. RT.
Fig. 9. MB removal mechanism of the EF process using FeCrAl@Fe3O4 as a cathode.
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Fig. 10. Reuse of catalyst. 2 V, 10 mg/l MB, 240 rpm, 3.5 cm, 0.05 M Na2SO4, 60 min of EF reaction.
10 ppm methylene blue was removed with > 99.8% removal rate after 60 min and with > 95.2% at 50 ppm after 120 min of reaction. Furthermore, those cathodes could be reused at least three times without performance degradation. Based on the experimental results, possible reaction pathways were suggested. Due to high degradation capability, simple recovery and high reusability, magnetite washcoated metal foams could be an effective cathode for electro-Fenton systems for removing dyes in wastewater.
removal efficiency was decreased. These results indicate that MB is mainly decomposed by the attack of %OH. 3.2.7. Degradation pathways On the basis of the results so far, we proposed a possible pathway for the MB degradation in our EF process with magnetite coated FeCrAl foam cathode (Fig. 9). Firstly, in acidic conditions, iron ions were leached from the Fe3O4 surface on the cathode into the solution. At the same time, oxygen in the solution adsorbed on the surface of the cathode metallic foam, and the oxygen was combined with proton to produce H2O2 in situ. Because leached iron ions and produced H2O2 are concentrated around the cathode, generation of hydroxyl radicals is facilitated. MB is a cationic dye [34] and is attracted towards cathode and reacts with hydroxyl radicals to be decomposed. There can be some possibility of direct anodic oxidation of the pollutant molecules. However, as MB is a cationic dye and MB molecules will be attracted towards cathode, anodic oxidation cannot be the main decomposition pathway. Decomposition of MB mainly occurs on cathode surface. One indication supporting our assumption is that the degradation efficiency varies significantly depending on the type of cathode (Fig. 7). If anodic oxidation of the dye were the main decomposition pathway, influence of the type of the cathode would have been negligible.
Acknowledgements This work was supported by a grant from the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2015R1A2A2A04004411) and the future R & D Program (2E26960) of KIST. References [1] F.L.Y. Lam, X. Hu, Ind. Eng. Chem. Res. 52 (2013) 6639–6646. [2] A. Rodríguez, G. Ovejero, J.L. Sotelo, M. Mestanza, J. García, Ind. Eng. Chem. Res. 49 (2009) 498–505. [3] M. Hartmann, S. Kullmann, H. Keller, J. Mater. Chem. 20 (2010) 9002–9017. [4] Z. Ai, T. Mei, J. Liu, J. Li, F. Jia, L. Zhang, J. Qiu, J. Phys. Chem. C 111 (2007) 14799–14803. [5] M.S. Lucas, J.A. Peres, Dyes Pigm. 71 (2006) 236–244. [6] J.H. Ramirez, F.J. Maldonado-Hódar, A.F. Pérez-Cadenas, C. Moreno-Castilla, C.A. Costa, L.M. Madeira, Appl. Catal. B: Environ. 75 (2007) 312–323. [7] S. Wang, Dyes Pigm. 76 (2008) 714–720. [8] E. Rosales, M. Pazos, M.A. Sanromán, Chem. Eng. Technol. 35 (2012) 609–617. [9] M. Zhou, Q. Yu, L. Lei, G. Barton, Sep. Purif. Technol. 57 (2007) 380–387. [10] E. Rosales, M. Pazos, M.A. Longo, M.A. Sanromán, Chem. Eng. J. 155 (2009) 62–67. [11] N. Daneshvar, S. Aber, V. Vatanpour, M.H. Rasoulifard, J. Electroanal. Chem. 615 (2008) 165–174. [12] A. Özcan, Y. Şahin, A.S. Koparal, M.A. Oturan, J. Hazard. Mater. 153 (2008) 718–727. [13] J. Zhang, J. Zhuang, L. Gao, Y. Zhang, N. Gu, J. Feng, D. Yang, J. Zhu, X. Yan, Chemosphere 73 (2008) 1524–1528. [14] L. Guo, F. Chen, X. Fan, W. Cai, J. Zhang, Appl. Catal. B: Environ. 96 (2010) 162–168. [15] J.S. Shen, J. Zhu, Y. Kong, T. Li, Z. Chen, Water Sci. Technol. 68 (2013) 1614–1621. [16] J. Chun, H. Lee, S.-H. Lee, S.-W. Hong, J. Lee, C. Lee, J. Lee, Chemosphere 89 (2012) 1230–1237. [17] S. Sun, H. Zeng, J. Am. Chem. Soc. 124 (2002) 8204–8205. [18] M. Luo, S. Yuan, M. Tong, P. Liao, W. Xie, X. Xu, Water Res. 48 (2014) 190–199. [19] P.V. Nidheesh, R. Gandhimathi, S. Velmathi, N.S. Sanjini, RSC Adv. 4 (2014) 5698–5708. [20] Z. He, C. Gao, M. Qian, Y. Shi, J. Chen, S. Song, Ind. Eng. Chem. Res. 53 (2014) 3435–3447. [21] P.V. Nidheesh, R. Gandhimathi, N.S. Sanjini, Sep. Purif. Technol. 132 (2014) 568–576. [22] D.H. Kim, B.Y. Yu, P.R. Cha, W.Y. Yoon, J.Y. Byun, S.H. Kim, Surf. Coat. Technol. 209 (2012) 169–176. [23] T.M. Do, J.Y. Byun, S.H. Kim, Res. Chem. Intermed. (2016), http://dx.doi.org/10. 1007/s11164-016-2440-z online published. [24] E. Brillas, I. Sirés, M.A. Oturan, Chem. Rev. 109 (2009) 6570–6631.
3.2.8. Reuse of catalyst In order to evaluate the reusability of FeCrAl@Fe3O4 cathode, reuse test was carried out under the optimal condition obtained so far. Fig. 10 shows the degradation efficiency of MB for repeated runs. It was found that the removal efficiency was maintained up to three cycles after which there was a slight decrease in the degradation rate (Fig. 10(b)), showing reasonable stability and reusability of our magnetite coated FeCrAl foam cathode. After 5–6 repeated decomposition studies, graphite anode seems to be slightly oxidized with part of anode surface slightly turned to gray. In such cases, decomposition performance recovered when we replace the anode. As it is neither difficult nor expensive to replace graphite anode, we believe that this degradation of graphite anode is not a significant problem for practical application of our system. 4. Conclusions An electro-Fenton system with magnetite washcoated metal foam as cathode and graphite as anode was successfully applied for the discoloration of methylene blue in aqueous solution. There was no need for addition of H2O2 and Fe2+ ions to the system. The effect of pH, applied voltage, supporting electrolyte, electrode inner space, and catalyst dosages were investigated and optimized. Using this cathode, 7
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