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Improved azo dye decolorization in a modified sleeve-type bioelectrochemical system
Bioresource Technology 143 (2013) 669–673
Contents lists available at SciVerse ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Short Communication
Improved azo dye decolorization in a modified sleeve-type bioelectrochemical system Fanying Kong a, Aijie Wang a,⇑, Bin Liang a, Wenzong Liu b, Haoyi Cheng a a b
State Key Laboratory of Urban Water Resource and Environment, School of Municipal and Environmental Engineering, Harbin Institute of Technology, Harbin 150090, China Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China
h i g h l i g h t s Azo dye was decolorizated in sleeve-type BES for the first time. The compact structure configuration showed an almost complete decolorization. EIS analysis demonstrated the decrease in internal resistance of sleeve-type BES. Efficient azo dye decolorization was related to the modified configuration.
a r t i c l e
i n f o
Article history: Received 9 May 2013 Received in revised form 11 June 2013 Accepted 14 June 2013 Available online 22 June 2013 Keywords: Azo dye decolorization Bioelectrochemical system (BES) Electrochemical impedance spectroscopy (EIS) Sleeve-type configuration
a b s t r a c t Bioelectrochemical system (BES) that removes recalcitrant pollutant out of wastewater is of special interest for practice. This study modified the configuration of BES to be a sleeve-type with compact structure. Azo dye (acid orange 7, AO7) in the outer cathode chamber performed a complete decolorization by electrons supplied from acetate oxidized with electricigens in the inner anode chamber. The AO7 decolorization efficiency (DEAO7) was enhanced to be higher than 98% from 0.14 to 2.00 mM. Electrochemical impedance spectroscopy (EIS) analysis showed that the internal resistance of anode, cathode and the whole cell was 26.4, 38.3, and 64.6 X, respectively, indicating that the modified configuration with large area and small distance between anode and cathode can result in a lower internal resistance and higher decolorization performance. This is the first study for azo dye decolorization using sleeve-type configuration with highly efficient decolorization by abiotic cathode BES. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction In recent years, bioelectrochemical technology has presented great potential in azo dyes decolorization due to their innovative features and environmental benefits (Fernando et al., 2012; Hou et al., 2012; Luo et al., 2011; Mu et al., 2009; Solanki et al., 2013). At the anode, substrate is oxidized by bacteria to produce protons and electrons, which are transferred to the cathode via membrane and external circuit, respectively. At the cathode, the azo bonds of dye are broken using proton and electron, resulting in the formation of colorless products (Solanki et al., 2013). However, the improvement of azo dye decolorization performance remains to be a challenging project. The highest revenue for BES application in the future can be obtained at the lowest internal resistance, thus considerable effort should be applied to the reduction of the internal resistance (Sleutels et al., 2012). Present BES reactor types vary from dual-chamber to single chamber ⇑ Corresponding author. Tel./fax: +86 451 86282195. E-mail address: [email protected] (A. Wang). 0960-8524/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biortech.2013.06.050
configurations (Fernando et al., 2012; Hou et al., 2012; Mu et al., 2009). Among the different types of reactor designs for azo dyes decolorization, the traditional dual-chamber BES suffers from high internal resistance (Dan Cui et al., 2012). Single chamber microbial fuel cells (MFCs) could perform advantageously because of less internal resistance (Hou et al., 2012; Sun et al., 2009), but be limited by the unavailable cathode role. This will make the loss of the maximum effective use of the whole BES reactor in wastewater treatment. Thus, the development of a modified configuration that exhibits not only a reduced internal resistance but also an effective use of cathode reduction is needed. Based on the above consideration, this study modified the BES configuration to be sleeve-type with an inner anode chamber and an outer cathode chamber to decolorize azo dye. The advantages of the sleeve-type BES are the reduced electrodes spacing and the large proton exchange area, both of which would lead to a lower internal resistance and therefore an improved system performance (Clauwaert et al., 2007; Huang et al., 2011).
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F. Kong et al. / Bioresource Technology 143 (2013) 669–673
This study aims at enhancing the electrochemical efficiency for azo dye (acid orange 7, AO7) decolorization in a modified sleevetype BES reactor. The decolorization efficiency of the sleeve-type BES was compared with the reported results using different configurations. Electrochemical impedance spectroscopy (EIS) was employed to determine the internal resistance of the sleeve-type configuration.
2. Methods 2.1. Construction of the sleeve-type bioelectrochemical reactor The BES reactor was constructed as sleeve-type (Fig. 1) (inner tube: ID 5 cm H 10 cm; outer tube: ID 8 cm H 10 cm) using Perspex with an inner anode chamber and an outer cathode chamber separated by a cation exchange membrane (Ultrex CMI7000, Membranes International Inc., USA). The inner tube was full of holes. The working volumes for cathode and anode chamber were both 200 mL plus a certain headspace after installation of anode and cathode electrodes. Both anode and cathode electrodes were carbon brush (ID 3 cm L 8 cm) made of carbon fibers (Jilin Carbon Plant, China). The anode, cathode and the reference electrode were connected to a data acquisition (Keithley 2700, Keithley Co. Ltd., USA) with external resistance of 10 X.
