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COD removal from real dyeing wastewater by electro-Fenton technology using an activated carbon fiber cathode
Desalination 253 (2010) 129–134
Contents lists available at ScienceDirect
Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
COD removal from real dyeing wastewater by electro-Fenton technology using an activated carbon fiber cathode Chih-Ta Wang a,⁎, Wei-Lung Chou b, Mei-Hui Chung a, Yi-Ming Kuo a a b
Department of Safety Health and Environmental Engineering, Chung Hwa University of Medical Technology, Tainan County 717, Taiwan Department of Safety Health and Environmental Engineering, HungKuang University, Sha-Lu, Taichung 433, Taiwan
a r t i c l e
i n f o
Article history: Received 5 September 2009 Received in revised form 12 November 2009 Accepted 13 November 2009 Available online 8 December 2009 Keywords: Real dyeing wastewater Electro-Fenton technology COD removal Hydrogen peroxide Activated carbon fiber
a b s t r a c t This study investigated the efficiency of COD removal from real dyeing wastewater by using Fe2+ in combination with electrogenerated hydrogen peroxide at the PAN (polyacrylonitrile) based activated carbon fiber cloth cathode. A systematic study of COD removal percentage was carried out at different oxygen sparging rates, applied current densities, Fe2+ concentrations, solution pHs and temperatures. The electroFenton technology can successfully remove COD from the complex real dyeing wastewater. Over 70% of COD can be removed after 240 min of treatment. The COD removal efficiency increased significantly with the oxygen sparging rates up to 150 cm3/min. The highest COD removal efficiency, 75.2%, was achieved at an applied current density of 3.2 mA/cm2; COD removal efficiency deteriorated at higher current densities because the rates of side reactions increased. The results showed that the optimal solution pH was 3. Increasing the solution pH rapidly decreased COD removal efficiency. Conversely, COD removal efficiency increased with increasing Fe2+ concentrations up to 2 mM. Temperature had a slightly negative effect on COD removal efficiency. © 2009 Elsevier B.V. All rights reserved.
1. Introduction The textile dyeing and finishing industry is a major industrial water consumer in Taiwan. However, satisfactory treatment of effluents from the dyeing industry is usually difficult because of the effluents' strong color and high COD. Although several traditional approaches—including chemical oxidation, coagulation, adsorption and biological treatment—are known to remove dye from wastewater, some limitations still exist. Recently, interest in the use of electrochemical methods to treat wastewater, such as direct or indirect electrooxidation, electroreduction, electrocoagulation and electrosorption, has been increasing [1–6]. The most efficient electrochemical degradation method proceeds by indirect or mediated oxidation of the treated pollutants. Traditional indirect oxidation deals with the use of chloride ions as redox mediators [7,8]. Although it is very effective in treating some organics and inorganics, some toxic substances likely formed during the initial treatment stage. However, these chlorinated compounds in the aqueous solution can be completely mineralized (electrochemical incineration) or oxidized to volatile compounds [9,10]. Recently, several researchers had used the in-situ electrogenerated hydrogen peroxide to treat wastewater [11,12]. However, the oxidative power of hydrogen peroxide is not strong enough to degrade the organic
⁎ Corresponding author. Tel.: +886 6 2674567 850; fax: +886 6 2675049. E-mail address: [email protected] (C.-T. Wang). 0011-9164/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2009.11.020
pollutants in some cases. Application in conjunction with UV irradiation and a metal catalyst can effectively increase the oxidative power of hydrogen peroxide due to the production of hydroxyl free radicals. Recently, the use of hydroxyl free radicals to partially or completely destroy the organic compounds has been promising. Fenton's reagent is an acidic mixture of hydrogen peroxide and Fe2+ [13]. Hydroxyl free radicals with high oxidative power are produced in Fenton's reagent. The reaction mechanism for the formation of hydroxyl free radicals in Fenton's reagent is complex. The main reaction for producing hydroxyl free radicals may be described as [13]: 2 +
Fe
3 +
+ H2 O2 →Fe
•
−
+ OH + OH
ð1Þ
Despite the high oxidative efficiency of Fenton's reagent, its application is limited by the storage and shipment of concentrated H2O2 (aq) and the production of Fe3+ sludge. The in-situ electrochemical production of H2O2 by reduction of oxygen and regeneration of Fe2+ on a cathode in acidic media can solve this problem. In recent years, several researchers have electrochemically produced considerable amounts of hydrogen peroxide by reducing oxygen in acidic solution. The technology combining the electrogenerated hydrogen peroxide and the added Fe2+ has been called electro-Fenton technology [14–20]. Obviously, the efficiency of hydrogen peroxide production at cathodes is crucial to the electro-Fenton process [19]. Oxygen can be reduced at cathodes in two ways, yielding hydrogen
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C.-T. Wang et al. / Desalination 253 (2010) 129–134
peroxide or water. The electroreduction of oxygen following the twoor four-electron process depends strongly on cathode materials [12]. þ
−
0
O2 + 2H + 2e →H2 O2 E = 0:67 V þ
−
0
H2 O2 + 2H + 2e →2H2 O E = 1:77 V
ð2Þ ð3Þ
The carbon-based electrode including graphite [21], vitreous carbon [15], carbon fiber [22] and carbon sponge [23] and the gasdiffusion electrode [24] have been used in the production of hydrogen peroxide. Electrochemical production of hydrogen peroxide has traditionally used graphite because of its low cost. However, due to poor oxygen solubility in aqueous solutions, causing mass transfer limitation, efficiency is not high. Recently the oxygen-fed carbon-PTFE diffusion cathode has been used, and is considered to be the most effective electrode for hydrogen peroxide yield [24]. However, the surface area of gas-diffusion cathode is relatively small and thus not suitable to apply to a large volume of wastewater. In addition, the high cost and instability of the gas-diffusion electrode for long term operation are major problems [24]. Because of this, inexpensive carbon fiber with a high surface area is most attractive for producing hydrogen peroxide in industrial applications. The hydrogen peroxide electrogeneration rate significantly affects the treatment efficiency when using electro-Fenton technology. Several series processes affect the electrogeneration of hydrogen peroxide [11,12,21]. Oxygen gas must first be dissolved in solution. Second, dissolved oxygen is transferred to cathode surface, adsorbed on cathode and finally reduced electrochemically to produce hydrogen peroxides in acidic media. Possible side reactions may occur simultaneously at the cathode and thus decrease the electrogeneration rate of hydrogen peroxide [21,24]. þ
