β-MnO2 modified graphite felt anode for enhancing recalcitrant phenol degradation in a bioelectrochemical system

Electrochimica Acta 244 (2017) 119–128

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Fabrication of polypyrrole/b-MnO2 modified graphite felt anode for enhancing recalcitrant phenol degradation in a bioelectrochemical system Dan Chena , Jinyou Shena,* , Xinbai Jianga , Yang Mub , Dehua Maa , Weiqing Hana , Xiuyun Suna , Jiansheng Lia , Lianjun Wanga,* a Jiangsu Key Laboratory of Chemical Pollution Control and Resources Reuse, School of Environmental and Biological Engineering, Nanjing University of Science and Technology, Nanjing 210094, Jiangsu Province, China b CAS Key Laboratory of Urban Pollutant Conversion, Department of Chemistry, University of Science and Technology of China, Hefei 230026, Anhui Province, China

A R T I C L E I N F O

Article history: Received 9 December 2016 Received in revised form 4 February 2017 Accepted 17 May 2017 Available online 17 May 2017 Keywords: Bioelectrochemical system Electrode modification Polypyrrole b-MnO2 Degradation pathway

A B S T R A C T

In order to develop a highly efficient anode material for recalcitrant phenol degradation in bioelectrochemical system (BES), fabrication of polypyrrole (PPy)/b-MnO2 composite onto graphite felt (GF) electrode through facile one-step electrodeposition was investigated in this study. The successful coating of PPy/b-MnO2 onto GF surface was verified by scanning electron microscopy, Raman spectrum and XPS. The improved electrochemical property of the GF electrode modified by PPy/b-MnO2 was confirmed by cyclic voltammetry analysis, chronoamperometric and electrochemical impedance spectra. The application of PPy/b-MnO2 modified GF electrodes in BES notarized the superior degradation performance towards phenol. Shorter startup time, higher mineralization efficiency and improved bacteria adhesion was achieved in BES using PPy/b-MnO2 modified GF as anode. Coulombic efficiencies of 17.3
0.5% in BES using PPy/b-MnO2 modified GF as anode was much higher than those in BES using PPy modified GF and blank GF as anode, which were as low as 12.1
2.4% and 6.6
1.3%, respectively. The key role of MnO2 and possible degradation pathway involved in phenol degradation was further proposed. The milder fabrication condition, improved electrochemical activity and increased phenol degradation efficiency suggest that PPy/b-MnO2/GF has a promising future in BES application for recalcitrant phenol catalytic degradation. © 2017 Elsevier Ltd. All rights reserved.

1. Introduction Bioelectrochemical systems (BESs) appeared as a promising technology, which could achieve double goals of wastewater treatment and electricity generation by the conversion of organic contaminants with the help of bacteria [1–3]. Most studies were performed with easily biodegradable organics such as volatile fatty acids and carbohydrates to generate extracellular electrons [4]. However, energy recovery from recalcitrant organics, especially aromatic compounds and heterocyclic compounds, was rather difficult in BESs [5]. One of the principal challenges could be attributed to the poor catalytic activity and limited conductivity of

* Corresponding authors. E-mail addresses: [email protected], [email protected] (J. Shen), [email protected] (L. Wang). http://dx.doi.org/10.1016/j.electacta.2017.05.108 0013-4686/© 2017 Elsevier Ltd. All rights reserved.

the electrode such as graphite felt (GF), which was normally employed as the anode of BESs. In BESs, biological oxidation of organic compounds occurred on anode, with electrons transferred from the microorganisms to the electrode surface [6,7]. As an alternative electron acceptor for electrochemically active microorganisms, anode played an important role in both substrate degradation and current generation [8]. Therefore, selection and preparation of high-performance anode materials is of crucial importance in BES design. Significant improvements in terms of power generation and degradation efficiency could be realized through the modification of electrode materials such as GF [9]. Transition metal oxides were widely used as electrode materials, considering fast and reversible redox reactions between the surface of active materials and electrolyte [10,11]. Among those transition metal oxides, MnO2 was generally regarded as a promising one owing to high abundance of Mn, low cost and tunable catalytic activity [12,13]. Zhang et al. [14] electrodeposited

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MnO2 on carbon felt to fabricate a novel anode, with maximum power density as high as 3580
130 mW m2 obtained in a BES equipped with the MnO2-coated anode. Lv et al. [15] indicated that modification of carbon felt electrode by transition metal oxide had turned out to effectively accelerate the extracellular electron transfer from electrogenic microbes to the anode. However, MnO2 usually delivered a relatively low surface area and poor electrical conductivity, which strongly depressed its catalytic activities [16]. In addition, loading of solid-state MnO2 was rather difficult if without any binder. To overcome these critical problems, efforts have been devoted to integrate MnO2 nanostructures with conductive polymers, which could act as conductive wrapping to anchor MnO2 onto the surface of electrode [17,18]. Recently, composite of transition metal oxides and conducting polymers such as polypyrrole (PPy) and polyaniline have been investigated in the application of supercapacitor, lithium-ion battery and BES [17,19–27]. The electrical conductivity, specific capacitance and stability of this composite were higher than each individual component, demonstrating a synergistic effect between transition metal oxides and conducting polymers [17]. Han et al. [24] used PPy as the intermediate layer to synthesize core-shellshell nanowire arrays of Co3O4@PPy@MnO2, which exhibited prominent electrochemical performance and a remarkable longterm cycling stability. Lu et al. [28] used PPy-manganese-carbon nanotube composite as a cathode catalyst in BES for power generation, with maximum power density as high as 213 mW m2 achieved. Carbon cloth coated by MnO2/polypyrrole/MnO2 multiwalled-nanotube through Nafion solution was used as anode material in a BES, with maximum power density as high as 32.7
3.0 W cm3 achieved [17]. These efforts on electrode modification by composite of transition metal oxides and conducting polymers have been mainly devoted to enhance power density [29]. However, the key role of composite of transition metal oxides and conducting polymers in enhancing the degradation of recalcitrant organics in BES anode chamber has never been reported. In addition, the traditional electrode modification method based on the coating of composite of transition metal oxides and conducting polymers onto electrode through Nafion solution deserved to be improved, considering the poor mechanical properties and solubility of recast Nafion in a variety of polar organic solvents at room temperature [30]. What’s more, Nafion solution commercially available was rather expensive, which hindered its practical applications. Herein, facile electrochemical fabrication of PPy and MnO2 composite onto graphite felt electrode and its application in catalytic degradation of phenol in BES was investigated in this study. b-MnO2 was chosen as the model, considering the superior catalytic activity of b-MnO2 due to its high BET surface area and average oxidation states [31]. Field emission scanning electron microscopy (FE-SEM), Raman spectrum, XPS, cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and chronoamperometric (CA) was conducted to characterize the modified GF electrode. Unmodified GF, GF modified with PPy alone and modified with PPy/b-MnO2 was used as the anodes in BESs, with their catalytic performance towards phenol compared. Moreover, the possible mechanism for enhanced phenol degradation by PPy and b-MnO2 modified GF electrode was explored preliminarily. 2. Materials and methods 2.1. Preparation of b-MnO2 A modified method based on the selected-control hydrothermal synthesis was used for the preparation of b-MnO2 [32]. Briefly, 0.024 mol analytical grade hydrate manganese sulfate (MnSO4H2O) and ammonium persulfate ((NH4)2S2O8) were added

