Application of response surface methodology for optimization of reactive black 5 removal by three dimensional electro-Fenton process

Journal of Environmental Chemical Engineering 6 (2018) 3418–3435

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Application of response surface methodology for optimization of reactive black 5 removal by three dimensional electro-Fenton process

T

⁎

Faeze Iranpoura, Hamidreza Pourzamanib, Nezamaddin Mengelizadeha, , Parisa Bahramia, Hamed Mohammadia a Environment Research Committee, Isfahan University of Medical Sciences, Isfahan, Iran, and Student Research Committee and Department of Environmental Health Engineering, School of Health, Isfahan University of Medical Sciences, Isfahan, Iran b Environment Research Center, Research Institute for Primordial Prevention of Non-communicable disease, Isfahan University of Medical Sciences, Isfahan, Iran, and Department of Environmental Health Engineering, School of Health, Isfahan University of Medical Sciences, Isfahan, Iran

A R T I C LE I N FO

A B S T R A C T

Keywords: Three dimensional electro-Fenton Ti/TiO2-RuO2-IrO2 anode MWCNTs/Fe3O4 Reactive black 5 Response surface methodology

The electrocatalytic degradation of reactive black 5 (RB5) from aqueous solutions was studied in a three dimensional electro-Fenton process (3DEF) using the Ti/TiO2-RuO2-IrO2 anode electrode in the presence of magnetic multi-walled carbon nanotube (MWCNTs/Fe3O4). The response surface methodology (RSM) was used to determine the effect of various parameters on the RB5 and COD removal efficiency, the production of H2O2, and the energy consumption. The results showed that the 3DEF system with MWCNTs/Fe3O4 worked at a wide range of pH from 4 to 6 without significant reduction in efficiency. Electrochemical production of H2O2 increased with increasing the concentrations of MWCNTs/Fe3O4 and decreasing the initial pH. The results also indicated an increase in the RB5 and COD removal efficiency in 3DEF system than various electrocatalytic processes. The efficiency enhancement is related to more production H2O2 and %OH on MWCNTs/Fe3O4 surface. The MWCNTs/Fe3O4 nanocomposites indicated a high degree of stability and reusability. The reactive oxygen species (ROSs) like %OH, HO2% and O2%− were generated in the reaction and %OH was the main oxidizer for the removal of RB5. To achieve maximum removal of RB5 and COD, optimized condition was found at solution pH of 5.13, MWCNTs/Fe3O4 concentration of 55.27 mg/L, current density of 15.86 mA/cm2, and electrolysis time of 57.91 min. A mechanism for production of ROSs and its catalytic decomposition using MWCNTs/Fe3O4 nanocomposites is proposed. The results of the GC–MS analysis showed that various types of acids such as oxalic and butyric acid can be produced in the 3DEF process.

1. Introduction The quality of water resources is affected by different types of stable pollutants produced by various industries such as textile [1]. Reactive dyes are known as one of the important pollutants in wastewater discharged from these industries [1,2]. Reactive dyes are mainly used in textile industry due to complex aromatic molecular structures, superior fastness and their easily attach to the textile fibers [3–5]. However, the relatively low stabilization of these colors on textile fibers during the dyeing process is resulted in the release of large amount of dye in the wastewater. Discharge of these effluents containing reactive dyes into the environment may cause the problems such as reducing the light penetration, reducing the photosynthesis of aqueous flora and creating anaerobic conditions in the aquatic ecosystem [4,6–8]. Therefore, the removal of reactive dyes from wastewater before discharging to the environment is essential. ⁎

Corresponding author. E-mail address: [email protected] (N. Mengelizadeh).

https://doi.org/10.1016/j.jece.2018.05.023 Received 7 February 2018; Received in revised form 28 April 2018; Accepted 10 May 2018 2213-3437/ © 2018 Elsevier Ltd. All rights reserved.

The conventional methods used for treating the wastewater containing dye include adsorption [9], biodegradation [10], electrocoagulation [11], coagulation [12], TiO2 photocatalytic [13], etc. However, due to the stability and low biodegradability of dyes, these technologies have various disadvantages, such as high operational costs, generation of sludge, limited applicability, and insufficient degradation [2,6,14]. In recent decades, advanced oxidation processes (AOPs) have been developed to solve these problems [15]. These processes are based on the formation of hydroxyl radicals (%OH) as a strong oxidant for organic and inorganic compounds [16]. Among the AOPs technologies, Fenton reaction with H2O2 and the ferrous ion (Eq. (1)) is one of the most important catalytic processes due to simplicity of technology, high degradation efficiency, insignificant toxicity of reactants, and inexpensive materials [16–19]. However, this method shows various drawbacks such as the large amount of iron sludge, the rapid consumption of ferrous ions, low stability of H2O2, requiring large

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Nomenclature 2DE 3DEF ANOVA AOPs COD EEC EF FTIR

GC–MS H2O2 MWCNTs RB5 ROSs RSM SEM UV–vis XRD λmax

Two dimensional electrochemical Three dimensional electro-Fenton Analysis of variance Advanced oxidation processes Chemical oxygen demand Electrical energy consumption Electro-Fenton Fourier transform infrared spectroscopy

anode for electrochemical degradation of reactive dyes. The RB5 dye was chosen as a reactive dye model due to the nature of recalcitrant and its extensive use in various industries [2]. The RSM coupled with central composite design (CCD) was used to evaluate the effect of experimental parameters such as initial pH, particle electrode concentration, electrolysis time and current density on the efficiency of RB5 removal, H2O2 production, and energy consumption. The electrocatalytic performance of the 3DEF process was compared with various electro-oxidation systems. A series of active species trapping experiments were conducted to probe the ROSs and to understand the mechanism of RB5 degradation during the 3DEF process. UV–vis spectrophotometer and gas chromatography–mass spectrometry (GC–MS) analysis were used to identify RB5 intermediates products in optimum conditions.

amounts of chemicals, and manpower for sludge management [20–22]. To solve these disadvantages, in recent years, electro-Fenton (EF) process is considered for treating the large quantities of reactive dyes from wastewater [23]. This technology has several advantages such as continuous electro-generation of H2O2 on cathode surface (Eq. (2)), short electrolysis time, and Fe2+ regeneration by direct reduction of Fe3+ on the cathode (Eq. (3)) [21]. Unfortunately, the above advantages of EF process could only occur in acidic conditions that would damage electrodes and increase the cost of EF due to the reduction of H2O2 production in acidic solutions through a series of parasitic reactions as shown in Eqs. (4) and (5) [24]. Recently, to overcome these problems and improve the oxidation at neutral pH researchers were proposed three dimensional electro-Fenton (3DEF) processes [25].