−
0
2H + 2e →H2 E = 0 V 3 +
Fe
−
2 +
+ e →Fe
0
E = 0:771 V
ð4Þ
local company and the sparging rates ranging from 50 to 250 cm3/min in this work. 2.2. Experimental setup and methods All experiments were performed in an undivided electrochemical cell equipped with two electrodes. The solution volume was 0.5 dm3. The anode was a platinum wire with a diameter of 0.05 cm. The cathode was designed as a hollow cylindrical structure, composed of one layer of PAN-based activated carbon fiber cloth with a dimension of 9 cm × 7 cm × 0.02 cm held by two plastic screens, as seen in Fig. 1. The diameter and the height of the hollow cylindrical cathode were 2.9 cm and 7 cm, respectively. The platinum wire anode was placed at the center of the hollow cylindrical cathode. One feature of this design makes the primary current or potential distribution more uniform. The oxygen gas from an oxygen cylinder was dispensed directly at the bottom of the hollow cylindrical cathode. In all experiments, the solution was stirred magnetically at a rate of 300 rpm and the temperature was kept constant with a thermostat bath. The pH of solution was measured and controlled using a pH-stat. The oxygen flow rate was controlled using a rotameter. The voltage was supplied by a DC power supply (GW, GPR-25H30D). To accurately measure the voltage and current, a voltmeter and ammeter were connected to the circuit. The COD of the wastewater were conducted using a Hach spectrophotometer (DR5000). The BET surface area (1558.1 m2/g) and pore volume (0.431 cm3/g) of activated carbon fiber were measured using a surface area analyzer (ThermoQuest, Qsurf Series). The wastewater samples used in this work were taken from a textile dyeing plant located at Rende Village in Taiwan, and then stored in a dark environment. Before conducting the electrochemical experiments, the suspended particles of colloidal ranges in the wastewater were removed by filtration with cellulose acetate filter papers (0.45 μm in pore diameter). Some characteristics of the wastewater were measured; these are given in Table 1. The treatment
ð5Þ
The electrogenerated hydrogen peroxide reacts with the ferrous ions, externally added or produced by the reduction of ferric ions or those originally present in the solution, producing hydroxyl free radicals with a high redox potential, according to Eq. (1). Most of the researchers used the electro-Fenton technology for treating synthetic wastewater with relatively less constituents and low dye concentrations. Comparatively, the studies focused on the treatment of real wastewater using electro-Fenton technology were limited. This work investigates the feasibility and efficiency of COD removal from real dyeing wastewater using the electro-Fenton technology. Hydrogen peroxide was in-situ generated at the quasithree-dimensional activated carbon fiber cloth cathode. Fe2+ was added externally. The constant-current mode was adopted to evaluate the efficiency of COD removal. We are concerned with the efficiency of the quasi-three-dimensional carbon fiber cloth cathode and the influences of several operating parameters such as the sparging oxygen rate, applied current density, solution pH, Fe2+ concentration and temperature on COD removal from real dyeing wastewater. 2. Experimental 2.1. Materials The Taiwan Carbon Technology Company, in Taiwan, supplied the PAN (polyacrylonitrile) based activated carbon fiber cloth. The platinum wire was purchased from a local metal company. All chemicals were of analytical grade and used as received without further purification. Oxygen gas (purity 99.9%) was obtained from a
Fig. 1. Schematic diagram of the experimental setup; 1: power supply; 2: activated carbon fiber cloth cathode; 3: anode; 4: magnetic stirrer; 5: gas dispenser; 6: pH-stat.
C.-T. Wang et al. / Desalination 253 (2010) 129–134 Table 1 Some characteristics of the wastewater after filtration. COD (mg dm− 3) BOD (mg dm− 3) TOC (mg dm− 3) pH Chloride concentration (mg dm− 3) Sulfate concentration (mg dm− 3) Conductivity (μs cm− 1)
1224.0 323.8 394.6 4.8 233.9 38.5 2913.5
time was 240 min in all of the experimental runs. The COD removal percentage was defined as: COD0 COD × 100% COD0
131
oxygen was higher than that by simple adsorption, oxidation with pollutants using oxygen seemed not to effectively remove the COD in the dyeing wastewater. The COD removal percentage gradually increased with time, reaching 10.9% after 240 min; however, the removal rate was still low using oxygen oxidation. Comparatively, the electro-Fenton method obtained high COD removal efficiency. The COD removal percentage was 75.2% after 240 min treatment, indicating that most of the COD was removed by the electro-Fenton process in the present study. Consequently, this carbon fiber cloth cathode proved to effectively produce hydrogen peroxide, and then react with Fe2+ to produce hydroxyl free radicals according to Eq. (1). 3.2. Effect of oxygen sparging rate
ð6Þ
where COD0 is the initial COD in the real dyeing wastewater. 3. Results and discussion 3.1. Comparison of adsorption, oxygen oxidation and electro-Fenton process With the exception of the electroreduction of oxygen to produce hydrogen peroxide, the other two processes—including the adsorption of pollutants and oxygen oxidation of pollutants—are involved in the removal of COD in the dyeing wastewater when using carbon fiber cloth as cathode in the electro-Fenton technology. To investigate the COD removal percentage using just the electro-Fenton method, the experiments on the COD removal percentages by the adsorption of pollutants on the activated carbon fiber and by the oxidation with sparged oxygen were conducted in this study. Fig. 2 shows the COD removal percentage as a function of time using pure adsorption, oxidation with oxygen and the electro-Fenton method. Fig 2 was obtained with three separate experiments. In the case of oxygen oxidation, only oxygen was sparged into the reactor without the electrodes therein. For investigating COD removal by adsorption, no oxygen was sparged into the reactor and the current was turned off. Findings showed that COD removal efficiency by simple adsorption was very poor, only reaching 2.8% after 240 min of treatment. Although the COD removal percentage by oxidation with sparged
Fig. 2. Variation of COD removal percentage with time at pH 3 and 20 °C. For electroFenton: applied current density: 3.2 mA/cm2; oxygen sparging rate: 150 cm3/min; added Fe2+: 2 mM. For oxygen oxidation: oxygen sparging rate: 150 cm3/min. For adsorption: no oxygen was purged and the current was turned off.
