- Email: [email protected]
Enhanced degradation of azo dye by a stacked microbial fuel cell-biofilm electrode reactor coupled system
Accepted Manuscript Enhanced degradation of azo dye by a stacked microbial fuel cell-biofilm electrode reactor coupled system Xian Cao, Hui Wang, Xiao-qi Li, Zhou Fang, Xian-ning Li PII: DOI: Reference:
S0960-8524(16)31713-8 http://dx.doi.org/10.1016/j.biortech.2016.12.043 BITE 17419
To appear in:
Bioresource Technology
Received Date: Revised Date: Accepted Date:
10 November 2016 8 December 2016 11 December 2016
Please cite this article as: Cao, X., Wang, H., Li, X-q., Fang, Z., Li, X-n., Enhanced degradation of azo dye by a stacked microbial fuel cell-biofilm electrode reactor coupled system, Bioresource Technology (2016), doi: http:// dx.doi.org/10.1016/j.biortech.2016.12.043
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Enhanced degradation of azo dye by a stacked microbial fuel cell-biofilm electrode reactor coupled system Xian Cao, Hui Wang, Xiao-qi Li, Zhou Fang, Xian-ning Li* School of Energy and Environment, Southeast University, Nanjing 210096, China.
*
Corresponding author address: School of Energy and Environment, Southeast
University, Nanjing 210096, China. Tel.: +86 13776650963; fax: +86 025 83795618. E-mail addresses: [email protected] (X.-n. Li)
Corresponding Author *
Xian-ning Li.
Tel.: +86 13776650963; fax: +86 025 83795618. E-mail addresses: [email protected] Abstract In this study, a microbial fuel cell (MFC)-biofilm electrode reactor (BER) coupled system was established for degradation of the azo dye Reactive Brilliant Red X-3B. In this system, electrical energy generated by the MFC degrades the azo dye in the BER without the need for an external power supply, and the effluent from the BER was used as the inflow for the MFC, with further degradation. The results indicated that the X-3B removal efficiency was 29.87% higher using this coupled system than in a control group. Moreover, a method was developed to prevent voltage reversal in stacked MFCs. Current was the key factor influencing removal efficiency
in the BER. The X-3B degradation pathway and the types and transfer processes of intermediate products were further explored in our system coupled with gas chromatography–mass spectrometry. Keywords Azo dye; coupled system; microbial fuel cell; biofilm electrode reactor; bioelectricity generation 1.Introduction Azo dyes, aromatic compounds with one or more R-N=N-R’ groups, are among the most widely used commercial synthetic chemical dyes in the world (Fang et al., 2013b). As they are very stable in the environment and resistant to oxidation and biodegradation (Sun et al., 2009), discharge of azo dye wastewater into surface water without appropriate treatment can lead to several environmental problems (Cui et al., 2012). A series of physicochemical and biological methods have been developed for azo dye treatment (Santos et al., 2007). In recent years, bioelectrochemical systems that combine biological and electrochemical methods have been developed and employed to remove refractory substances (Kong et al., 2014; Liang et al., 2013; Pant et al., 2010; Zhang et al., 2016). Biofilm electrode reactors (BERs) have received much attention for their usefulness in the degradation of azo dyes (Cardenas-Robles et al., 2013; Shen et al., 2014; Wang et al., 2013; Zhang et al., 2013). To enhance the capabilities of BERs in removing refractory substances, optimal operational parameters have been investigated, such as
reactor materials (Kong et al., 2014), voltage application (Mu et al., 2009), co-substrates (Sun et al., 2016) and hydraulic retention time (Sun et al., 2015). Energy consumption is still an unavoidable issue, although it is lower in BERs than in traditional physicochemical technologies. Microbial fuel cells (MFCs), which use microorganisms as catalysts for oxidation and/or reduction reactions in electrodes, have shown potential for the degradation of azo dyes and for energy generation (Cao et al., 2010; Xu et al., 2016; Yadav et al., 2012; Yang et al., 2016). An MFC-BER coupled system can potentially be used for efficient degradation of azo dyes without requiring additional energy input (Sun et al., 2008; Yang et al., 2016; Zhao et al., 2012). In this coupled system, electrical energy is generated by the MFC, while the BER degrades the azo dye without requiring an external power supply. Moreover, the effluent from the BER can be used as the inflow for the MFC for further degradation, such that the BER is effectively a pretreatment that enhances azo dye degradation in the coupled system. In our previous study, we found processing was effective when the voltage in the BER was greater than 0.7 V (Liu et al., 2015). However, a single MFC typically produces an open circuit voltage (OCV) of less than 0.8 V, and a working voltage of approximately 0.5 V as a result of energy use by bacteria, electrode overpotentials and high internal resistance (Oh & Logan, 2007). A stacked MFC was built (i.e., linking MFCs together in a series) to generate the required voltage. In this study, a stacked MFC-BER-coupled bioelectrochemical system was built
for the degradation of azo dyes. Reactive Brilliant Red X-3B was chose as a model azo dye. In our system, the electrical energy generated by the stacked MFC was inputted into the BER, and the removal efficiencies of both the MFC and BER were measured. A method for preventing voltage reversal in stacked MFCs was developed. The influence of current on the removal efficiencies of the BER was also analyzed. Furthermore, the X-3B degradation pathway and the types and transfer processes of intermediate products were explored in our coupled system. 2. Materials and methods 2.1 Reactor configuration The polycarbonate plastic BER used in this study was 15 cm in diameter by 25 cm high, with water inlet at the bottom and a water outlet at the top. The cathode was composed of a stainless steel (1 mm thickness) ring and activated carbon fiber (ACF). The ACF (1.0 mm in thickness) was attached to both the inner and the outer surfaces of the stainless steel ring using conductive adhesive (Nanjing Xilite Adhesive Co., Ltd., Nanjing, China). A graphite rod (18 cm in length and 2 cm in diameter) was installed in the center of each type of reactor as an anode, with the cathode surrounding it. The cathode and anode were connected by titanium wire (1 mm in diameter). The effective working volume of the reactor was 3.3 L. A timed electric mixer provided stirring for 15 min every 6 h. X-3B artificial wastewater was pumped into the reactors continuously through the bottom water intake by a peristaltic pump (BT-100; Baoding Longer Precision Pump Co., Ltd, Baoding, China). The hydraulic
retention time was 2 days. The MFC, also made of polycarbonate plastic, was 40 cm in diameter by 50 cm high. The total liquid volume of the reactor was 12.4 L. From the bottom upward, there were four layers: a 20-cm-deep bottom gravel layer (3–6 mm diameter); a 10-cm-deep anode layer of granular activated carbon (GAC) (3–5 mm diameter, specific area 500–900 m2/g); a 20-cm-deep middle gravel layer; and a 3-mm-deep air-cathode layer of GAC. A titanium wire 1 mm in diameter led out of the anode, and epoxy was used to prevent the Titanium wire from making a direct electrical contact with the cathode electrode. The external circuit was connected to the BER unit with titanium wires. 2.2 Operation and evaluation The experiment was conducted in two stages. In the first stage, the MFC unit was coupled to the BER unit, and the electricity generated by the MFC unit was supplied to the BER unit. The coupled system was formed by connecting the cathode of a series of two MFCs to the anode of the BER unit, and the anode of a series of two MFCs to the cathode of the BER unit (Fig. 1A). The structure of the two MFCs was the same. The X-3B concentration in the effluent of the BER unit was 200 mg/L, and the chemical composition of the nutrient solution per liter was as follows: 400 mg glucose, 330 mg NaCl, 134 mg NH4Cl, 33 mg NaH2PO4, 18 mg Na2HPO4, 340 mg NaHCO3, 15 mg MgSO4·7H2O, 2 mg ZnSO4·7H2O, 2.2 mg MnSO4·H2O, 1 mg FeSO4, 0.24 mg CoCl2·6H2O, 15 mg CaCl2 and 1.17 mg (NH4)6Mo7O24·4H2O. The