2.3. Analytical methods The AO7 concentrations were measured at kmax 484 nm by UV–vis scanning spectrophotometer (Shimadzu UV2550, Japan). After filtering through a 0.45 lm filter, the reduced products were identified and quantified using high performance liquid chromatography (HPLC, model e2695, Waters Co., USA) performed with a C18 column (5 lm; 5 mm 250 mm, Waters Co., USA) as previously described (Mu et al., 2009). AO7 decolorization efficiency (DEAO7) was calculated according to (Fernando et al., 2012). The kinetic of AO7 decolorization was modeled using first order kinetic model C = C0 ekt, where C is AO7 concentration (mM) at time t (h), and C0 is the initial AO7 concentration (mM). The kinetic of major product (sulfanilic acid, SA) was assumed to model as C 0 ¼ a ð1 C 00 ekt Þ, where C0 is the SA concentration (mM) at time t (h), and C 00 is the maximal SA concentration (mM). Cyclic Voltammetry (CV) was measured as described previously with saturated calomel reference electrode (SCE, model-217, Shanghai Precise. Sci. Instru. Co., Ltd., China; 0.247 V vs. standard hydrogen electrode, SHE) as the reference electrode (Wang et al., 2011). Electrochemistry impedance spectroscopy (EIS) measurements were carried at open circuit condition in a frequency range of 0.01–105 Hz with a perturbation signal of 10 mV using electrochemical workstation (model-660D, CH Instruments Inc., U.S.). 3. Results and discussion
2.2. Operational conditions 3.1. The sleeve-type BES performance in AO7 decolorization Sodium acetate was used as sole electron donor for anodophilic bacteria in the anode, and azo dye, acid orange 7 (AO7) was used as electron accepter in the cathode. The anolyte compositions and preparation process was described as previously (Wang et al., 2011). The catholyte was AO7 (from 0.14 to 2.00 mM) and PBS (50 mM, pH 7.0). In order to investigate the decolorization performance of the sleeve-type reactor in good comparison with the other configurations reported with respect to AO7 decolorization in BES, the AO7 concentration of 0.14 mM (50 mg L1) (Luo et al., 2011), 0.20 mM (70 mg L1) (Fernando et al., 2012), 0.57 mM (200 mg L1) (Fernando et al., 2012), 0.71 mM (250 mg L1) (Mu et al., 2009), 1.00 mM (350 mg L1) (Fernando et al., 2012) were chosen here. The concentration of 2.00 mM (700 mg L1) was added to test the tolerance of the reactor.
To improve DEAO7 in BES, the first and foremost focus should be on a high decolorization performance. Complete decolorization (over 98%) can be observed in the sleeve-type BES reactor in this study (Fig. 2A). The DEAO7 ranged from 91.55 ± 2.66% (3 h) to 91.0 ± 3.55% (12 h) as AO7 concentration increased from 0.14 to 1.00 mM. The AO7 reduction fitted first-order kinetic model very well (R2 > 0.98) with the rate constant k decreased from 0.85 to 0.21 h1 respectively. The DEAO7 and k in this study were comparable or even higher than the recent reported related researches (Table 1). It was about 9.6% increased in the sleeve-type BES (91.55 ± 2.66%) at 0.14 mM within 3 h in comparison with the DEAO7 for Fenton-like reaction at cathode in dual-chamber BES (82%) (Luo et al., 2011). The better performance than Fenton-like
Fig. 1. Construction of the sleeve-type bioelectrochemical reactor.
F. Kong et al. / Bioresource Technology 143 (2013) 669–673
AO7 concentration / mM
cathode reaction might be that the proton exchange and electron transfer were favorable for cathode reaction. The k was 0.75 to 0.21 h1 in this study, which was 1.3 to 4.2-fold higher than that with co-substrate in anode chamber (0.587 to 0.050 h1) as AO7 concentration from 0.20 to 1.00 mM (Fernando et al., 2012). The cathode chamber of sleeve-type BES showed good decolorization at high AO7 concentration (2.00 mM). The advantages of the sleeve-type BES might be due to the reduction in the distance between anode and cathode and the large proton exchange area, both of which would lower the internal resistance of BES and then improve the DEAO7. The possible mechanism was similar to the results from (Mu et al., 2009) (Fig. 1B). According to the chemical structure of AO7, it was assumed that the azo bond would be destroyed to produce sulfanilic acid (SA) and 1-amino-2-naphthol (AN). The samples
A
2.0 1.5
0.14 mM 0.20 mM 0.57 mM 0.71 mM 1.00 mM 2.00 mM
1.0 0.5 0.0 0
5
10
15
20
25
SA concentration / mM
Time / h 2.0 1.5 1.0 0.14 mM 0.20 mM 0.57 mM 0.71 mM 1.00 mM 2.00 mM
0.5 0.0 5
10
15
20
25
Current / mA
Time / h 0.10 0.08 0.06 0.04 0.02 0.00 -0.02 -0.04 -0.06 -0.08
taken from the cathode at different hours were measured using UV–vis spectrophotometer. The UV–vis absorption spectra showed that the characteristic absorbance peak of AO7 (484 nm) was significantly decreased until disappeared, while a new peak appeared simultaneously at 245 nm (data not shown). These results preliminarily indicated the cleavage of the azo bond and the formation of products at the cathode of BES. The cathode samples were further analyzed by HPLC to identify the reduced products of the AO7 decolorization based on the standard samples. Results demonstrated the formation of break down products SA and AN, and the azo double bond of AO7 was destroyed at the cathode of BES. While the AN was easily undergo autoxidation reactions even in the low amounts of oxygen (Mu et al., 2009), thus in this study the formation of SA was analyzed to calculate the product formation efficiency. The SA showed higher formation efficiencies with the increased AO7 concentration, increased from 85.71 ± 1.35% to 94.5 ± 3.63% as AO7 concentration from 0.14 to 2.00 mM, achieving over 98% AO7 decolorization simultaneously (Fig. 2B). Further evidence from cyclic voltammetry supported the reduction pathway (Fig. 2C). In the absence of AO7 (50 mM PBS), no obviously peak was observed. Upon AO7 addition, an obvious irreversible cathodic peak (Ia) appeared at 0.493 V in the first cycle, suggesting AO7 reduction at the potential of Ia, which gave rise to formation of the new products that can be oxidized in the reverse scan at 0.207 V (Ib) (AO-Cycle 1). When it was changed toward negative potentials again in the second cycle, a different CV curve with a new reduction peak Ic (0.077 V) appeared, forming quasi-reversible pair with Ib. These redox peaks (Ib and Ic) resulted from the oxidation of the amine products generated during the disruption of azo bond (Andrea Zille et al., 2004). This might be the reason that the product SA was not completely formed from AO7 decolorization. 3.2. The performance of sleeve-type BES in electrochemical characterization
B
0
671
C
Ia Ic
PBS AO-Cycle 1 AO-Cycle 2
Ib
0.6 0.4 0.2 0.0 -0.2 -0.4 -0.6 -0.8 -1.0 -1.2
Cathode potential vs. SHE / V Fig. 2. (A) Acid orange 7 (AO7) decolorization, (B) sulfanilic acid (SA) formation at different initial AO7 concentrations, and (C) Cyclic voltammograms (CV) of abiotic cathode for AO7 decolorization.