Fig. 3 displays the experimental results for the oxygen sparging rates ranging from 50 to 250 cm3/min. The COD removal percentages were about 59.3%, 69.5%, 75.2%, 76.3% and 77.1% for the oxygen sparging rates of 50, 100, 150, 200 and 250 cm3/min, respectively, at an applied current density of 3.2 mA/cm2. Obviously, the COD removal percentage increased with the oxygen sparging rate in the electroFenton technology because the electrogeneration of hydrogen peroxide increased. The COD removal efficiency would depend on the electrogenerated hydrogen peroxide concentration. That is, increasing the oxygen sparging rate can increase the dissolved oxygen concentration and the mass transfer rate of dissolved oxygen and finally increase the production of hydrogen peroxide. This finding is consistent with the reports from Do et al. [25] using a CSTER (continuous stirred tank electrochemical reactor) and our previous study [21]. Furthermore, the COD removal percentage only slightly increased after the oxygen sparging rate was over 150 cm3/min. The results indicated that the electrochemical kinetics of the hydrogen peroxide production controlled the COD removal rate when the oxygen sparging rate exceeded 150 cm3/min in this work. Clearly, an oxygen sparging rate of 150 cm3/min at an applied current density of 3.2 mA/cm2 was adequate for further study in the present work. 3.3. Effect of applied current density To choose the suitable applied current density for optimal COD removal, several experiments at an applied current density range from 0.8 mA/cm2 to 4.8 mA/cm2 were performed. Fig. 4 shows the
Fig. 3. COD removal percentage at various oxygen sparging rates ranging from 50 to 250 cm3/min; applied current density: 3.2 mA/cm2; pH: 3; added Fe2+: 2 mM; temperature: 20 °C.
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Fig. 4. COD removal percentage at various applied current densities; oxygen sparging rate: 150 cm3/min; pH: 3; added Fe2+: 2 mM; temperature: 20 °C.
Fig. 5. COD removal percentage at various solution pHs; applied current density: 3.2 mA/ cm2; oxygen sparging rate: 150 cm3/min; added Fe2+: 2 mM; temperature: 20 °C.
effect of applied current densities on the COD removal percentage in the wastewater. The COD removal percentages were 19.2%, 49.9%, 62.8%, 75.2%, 73.0% and 64.8% for applied current densities of 0.8, 1.6, 2.4, 3.2, 4.0 and 4.8 mA/cm2, respectively. The highest COD removal percentage was achieved at an applied current density of 3.2 mA/cm2. In the cases where less than 3.2 mA/cm2 were applied, the COD removal percentage improved with increasing applied current density, because increasing the applied current density can enlarge the production rate of hydrogen peroxide and consequently boost the concentration of hydroxyl free radicals. However, the findings further indicated that the COD removal percentage dropped when the applied current density exceeded 3.2 mA/cm2, implying that the electrogeneration rate of hydrogen peroxide decreased. Higher applied current density means higher applied voltage on the electrochemical system. Therefore, the side reaction in Eq. (3) is apt to occur at the cathode when applying a higher voltage [23]. Consequently, more electricity was wasted at applied current density greater than 3.2 mA/cm2.
pH 3 because two side reactions (Eqs. (3) and (4)) simultaneously occurred at the cathode [26]. In addition, at pH below 3, hydrogen peroxide would remain steady according to the formation of oxonium ion [26]: þ
þ
Η2 Ο2 + Η →Η3 Ο2
ð7Þ
Consequently, the optimal pH for COD removals was obtained under pH 3, reaching 75.2% after 240 min treatment, due to the combined action of H2O2 electrogeneration and Fenton chemistry. This result was consistent with that reported previously [12,26]. 3.5. Effect of initial Fe 2+ concentration Fig. 6 plots the effect of the concentration of the externally added Fe2+ on COD removal percentage. The COD removal percentage was poor (19.8%) when no Fe2+ was added to the solution, indicating the
3.4. Effect of solution pH For electro-Fenton oxidation process, the solution pH is not only a crucial factor for Fenton oxidation efficiency, but also a chief parameter for electrogeneration of H2O2. Therefore, it is important to investigate the pH effect on the COD removal percentage in this work. In traditional Fenton process, iron species begin to precipitate as ferric hydroxides at higher pH values. On the other hand, iron species form stable complexes with H2O2 at lower pH values, leading to deactivation of catalysts. Consequently, the oxidation efficiency dramatically decreases. Therefore, Fenton reaction is generally conducted in acidic solution with pH values ranging from 2 to 4. The optimal traditional Fenton treatment process has been found to be at ∼pH 3. In the aspect of H2O2 production, a low pH is theoretically favorable for the production of hydrogen peroxide because the conversion of dissolved oxygen to hydrogen peroxide consumes protons in acidic solution, as shown in Eq. (2). However, a low pH also promotes hydrogen evolution, as shown in Eq. (4), reducing the number of active sites for generating hydrogen peroxide. Therefore, an optimal solution pH might be expected in this work. The effect of the solution pH on COD removal percentage is shown in Fig. 5. When the solution pH increased from 3 to 5, the COD removal percentage significantly decreased. In addition, when keeping the solution pH at 2, the COD removal percentage was 72.4%, slightly lower than that at
Fig. 6. COD removal percentage at various Fe2+ concentrations; applied current density: 3.2 mA/cm2; oxygen sparging rate: 150 cm3/min; pH: 3; temperature: 20 °C.