effluent of the BER unit was used as the inflow for the MFC unit, after filtering out suspended impurities and adding 200 mg/L glucose as a co-substrate. In the control group, the BER and MFC units were not connected; thus, the BER and MFC units both functioned as pure open bioreactors. Another BER unit identical in setup to the experimental BER unit was connected to a DC power supply (1 V DC) and designated as the 1 V DC control group. Table 1 Resistance values situation
R1
R2
initial situation
0Ω
0Ω
“1” situation
1500Ω
500Ω
“2” situation
1000Ω
1000Ω
In the second stage, we investigated the influence of current on the BER unit. The voltage and current in the BER unit were changed by altering the series and parallel external resistances, as depicted in Fig. 1B. The resistance of the BER unit in parallel was designated R1, and that with the BER in series was designated R2. Resistance values are given in Table 1. 2.3 Analytics and calculations The X-3B decolorization products were detected by a Gas Chromatograph–Mass Spectrometer (GC–MS, Thermo Fisher Scientific Co., Ltd., USA). The capillary column is DB-5MASS (inner diameter 0.25 mm, length 30 m). High purity Helium was employed as the carrier gas at a flow rate of 1 mL/ min. The temperature program was as following: the gasification compartment temperature was firstly set to 60 ◦C for 0.5 min, and then raised linearly to 235 ◦ C at a rate of 25 ◦C/min. After
maintaining at 235 ◦ C for 2 min, it was further raised linearly to 250 ◦C at a rate of 2 ◦
C/min and then maintained at 250 ◦ C for 5 min. Finally, the temperature of
gasification component was increased linearly to 280 ◦C at a rate of 15 ◦ C/min and maintained for 5 min. The molecular weight was scanned in the range of 45 m/z–600 m/z. The retention time and sampling volume was 4 min and 1 µL, respectively. The sample was injected with no diversion. The ionization way of mass spectroscopy was electron impact (EI) with the voltage of electron multiplier at 1605 V. The temperatures of the ion source and the interface were 230 ◦C and 280 ◦C, respectively. Separated ingredients were analyzed with a reference to the NIST MS Search 2.0mass spectral library database. The analysis of DMPD used GC–MS choice ion pattern (SIM). The temperature program was the same as above. The peak areas of 93 m/z, 121 m/z, and 136 m/z were used to generate standard curve (Cao et al., 2015; Fang et al., 2016). To determine the X-3B removal efficiency, influent absorbance values (A1) and effluent absorbance values (A2) were measured with by UV–Vis Spectrophotometer (UV9100, Lab Tech Ltd, Beijing, China) under 540 nm (the maximum absorbing wavelength of RBR X-3B), and the X-3B removal efficiency (E) was calculated as shown in Eq. (1) E=(A1-A2)/A1 x 100%
(1)
The influent Chemical oxygen demand (COD) concentrations (C1) and effluent COD concentrations (C2) were measured according to standard methods (APHA, 1995). The COD removal efficiency (C) was calculated as shown in Eq. (2) C=(C1-C2)/C1 x 100%
(2)
All samples were filtered through a 0.45 lm syringe filter to remove suspended solids from the liquid media prior to the measurements (Fang et al., 2013a). The power density (W/m3) was calculated according to Eq. (3) P = IU/V
(3)
Where I is the current, U is the voltage, and V is the working volume of the anode. The internal resistance was calculated by the linear region of polarization curve (Puig et al., 2012). 3. Results 3.1 The electrical properties of the BER-MFC coupled system We first evaluated the electrical properties of the BER-MFC coupled system. The MFC units of both the experimental and control groups were run for 30 days before coupling, which stabilized the output voltage at approximately 0.40–0.50 V and the current at approximately 0.40–0.50 mA. Figs. 2A and 2B provide the voltage and current of the coupled system, respectively; the total output voltage was relatively stable within a range of 0.72–0.95 V and the current stabilized at approximately 0.20 mA. We also found that the electricity production of the two MFCs differed, at
approximately 0.52 V for MFC 1# and approximately 0.31 V for MFC 2#. A polarization curve was obtained on day 123, when the output voltage and current of the coupled system remained stable. As shown in Fig. 2C, the highest observed power density was 0.257 W/m3 at an external resistance of 1000 Ω, while the internal resistance was 1279.50 Ω. As shown in Fig. 2A, there was an obvious difference in output voltage between the two MFCs in the coupled systems. In general, voltage reversal is common in stacked MFCs. Once this occurs, undesirable electrochemical reactions can adversely affect fuel cell components (Min et al., 2005) and the activity of anode electrical bacteria. Even using the same device structure, the amounts of individually generated protons and electrons in MFCs are inconsistent as a result of differences in properties such as internal resistance and substrate consumption rate. Additionally, electrons can often be transferred between stacked MFCs, whereas protons cannot. This results in unequal numbers of protons and electrons, leading to a rise in the potential of the cathode or anode and voltage reversal. In this study, the voltage reversal phenomenon that is common in stacked MFCs did not occur. In previous studies, voltage reversal was often observed at high current densities (Aelterman et al., 2006), and a relatively large current capacity was required to prevent it (Kim et al., 2015). During the acclimatization stage in our study, single MFCs were connected to a smaller external resistance to obtain a higher current of 0.4–0.5 mA, and then the current was decreased to 0.13–0.24 mA in the coupled
MFCs. The current change in the MFCs from high to low increased the current capacity to a certain extent. Therefore, voltage reversal in stacked MFCs can be avoided by increasing the current capacity via an acclimitazation stage, in which a smaller external resistance is connected to single MFCs. 3.2 X-3B degradation in the BER-MFC coupled system Next we evaluated X-3B degradation in the BER-MFC coupled system. The average X-3B removal efficiencies of the BER and MFC units are provided in Figs. 3A and 3B. The average X-3B removal efficiencies of the BER units in the test group, 1 V DC control group and blank control group were 47.45%, 50.23% and 38.64%, respectively. The average X-3B removal efficiency in the test group reached a level similar to that in the control group: 1 V DC. The average X-3B removal efficiencies of the two MFCs were 50.37% and 50.28% in the test group and 35.93% and 34.77% in the control groups, respectively. The X-3B removal efficiency contribution rates of the BER and MFC units in the coupled system are shown in Fig. 3C. In the coupled system, the contribution rates of the BER and MFC units were 47.45% and 50.32%, respectively, for a total average removal efficiency of 97.77%. In contrast, the contribution rates of the BER and MFC units were 32.55% and 35.35%, respectively, in the control group system, giving a total average removal efficiency of 67.90%. The average X-3B removal efficiencies in the BER and MFC units increased by 14.90% and 14.97%, respectively, after coupling, improving the total removal efficiency by 29.87%.