Fe2 + + H2 O2 → Fe3 + + ·O H+ OH− O2 +

2H+

+

2e−

→ H2 O2

Gas chromatography–mass spectrometry Hydrogen peroxide Multi-walled carbon nanotubes Reactive black 5 Reactive oxygen species Response surface methodology Scanning electron microscope Ultraviolet-visible X-ray diffraction Maximum wavelength

(1)

2. Materials and methods

(2)

Fe3 + + e− → Fe2 +

(3)

H2 O2 + 2H+ + 2e− → 2H2 O

(4)

2H+ + 2 e→ H2

(5)

2.1. Materials Reactive black 5 (RB5, C26H21N5Na4O19S6, ≥50% content, λmax = 600 nm, see the structure in Fig. 1) was supplied by Sigma Aldrich (USA). Multi-walled carbon nanotubes (MWCNTs ≥95%purity; diameter, 20–30 nm; length, 10–30 μm; SSA:≥110 m2/g) were purchased from US research nanomaterials (Houston, USA). Ferric chloride hexahydrate (FeCl3·6H2O, 98.0%), ferrous chloride tetrahydrate (FeCl2·4H2O, 98.0%) and ammonium hydroxide (NH4OH, 28% of ammonia) were purchased from Sigma Aldrich Company. Sodium hydroxide (NaOH, 99%) and hydrochloric acid (HCl, 37.0%) were obtained from Merck Company. RB5 stock solutions were prepared with distilled water. To investigate the proposed electro-oxidation treatment, we used a real wastewater sample from a local textile factory. The properties of real wastewater have shown in Table 1.

3DEF process is a system that uses electrode particles between anode and cathode to improve treatment of pollutants. In this process, the electrodes are easily polarized to the form of the charged microelectrodes at an appropriate voltage, which not only shorten the distance between the reactant and electrode, but also increase the conductivity, mass transfer, create another anode and cathode, and increase the adsorption of pollutant. Several studies were reported that the 3DEF process has a higher efficiency in compared to EF at different pH range. This high removal rate is due to various mechanisms such as adsorption/electro-sorption, and direct and indirect oxidation [24–27]. In 3DEF system, given the importance of corrosion resistance and high activity of particle electrodes under the highly oxidizing conditions, carbon blacks, and nickel foam were frequently considered as electrocatalyst [21,24,25]. Compared to with its electrodes, MWCNTs/ Fe3O4 have the properties including the large specific surface area, good electrical conductivity, high thermal & chemical stability, the suitable biocompatibility, electrocatalyse effect, easily separation by magnet, and also the provision of iron. But less attention has been paid to MWCNTs/Fe3O4 as the particle electrode for production of ROSs such as H2O2 and %OH [27–31]. In the electrochemical oxidation processes, also the anode characteristics are another one of the most effective factors. Recently, various materials have been studied as anode such as PbO2 [32], SnO2 [33], RuO2, and IrO2 [34]. However, these anodes have shown a low efficiency in oxidation processes due to limited service life, instability for anodic oxygen evolution reaction and heavy corrosion in acidic media. Compared with these anodes, Ti/TiO2-RuO2-IrO2 has the characteristics, e.g., relatively long lifetimes, high electrocatalytic activity, and inexpensive [26,35]. Based on the above studies and the excellent features reported for the 3DEF process, this study examined the use of the 3DEF system with MWCNTs/Fe3O4 as a particle electrode and Ti/TiO2-RuO2-IrO2 as

2.2. Preparation of MWCNTs/Fe3O4 nanocomposites Further purification of MWCNTs was conducted by stirring the nanotubes in concentrated nitric acid at 50 °C for 12 h. Subsequently, the obtained MWCNTs were dried at 60 °C over night. The MWCNTs/Fe3O4 nanocomposite was prepared by the chemical co-precipitation in alkaline solution. 1000 mg MWCNTs was added to 100 mL solution containing 0.425 g ferrous chloride and 0.6275 g ferric chloride and mixed by mechanical stirrer. While the mixture was stirred, 2.5 mL of ammonium hydroxide solution was added drop wise to it under nitrogen

Fig. 1. Chemical structure of reactive black 5 dye. 3419

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Table 1 Properties of the real wastewater used. Property

Range

pH COD (mg/L) TSS (mg/L) Fe (mg/L)

7.6 ± 0.5 512 245 0.12

Table 2 Experimental range and levels of the independent variables. Independent variable

Coded variable

Coded levels −1

+1

Actual levels Initial pH MWCNTs/Fe3O4 concentration (mg/L) Electrolysis time (min) Current density (mA/cm2)

atmosphere. The pH of the final mixture was controlled in the range of 10–11. To improvement of the Fe3O4 crystals on MWCNTs, the reaction was carried out at 50 °C for 30 min under constant stirring and nitrogen atmosphere. Then the mixture was filtered and washed with 100 mL deionized water, and dried at 50 °C [36]. The morphology and characterization of the formed nanocomposites are identified using scanning electron microscope (SEM, Philips XI30, Netherland) and X-ray diffraction ((X-Ray Diffractometer, Bruker, D8ADVANCE, Germany(X-Ray Tube Anode: Cu, Wavelength: 1.5406 Å (Cu Kα),Filter: Ni)). Fourier transform infrared spectroscopy (FTIR, Thermo, USA) was used for qualitative detection of chemical functional groups of the particle electrode.

A B C D

3 20 30 10

6 60 70 20

experiments [37]. The CCD based RSM is an efficient statistical tool for the experimental design, modeling and optimization of oxidation methods. In present study, the experimental design and statistical analysis were conducted according to RSM using Design-Expert software (trial version 10, Stat-Ease). Central composite response surface design was used to optimize 3DEF process variables such as initial pH, MWCNTs/ Fe3O4 concentration, current density, and electrolysis time on the RB5 removal, COD removal, H2O2 production and energy consumption as responses. The upper and lower limits of process variables for the responses are presented in Table 2. The empirical quadratic polynomial equation was used for relationship between the four independent variables and responses, which is shown as:

2.3. Experimental setup The batch experiments were performed at 25 °C temperature using a 500 mL glass cylindrical reactor shown in Fig. 2. Ti/TiO2-RuO2-IrO2 and graphite felt electrodes (50 cm2 area) were selected as the anode and the cathode, respectively. The distance between anode and cathode electrodes was fixed at 1 cm 0.05 M sodium sulfate (Na2SO4) was used as supporter electrolyte. The pH of the solution was adjusted using 0.01 M sodium hydroxide or hydrochloric acid. In order to increase H2O2 production and mass transfer, the reactor was aerated using an air compressor (Hailea, Model: ACO-5501, China) with a flow rate of 1 L/ min. Oxidation was conducted with a DC power supply and the sample was continuously mixing using the magnetic stirrer. To decrease the effect of the adsorption process, prior to oxidation process, the adsorbent saturation process was performed by high concentration of the dye. During the treatment process, samples were withdrawn and filtered through 0.45 μm membrane filter to perform the analysis of COD and RB5 by UV–vis spectrophotometer. Finally, the process of degradation was also performed on samples of real wastewater. All of the experiments were repeated three times and only the average values were reported.