To investigate the resistance of the modified sleeve-type BES reactor, EIS was measured. The Nyquist and Bode plots of anode, cathode and the whole cell were fitted to the equivalence circuits of R(QR)(QR). There were very good uniformity between the measured and the fitted impedance data (Fig. 3). The sleeve-type configuration resulted in the total internal resistances of 64.6 X, which was 32.5-fold lower than a rectangular-type dual-chamber BES with the same operation conditions as control (2100 X, data not shown), and was 11.5-fold lower than the rectangular-type biocathode dual-chamber BES (744 X) for 50 mg L1 alizarin yellow R (AYR) decolorization (Dan Cui et al., 2012). It suggested that the total resistance was much lower with the modified sleeve-type BES compared with the rectangular-type dual-chamber BES. This was consistent with the azo dye decolorization results, suggesting that the sleeve-type configuration was superior to the traditional dual chamber configurations both in the low internal resistance and high azo dye decolorization. Three contributions, ohmic resistance (Rohm), charge transfer resistance (Rct), and diffusion resistance (Rdiff) obtained from the Nyquist plot were analyzed to evaluate the EIS characterization of the sleeve-type BES and find out the interaction between internal resistance and decolorization process in the modified BES. Rohm for anode, cathode and whole cell was 5.8, 4.5 and 11.1 X, the corresponding Rct was 9.2, 19.5 and 32.9 X, and Rdiff was 11.4, 14.3 and 20.6 X, respectively, resulting in the total resistances of 26.4, 38.3 and 64.6 X for anode, cathode and whole cell respectively with the sleeve-type configuration (Fig. 3A). The cathode performed low resistance, and it implied that the modified sleeve-type configuration might lead to an increased decolorization rate of AO7 as the larger active surface area between anode and cathode.
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Table 1 Comparison of AO7 decolorization with different initial AO7 concentrations in different reactor types. AO7 conc.
Reactor type (working chamber)
Condition 1
1
DEAO7 (time)
k (h1)
Reference
82% (3 h) 42% (60 h) 91.55 ± 2.66% (3 h)
– – 0.85
Luo et al., 2011 Luo et al., 2011 This study
0.14 mM
Dual chamber (cathode) Dual chamber (cathode) Sleeve-type (cathode)
1 g L FeVO4, 20 g L 0.14 mM AO7 0.14 mM AO7
0.20 mM
H-type (anode) Sleeve-type (cathode)
Shewanella oneidensis, co-substrate pyruvate, 0.20 mM AO7 0.20 mM AO7
– 99.00 ± 1.95% (6 h)
0.587 0.75
Fernando et al., 2012 This study
0.57 mM
H-type (anode) Sleeve-type (cathode)
Shewanella oneidensis, co-substrate pyruvate, 0.57 mM AO7 0.57 mM AO7
– 96.95 ± 2.82% (8 h)
0.352 0.43
Fernando et al., 2012 This study
0.71 mM
Dual chamber (cathode) Sleeve-type (Cathode)
0.70 mM AO7 continues 0.71 mM AO7
35.00 ± 2.00% (1.44 h) 29.16 ± 3.46% (1 h) 97.52 ± 1.87% (12 h)
– 0.30
Mu et al., 2009 This study
1.00 mM
H-type (Anode) Sleeve-type (Cathode)
Shewanella oneidensis, co-substrate pyruvate, 1.00 mM AO7 1.00 mM AO7
– 91.0 ± 3.55% (12 h)
0.05 0.21
Fernando et al., 2012 This study
2.00 mM
Sleeve-type (Cathode)
2.00 mM AO7
89.01 ± 2.34% (12 h)
0.19
This study
60
A
-Z'' / ohm
50
Na2SO4, 0.14 mM AO7
suggested that BES performance for azo dye decolorization could be further improved by modifying cathode electrode to decrease the higher Rct. Decreasing the internal resistance, especially ohmic resistance can enhance the electron transfer process (Karra et al., 2013). The sleeve-type BES performed compact structure with small distance and large area between anode and cathode, which would increase the area of proton exchange and decrease the internal resistance. Thus this indicated that the improvement of good performance in proton exchange and electron transfer process of BES reactor was prerequisite, which finally aided in increasing the azo dye wastewater treatment efficiency and stability.
Anode Cathode Whole cell
40 30 20 10 0 0
10
20
30
40
50
60
70
Z' / ohm 2.0
Log |Z|/ ohm
1.8
B
Anode Cathode Whole cell
1.6 1.4 1.2 1.0 0.8 0.6 -3
-2
-1
0
1
2
3
4
5
6
Log f / Hz Fig. 3. (A) Nyquist plots and (B) Bode plots of anode, cathode and the whole cell in the sleeve-type BES, performed with 0.14 mM AO7 and 50 mM PBS.