After coupling, the electron flow between the two units created current, effectively forming a complete circuit. The stable current generated by the stacked MFCs was provided to the BER unit. The removal of azo dye X-3B is a reductive process requiring electrons from the activity of microorganisms under anaerobic conditions; thus, the supply of external electrons from the MFC unit improved the degradation efficiency of X-3B in the BER unit. Concurrently, the BER unit funtioned as external resistance, forming an effective circuit in the coupled system, so that electrons generated by microorganisms degrading glucose substrates in the anode of MFCs were able to flow faster and easier, providing more electrons to reduce X-3B in the MFC unit, thereby improving the removal efficiency. A coupled system was formed comprising two units easily connected into a circuit. The electrical energy generated by the MFC unit was provided to the BER unit, and the effluent from the BER unit was used as the inflow for the MFC unit resulting in further degradation. Without the support of any external energy source, X-3B removal efficiency was significantly improved in this coupled system. 3.3 Identification of degradation products The results of gas chromatography–mass spectrometry (GC-MS) chromatographic analysis of the effluent from the BER and MFC units in the coupled and control group systems showed that X-3B was broken down into more than 20 metabolites. The chemical formulas of the metabolites are given in Table 2. The dye degradation products from the coupled system were classified as alcohols (C6H14O2),
aldehydes (C5 H6O, C10H16O), aromatic amines (C6 H7N), hydrocarbons (C8H16, C10H18, C14H26, C17H36, C18H38), phenols (C14H22O, C15H24O, C20H34O) and esters (C18H26O4, C14H26O4). The products from the control group system were classified as aldehydes (C5H6O, C9H16O, C10H16O), aromatic amines (C6H7N, C16H14N2O2S, C16H16N2O4, C19H22N2O2, C18H31N), ketones (C8H12O), hydrocarbons (C8H16, C9H16, C10H16, C13H24, C23H48) and esters (C10H16O2, C14H26O4, C16H21NO3, C18H26O4, C19H34O2). Table 2 GC–MS analysis results of the effluent from the BER and MFC units in the coupled and control group systems Retention
Molecular
time (min)
formula
BER unit in
4.84
C5 H6O
2-Pentynal
coupled
5.93
C10H16O
2,2-Dimethyl-3,4-octadienal
system
6.18
C6H14O2
Hexylene glycol
7.85
C6 H7N
Aniline
10.62
C14 H26
3,5-Octadiene,4,5-diethyl-3.6-dimethyl
14.71
C14H22O
Phenol,2,4-bis(1,1-dimethylethyl)
15.22
C15H24O
Nonylphenol
17.68
C17 H36
Heptadecane
18.74
C18 H38
Octadecane
18.78
C18H26O4
Phthalic acid,hex-3-yl isobutyl ester
19.77
C20H34O
2,4,7,14-Tetramethyl-4-vinyl-tricyclo
MFC unit in
4.84
C5 H6O
2-Pentynal
coupled
7.59
C10 H18
Cyclopropane,tetramethylpropylidene
system
7.93
C8H16
Z-3,4,4-Trimethyl-2-pentene
10.62
C14 H26
3,5-Octadiene,4,5-diethyl-3,6-dimethyl
4.84
C5 H6O
2-Pentynal
Systems
BER unit in
Degraded products name
control group
5.55
C8H12O
5-Hexen-2-one, 5-methyl-3-methylene
system
5.93
C10H16O
2,2-Dimethyl-3,4-octadienal
6.06
C13 H24
7-Ethenyl-5-undecene
7.85
C6 H7N
Aniline
7.93
C8H16
Z-3,4,4-Trimethyl-2-pentene
11.20
C10H16O2
Cyclohex
11.36
C9H16
2,3-Dimethyl-1,3-heptadiene
12.33
C10 H16
Alloocimene
15.11
C18H31N
4-Dodecylaniline ndole-3-carboxylic acid,
17.08
C16H21NO3
5-hydroxy-2-methyl-1-(2-methylpropyl)-, ethyl ester
18.78
C18H26O4
Phthalic acid,hex-3-yl isobutyl ester
19.74
C16H16N2O4
Ethane,1,2-bis(2-methyl-5-nitrophenyl)
21.83
C23 H48
Heptadecane, 9-hexyl
23.89
C14H26O4
Hexanedioic acid,mono(2-ethylhexy)ester MFC unit in
4.84
C5 H6O
2-Pentynal
control
5.62
C9H16O
3-Nonyn-2-ol
group system
7.93
C8H16
Z-3,4,4-Trimethyl-2-pentene
10.86
C16H14N2O2S
mefenacet
12.33
C10 H16
Alloocimene
12.96
C19H22N2O2
Butanediamide,N-benzyl-N-(4-ethylphenyl)
14.92
C19H34O2
9-Octadecynoic acid, methyl ester
15.11
C18H31N
4-Dodecylaniline Indole-3-carboxylic acid,
17.08
C16H21NO3
5-hydroxy-2-methyl-1-(2-methylpropyl)-, ethyl ester
21.83
C23 H48
Heptadecane, 9-hexyl
23.89
C14H26O4
Hexanedioic acid,mono(2-ethylhexy)ester
We confirmed that X-3B was actually broken down into simple molecules in both the coupled and control group systems. These products are not harmful to the environment and can be broken down further into mineralized products such as CO2 and H2O (Khan et al., 2012; Liu et al., 2015). When comparing the intermediate products between the coupled and control group systems, the products in the control group system were more complex and diverse no matter the BER or MFC unit. The intermediate products in the coupled system, particularly in the MFC unit, were rare, and all had simple structures. The metabolites in the BER unit were not found in the MFC unit, except for C5H6O in the coupled system. However, the metabolites C5 H6O, C8H16, C10H16, C18H31N, C16H21NO3, C23H48 and C14H26O1 in the BER unit all appeared in the MFC unit in the control group system. Due to the lack of current in the control group system, the MFC unit exhibited a poor removal efficiency for X-3B degradation products, especially macromolecular products. When comparing the products of the BER and MFC unit in the coupled system, the degradation products of X-3B in the MFC unit had simpler structures with lower molecular weights. This indicated that X-3B was broken down into simple compounds in the BER unit initially, and those compounds continued to be broken down into simpler structures in the MFC. The two units exhibited not only a coupled energetic relationship but also a progressive relationship in terms of the degradation of X-3B.
3.4 Effects of current on X-3B and chemical oxygen demand (COD) removal efficiencies in the BER unit The current in the BER unit was altered by varying series and parallel resistances in connected external circuits. Fig. 4A shows the voltage and current of the BER unit, revealing that the voltage of the BER unit was stable at 0.40–0.50 V, and the current was stable at 0.40–0.50 mA in the initial state. The voltage dropped to 0.5 V and the current to 0.07 mA after the first change in resistance (Table 1). The voltage dropped to 0.1 V and the current to 0.3 mA and then trended to a certain stable level after the second change (Table 1). As shown in Fig. 4B, the average removal efficiencies of X-3B and COD initially decreased, then increased, and became stable over time. The average removal efficiency of X-3B was 44.03–60.70%, and that of COD was 73.00–88.62%. Compared with the results presented in Fig. 4A, it appears that the change in the average removal efficiencies of X-3B and COD in the BER unit exhibited almost the same trend as the change in current. This suggests that the current affected the removal efficiencies of the BER unit. As mentioned above, the removal of X-3B is a reductive process that requires electrons. Thus, an increase or decrease in the current implies an increase or decrease in the number of electrons that can be used by microorganisms to reduce X-3B, and the removal efficiencies of X-3B and COD change accordingly. 4. Conclusions We determined that a system of stacked MFCs coupled with a BER can
significantly improve the efficiency of azo dye X-3B removal. The X-3B removal efficiency in this coupled system was 29.87% greater than that in the non-coupled control group. The highest power density in the stacked MFCs was 0.257 W/m3, and the internal resistance was 1279.50 Ω. We also identified a method for preventing voltage reversal in stacked MFCs. We identified current as the key factor influencing the removal efficiency in the BER unit. The BER and MFC units showed a progressive relationship in the degradation of X-3B. Acknowledgments We thank the National Natural Science Foundation of China (21277024), the Provincial Natural Science Foundation of Jiangsu, China (BK20141330), the Fundamental Research Funds for the Central Universities and Scientific Research Foundation of Graduate School of Southeast University for financial support.