n

Y = β0 +

∑ i=1

n=1

n

βi Xi +

∑ i=1

βii Xi2

n

∑ ∑ i=1

i=1

βij Xi Xj

(6)

where Y is the response variable, β0 is constant, βi is the linear coefficient, βii is the second-order coefficient, βij is interaction coefficient, Xi and Xj are the coded values of the parameters, n is the number of parameters. 2.5. Analytical methods The initial and final RB5 concentration was determined by UV–vis spectrophotometer with maximum wavelength (λmax) of 600 nm. Hydrogen peroxide (H2O2) concentration was determined using the KMnO4 titration. Chemical oxygen demand (COD) was determined using a UV-vis spectrophotometer (HACH, DR 5000, USA) according to standard method. Identification of intermediates was performed using GC–MS analysis. Before analysis, samples were extracted by dispersive liquid–liquid microextraction. 5 mL of sample solution was placed in a 10 mL conical centrifuge Tubes. 1 mL acetone (dispersing solvent) containing 80 μL of chloroform (extracting solvent) was injected rapidly into sample solution using a 2 mL syringe. Then, the mixture was centrifuged for 5 min at 5000 rpm. The dispersed fine particles of extraction phase were sedimented in the bottom of the conical centrifuge tubes. Finally, the sedimented phase was withdrawn with a 50 μL

2.4. Experimental design The RSM is a series of mathematical methods that determine the relationship between one or more response variable with several independent variables. The main purpose of this method is to identify and analyze the effect of variables on outputs with the least number of

Fig. 2. Schematic diagram of the experimental setup for the 3DEF degradation. 3420

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syringe and 2 μL was injected into GC–MS system (Agilent, USA) for analysis. The GC was equipped with a DB-5 column with 25 μm film thicknesses and interfaced directly to the mass spectrometer (5975A inert MSD). The GC column was operated for 4 min at a temperature of 80 °C, which was then increased to 300 °C at the rate of 15 °C/min. The ionization mode was electron impact (70 eV) and data was collected in the full scan mode (m/z 50–1000). The other experimental conditions were: helium (99.999% purity) as carrier gas at a flow-rate of 2 mL/ min, injection temperature 280 °C, and MS source temperature 230 °C. The RB5 degradation efficiency and electrical energy consumption (EEC, kWh/m3) was evaluated using the following equations:

%Degradation =

EEC(kWh/m3) =

C0 − Ct × 100 C0 IVt Vs

3.2. Experimental design for the electro-oxidation of RB5 and COD 3.2.1. Statistical analysis The statistical combinations of the main variables, such as initial pH, particle electrode concentration, current density, and electrolysis time, with the maximum actual and predicted degradation efficiency are listed in Table 3. According to the experimental design results the following regression model were developed to the correlation between the removal of RB5 or COD and the independent factors: % RB5 removal = 97.12 + 0.073A + 3.06B + 2.58C + 2.32D + 0.092AB − 1.01AC + 0.41AD + 1.03BC − 0.30BD − 0.68CD −1.75A − 1.65B2 − 2.63C2 − 2.15D2 (9)

(7)

% COD removal = 92.66 − 0.15A + 0.70B + 0.88C + 2.68D − 0.10AB − 1.01AC + 1.50AD + 1.03BC − 1.25BD − 1.49CD − 1.01A2 − 4.06B2 − 4.87C2 − 2.99D2 (10)

(8)

The R2 value of the response model, the F and P > F values for RB5 dye and COD removal efficiencies are showed in Table 4. The results of this table show that the CCD model was significant for RB5 and COD removal efficiency with R2 value of 0.99 and 0.98, respectively. This implies that less than 1% and 2% of the variations for RB5 and COD removal efficiency could not be explained by the model. For this model, values of P > F is less than 0.05, indicating that model was significant. Moreover, for COD and RB5 removal, B, C, D were significant factors, AC, AD, BC, BD, DC, A2, B2, C2, and D2 were significant model interactions. Interactions results are confirmed by interaction plot (Fig. 6). According to Zhang et al. [38], parallel lines indicate that the interaction between the corresponding variables is negligible. The crossing lines indicate that the interaction between the corresponding variables is significant. The value of the adjusted determination coefficient also showed that the model was highly significant for both cases. The fifth column of this table indicates the percent contribution of the each factor in the model. According to the column, the main terms presented the greater contribution than their interactions for dye removal. However for COD removal, the contribution percentage of interactions between variables and second-order variables are greater than the main factors. These results were confirmed with the results of the Pareto chart (Fig. 7). The “Lack of Fit F-value” of 0.3 and 3.00 implies the Lack of Fit is not significant relative to the pure error. The comparison of actual and predicted results of the process efficiency shows that the predicted data are in good agreement with the experimental data (Fig. 8(a)). Points and points clusters in Fig. 8(b) show that experimental values are distributed relatively near to the straight line and show satisfactory correlation between these values.

where C0 and Ct are input and output concentration of RB5 (mg/L), respectively; I is the average applied current (A); V is the cell voltage (V); t is the treatment time (h); and Vs is the treated volume (m3).

3. Results and discussion 3.1. Characterization of the MWCNTs/Fe3O4 nanocomposite The X-ray diffraction (XRD) pattern of the MWCNTs/Fe3O4 nanocomposite is shown in Fig. 3. According to this figure, peaks are observed at 2Ө = 26.84°, 30.55°,35.86°, 43.71°, 54.03°, 57.88°, and 63.25° that can be related to MWCNTs, magnetite (Fe3O4), maghemite (Fe2O3), and hematite (Fe2O3). Of these, only Fe3O4 and Fe2O3 are magnetic. These XRD results presented that iron successfully deposited on the MWCNTs. Fig. 4 shows the scanning electron microscopy - energy dispersive X-ray spectrometer (SEM-EDS) analysis of the prepared MWCNTs/Fe3O4 nanocomposite. The result of SEM-EDS indicated that the iron is deposited on MWCNTs. Since the catalytic performance and easy separation of MWCNTs/ Fe3O4 in 3DEF process could be related to Fe-O-Fe groups on MWCNTs surface, therefore, MWCNTs and MWCNTs/Fe3O4 composites were analyzed by FTIR spectroscopy throughout the range of 500–4000 cm−1 as shown in Fig. 5(a). Compared to the FTIR spectrum of MWCNTs, MWCNTs-Fe3O4 are contains an independent peak at 585 cm−1, which is related to the Fe-O stretching band. Fig. 5(b) confirms the magnetic properties of MWCNTs containing Fe3O4 nanoparticles. The result of FTIR spectra was consistent with the result of EDS and XRD.