into distilled water at room temperature to form a homogeneous solution, which was then transferred into a Teflon-lined stainless steel autoclave, sealed and maintained at 135  C for 12 h. The difference from Wang and Li [32] was that, reaction temperature increased from 120  C to 135  C in this study, in order to get b-MnO2 of high purity. After the reaction was completed, the resulting black solid was filtered, washed with distilled water to remove ions remained, and finally dried at 50  C in the vacuum oven. The as-prepared b-MnO2 was then added into deionized water at concentration of 10 g L1, and then treated by ultrasonic for 80 min to obtain a homogeneous b-MnO2 suspension. 2.2. GF electrode modification Polyacrylonitrile based graphite felt (Chemshine Carbon CO., China) was cut into small pieces of 2.0 cm  2.0 cm  0.30 cm prior to use. All pieces were immersed in acetone solution and utterly cleaned in an ultrasonic cleaner for 30 minutes. Then the GF pieces were ultrasonicated in 30% H2O2 solution for 30 minutes in order to remove impurities, followed by thorough rinse with enough deionized water and drying in a vacuum oven at 50  C for 48 h. According to Lv et al. [8], a facile one-step electrodeposition method was adopted for GF modification. The room-temperature electrodeposition was performed in a three-electrode electrochemical cell consisting of a working electrode (GF), a reference electrode (Ag/AgCl electrode) and a counter electrode (Pt). A potentiostat (Bio-Logic Science Instruments, VMP3, France) was used for controlling the current applied on the working electrode. Briefly, 0.2 M pyrrole and 0.1 M NaClO4 was added into 10 g L1 b-MnO2 suspension prepared as above. The mixture was added into three-electrode electrochemical cell and then electropolymerization was carried out at a constant potential of 0.80 V, until the total charge density passed reached 10C cm2. GF electrode loaded by PPy/b-MnO2 composites was marked as PPy/b-MnO2/GF. For comparisons, PPy modified GF electrode was prepared with an identical total charge density passed and marked as PPy/GF. The GF treated as the above process in the absence of PPy and b-MnO2 was marked as GF. These samples were then carefully rinsed by deionized water to remove any absorbed species and dried at 50  C in the vacuum oven. It should be noted that all the potentials reported throughout this paper were referred to the Ag/AgCl electrode (assumed + 0.197 V vs. SHE), if not otherwise stated. 2.3. Physical and electrochemical characterizations The X-ray difftaction (XRD) patterns were recorded on an X-ray diffractometer (D8 Advance, Bruker, Germany) by using Cu Ka radiation (l = 1.54 Å) at 40 kV and 30 mA. Scanning electron microscopy (SEM) was examined with a field emission scanning electron microscopy (FE-SEM, FEI Quanta 250FEG). The surface morphology of the MnO2 was examined with FE-SEM at high voltage of 20 kV and spot of 3.5. The preparation of bioanode sample for FE-SEM observation was referred to Lv et al. [8]. Transmission electron microscopy (TEM) analysis was conducted on a TECNAI G2 20 LaB6 electron microscope operated at 200 kV. Raman spectrum was obtained by integrated confocal Raman Microscopy System (LabRAM Aramis, HORIBA Jobin Yvon, France) from 500 to 2000 cm1 using a 532 nm laser beam. XPS analysis was operated on a Thermo ESCALAB 250 instrument. The nitrogen absorption measurement was carried out by using a BrunauerEmmett-Teller (BET) apparatus (Micromeritics, ASAP 2020, USA) at liquid nitrogen temperature (–196  C). Electrochemical analysis was performed with a potentiostat (Bio-Logic Science Instruments, VMP3, France) by using a standard three-electrode system. CV of the modified electrodes in N2-saturated pH 7.0 buffer with and without containing 100 mg L1 phenol was recorded at a potential

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scan rate of 1 mV s1. CA experiments were conducted to explore the stability of modified electrodes under a constant potential of 0.70 V for 1200 s. EIS of the anodes, which were inoculated with bacteria, was performed in 0.1 M NaCl by a three-electrode mode, with modified electrode, Pt electrode and Ag/AgCl electrode used as working electrode, counter electrode and reference electrode, respectively. The impedance spectra were recorded at open circuit potential and in frequency ranged from 100,000 to 0.1 Hz, with a sinusoidal excitation signal of 10 mV. The impedance data were fitted to an equivalent electrical circuit using Zview software. Coulombic efficiency (CE) was calculated according to Cheng et al. [5]. 2.4. Application of the modified electrodes in BESs Dual-chamber BESs consisting of a bare GF cathode and the modified GF anode described above were constructed, separated by a cation exchange membrane (Membrane International, Ultrex CMI-7000, USA). Each cell chamber made of Perspex flames has an effective volume of 15 mL. Ti wires (0.80 mm in diameter) woven into the graphite felts were used to allow the external circuit connection. The anode half-cell potential was measured by placing an Ag/AgCl reference electrode in the anode compartment. A 1000 V external resistance was used to connect the external circuit with Ti wires. The generated current was recorded using a data acquisition system (Bio-Logic Science Instruments, VMP3, France), which automatically converted the measured voltage to current based on the ohm's law. The influent composition of the anode was as follows: NaH2PO42H2O (5.6 g L1), Na2HPO412H2O (6.1 g L1), NH4Cl (0.31 g L1), KCl (0.1 g L1), phenol (0.2 g L1) and SL-4 solution (10 mL L1) [33]. Co-substrate of sodium acetate (0.1 g L1) was added into anode influent to reduce potential toxicity from phenol during startup period. The cathode medium contained: KH2PO4 (2.7 g L1), Na2HPO412H2O (10.8 g L1) and K3[Fe(CN)6] (16.5 g L1) [34]. Before use, the anode influent was purged with nitrogen gas for 15 minutes to remove any dissolved oxygen. All the BES anode chambers were inoculated with the sludge collected from a coupled UASB-BES system treating the chloronitrobenzenescontaining wastewater. Ten mL sludge at mixed liquid suspended solid (MLSS) concentration of 4.5 g L1 was inoculated into each anode chamber, resulting initial MLSS concentration of 3.0 g L1. Then the BES reactors equipped with the modified and unmodified GF anodes were operated under fed-batch mode at constant temperature of 35  C. When the phenol was totally removed, the chambers were refilled. The removal rate (R, mg L1 phenol d1 cm2) of BESs in different conditions was calculated according to the following equation: R¼