C.-T. Wang et al. / Desalination 253 (2010) 129–134
oxidizing power of hydrogen peroxide was not enough to destroy large molecules, such as dyestuffs in real dyeing wastewater. Adding Fe2+ to real wastewater and combining electrogenerated hydrogen peroxide can produce hydroxyl free radicals according to Fenton chemistry. The added Fe2+ concentration was related to the amounts of hydroxyl free radicals produced, according to Eq. (1). Therefore, a series of experiments were performed to investigate the effects of different Fe2+ concentrations added to the COD removal percentage at the start of the electrolysis. In Fig. 6, the presence of Fe2+ significantly affected the COD removal percentage. The COD removal percentage markedly increased from 19.8% to 43.1% by externally adding a Fe2+ concentration of 0.33 mM. Clearly, the COD removal percentage increased with added Fe2+ concentrations up to 2 mM. In comparison with the results obtained from the treatment of simply synthetic wastewater containing only dyes, higher Fe2+ concentration was added to treat the real wastewater. This can possibly be explained by the formation of complexes of Fe2+ and Fe2+ with acids, alcohols, etc. present in the real dyeing effluent or formed during degradation [21]. In contrast, the Fe2+ concentration negatively affected the COD removal percentage when the Fe2+ concentration added was more than 2 mM. A possible explanation is that when the Fe2+ concentration is high, the Fe2+ can react with the useful hydroxyl free radicals, leading to a decrease in COD removal, according to [13]: 2 +
Fe
•
3 +
+ ΟΗ →Fe
+ ΟΗ
ð8Þ
Hence, the excess Fe2+ consumed the hydroxyl free radicals. Consequently, the COD removal percentage decreased when the Fe2+ concentration was over 2 mM in the present work. Additionally, the active sites on the cathode surface are presumably occupied by Fe3+ probably generated according to Eqs. (1) and (8), leading to the reduction of the number of effective sites on the cathode surface for producing hydrogen peroxide. Therefore, the electrogeneration rate of hydrogen peroxide and the COD removal percentage decreased. 3.6. Effect of temperature To investigate the temperature effect in the electro-Fenton process, the electrolyses were conducted with five temperatures ranging from 20 °C to 40 °C. Fig. 7 displays the effect of temperature on the COD removal percentage in this work. The result shows that the temperature negatively affected the COD removal efficiency. The
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COD removal percentages decreased from 75.2% to 68.1% as the temperature increased from 20 °C to 40 °C. Increasing the temperature would decrease the electrogeneration rate of hydrogen peroxide since the COD was removed by hydroxyl free radicals, which were essentially related to the electrogeneration rate of hydrogen peroxide. The negative effect of temperature on the COD removal percentage can be explained by the lower concentration of dissolved oxygen and the self-decomposition of hydrogen peroxide at higher temperatures. Namely, the concentration of hydrogen peroxide decreased as the temperature increased because increasing temperatures can decrease oxygen solubility in the wastewater. In addition, the rate of selfdecomposition of the hydrogen peroxide to water and oxygen increased with the temperature. A 10 °C rise resulted in a decomposition rate about 2.3 times that at the lower temperature [27]. In this respect, a lower temperature favored the electrogeneration and accumulation of hydrogen peroxide, thereby increasing the COD removal rate. 4. Conclusion This study designed a hollow cylindrical cathode composed of PANbased activated carbon fiber cloth to remove COD from real dyeing wastewater. The hydroxyl free radicals produced by the electrogenerated hydrogen peroxide combined with added Fe2+ (electro-Fenton process) was the main mechanism to removal the COD in the wastewater. The COD removal percentage increased with the oxygen sparging rate. The results indicated that the kinetics of generating hydrogen peroxide controlled the COD removal percentage when the oxygen sparging rate exceeded 150 cm3/min for the reactor configuration adopted herein. The highest COD removal percentage was achieved at an applied current density of 3.2 mA/cm2. Applying a current density greater than 3.2 mA/cm2 decreased the COD removal percentage because the rates of side reactions increased. Findings showed that the optimal solution pH for the present study was 3 and that increasing the solution pH rapidly decreased the COD removal percentage. The COD removal percentage improved with increasing Fe2+ concentration up to 2 mM. Temperature had a slightly negative effect on the COD removal percentage. COD removal percentage decreased from 75.2% to 68.1% as the temperature increased from 20 °C to 40 °C. Acknowledgement The authors would like to thank the National Science Council, Taiwan, ROC, for its financial support (under grant NSC96-2221-E273-002- MY3) of this study. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
Fig. 7. COD removal percentage at various temperatures; applied current density: 3.2 mA/cm2; oxygen sparging rate: 150 cm3/min; pH: 3; added Fe2+: 2 mM.
[17] [18]
E. Brillas, J.C. Calpe, J. Casado, Water Res. 34 (2000) 2253–2262. A.M. Palcaro, S. Palmas, F. Renoldi, M. Mascia, Electrochim. Acta 46 (2000) 389–394. G. Chen, Sep. Purif. Technol. 38 (2004) 11–41. L. Szpyrkowicz, M. Radaelli, S. Daniele, Catal. Today 100 (2005) 425–429. A.S. Koparal, Y. Yavuz, C. Gürel, Ü.B. Öğütveren, J. Hazard. Mater. 145 (2007) 100–108. C.T. Wang, W.L. Chou, Y.M. Kuo, J. Hazard. Mater. 164 (2009) 81–86. G.P.R. Malpass, D.W. Miwa, D.A. Mortari, S.A.S. Machado, A.J. Motheo, Water Res. 41 (2007) 2969–2977. J.L. Hu, W.L. Chou, C.T. Wang, Y.M. Kuo, Environ. Eng. Sci. 25 (2008) 1009–1015. M. Panizza, G. Cerisola, Electrochim. Acta 48 (2003) 1515–1519. F. Bonfatti, S. Ferro, F. Lavezzo, M. Malacarne, G. Lodi, A. DeBattisti, J. Electrochem. Soc. 147 (2000) 592–596. J.S. Do, C.P. Chen, Ind. Eng. Chem. Res. 33 (1994) 387–394. Z. Qiang, J.H. Chang, C.P. Huang, Water Res. 36 (2002) 85–94. Y.W. Kang, K. Hwang, Water Res. 34 (2000) 2786–2790. T. Harrington, D. Pletcher, J. Electrochem. Soc. 46 (1999) 2983–2989. A. Alverez-Gallegos, D. Pletcher, Electrochemi. Acta 44 (1999) 2483–2492. E. Brillas, E. Mur, R. Sauleda, L. Sanchez, J. Peral, X. Domenech, J. Casado, Appl. Catal., B Environ. 16 (1998) 31–42. A. Da, P.F. Pozzo, M. Merli, E. Petrucce, J. Appl. Electrochem. 35 (2005) 391–398. C. Flox, S. Ammar, C. Arias, E. Brillas, A.V. Vargas-Zavala, R. Abdelhedi, Appl. Catal., B Environ. 67 (2006) 93–104.
