comparative study of dye solution treatment by electro

© by PSP Volume 21 – No 12b. 2012

Fresenius Environmental Bulletin

COMPARATIVE STUDY OF DYE SOLUTION TREATMENT BY ELECTRO-FENTON PROCESS USING CARBON PAPER AND CARBON PAPER MODIFIED WITH CARBON NANOTUBES AS CATHODE Nader Djafarzadeh1,*, Alireza Khataee2, Morteza Khosravi1and Mahmoud Reza Sohrabi1 1

Faculty of Chemistry, North Tehran Branch, Islamic Azad University, Tehran, Iran 2 Research Laboratory of Advanced Water and Wastewater Treatment Processes, Department of Applied Chemistry, Faculty of Chemistry,University of Tabriz, Tabriz, Iran

ABSTRACT The Electro-Fenton (EF) process was used for decolorization of an anthraquinone dye, Reactive Blue 69 (RB69) and a real textile wastewater. Hydrogen peroxide was electro-generated by reduction of dissolved oxygen in acidic solution. Carbon paper (CP) and carbon paper modified with carbon nanotubes (CP-CNT) were used as the cathode and a Pt sheet was used as the anode electrode. Hydrogen peroxide was continuously generated by the two-electron reduction of dissolved oxygen at cathode electrode while Fe3+ was added to the solution. The results indicated that the decolorization efficiency using CP-CNT cathode was higher than that of CP electrode. The effect of operational parameters such as applied current density, initial pH and electrolyte type was studied in an attempt to reach the higher dye removal efficiency. The results of COD measurements indicated that EF with CP-CNT led to 73% degradation after 300 min of electrolysis time. Also, for real textile wastewater containing reactive dyes, the results of COD measurements indicated that EF process caused 75% reduction after 420 min electrolysis time. KEYWORDS: Textile wastewater, Electro-Fenton, Carbon nanotubes, Advanced oxidation processes.

1 INTRODUCTION In the textile industry, dyeing process generates large volume of wastewater containing most un-reacted colored dyestuffs. The presence of even a small amount of dye in water is highly visible and affects the water transparency and the gas solubility of water bodies [1-7]. Recently, increasing attention has been focused on complete oxidation of organic compounds to harmless inorganic products. Ad* Corresponding author

vanced oxidation processes (AOPs) methods, based on the generation of hydroxyl radicals (•OH), have been applied to degradation of toxic organic pollutants due to the high oxidative power of these radicals [8-11]. The development of a new process ensuring an in situ production of the Fenton reagent (Fe2+/H2O2) by electro-Fenton has been considered. In this technique, H2O2 is continuously supplied to the contaminated solution from the two-electron reduction of O2 usually at carbon-felt and its components cathodes [12]: O2 + 2H+ + 2e−→ H2O2 (1) The oxidizing power of the hydrogen peroxide is highly enhanced by the addition of Fe2+ generating the Fenton reaction [13]: Fe2+ + H2O2 → Fe3+ + •OH + OH(2) The EF method utilizes a Pt anode in an undivided cell, while Fe2+ or Fe3+ is added to the solution to permit degradation of pollutants by •OH generated through reaction (2) in the medium. The electro-Fenton has successfully been used for the treatment of wastewaters including phosphonate herbicides [14], Metomyl insecticide [15], cellulose [16], tannery industrial wastewater [17], landfill leachate waste [18] and 2,4-dichlorophenol [19]. Meanwhile, EF process has widely been used in decolorization of various structurally different dye containing solutions and wastewaters [7, 8, 20, 21, 22]. In this work, EF process was preformed using carbon paper (CP) and carbon paper modified with carbon nanotubes (CP-CNT) as cathode electrode for removal of Reactive Blue 69 as an anthraquinone dye from aqueous solutions. Reactive Blue 69 is soluble in water and idoneous for acrylic fiber dyeing and is used in the wool, towel and blanket factories, so their effluents have a great deal of this dye. Also, treatment of a real textile wastewater containing mixture of four reactive textile dyes by electro-Fenton process using CP-CNT electrode was studied. The presence of the reactive textile dyes in wastewater are of a particular environmental concern, since they give an undesirable color to the water and can originate dangerous byproducts through oxidation or other chemical reactions.