CE tS

ð1Þ

where C (mg L1) is the initial concentration of phenol in anolyte, E (%) is the removal efficiency of phenol, t (d) is the corresponding degradation time and S (cm2) is the effective surface area of the anode. 2.5. Analytical methods The concentration of phenol was measured by Waters 2695 HPLC equipped with an XBridge C18 (5 mm, 4.6 mm  250 mm) column and a 2998 PDA detector. The mobile phase contained a mixture of 50% methanol and 50% ultrapure water with a rate of 1 mL min1 at isocratic mode. The sample injection volume was 10 mL. The analysis was performed at 254 nm, with column temperature of 35  C. Before analysis, the sample was filtrated through a 0.22 mm pore-size membrane filter. Chemical oxygen

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demand (COD) was analyzed according to Han et al. [35]. The intermediates of phenol degradation were analyzed by GC/MS (Agilent 7890A GC/5975C MSD, HP-5MS column) and some carboxylic acids were detected by HPLC (Agilent 1200). Prior to GC/MS analysis, the samples were concentrated by solid phase extraction according to Ma et al. [36]. The dehydrated extracts were concentrated by a rotary evaporator at 40  C and redissolved in ethyl acetate. Then 0.1 mL N,O-bis(trimethylsilyl)acetamide (BSA) was added into the obtained organic solution and was left for 30 min at 60  C. The derivatized samples were then transferred to an autosampler vial for the for GC/MS analysis. The oven temperature was initially maintained at 60  C for 1 min, programmed to 200  C at 5  C min1 (hold 5 min) and then increased to 280  C at 15  C min1 (hold 5 min) at last. The transfer line and ion source temperatures were 250  C and 200  C, respectively, and the electron energy was 70 eV.

3. Results and discussion 3.1. Characterization of b-MnO2 The crystal phase of the as-synthesized b-MnO2 sample was analyzed by XRD, as depicted in Fig. 1a. All reflections in the XRD pattern could be readily indexed to a pure tetragonal phase of b-MnO2 with lattice constants a = 4.399 Å and c = 2.874 Å (JCPDS 24-0735). This result indicated that the product synthesized through modified selected-control hydrothermal synthesis method can be assigned to b-MnO2, which was of high purity and in good crystallinity [32]. Besides, the BET surface area of the obtained product was determined to be 84.2 m2 g1, which was comparable with Zhao et al. [37]. The SEM observation showed that b-MnO2 appeared in loosen and claviform form with high crystallinity (Fig. 1b), which was consistent with a previous study [31]. The high crystallinity observed through SEM was in agreement with the XRD peak shapes, which were rather strait and high. From TEM images, it could be seen that the diameter of the nanorods was around 40–60 nm while the length was in the range of 800–1000 nm (Fig. 1c and d).

3.2. Characterization of the as-modified GF 3.2.1. Morphology and structure Surface morphology of the blank GF and the modified GF were characterized by FE-SEM, as exhibited in Fig. 2. The blank GF presents smooth fibrous structure and obvious indentation on its surface (Fig. 2a). PPy/GF also presented fibrous structure, but with a globular PPy film uniformly covered on electrode surface (Fig. 2c). However, the morphology of PPy/b-MnO2/GF surface was significantly different from that of PPy/GF, as shown in Fig. 2e. The rod-like MnO2 could be uniformly coated on the individual GF, which was attributed to the incorporation of MnO2 into the PPy film during the in-situ electropolymerization process [16,34]. The specifical and homogeneous growth of PPy/b-MnO2 composites onto the fiber surfaces provided enough roughness for bacteria adhesion and enabled more accessible surface active sites for redox reactions and thus helped to promote the anode performance [38]. Furthermore, BET surface area of PPy/b-MnO2/GF was found to be as high as 10.3 m2 g1, however, BET surface area of both PPy/GF and blank GF were non-detectable, which were in accordance with the reported literature [34]. The rough surface of PPy/b-MnO2/GF clearly indicated that the introduction of b-MnO2 onto GF surface resulted in a substantial increase in specific surface area, due to the loofah-like structure of MnO2 incorporated into the PPy film, as indicated in Fig. 2e.

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Fig. 1. XRD patterns (a), FE-SEM image (b) and TEM images (c, d) of the as-synthesized b-MnO2 nanostructures.

3.2.2. Raman and XPS Raman spectra of MnO2, blank GF, PPy/GF, and PPy/b-MnO2/GF were illustrated in Fig. 3. The GF electrode exhibited two peaks at around 1350 cm1 and 1592 cm1, corresponding to the welldocumented D band and G band [8,39], respectively. In the Raman spectrum of PPy/GF, both D and G bands were also obviously observed, with G band higher than D band. In addition, two more peaks at 989 cm1 and 932 cm1 were visible in the Raman spectrum of PPy/GF, which could be assigned to ring deformation associated with dication (bipolaron) and radical cation (polaron), respectively [8]. One more peak at 640 cm1 observed in the Raman spectrum of PPy/b-MnO2/GF was in accordance to the Raman spectrum of MnO2, which was attributed to the Mn-O stretching vibration in the basal plane of MnO6 octahedra [40]. From these Raman peaks, it could be inferred that both PPy and MnO2 were successfully loaded onto GF surface. To confirm the Raman results, the PPy/b-MnO2 layer was scraped off from the modified electrode and further characterized with XPS. The existence of C, N, O and Mn in PPy/b-MnO2 composite could be confirmed by Fig. 4a. Both Mn 2p3/2 peak at 642.0 eV and Mn 2p1/2 peak at 653.7 eV could be clearly observed in Mn 2p spectrum (Fig. 4b), which were in good agreement with the energy splitting of the standard spectrum of MnO2. The spinenergy separation between Mn 2p1/2 and Mn 2p3/2 level was 11.7 eV, which was consistent with the literature [40,41]. As shown in Fig. 4c, the XPS peaks of N 1 s were further decomposed into three Gaussian peaks with binding energies of 398.6 (-N = ), 399.7 (-NH-) and 400.1 (-N+-), corresponding to the imine-like structure, the neutral and amine-like structure and positively charged

structure [41–43]. The XPS analysis further confirmed the existence of PPy/b-MnO2 compositions on the GF. 3.3. Electrochemical analysis 3.3.1. CV analysis The CV curves for the modified GF electrodes and blank GF electrode tested in the buffer solution (pH 7.0) containing 100 mg L1 phenol were shown in Fig. 5a. An open curve could be observed for all the electrodes. However, in the buffer solution (pH 7.0) without phenol, coherent curves were available for all the electrodes, as indicated in Fig. S1. This intriguing phenomenon might be ascribed to the change in solution composition during CV tests, which was confirmed by HPLC analysis (Fig. S2). After CV tests, several peaks appeared in the HPLC chromatogram of buffer solution containing phenol, which could be assigned to 4benzoquinone and some carboxylic acids. In Fig. S1, there were no clear redox peaks and all CV curves indicated a typical capacitive behavior, which could be attributed to the charge and discharge process of PPy conducting polymer and pseudocapacitive behaviors [44,45]. Nevertheless, from Fig. 5a, an obvious oxidation peak could be observed but no obvious reduction peak could be observed for all the electrodes. The oxidation peaks were mainly due to the transformation of phenol to 4-benzoquinone [46–48]. Within the potential region of 0.65 to 1.0 V, the peak area of PPy/b-MnO2/GF was significantly larger than those of PPy/GF and GF. In addition, an oxidation peak for blank GF could be observed at 0.72 V but with a low current response of 0.51 mA, suggesting