References Aelterman, P., Rabaey, K., Pham, H.T., Boon, N., Verstraete, W. 2006. Continuous electricity generation at high voltages and currents using stacked microbial fuel cells. Environmental science & technology, 40(10), 3388-3394. Cao, X., Song, H.-l., Yu, C.-y., Li, X.-n. 2015. Simultaneous degradation of toxic refractory organic pesticide and bioelectricity generation using a soil microbial fuel cell. Bioresource technology, 189, 87-93. Cao, Y., Hu, Y., Jian, S., Hou, B. 2010. Explore various co-substrates for simultaneous electricity generation and Congo red degradation in air-cathode
single-chamber microbial fuel cell. Bioelectrochemistry, 79(1), 71-76. Cardenas-Robles, A., Martinez, E., Rendon-Alcantar, I., Frontana, C., Gonzalez-Gutierrez, L. 2013. Development of an activated carbon-packed microbial bioelectrochemical system for azo dye degradation. Bioresource Technology, 127(1), 37-43. Cui, D., Guo, Y.Q., Cheng, H.Y., Liang, B., Kong, F.Y., Lee, H.S., Wang, A.J. 2012. Azo dye removal in a membrane-free up-flow biocatalyzed electrolysis reactor coupled with an aerobic bio-contact oxidation reactor. Journal of Hazardous Materials, 239-240(18), 257-264. Fang, Z., Song, H.-L., Cang, N., Li, X.-N. 2013a. Performance of microbial fuel cell coupled constructed wetland system for decolorization of azo dye and bioelectricity generation. Bioresource technology, 144, 165-171. Fang, Z., Song, H., Yu, R., Li, X. 2016. A microbial fuel cell-coupled constructed wetland promotes degradation of azo dye decolorization products. Ecological Engineering, 94, 455-463. Fang, Z., Song, H.L., Ning, C., Li, X.N. 2013b. Performance of microbial fuel cell coupled constructed wetland system for decolorization of azo dye and bioelectricity generation. Bioresource Technology, 144(6), 165-171. Khan, R., Bhawana, P., Fulekar, M.H. 2012. Microbial decolorization and degradation of synthetic dyes: a review. Reviews in Environmental Science & Bio/technology, 12(1), 75-97.
Kim, B., Lee, B.G., Kim, B.H., Chang, I.S. 2015. Assistance current effect for prevention of voltage reversal in stacked microbial fuel cell systems. ChemElectroChem, 2(5), 755-760. Kong, F., Wang, A., Ren, H.-Y. 2014. Improved 4-chlorophenol dechlorination at biocathode in bioelectrochemical system using optimized modular cathode design with composite stainless steel and carbon-based materials. Bioresource technology, 166, 252-258. Liang, B., Cheng, H.-Y., Kong, D.-Y., Gao, S.-H., Sun, F., Cui, D., Kong, F.-Y., Zhou, A.-J., Liu, W.-Z., Ren, N.-Q. 2013. Accelerated reduction of chlorinated nitroaromatic antibiotic chloramphenicol by biocathode. Environmental science & technology, 47(10), 5353-5361. Liu, S., Song, H., Wei, S., Liu, Q., Li, X., Qian, X. 2015. Effect of direct electrical stimulation on decolorization and degradation of azo dye reactive brilliant red X-3B in biofilm-electrode reactors. Biochemical Engineering Journal, 93, 294-302. Min, B., Kim, J., Oh, S., Regan, J.M., Logan, B.E. 2005. Electricity generation from swine wastewater using microbial fuel cells. Water research, 39(20), 4961-4968. Mu, Y., Rabaey, K., Rozendal, R.A., Yuan, Z., Keller, J. 2009. Decolorization of azo dyes in bioelectrochemical systems. Environmental science & technology, 43(13), 5137-5143.
Oh, S.E., Logan, B.E. 2007. Voltage reversal during microbial fuel cell stack operation. Journal of Power Sources, 167(1), 11-17. Pant, D., Bogaert, G.V., Diels, L., Vanbroekhoven, K. 2010. A review of the substrates used in microbial fuel cells (MFCs) for sustainable energy production. Bioresource Technology, 101(6), 1533-1543. Puig, S., Coma, M., Desloover, J., Boon, N., Colprim, J.s., Balaguer, M.D. 2012. Autotrophic denitrification in microbial fuel cells treating low ionic strength waters. Environmental science & technology, 46(4), 2309-2315. Santos, A.B.D., Cervantes, F.J., Lier, J.B.V. 2007. Review paper on current technologies for decolourisation of textile wastewaters: Perspectives for anaerobic biotechnology. Bioresource Technology, 98(12), 2369-85. Shen, J., Xu, X., Jiang, X., Hua, C., Zhang, L., Sun, X., Li, J., Yang, M., Wang, L. 2014. Coupling of a bioelectrochemical system for p -nitrophenol removal in an upflow anaerobic sludge blanket reactor. Water Research, 67(67C), 11-18. 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 single-chamber microbial fuel cell. Bioresource Technology, 100(13), 3185-92. Sun, M., Sheng, G.P., Zhang, L., Xia, C.R., Mu, Z.X., Liu, X.W., Wang, H.L., Yu, H.Q., Qi, R., Yu, T. 2008. An MEC-MFC-Coupled System for Biohydrogen Production from Acetate. Environmental Science & Technology, 42(21),
8095-100. Sun, Q., Li, Z.-L., Wang, Y.-Z., Yang, C.-X., Chung, J.S., Wang, A.-J. 2016. Cathodic bacterial community structure applying the different co-substrates for reductive decolorization of Alizarin Yellow R. Bioresource technology, 208, 64-72. Sun, Q., Li, Z., Wang, Y., Cui, D., Liang, B., Thangavel, S., Chung, J.S., Wang, A. 2015. A horizontal plug-flow baffled bioelectrocatalyzed reactor for the reductive decolorization of Alizarin Yellow R. Bioresource Technology, 195, 73-77. Wang, Y.Z., Wang, A.J., Liu, W.Z., Sun, Q. 2013. Enhanced azo dye removal through anode biofilm acclimation to toxicity in single-chamber biocatalyzed electrolysis system. Bioresource Technology, 142(4), 688-692. Xu, F., Mou, Z., Geng, J., Zhang, X., Li, C.Z. 2016. Azo dye decolorization by a halotolerant exoelectrogenic decolorizer isolated from marine sediment. Chemosphere, 158, 30-36. Yadav, A.K., Dash, P., Mohanty, A., Abbassi, R., Mishra, B.K. 2012. Performance assessment of innovative constructed wetland-microbial fuel cell for electricity production and dye removal. Ecological Engineering, 47(5), 126–131. Yang, L., Yang, H.Y., Shen, J.Y., Yang, M., Yu, H.Q. 2016. Enhancement of azo dye decolourization in a MFC–MEC coupled system. Bioresource Technology, 202, 93-100.
Zhang, J., Zhang, Y., Xie, Q., Chen, S., Afzal, S. 2013. Enhanced anaerobic digestion of organic contaminants containing diverse microbial population by combined microbial electrolysis cell (MEC) and anaerobic reactor under Fe(III) reducing conditions. Bioresource Technology, 136(5), 273-280. Zhang, S., Song, H.L., Yang, X.L., Yang, K.Y., Wang, X.Y. 2016. Effect of electrical stimulation on the fate of sulfamethoxazole and tetracycline with their corresponding resistance genes in three-dimensional biofilm-electrode reactors. Chemosphere, 164, 113-119. Zhao, H., Zhang, Y., Zhao, B., Chang, Y., Li, Z. 2012. Electrochemical reduction of carbon dioxide in an MFC-MEC system with a layer-by-layer self-assembly carbon nanotube/cobalt phthalocyanine modified electrode. Environmental Science & Technology, 46(9), 5198-204.
Figure 1. Configuration of the stacked microbial fuel cell-biofilm electrode reactor coupled system Figure 2. Electricity generation of MFC-BER coupled system: (A) the voltage of total output, MFC 1# and MFC 2#; (B) the current; (C) power density curve and the polarization curve Figure 3. The average X-3B removal efficiencies in BER-MFC coupled system: (A) in BER unit; (B) in MFC unit; (C) the X-3B removal efficiency contribution rates Figure 4. Electricity generation and effects on X-3B removal in BER unit: (A) the voltage and current; (B) the average removal efficiencies of X-3B and COD
Figure 1
Figure 2
Figure 3
Figure 4
Highlights •
MFC-BER coupled system was established in a simple method.
