Application of Carbon Nanotubes Coated Electrodes and Immobilized

Article pubs.acs.org/IECR

Application of Carbon Nanotubes Coated Electrodes and Immobilized TiO2 for Dye Degradation in a Continuous Photocatalytic-Electro-Fenton Process Elmira Pajootan, Mokhtar Arami,* and Mehdi Rahimdokht Textile Engineering Department, Amirkabir University of Technology, 424 Hafez Avenue, Tehran, 15875-4413, Iran S Supporting Information *

ABSTRACT: In this research, the decolorization of acid dyes in binary solutions via a continuous photocatalytic-electro-Fenton (PEF−TiO2) process was studied using a modified graphite electrode with carbon nanotubes and immobilized TiO2 with adequate stabilities. The effect of the operating parameters including current density, initial dye concentration, iron ion concentration, and time on the degradation process was studied applying response surface methodology (RSM) using NaCl and Na2SO4 as the supporting electrolytes. The investigation of kinetic parameters indicated that the dye decolorization had followed a pseudo-second order kinetic model. The color and chemical oxygen demand removal (%) and electrical energy consumption were chosen as responses to optimize the process. The experimental results greatly agreed with the predicted data, illustrating that the RSM can be effectively used for the optimization of the removal procedure. It was then concluded that PEF−TiO2 is a promising method with high efficiencies and reasonable energy consumption for dye elimination from multicomponent effluents.

1. INTRODUCTION The colored wastewaters generated by the textile, paper, plastic, leather, food, and mineral processing industries have been a serious environmental problem for years. Dying effluents produced in large amounts from the textile industry contain detergents, toxic matter, and nonbiodegradable matter with strong color. It is therefore necessary to treat textile effluents, and recently various methods have been suggested for effective color removal.1−6 These include adsorption, precipitation, chemical degradation, biodegradation, and chemical coagulation. However, most of these treatment methods have shown limited success and/or they are limited to low concentrations of the pollutant. Furthermore, most of these treatment methods have disadvantages, such as high costs, inefficiency in the degradation of complex compounds, formation of secondary pollution, and sludge separation, which make their large-scale adaptation nonfeasible from the practical point of view.7−11 Since the diversity of textile products increases, different dyestuffs with highly varying chemical characteristics are used to only complicate the further treatment of textile wastewaters.9 Therefore, there is an urgent need to develop more efficient and inexpensive methods that require minimum chemical and energy consumptions as well as minimum installation space. During the past few decades, advanced oxidation processes (AOPs) such as ozone (O3), combination of ozone with hydrogen peroxide or with ultraviolet light (O3−H2O2/UV), Fenton process (H2O2−Fe2+), photo-Fenton process (H2O2− Fe2+−UV), and photoelectro-Fenton process (PE-Fenton) have been successfully applied to industrial wastewaters.12−16 On the other hand, electrooxidation using modified electrodes has proven to be very effective in degradation of dyes and other organic matter existing in industrial effluents.17−19 Also, many studies have lately focused on the modification and/or fabrication of electrodes with nanomaterials to generate in situ hydrogen peroxide at the surface of the cathode in © XXXX American Chemical Society

electrochemical processes. In these cases, hydrogen peroxide is generated by the electrochemical reduction of the dissolved oxygen at the surface of the cathode (0.695 V = NHE) and by the simple addition of iron ions to the solution; the electroFenton process can effectively degrade the contaminants.20−22 Carbon nanotubes (CNTs) are interesting nanomaterials that have found a special place in electrochemistry owing to their unique properties such as their high surface area, geometry, high electrical conductivity, relative chemical inertness, and mechanical strength.23 Several studies focused on the electrochemical processes using CNTs to modify the surface of the electrode for various applications such as sensors, biosensors, electrical enhancement, solar cells, etc. are listed in Table S1 in the Supporting Information. It has been reported that simultaneous UV irradiation with the electro-Fenton process can improve the performance of the procedure via photolysis of Fe(OH)2 to generate hydroxyl radicals (HO•), and further combination of this method with photocatalytic process will also enhance the degradation efficiency.24,25 However, employing TiO2 nanoparticles in photocatalytic degradation of pollutants has the major disadvantage of separating and recycling the nanoparticles from the treated wastewaters to prevent them from entering the water streams. Therefore, different methods such as sol−gel, chemical vapor deposition (CVD), etc. are introduced for the immobilization of TiO2 nanoparticles on the surface of various supporters.26 Since the textile effluents consist of a large number of various dyes, the photocatalytic-electro-Fenton (PEF−TiO2) process can be employed to investigate the indirect oxidation of binary Received: June 18, 2014 Revised: September 27, 2014 Accepted: September 30, 2014