The frequency dependence of the impedance was shown in Bode plot, and the logarithm of the impedance modulus |Z| is plotted against the logarithm of the frequency f of the applied AC signal (Fig. 3B). The low- (0.01 Hz) and high- (105 Hz) frequency data can be easily determined from the plot, representing Rohm + Rct + Rdiff and Rohm, respectively (He and Mansfeld, 2009). The difference between the low- and high- frequency data is Rct + Rdiff, while the low frequency region of the bode plots is Rct and the high frequency region of the bode plots is Rdiff. The low frequency region of the bode plots showed that the Rct (affected by the kinetics of the electrode reactions) of the cathode was higher than that of the anode in BES, indicating that the AO7 reduction rate at cathode was slower than the acetate oxidation rate of anode (Fig. 3B). It
3.3. Potential and challenge According to previous studies, the limited factors that influence the treatment efficiency and the scale up of BES were the electrode distance, the area of membrane and the position between electrode and membrane (Davis and Higson, 2007; Oh and Logan, 2006; Sharma and Li, 2010). Studies showed that shortening the electrode distance can effectively reduce the internal resistance (Sharma and Li, 2010), increasing the relative area between the membrane and electrode can reduce the mass transfer resistance (Oh and Logan, 2006), and the reasonable arrangement is conductive to the process of amplification and practical application aims (Davis and Higson, 2007). Therefore, the development of compact structure BES with small distance between anode and cathode is critical to accelerate the popularization and application of this technology in practical refractory wastewaters. Modifying the reactor to be sleeve-type BES could effectively compress the distance between anode and cathode, further improve the effective utilization of the membrane, then reduce the electron and ion transport resistance, and finally improve the operating results of the process. This study clearly proved that the modified reactor configuration (Fig. 1) significantly reduced the internal resistance and enhanced the electron transfer reaction (Fig. 3) and finally enhanced the azo dye decolorization performance (Fig. 2). Further improvement will be realized with the low-cost electrode materials and catalysts (such as biocathode) in the electrode. The biocathode with electron transfer between cathode electrode surface and microorganisms will also be explored to the potential in azo dye decolorization using sleeve-type BES. 4. Conclusions This study clearly proved that modifying the bioelectrochemical reactor to be sleeve-type would lead to pretty superiority for azo
F. Kong et al. / Bioresource Technology 143 (2013) 669–673
dye treatment. The sleeve-type configuration with increasing area and decreasing distance between anode and cathode lowered the internal resistance, which was beneficial to the proton exchange and electron transfer process, finally contributing to the good azo dye decolorizaiton efficiency. Development of compact structure for BES with small distance between anode and cathode will be meaningful to accelerate the popularization and final application of this bioelectrochemical technology for the practical refractory wastewaters treatment. Acknowledgements This research was supported by the National Natural Science Foundation of China (NSFC, Grant No. 51078100), National Science Foundation for Distinguished Young Scholars (Grant No. 51225802), Science Fund for Creative Research Groups of the National Natural Science Foundation of China (Grant No. 51121062), and National Science Foundation for Distinguished Young Scholars (Grant No. 51225802). References Andrea Zille, P.R., Tzanko Tzanov, Roy Millward, V.A., 2004. Predicting dye biodegradation from redox potentials. Biotechnol. Prog., 1588–1592. Clauwaert, P., Rabaey, K., Aelterman, P., De Schamphelaire, L., Pham, T.H., Boeckx, P., Boon, N., Verstraete, W., 2007. Biological denitrification in microbial fuel cells. Environ. Sci. Technol. 41, 3354–3360. Dan Cui, F.Y.K., Liang, Bin, Cheng, Hao Yi, Liu, Dan, Sun, Qian, Wang, Ai Jie, 2012. Decolorization of azo dyes in dual-chamber biocatalyzed electrolysis systems seeding with enriched inoculum. J. Environ. Anal. Toxicol. S3, 1–6. Davis, F., Higson, S.P.J., 2007. Biofuel cells – recent advances and applications. Biosens. Bioelectron. 22, 1224–1235.