Fig. 3. XRD pattern of the prepared MWCNTs/Fe3O4 nanocomposite. 3421

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Fig. 4. SEM-EDS image of the prepared MWCNTs/Fe3O4 nanocomposite.

due to an increase in the collision rate between %OH and intermediate compared to %OH and RB5 molecules. In addition, with prolongation of electrolysis time, the pores of graphite felt cathode is occupied by various intermediate and the production of H2O2 is reduced. Similar results were obtained by Nidheesh et al. [39] for the degradation of Rhodamine B from aqueous solutions. The effect of interaction between MWCNTs/Fe3O4 concentration and current density on removal efficiency at a constant electrolysis time of 50 min, pH of 5 and an RB5 concentration of 50 mg/L also is shown in the 3D plot of Fig. 9(b). The RB5 and COD removal efficiency increased with increasing the particle electrode concentration at the various current densities. This increase in the removal rate can be related to adsorb dissolved oxygen and reduce its into H2O2 by MWCNTs/ Fe3O4 nanocomposite [27]. In addition, based on the high values of the particle electrode, more iron ions can react with H2O2 for producing % OH. Fig. 9(b) also shows that by further increase of the particle electrode from 50 mg/L to 60 mg/L, the COD removal efficiency decreases. This may be due to the production of high levels of iron species that can react with H2O2 and %OH and reduce the concentration of these reactive species. In addition, the active sites on the surface of graphite probably occupied by ferric ions which it leads to a reduction in the active sites for the production of H2O2. This is proven by the results reported by Tian et al. [40]. The authors reported that the pollutant removal efficiency decreases by increasing the concentration of the catalyst due to

3.2.2. Effect of variables on RB5 and COD removal efficiency Fig. 9(a) and (b) indicates the effect of the various parameters on RB5 and COD removal efficiency in 3D plots according to Eqs. (9) and (10). Fig. 9(a) shows the effect of pH and electrolysis time on RB5 and COD removal at an initial concentration dye of 50 mg/L, current density of 15 mA/cm2, and particle electrode concentration of 40 mg/L. As shown in these figure, at the first, by increasing initial pH, the RB5 and COD removal percentage is increased and then decreased; so that, the maximum removal of RB5 and COD was obtained in pH 5. However, Effect of pH on RB5 and COD removal efficiency by 3DEF process was inconspicuous. This behavior was related to the fact that the effects of pH on the Fenton reactants were weakened in 3DEF system. The precipitation of iron never occurred in 3DEF system, and the effect of decomposition of H2O2 with increasing pH was attenuated because the H2O2 was frequently electro-generated on the surface of MWCNTs/ Fe3O4 [25]. These results are in agreement with those reported by Hou et al. [25], who report that the highest of TOC removal could be occurred at a wide pH range. The results of the effect of electrolysis time on the removal of pollutants show that the removal efficiency of RB5 and COD increases by enhancing the electrolysis time from 15 to 50 min and then remains constant at highly electrolysis time. This increase could be due to more production of H2O2 and %OH on nanocomposite surface and the probability of ROSs interaction with RB5 molecule [24]. Stabilization of the removal efficiency in higher reaction times can be

Fig. 5. (a) FTIR spectra of MWCNTs and MWCNTs/Fe3O4, (b) magnetic properties of MWCNTs/Fe3O4. 3422

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Table 3 The actual design of experiments and responses for RB5 and COD removal and energy consumption by 3DEF optimization. Run

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

A pH

4 5 6 4 4 5 6.71 6 6 5 5 6 5 4 4 5 5 3.28 5 6 5 5 4 5 6 4 6 6 4 5

B MWCNTs/Fe3O4 concentration (mg/L)

60 40 20 20 20 40 40 60 60 40 40 20 40 60 20 5.62 74.37 40 40 20 40 40 60 40 20 20 60 60 60 40

C Time (min)

30 50 30 30 30 50 50 30 70 50 84.37 70 50 70 70 50 50 50 50 30 50 15.62 30 50 70 70 30 70 70 50

D Current density (mA/cm2)

10 6.40 20 10 20 15 15 10 20 23.59 15 10 15 10 10 15 15 15 15 10 15 15 20 15 20 20 20 10 20 15

the scavenging activity of the additional species of iron for %OH. Current density is another of the most important factors on electrochemical oxidation and the polarization behavior of particle electrodes [26]. The results plots indicate the RB5 and COD removal efficiency increased with the increase of current density. This enhancement in removal efficiency with current density can be related to more production of H2O2 through reaction (2) on the surface of both electrode of graphite felt and MWCNTs/Fe3O4 [41,42]. Increasing the current density also causes the electrogeneration of Fe2+ from Fe3+. Moreover,

RB5 removal (%)

COD removal (%)

EEC (KWh/m3)

Actual

Predicted

Actual

Predicted

Actual

Predicted

84.94 86.94 88.94 80.09 86.28 96.92 92.09 86.22 96.92 94.54 93.90 83.68 96.55 95.65 86.98 86.97 97.49 91.76 97.55 82.1 97.94 84.78 89.94 96.14 88.66 90.25 92.57 92.89 96.81 97.63

84.92 86.77 89.03 80.44 86.23 97.12 92.06 86.44 96.51 94.75 93.78 84.05 97.12 95.51 86.92 86.99 97.51 91.81 97.12 81.6 97.12 84.92 89.52 97.12 88.74 89.97 92.68 93 97.37 97.12

75.87 79.34 86.97 74.83 82.29 92.91 89.06 74.54 81.09 89.02 80.90 73.67 92.08 84.32 78.83 78.57 83.43 91.01 93.56 73.82 91.87 76.36 78.53 92.67 82.63 80.29 84.26 79.87 81.84 92.12

76.35 79.22 87.05 74.3 82.14 92.66 89.42 74.86 82.52 88.44 79.79 73.9 92.66 85.14 78.99 79.44 81.85 89.94 92.66 73.22 92.66 76.77 79.21 92.66 81.77 80.88 83.71 79.63 82.05 92.66

5.95 7.71 6.67 1.42 6.55 6.86 7.1 1.96 5.76 18.57 15.56 26.42 5.93 2.02 20.32 12.2 1.6 7.05 2.18 4.91 4.81 20.48 8.1 26.59 24.67 14.71 1.9 9.71 8.81 5.95

6.47 7.95 6.9 0.047 7.4 6.42 7.12 2.89 6.64 19.18 16.57 26.4 6.42 1.29 21.09 12.86 1.79 7.87 2.15 6.42 4.65 20 6.42 25.94 23.24 13.82 0.92 9.62 6.42 6.47