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[19] B. Gozmen, M.A. Oturan, N. Oturan, O. Erbatur, Environ. Sci. Technol. 37 (2003) 3716–3723. [20] S. Hammami, N. Oturan, N. Bellakhal, M. Dachraoui, M.A. Oturan, J. Electroanal. Chem. 660 (2007) 75–84. [21] C.T. Wang, J.L. Hu, W.L. Chou, Y.M. Kuo, J. Hazard. Mater. 152 (2008) 601–606. [22] A. Wang, J. Qu, J. Ru, H. Liu, J. Ge, Dyes Pigm. 65 (2005) 227–233.
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A. Özcan, Y. Şahin, A.S. Koparal, M.A. Oturan, J. Electroanal. Chem. 616 (2008) 71–78. A. Da, L.D.P. Pozzo, C. Merli, E. Petrucci, J. Appl. Electrochem. 35 (2005) 413–419. J.S. Do, I.Y. Chao, J. Chin. Inst. Chem. Eng. 30 (1999) 329–338. M. Zhou, Q. Yu, L. Lei, G. Barton, Sep. Purif. Technol. 57 (2007) 380–387. C.W. Jones, Applications of Hydrogen Peroxide and Derivatives, 1st Ed., MPG Books, UK, 1999.
Contents lists available at ScienceDirect
Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
COD removal from real dyeing wastewater by electro-Fenton technology using an activated carbon fiber cathode Chih-Ta Wang a,⁎, Wei-Lung Chou b, Mei-Hui Chung a, Yi-Ming Kuo a a b
Department of Safety Health and Environmental Engineering, Chung Hwa University of Medical Technology, Tainan County 717, Taiwan Department of Safety Health and Environmental Engineering, HungKuang University, Sha-Lu, Taichung 433, Taiwan
a r t i c l e
i n f o
Article history: Received 5 September 2009 Received in revised form 12 November 2009 Accepted 13 November 2009 Available online 8 December 2009 Keywords: Real dyeing wastewater Electro-Fenton technology COD removal Hydrogen peroxide Activated carbon fiber
a b s t r a c t This study investigated the efficiency of COD removal from real dyeing wastewater by using Fe2+ in combination with electrogenerated hydrogen peroxide at the PAN (polyacrylonitrile) based activated carbon fiber cloth cathode. A systematic study of COD removal percentage was carried out at different oxygen sparging rates, applied current densities, Fe2+ concentrations, solution pHs and temperatures. The electroFenton technology can successfully remove COD from the complex real dyeing wastewater. Over 70% of COD can be removed after 240 min of treatment. The COD removal efficiency increased significantly with the oxygen sparging rates up to 150 cm3/min. The highest COD removal efficiency, 75.2%, was achieved at an applied current density of 3.2 mA/cm2; COD removal efficiency deteriorated at higher current densities because the rates of side reactions increased. The results showed that the optimal solution pH was 3. Increasing the solution pH rapidly decreased COD removal efficiency. Conversely, COD removal efficiency increased with increasing Fe2+ concentrations up to 2 mM. Temperature had a slightly negative effect on COD removal efficiency. © 2009 Elsevier B.V. All rights reserved.
1. Introduction The textile dyeing and finishing industry is a major industrial water consumer in Taiwan. However, satisfactory treatment of effluents from the dyeing industry is usually difficult because of the effluents' strong color and high COD. Although several traditional approaches—including chemical oxidation, coagulation, adsorption and biological treatment—are known to remove dye from wastewater, some limitations still exist. Recently, interest in the use of electrochemical methods to treat wastewater, such as direct or indirect electrooxidation, electroreduction, electrocoagulation and electrosorption, has been increasing [1–6]. The most efficient electrochemical degradation method proceeds by indirect or mediated oxidation of the treated pollutants. Traditional indirect oxidation deals with the use of chloride ions as redox mediators [7,8]. Although it is very effective in treating some organics and inorganics, some toxic substances likely formed during the initial treatment stage. However, these chlorinated compounds in the aqueous solution can be completely mineralized (electrochemical incineration) or oxidized to volatile compounds [9,10]. Recently, several researchers had used the in-situ electrogenerated hydrogen peroxide to treat wastewater [11,12]. However, the oxidative power of hydrogen peroxide is not strong enough to degrade the organic
⁎ Corresponding author. Tel.: +886 6 2674567 850; fax: +886 6 2675049. E-mail address: [email protected] (C.-T. Wang). 0011-9164/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2009.11.020
pollutants in some cases. Application in conjunction with UV irradiation and a metal catalyst can effectively increase the oxidative power of hydrogen peroxide due to the production of hydroxyl free radicals. Recently, the use of hydroxyl free radicals to partially or completely destroy the organic compounds has been promising. Fenton's reagent is an acidic mixture of hydrogen peroxide and Fe2+ [13]. Hydroxyl free radicals with high oxidative power are produced in Fenton's reagent. The reaction mechanism for the formation of hydroxyl free radicals in Fenton's reagent is complex. The main reaction for producing hydroxyl free radicals may be described as [13]: 2 +
Fe
3 +
+ H2 O2 →Fe
•
−
+ OH + OH
ð1Þ
Despite the high oxidative efficiency of Fenton's reagent, its application is limited by the storage and shipment of concentrated H2O2 (aq) and the production of Fe3+ sludge. The in-situ electrochemical production of H2O2 by reduction of oxygen and regeneration of Fe2+ on a cathode in acidic media can solve this problem. In recent years, several researchers have electrochemically produced considerable amounts of hydrogen peroxide by reducing oxygen in acidic solution. The technology combining the electrogenerated hydrogen peroxide and the added Fe2+ has been called electro-Fenton technology [14–20]. Obviously, the efficiency of hydrogen peroxide production at cathodes is crucial to the electro-Fenton process [19]. Oxygen can be reduced at cathodes in two ways, yielding hydrogen
