Decolorization of azo dyes wastewater by electrochemical oxidation Xu Hao, Yan Wei Department of Environmental Science and Engineering Xi’an Jiaotong University Xi’an, 710049, P. R. China
[email protected] Abstract—The electrochemical oxidation decolorization of the azo dye wastewater containing Acid Red G (5-acetylamino-4hydroxy-3-phenylazo-naphthalene-2,7-disulfonic sodium) was investigated. The effect of the operating parameters, such as applied voltage, dye concentration, electrolyte concentration and temperature, on the dye wastewater decolorization efficiency was analyzed. The reaction dynamics equation was obtained. The electrolysis products were characterized by GC/MS. Keywords- electrochemical oxidation; decolorization; Acid Red G
I.
INTRODUCTION
Azo dyes, constituting the largest class among the synthetic colorants, are considered as the widespread environmental pollutants associated with many important industries such as textile, food colorants, printing and cosmetic manufacturing [1]. Azo dye wastewater, which has strong color, high chemical oxygen demand (COD) and low biodegradability, causes many environmental problems. For example, they can overload self-purification mechanisms, they can reduce or prevent photosynthetic processes, and they can have toxic or carcinogenic effects on the aquatic environment. Hence, to solve these environmental problems, azo dye wastewater treatment is stringently necessary and it is urgent to develop sound and cost-effective treatment technologies to reach related standards before being discharged to the environment. Besides decreasing wastewater volume, dye removal in receiver basins before discharge is an indispensable aspect of the treatment. The classical processes of physicochemical and biological oxidation used in their degradation are not always sufficient, and so it becomes necessary to introduce new, more efficient methods [2]. Advanced oxidation process (AOP), through which the highly oxidizing species such as hydroxyl radicals are produced, can provide innovative methods for efficient treatment of wastewater, and is proved to be useful for substances resistant to conventional technologies. Various AOPs have been attempted to dyes degradation; among which electrochemical oxidation (EO) seem to be the cleanest one because it does not involve the use of any harmful chemicals but clean reagent of “electron” [3]. EO treatment of wastewater offers high removal efficiencies and has lower temperature requirements compared to non-electrochemical treatment [4-8]. In addition, it could prevent the production of unwanted side-
products and there is no need for addition of chemicals to the treated wastewaters. The aim of this work was to investigate the EO decolorization of an organic pollutant from a simulated wastewater, using the graphite as the anodes. Acid Red G (ARG, 5-acetylamino-4-hydroxy-3-phenylazo-naphthalene2,7-disulfonic sodium) was chosen as a model dye, because it was a commercially common azo dye, containing aromatic ring, naphthalene ring and sulfonic groups, making its treatment with traditional processes difficult. The factors of the main operating parameters, such as applied voltage, concentration of the azo dye, concentration of conductive supporting electrolyte and temperature, on the dye removal efficiency was analyzed. The reactive dynamics kinetics and the degradation produces were also discussed. II.
MATERIALS AND METHODS
A. Materials and chemicals Acid Red G (ARG, MW= 531 nm) was industrial dye and used without further purification. The molecular structure of the Acid Red G is illustrated in Fig 1.The synthetic wastewater was prepared by the dye (500–2000 mg.L-1) and an inert electrolyte of sodium sulfate which was the common salt used for the dyeing process. Other chemicals used as received. OH
N
NHCOCH3
N
NaO3S
SO3Na
Figure1. Chemical structure of Acid Red G azo dye
B. Equipments and procedures The experiment equipment was schematically shown in Fig2. The stainless steel reactor was used as the cathode, and the graphite was used as the anodes, respectively. The distance between the electrodes was 1.0cm. The solution’s temperature was controlled by the water bath and thermostat. The power was supplied by WYK-303B electrical source. In the process, after the synthetic dye wastewater (700ml) was added into the stainless steel reactor, the stirring and controller system was turned on. The process was carried out under a constant voltage and the electrolysis time was 6h.
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1
During the process, the wastewater sample was got every hour for the UV-vis analyses.
60
50
Absorbance
40
30
20
10
0 0
500
1000
1500
2000
-1
Concentration(mg.L )
Figure3. Dye concentration standard curve
3.0
Figure2. Schematic diagram of the experiment equipment 1-stainless steel reactor; 2-grapite anode; 3-stirrer; 4-water bath; 5-power; 6-thermostat
III.