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Fig. 2. FE-SEM images of GF anodes modified with different catalysts before (a, c and e) and after (b, d and f) inoculation: (a, b) the blank GF electrode, (c, d) PPy/GF electrode and (e, f) PPy/b-MnO2/GF electrode. Insets showed the magnified images from the corresponding electrodes.

limited electrochemical activity of GF electrode. In contrast, the oxidation peaks for PPy/GF and PPy/b-MnO2/GF were observed at the potential of 0.69 V and 0.58 V with peak currents of 1.64 mA and 3.01 mA, respectively. The redox current produced on PPy/ b-MnO2/GF was almost 6 and 2 times larger than those on blank GF and PPy/GF. The significant increase in both current intensity and electroactive area indicated that PPy/b-MnO2/GF electrode had a higher electrochemical activity as more redox mediators were involved in electron transfer. What’s more, the oxidation peak potential shifted negatively after the GF was loaded with PPy/ b-MnO2 composites, strongly indicating the electrocatalytic oxidation of phenol by PPy/b-MnO2/GF could be much more easily achieved at lower potential compared with blank GF. These results indicated that the PPy/b-MnO2/GF electrode had higher specific capacitance and electrocatalytic activity, which might result from the unique crystallographic structure of b-MnO2. In addition, its interaction with PPy could provide high electrical conductivity and more accessible surface active sites, as well as special fibrous and porous structure [42].

constant model [8]. Except for GF, the impedance data of the modified electrodes were fitted to equivalent electrical circuit with Warburg’s diffusion element (W) (shown in the inset of Fig. 5b). The values of charge transfer resistance (Rct) for GF, PPy/GF and PPy/b-MnO2/GF electrodes were estimated to be 186.3, 27.7 and 17.1 V, respectively. It could be inferred that the modification of GF electrode caused a significant decrease in Rct compared to the blank GF electrode, indicating much higher electron transfer efficiency of modified GF electrode. The lowest Rct obtained for PPy/b-MnO2/GF might be explained by pronounced synergistic effect between PPy and MnO2, which afforded efficient pathways for ion diffusion and electron transfer throughout the electrode [41]. Furthermore, PPy/b-MnO2/GF had a more vertical shape at low frequencies than PPy/GF, indicating its better capacitive behavior with lower diffusion resistance, which was consistent with CV results. Considering its low charge transfer resistance and excellent capacitive behavior, PPy/b-MnO2/GF seemed to be most promising to catalyze phenol degradation, compared with PPy/GF and blank GF.

3.3.2. EIS analysis EIS was a useful technique to estimate the resistance data [49]. The Nyquist impedance plots of the electrodes were presented in Fig. 5b. A semicircle was observed in each curve, suggesting that the impedance spectra of all anodes followed the one-time

3.3.3. Stability tests by CA In this study, the short-term stability of GF, PPy/GF and PPy/ b-MnO2/GF was evaluated by CA tests, as shown in Fig. 5c. The initial polarization current of the PPy/b-MnO2/GF was about 45 mA while it was only 13 mA and 0.03 mA for PPy/GF and GF,

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Fig. 3. Raman spectra of the blank GF, b-MnO2, PPy/GF and PPy/b-MnO2/GF.

respectively. Gradual decrease of the oxidation current with time could be seen for both PPy/GF and PPy/b-MnO2/GF electrodes, although the blank GF always exhibited extremely low polarization current. After 1200 s, the oxidation current of PPy/b-MnO2/GF was still higher than those of PPy/GF and blank GF. PPy/b-MnO2/GF exhibited higher current but lower decay rate as compared to PPy/ GF, suggesting its superior stability and catalytic activity, which was beneficial for phenol oxidation [50]. The higher stability for PPy/b-MnO2/GF indicated by CA tests could be attributed to the synergistic effect of PPy and MnO2. 3.4. Application of the modified electrodes in BESs Three two-chamber BES reactors equipped with GF, PPy/GF and PPy/b-MnO2/GF anodes, which were remarked as R-GF, R-PPy/GF and R-PPy/b-MnO2/GF, respectively, were used to verify the performance of the modified electrodes in BESs. The anode potential of R-PPy/b-MnO2/GF, R-PPy/GF and R-GF reached below 400 mV within 7 days, 10 days and 14 days, respectively. Thereafter, the anode potentials of BESs were well maintained at around 400 mV, suggesting the high activity of the electrochemically active microorganisms on the anode in acetate oxidation and electron transfer [33]. PPy/b-MnO2/GF showed excellent biocompatibility as the anode potential of R-PPy/ b-MnO2/GF reached below 400 mV within as short as 7 days [38]. 30 days after the start-up of BESs, phenol was used as the sole electron donor and carbon source to investigate its biodegradation and power generation. As shown in Fig. 6a, complete removal of 200 mg/L phenol could be achieved within 4 days in R-PPy/b-MnO2/GF (bio-anode). RPPy/b-MnO2/GF without microorganisms in the anode was operated as control. As shown in Fig. S3, phenol removal efficiency less than 7% was achieved in the control reactor after 7 days. The significant difference in terms of phenol removal at the presence or absence of microorganisms indicated that biodegradation mainly contributed to the phenol removal in the BES. However, in R-GF (bio-anode) and R-PPy/GF (bio-anode), complete phenol removal could be achieved within 7 days and 6 days, respectively. Therefore, the superior removal performance of R-PPy/b-MnO2/GF was attributed to the strong synergetic effect of PPy/b-MnO2/GF and

Fig. 4. Surface scanning XPS full spectra (a), Mn 2p XPS (b) and N 1 s core level XPS (c) of the PPy/b-MnO2 composite.

phenol-degrading bacteria rather than the modified electrode or the microorganisms alone. COD removal followed the similar trend, as indicated in Fig. 6b. Within a typical cycle of 7 days, COD removal in R-PPy/b-MnO2/GF, R-PPy/GF and R-GF reached 99.6
0.2%, 91.0
3.6% and 72.5
4.7%, respectively. It was quite interesting that COD removal in R-PPy/b-MnO2/GF was always significantly higher than those in R-PPy/GF and R-GF, indicating much higher mineralization capacity of BES catalyzed by PPy/ b-MnO2/GF. After 6 months’ operation, R-PPy/b-MnO2/GF still showed good phenol degradation and higher COD removal compared to R-GF and R-PPy/GF, demonstrating the feasibility of PPy/b-MnO2/GF for long-term application in BES.