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2 MATERIALS AND METHODS 2.1. Chemicals

The commercial dye (Reactive Blue 69) was purchased from Ciba-Geigy (Switzerland). The dyestuff was used without further purification. Dye solution was prepared by dissolving dye in distilled water. The Na2SO4 was used as support electrolyte. Real wastewater was obtained from Tabriz Carpet Factory, Tabriz, Iran. The chemical characteristics of the real wastewater and containing dyestuffs are reported in Tables 1 and 2, respectively. All chemicals used in this study were of the highest purity obtained from Merck (Germany). 2.2. Fabrication of the Cathode Electrode

Carbon paper (TGP-H-060, thick: 190 µm, conductivity: 12.5 S/cm, bulk density: 0.44 g/cm3 and porosity: 80%) and multi walled carbon nanotube (outer diameter: 10-

20 nm, inside diameter: 3-5nm, length: 10-30 µm, bulk density: 0.22 g/cm3, conductivity: >100 S/cm and purity: >95 wt%) were purchased from Toray (Japan) and Cheap Tubes Inc. (USA), respectively. Scanning electron microscopy (SEM) was carried out on a Tescan VEGA device (Czech). For immobilization of carbon nanotube on the surface of carbon paper, appropriate amounts (0.1 g) of CNT, 0.42 g Polytetrafluoroethylene (PTFE), 60 mL distilled water and 3% n-butanol were mixed in an ultrasonic bath (Grant, England) for 20 min to obtain a highly disTABLE 1 - Chemical characteristics of the real wastewater. Characteristics Initial COD (mg/L) Initial pH Electrical conductivity (mS/cm) TDS (mg/L) Color

Value 3960 4.85 13.35 8240 Olivaceous

TABLE 2 - Characteristics of the commercial dyestuffs in the real wastewater. Dye

Chemical class

Reactive Blue 69

Anthraquinone

Reactive Yellow 39

Monoazo

Reactive Red 84

Monoazo

Chemical structure

FIGURE 1 - (a) SEM image of applied carbon paper; (b) SEM image of fabricated CNT-Carbon paper electrode.

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persed mixture. The resulting mixture was heated at 80○C until resembled an ointment in appearance. The ointment was bonded to 50% PTFE-loaded carbon papers and sintered at 350 ○C for 30 min. Fig.1 shows the scanning electron microscopy (SEM) image of applied cathode electrode.

oxygen demand (COD) of the dye solutions was measured according to the standard methods for examination of water and wastewater by open reflax methods [23]. The COD removal percentage was defined as:

COD removal (%) =

2.3. Instruments and Chemical Analysis

The experiments were conducted in an open, undivided cell with a DC power supply (Micro, PW-4053S, Iran). In each run, 250 mL of the dye solution with initial concentration of 100 mg/L RB69 containing 0.15 mM of Fe+3 ions and 0.05 M of Na2SO4 were decanted into the electrolytic cell. All the runs were performed at room temperature and stirred magnetically at a rate of 200 rpm. In all experiments, air was injected to the dye solution by an air pump constant flow rates near the cathode and anode electrodes for the production of H2O2 through Eq. (1). The batch experimental cell is shown in Fig. 2.

COD0 − CODt ×100 COD0

(4)

where COD0 and CODt are chemical oxygen demands at times t = 0 (initial) and t (reaction time) in gO2/L. Hydrogen peroxide concentration was determined spectrophotometrically by the iodide method (detection limit of ≈ 10-6 M) as follow [7, 22]. 3 mL of iodide reagent (0.4 M potassium iodide, 0.06 M NaOH and 10-4 M ammonium molybdate) and 3 mL of 0.1 M potassium biphthalate were added to 23 mL of samples and were diluted to 10 mL. Then, the absorbance of the treated solution was measured with a UVVis spectrophotometer at λ=351 nm. 3 RESULTS AND DISCUSSION 3.1. Influence of Cathode Electrode Type on the Hydrogen peroxide Production

In electro-Fenton process, the concentration of hydrogen peroxide is the important parameters as it is the source of •OH. So, Hydrogen peroxide production and stability depend on factors such as cell configuration, cathode properties and operation conditions [7, 9, 22].