A

dx.doi.org/10.1021/ie5024589 | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

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experiments. Graphite electrodes (purity >95%) having a total contact surface area of 17 cm2 were used as anode and cathode. 2.2. Preparation of the Modified Electrode. As explained in our previous research, in order for the graphite electrodes to be modified, they were first abraded with sandpaper and then washed with distilled water, NaOH (10% w/v), HNO3 (50% v/v), and acetone, each for 5 min, respectively. Then they were rinsed with distilled water. Prior to electrodeposition, the solution containing CNTs/CTAB, 1.5:1 (w:w), was sonicated for 60 min using a Delta D68H ultrasonic cleaner, and then a dc voltage of 17.5 (V) was applied to the solution. The arranged cationic hydrophilic group of the surfactants on the surface of the CNTs would transfer the positively charged nanotubes from bulk to the surface of the negatively charged cathode electrode during the electrodeposition process. The modified electrodes were washed with distilled water and dried in an oven for 15 min at 60 °C. The structural and elechtrochemical properties of the electrodes and the determination of methylene blue (MB) in the aqueous solutions, before and after the modification process, had been reported in our previous study.27 2.3. Preparation of Immobilized TiO2. In order to immobilize the TiO2 nanoparticles, the inner wall of the reactor was washed with acetone and distilled water and then dried in air. TiO2 nanoparticles (0.5 g) were attached to the inner walls using a thin layer of a UV resistant silicone polymer (with the reaction temperature of 25 °C and preparation time of 1 h), and finally, the reactor was dried at room temperature for 24 h.36 2.4. PEF−TiO2 Process. A continuous PEF−TiO2 bubble reactor (a cylindrical Plexiglas reactor) having a working volume of 1 L was used for the removal process (Figure 1). A

dye containing solutions at acidic media. In this study, the simultaneous application of the modified electrochemical process employing CNT coated graphite cathodes along with the immobilized photocatalytic process using TiO2 nanoparticles in a continuous cylindrical reactor to remove two anionic dyes (Acid Red 14 and Acid Blue 92) from the synthesized colored textile wastewaters (binary solutions) was examined through using the extension of the Beer−Lambert law. In this regard, TiO2 nanoparticles were immobilized on a UV resistant polymeric substrate to exclude their timeconsuming and expensive separation from the solutions. On the other hand, in order to overcome the affinity of the CNTs to aggregate, which is one of the major difficulties ahead of their usage in various processes, a cationic surfactant with amphiphilic molecular structure and the ability to be adsorbed at the interfaces was employed.27 The availability, low resistivity, chemical inertness, low cost, and high hydrogen evolution overpotential were the main advantages of graphite electrodes that were chosen as the anode and cathode in the present study.27,28 Also, the efficient electrogeneration of H2O2 can be achieved by graphite cathode material rather than metal.29 Table S2 (Supporting Information) displays some investigations, in which different modified carbonaceous electrodes were employed to remove organic pollutants by electrochemical processes. Response surface methodology (RSM), which has been extensively used in different experimental and industrial processes, was employed as a statistical tool for design and further analysis of the results including the interactive effects among the parameters. Decreasing the number of necessary experiments and reducing the time and the consumption of materials are other priorities of RSM over classical methods.30−35 The initial dye concentration, current density (CD), time, and the iron ion concentrations were chosen as the main parameters to be evaluated in the dye decolorization in binary solutions. The multiresponse optimization of the PEF− TiO2 process considering the high decolorization efficiency as well as the high chemical oxygen demand (COD) removal and minimum electrical energy consumption to achieve the maximum performance and the best possible response was conducted. In addition, the effect of the flow rate on the PEF− TiO2 performance, the kinetics of dye decolorization, and the stability of nanotubes and TiO2 nanoparticles on the surface of the graphite electrode and polymeric support were studied.