RESULT AND DISCUSSION
A. Dye concentration standard curve The experimental results showed that the linear relationship between the absorbance (A) at 531 nm and the ARG (C) (mg.L-1) could be represented approximately by the equation: A = 0.02757C − 0.31389 (2) The concentration standard curve was shown in Fig3. The red beeline was the fitting curve, and R=0.99863. B. UV-vis absorption spectra The UV–vis absorption spectra of ARG degradation in the EO processes at typical time were investigated. As shown in Fig. 4, before treatment ARG was characterized by two bands in the visible region with the peak absorbance at 490nm560nm(azo linkage), and the other two bands in the ultraviolet region situated at around 300–330 nm(benzene ring) and 350–
Absorbance
C. Analysis The concentration of ARG aqueous solution was determined by measuring the absorbance at 531 nm with an UV–vis spectrophotometer (Agilent 8453, USA). The color removal efficiency (η) was calculated by the following formula: (1) η = (c0-ct)/c0*100% Where c0 was the initial dye concentration and ct was the remaining dye concentration at given time t. The degradation products were analyzed using a Agilent 5973/Aglient 6890 GC/MS system (Agilent 5973/Aglient 6890, Agilent, USA) with HP-5 capillary column (30 m×0.32 mm, film thickness 0.25μm). The oven temperature was initially at 50℃, rising at 5℃/min to a final temperature of 230℃. The carrier gas was helium.
2.5
0h 1h 2h 3h 4h 5h 6h
2.0
1.5
1.0
0.5
0.0 300
400
500
600
700
Wavelenghth(nm)
Figure4. The UV-vis absorption spectra of Acid Red G
370 nm(naphthalene ring) [1,8]. During treatment, the general trend was that the residual absorbency at visible light region decreased quickly as while as the absorbency at ultraviolet region, indicating that azo bond, benzene ring and naphthalene ring were destroyed. C. Operating parameters 1) Applied voltage Fig.5 shows the effect of applied voltage on decolorization efficiency. Obviously, high applied voltage promoted the color removal. This might be due to the electro generation rate of hydroxyl radical increased with the applied voltage, and thus in turn enhanced the ARG molecular degradation. At time 1h, the decolorization efficiency for voltage 3V was 43.26%, but at voltage 6V, it was 74.53%. The considerable promotion on performance in the first hour indicated that the applied voltage was a crucial parameter for the decolorization efficiency. But at time 6h, the decolorization efficiency for voltage 3V and 6V was 87.63% and 96.1%, respectively. That means if the electrolysis time is long enough, the decolorization efficiency would be high enough. These results indicated that at a low applied voltage the degradation would be cost-effective but need enough treatment time, while at a high voltage it was of highly decolorization efficiency but costly [1]. And if the voltage is excess, a large mount of the current supplied will be wasted to produce oxygen, leading to a low current efficiency
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[6]
. So according to the removal efficiency and the power consumption, the voltage 5.0V would be a good choice.