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Fernando, E., Keshavarz, T., Kyazze, G., 2012. Enhanced bio-decolourisation of acid orange 7 by Shewanella oneidensis through co-metabolism in a microbial fuel cell. Int. Biodeterior. Biodegrad. 72, 1–9. He, Z., Mansfeld, F., 2009. Exploring the use of electrochemical impedance spectroscopy (EIS) in microbial fuel cell studies. Energy Environ. Sci. 2, 215–219. Hou, B., Hu, Y., Sun, J., 2012. Performance and microbial diversity of microbial fuel cells coupled with different cathode types during simultaneous azo dye decolorization and electricity generation. Bioresour. Technol. 111, 105– 110. Huang, L., Chai, X., Cheng, S., Chen, G., 2011. Evaluation of carbon-based materials in tubular biocathode microbial fuel cells in terms of hexavalent chromium reduction and electricity generation. Chem. Eng. J. 166, 652–661. Karra, U., Manickam, S.S., McCutcheon, J.R., Patel, N., Li, B., 2013. Power generation and organics removal from wastewater using activated carbon nanofiber (ACNF) microbial fuel cells (MFCs). Int. J. Hydrogen Energy 38, 1588–1597. Luo, Y., Zhang, R., Liu, G., Li, J., Qin, B., Li, M., Chen, S., 2011. Simultaneous degradation of refractory contaminants in both the anode and cathode chambers of the microbial fuel cell. Bioresour. Technol. 102, 3827–3832. Mu, Y., Rabaey, K., Rozendal, R.A., Yuan, Z., Keller, J.r., 2009. Decolorization of azo dyes in bioelectrochemical systems. Environ. Sci. Technol. 43, 5137–5143. Oh, S.-E., Logan, B., 2006. Proton exchange membrane and electrode surface areas as factors that affect power generation in microbial fuel cells. Appl. Microbiol. Biotechnol. 70, 162–169. Sharma, Y., Li, B., 2010. The variation of power generation with organic substrates in single-chamber microbial fuel cells (SCMFCs). Bioresour. Technol. 101, 1844– 1850. Sleutels, T.H., Ter Heijne, A., Buisman, C.J., Hamelers, H.V., 2012. Bioelectrochemical systems: an outlook for practical applications. ChemSusChem 5, 1012–1019. Solanki, K., Subramanian, S., Basu, S., 2013. Microbial fuel cells for azo dye treatment with electricity generation: a review. Bioresour. Technol. 131, 564–571. Sun, J., Hu, Y.-y., Bi, Z., Cao, Y.-q., 2009. Simultaneous decolorization of azo dye and bioelectricity generation using a microfiltration membrane air-cathode singlechamber microbial fuel cell. Bioresour. Technol. 100, 3185–3192. Wang, A.-J., Cheng, H.-Y., Liang, B., Ren, N.-Q., Cui, D., Lin, N., Kim, B.H., Rabaey, K., 2011. Efficient reduction of nitrobenzene to aniline with a biocatalyzed cathode. Environ. Sci. Technol. 45, 10186–10193.
Contents lists available at SciVerse ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Short Communication
Improved azo dye decolorization in a modified sleeve-type bioelectrochemical system Fanying Kong a, Aijie Wang a,⇑, Bin Liang a, Wenzong Liu b, Haoyi Cheng a a b
State Key Laboratory of Urban Water Resource and Environment, School of Municipal and Environmental Engineering, Harbin Institute of Technology, Harbin 150090, China Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China
h i g h l i g h t s Azo dye was decolorizated in sleeve-type BES for the first time. The compact structure configuration showed an almost complete decolorization. EIS analysis demonstrated the decrease in internal resistance of sleeve-type BES. Efficient azo dye decolorization was related to the modified configuration.
a r t i c l e
i n f o
Article history: Received 9 May 2013 Received in revised form 11 June 2013 Accepted 14 June 2013 Available online 22 June 2013 Keywords: Azo dye decolorization Bioelectrochemical system (BES) Electrochemical impedance spectroscopy (EIS) Sleeve-type configuration
a b s t r a c t Bioelectrochemical system (BES) that removes recalcitrant pollutant out of wastewater is of special interest for practice. This study modified the configuration of BES to be a sleeve-type with compact structure. Azo dye (acid orange 7, AO7) in the outer cathode chamber performed a complete decolorization by electrons supplied from acetate oxidized with electricigens in the inner anode chamber. The AO7 decolorization efficiency (DEAO7) was enhanced to be higher than 98% from 0.14 to 2.00 mM. Electrochemical impedance spectroscopy (EIS) analysis showed that the internal resistance of anode, cathode and the whole cell was 26.4, 38.3, and 64.6 X, respectively, indicating that the modified configuration with large area and small distance between anode and cathode can result in a lower internal resistance and higher decolorization performance. This is the first study for azo dye decolorization using sleeve-type configuration with highly efficient decolorization by abiotic cathode BES. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction In recent years, bioelectrochemical technology has presented great potential in azo dyes decolorization due to their innovative features and environmental benefits (Fernando et al., 2012; Hou et al., 2012; Luo et al., 2011; Mu et al., 2009; Solanki et al., 2013). At the anode, substrate is oxidized by bacteria to produce protons and electrons, which are transferred to the cathode via membrane and external circuit, respectively. At the cathode, the azo bonds of dye are broken using proton and electron, resulting in the formation of colorless products (Solanki et al., 2013). However, the improvement of azo dye decolorization performance remains to be a challenging project. The highest revenue for BES application in the future can be obtained at the lowest internal resistance, thus considerable effort should be applied to the reduction of the internal resistance (Sleutels et al., 2012). Present BES reactor types vary from dual-chamber to single chamber ⇑ Corresponding author. Tel./fax: +86 451 86282195. E-mail address: [email protected] (A. Wang). 0960-8524/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biortech.2013.06.050
configurations (Fernando et al., 2012; Hou et al., 2012; Mu et al., 2009). Among the different types of reactor designs for azo dyes decolorization, the traditional dual-chamber BES suffers from high internal resistance (Dan Cui et al., 2012). Single chamber microbial fuel cells (MFCs) could perform advantageously because of less internal resistance (Hou et al., 2012; Sun et al., 2009), but be limited by the unavailable cathode role. This will make the loss of the maximum effective use of the whole BES reactor in wastewater treatment. Thus, the development of a modified configuration that exhibits not only a reduced internal resistance but also an effective use of cathode reduction is needed. Based on the above consideration, this study modified the BES configuration to be sleeve-type with an inner anode chamber and an outer cathode chamber to decolorize azo dye. The advantages of the sleeve-type BES are the reduced electrodes spacing and the large proton exchange area, both of which would lead to a lower internal resistance and therefore an improved system performance (Clauwaert et al., 2007; Huang et al., 2011).
670
F. Kong et al. / Bioresource Technology 143 (2013) 669–673
This study aims at enhancing the electrochemical efficiency for azo dye (acid orange 7, AO7) decolorization in a modified sleevetype BES reactor. The decolorization efficiency of the sleeve-type BES was compared with the reported results using different configurations. Electrochemical impedance spectroscopy (EIS) was employed to determine the internal resistance of the sleeve-type configuration.