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Fig. 5. CV curves of the modified electrodes at 1 mV s1 in 100 mg L1 phenol solution (a), Nyquist plots (b) and CA curves (c) of modified electrodes and blank GF electrode. The inset showed the equivalent circuit.

Constant and repeatable power cycles were observed during every refill of BES anode chamber. As indicated in Fig. 6c, at the beginning of a typical cycle, the current produced increased slowly, probably due to the refractory characteristics and high toxicity of phenol. 2 days later, current produced increased significantly, with peak current reached. The peak current obtained in R-GF, R-PPy/GF and R-PPy/b-MnO2/GF was 0.023, 0.030 and 0.047 mA within 66, 56 and 50 h, respectively. With the further decrease of phenol and COD concentrations, current produced decreases and finally reached a plateau. However, the current obtained in R-PPy/ b-MnO2/GF was always higher than those in R-GF and R-PPy/GF. The current obtained followed in the order of R-PPy/b-MnO2/GF > R-PPy/GF > R-GF. Furthermore, the CEs of the BESs were calculated to be 6.6
1.3%, 12.1
2.4% and 17.3
0.5%, when using GF, PPy/GF

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Fig. 6. Removal efficiencies of phenol (a) and COD (b) in BESs as a function of reaction time, and a representative cycle of current generation (c) in different BESs.

and PPy/b-MnO2/GF electrode as the bio-anode, respectively. Compared to R-GF and R-PPy/GF, BES equipped with PPy/b-MnO2/ GF anode resulted in the increase of CE by 2.6 and 1.4 fold, indicating the effective utilization of electron as electricity. 200 days after the start-up of BESs, SEM analysis on the bioanodes in R-GF, R-PPy/GF and R-PPy/b-MnO2/GF was conducted, as shown in Fig. 2b, d and f. The bacteria grown on the anode of all the three BESs exhibited rod shape. Only sparse and uneven bacterial community could be found on blank GF electrode, probably due to the limited BET surface and poor biocompatibility of GF. However, abundant rod-shaped bacteria could be seen on PPy/b-MnO2/GF, probably due to the porous structure and good biocompatibility of PPy/b-MnO2/GF. PPy/b-MnO2 composites were specifically and homogeneously grown on the fiber surfaces, which provided enough roughness for bacteria adhesion. The increased electron transfer efficiency between bacteria and anode

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was a consequence of increased interfacial surface area and improved adhesion of bacteria on the anode material [8]. 3.5. Phenol degradation pathway Both GC/MS and HPLC were used to identify the intermediates during phenol degradation. The mass spectrum of catechol, resorcine, hydroquinone, phloroglucinol, glycerol and hydroxyacetic acid after derivatization by BSA was shown in Fig. S4. In addition, pyruvate and formic acid was detected by HPLC shown in Fig. S5. However, fumaric acid, which was also presumed to be involved in phenol degradation, was not detectable in this study. Based on the identified intermediates, the possible degradation pathways of phenol on PPy/b-MnO2/GF anode in BES were proposed, as demonstrated in Fig. 7. On one hand, OH radical produced on the surface of MnO2 could attack the phenyl ring [51], yielding intermediates such as catechol, resorcine, hydroquinone and phloroglucinol. Then the phenyl rings in these compounds broke up to give fumaric acid and then short-chain organic compounds such as glycerol, hydroxy-acetic acid, formic acid, and finally to CO2. This degradation pathway of phenol was similar to that in TiO2 photocatalysis system mentioned by Grabowska et al. [52]. On the other hand, phenol was firstly transferred to catechol catalyzed by hydroxylase [53], then further biodegradation of catechol led to phenol ring cleavage and formation of low molar mass compounds such as pyruvate, formic acid, and so on [54]. Both route 1 and route 2 described above contributed to the enhanced phenol removal on PPy/b-MnO2/GF anode in BES. 3.6. The key role of b-MnO2 in phenol degradation and power generation As was indicated previously, the participation of b-MnO2 could improve the BET surface of the GF electrode, which provided enough roughness for bacteria adhesion and enables more accessible surface active sites for redox reactions and thus helped to promote the anode performance. b-MnO2 played a key role in both phenol degradation and power generation. However, the mechanism involved was still unclear and deserves to be

investigated throughly. According to Grebel et al. [55], manganese oxides were capable of catalyzing and oxidizing a variety of aromatic Lewis bases, including phenols, aromatic amines and aromatic thiols. Li et al. [56] indicated that MnO2 showed a fast charge-discharge capability, analogous to non-Faradaic energy storage behavior. Considering the chemical and electrochemical characteristic of MnO2, the role of b-MnO2 involved in phenol degradation and power generation was proposed. On one hand, b-MnO2 was an efficient oxidant capable of oxidizing various recalcitrant compounds even at neutral pH condition [51]. Generally, the oxidation mechanism of MnO2 was ascribed to radicals formed via electron transfer [51,57]. Therefore, it could be inferred that oxidation capacity of b-MnO2 on GF surface would contribute to phenol removal. What’s more, the potential of w(MnO2/Mn2+) was as high as 1.23 V, which was high enough to oxidize pyrrole monomers as the oxidation potential of pyrrole was no more than 0.7 V [58]. In the process of electropolymerzation, some pyrrole monomers would adsorb uniformly on the exterior surfaces of solid b-MnO2, resulting in the formation of PPy nanolayers sheathed on the surfaces of b-MnO2 nanorods. b-MnO2 was served as rigid backbones to support PPy by interlinking the polymer chains and then special coaxial nanorods were obtained. Similar processes were also reported by many researches [16,17,21,23,59]. b-MnO2 cores had an intimate electrical connection to the PPy nanosheath, which built a reliable conductive network for fast electron transport throughout the electrode. The nanostructure provided short solid-state pathways for the ion diffusion and rapid charge collection/transfer. Accordingly, a maximum harvest of pseudocapacitance from b-MnO2 and PPy components was promoted. As a consequence, the combination of b-MnO2 and PPy would give rise to a strong synergetic effect for improving electrochemical performance. On the other hand, at the presence of anaerobic phenol degrading species, phenol and its intermediates was biodegraded in the anode of BES, with electron released and transferred to anode by potential electroactive species. As an alternative way, partial electrons could be stored by MnO2, according to Eq. (2) [56]: MnO2 + xM+ + yH+ + (x + y)e $ MnOOMxHy (2)

Fig. 7. Phenol degradation pathways in BES anode using PPy/b-MnO2/GF as anodic electrode.