Figure 1. Cylindrical PEF−TiO2 reactor for dye degradation in continuous mode.

2. MATERIALS AND METHODS 2.1. Chemicals and Materials. In this study, all the reagents used were of analytical grade and obtained from Merck Co. Titania nanoparticles (Degussa P25) were utilized as a photocatalyst (average particle size 30 nm, purity >97%, and 80:20 anatase to rutile). Multiwalled carbon nanotubes (CNTs; purity >95%, length 10−20 μm, and diameter 30−50 nm) was obtained from Neutrino Co. Cetyltrimethylammonium bromide (CTAB) was used as the cationic surfactant for the dispersion of CNTs. Two acid dyes (C.I. Acid Red 14 (AR14) and C.I. Acid Blue 92 (AB92)) purchased from Ciba Co. were used as the synthetic model dyes (Supporting Information, Table S3). Dyes were dissolved in distilled water containing Na2SO4 (0.05 M) or NaCl (1 g/L) as the supporting electrolyte for the indirect oxidation. The pH of the solutions was adjusted by the addition of H2SO4 (1 M) or NaOH (1 M). Fe2(SO4)3·7H2O was also used as the Fenton reagent in all

UV lamp (Philips, 9 W) in a quartz tube was placed at the center of the reactor to provide near-UV radiation; the anode and modified cathode plates were placed 10 mm from the UV lamp. A constant current density in the range 8.2−17.6 mA/cm2 (current intensity from 0.14 to 0.30 A) was applied by a power supply (ESCORT, 3060 TD Dual Tracking, Taiwan) and the dye solution was pumped through the reactor. The samples were collected over specific time intervals, and the absorbance was measured using a UNICO 2100 spectrophotometer at the maximum wavelength of each dye. The decolorization efficiency was determined according to eq 1: decolorization (%) = [(A 0 − A)/A 0]·100 B

(1)



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where A0 and A are the absorbances of the dye solution before and after treatment, respectively. For the binary solutions, the extension of the Beer−Lambert law was used and the equations are presented in the Supporting Information (eqs S1−S3). The electrical energy consumption (EEC) per unit COD decay (kWh/gCOD) was calculated with eq 2: EEC =

[(Ecell I ) + (E UV )]t ·100 V (COD0 − CODt )

electrochemical treatment of dye containing wastewater can be carried out with a good repeatability especially at the reaction time over 60 min (RSD < 3%). Error bars represent the RSD for all data in Figure 2. Therefore, the minimum time for the RSM experiments was chosen as 60 min. 3.2. Response Surface Methodology (RSM). The central composite design (CCD) was applied in the present study to investigate the effect of important parameters in the PEF−TiO2 process (using Na2SO4 or NaCl as the supporting electrolyte) including current density, initial dye concentration, Fe3+ dosage, and time. In this regard, a four-factor and a five-level CCD consisting of 31 experimental runs was performed. The considered four variables, their values, and the resulting data for decolorization (%), COD removal (%), and EEC (kWh/ gCOD) are represented in Table S4 in the Supporting Information. The variable ranges were selected according to our previous study. In the studied CD ranges, no anode corrosion was observed to produce any secondary pollution, and a small amount of Fe3+ was used to reach high efficiencies.37 The obtained experimental results were analyzed using the statistical software Minitab, version 16.2.4, to fit the equation and to evaluate the statistical significance of the equations. The coefficients of the response functions and P values of the parameters for different responses are illustrated in the Supporting Information (Table S5). The factors with P values (P ≤ 0.05) are significant to the model, but the others are insignificant to the response.40 The response functions (Yi) are related to the parameters by applying eq 3:

