Treatment of Reactive Blue 69 solution by electro-fenton process using carbon nanotubes based cathode

2011 International Conference on Biology, Environment and Chemistry IPCBEE vol.24 (2011) (2011)IACSIT Press, Singapoore Treatment of Reactive Blue 69 solution by electro-fenton process using carbon nanotubes based cathode Nader Djafarzadeh 1+ and Alireza Khataee 2 1 Department of Chemistry, Miyaneh Branch, Islamic Azad University, Miyaneh, IRAN. 2 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). Hydrogen peroxide was electro-generated by reduction of dissolved oxygen in acidic solution. EF process allows the production of active intermediates, which react with the organic compounds leading to their mineralization. In the electrochemical cell carbon paper modified with carbon nanotubes (CP- CNT) were used as cathode and a Pt sheet was used as anode. The experiments were conducted at room temperature in an open, undivided and cylindrical glass cell of 500 ml capacity. H 2 O 2 was continuously generated from the two-electron reduction of O 2 at cathode electrode while Fe 3+ was added to the solution. The effect of operational parameters such as applied current, initial ph, electrolyte type and reaction time was studied in an attempt to reach the higher dye removal efficiency. The degradation of RB69 was followed by chemical oxygen demand (COD) analysis. The results of COD measurements indicated that electro- Fenton with carbon paper modified with carbon nanotubes allowed 70% degradation after 5 h of electrolysis. Keywords: Reactive Blue 69, Electro-Fenton, Carbon nanotubes, Water treatment, COD reduction. 1. Introduction In the textile industry, dyeing process generated large volume of wastewater containing most un-reacted colored dyestuffs. Dyes present in wastewater are of a particular environmental concern since they give an undesirable color to the waters and in some cases they are harmful compounds and can originate dangerous byproducts through oxidation, hydrolysis, or other chemical reactions taking place in the waste phase [1,2]. Recently, increasing attention has been focused on complete degradation of organic compounds to harmless products such as CO 2 and H 2 O. The treatment methods, based on the generation of hydroxyl radicals ( OH), known as advanced oxidation processes (AOPs), have been applied for degradation of toxic organic pollutants. AOPs consist of the production of hydroxyl radicals from different systems, such as the Fenton, photo-fenton, electrochemical (anodic oxidation) and electro-fenton processes [3,4]. The development of a new process ensuring an in-situ production of the Fenton reagent (Fe 2+ /H 2 O 2 ) by electro-fenton has been considered therefore. Electro-Fenton process uses as reagent the compressed air and a catalytic amount of ferric ions. In this technique, H 2 O 2 is continuously supplied to the contaminated solution from the twoelectron reduction of O 2 usually at carbon-felt and its components cathodes [5]: O 2 + 2H + + 2e H 2 O 2 (1) The oxidizing power of the hydrogen peroxide is highly enhanced by the addition of Fe 2+ generating the Fenton reaction [6]: Fe 2+ + H 2 O 2 Fe 3+ + OH + OH - (2) + Corresponding author. Tel.: +98 423 2237040-44; fax: +98 423 2227290 E-mail address: n.jafarzadeh@gmail.com 479

The EF method utilizes a Pt anode in an undivided cell, while Fe3+ is added to the solution to permit degradation of pollutants by OH generated from reaction (2). Soluble Fe3+ can be reduced to Fe2+ through reaction (3) on cathode [5]: Fe3+ + e Fe2+ (3) The optimal ph for the electro-fenton process is around 3.0 because at this ph the main species, Fe(OH)2+(H2O)5, has the largest light absorption coefficient and quantum yield for OH production, along with Fe(II) regeneration in the range of 280 370 nm [4]. The electro-fenton has successfully been used for the treatment of wastewaters including phosphonate herbicides [7] and decolorization of various structurally different dye in wastewaters [2,3,8]. In the Electro-Fenton process property of cathode electrode is one of the affecting factors on the efficiency of process. The protection of hydroxyl radicals is increased with increasing the conductivity and specific surface of the cathode which causes the enhancement of removal of pollutants. In this work, EF process was preformed using carbon paper modified with carbon nanotubes as cathode electrode for removal of Reactive Blue 69 as an anthraquinone dye from aqueous solutions. This electrode is very resistant in aqueous solutions and it has particular characterizes such as high conductivity and large specific surface. 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. Moreover, the effects of operational parameters such as applied current, initial ph, electrolysis time and initial dye concentration on dye removal efficiency have been investigated in this study. 2. Experimental 2.1. Cathode electrode fabrication Carbon paper (TGP-H-060, thick: 190 μm, conductivity: 12.5 S/cm and porosity: 80%) and Multi walled carbon nanotube (OD:10-20 nm, ID: 3-5nm, length: 10-30 μm and purity: >95 wt%) were purchased from Toray, Japan and Cheap Tubes Inc., USA, respectively. 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 create a highly dispersed mixture. The resulting mixture was heated at 80 C until it 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. Fig. 1. (a) SEM image of used carbon paper; (b) SEM image of fabricated CNT-Carbon paper electrode. 2.2. Materials and methods The commercial dye (Reactive Blue 69) used in this project was purchased from Ciba-Geigy, Switzerland. The chemical structure and other characteristic of this dye are shown in Table 1, and were used without further purification. Dye solution was prepared by dissolving dye in distilled water. All chemicals used in this study were of the highest purity available from Merck. The ph of the solutions was measured by ph meter (WTW 720i, Germany) and adjusted by adding H2SO4 solutions. The Na2SO4 was used as support electrolyte. All the runs were performed at room temperature. In each run, 250 ml of the dye solution containing 0.15 mm of Fe+3 ions was decanted into the electrolytic cell. The batch experimental cell is shown in Fig. 2. Pt sheet of 25 cm2 area was used as anode and modified carbon paper of 40 cm2 area was used as cathode. Table 1: Characteristics of the commercial dye 480