•
Electrical energy generated by the MFC was used in the BER.
•
The effluent from the BER was used as the inflow for the MFC.
•
Current was the key factor influencing removal efficiency in the BER.
S0960-8524(16)31713-8 http://dx.doi.org/10.1016/j.biortech.2016.12.043 BITE 17419
To appear in:
Bioresource Technology
Received Date: Revised Date: Accepted Date:
10 November 2016 8 December 2016 11 December 2016
Please cite this article as: Cao, X., Wang, H., Li, X-q., Fang, Z., Li, X-n., Enhanced degradation of azo dye by a stacked microbial fuel cell-biofilm electrode reactor coupled system, Bioresource Technology (2016), doi: http:// dx.doi.org/10.1016/j.biortech.2016.12.043
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Enhanced degradation of azo dye by a stacked microbial fuel cell-biofilm electrode reactor coupled system Xian Cao, Hui Wang, Xiao-qi Li, Zhou Fang, Xian-ning Li* School of Energy and Environment, Southeast University, Nanjing 210096, China.
*
Corresponding author address: School of Energy and Environment, Southeast
University, Nanjing 210096, China. Tel.: +86 13776650963; fax: +86 025 83795618. E-mail addresses: [email protected] (X.-n. Li)
Corresponding Author *
Xian-ning Li.
Tel.: +86 13776650963; fax: +86 025 83795618. E-mail addresses: [email protected] Abstract In this study, a microbial fuel cell (MFC)-biofilm electrode reactor (BER) coupled system was established for degradation of the azo dye Reactive Brilliant Red X-3B. In this system, electrical energy generated by the MFC degrades the azo dye in the BER without the need for an external power supply, and the effluent from the BER was used as the inflow for the MFC, with further degradation. The results indicated that the X-3B removal efficiency was 29.87% higher using this coupled system than in a control group. Moreover, a method was developed to prevent voltage reversal in stacked MFCs. Current was the key factor influencing removal efficiency
in the BER. The X-3B degradation pathway and the types and transfer processes of intermediate products were further explored in our system coupled with gas chromatography–mass spectrometry. Keywords Azo dye; coupled system; microbial fuel cell; biofilm electrode reactor; bioelectricity generation 1.Introduction Azo dyes, aromatic compounds with one or more R-N=N-R’ groups, are among the most widely used commercial synthetic chemical dyes in the world (Fang et al., 2013b). As they are very stable in the environment and resistant to oxidation and biodegradation (Sun et al., 2009), discharge of azo dye wastewater into surface water without appropriate treatment can lead to several environmental problems (Cui et al., 2012). A series of physicochemical and biological methods have been developed for azo dye treatment (Santos et al., 2007). In recent years, bioelectrochemical systems that combine biological and electrochemical methods have been developed and employed to remove refractory substances (Kong et al., 2014; Liang et al., 2013; Pant et al., 2010; Zhang et al., 2016). Biofilm electrode reactors (BERs) have received much attention for their usefulness in the degradation of azo dyes (Cardenas-Robles et al., 2013; Shen et al., 2014; Wang et al., 2013; Zhang et al., 2013). To enhance the capabilities of BERs in removing refractory substances, optimal operational parameters have been investigated, such as
reactor materials (Kong et al., 2014), voltage application (Mu et al., 2009), co-substrates (Sun et al., 2016) and hydraulic retention time (Sun et al., 2015). Energy consumption is still an unavoidable issue, although it is lower in BERs than in traditional physicochemical technologies. Microbial fuel cells (MFCs), which use microorganisms as catalysts for oxidation and/or reduction reactions in electrodes, have shown potential for the degradation of azo dyes and for energy generation (Cao et al., 2010; Xu et al., 2016; Yadav et al., 2012; Yang et al., 2016). An MFC-BER coupled system can potentially be used for efficient degradation of azo dyes without requiring additional energy input (Sun et al., 2008; Yang et al., 2016; Zhao et al., 2012). In this coupled system, electrical energy is generated by the MFC, while the BER degrades the azo dye without requiring an external power supply. Moreover, the effluent from the BER can be used as the inflow for the MFC for further degradation, such that the BER is effectively a pretreatment that enhances azo dye degradation in the coupled system. In our previous study, we found processing was effective when the voltage in the BER was greater than 0.7 V (Liu et al., 2015). However, a single MFC typically produces an open circuit voltage (OCV) of less than 0.8 V, and a working voltage of approximately 0.5 V as a result of energy use by bacteria, electrode overpotentials and high internal resistance (Oh & Logan, 2007). A stacked MFC was built (i.e., linking MFCs together in a series) to generate the required voltage. In this study, a stacked MFC-BER-coupled bioelectrochemical system was built
for the degradation of azo dyes. Reactive Brilliant Red X-3B was chose as a model azo dye. In our system, the electrical energy generated by the stacked MFC was inputted into the BER, and the removal efficiencies of both the MFC and BER were measured. A method for preventing voltage reversal in stacked MFCs was developed. The influence of current on the removal efficiencies of the BER was also analyzed. Furthermore, the X-3B degradation pathway and the types and transfer processes of intermediate products were explored in our coupled system. 2. Materials and methods 2.1 Reactor configuration The polycarbonate plastic BER used in this study was 15 cm in diameter by 25 cm high, with water inlet at the bottom and a water outlet at the top. The cathode was composed of a stainless steel (1 mm thickness) ring and activated carbon fiber (ACF). The ACF (1.0 mm in thickness) was attached to both the inner and the outer surfaces of the stainless steel ring using conductive adhesive (Nanjing Xilite Adhesive Co., Ltd., Nanjing, China). A graphite rod (18 cm in length and 2 cm in diameter) was installed in the center of each type of reactor as an anode, with the cathode surrounding it. The cathode and anode were connected by titanium wire (1 mm in diameter). The effective working volume of the reactor was 3.3 L. A timed electric mixer provided stirring for 15 min every 6 h. X-3B artificial wastewater was pumped into the reactors continuously through the bottom water intake by a peristaltic pump (BT-100; Baoding Longer Precision Pump Co., Ltd, Baoding, China). The hydraulic
retention time was 2 days. The MFC, also made of polycarbonate plastic, was 40 cm in diameter by 50 cm high. The total liquid volume of the reactor was 12.4 L. From the bottom upward, there were four layers: a 20-cm-deep bottom gravel layer (3–6 mm diameter); a 10-cm-deep anode layer of granular activated carbon (GAC) (3–5 mm diameter, specific area 500–900 m2/g); a 20-cm-deep middle gravel layer; and a 3-mm-deep air-cathode layer of GAC. A titanium wire 1 mm in diameter led out of the anode, and epoxy was used to prevent the Titanium wire from making a direct electrical contact with the cathode electrode. The external circuit was connected to the BER unit with titanium wires. 2.2 Operation and evaluation The experiment was conducted in two stages. In the first stage, the MFC unit was coupled to the BER unit, and the electricity generated by the MFC unit was supplied to the BER unit. The coupled system was formed by connecting the cathode of a series of two MFCs to the anode of the BER unit, and the anode of a series of two MFCs to the cathode of the BER unit (Fig. 1A). The structure of the two MFCs was the same. The X-3B concentration in the effluent of the BER unit was 200 mg/L, and the chemical composition of the nutrient solution per liter was as follows: 400 mg glucose, 330 mg NaCl, 134 mg NH4Cl, 33 mg NaH2PO4, 18 mg Na2HPO4, 340 mg NaHCO3, 15 mg MgSO4·7H2O, 2 mg ZnSO4·7H2O, 2.2 mg MnSO4·H2O, 1 mg FeSO4, 0.24 mg CoCl2·6H2O, 15 mg CaCl2 and 1.17 mg (NH4)6Mo7O24·4H2O. The