(2)

where Ecell and EUV are the average applied potential (V) for the electrical current and the UV lamp, respectively; I is the electrical current intensity (A); COD0 and CODt are the chemical oxygen demands (g/L) of the solution before and after time t of the treatment; and V is the volume of the electrolyte (1 L).37

3. RESULTS AND DISCUSSION 3.1. Repeatability Test. The repeatability of the PEF− TiO2 process using the modified electrodes was evaluated. In this procedure, five PEF−TiO2 experiments were carried out under the same experimental conditions (CD, 10.6 mA/cm2; pH 3; Na2SO4, 0.05 M; initial AR14 concentration, 50 mg/L; and Fe, 0.1 mM) to estimate the relative standard deviation during the decolorization of AR14. The relative standard deviation (RSD) can describe the repeatability of an experiment. The RSD is the value of the standard deviation in a ratio of the mean of the data. If the RSD values are lower than 3%, the repeatability of the experiment will be acceptable.38,39 The results in Figure 2 indicate that the

4

Yi = b0 +

4

3

4

∑ bixi +

∑ biixi 2 +

∑ ∑

i=1

i=1

i=1 j=i+1

bijxixj (3)

where Yi is the observable response variable, b0 is the constant coefficient, bi are the regression coefficients for linear effects, bii are the quadratic coefficients, bij are the interaction coefficients, and xi and xj are the coded values of input factors.40 The analysis of variance (ANOVA) and R2 (coefficient of determination) statistics were also used to check the adequacy of the developed model. The P value (P ≤ 0.05) justified the reliability of the fitted polynomial model with a confidence level of 95%, and high R2 values (>90%) validated the accuracy and statistical significance of the model (data not shown). The three-dimensional (3D) surface plots indicating the simultaneous effect of the parameters on the responses are shown in Figures 3−5. Figure 3 shows the changes of AB92 decolorization (%) by the variation of the key factors using NaCl as the supporting electrolyte. Higher values of CD will

Figure 2. Decolorization of AR14 by PEF−TiO2 process. Error bars for each point indicate their RSD.

Figure 3. Three-dimensional plots of parameters affecting AB92 decolorization (%) using NaCl as the supporting electrolyte. C



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Figure 4. Three-dimensional plots of parameters affecting COD removal (%) using Na2SO4 as the supporting electrolyte.

Figure 5. Three-dimensional plots of parameters affecting EEC (kWh/gCOD) using NaCl as the supporting electrolyte.

react with the large amount of dye molecules to oxidize their structure. On the other hand, at low dye concentrations, the increasing of the CD will lead to the formation of more hydroxyl radicals, but there are not enough dye molecules to be oxidized, and the parasitic reactions (the recombination of two hydroxyl radicals and/or the reaction of a hydroxyl radical with H2O2 to produce weaker oxidants such as perhydroxyl radicals) will occur, which reduces the COD removal efficiency.42,43 The effect of Fe3+ and time on the PEF−TiO2 performance is also represented in Figure 4. It is evident that the long time of the PEF−TiO2 process has increased the COD removal efficiencies, because the contact time of dye molecules with various oxidizing species formed during the reaction has increased and, thus, higher efficiencies are achieved.37 The effect of selected variables on the electrical energy consumption (EEC) is illustrated in Figure 5. At higher CD values, more electrical current is consumed for the removal process and, as a result, more electrical energy is consumed, but when the initial dye concentration is low, increasing the CD does not affect the EEC significantly. This may be due to the higher grams of COD removed at higher CD which, according to eq 2, results in almost constant values of EEC. More interaction of hydroxyl radicals and dye molecules leads to the removal of a higher amount of COD (g/L) resulting in low values of EEC at high dye concentration and low CD values, but at low dye concentrations, the grams of COD removed is not high and leads to higher EEC values.42 Also, Figure 5 exhibits that increasing the reaction time will lead to higher values of EEC because of the consumption of more electrical current both for the UV lamp and for electrodes. As can be seen in Figure 5, when the dosage of Fe3+ is not optimum, the formation of hydroxyl radicals from Fenton’s reaction is low; thus the COD removal efficiency is decreased and hence EEC is increased.42