Dye Cas. No. Molecular formula λ max Chemical class Chemical Structure Reactive Blue 69 70209-99-3 C 23 H 14 BrN 3 Na 2 O 9 S 2 610 nm Anthraquinone Fig. 2. An apparatus electrochemical cell. The dye concentration was determined from them absorbance characteristics in the UV Vis range with the calibration method. A Hach UV Vis spectrophotometer (DR 5000, USA) was used. For the measurement the maximum absorption (λ max =610 nm) wavelength of dye was determined by measurement of it absorbance at various wavelengths. The calculation of dye removal efficiency after electro-fenton treatment was performed using Eq. 4: C C (4) DR (%) = 0 C 0 where C 0 and C are concentrations of dye before and after decolorization in mg/l, respectively. The chemical oxygen demand (COD) of dye solution was measured according to the standard methods for examination of water and wastewater by open reflax methods [9]. The COD decay percentage was defined as: COD CODt COD Decay (%) = 0 100 COD (5) 0 where COD 0 and COD t are chemical oxygen demands at times t = 0 (initial) and t (reaction time) in go 2 /dm 3. 3. Results and discussion 3.1. Influence of electrolysis time on the dye removal Reaction time influences the treatment efficiency of the electrochemical process. The dye removal efficiency depends directly on the generation of hydroxyl radicals on the dyestuff wastewater. To compare the performance of CP-CNT electrode, dye removal process was done with only CP electrode in the same conditions. Fig. 3 shows the relationship between the dye removal efficiency and electrolysis time for two different cathode electrodes. According to the results, dye removal percent with CP-CNT electrode was around 15% more than only CP electrode. In the EF with CP-CNT electrode at the time of 300 min, DR % was around 90%. 100 Fig. 3. Influence of electrolysis time on the dye removal ([Dye] 0 =100 mg/l, I =250 ma, ph=3.0 and [NaSO 4 ]=0.05M) 481 Fig. 4. Influence of applied current on the dye removal ([Dye] 0 =100 mg/l, t EF =300 min, ph=3.0 and [NaSO 4 ]=0.05M)