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C.-T. Wang et al. / Desalination 253 (2010) 129–134
peroxide or water. The electroreduction of oxygen following the twoor four-electron process depends strongly on cathode materials [12]. þ
−
0
O2 + 2H + 2e →H2 O2 E = 0:67 V þ
−
0
H2 O2 + 2H + 2e →2H2 O E = 1:77 V
ð2Þ ð3Þ
The carbon-based electrode including graphite [21], vitreous carbon [15], carbon fiber [22] and carbon sponge [23] and the gasdiffusion electrode [24] have been used in the production of hydrogen peroxide. Electrochemical production of hydrogen peroxide has traditionally used graphite because of its low cost. However, due to poor oxygen solubility in aqueous solutions, causing mass transfer limitation, efficiency is not high. Recently the oxygen-fed carbon-PTFE diffusion cathode has been used, and is considered to be the most effective electrode for hydrogen peroxide yield [24]. However, the surface area of gas-diffusion cathode is relatively small and thus not suitable to apply to a large volume of wastewater. In addition, the high cost and instability of the gas-diffusion electrode for long term operation are major problems [24]. Because of this, inexpensive carbon fiber with a high surface area is most attractive for producing hydrogen peroxide in industrial applications. The hydrogen peroxide electrogeneration rate significantly affects the treatment efficiency when using electro-Fenton technology. Several series processes affect the electrogeneration of hydrogen peroxide [11,12,21]. Oxygen gas must first be dissolved in solution. Second, dissolved oxygen is transferred to cathode surface, adsorbed on cathode and finally reduced electrochemically to produce hydrogen peroxides in acidic media. Possible side reactions may occur simultaneously at the cathode and thus decrease the electrogeneration rate of hydrogen peroxide [21,24]. þ
−
0
2H + 2e →H2 E = 0 V 3 +
Fe
−
2 +
+ e →Fe
0
E = 0:771 V
ð4Þ
local company and the sparging rates ranging from 50 to 250 cm3/min in this work. 2.2. Experimental setup and methods All experiments were performed in an undivided electrochemical cell equipped with two electrodes. The solution volume was 0.5 dm3. The anode was a platinum wire with a diameter of 0.05 cm. The cathode was designed as a hollow cylindrical structure, composed of one layer of PAN-based activated carbon fiber cloth with a dimension of 9 cm × 7 cm × 0.02 cm held by two plastic screens, as seen in Fig. 1. The diameter and the height of the hollow cylindrical cathode were 2.9 cm and 7 cm, respectively. The platinum wire anode was placed at the center of the hollow cylindrical cathode. One feature of this design makes the primary current or potential distribution more uniform. The oxygen gas from an oxygen cylinder was dispensed directly at the bottom of the hollow cylindrical cathode. In all experiments, the solution was stirred magnetically at a rate of 300 rpm and the temperature was kept constant with a thermostat bath. The pH of solution was measured and controlled using a pH-stat. The oxygen flow rate was controlled using a rotameter. The voltage was supplied by a DC power supply (GW, GPR-25H30D). To accurately measure the voltage and current, a voltmeter and ammeter were connected to the circuit. The COD of the wastewater were conducted using a Hach spectrophotometer (DR5000). The BET surface area (1558.1 m2/g) and pore volume (0.431 cm3/g) of activated carbon fiber were measured using a surface area analyzer (ThermoQuest, Qsurf Series). The wastewater samples used in this work were taken from a textile dyeing plant located at Rende Village in Taiwan, and then stored in a dark environment. Before conducting the electrochemical experiments, the suspended particles of colloidal ranges in the wastewater were removed by filtration with cellulose acetate filter papers (0.45 μm in pore diameter). Some characteristics of the wastewater were measured; these are given in Table 1. The treatment
ð5Þ
The electrogenerated hydrogen peroxide reacts with the ferrous ions, externally added or produced by the reduction of ferric ions or those originally present in the solution, producing hydroxyl free radicals with a high redox potential, according to Eq. (1). Most of the researchers used the electro-Fenton technology for treating synthetic wastewater with relatively less constituents and low dye concentrations. Comparatively, the studies focused on the treatment of real wastewater using electro-Fenton technology were limited. This work investigates the feasibility and efficiency of COD removal from real dyeing wastewater using the electro-Fenton technology. Hydrogen peroxide was in-situ generated at the quasithree-dimensional activated carbon fiber cloth cathode. Fe2+ was added externally. The constant-current mode was adopted to evaluate the efficiency of COD removal. We are concerned with the efficiency of the quasi-three-dimensional carbon fiber cloth cathode and the influences of several operating parameters such as the sparging oxygen rate, applied current density, solution pH, Fe2+ concentration and temperature on COD removal from real dyeing wastewater. 2. Experimental 2.1. Materials The Taiwan Carbon Technology Company, in Taiwan, supplied the PAN (polyacrylonitrile) based activated carbon fiber cloth. The platinum wire was purchased from a local metal company. All chemicals were of analytical grade and used as received without further purification. Oxygen gas (purity 99.9%) was obtained from a
Fig. 1. Schematic diagram of the experimental setup; 1: power supply; 2: activated carbon fiber cloth cathode; 3: anode; 4: magnetic stirrer; 5: gas dispenser; 6: pH-stat.