2. Methods 2.1. Construction of the sleeve-type bioelectrochemical reactor The BES reactor was constructed as sleeve-type (Fig. 1) (inner tube: ID 5 cm H 10 cm; outer tube: ID 8 cm H 10 cm) using Perspex with an inner anode chamber and an outer cathode chamber separated by a cation exchange membrane (Ultrex CMI7000, Membranes International Inc., USA). The inner tube was full of holes. The working volumes for cathode and anode chamber were both 200 mL plus a certain headspace after installation of anode and cathode electrodes. Both anode and cathode electrodes were carbon brush (ID 3 cm L 8 cm) made of carbon fibers (Jilin Carbon Plant, China). The anode, cathode and the reference electrode were connected to a data acquisition (Keithley 2700, Keithley Co. Ltd., USA) with external resistance of 10 X.
2.3. Analytical methods The AO7 concentrations were measured at kmax 484 nm by UV–vis scanning spectrophotometer (Shimadzu UV2550, Japan). After filtering through a 0.45 lm filter, the reduced products were identified and quantified using high performance liquid chromatography (HPLC, model e2695, Waters Co., USA) performed with a C18 column (5 lm; 5 mm 250 mm, Waters Co., USA) as previously described (Mu et al., 2009). AO7 decolorization efficiency (DEAO7) was calculated according to (Fernando et al., 2012). The kinetic of AO7 decolorization was modeled using first order kinetic model C = C0 ekt, where C is AO7 concentration (mM) at time t (h), and C0 is the initial AO7 concentration (mM). The kinetic of major product (sulfanilic acid, SA) was assumed to model as C 0 ¼ a ð1 C 00 ekt Þ, where C0 is the SA concentration (mM) at time t (h), and C 00 is the maximal SA concentration (mM). Cyclic Voltammetry (CV) was measured as described previously with saturated calomel reference electrode (SCE, model-217, Shanghai Precise. Sci. Instru. Co., Ltd., China; 0.247 V vs. standard hydrogen electrode, SHE) as the reference electrode (Wang et al., 2011). Electrochemistry impedance spectroscopy (EIS) measurements were carried at open circuit condition in a frequency range of 0.01–105 Hz with a perturbation signal of 10 mV using electrochemical workstation (model-660D, CH Instruments Inc., U.S.). 3. Results and discussion
2.2. Operational conditions 3.1. The sleeve-type BES performance in AO7 decolorization Sodium acetate was used as sole electron donor for anodophilic bacteria in the anode, and azo dye, acid orange 7 (AO7) was used as electron accepter in the cathode. The anolyte compositions and preparation process was described as previously (Wang et al., 2011). The catholyte was AO7 (from 0.14 to 2.00 mM) and PBS (50 mM, pH 7.0). In order to investigate the decolorization performance of the sleeve-type reactor in good comparison with the other configurations reported with respect to AO7 decolorization in BES, the AO7 concentration of 0.14 mM (50 mg L1) (Luo et al., 2011), 0.20 mM (70 mg L1) (Fernando et al., 2012), 0.57 mM (200 mg L1) (Fernando et al., 2012), 0.71 mM (250 mg L1) (Mu et al., 2009), 1.00 mM (350 mg L1) (Fernando et al., 2012) were chosen here. The concentration of 2.00 mM (700 mg L1) was added to test the tolerance of the reactor.
To improve DEAO7 in BES, the first and foremost focus should be on a high decolorization performance. Complete decolorization (over 98%) can be observed in the sleeve-type BES reactor in this study (Fig. 2A). The DEAO7 ranged from 91.55 ± 2.66% (3 h) to 91.0 ± 3.55% (12 h) as AO7 concentration increased from 0.14 to 1.00 mM. The AO7 reduction fitted first-order kinetic model very well (R2 > 0.98) with the rate constant k decreased from 0.85 to 0.21 h1 respectively. The DEAO7 and k in this study were comparable or even higher than the recent reported related researches (Table 1). It was about 9.6% increased in the sleeve-type BES (91.55 ± 2.66%) at 0.14 mM within 3 h in comparison with the DEAO7 for Fenton-like reaction at cathode in dual-chamber BES (82%) (Luo et al., 2011). The better performance than Fenton-like
Fig. 1. Construction of the sleeve-type bioelectrochemical reactor.