3.2. Influence of applied current on the dye removal In all electrochemical processes, current density is the most important parameter for controlling the reaction rate within the reactor. The influence of applied current on dye oxidation has been investigated in the range of 50-350 ma with CP-CNT electrode and the results are reported in Fig. 4. This results shows a gradual increase in dye removal efficiency with raising current. This enhancement of the oxidation power can only be associated with a great production of H 2 O 2 from reaction leading to the generation of high amount of hydroxyl radicals from Fenton s reaction. Dye removal percent was obtained around 90 % in the 250 ma current and for saving the electrical energy consumption, current upper than this data was not appropriate. 3.3. Influence of initial dye concentration on the dye removal The dye solution with different initial concentration in the range of 25 250 mg/l were treated by EF in optimized current and time of electrolysis values. The initial dye concentration was plotted against related dye removal percentage (Fig. 5). As can be seen in Fig. 5, the dye removal percent is decreased with increasing the initial dye concentration. Fig. 5. Influence of dye concentration on the dye removal Fig. 6. Influence of electrolyte type on the dye removal (I =250 ma, t EF =300 min, ph=3.0 and [NaSO 4 ]=0.05M) (I=250 ma, [Dye] 0 =100 mg/l and ph=3.0) 3.4. Influence of electrolyte type on the efficiency of dye removal For studying of the influence of electrolyte type, four different salts CaCO 3, Na 2 SO 4, Na 2 SO 3 and MgSO 4 with concentration of 0.05 M were employed (Fig. 6). According to the obtained results, electrolyte CaCO 3 was not suitable. However, Na 2 SO 4 electrolyte was chosen because MgSO 4 electrolyte was precipitating to the cathode electrode surface and with Na 2 SO 3 electrolyte, the initial ph of dye solution was around 8.90. 3.5. Influence of initial ph on the efficiency of dye removal The initial ph of solutions was selected 3.0 as the optimum ph to carry out Fenton s reaction (Eq. 2), according to several studies on electro-fenton process [4]. It is maintained about 2.8-3 during the treatment (H + consumed by Eq. 1 being compensated by oxidation of water at the anode: 2H 2 O O 2 + 4H + + 4e ). To clarify the effect of ph, dye solution with initial ph of 2.0, 3.0, 4.5 and 6.0 were electrolyzed at CP-CNT electrode and the results are illustrated in Fig. 7. For the 100 mg/l of dyestuff containing RB69, the natural ph was 3.85 and after 300 min electrolysis time for the initial ph 3.0, dye removal percent was around 90%. Fig. 7. Influence of initial ph on the dye removal (I=250 ma, [Dye] 0 =100 mg/l and [NaSO 4 ]=0.05M) Fig. 8. Influence of electrolysis time on the energy consumption (I=250 ma, [Dye] 0 =100 mg/l, ph=3.0 and [NaSO 4 ]=0.05M) 482

3.6. Electrical energy consumption Electrical energy consumption is very important economical parameter in electro-fenton process. Energy consumption amount of COD destroyed can be obtained through Eq. 6 [7, 8]: Electricalenergy consumption (kwh/kg COD IVt ) = (6) ( ΔCODV ) S where I is the average applied current (A), V is the average cell voltage (V), t is the electrolysis time (h), V s is the solution volume (dm 3 ) and COD is the decay in COD (g/dm 3 ). As can be seen in Fig. 8, electrical energy consumption for electrolysis time of 120 to 360 min was around 400 kwh/kg COD. 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, [Dye] 0 =100 mg/l and [NaSO 4 ]=0.05M) were shown in Figs. 9 and 10, respectively. As can be seen in Fig. 9, COD decay percent at 300 min electrolysis time was 70%. According to the Fig. 9, EC process caused almost complete dye removal after 360 min. Fig. 9. Influence of electrolysis time on the COD reduction. (I=250 ma, ph=3.0, [Dye] 0 =100 mg/l and [NaSO 4 ]=0.05M) 4. Conclusions Fig. 10. UV Vis spectra changes for RB69 solution by EF (I=250 ma, ph=3.0, [Dye] 0 =100 mg/l and [NaSO 4 ]=0.05M) In this work, a CP-CNT electrode was prepared as cathode and compared with carbon paper electrode. The resulting electrodes were examined by SEM image. CP-CNT electrode as cathode was used for treatment of dye solution containing Reactive Blue 69 by electro-fenton process in batch electrochemical cell. The effect of operational parameters such as applied current, initial ph, support electrolyte type and initial dye concentration was studied. Our results showed that for decolorization of 250 ml of 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 was around 400 kwh/kg COD. The reduction of COD was 70% and dye removal percent was 90%, at the optimum conditions. 5. References [1] N. Daneshvar, A. Oladegaragoze, N. Djafarzadeh, Decolorization of basic dye solutions by electrocoagulation: an investigation of the effect of operational parameters, J. Hazard. Mater. 2006, 129: 116 122. [2] A. Wang, J. Qu, J. Ru, H. Liu, J. Ge, Mineralization of an azo dye Acid Red 14 by electro-fenton s reagent using an activated carbon fiber cathode, Dyes Pigments, 2005, 65: 227-233. [3] S. Hammami, N. Oturan, N. Bellakhal, M. Dachraoui, M. A. Oturan, Oxidative degradation of direct orange 61 by electro-fenton process using a carbon felt electrode: Application of the experimental design methodology, J. Electroanal. Chem., 2007, 610: 75 84. [4] E. Brillas, I. Sires, M. A. Oturan, Electro-Fenton process and related electrochemical technologies based on Fenton s reaction chemistry, Chem. Rev. 2009,109: 6570 6631. [5] A. K. Abdessalem, N. Bellakhal, N. Oturan, M. Dachraoui, M. A. Oturan, Treatment of a mixture of three pesticides by photo- and electro-fenton processes, Desalination, 2010, 250: 450 455. 483

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