(M+ = Na+, K+, et al)

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127

Table 1 Comparison of phenol degradation performance in BESs using different anode materials. Anode

Substrate

Removal efficiency (%)

Removal rate (mg L1 phenol d1 cm2) CE (%)

Reference

PPy/b-MnO2/GF Carbon paper Carbon felt Activated carbon fibers Graphite rods Graphite sheet

200 mg L1 phenol 400 mg L1 phenol 600 mg L1 phenol 200 mg L1 phenol 13.2 mg L1 phenol Retting wastewater containing 320
60 mg L1 phenol 100 mg L1 acetate + 200 mg L1 phenol

99.1
0.6 95.5
1.7 88.9 57.2 92.9 80

12.39
0.03 2.55 4.68 6.36 0.38 0.15

17.3
0.5 <10.0 3.68 3.85 22.7 19

This study [61] [62] [63] [64] [65]

61
5

1.15
0.09

13
5

[66]

Carbon paper

In this case, electrons liberated from the degradation of phenol and its intermediates were stored in the form of MnOOMxHy, when electron transfer was hindered by the immediate unavailability of electron acceptor [15]. MnOOMxHy, which was unstable, could be easily converted to MnO2, with electron released and transferred onto anode electrode. The storage of electron by MnO2 and release of electron by MnOOMxHy resulted into the high electron recovery and thus efficient power generation. Efficient electron transfer could promote substrate degradation in turn [60], which could be another explanation of enhanced phenol removal in BES using PPy/ b-MnO2/GF as anode. 3.7. Implications of this work Although recalcitrant organics such as phenol has been reported to be biologically degraded in the anode of BESs, the removal performance and CEs were usually very low [5,61–66]. However, improved phenol removal performance and CEs was observed in the case of PPy/b-MnO2/GF used as anode. As shown in Table 1, the maximum phenol removal efficiencies and phenol removal rates by PPy/b-MnO2/GF anode were 99.1
0.6% and 12.39
0.03 mg L1 phenol d1 cm2, respectively, which were higher than those of conventional electrodes such as carbon paper and carbon felt [61–64]. In addition, CEs in BES using PPy/b-MnO2/ GF anode was as high as 17.3
0.5% when phenol was used as the sole electron donor, which was significantly higher than those of conventional electrodes [61–63]. Due to the relatively high CEs, the BESs using PPy/b-MnO2/GF anode showed higher electron utilization efficiency, even compared with the BESs using acetate as supplementary electron donor [1,66]. What’s more, as indicated by Fig. S6, the time needed for the fabrication of PPy/b-MnO2 composite onto GF electrode was less than 7 min, indicating the convenience and effectiveness for the fabrication of PPy/b-MnO2/ GF anode. Therefore, PPy/b-MnO2/GF could be a promising alternative to the conventional electrodes for the removal of recalcitrant organics such as phenol in BESs. Our study provides a platform towards the design of simple, inexpensive materials with extraordinary electron pathways and catalytic performance, which can be used as high-performance BES anodes for wastewater treatment. Further study is needed to examine the dynamic change of anodic microbial community during the start-up and long-term operation of the BESs, with the effect of polypyrrole/b-MnO2 on microbial community emphasized. In addition, the effect of polypyrrole/b-MnO2 proportion and MnO2 type (i.e., a-, b and g -MnO2) on phenol catalytic performance will be investigated in our further study. 4. Conclusions This study proposed a convenient and effective electropolymerzation method for the fabrication of PPy/b-MnO2 composite onto GF electrode. R-PPy/b-MnO2/GF showed smaller internal resistance, shorter startup time, improved bacteria adhesion,

better phenol degradation activity, higher mineralization efficiency and higher CE, compared to R-GF and R-PPy/GF. The superior performance of PPy/b-MnO2/GF could be attributed to the key role of b-MnO2 as efficient energy storage, oxidant and catalyst. PPy/ b-MnO2/GF anode should have a promising future in BES application for recalcitrant phenol degradation. Acknowledgements This work was financed by Major Project of Water Pollution Control and Management Technology of China (No. 2012ZX07101003-001), National Natural Science Foundation of China (NSFC) (No. 51378261) and Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.electacta.2017. 05.108. References [1] J. Shen, Y. Zhang, X. Xu, C. Hua, X. Sun, J. Li, Y. Mu, L. Wang, Role of molecular structure on bioelectrochemical reduction of mononitrophenols from wastewater, Water Res. 47 (2013) 5511–5519. [2] J. Villaseñor, P. Capilla, M.A. Rodrigo, P. Cañizares, F.J. Fernández, Operation of a horizontal subsurface flow constructed wetland – Microbial fuel cell treating wastewater under different organic loading rates, Water Res. 47 (2013) 6731– 6738. [3] H. Tang, Y. Zeng, Y. Zeng, R. Wang, S. Cai, C. Liao, H. Cai, X. Lu, P. Tsiakaras, Ironembedded nitrogen doped carbon frameworks as robust catalyst for oxygen reduction reaction in microbial fuel cells, Appl. Catal. B—Environ. 202 (2017) 550–556. [4] D. Pant, G. Van Bogaert, L. Diels, K. Vanbroekhoven, A review of the substrates used in microbial fuel cells (MFCs) for sustainable energy production, Bioresource Technol. 101 (2010) 1533–1543. [5] H.-Y. Cheng, B. Liang, Y. Mu, M.-H. Cui, K. Li, W.-M. Wu, A.-J. Wang, Stimulation of oxygen to bioanode for energy recovery from recalcitrant organic matter aniline in microbial fuel cells (MFCs), Water Res. 81 (2015) 72–83. [6] C.M. Werner, C. Hoppe-Jones, P.E. Saikaly, B.E. Logan, G.L. Amy, Attenuation of trace organic compounds (TOrCs) in bioelectrochemical systems, Water Res. 73 (2015) 56–67. [7] R. Burkitt, T.R. Whiffen, E.H. Yu, Iron phthalocyanine and MnOx composite catalysts for microbial fuel cell applications, Appl. Catal. B—Environ. 181 (2016) 279–288. [8] Z. Lv, Y. Chen, H. Wei, F. Li, Y. Hu, C. Wei, C. Feng, One-step electrosynthesis of polypyrrole/graphene oxide composites for microbial fuel cell application, Electrochim. Acta 111 (2013) 366–373. [9] C. Feng, F. Li, K. Sun, Y. Liu, L. Liu, X. Yue, H. Tong, Understanding the role of Fe (III)/Fe(II) couple in mediating reductive transformation of 2-nitrophenol in microbial fuel cells, Bioresource Technol. 102 (2011) 1131–1136. [10] J. Ma, C. Wang, H. He, Transition metal doped cryptomelane-type manganese oxide catalysts for ozone decomposition, Appl. Catal. B—Environ. 201 (2017) 503–510. [11] J. Lin, Y. Zheng, Q. Du, M. He, Z. Deng, Synthesis and electrochemical properties of graphene/MnO2/conducting polymer ternary composite for supercapacitors, Nano 08 (2013) 1350004. [12] N.D. Wasalathanthri, T.M. SantaMaria, D.A. Kriz, S.L. Dissanayake, C.-H. Kuo, S. Biswas, S.L. Suib, Mesoporous manganese oxides for NO2 assisted catalytic soot oxidation, Appl. Catal. B—Environ. 201 (2017) 543–551.