RB5 molecules are more likely to be absorbed into the graphite cathode by increasing the current density, thus the collision between %OH and RB5 molecules will increase. Similar results have been reported for the degradation of pollutants by 3DE process and 3DEF process [24,43]. 3.3. Experimental design for the energy consumption 3.3.1. Statistical analysis Experimental and predicted values obtained for energy consumption at

Table 4 ANOVA results for the quadratic model of RB5 and COD removal and energy consumption in 3DEF process. Source

Model A-pH B: MWCNTs/Fe3O4 C-time D-current density AB AC AD BC BD CD A2 B2 C2 D2 Residual Lack of Fit Pure Error R2 R2adj R2pre

DF

14 1 1 1 1 1 1 1 1 1 1 1 1 1 1 15 10 5

F Value

p-value Prob > F

Percent contribution (%)

RB5

COD

EEC

RB5

COD

EEC

223.38 0.44 785.76 556.55 452.64 0.52 62.12 10.32 64.62 5.4 28.68 205.73 182.03 462.48 310.03

89.96 0.52 11.39 17.86 166.91 0.18 17.1 37.92 17.8 26.26 37.55 18.73 305.09 437.31 164.88

67.3 0.6 128.21 148.68 132.2 18.22 5.8 3.84 388.24 21.53 15.3 1.3 0.93 19.79 57.61

< 0.0001 0.515 < 0.0001 < 0.0001 < 0.0001 0.4825 < 0.0001 0.0058 < 0.0001 0.0347 < 0.0001 < 0.0001 < 0.0001 < 0.0001 < 0.0001

< 0.0001 0.4813 0.0042 0.0007 < 0.0001 0.6763 0.0009 < 0.0001 0.0007 0.0001 < 0.0001 0.0006 < 0.0001 < 0.0001 < 0.0001

< 0.0001 0.4495 < 0.0001 < 0.0001 < 0.0001 0.0007 0.0293 0.0689 < 0.0001 0.0003 0.0014 0.272 0.3513 0.0005 < 0.0001

0.3

3

0.47

0.9488

0.1188

0.8536

0.9952 0.9908 0.9844

0.9882 0.9772 0.9414

0.9843 0.9697 0.9423

3423

RB5

COD

EEC

0.01 25.13 17.8 14.47 0.02 1.99 0.33 2.07 0.17 0.92 6.58 5.82 14.79 9.91 0.48 0.18 0.3

0.04 0.9 1.42 13.25 0.01 1.36 3.01 1.41 2.09 2.98 1.49 24.22 34.72 13.09 1.19 1.02 0.17

0.06 13.19 15.29 13.6 1.87 0.6 0.39 39.93 2.21 1.57 0.13 0.1 2.04 5.93 1.54 0.75 0.79

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Fig. 6. Interaction plots for RB5 and COD removal efficiency ((A) pH; (B) initial concentration of MWCNTs/Fe3O4; (C) electrolysis time; (D) current density).

Points and points’ clusters in Fig. 12a indicate that experimental values are distributed relatively near to the straight line and show satisfactory correlation between these values. The plot of predicted and actual energy consumption (Fig. 12b) showed that the model is reliable to describe the effect of the factors on energy consumption.

the design points are presented in Table 3. Energy consumption model for RB5 degradation evaluated using Eq. (11) with R2 = 0.9843, Radj2 = 0.9697, and C.V = 13.70%. These model coefficients show that the model is sufficient and can be used for the process analysis. The ANOVA analysis results in Table 4 show that the relationship between the variables and the response was very good. Significance of each variable was determined by using of p-values (Table 4). The linear effect of concentration of MWCNTs/Fe3O4 (p < 0.0001), electrolysis time (p < 0.0001) and current density (p < 0.0001) are significant, while pH (p = 0.4495) is insignificant, which means that the energy consumption in 3DEF system is more related to the concentration of MWCNTs/Fe3O4, electrolysis time and current density. The interactive effects factors and their square interactions (P < 0.05) except for interaction of pH–current density (AD), pH–pH (A2) and MWCNTs/Fe3O4- MWCNTs/Fe3O4 (B2) were significant at the 95% confidence level. Interactions results are confirmed by interaction plot (Fig. 10). Based on the percent contribution results, among the variables, MWCNTs/Fe3O4 concentration and electrolysis time interaction (39.93%), electrolysis time (15.29%), current density (13.60%), and variable of MWCNTs/Fe3O4 concentration (13.19%), had the highest effects on energy consumption by the studied 3DEF process. These results were confirmed by the results of the Pareto chart (Fig. 11).

3.3.2. Effect of variables on energy consumption Fig. 13(a) shows the interaction of pH and electrolysis time at current density of 15 mA/cm2, particle electrode concentration of 40 mg/L, and initial dye concentration of 50 mg/L. According to this figure, with increasing electrolysis time, the energy consumption increased during oxidation of dye by 3DEF system. However, the effect of pH on energy consumption in the 3DEF process was negligible. The energy consumption decreased only 0.4415 KWh/m3 when the pH increased from 4 to 6. This behavior is due to the fact that the particle electrode MWCNT/Fe3O4, by increasing the mass transfer, creates a very good situation for oxidation in solution at the studied pH values. Yang et al. [44] studied the electrochemical improvement of wastewater through a carbon nanotube electrode reactor. They observed that the carbon nanotubes reduce the energy consumption compared to conventional electrochemical reactors. Fig. 13(b) shows the interaction of concentration of MWCNTs/ Fe3O4 and current density at initial pH of 5, electrolysis time of 50 min, and initial dye concentration of 50 mg/L. As can be seen energy consumption increased with decreasing the MWCNTs/Fe3O4 concentration

EC = 6.42 − 0.22A − 3.22B + 3.47C + 3.27D − 1.42AB + 0.80AC −0.65AD − 6.56BC + 1.54BD − 1.30CD + 0.36A2 + 0.31B2 + 1.42C2 +2.42D2 (11)

Fig. 7. Pareto chart standardized showing the effect of all factors on RB5 and COD removal. 3424

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Fig. 8. (a) Predicted vs. actual values plot, (b) normal probability plot of residual for RB5 and COD removal.

Significance of interaction between variables is confirmed by interaction plot (Fig. 14). The standardized effects of the variables and their interactions on H2O2 production were investigated by Pareto plot analysis (Fig. 15). The results of Pareto analysis showed that concentration of MWCNTs/ Fe3O4 is the most important factor on electrochemical production of H2O2 in the 3DEF system. This result is confirmed by the regression coefficients in the quadratic model Eq. (12). The percent contribution of each of the independent variables on H2O2 production was calculated accordingly, as shown in Eq. (13).

and increasing the current density. This reduction in energy consumption of 3DEF system by the addition of MWCNTs/Fe3O4 is mostly due to increasing the mass transfer in the presence of large specific surface area and high electro-activity of MWCNTs/Fe3O4 particles. Sowmiya et al. [42], Körbahti and Taşyürek [45], and Wang et al. [46] investigated the electrochemical treatment of different pollutants, and they reported that increasing the current density and decreasing the particle electrode increases the energy consumption.