80
60
40
3V 4V 5V 6V
20
0 0
1
2
3
4
5
6
Time(h)
Figure5. Decolorization efficiency of ARG at different voltage, t=25℃, cARG=1000mg•L-1, cNa2SO4=0.2 mol•L-1
100
Decolorization efficiency(%)
80
60
40 -1
500mg.L -1 1000mg.L -1 1500mg.L -1 2000mg.L
20
0 0
1
2
3
4
5
6
Time(h)
Figure6. Decolorization efficiency of ARG at different dye concentration, t=25℃, U=5.0V, cNa2SO4=0.2 mol•L-1
Decolorization efficiency(%)
100
80
60
3) Temperature Fig.7. shows the effect of temperature on color removal. The color removal efficiency increased significantly from temperature 5℃, but when the temperature above 25℃ (the room temperature) the effect was not obvious. Temperature has little impact on electrochemical oxidation with •OH radicals [2]. So the above result might be explained that high temperature promoted the dye degradation reaction and also the generation of some hard to treatment intermediates, which resulted in competitive reactions among color removal and intermediates degradation [1]. These outcomes indicated the temperature was not a crucial parameter for the dye degradation process. On the point of view to save energy, it was better to chose 25℃ as the optimal temperature. 4) Electrolyte concentration Fig.8. shows the effect of Na2SO4 concentration on decolorization efficiency. It was observed that the removal efficiency increased with the Na2SO4 concentration increased. But there were no big difference in the investigated ranges which was similar to the literature [1]. These indicated that a small quantity of salt was needed to start the degradation but the increasing of the salt concentration would do nothing to improve the performance. 100
40 o
5C o 25 C o 35 C o 50 C
20
0 0
1
2
3
4
5
6
Time(h)
Figure7. Decolorization efficiency of ARG at different temperature, U=5.0V, cARG=1000mg•L-1,cNa2SO4=0.2 mol•L-1
Decolorization efficiency(%)
Decolorization efficiency(%)
100
the higher color removal, and that the color removal was effective in the initial period time. When the initial concentration was 500 mg•L-1, the removal efficiency was 97.98% within 6h. As the concentration increased, the removal efficiency decreased to 61.48% for that of 2000 mg•L-1. In the EO process, the electro generated hydroxyl radical is expected to be more strongly adsorbed on the anode’s surface [10]. With the increase of the initial dye concentration, much more dye molecules would be adsorbed on the surface of the electrode, which would reduce the possibility of water adsorption on the surface and thus lower the generation of hydroxyl radical. More badly, it might lead to the electrode fouling [11], and thus the performance decayed. Although the removal efficiency decreased with initial concentration, the absolute removal amount increased. When ARG concentration increased from 500 to 2000mg•L-1, the removal amount was increased from 489.91 to about 1229.6mg•L-1. So this process seemed to be a good alternative for high concentration dye wastewater pretreatment [1].
80
60
40 -1
0.01mol.L -1 0.05mol.L -1 0.1mol.L -1 0.2mol.L
20
0
2) Dye concentration Fig.6. shows the effect of initial dye concentration on color removal. It was observed that lower dye concentration led to
0
1
2
3
4
5
6
Time(h)
Figure8. Decolorization efficiency of ARG at different electrolyte concentration, t=25℃,U=5.0V, cARG=1000mg•L-1
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TABLE I. 1200
1000
Name
4
800
-1
Benzene-1,2,3,4tetraol
600
2
400
Molecular formula C6H6O4
Structure HO
ln(co/ct)
C(mg.L )
THE MAIN DEGRADATION PRODUCTS OF ARG BY EO PROCESS OH
HO
Phenol
200
OH
C6H6O
0
OH
0 0
2
4
6
Time(h)
Benzoic acid
C7H6O2
Figure9. The ct-t and ln(co/ct)-t
COOH
D. Reaction dynamics equation In order to gain the reaction dynamics equation of the graphite anode, the ct-t and -ln(ct/c0)-t was shown in Fig9. From figure we could find that the -ln(cA/c0)-t curve was more similar as the beeline. So it can presume that the decoloration reaction of the dye followed pseudo first-order kinetics. The reaction dynamics equations were ln(co/ct)=0.65271t-0.12268. E. Degradation products To further explore possible mechanisms for ARG degradation by EO processes, the degradation products were detected by GC/MS as listed in Table 1. From Table1, we can find that the most products are alcohols and acids. That can explain that why the residual solution was acidic and also demonstrate the dye molecular is destructed by the hydroxyl radical indirectly. IV.
CONCLUSIONS
Electrochemical oxidation treatment of a synthetic dye wastewater containing Acid Red G was investigated using a graphite electrode. The color removal efficiency was significantly affected by the current density, initial dye concentration and temperature. At low applied voltage, the performance would be as well as the situation at the high applied voltage if the electrolysis time was enough. The decolorization efficiency increase with the raise of temperature but decreased with the enhancement of the initial dye concentration. It seemed that the electrolyte concentration did not affect the color removal efficiency. The electrochemical degradation of ARG dye wastewater followed pseudo firstorder kinetics and the reaction dynamics equations were ln(co/ct)=0.65271t-0.12268. The most products were alcohols and acids which can demonstrate the dye molecular is degraded by the hydroxyl radical. ACKNOWLEDGMENT The authors gratefully acknowledge the financial support from the Program for New Century Excellent Talents in University (NCET-07-0683) .
1-Butanol
C4H10O
Glycerin
C3H9O3
HO HO
Acetic acid
C2H4O2
OH
CH3
OH
COOH
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