128

D. Chen et al. / Electrochimica Acta 244 (2017) 119–128

[13] G. Yu, L. Hu, N. Liu, H. Wang, M. Vosgueritchian, Y. Yang, Y. Cui, Z. Bao, Enhancing the supercapacitor performance of graphene/MnO2 nanostructured electrodes by conductive wrapping, Nano Lett. 11 (2011) 4438–4442. [14] C. Zhang, P. Liang, Y. Jiang, X. Huang, Enhanced power generation of microbial fuel cell using manganese dioxide-coated anode in flow-through mode, J. Power Sources 273 (2015) 580–583. [15] Z. Lv, D. Xie, X. Yue, C. Feng, C. Wei, Ruthenium oxide-coated carbon felt electrode: A highly active anode for microbial fuel cell applications, J. Power Sources 210 (2012) 26–31. [16] J.-G. Wang, Y. Yang, Z.-H. Huang, F. Kang, MnO2/polypyrrole nanotubular composites: reactive template synthesis, characterization and application as superior electrode materials for high-performance supercapacitors, Electrochim. Acta 130 (2014) 642–649. [17] H. Yuan, L. Deng, Y. Chen, Y. Yuan, MnO2/Polypyrrole/MnO2 multi-wallednanotube-modified anode for high-performance microbial fuel cells, Electrochim. Acta 196 (2016) 280–285. [18] J.F. Rivera, C. Bucher, E. Saint-Aman, B.L. Rivas, M.d.C. Aguirre, J. Sanchez, I. Pignot-Paintrand, J.-C. Moutet, Removal of arsenite by coupled electrocatalytic oxidation at polymer-ruthenium oxide nanocomposite and polymer-assisted liquid phase retention, Appl. Catal. B—Environ. 129 (2013) 130–136. an, F. Köleli, Electrochemical reduction of carbon dioxide on [19] R. Aydın, H.Ö. Dog polypyrrole coated copper electro-catalyst under ambient and high pressure in methanol, Appl. Catal. B—Environ. 140–141 (2013) 478–482. [20] S. Ameen, M.S. Akhtar, H.-K. Seo, H.S. Shin, Distinctive polypyrrole nanobelts as prospective electrode for the direct detection of aliphatic alcohols: Electrocatalytic properties, Appl. Catal. B—Environ. 144 (2014) 665–673. [21] A. Bahloul, B. Nessark, E. Briot, H. Groult, A. Mauger, K. Zaghib, C.M. Julien, Polypyrrole-covered MnO2 as electrode material for supercapacitor, J. Power Sources 240 (2013) 267–272. [22] K. Liang, T. Gu, Z. Cao, X. Tang, W. Hu, B. Wei, In situ synthesis of SWNTs@MnO2/ polypyrrole hybrid film as binder-free supercapacitor electrode, Nano Energy 9 (2014) 245–251. [23] W. Yao, H. Zhou, Y. Lu, Synthesis and property of novel MnO2@polypyrrole coaxial nanotubes as electrode material for supercapacitors, J. Power Sources 241 (2013) 359–366. [24] L. Han, P. Tang, L. Zhang, Hierarchical Co3O4@PPy@MnO2 core-shell-shell nanowire arrays for enhanced electrochemical energy storage, Nano Energy 7 (2014) 42–51. [25] T. Jiang, Y. Wang, K. Wang, Y. Liang, D. Wu, P. Tsiakaras, S. Song, A novel sulfurnitrogen dual doped ordered mesoporous carbon electrocatalyst for efficient oxygen reduction reaction, Appl. Catal. B—Environ. 189 (2016) 1–11. [26] D.B. Kayan, F. Köleli, Simultaneous electrocatalytic reduction of dinitrogen and carbon dioxide on conducting polymer electrodes, Appl. Catal. B—Environ. 181 (2016) 88–93. [27] J. Zhang, H. Yang, S. Xu, L. Yang, Y. Song, L. Jiang, Y. Dan, Dramatic enhancement of visible light photocatalysis due to strong interaction between TiO2 and endgroup functionalized P3HT, Appl. Catal. B—Environ. 174–175 (2015) 193–202. [28] M. Lu, L. Guo, S. Kharkwal, H.N. Wu, H.Y. Ng, S.F.Y. Li, Manganese-polypyrrolecarbon nanotube, a new oxygen reduction catalyst for air-cathode microbial fuel cells, J. Power Sources 221 (2013) 381–386. [29] J. Yang, M. Zhou, Y. Zhao, C. Zhang, Y. Hu, Electrosorption driven by microbial fuel cells to remove phenol without external power supply, Bioresource Technol. 150 (2013) 271–277. [30] R.B. Moore, C.R. Martin, Chemical and morphological properties of solutioncast perfluorosulfonate ionomers, Macromolecules 21 (1988) 1334–1339. [31] L. Zhang, C. Liu, L. Zhuang, W. Li, S. Zhou, J. Zhang, Manganese dioxide as an alternative cathodic catalyst to platinum in microbial fuel cells, Biosens. Bioelectron. 24 (2009) 2825–2829. [32] X. Wang, Y. Li, Selected-control hydrothermal synthesis of a- and b-MnO2 single crystal nanowires, J. Am. Chem. Soc. 124 (2002) 2880–2881. [33] J. Shen, X. Xu, X. Jiang, C. Hua, L. Zhang, X. Sun, J. Li, Y. Mu, L. Wang, Coupling of a bioelectrochemical system for p-nitrophenol removal in an upflow anaerobic sludge blanket reactor, Water Res. 67 (2014) 11–18. [34] X. Jiang, S. Lou, D. Chen, J. Shen, W. Han, X. Sun, J. Li, L. Wang, Fabrication of polyaniline/graphene oxide composite for graphite felt electrode modification and its performance in the bioelectrochemical system, J. Electroanal. Chem. 744 (2015) 95–100. [35] W. Han, C. Zhong, L. Liang, Y. Sun, Y. Guan, L. Wang, X. Sun, J. Li, Electrochemical degradation of triazole fungicides in aqueous solution using TiO2-NTs/SnO2Sb/PbO2 anode: Experimental and DFT studies, Electrochim. Acta 130 (2014) 179–186. [36] D. Ma, L. Chen, Y. Wu, R. Liu, Evaluation of the removal of antiestrogens and antiandrogens via ozone and granular activated carbon using bioassay and fluorescent spectroscopy, Chemosphere 153 (2016) 346–355. [37] B. Zhao, R. Ran, X. Wu, D. Weng, Phase structures morphologies, and NO catalytic oxidation activities of single-phase MnO2 catalysts, Appl. Catal. A—Gen. 514 (2016) 24–34. [38] J. Hui, X. Jiang, H. Xie, D. Chen, J. Shen, X. Sun, W. Han, J. Li, L. Wang, Laccasecatalyzed electrochemical fabrication of polyaniline/graphene oxide composite onto graphite felt electrode and its application in bioelectrochemical system, Electrochim. Acta 190 (2016) 16–24. [39] S. He, W. Chen, Application of biomass-derived flexible carbon cloth coated with MnO2 nanosheets in supercapacitors, J. Power Sources 294 (2015) 150–158.