FIGURE 2 - An apparatus electrochemical cell.

Platinum sheet of 25 cm2 area was used as anode and CP or CP-CNT of 40 cm2 area was used as cathode. The pH of the solutions was measured by pH meter (WTW 720i, Germany) and adjusted by adding H2SO4 solutions. The dye concentration was determined from absorbance characteristics of the dye (λmax=610 nm) in the UV-Vis range with the calibration method. So, at certain time intervals, 2 mL sample was taken to measure the dye concentration. A Hach UV-Vis spectrophotometer (DR 5000, USA) was used. The calculation of dye removal efficiency after electro-Fenton treatment was performed using Equation 3 at λmax=610 nm:

DR (%) =

C0 − C ×100 C0

Therefore, electrolyses were performed with 250 ml solution in order to determine the amount of hydrogen peroxide generated by the graphite felt, CP and CP-CNT electrodes when the electrodes were fed with Air. A Pt sheet of 25 cm2 area was used as anode in these electrolyses. Fig. 3 shows the H2O2 concentration produced on the graphite felt, CP and CP-CNT as a function of time. As can be seen from the Fig. 3, the concentration of H2O2 obtained via graphite felt, CP and CP- CNT electrodes was 0.84, 2.98 and 3.92 mM, respectively at the end of 300 min electrolysis. The amount of electrogenerated H2O2 obtained with CP-CNT electrode was nearly three and two times higher than that of graphite felt and CP electrodes, respectively.

(3)

where C0 and C are concentrations of dye before and after decolorization in mg/L, respectively. The chemical

FIGURE 3 - Influence of cathode electrode type on the hydrogen peroxide production (I=250 mA, Fe+3= 0.15 mM, [Na2SO4]= 0.05 M and pH=3.0)

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3.2. Influence of Cathode Electrode Type on the Dye Removal

In the EF process, type and property of cathode electrode are two of the main factors affecting process efficiency [9]. Previous researchers usually have used the carbon components such as graphite sheet [4, 16, 22], carbon felt [1, 12, 14, 15], activated carbon fiber [7] and graphitefelt as a cathode [9, 23]. Carbon nanotube, possessing unique properties such as high electrical conductivity, high surface area and chemical stability, is considered to be the promising electrode material [26]. In the first part of this work, CP-CNT electrode was fabricated as cathode. To compare the performance of CP-CNT electrode with CP cathode, dye removal process was performed with graphite sheet electrode of 40 cm2 area under the same conditions. Fig. 4 shows the relationship between the dye removal efficiency and electrolysis time for three different cathode electrodes. According to the results, dye removal efficiency with CPCNT electrode is 14% and 32% more than unmodified CP electrode and graphite electrode, respectively. Reaction time also influences the treatment efficiency of the electrochemical process. In the electro-Fenton process, dye removal efficiency depends directly on the generation of hydroxyl radicals on the dyestuff wastewater. According to the results are shown in Fig. 4, in the EF with CP-CNT electrode at the time of 300 min, dye removal efficiency was about 90%.

hydroxyl radicals from Fenton’s reaction 2. The dye removal efficiency was obtained 89 % and 75% using CPCNT and CP electrode, respectively, with the 250 mA current. So, in order to save the electrical energy consumption, current upper than this optimum amount was not appropriate.

FIGURE 5 - Influence of applied current on the dye removal (tEF =300 min , Fe+3= 0.15 mM, [Na2SO4]= 0.05 M and pH=3.0)

FIGURE 6 - Influence of electrolysis time on current efficiency (I=250 mA, Fe+3= 0.15 mM, [Na2SO4]= 0.05 M and pH=3.0) 3.4. Influence of Electrolysis Time on the Current Efficiency FIGURE 4 - Influence of cathode electrode type on the dye removal (I=250 mA, Fe+3= 0.15 mM, [Na2SO4]= 0.05 M and pH=3.0) 3.3. Influence of Applied Current Density on the Efficiency of Dye Removal