effluent of the BER unit was used as the inflow for the MFC unit, after filtering out suspended impurities and adding 200 mg/L glucose as a co-substrate. In the control group, the BER and MFC units were not connected; thus, the BER and MFC units both functioned as pure open bioreactors. Another BER unit identical in setup to the experimental BER unit was connected to a DC power supply (1 V DC) and designated as the 1 V DC control group. Table 1 Resistance values situation
R1
R2
initial situation
0Ω
0Ω
“1” situation
1500Ω
500Ω
“2” situation
1000Ω
1000Ω
In the second stage, we investigated the influence of current on the BER unit. The voltage and current in the BER unit were changed by altering the series and parallel external resistances, as depicted in Fig. 1B. The resistance of the BER unit in parallel was designated R1, and that with the BER in series was designated R2. Resistance values are given in Table 1. 2.3 Analytics and calculations The X-3B decolorization products were detected by a Gas Chromatograph–Mass Spectrometer (GC–MS, Thermo Fisher Scientific Co., Ltd., USA). The capillary column is DB-5MASS (inner diameter 0.25 mm, length 30 m). High purity Helium was employed as the carrier gas at a flow rate of 1 mL/ min. The temperature program was as following: the gasification compartment temperature was firstly set to 60 ◦C for 0.5 min, and then raised linearly to 235 ◦ C at a rate of 25 ◦C/min. After
maintaining at 235 ◦ C for 2 min, it was further raised linearly to 250 ◦C at a rate of 2 ◦
C/min and then maintained at 250 ◦ C for 5 min. Finally, the temperature of
gasification component was increased linearly to 280 ◦C at a rate of 15 ◦ C/min and maintained for 5 min. The molecular weight was scanned in the range of 45 m/z–600 m/z. The retention time and sampling volume was 4 min and 1 µL, respectively. The sample was injected with no diversion. The ionization way of mass spectroscopy was electron impact (EI) with the voltage of electron multiplier at 1605 V. The temperatures of the ion source and the interface were 230 ◦C and 280 ◦C, respectively. Separated ingredients were analyzed with a reference to the NIST MS Search 2.0mass spectral library database. The analysis of DMPD used GC–MS choice ion pattern (SIM). The temperature program was the same as above. The peak areas of 93 m/z, 121 m/z, and 136 m/z were used to generate standard curve (Cao et al., 2015; Fang et al., 2016). To determine the X-3B removal efficiency, influent absorbance values (A1) and effluent absorbance values (A2) were measured with by UV–Vis Spectrophotometer (UV9100, Lab Tech Ltd, Beijing, China) under 540 nm (the maximum absorbing wavelength of RBR X-3B), and the X-3B removal efficiency (E) was calculated as shown in Eq. (1) E=(A1-A2)/A1 x 100%
(1)
The influent Chemical oxygen demand (COD) concentrations (C1) and effluent COD concentrations (C2) were measured according to standard methods (APHA, 1995). The COD removal efficiency (C) was calculated as shown in Eq. (2) C=(C1-C2)/C1 x 100%
(2)
All samples were filtered through a 0.45 lm syringe filter to remove suspended solids from the liquid media prior to the measurements (Fang et al., 2013a). The power density (W/m3) was calculated according to Eq. (3) P = IU/V
(3)
Where I is the current, U is the voltage, and V is the working volume of the anode. The internal resistance was calculated by the linear region of polarization curve (Puig et al., 2012). 3. Results 3.1 The electrical properties of the BER-MFC coupled system We first evaluated the electrical properties of the BER-MFC coupled system. The MFC units of both the experimental and control groups were run for 30 days before coupling, which stabilized the output voltage at approximately 0.40–0.50 V and the current at approximately 0.40–0.50 mA. Figs. 2A and 2B provide the voltage and current of the coupled system, respectively; the total output voltage was relatively stable within a range of 0.72–0.95 V and the current stabilized at approximately 0.20 mA. We also found that the electricity production of the two MFCs differed, at
approximately 0.52 V for MFC 1# and approximately 0.31 V for MFC 2#. A polarization curve was obtained on day 123, when the output voltage and current of the coupled system remained stable. As shown in Fig. 2C, the highest observed power density was 0.257 W/m3 at an external resistance of 1000 Ω, while the internal resistance was 1279.50 Ω. As shown in Fig. 2A, there was an obvious difference in output voltage between the two MFCs in the coupled systems. In general, voltage reversal is common in stacked MFCs. Once this occurs, undesirable electrochemical reactions can adversely affect fuel cell components (Min et al., 2005) and the activity of anode electrical bacteria. Even using the same device structure, the amounts of individually generated protons and electrons in MFCs are inconsistent as a result of differences in properties such as internal resistance and substrate consumption rate. Additionally, electrons can often be transferred between stacked MFCs, whereas protons cannot. This results in unequal numbers of protons and electrons, leading to a rise in the potential of the cathode or anode and voltage reversal. In this study, the voltage reversal phenomenon that is common in stacked MFCs did not occur. In previous studies, voltage reversal was often observed at high current densities (Aelterman et al., 2006), and a relatively large current capacity was required to prevent it (Kim et al., 2015). During the acclimatization stage in our study, single MFCs were connected to a smaller external resistance to obtain a higher current of 0.4–0.5 mA, and then the current was decreased to 0.13–0.24 mA in the coupled
MFCs. The current change in the MFCs from high to low increased the current capacity to a certain extent. Therefore, voltage reversal in stacked MFCs can be avoided by increasing the current capacity via an acclimitazation stage, in which a smaller external resistance is connected to single MFCs. 3.2 X-3B degradation in the BER-MFC coupled system Next we evaluated X-3B degradation in the BER-MFC coupled system. The average X-3B removal efficiencies of the BER and MFC units are provided in Figs. 3A and 3B. The average X-3B removal efficiencies of the BER units in the test group, 1 V DC control group and blank control group were 47.45%, 50.23% and 38.64%, respectively. The average X-3B removal efficiency in the test group reached a level similar to that in the control group: 1 V DC. The average X-3B removal efficiencies of the two MFCs were 50.37% and 50.28% in the test group and 35.93% and 34.77% in the control groups, respectively. The X-3B removal efficiency contribution rates of the BER and MFC units in the coupled system are shown in Fig. 3C. In the coupled system, the contribution rates of the BER and MFC units were 47.45% and 50.32%, respectively, for a total average removal efficiency of 97.77%. In contrast, the contribution rates of the BER and MFC units were 32.55% and 35.35%, respectively, in the control group system, giving a total average removal efficiency of 67.90%. The average X-3B removal efficiencies in the BER and MFC units increased by 14.90% and 14.97%, respectively, after coupling, improving the total removal efficiency by 29.87%.