F. Kong et al. / Bioresource Technology 143 (2013) 669–673
AO7 concentration / mM
cathode reaction might be that the proton exchange and electron transfer were favorable for cathode reaction. The k was 0.75 to 0.21 h1 in this study, which was 1.3 to 4.2-fold higher than that with co-substrate in anode chamber (0.587 to 0.050 h1) as AO7 concentration from 0.20 to 1.00 mM (Fernando et al., 2012). The cathode chamber of sleeve-type BES showed good decolorization at high AO7 concentration (2.00 mM). The advantages of the sleeve-type BES might be due to the reduction in the distance between anode and cathode and the large proton exchange area, both of which would lower the internal resistance of BES and then improve the DEAO7. The possible mechanism was similar to the results from (Mu et al., 2009) (Fig. 1B). According to the chemical structure of AO7, it was assumed that the azo bond would be destroyed to produce sulfanilic acid (SA) and 1-amino-2-naphthol (AN). The samples
A
2.0 1.5
0.14 mM 0.20 mM 0.57 mM 0.71 mM 1.00 mM 2.00 mM
1.0 0.5 0.0 0
5
10
15
20
25
SA concentration / mM
Time / h 2.0 1.5 1.0 0.14 mM 0.20 mM 0.57 mM 0.71 mM 1.00 mM 2.00 mM
0.5 0.0 5
10
15
20
25
Current / mA
Time / h 0.10 0.08 0.06 0.04 0.02 0.00 -0.02 -0.04 -0.06 -0.08
taken from the cathode at different hours were measured using UV–vis spectrophotometer. The UV–vis absorption spectra showed that the characteristic absorbance peak of AO7 (484 nm) was significantly decreased until disappeared, while a new peak appeared simultaneously at 245 nm (data not shown). These results preliminarily indicated the cleavage of the azo bond and the formation of products at the cathode of BES. The cathode samples were further analyzed by HPLC to identify the reduced products of the AO7 decolorization based on the standard samples. Results demonstrated the formation of break down products SA and AN, and the azo double bond of AO7 was destroyed at the cathode of BES. While the AN was easily undergo autoxidation reactions even in the low amounts of oxygen (Mu et al., 2009), thus in this study the formation of SA was analyzed to calculate the product formation efficiency. The SA showed higher formation efficiencies with the increased AO7 concentration, increased from 85.71 ± 1.35% to 94.5 ± 3.63% as AO7 concentration from 0.14 to 2.00 mM, achieving over 98% AO7 decolorization simultaneously (Fig. 2B). Further evidence from cyclic voltammetry supported the reduction pathway (Fig. 2C). In the absence of AO7 (50 mM PBS), no obviously peak was observed. Upon AO7 addition, an obvious irreversible cathodic peak (Ia) appeared at 0.493 V in the first cycle, suggesting AO7 reduction at the potential of Ia, which gave rise to formation of the new products that can be oxidized in the reverse scan at 0.207 V (Ib) (AO-Cycle 1). When it was changed toward negative potentials again in the second cycle, a different CV curve with a new reduction peak Ic (0.077 V) appeared, forming quasi-reversible pair with Ib. These redox peaks (Ib and Ic) resulted from the oxidation of the amine products generated during the disruption of azo bond (Andrea Zille et al., 2004). This might be the reason that the product SA was not completely formed from AO7 decolorization. 3.2. The performance of sleeve-type BES in electrochemical characterization
B
0
671
C
Ia Ic
PBS AO-Cycle 1 AO-Cycle 2
Ib
0.6 0.4 0.2 0.0 -0.2 -0.4 -0.6 -0.8 -1.0 -1.2
Cathode potential vs. SHE / V Fig. 2. (A) Acid orange 7 (AO7) decolorization, (B) sulfanilic acid (SA) formation at different initial AO7 concentrations, and (C) Cyclic voltammograms (CV) of abiotic cathode for AO7 decolorization.
To investigate the resistance of the modified sleeve-type BES reactor, EIS was measured. The Nyquist and Bode plots of anode, cathode and the whole cell were fitted to the equivalence circuits of R(QR)(QR). There were very good uniformity between the measured and the fitted impedance data (Fig. 3). The sleeve-type configuration resulted in the total internal resistances of 64.6 X, which was 32.5-fold lower than a rectangular-type dual-chamber BES with the same operation conditions as control (2100 X, data not shown), and was 11.5-fold lower than the rectangular-type biocathode dual-chamber BES (744 X) for 50 mg L1 alizarin yellow R (AYR) decolorization (Dan Cui et al., 2012). It suggested that the total resistance was much lower with the modified sleeve-type BES compared with the rectangular-type dual-chamber BES. This was consistent with the azo dye decolorization results, suggesting that the sleeve-type configuration was superior to the traditional dual chamber configurations both in the low internal resistance and high azo dye decolorization. Three contributions, ohmic resistance (Rohm), charge transfer resistance (Rct), and diffusion resistance (Rdiff) obtained from the Nyquist plot were analyzed to evaluate the EIS characterization of the sleeve-type BES and find out the interaction between internal resistance and decolorization process in the modified BES. Rohm for anode, cathode and whole cell was 5.8, 4.5 and 11.1 X, the corresponding Rct was 9.2, 19.5 and 32.9 X, and Rdiff was 11.4, 14.3 and 20.6 X, respectively, resulting in the total resistances of 26.4, 38.3 and 64.6 X for anode, cathode and whole cell respectively with the sleeve-type configuration (Fig. 3A). The cathode performed low resistance, and it implied that the modified sleeve-type configuration might lead to an increased decolorization rate of AO7 as the larger active surface area between anode and cathode.
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Table 1 Comparison of AO7 decolorization with different initial AO7 concentrations in different reactor types. AO7 conc.
Reactor type (working chamber)
Condition 1
1
DEAO7 (time)
k (h1)
Reference
82% (3 h) 42% (60 h) 91.55 ± 2.66% (3 h)
– – 0.85
Luo et al., 2011 Luo et al., 2011 This study
0.14 mM
Dual chamber (cathode) Dual chamber (cathode) Sleeve-type (cathode)
1 g L FeVO4, 20 g L 0.14 mM AO7 0.14 mM AO7
0.20 mM
H-type (anode) Sleeve-type (cathode)
Shewanella oneidensis, co-substrate pyruvate, 0.20 mM AO7 0.20 mM AO7
– 99.00 ± 1.95% (6 h)
0.587 0.75
Fernando et al., 2012 This study
0.57 mM
H-type (anode) Sleeve-type (cathode)
Shewanella oneidensis, co-substrate pyruvate, 0.57 mM AO7 0.57 mM AO7
– 96.95 ± 2.82% (8 h)
0.352 0.43
Fernando et al., 2012 This study
0.71 mM
Dual chamber (cathode) Sleeve-type (Cathode)
0.70 mM AO7 continues 0.71 mM AO7
35.00 ± 2.00% (1.44 h) 29.16 ± 3.46% (1 h) 97.52 ± 1.87% (12 h)
– 0.30
Mu et al., 2009 This study
1.00 mM
H-type (Anode) Sleeve-type (Cathode)
Shewanella oneidensis, co-substrate pyruvate, 1.00 mM AO7 1.00 mM AO7
– 91.0 ± 3.55% (12 h)
0.05 0.21
Fernando et al., 2012 This study
2.00 mM
Sleeve-type (Cathode)
2.00 mM AO7
89.01 ± 2.34% (12 h)
0.19
This study
60
A
-Z'' / ohm
50
Na2SO4, 0.14 mM AO7
suggested that BES performance for azo dye decolorization could be further improved by modifying cathode electrode to decrease the higher Rct. Decreasing the internal resistance, especially ohmic resistance can enhance the electron transfer process (Karra et al., 2013). The sleeve-type BES performed compact structure with small distance and large area between anode and cathode, which would increase the area of proton exchange and decrease the internal resistance. Thus this indicated that the improvement of good performance in proton exchange and electron transfer process of BES reactor was prerequisite, which finally aided in increasing the azo dye wastewater treatment efficiency and stability.