C.-T. Wang et al. / Desalination 253 (2010) 129–134 Table 1 Some characteristics of the wastewater after filtration. COD (mg dm− 3) BOD (mg dm− 3) TOC (mg dm− 3) pH Chloride concentration (mg dm− 3) Sulfate concentration (mg dm− 3) Conductivity (μs cm− 1)
1224.0 323.8 394.6 4.8 233.9 38.5 2913.5
time was 240 min in all of the experimental runs. The COD removal percentage was defined as: COD0 COD × 100% COD0
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oxygen was higher than that by simple adsorption, oxidation with pollutants using oxygen seemed not to effectively remove the COD in the dyeing wastewater. The COD removal percentage gradually increased with time, reaching 10.9% after 240 min; however, the removal rate was still low using oxygen oxidation. Comparatively, the electro-Fenton method obtained high COD removal efficiency. The COD removal percentage was 75.2% after 240 min treatment, indicating that most of the COD was removed by the electro-Fenton process in the present study. Consequently, this carbon fiber cloth cathode proved to effectively produce hydrogen peroxide, and then react with Fe2+ to produce hydroxyl free radicals according to Eq. (1). 3.2. Effect of oxygen sparging rate
ð6Þ
where COD0 is the initial COD in the real dyeing wastewater. 3. Results and discussion 3.1. Comparison of adsorption, oxygen oxidation and electro-Fenton process With the exception of the electroreduction of oxygen to produce hydrogen peroxide, the other two processes—including the adsorption of pollutants and oxygen oxidation of pollutants—are involved in the removal of COD in the dyeing wastewater when using carbon fiber cloth as cathode in the electro-Fenton technology. To investigate the COD removal percentage using just the electro-Fenton method, the experiments on the COD removal percentages by the adsorption of pollutants on the activated carbon fiber and by the oxidation with sparged oxygen were conducted in this study. Fig. 2 shows the COD removal percentage as a function of time using pure adsorption, oxidation with oxygen and the electro-Fenton method. Fig 2 was obtained with three separate experiments. In the case of oxygen oxidation, only oxygen was sparged into the reactor without the electrodes therein. For investigating COD removal by adsorption, no oxygen was sparged into the reactor and the current was turned off. Findings showed that COD removal efficiency by simple adsorption was very poor, only reaching 2.8% after 240 min of treatment. Although the COD removal percentage by oxidation with sparged
Fig. 2. Variation of COD removal percentage with time at pH 3 and 20 °C. For electroFenton: applied current density: 3.2 mA/cm2; oxygen sparging rate: 150 cm3/min; added Fe2+: 2 mM. For oxygen oxidation: oxygen sparging rate: 150 cm3/min. For adsorption: no oxygen was purged and the current was turned off.
Fig. 3 displays the experimental results for the oxygen sparging rates ranging from 50 to 250 cm3/min. The COD removal percentages were about 59.3%, 69.5%, 75.2%, 76.3% and 77.1% for the oxygen sparging rates of 50, 100, 150, 200 and 250 cm3/min, respectively, at an applied current density of 3.2 mA/cm2. Obviously, the COD removal percentage increased with the oxygen sparging rate in the electroFenton technology because the electrogeneration of hydrogen peroxide increased. The COD removal efficiency would depend on the electrogenerated hydrogen peroxide concentration. That is, increasing the oxygen sparging rate can increase the dissolved oxygen concentration and the mass transfer rate of dissolved oxygen and finally increase the production of hydrogen peroxide. This finding is consistent with the reports from Do et al. [25] using a CSTER (continuous stirred tank electrochemical reactor) and our previous study [21]. Furthermore, the COD removal percentage only slightly increased after the oxygen sparging rate was over 150 cm3/min. The results indicated that the electrochemical kinetics of the hydrogen peroxide production controlled the COD removal rate when the oxygen sparging rate exceeded 150 cm3/min in this work. Clearly, an oxygen sparging rate of 150 cm3/min at an applied current density of 3.2 mA/cm2 was adequate for further study in the present work. 3.3. Effect of applied current density To choose the suitable applied current density for optimal COD removal, several experiments at an applied current density range from 0.8 mA/cm2 to 4.8 mA/cm2 were performed. Fig. 4 shows the
Fig. 3. COD removal percentage at various oxygen sparging rates ranging from 50 to 250 cm3/min; applied current density: 3.2 mA/cm2; pH: 3; added Fe2+: 2 mM; temperature: 20 °C.
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Fig. 4. COD removal percentage at various applied current densities; oxygen sparging rate: 150 cm3/min; pH: 3; added Fe2+: 2 mM; temperature: 20 °C.
Fig. 5. COD removal percentage at various solution pHs; applied current density: 3.2 mA/ cm2; oxygen sparging rate: 150 cm3/min; added Fe2+: 2 mM; temperature: 20 °C.
effect of applied current densities on the COD removal percentage in the wastewater. The COD removal percentages were 19.2%, 49.9%, 62.8%, 75.2%, 73.0% and 64.8% for applied current densities of 0.8, 1.6, 2.4, 3.2, 4.0 and 4.8 mA/cm2, respectively. The highest COD removal percentage was achieved at an applied current density of 3.2 mA/cm2. In the cases where less than 3.2 mA/cm2 were applied, the COD removal percentage improved with increasing applied current density, because increasing the applied current density can enlarge the production rate of hydrogen peroxide and consequently boost the concentration of hydroxyl free radicals. However, the findings further indicated that the COD removal percentage dropped when the applied current density exceeded 3.2 mA/cm2, implying that the electrogeneration rate of hydrogen peroxide decreased. Higher applied current density means higher applied voltage on the electrochemical system. Therefore, the side reaction in Eq. (3) is apt to occur at the cathode when applying a higher voltage [23]. Consequently, more electricity was wasted at applied current density greater than 3.2 mA/cm2.