3.4. Experimental design for the H2O2 production

H2O2 production = 47.53 − 7.45A + 8.84B + 2.56C + 2.25D + 2.40AB + 1.14AC + 0.65AD + 1.30BC − 0.63BD + 0.50CD − 1.24A2− 2.42B2 − 2.67C2 − 8.67D2 (12)

3.4.1. Statistical analysis The experimental data for electrochemical production of H2O2 in the 3DEF system were statistically investigated by ANOVA and the results are shown in Table 5. H2O2 production model evaluated using Eq. (12) with R2 = 0.9471, Radj2 = 0.8976, and C.V = 11.69%. These model coefficients reveal that the model is sufficient and can be used for analysis of production process of H2O2. Significance of each variable was determined by applying F value and p-values (Table 5). According to this table, the p-values of the all the variables, except for AC, AD, BC, BD, CD, and A2 are p < 0.05, imply that these are significant.

Percent contribution =

sum of squares

∑

sum of squares

× 100 (13)

As evident from Table 5, the concentration of MWCNTs/Fe3O4 showed the highest level of significance as compared to other factors, which was confirmed by Pareto chart analysis (Fig. 15). Fig. 16(a) illustrates the normal probability plot of residual values 3425

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Fig. 9. Effect of the interaction between factors on RB5 and COD removal efficiency.

empirical second-order polynomial model was appropriate in fitting the H2O2 production for the experimental data.

for the H2O2 production. Points and points clusters in Fig. 16(a) show that experimental values are distributed relatively near to the straight line and show satisfactory correlation between these values. Furthermore, Fig. 16(b) shows an excellent relationship between the experimental and predicted values of the response, and demonstrates that the

3.4.2. Effect of different variables on H2O2 production Fig. 17(a) shows the interaction of pH and electrolysis time at

Fig. 10. Interaction plots for the energy consumption ((A) pH; (B) initial concentration of MWCNTs/Fe3O4; (C) electrolysis time; (D) current density).

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Fig. 11. Pareto chart standardized showing the effect of all factors on energy consumption.

current density of 15 mA/cm2, particle electrode concentration of 40 mg/L, and initial dye concentration of 50 mg/L. According to this figure, the H2O2 generation decreased with increasing the initial pH. This reduction in the production of H2O2 is due to inefficient protons and instability of H2O2 in high pH. These results are in good agreement with other studies [47–49]. The 3D plot also shows that with increasing electrolysis time, H2O2 production first increased and then remained constant. This stabilization of H2O2 production is due to the balance between production and consumption of H2O2. Similar results were obtained by Mounia and Djilali [50] for electrochemical production of H2O2 by a graphite cathode containing gold nanoparticles. GutiérrezHernández et al. [51] reported that the production of H2O2 remains constant with increasing electrolysis time. They explained that H2O2 produced by the reduction of O2 on the surface of the cathode is simultaneously consumed by parasitic reactions in the same cathode (Eqs. (4) and (14)) and oxidation in the anode (Eq.(15)).

2H2 O2 → O2 + 2H2 O

(14)

H2 O2 → O2 + 2H+ + 2e−

(15)

numerous microelectrodes in the suitable current density, which could reduce O2 to H2O2 [42]. Moreover, it can be seen in Fig. 17 (b) that, during 50 min of electrolysis, the H2O2 produced concentration increased with increasing the current density from 10 to 15 mA/cm2, and then decreased with the further increase of current density. This enhancement in production of H2O2 is related to the improvement of the reaction (2) on the surface of both electrode of graphite felt and MWCNTs/Fe3O4 by increasing of current density. In addition, reduction in the production of H2O2 in high current density is due to the 4-electron reduction of oxygen to water and the decomposition of H2O2 according to the related Eq. (4). This is in agreement with results Lei et al. [52] and Luo et al. [53]. 3.5. Process optimization The principal purpose of this research is to find the optimum conditions of factors in order to achieve the maximum degradation of RB5 dye and the lowest energy consumption based on the experimental results. Therefore the numerical optimization was conducted by CCD model. Using the Design Expert software the maximum RB5 and COD removal (97.12 and 92.65%, respectively) and minimum energy consumption (4.37 Kwh/m3) were predicted under oxidation conditions of 57.91 min, pH of 5.13, current density of 15.86 mA/cm2 using amount of MWCNTs/Fe3O4 nanocomposite of 55.27 mg/L. Based on predicted optimum conditions, the actual experiments were performed and the RB5 and COD removal efficiency was respectively observed to be

Fig. 17(b) shows the interaction of concentration of MWCNTs/ Fe3O4 and current density at initial pH of 5, particle electrode concentration of 40 mg/L, and initial dye concentration of 50 mg/L. It is clear from this figure that with increasing concentration of particle electrode increased H2O2 generation. This enhanced H2O2 production with the addition of MWCNTs/Fe3O4 is mostly due to the formation of

Fig. 12. (a) Normal probability plot of residual, (b) predicted vs. actual values plot for energy consumption. 3427

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Fig. 13. Effect of the interaction between factors on energy consumption.

98.20% and 91.96% after 57.9 min of electrolysis time while the energy consumption was 4.86 Kwh/m3, which agreed well with the model predictions.

Table 5 ANOVA results for the quadratic model of H2O2 production during 3DEF process. Source

Sum of Squares

df

Mean Square

F Value

p-value (Prob > F)

Percent Contribution (%)

Model

4903.59

14

350.26

19.17

< 0.0001

A-pH B: MWCNTs/ Fe3O4 C-time D-current density AB AC AD BC BD CD A2 B2 C2 D2 Residual Lack of Fit Pure Error R2 R2adj R2pre

1214.74 1713.08

1 1

1214.74 1713.08

66.47 93.74

< 0.0001 < 0.0001

24.77 34.94

143.09 110.46

1 1

143.09 110.46

7.83 6.04

0.0135 0.0266

2.92 2.25

91.97 20.61 6.84 26.88 6.45 4.04 26.81 102.56 124.22 1311.84 274.13 165.79 108.34 0.9471 0.8976 0.8029