increase the decolorization efficiency at higher concentrated dye solutions. When the CD rises, more H2O2 is electrogenerated at the surface of the cathode which will react with the iron ions to produce more OH• (other than the ones produced by the UV irradiation at the surface of the immobilized TiO2) through the Fenton reaction that will react with the dye molecules, causing a higher removal efficiency to be obtained. However, when the initial dye concentration is decreased, the removal efficiency by increasing the CD is decreased. This may be due to parasitic reactions between the radicals or H2O2 to lower the decolorization efficiency.41 The simultaneous effect of iron ion with time of the reaction on the decolorization of AB92 indicates that the addition of Fe3+ to the solution up to a maximum value could enhance the decolorization efficiency, but the excess of this value will not affect the removal (%). Fe3+ ions at the surface of the modified cathode will produce Fe2+ (0.77 V = NHE), which will react with the in situ H2O2 to improve the removal efficiency by the formation of hydroxyl radicals from Fenton’s reaction. The reduction of removal percent by the excessive amount of Fe3+ could be due to the parallel reaction of Fe2+ with the hydroxyl radicals and to the regeneration of the Fe3+; however, the required iron ion concentration in this study was very small due to the regeneration at the electrode surface.37,42,43 According to Figure 4, at low CD values, when the initial dye concentration is high, the COD removal efficiencies are very low due to less H2O2 production at the surface of the cathode and, therefore, a small amount of hydroxyl radicals is formed to degrade the dye structure. Also, it is obvious that, at high initial dye concentration when the CD is increased, the COD removal (%) increases too, but increasing the CD at lower initial dye concentration does not lead to the higher COD removal (%). A possible explanation for such results could be that increasing the CD at a high concentration of dyes will produce more H2O2 and consequently more hydroxyl radicals are available to D



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Figure 6. Schematic view of reactions in the PEF−TiO2 reactor using NaCl and Na2SO4 as the supporting electrolytes.

hypochlorite and chlorate ions to be reduced at the surface of the cathode to generate chloride radicals. Among these oxidants, the oxidation rates of chlorine (E° = 1.36 V) and hypochlorous acid (E° = 1.49 V) are higher than that of hypochlorite ions (E° = 0.89 V) in acidic media. The mechanism of degradation of dye molecules by hypochlorite ions can be written as44−48

According to the presented data in Table S4 in the Supporting Information, most of the results for decolorization and COD removal efficiencies are higher in the case of using NaCl as the electrolyte. This can be explained by the existence of active chlorine in the bulk solution. The active chlorine is electrogenerated from the anodic oxidation of the chloride ions and is responsible for the indirect electrooxidation of dyes. Different chloro species such as hypochlorous acid (HOCl), hypochlorite (HClO−), chlorite (ClO2−), chlorate (ClO3−), and perchlorate (ClO4−) can be formed in aqueous solution at various pH values. Up to about pH 3, the dominant chloro species is soluble chlorine.10,41−45 2Cl− → Cl 2(aq) + 2e−