After coupling, the electron flow between the two units created current, effectively forming a complete circuit. The stable current generated by the stacked MFCs was provided to the BER unit. The removal of azo dye X-3B is a reductive process requiring electrons from the activity of microorganisms under anaerobic conditions; thus, the supply of external electrons from the MFC unit improved the degradation efficiency of X-3B in the BER unit. Concurrently, the BER unit funtioned as external resistance, forming an effective circuit in the coupled system, so that electrons generated by microorganisms degrading glucose substrates in the anode of MFCs were able to flow faster and easier, providing more electrons to reduce X-3B in the MFC unit, thereby improving the removal efficiency. A coupled system was formed comprising two units easily connected into a circuit. The electrical energy generated by the MFC unit was provided to the BER unit, and the effluent from the BER unit was used as the inflow for the MFC unit resulting in further degradation. Without the support of any external energy source, X-3B removal efficiency was significantly improved in this coupled system. 3.3 Identification of degradation products The results of gas chromatography–mass spectrometry (GC-MS) chromatographic analysis of the effluent from the BER and MFC units in the coupled and control group systems showed that X-3B was broken down into more than 20 metabolites. The chemical formulas of the metabolites are given in Table 2. The dye degradation products from the coupled system were classified as alcohols (C6H14O2),
aldehydes (C5 H6O, C10H16O), aromatic amines (C6 H7N), hydrocarbons (C8H16, C10H18, C14H26, C17H36, C18H38), phenols (C14H22O, C15H24O, C20H34O) and esters (C18H26O4, C14H26O4). The products from the control group system were classified as aldehydes (C5H6O, C9H16O, C10H16O), aromatic amines (C6H7N, C16H14N2O2S, C16H16N2O4, C19H22N2O2, C18H31N), ketones (C8H12O), hydrocarbons (C8H16, C9H16, C10H16, C13H24, C23H48) and esters (C10H16O2, C14H26O4, C16H21NO3, C18H26O4, C19H34O2). Table 2 GC–MS analysis results of the effluent from the BER and MFC units in the coupled and control group systems Retention
Molecular
time (min)
formula
BER unit in
4.84
C5 H6O
2-Pentynal
coupled
5.93
C10H16O
2,2-Dimethyl-3,4-octadienal
system
6.18
C6H14O2
Hexylene glycol
7.85
C6 H7N
Aniline
10.62
C14 H26
3,5-Octadiene,4,5-diethyl-3.6-dimethyl
14.71
C14H22O
Phenol,2,4-bis(1,1-dimethylethyl)
15.22
C15H24O
Nonylphenol
17.68
C17 H36
Heptadecane
18.74
C18 H38
Octadecane
18.78
C18H26O4
Phthalic acid,hex-3-yl isobutyl ester
19.77
C20H34O
2,4,7,14-Tetramethyl-4-vinyl-tricyclo
MFC unit in
4.84
C5 H6O
2-Pentynal
coupled
7.59
C10 H18
Cyclopropane,tetramethylpropylidene
system
7.93
C8H16
Z-3,4,4-Trimethyl-2-pentene
10.62
C14 H26
3,5-Octadiene,4,5-diethyl-3,6-dimethyl
4.84
C5 H6O
2-Pentynal
Systems
BER unit in
Degraded products name
control group
5.55
C8H12O
5-Hexen-2-one, 5-methyl-3-methylene
system
5.93
C10H16O
2,2-Dimethyl-3,4-octadienal
6.06
C13 H24
7-Ethenyl-5-undecene
7.85
C6 H7N
Aniline
7.93
C8H16
Z-3,4,4-Trimethyl-2-pentene
11.20
C10H16O2
Cyclohex
11.36
C9H16
2,3-Dimethyl-1,3-heptadiene
12.33
C10 H16
Alloocimene
15.11
C18H31N
4-Dodecylaniline ndole-3-carboxylic acid,
17.08
C16H21NO3
5-hydroxy-2-methyl-1-(2-methylpropyl)-, ethyl ester
18.78
C18H26O4
Phthalic acid,hex-3-yl isobutyl ester
19.74
C16H16N2O4
Ethane,1,2-bis(2-methyl-5-nitrophenyl)
21.83
C23 H48
Heptadecane, 9-hexyl
23.89
C14H26O4
Hexanedioic acid,mono(2-ethylhexy)ester MFC unit in
4.84
C5 H6O
2-Pentynal
control
5.62
C9H16O
3-Nonyn-2-ol
group system
7.93
C8H16
Z-3,4,4-Trimethyl-2-pentene
10.86
C16H14N2O2S
mefenacet
12.33
C10 H16
Alloocimene
12.96
C19H22N2O2
Butanediamide,N-benzyl-N-(4-ethylphenyl)
14.92
C19H34O2
9-Octadecynoic acid, methyl ester
15.11
C18H31N
4-Dodecylaniline Indole-3-carboxylic acid,
17.08
C16H21NO3
5-hydroxy-2-methyl-1-(2-methylpropyl)-, ethyl ester
21.83
C23 H48
Heptadecane, 9-hexyl
23.89
C14H26O4
Hexanedioic acid,mono(2-ethylhexy)ester
We confirmed that X-3B was actually broken down into simple molecules in both the coupled and control group systems. These products are not harmful to the environment and can be broken down further into mineralized products such as CO2 and H2O (Khan et al., 2012; Liu et al., 2015). When comparing the intermediate products between the coupled and control group systems, the products in the control group system were more complex and diverse no matter the BER or MFC unit. The intermediate products in the coupled system, particularly in the MFC unit, were rare, and all had simple structures. The metabolites in the BER unit were not found in the MFC unit, except for C5H6O in the coupled system. However, the metabolites C5 H6O, C8H16, C10H16, C18H31N, C16H21NO3, C23H48 and C14H26O1 in the BER unit all appeared in the MFC unit in the control group system. Due to the lack of current in the control group system, the MFC unit exhibited a poor removal efficiency for X-3B degradation products, especially macromolecular products. When comparing the products of the BER and MFC unit in the coupled system, the degradation products of X-3B in the MFC unit had simpler structures with lower molecular weights. This indicated that X-3B was broken down into simple compounds in the BER unit initially, and those compounds continued to be broken down into simpler structures in the MFC. The two units exhibited not only a coupled energetic relationship but also a progressive relationship in terms of the degradation of X-3B.
3.4 Effects of current on X-3B and chemical oxygen demand (COD) removal efficiencies in the BER unit The current in the BER unit was altered by varying series and parallel resistances in connected external circuits. Fig. 4A shows the voltage and current of the BER unit, revealing that the voltage of the BER unit was stable at 0.40–0.50 V, and the current was stable at 0.40–0.50 mA in the initial state. The voltage dropped to 0.5 V and the current to 0.07 mA after the first change in resistance (Table 1). The voltage dropped to 0.1 V and the current to 0.3 mA and then trended to a certain stable level after the second change (Table 1). As shown in Fig. 4B, the average removal efficiencies of X-3B and COD initially decreased, then increased, and became stable over time. The average removal efficiency of X-3B was 44.03–60.70%, and that of COD was 73.00–88.62%. Compared with the results presented in Fig. 4A, it appears that the change in the average removal efficiencies of X-3B and COD in the BER unit exhibited almost the same trend as the change in current. This suggests that the current affected the removal efficiencies of the BER unit. As mentioned above, the removal of X-3B is a reductive process that requires electrons. Thus, an increase or decrease in the current implies an increase or decrease in the number of electrons that can be used by microorganisms to reduce X-3B, and the removal efficiencies of X-3B and COD change accordingly. 4. Conclusions We determined that a system of stacked MFCs coupled with a BER can
significantly improve the efficiency of azo dye X-3B removal. The X-3B removal efficiency in this coupled system was 29.87% greater than that in the non-coupled control group. The highest power density in the stacked MFCs was 0.257 W/m3, and the internal resistance was 1279.50 Ω. We also identified a method for preventing voltage reversal in stacked MFCs. We identified current as the key factor influencing the removal efficiency in the BER unit. The BER and MFC units showed a progressive relationship in the degradation of X-3B. Acknowledgments We thank the National Natural Science Foundation of China (21277024), the Provincial Natural Science Foundation of Jiangsu, China (BK20141330), the Fundamental Research Funds for the Central Universities and Scientific Research Foundation of Graduate School of Southeast University for financial support.