Anode Cathode Whole cell
40 30 20 10 0 0
10
20
30
40
50
60
70
Z' / ohm 2.0
Log |Z|/ ohm
1.8
B
Anode Cathode Whole cell
1.6 1.4 1.2 1.0 0.8 0.6 -3
-2
-1
0
1
2
3
4
5
6
Log f / Hz Fig. 3. (A) Nyquist plots and (B) Bode plots of anode, cathode and the whole cell in the sleeve-type BES, performed with 0.14 mM AO7 and 50 mM PBS.
The frequency dependence of the impedance was shown in Bode plot, and the logarithm of the impedance modulus |Z| is plotted against the logarithm of the frequency f of the applied AC signal (Fig. 3B). The low- (0.01 Hz) and high- (105 Hz) frequency data can be easily determined from the plot, representing Rohm + Rct + Rdiff and Rohm, respectively (He and Mansfeld, 2009). The difference between the low- and high- frequency data is Rct + Rdiff, while the low frequency region of the bode plots is Rct and the high frequency region of the bode plots is Rdiff. The low frequency region of the bode plots showed that the Rct (affected by the kinetics of the electrode reactions) of the cathode was higher than that of the anode in BES, indicating that the AO7 reduction rate at cathode was slower than the acetate oxidation rate of anode (Fig. 3B). It
3.3. Potential and challenge According to previous studies, the limited factors that influence the treatment efficiency and the scale up of BES were the electrode distance, the area of membrane and the position between electrode and membrane (Davis and Higson, 2007; Oh and Logan, 2006; Sharma and Li, 2010). Studies showed that shortening the electrode distance can effectively reduce the internal resistance (Sharma and Li, 2010), increasing the relative area between the membrane and electrode can reduce the mass transfer resistance (Oh and Logan, 2006), and the reasonable arrangement is conductive to the process of amplification and practical application aims (Davis and Higson, 2007). Therefore, the development of compact structure BES with small distance between anode and cathode is critical to accelerate the popularization and application of this technology in practical refractory wastewaters. Modifying the reactor to be sleeve-type BES could effectively compress the distance between anode and cathode, further improve the effective utilization of the membrane, then reduce the electron and ion transport resistance, and finally improve the operating results of the process. This study clearly proved that the modified reactor configuration (Fig. 1) significantly reduced the internal resistance and enhanced the electron transfer reaction (Fig. 3) and finally enhanced the azo dye decolorization performance (Fig. 2). Further improvement will be realized with the low-cost electrode materials and catalysts (such as biocathode) in the electrode. The biocathode with electron transfer between cathode electrode surface and microorganisms will also be explored to the potential in azo dye decolorization using sleeve-type BES. 4. Conclusions This study clearly proved that modifying the bioelectrochemical reactor to be sleeve-type would lead to pretty superiority for azo
F. Kong et al. / Bioresource Technology 143 (2013) 669–673
dye treatment. The sleeve-type configuration with increasing area and decreasing distance between anode and cathode lowered the internal resistance, which was beneficial to the proton exchange and electron transfer process, finally contributing to the good azo dye decolorizaiton efficiency. Development of compact structure for BES with small distance between anode and cathode will be meaningful to accelerate the popularization and final application of this bioelectrochemical technology for the practical refractory wastewaters treatment. Acknowledgements This research was supported by the National Natural Science Foundation of China (NSFC, Grant No. 51078100), National Science Foundation for Distinguished Young Scholars (Grant No. 51225802), Science Fund for Creative Research Groups of the National Natural Science Foundation of China (Grant No. 51121062), and National Science Foundation for Distinguished Young Scholars (Grant No. 51225802). References Andrea Zille, P.R., Tzanko Tzanov, Roy Millward, V.A., 2004. Predicting dye biodegradation from redox potentials. Biotechnol. Prog., 1588–1592. Clauwaert, P., Rabaey, K., Aelterman, P., De Schamphelaire, L., Pham, T.H., Boeckx, P., Boon, N., Verstraete, W., 2007. Biological denitrification in microbial fuel cells. Environ. Sci. Technol. 41, 3354–3360. Dan Cui, F.Y.K., Liang, Bin, Cheng, Hao Yi, Liu, Dan, Sun, Qian, Wang, Ai Jie, 2012. Decolorization of azo dyes in dual-chamber biocatalyzed electrolysis systems seeding with enriched inoculum. J. Environ. Anal. Toxicol. S3, 1–6. Davis, F., Higson, S.P.J., 2007. Biofuel cells – recent advances and applications. Biosens. Bioelectron. 22, 1224–1235.
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