pH 3 because two side reactions (Eqs. (3) and (4)) simultaneously occurred at the cathode [26]. In addition, at pH below 3, hydrogen peroxide would remain steady according to the formation of oxonium ion [26]: þ
þ
Η2 Ο2 + Η →Η3 Ο2
ð7Þ
Consequently, the optimal pH for COD removals was obtained under pH 3, reaching 75.2% after 240 min treatment, due to the combined action of H2O2 electrogeneration and Fenton chemistry. This result was consistent with that reported previously [12,26]. 3.5. Effect of initial Fe 2+ concentration Fig. 6 plots the effect of the concentration of the externally added Fe2+ on COD removal percentage. The COD removal percentage was poor (19.8%) when no Fe2+ was added to the solution, indicating the
3.4. Effect of solution pH For electro-Fenton oxidation process, the solution pH is not only a crucial factor for Fenton oxidation efficiency, but also a chief parameter for electrogeneration of H2O2. Therefore, it is important to investigate the pH effect on the COD removal percentage in this work. In traditional Fenton process, iron species begin to precipitate as ferric hydroxides at higher pH values. On the other hand, iron species form stable complexes with H2O2 at lower pH values, leading to deactivation of catalysts. Consequently, the oxidation efficiency dramatically decreases. Therefore, Fenton reaction is generally conducted in acidic solution with pH values ranging from 2 to 4. The optimal traditional Fenton treatment process has been found to be at ∼pH 3. In the aspect of H2O2 production, a low pH is theoretically favorable for the production of hydrogen peroxide because the conversion of dissolved oxygen to hydrogen peroxide consumes protons in acidic solution, as shown in Eq. (2). However, a low pH also promotes hydrogen evolution, as shown in Eq. (4), reducing the number of active sites for generating hydrogen peroxide. Therefore, an optimal solution pH might be expected in this work. The effect of the solution pH on COD removal percentage is shown in Fig. 5. When the solution pH increased from 3 to 5, the COD removal percentage significantly decreased. In addition, when keeping the solution pH at 2, the COD removal percentage was 72.4%, slightly lower than that at
Fig. 6. COD removal percentage at various Fe2+ concentrations; applied current density: 3.2 mA/cm2; oxygen sparging rate: 150 cm3/min; pH: 3; temperature: 20 °C.
C.-T. Wang et al. / Desalination 253 (2010) 129–134
oxidizing power of hydrogen peroxide was not enough to destroy large molecules, such as dyestuffs in real dyeing wastewater. Adding Fe2+ to real wastewater and combining electrogenerated hydrogen peroxide can produce hydroxyl free radicals according to Fenton chemistry. The added Fe2+ concentration was related to the amounts of hydroxyl free radicals produced, according to Eq. (1). Therefore, a series of experiments were performed to investigate the effects of different Fe2+ concentrations added to the COD removal percentage at the start of the electrolysis. In Fig. 6, the presence of Fe2+ significantly affected the COD removal percentage. The COD removal percentage markedly increased from 19.8% to 43.1% by externally adding a Fe2+ concentration of 0.33 mM. Clearly, the COD removal percentage increased with added Fe2+ concentrations up to 2 mM. In comparison with the results obtained from the treatment of simply synthetic wastewater containing only dyes, higher Fe2+ concentration was added to treat the real wastewater. This can possibly be explained by the formation of complexes of Fe2+ and Fe2+ with acids, alcohols, etc. present in the real dyeing effluent or formed during degradation [21]. In contrast, the Fe2+ concentration negatively affected the COD removal percentage when the Fe2+ concentration added was more than 2 mM. A possible explanation is that when the Fe2+ concentration is high, the Fe2+ can react with the useful hydroxyl free radicals, leading to a decrease in COD removal, according to [13]: 2 +
Fe
•
3 +
+ ΟΗ →Fe
+ ΟΗ
ð8Þ
Hence, the excess Fe2+ consumed the hydroxyl free radicals. Consequently, the COD removal percentage decreased when the Fe2+ concentration was over 2 mM in the present work. Additionally, the active sites on the cathode surface are presumably occupied by Fe3+ probably generated according to Eqs. (1) and (8), leading to the reduction of the number of effective sites on the cathode surface for producing hydrogen peroxide. Therefore, the electrogeneration rate of hydrogen peroxide and the COD removal percentage decreased. 3.6. Effect of temperature To investigate the temperature effect in the electro-Fenton process, the electrolyses were conducted with five temperatures ranging from 20 °C to 40 °C. Fig. 7 displays the effect of temperature on the COD removal percentage in this work. The result shows that the temperature negatively affected the COD removal efficiency. The
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COD removal percentages decreased from 75.2% to 68.1% as the temperature increased from 20 °C to 40 °C. Increasing the temperature would decrease the electrogeneration rate of hydrogen peroxide since the COD was removed by hydroxyl free radicals, which were essentially related to the electrogeneration rate of hydrogen peroxide. The negative effect of temperature on the COD removal percentage can be explained by the lower concentration of dissolved oxygen and the self-decomposition of hydrogen peroxide at higher temperatures. Namely, the concentration of hydrogen peroxide decreased as the temperature increased because increasing temperatures can decrease oxygen solubility in the wastewater. In addition, the rate of selfdecomposition of the hydrogen peroxide to water and oxygen increased with the temperature. A 10 °C rise resulted in a decomposition rate about 2.3 times that at the lower temperature [27]. In this respect, a lower temperature favored the electrogeneration and accumulation of hydrogen peroxide, thereby increasing the COD removal rate. 4. Conclusion This study designed a hollow cylindrical cathode composed of PANbased activated carbon fiber cloth to remove COD from real dyeing wastewater. The hydroxyl free radicals produced by the electrogenerated hydrogen peroxide combined with added Fe2+ (electro-Fenton process) was the main mechanism to removal the COD in the wastewater. The COD removal percentage increased with the oxygen sparging rate. The results indicated that the kinetics of generating hydrogen peroxide controlled the COD removal percentage when the oxygen sparging rate exceeded 150 cm3/min for the reactor configuration adopted herein. The highest COD removal percentage was achieved at an applied current density of 3.2 mA/cm2. Applying a current density greater than 3.2 mA/cm2 decreased the COD removal percentage because the rates of side reactions increased. Findings showed that the optimal solution pH for the present study was 3 and that increasing the solution pH rapidly decreased the COD removal percentage. The COD removal percentage improved with increasing Fe2+ concentration up to 2 mM. Temperature had a slightly negative effect on the COD removal percentage. COD removal percentage decreased from 75.2% to 68.1% as the temperature increased from 20 °C to 40 °C. Acknowledgement The authors would like to thank the National Science Council, Taiwan, ROC, for its financial support (under grant NSC96-2221-E273-002- MY3) of this study. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
Fig. 7. COD removal percentage at various temperatures; applied current density: 3.2 mA/cm2; oxygen sparging rate: 150 cm3/min; pH: 3; added Fe2+: 2 mM.
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