In all electrochemical processes, current density is one of the most important parameter for controlling the reaction rate within the reactor. The influence of applied current density on the dye oxidation has been investigated in the range of 50-350 mA with CP and CP-CNT electrodes and the results are reported in Fig. 5. A gradual increase in dye removal efficiency with raising current is seen for both electrodes. This enhancement of the oxidation power can be associated with a great production of H2O2 through reaction 1, leading to the generation of high amount of

Current efficiency (CE) is an important factor for electro-Fenton process efficiency, which evaluates the performance of the electrolytic cell, and is dramatically affected by the applied current. In the EF process, COD data are used to calculate the current efficiency at a given electrolysis time in batch operation mode at constant current through Eq. 5 [22, 25, 27]: (COD 0 − COD t )FVs ×100 (5) Current efficiency (%) = 8 It where COD0 and CODt are chemical oxygen demands at times t = 0 (initial) and t (reaction time) in gO2/L, I is the current (A), t is the reaction time (s), F is the Faraday constant (96487 C/mol), Vs is the electrolyte volume (dm3), and 8 is the oxygen equivalent mass (g/equivalent). The

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electrolysis time was plotted against related current efficiency percentage for CP and CP-CNT electrode (Fig. 6). As can be seen in Fig. 6, the current efficiency is increased with increasing the electrolysis time for both electrodes. Also, values of current efficiency percentage for CP-CNT electrode is more than CP electrode. 3.5. Influence of Initial pH on the Efficiency of Dye Removal

The solution pH is an important control parameter for maintaining the effectiveness of the EF process. Several authors [9, 15, 28] have reported its maximum efficiency in undivided cells with carbon-felt and gas diffusion (GDE) cathodes at pH 3.0, which is close to pH 2.8 where the maximum production of •OH is expected from Fenton’s reaction (2) [29]. When Reaction (1) takes place in acidic medium, lower pH values lead to a higher H2O2 yield. It is well known that the Fenton’s reactions occur at low pH values. The natural pH was 4.85 for the 100 mg/L of RB69 solution. The effect of initial pH on the dye removal investigated with CP-CNT electrode and the results are illustrated in Fig.7. According to the results, a notable pH effect was found for the electro-Fenton process, reaching its faster degradation at pH 3.0 with a maximum dye removal of 89% at 300 min. So, initial pH of 3.0 was selected the optimum pH.

cost with increasing electrolysis time from 60 to 120 min was found for both tested electrodes using Eq. 7. As can be seen in Fig. 8a, electrical energy consumption for CPCNT electrode at the electrolysis time of 300 min was around 20 kWh/kgdye less than CP electrode at same conditions. Fig. 8b shows a decrease in energy consumption with increasing electrolysis time for the both applied electrodes. As can be seen in Fig. 8b, the electrical energy consumption for CP-CNT electrode at the electrolysis time of 300 min was 100 kWh/kgCOD less than CP electrode at the same conditions. So, the application of CP-CNT cathode is economical than CP electrode for decolorization of RB69 dye solution. 3.7. COD Reduction and Absorbance Spectra of the Dye Solution

Influence of electrolysis time on the COD reduction and UV-Vis spectra changes at the optimized conditions (I=250 mA, pH=3.0 and [Dye]0 =100 mg/L) were shown in Figs. 9 and 10, respectively. As can be seen in Fig.9, COD decay percent at the electrolysis time of 300 min is 73% and 57% for CP-CNT and CP electrode, respectively. Fig. 10 illustrates the UV-Vis spectra of dye solution during EF process with CP-CNT electrode. According to the Fig. 10, EF process leads to almost complete decolorization after 360 min.