OCl− + dye → CO2 + inorganic ions + H+ + e−

However, it should be mentioned that the formation of carcinogens and toxic chlorinated compounds is the major disadvantage of the presence of chloride ions in the oxidation solution.6,44−48 In the case of using Na2SO4 as the electrolyte, the degradation of dye molecules could also be the result of oxidation by the persulfate ions which are formed from the anodic oxidation of sulfate ions.49 Various reactions occurring in the PEF−TiO2 process that lead to the generation of different types of oxidants are schematically shown in Figure 6. The two anodic reactions at the surface of the anode are related to the presence of NaCl and Na2SO4 as the electrolytes in the solution. 3.3. Multiresponse Optimization. In this study, the multiresponse optimization of the PEF−TiO2 process was conducted by Response Optimizer considering the AR14 decolorization (%), AB92 decolorization (%), COD (%), and EEC (kWh/gCOD ) as four responses. The desirability (Derringer) function is the most employed multicriterion methodology in the optimization of various processes, which favors the multiresponse optimization economically, objectively and in terms of efficiency. First, an individual desirability

(4)

The electrogenerated chlorine at the surface of the anode at pH 8, respectively. The other mentioned species (chlorite, chlorate, and perchlorate) are produced in alkaline media by the anodic oxidation of hypochlorite ions. It is also possible for E



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Table 1. Optimum Values for PEF−TiO2 Procedure AR14 decolorization (%)

AB92 decolorization (%)

electrolyte

des

pred

exp

des

pred

exp

des

COD removal (%) pred

exp

des

EEC (kWh/gCOD) pred

exp

Na2SO4a NaClb

60−65 65−70

66.18 70.95

65.67 68.78

70−75 75−80

75.93 83.60

74. 49 80.34

60−65 60−65

65.00 65.08

63.067 62.91

0.35−0.4 0.3−0.35

0.35 0.285

0.36 0.286

a

Optimum conditions: [C0], 47.17 mg/L; t, 120 min; [Fe3+], 0.23 mM; CD, 9.58 mA/cm2. bOptimum conditions: [C0], 48.70 mg/L; t, 80.40 min; [Fe3+], 0.15 mM; CD, 12.95 mA/cm2.

Figure 7. UV−vis spectra along with TOC removal (%) during the PEF−TiO2 procedure at the RSM optimum conditions.

Table 2. Kinetic Constants of AR14 and AB92 Decolorization Using NaCl and Na2SO4 as Electrolytea [C0] (mg/L)

a

k1

[C0] [1.5C0] [2C0] [2.5C0] [3C0]

0.013 0.010 0.08 0.008 0.006

[C0] [1.5C0] [2C0] [2.5C0] [3C0]

0.030 0.021 0.007 0.004 0.004

pseudo-first order

pseudo-second order

pseudo-first order

2

2

2

R

t1/2

k2

R

t1/2

AR14 Decolorization (%), NaCl 0.875 53.32 0.029 0.988 0.884 69.31 0.014 0.990 0.862 8.66 0.008 0.978 0.912 86.64 0.007 0.994 0.943 115.52 0.004 0.995 AR14 Decolorization (%), Na2SO4 0.695 23.10 0.065 0.904 0.805 33.01 0.029 0.941 0.556 99.02 0.007 0.794 0.742 173.29 0.003 0.865 0.683 173.29 0.003 0.865

k1

44.21 58.55 78.03 75.35 111.41

0.012 0.009 0.007 0.006 0.005

21.13 31.29 97.85 202.51 166.67

0.012 0.009 0.004 0.003 0.003

R

pseudo-second order t1/2

k2

R2

AB92 Decolorization (%), NaCl 0.943 57.76 0.047 0.996 0.896 77.02 0.021 0.977 0.909 99.02 0.013 0.989 0.933 115.52 0.009 0.993 0.920 138.63 0.005 0.985 AB92 Decolorization (%), Na2SO4 0.532 57.76 0.045 0.827 0.530 77.02 0.020 0.787 0.578 173.29 0.006 0.748 0.895 ND 0.003 0.942 0.816 ND 0.003 0.916

t1/2 52.53 71.18 80.97 98.33 138.89 52.91 70.03 180.18 ND ND

ND, not determined.