1 1 1 1 1 1 1 1 1 1 15 10 5

91.97 20.61 6.84 26.88 6.45 4.04 26.81 102.56 124.22 1311.84 18.28 16.58 21.67

5.03 1.13 0.37 1.47 0.35 0.22 1.47 5.61 6.80 71.78

0.0404 0.3050 0.5499 0.2439 0.5613 0.6450 0.2446 0.0317 0.0198 < 0.0001

1.88 0.42 0.14 0.55 0.13 0.08 0.55 2.09 2.53 26.75

0.77

0.6649

3.6. Electro-catalytic performance of MWCNTs/Fe3O4 based 3DEF system In order to obtain the electrocatalytic performance of the MWCNTs/ Fe3O4-based 3DEF system, the changes in the RB5 removal efficiency in various electrocatalysis processes were investigated. As shown in Fig. 18(a), the removal efficiency using the two dimensional electrochemical (2DE) process and conventional EF in the 60 min is 55% and 67%, respectively, while RB5 removal efficiency using the MWCNTsbased 3D system reaches 76.15%. These results indicate that the addition of MWCNTs improves the electrocatalytic performance of the 3D process. By adding the particle electrode and selecting the appropriate current density, the particle electrode turns into electrodes with opposite charges and, thus, it develops the reaction surface, facilitates the mass transfer and increases the production of ROSs. Compared to these processes, approximately 90.11% of RB5 were removed by MWCNTs/ Fe3O4-based 3DEF system in 30 min and then, it was reached 98.5% by prolongation of reaction time. These results indicate that the addition of MWCNTs containing Fe3O4 improves the electrocatalytic efficacy of the 3DEF process. Shen et al. [27] reported that the addition of CNTs/Fe3O4 to the solution, in addition to increase the current efficiency, provides an iron source for the Fenton reaction. Furthermore, carbon nanotube, like active carbon catalyst, can produce H2O2 by two electron reduction

Fig. 14. Interaction plots for the H2O2 production ((A) pH; (B) initial concentration of MWCNTs/Fe3O4; (C) electrolysis time; (D) current density). 3428

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Fig. 15. Pareto chart standardized showing the effect of all factors on of H2O2 production.

Fig. 16. (a) Normal probability plot of residual (b), predicted vs. actual values plot for H2O2 production.

Fig. 17. Effect of the interaction between factors on H2O2 production.

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Fig. 18. Comparison between electrocatalyst processes on (a) RB5 removal, (b) COD removal, (c) production of H2O2 and (d) energy consumption in optimum conditions obtained by RSM.

amount of H2O2 was detected for 2DE and EF systems. In contrast, H2O2 concentration in the 3D system based on MWCNTs was 60.12 mg/L. The high concentration of H2O2 in this system was due to this fact that MWCNTs can produce H2O2 by absorbing and electrochemical reduction of oxygen. When iron is loaded on the MWCNTs, the amount of H2O2 accumulation decreased dramatically compared with the 3D process. This decrease is related to rapid decomposition of H2O2 into %

of O2. These events prove the benefits of the 3DEF process compared to other electro-oxidation processes. In this study, COD removal efficiency was also evaluated by various electrocatalytic systems. The results of Fig. 18(b) show that the efficiency of removing the 3DEF system was higher than other electrocatalyst systems. Along with RB5 degradation, H2O2 accumulation was determined in different electrocatalytic processes. As shown in Fig. 18(c), a small Table 6 Comparison with other AOPs for the removal of RB5. AOPs type

Condition

Photocatalytic Electrochemical Fenton Photocatalytic Solar photocatalytic 3DEF 3DE EF 2DE

pH: pH: pH: pH: pH: pH:

4.5, C0: 50 mg/L, TiO2: 1 g/L, Time: 350 min 6.29, DC: 1.6 mA/cm2, electrolyte: 0.15 g/L, flow rate: 11.47 mL/min 3, C0: 100 mg/L, T: 30C, H2O2: 4 mM, catalyst=0.5 g/L. 5, C0: 1 mg/L, TiO2: 0.1 g/L, Time: 150 min 6, C0: 10 mg/L, Time: 300 min 5.13, C0: 50 mg/L, DC:15.86 mA/cm2, Time: 57.91 min

Note: T = temperature; DC = current density; C0 = initial RB5 concentration.

3430

RB5 removal (%)

Reference

89% 81.62% 89.18% 97% 78.4% 98.20% 76.15% 67% 55%

[54] [2] [55] [13] [56] This study This study This study This study

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Fig. 19. Stability of MWCNTs/Fe3O4 nanocomposites in repeated batch RB5 and COD removal.

OH in the presence of iron ions. However, the amount of %OH production in the 3DEF process is higher than the other process that it is resulted in increasing the RB5 degradation efficiency. In this study, the energy consumption of 3DEF process was compared to other electrocatalyst processes to evaluate the cost of electrochemical processes. The results of Fig. 18(d) show that energy consumption in the 3DEF process is lower than other electrocatalytic systems. Finally, considering these results, the 3DEF process is found to be the cost effective and has the high efficiency compared to the other electro-oxidation process. In addition to the above results, RB5 removal efficiency obtained in other AOPs reported in previous studies was compared with 3DEF system. From the results of Table 6, it can be seen that the removal efficiency using the MWCNTs/Fe3O4-based 3DEF process is higher than other AOPs and requires lower reaction time than other processes. Thus, the 3DEF process can act as excellent AOPs to remove the RB5 dye.

have been due to the adsorption of intermediates on the surface of electrode particle [57] and the iron leaching from the catalyst surface [23]. The stability of the particle electrode after fourth run of the RB5 degradation was investigated by analyzing the FTIR shown in Fig. 20. The FTIR analysis showed a structure similar to a particle electrode before and after the electrochemical process. Therefore, the MWCNTs/ Fe3O4 particle electrode has a good stability and can be recycled without the loss of electro-catalytic activity in initial cycle for the degradation of organic compounds. 3.8. Mechanism of MWCNTs/Fe3O4 based 3DEF system In order to study the species of generated radicals during the electrochemical process, radical quenching tests were performed. tert-butyl alcohol and chloroform was used as scavengers of %OH and O2%−, respectively [49]. As shown in Fig. 21(a), with increasing the concentration of tert-butyl alcohol scavenger from 2 mmo/L to 4 mmol/L, the removal efficiency of RB5 decreased from 67.11% to 35.76% in electrolysis time of 60 min, while at the same time removal efficiency was increased to 95.5% in the absence of scavenger. These results indicate that %OH was generated in the electrochemical system and was the dominant ROSs for the RB5 degradation. Meanwhile, the results shown in Fig. 21(b) demonstrate that the addition of chloroform into the process had little effect on the RB5 removal. O2%− was generated in the reaction but its role in the RB5 degradation was limited. Based on the results and studies reported by Hou et al. [58], reaction mechanism is proposed (Fig. 22). At the first, molecular oxygen is adsorbed onto the surface of nanocomposites, and then it generates the ROSs like H2O2 (Eq. (2)) [27]. At the same time, graphite felt as cathode can produce H2O2 from oxygen reduction through reaction (2). This value of electrogenerated H2O2 catalytically decomposed to %OH by embedded iron oxide on the nanocomposites surface (Eq. (1)) [58].