FIGURE 7 - Influence of initial pH on the dye removal with CPCNT electrode (I=250 mA, Fe+3= 0.15 mM and [Na2SO4]= 0.05 M) 3.6. Electrical Energy Consumption

Electrical energy consumption (EEC) is very important economical parameter in electro-Fenton process. Energy consumption per amount of removed dye mass (kWh/kg Dye) and destroyed COD (kWh/kg COD) can be obtained through Eqs. 6 and 7, respectively [9, 25]:

Energy consumption(kWh/kg dye) =

IVt ∆mdye

Energy consumption(kWh/kg COD) =

(6)

IVt (7) (∆COD)VS

where I is the applied current (A), V is the average cell voltage (V), t is the electrolysis time (h), Vs is the solution volume (dm3), ∆COD is the decay in COD (g/dm3) and ∆mdye is the dye mass removed (g). An increase in energy

FIGURE 8 - Influence of electrolysis time on the energy consumption of (a) amount of dye mass removed and (b) COD destroyed (I=250 mA, Fe+3= 0.15 mM, [Na2SO4]= 0.05 M and pH=3.0)

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FIGURE 9 - Influence of electrolysis time on the efficiency of COD removal (I=250 mA, Fe+3= 0.15 mM, [Na2SO4]= 0.05 M and pH=3.0)

FIGURE 10 - UV-Vis spectra of RB69 recorded during EF process at different electrolysis times with CP-CNT electrode (I=250 mA, Fe+3= 0.15 mM, [Na2SO4]= 0.05 M and pH=3.0)

sodium sulfate salt. The mixture of these dyestuffs generated olivaceous color which used in dying of wool and the handmade carpet fibers. This wastewater had ~0.5 mol/L sodium sulfate. This is very interesting, because sodium sulfate is used as supporting electrolyte in the electroFenton process [9, 25]. All experiments for treatment of 250 mL of real wastewater carried out with CP-CNT electrode at the conditions of I=800 mA, initial pH=3 and [Fe+3]=0.3 mM. Influence of electrolysis time on the COD reduction of wastewater at the mention conditions was shown in Fig. 11. As can be seen in Fig. 11, COD decay percent at 420 min electrolysis time is 76%. The UV–Vis spectra change of the real wastewater was depicted in Fig. 12. As can be seen, EF causes almost complete color removal of the wastewater after 420 min. This result proves that the EF process with CP-CNT electrode is an effective method in treatment of a real colorful wastewater.

FIGURE 12 - UV-Vis spectra of real wastewater recorded during EF process at different electrolysis times with CP-CNT electrode (I=800 mA, pH=3.0 and [Fe+3]=0.3 mM)

4 CONCLUSIONS

FIGURE 11 - Influence of electrolysis time on the efficiency of COD removal from real wastewater with CP-CNT electrode (I=800 mA, pH=3.0 and [Fe+3]=0.3 mM) 3.8. Treatment of a Real Textile Wastewater

The real wastewater, obtained from Tabriz Carpet Factory, contains three anthraquinone and azo reactive dyes (yellow 39, red 84 and blue 69) and other additive such as ALBEGAL B as a stabilizer and commercial

In this work, CP or CP-CNT electrode as cathode were used for treatment of dye solution containing Reactive Blue 69 by electro-Fenton process in batch electrochemical cell. The efficiency of EF process with CP-CNT cathode was compared with carbon paper electrode. This study showed that the decolorization efficiency of CPCNT electrode was better than unmodified CP electrode. The effect of operational parameters such as applied current, initial pH, support electrolyte type and initial dye concentration was studied. Results showed that for decolorization of 250 mL RB69 solution with initial concentration of 100 mg/L, applied current was 250 mA, electrolysis time was 300 min and initial pH was 3. Electrical energy consumption in the above conditions with the CPCNT electrode was around 400 kWh/kgCOD. The reduction of COD was 73% and dye removal percent was 90% using CP-CNT electrode. Also, the EF process with CPCNT electrode was used successfully for treatment of real wastewater containing four reactive dyestuffs.

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ACKNOWLEDGEMENT The authors thank the Islamic Azad University, Tehran North Branch, Iran for financial and other supports.

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Received: May 30, 2012 Revised: July 02, 2012 Accepted: July 18, 2012

CORRESPONDING AUTHOR Nader Djafarzadeh Islamic Azad University Faculty of Chemistry North Tehran Branch Tehran IRAN E-mail: [email protected] FEB/ Vol 21/ No 12b/ 2012 – pages 4022 - 4029

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