function (di) is considered for each response. The individual desirability function varies in the range 0 ≤ d ≤ 1, in which 0 refers to a completely undesirable response and 1 is related to a fully desired response. Then the overall desirability function (D) is expressed as the geometric average of individual desirability functions (eq 9), with n being the number of responses:30 D=

n

d1d 2 ··· dn

for the multiresponse optimization process with the individual and overall desirability functions as one are given in Table 1. The UV−vis (200−670 nm) absorption spectra of the binary solutions at the optimum conditions obtained from the RSM experiments in specific time intervals are exhibited in Figure 7. The spectra were measured at times of 0, 20, 40, 60, 80, 100, 120, and 140 min of the PEF−TiO2 process. The appearance of a peak in the UV region of the spectra is regarding the existence of aromatic rings in the molecular structures of the dyes. The peak appearing in the visible range is probably related to the azoic group in the structure of both dyes. According to Figure 7, the intensity of the absorbance peaks in both the UV and visible regions decreased gradually after the PEF−TiO2 process. Also, the mineralization efficiency of the process is presented in Figure 7 (insets). It can be seen that the total organic carbon (TOC; %) of the process using NaCl (∼66%) as the electrolyte has become higher than that using Na2SO4 (∼51%) due to the

(9)

The desired values of the responses to optimize the process are presented in Table 1. The verification of the optimized results was investigated by performing three experiments under the predicted optimum conditions. The experimental values for each response were in good agreement with the results obtained from RSM and thus validated the predicted data of RSM. The values of both the predicted and experimental values F



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Figure 8. Effect of flow rate on the decolorization of AR14 and AB92 at the optimum conditions obtained from RSM after 140 min using NaCl and Na2SO4 as electrolytes.

Figure 9. Stability investigation of (a) immobilized TiO2 and (b) deposited CNTs (electrolyte, NaCl; [C0], 48.70 mg/L; t, 140 min; [Fe3+], 0.15 mM; CD, 12.95 mA/cm2). The dotted lines indicate the average decolorization (%) before the decrease in efficiencies.

The results indicated that the color removal efficiencies increased when the flow rate increased up to 0.825 L/h for both dyes, and further increasing of this parameter decreased the dye decolaration (%). By increasing the flow rate, the amount of dye molecules in contact with the various oxidant species in the PEF−TiO2 reactor increased and the desired reaction of hydroxyl radicals and dye molecules occurred in the solution. Also, the rate of waste reactions such as the combination of hydroxyl radicals with each other or with H2O2 formed at the surface of the cathode, which will produce H2O2 and perhydroxyl radicals (weaker oxidant than hydroxyl radical), is minimum. Increasing the flow rate to 0.99 L/h will decrease the removal efficiency due to the lower contact time between the dye molecules and oxidants generating in the PEF−TiO2 cell.52 Table S6 in the Supporting Information contains the obtained data in this study in comparison with other investigations on dye decolorization. It should be mentioned that the types of electrode (anode and cathode), the initial dye concentration, the electrolyte type and concentration, the iron ion dosage, the geometry of reactor, whether the process is batch or continuous, the residence time, etc. affect the performance of a system, and the presented studies have similarities to and differences from our current research. 3.6. Stability of CNT Fabricated Graphite and Immobilized TiO2. The decolorization of AR14 and AB92 in binary solution was carried out for 11 cycles at the RSM optimum conditions with NaCl as the supporting electrolyte and a flow rate of 0.825 L/h for 140 min using a new modified

presence of active chlorine and hypochlorous acid in the solution that will oxidize the molecular structures of the dyes.37 3.4. PEF−TiO2 Kinetic Study. The pseudo-first order and pseudo-second order kinetic models were employed to survey the decolorization kinetic of AR14 and AB92 by raising the initial dye concentrations up to 3 times (C0, 1.5C0, 2C0, 2.5C0, and 3C0) at the optimum conditions from the RSM for 210 min. The pseudo-first order kinetic model can be written as eq 10: ln(A t ) = −k1t + ln(A 0)