3.7. The stability and reusability test The stability and reusability of the MWCNTs/Fe3O4 nanocomposites were studied at pH of 5.13, current density of 15.86 mA/cm2, initial dye concentration of 50 mg/L, and MWCNTs/Fe3O4 concentration of 55.27 mg/L. The experiments for the stability and reusability of catalyst were performed ten times, each time using the same catalyst. The results are shown in Fig. 19. As can be seen, with the initial increase of the number of cycle runs, the removal efficiency of RB5 and COD decreased from 97.5% to 92.85% and from 92.2% to 87.9%, respectively. Both of these changes indicated the stability of MWCNTs/Fe3O4 nanocomposites in initial cycle of 3DEF process. With a further increase in the number of cycle runs, the removal efficiency of RB5 and COD decreased by 29.12% and 29.4%. This decrease in the removal efficiency of RB5 and COD may

Fig. 20. FTIR transmission spectra of MWCNTs/Fe3O4 before and after oxidation of RB5. 3431

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Fig. 21. Influence of tert-butyl alcohol (a) and chloroform (b) scavengers on degradation of RB5.

Meanwhile, Fe3+ in Fe3O4 can be reduced to Fe2+ by H2O2, HOȮ and O2%− (Eqs. (16)–(19)) [57,59,60]. In addition, the homogeneous process may also occur in the solution. This proceeds through Fenton reactions of electrogenerated H2O2 with dissolved iron from nanocomposites. Finally, the RB5 were degraded by the generated %OH (Eq. (20)). In addition, RB5 dye can be degraded by direct oxidation of the Ti/TiO2-RuO2-IrO2 anode and also indirectly oxidized by the %OH produced in the anode. According to these results, the possible mechanism for RB5 degradation by the 3DEF process is direct and indirect oxidation.

Fe3 + + H2 O2 → Fe2 + + HOO· + H+

(16)

Fe3 + + HOO· → Fe2 + + O2 + H+

(17)

HOO· → H+ + O·2−

(18)

O·2−

+

Fe3 +

→

Fe2 +

+ O2

products generated during the electro-catalytic oxidation of RB5. Fig. 23 indicates the UV–vis absorption spectra at different time intervals during the dye removal by 3DEF process. UV–vis spectra of RB5 dye shows three characteristic absorption peaks at 600, 310, and 254 nm. The peak at 600 nm corresponds to chromophonic group eNeNe, and the peaks at 310 and 254 nm are related to naphthalene and benzene ring structure. As shown in Fig. 23, with increasing the electrolysis time, the intensity of 600 nm and 254 nm band decreased quickly, whereas the peak at 310 nm decreases gradually with increase of electrolysis time. The slower decrease of the intensity at the 310 nm peak may be attributed to insufficient ROSs, the resistance of the compounds to ROSs attack and the formation of intermediates [61]. The obtained data are in good agreement with the results presented by Mook et al. [11]. Fig. 24 indicates the probable intermediates corresponding to sample withdrawn from the reaction vessel containing 50 mg/L dye after 60 min. It is clear from Fig. 24, that with the use of 3DEF process, the compounds containing aromatic rings as well as various acids are produced during RB5 electrochemical degradation. The intermediates detected above were similar with byproducts identified in other researches using photocatalytic using TiO2 [62], photocatalytic using SrTiO3/CeO2 [63], sonochemical [64], sonophotocatalysis [65], combination of sonolysis, and ozonation [6].

(19)

·

O H+ RB5 → Intermediate + CO2 + H2 O

(20)

3.9. Possible intermediates generated in during degradation of RB5 UV–vis combined with GC–MS analysis were used to identify the by-

Fig. 22. Proposed mechanism for generation of %OH and degradation of RB5 in 3DEF system. 3432

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Fig. 23. UV–vis spectral change of RB5 with different electrolysis time.

Fig. 24. The intermediates formed during RB5 degradation using 3DEF system (experimental conditions: RB5 concentration = 50 m/L, MWCNT/Fe3O4 concentration = 55.27 mg/L; electrolyte concentration = 0.05 M; current density = 15.86 mA/cm2; electrolysis time = 60 min).

3.10. Electrocatalytic degradation of real textile wastewater To investigate the possibility of the use of the 3DEF process based on MWCNTs/Fe3O4 for treatment of three real textile wastewater samples, the electrocatalytic degradation experiments were carried out at the current density of 15 mA/cm2 and the particle electrode concentration of 50 mg/L. As shown in Fig. 25, the removal efficiency of dye in actual wastewater is 25–30% lower than synthetic wastewater at 60 min. This may be due to the presence of organic compounds in the matrix, which reduces the dye removal rate due to competitive reactions. With increasing electrolysis time, the dye removal efficiency is increased due to the adsorption of pollutants by the particle electrode and the participation of more iron ions in the development of electrochemical reactions. Fig. 25. The RB5 removal efficiency in real textile wastewater samples.

4. Conclusion the 3DEF process. These finding can be attributed due to formation of many microelectrodes in 3DEF system under the appropriate voltage. The electrocatalytic stability of MWCNTs/Fe3O4 more than 4 times showed the excellent recycling capacity for long-term degradation of RB5. Moreover, MWCNTs/Fe3O4 catalyst was useful for catalytic activity and increasing the production of reactive species such as O2·−, H2O2 and %OH in the electrochemical reactor. The GC–MS analysis confirmed the complete degradation of RB5 into various acid types. Further study on treatment of actual wastewater treatment confirms the possibility to use the 3DEF process catalyzed with MWCNTs/Fe3O4 to

In the present study, the RB5 and COD removal from synthetic solutions has been investigated by 3DEF process with graphite felt and Ti/ TiO2-RuO2-IrO2 electrodes as cathode and anode, respectively. The experimental results show that the rate of dye degradation is influenced by the initial concentration of MWCNTs/Fe3O4, current density, and electrolysis time. Moreover, the results of the present study show that energy consumption increased with decreasing MWCNTs/Fe3O4 concentration and increasing the current density. Comparative experiments between various electrocatalytic degradation processes proved the cost-effectiveness and high efficiency of 3433

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treat the wastewater containing the dye during a high reaction time. Finally, the experimental studies indicated that 3DEF system have a potential application in RB5 dye treatment due to its high degradation efficiency, the reusability and the stability of MWCNTs/Fe3O4 nanocomposite, repeat utilization and no secondary pollution.

[23]

[24]

Conflict of interest

[25]

None. [26]

Acknowledgment [27]

We are grateful for the financial support provided by the student research committee of Isfahan University of Medical Sciences.

[28]

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