[40] W. Ma, S. Chen, S. Yang, W. Chen, Y. Cheng, Y. Guo, S. Peng, S. Ramakrishna, M. Zhu, Hierarchical MnO2 nanowire/graphene hybrid fibers with excellent electrochemical performance for flexible solid-state supercapacitors, J. Power Sources 306 (2016) 481–488. [41] J.-G. Wang, Y. Yang, Z.-H. Huang, F. Kang, Rational synthesis of MnO2/ conducting polypyrrole@carbon nanofiber triaxial nano-cables for highperformance supercapacitors, J. Mater. Chem. 22 (2012) 16943–16949. [42] M. Lu, S. Kharkwal, H.Y. Ng, S.F.Y. Li, Carbon nanotube supported MnO2 catalysts for oxygen reduction reaction and their applications in microbial fuel cells, Biosens. Bioelectron. 26 (2011) 4728–4732. [43] Y. Yang, M.H. Diao, M.M. Gao, X.F. Sun, X.W. Liu, G.H. Zhang, Z. Qi, S.G. Wang, Facile preparation of graphene/polyaniline composite and its application for electrocatalysis hexavalent chromium reduction, Electrochim. Acta 132 (2014) 496–503. [44] X. Li, Y. Wu, F. Zheng, M. Ling, F. Lu, Preparation and characterization of RuO2/ polypyrrole electrodes for supercapacitors, Solid State Commun. 197 (2014) 57–60. [45] P. Li, B. Ding, L. Wang, Y. Guo, X. Wang, W. Zhang, Investigation of capacitance storage and cyclic voltammetry for the electrochemical oxidation of phenol on iridium and lead oxide electrodes, J.Solid State Electr. 20 (2016) 3429–3436. [46] R. Singh, D. Mishra, Pd-1% Ni composite electrodes for electrooxidation of phenol in acid solution, Int. J. Electrochem. Sci. 4 (2009) 1638–1649. [47] I. Uskova, O. Bulgakova, Cyclic voltammetry of phenol, J. Anal. Chem. 69 (2014) 542–547. [48] C.-Y. Yang, S.-M. Chen, T.-H. Tsai, B. Unnikrishnan, Poly (Diphenylamine) with multi-walled carbon nanotube composite film modified electrode for the determination of phenol, Int. J. Electrochem. Sci 7 (2012) 12796–12807. [49] R.T. Vinny, K. Chaitra, K. Venkatesh, N. Nagaraju, N. Kathyayini, An excellent cycle performance of asymmetric supercapacitor based on bristles like a-MnO2 nanoparticles grown on multiwalled carbon nanotubes, J. Power Sources 309 (2016) 212–220. [50] H.J. Kim, S.M. Choi, M.H. Seo, S. Green, G.W. Huber, W.B. Kim, Efficient electrooxidation of biomass-derived glycerol over a graphene-supported PtRu electrocatalyst, Electrochem. Commun. 13 (2011) 890–893. [51] X. Liao, C. Zhang, Y. Liu, Y. Luo, S. Wu, S. Yuan, Z. Zhu, Abiotic degradation of methyl parathion by manganese dioxide: Kinetics and transformation pathway, Chemosphere 150 (2016) 90–96.  ska, A. Zaleska, Mechanism of phenol [52] E. Grabowska, J. Reszczyn photodegradation in the presence of pure and modified-TiO2: A review, Water Res. 46 (2012) 5453–5471. [53] E. Heinaru, E. Naanuri, M. Grünbach, M. Jõesaar, A. Heinaru, Functional redundancy in phenol and toluene degradation in Pseudomonas stutzeri strains isolated from the Baltic Sea, Gene 589 (2016) 90–98. [54] E. Díaz, J.I. Jiménez, J. Nogales, Aerobic degradation of aromatic compounds, Curr. Opin. Biotech. 24 (2013) 431–442. [55] J.E. Grebel, J.A. Charbonnet, D.L. Sedlak, Oxidation of organic contaminants by manganese oxide geomedia for passive urban stormwater treatment systems, Water Res. 88 (2016) 481–491. [56] L. Li, Z.A. Hu, N. An, Y.Y. Yang, Z.M. Li, H.Y. Wu, Facile Synthesis of MnO2/CNTs composite for supercapacitor electrodes with long cycle stability, J. Phys. Chem. C 118 (2014) 22865–22872. [57] A.T. Stone, Reductive dissolution of manganese(III/Iv) oxides by substituted phenols, Environ. Sci. Technol. 21 (1987) 979–988. [58] L. Pan, L. Pu, Y. Shi, S. Song, Z. Xu, R. Zhang, Y. Zheng, Synthesis of polyaniline nanotubes with a reactive template of manganese oxide, Adv. Mater. 19 (2007) 461. [59] H. Yuan, L. Deng, J. Tang, S. Zhou, Y. Chen, Y. Yuan, Facile synthesis of MnO2/ Polypyrrole/MnO2 multiwalled nanotubes as advanced electrocatalysts for the oxygen reduction reaction, ChemElectroChem 2 (2015) 1152–1158. [60] Y. Yang, Y. Xiang, C. Xia, W.-M. Wu, G. Sun, M. Xu, Physiological and electrochemical effects of different electron acceptors on bacterial anode respiration in bioelectrochemical systems, Bioresource Technol. 164 (2014) 270–275. [61] H. Luo, G. Liu, R. Zhang, S. Jin, Phenol degradation in microbial fuel cells, Chem. Eng. J. 147 (2009) 259–264. [62] T.-S. Song, X.-Y. Wu, C.C. Zhou, Effect of different acclimation methods on the performance of microbial fuel cells using phenol as substrate, Bioproc. Biosyst. Eng. 37 (2014) 133–138. [63] J. Yang, Y. Zhao, C. Zhang, Y. Hu, M. Zhou, Electrosorption driven by microbial fuel cells without electric grid energy consumption for simultaneous phenol removal and wastewater treatment, Electrochem. Commun. 34 (2013) 121– 124. [64] D. Zhang, Z. Li, C. Zhang, X. Zhou, Z. Xiao, T. Awata, A. Katayama, Phenoldegrading anode biofilm with high coulombic efficiency in graphite electrodes microbial fuel cell, J. Biosci. Bioeng. 123 (2017) 364–369. [65] C. Jayashree, P. Arulazhagan, S. Adish Kumar, S. Kaliappan, I.T. Yeom, J. Rajesh Banu, Bioelectricity generation from coconut husk retting wastewater in fed batch operating microbial fuel cell by phenol degrading microorganism, Biomass Bioenergy 69 (2014) 249–254. [66] G. Buitrón, I. Moreno-Andrade, Performance of a single-chamber microbial fuel cell degrading phenol: Effect of phenol concentration and external resistance, Appl. Biochem. Biotech. 174 (2014) 2471–2481.