The pseudo-second order kinetic model is as follows: 1 1 = k 2t + At A0

(10) 29,50

(11)

where k1 is the pseudo-first order kinetic constant and k2 is the pseudo-second order kinetic constant. Table 2 represents the calculated kinetic constants (k1 and k2) along with their correlation coefficients (R2) and t1/2, which is the related time to decolorize 50% of the initial dye concentration.51 The results demonstrate that the experimental data are better fitted to the pseudo-second order kinetic model considering their high correlation coefficients. 3.5. Effect of Flow Rate. The effect of the flow rate on AR14 and AB92 decolorization in the binary solution after 140 min of oxidation is presented in Figure 8. This effect was investigated in the range 0.33−0.99 L/h (residence time of 3−1 h) at the optimum conditions obtained from the RSM results. G



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cathode for each cycle to investigate the stability of the immobilized TiO2 nanoparticles. The results in Figure 9 indicate that the TiO2 nanoparticles were stable for 8 cycles (1120 min), and after 11 cycles, the decolorization efficiency dropped to around 86 and 83% for AB92 and AR14, respectively. Similar experiments were also conducted for eight cycles without exchanging the modified electrode to investigate the stability of nanotubes on the surface of the graphite electrode. The results showed that the nanotubes were stable on the graphite surface for six cycles (840 min), and further usage of the same electrode will decrease the maximum accessible efficiency to 83% for AB92 and 78% for AR14.

comparison of the present study with other researches; eqs S1−S3 regarding the binary solution calculations. This material is available free of charge via the Internet at http://pubs.acs.org.

4. CONCLUSIONS In this study, the indirect oxidation of two acid dyes (C.I. Acid Red 14 and C.I. Acid Blue 92) in acidic media was investigated through the PEF−TiO2 process in a cylindrical continuous cell. A graphite electrode modified by the simple and uniform deposition of CNTs was employed as the cathode material for the in situ electrogeneration of hydrogen peroxide in the solution. The TiO2 nanoparticles were immobilized on a UV resistant silicone polymer, which will prevent their entrance to water streams and overcome their difficult separation and recycling from the treated solutions. NaCl and Na2SO4 were selected as two types of electrolyte that are mostly used in various textile processes. Since the real wastewaters often include more than one component, the study of the decolorization of wastewaters containing a mixture of dyes like binary solutions could be very useful for the treatment of real industrial wastewaters. In this regard, RSM was employed by utilizing the CCD to investigate the effect of important parameters including the time of reaction, iron ion concentration, current density, and initial dye concentration on the decolorization (%), COD (%), and electrical energy consumption per COD decay. The analysis of variance indicated that RSM can be efficiently used for the modeling of an oxidation process with high correlation coefficients. Also, the multiresponse optimization was conducted considering the overall desirability function. Two kinetic models (pseudo-first order and pseudo-second order) were applied to the data, and the results clearly demonstrated that the PEF−TiO2 process could be better explained by the pseudo-second order kinetic model. The deposited nanotubes and immobilized TiO2 nanoparticles demonstrated appropriate stability for the continuous treatment of colored wastewater. The results illustrated that using NaCl as the supporting electrolyte would improve the efficiency of the process due to the formation of active chlorine and hypochlorous acid in the bulk solution. Decolorizations of 99 and 95% were obtained for AB92 and AR14 after 140 min of the process at the RSM optimum conditions with a residence time of 72 min. Therefore, it can be concluded that PEF−TiO2 is a viable technique and a successful promising process with a high efficiency for color removal from wastewaters containing more than one component.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +98 2164542614. Fax: +98 2166400245. E-mail: arami@ aut.ac.ir. Notes

The authors declare no competing financial interest.

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ASSOCIATED CONTENT

S Supporting Information *

Tables S1−S6 showing results of studies using various electrodes modified with CNTs and modified graphite electrodes for electrochemical wastewater treatment, dye structures, CCD matrix for the PEF−TiO2 process, coefficients of significant factors to the response functions, and the H

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Application of Carbon Nanotubes Coated Electrodes and Immobilized

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