References Aelterman, P., Rabaey, K., Pham, H.T., Boon, N., Verstraete, W. 2006. Continuous electricity generation at high voltages and currents using stacked microbial fuel cells. Environmental science & technology, 40(10), 3388-3394. Cao, X., Song, H.-l., Yu, C.-y., Li, X.-n. 2015. Simultaneous degradation of toxic refractory organic pesticide and bioelectricity generation using a soil microbial fuel cell. Bioresource technology, 189, 87-93. Cao, Y., Hu, Y., Jian, S., Hou, B. 2010. Explore various co-substrates for simultaneous electricity generation and Congo red degradation in air-cathode
single-chamber microbial fuel cell. Bioelectrochemistry, 79(1), 71-76. Cardenas-Robles, A., Martinez, E., Rendon-Alcantar, I., Frontana, C., Gonzalez-Gutierrez, L. 2013. Development of an activated carbon-packed microbial bioelectrochemical system for azo dye degradation. Bioresource Technology, 127(1), 37-43. Cui, D., Guo, Y.Q., Cheng, H.Y., Liang, B., Kong, F.Y., Lee, H.S., Wang, A.J. 2012. Azo dye removal in a membrane-free up-flow biocatalyzed electrolysis reactor coupled with an aerobic bio-contact oxidation reactor. Journal of Hazardous Materials, 239-240(18), 257-264. Fang, Z., Song, H.-L., Cang, N., Li, X.-N. 2013a. Performance of microbial fuel cell coupled constructed wetland system for decolorization of azo dye and bioelectricity generation. Bioresource technology, 144, 165-171. Fang, Z., Song, H., Yu, R., Li, X. 2016. A microbial fuel cell-coupled constructed wetland promotes degradation of azo dye decolorization products. Ecological Engineering, 94, 455-463. Fang, Z., Song, H.L., Ning, C., Li, X.N. 2013b. Performance of microbial fuel cell coupled constructed wetland system for decolorization of azo dye and bioelectricity generation. Bioresource Technology, 144(6), 165-171. Khan, R., Bhawana, P., Fulekar, M.H. 2012. Microbial decolorization and degradation of synthetic dyes: a review. Reviews in Environmental Science & Bio/technology, 12(1), 75-97.
Kim, B., Lee, B.G., Kim, B.H., Chang, I.S. 2015. Assistance current effect for prevention of voltage reversal in stacked microbial fuel cell systems. ChemElectroChem, 2(5), 755-760. Kong, F., Wang, A., Ren, H.-Y. 2014. Improved 4-chlorophenol dechlorination at biocathode in bioelectrochemical system using optimized modular cathode design with composite stainless steel and carbon-based materials. Bioresource technology, 166, 252-258. Liang, B., Cheng, H.-Y., Kong, D.-Y., Gao, S.-H., Sun, F., Cui, D., Kong, F.-Y., Zhou, A.-J., Liu, W.-Z., Ren, N.-Q. 2013. Accelerated reduction of chlorinated nitroaromatic antibiotic chloramphenicol by biocathode. Environmental science & technology, 47(10), 5353-5361. Liu, S., Song, H., Wei, S., Liu, Q., Li, X., Qian, X. 2015. Effect of direct electrical stimulation on decolorization and degradation of azo dye reactive brilliant red X-3B in biofilm-electrode reactors. Biochemical Engineering Journal, 93, 294-302. Min, B., Kim, J., Oh, S., Regan, J.M., Logan, B.E. 2005. Electricity generation from swine wastewater using microbial fuel cells. Water research, 39(20), 4961-4968. Mu, Y., Rabaey, K., Rozendal, R.A., Yuan, Z., Keller, J. 2009. Decolorization of azo dyes in bioelectrochemical systems. Environmental science & technology, 43(13), 5137-5143.
Oh, S.E., Logan, B.E. 2007. Voltage reversal during microbial fuel cell stack operation. Journal of Power Sources, 167(1), 11-17. Pant, D., Bogaert, G.V., Diels, L., Vanbroekhoven, K. 2010. A review of the substrates used in microbial fuel cells (MFCs) for sustainable energy production. Bioresource Technology, 101(6), 1533-1543. Puig, S., Coma, M., Desloover, J., Boon, N., Colprim, J.s., Balaguer, M.D. 2012. Autotrophic denitrification in microbial fuel cells treating low ionic strength waters. Environmental science & technology, 46(4), 2309-2315. Santos, A.B.D., Cervantes, F.J., Lier, J.B.V. 2007. Review paper on current technologies for decolourisation of textile wastewaters: Perspectives for anaerobic biotechnology. Bioresource Technology, 98(12), 2369-85. Shen, J., Xu, X., Jiang, X., Hua, C., Zhang, L., Sun, X., Li, J., Yang, M., Wang, L. 2014. Coupling of a bioelectrochemical system for p -nitrophenol removal in an upflow anaerobic sludge blanket reactor. Water Research, 67(67C), 11-18. 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 single-chamber microbial fuel cell. Bioresource Technology, 100(13), 3185-92. Sun, M., Sheng, G.P., Zhang, L., Xia, C.R., Mu, Z.X., Liu, X.W., Wang, H.L., Yu, H.Q., Qi, R., Yu, T. 2008. An MEC-MFC-Coupled System for Biohydrogen Production from Acetate. Environmental Science & Technology, 42(21),
8095-100. Sun, Q., Li, Z.-L., Wang, Y.-Z., Yang, C.-X., Chung, J.S., Wang, A.-J. 2016. Cathodic bacterial community structure applying the different co-substrates for reductive decolorization of Alizarin Yellow R. Bioresource technology, 208, 64-72. Sun, Q., Li, Z., Wang, Y., Cui, D., Liang, B., Thangavel, S., Chung, J.S., Wang, A. 2015. A horizontal plug-flow baffled bioelectrocatalyzed reactor for the reductive decolorization of Alizarin Yellow R. Bioresource Technology, 195, 73-77. Wang, Y.Z., Wang, A.J., Liu, W.Z., Sun, Q. 2013. Enhanced azo dye removal through anode biofilm acclimation to toxicity in single-chamber biocatalyzed electrolysis system. Bioresource Technology, 142(4), 688-692. Xu, F., Mou, Z., Geng, J., Zhang, X., Li, C.Z. 2016. Azo dye decolorization by a halotolerant exoelectrogenic decolorizer isolated from marine sediment. Chemosphere, 158, 30-36. Yadav, A.K., Dash, P., Mohanty, A., Abbassi, R., Mishra, B.K. 2012. Performance assessment of innovative constructed wetland-microbial fuel cell for electricity production and dye removal. Ecological Engineering, 47(5), 126–131. Yang, L., Yang, H.Y., Shen, J.Y., Yang, M., Yu, H.Q. 2016. Enhancement of azo dye decolourization in a MFC–MEC coupled system. Bioresource Technology, 202, 93-100.
Zhang, J., Zhang, Y., Xie, Q., Chen, S., Afzal, S. 2013. Enhanced anaerobic digestion of organic contaminants containing diverse microbial population by combined microbial electrolysis cell (MEC) and anaerobic reactor under Fe(III) reducing conditions. Bioresource Technology, 136(5), 273-280. Zhang, S., Song, H.L., Yang, X.L., Yang, K.Y., Wang, X.Y. 2016. Effect of electrical stimulation on the fate of sulfamethoxazole and tetracycline with their corresponding resistance genes in three-dimensional biofilm-electrode reactors. Chemosphere, 164, 113-119. Zhao, H., Zhang, Y., Zhao, B., Chang, Y., Li, Z. 2012. Electrochemical reduction of carbon dioxide in an MFC-MEC system with a layer-by-layer self-assembly carbon nanotube/cobalt phthalocyanine modified electrode. Environmental Science & Technology, 46(9), 5198-204.
Figure 1. Configuration of the stacked microbial fuel cell-biofilm electrode reactor coupled system Figure 2. Electricity generation of MFC-BER coupled system: (A) the voltage of total output, MFC 1# and MFC 2#; (B) the current; (C) power density curve and the polarization curve Figure 3. The average X-3B removal efficiencies in BER-MFC coupled system: (A) in BER unit; (B) in MFC unit; (C) the X-3B removal efficiency contribution rates Figure 4. Electricity generation and effects on X-3B removal in BER unit: (A) the voltage and current; (B) the average removal efficiencies of X-3B and COD
Figure 1
Figure 2
Figure 3
Figure 4
Highlights •
MFC-BER coupled system was established in a simple method.
•
Electrical energy generated by the MFC was used in the BER.
•
The effluent from the BER was used as the inflow for the MFC.
•
Current was the key factor influencing removal efficiency in the BER.