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Carbon filtration cathode in microbial fuel cell to enhance wastewater treatment
Bioresource Technology 185 (2015) 426–430
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Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Short Communication
Carbon filtration cathode in microbial fuel cell to enhance wastewater treatment Kuichang Zuo, Shuai Liang, Peng Liang 1, Xuechen Zhou, Dongya Sun, Xiaoyuan Zhang, Xia Huang ⇑ State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing 100084, PR China
h i g h l i g h t s A homogeneous carbon filtration membrane was fabricated without using noble metals. Excellent effluent quality and power generation were achieved in the filtration MFC. The performance of filtration MFC kept relative stable during 20 days operation.
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Article history: Received 3 January 2015 Received in revised form 25 February 2015 Accepted 27 February 2015 Available online 6 March 2015 Keywords: Carbon filtration membrane Microbial fuel cell Advanced wastewater treatment Energy recovery
a b s t r a c t A homogeneous carbon membrane with multi-functions of microfiltration, electron conduction, and oxygen reduction catalysis was fabricated without using noble metals. The produced carbon membrane has a pore size of 553 nm, a resistance of 6.0 ± 0.4 X cm2/cm, and a specific surface area of 32.2 m2/g. After it was assembled in microbial fuel cell (MFC) as filtration air cathode, a power density of 581.5 mW/m2 and a current density of 1671.4 mA/m2 were achieved, comparable with previous Pt air cathode MFCs. The filtration MFC was continuously operated for 20 days and excellent wastewater treatment performance was also achieved with removal efficiencies of TOC (93.6%), NH+4–N (97.2%), and total nitrogen (91.6%). In addition, the carbon membrane was much cheaper than traditional microfiltration membrane, suggesting a promising multi-functional material in wastewater treatment field. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Microbial fuel cell (MFC) is an electrochemical device that utilizes electroactive bacteria to recover energy in the form of electricity through oxidization of organic matter. As it couples electricity generation to wastewater treatment, MFC has drawn a great attention and received significant improvement in structure and performance in recent years. During the past decade, MFC has achieved a 105 times increase in power density from 0.01 mW/m2 in 1999 (Kim et al., 1999) to 2.7 W/m2 in 2007 (Fan et al., 2007) and various configuration with reactor size ranged from mL to >50 L (Liang et al., 2013) were reported. However, the present MFC is still impotent in achieving high quality effluent (Ge et al., 2013), due to the limited biomass retention (Logan, 2008) and low reaction rate under anaerobic condition in anode.
⇑ Corresponding author. Tel.: +86 10 62772324. E-mail addresses: [email protected] (P. Liang), [email protected]. cn (X. Huang). 1 Co-corresponding author. Tel.: +86 10 62796790. http://dx.doi.org/10.1016/j.biortech.2015.02.108 0960-8524/Ó 2015 Elsevier Ltd. All rights reserved.
Integration of MFC with filtration technology may provide a potential solution to the existing problems. In a filtration bioelectrochemical system, energy could be ideally extracted in the form of electricity through oxidation of organic matter, while a high quality permeate could be obtained due to the filtration of membrane, achieving simultaneous energy recovery and advanced wastewater treatment. In previous studies, microfiltration (MF) or ultrafiltration (UF) membranes have been used as separation materials instead of expensive ion-exchange membranes in MFCs to intercept organic matter and biomass in anode (Kyoung Yeol et al., 2013). However, these traditional MF or UF membranes presented low mass transfer due to their low ion conductivity, resulting in a high internal resistance (Kim et al., 2007) of the integrated filtration MFC system. Another combination approach could be realized by using a conductive filtration membrane as air- or biocathode of a single-chamber MFC, thereby avoiding the requirements for separator or aeration in catholyte. Stainless steel mesh (Wang et al., 2011) and carbon-nanotube-deposited UF membrane (Malaeb et al., 2013) were employed as such conductive membranes to constitute a filtration cathode, and the feasibility and capacity of this configuration for simultaneous wastewater
K. Zuo et al. / Bioresource Technology 185 (2015) 426–430
treatment and energy recovery had been confirmed. Nevertheless, these fabricated filtration cathode materials had the following shortages: (I) nonuniform or much bigger pore size compared to MF or UF membrane (Wang et al., 2011, 2013); (II) utilization of noble metal (Pt) as catalyst (Malaeb et al., 2013); (III) relative high cost for large scale application (Malaeb et al., 2013; Wang et al., 2013). In this study, conductive activated carbon powder, which possessed high specific surface area, was used as raw material for preparation of a homogeneous MF membrane. The obtained carbon MF membrane was then set as an air cathode to establish a filtration microbial fuel cell (F-MFC), which exhibited combined functions of MFC and membrane bioreactor (MBR). The physicochemical characteristics of the carbon filtration membrane were investigated before assembled in MFC. The power production and pollutants removal were evaluated during 20 days operation. And the performance of polluted filtration cathode in terms of power generation and flux were further investigated. In addition, the state-of-the-art progress in filtration MFC fabrication and performance was also summarized.
2. Methods 2.1. Fabrication of carbon filtration membrane Carbon powder, originated from milling of activated carbon granule (Weishimei Environmental Technology Co., Ltd., Beijing, China), was screened with a 70-mesh sieve. Then 2.4 g screenthroughs were adequately mixed with 0.3 g sodium carboxymethyl cellulose (binder) and 1.2 mL H2O (solvent) by stirring with a glass rod until a fluffy mixture was obtained. After that the fluffy mixture was transferred into a mould and pressed at 25 MPa for 2 min using a press machine (FW-4, Tianjin Optical Instrument Factory, China). The resulted carbon wafer was dewatered in a drying oven and at last carbonized in a vacuum tube furnace (GSL1700X, Heifeikejing, China) at 900 °C for 90 min under argon protection. After cooling down, the obtained carbon membrane was stored at room temperature before use.
2.2. MFC assembly and operation As illustrated in Fig. S1, the filtration MFC is a cylindrical bioreactor (total volume: 28 mL) comprising a carbon brush anode and a carbon filtration air cathode. The anode brushes are 3 cm in length and 3 cm in diameter, which were inoculated with anolyte of a mature MFC that has been operated for several years in our laboratory. The effective area of the cathode is 7 cm2 and the shortest distance between cathode and anode carbon is 1 cm. An O-ring titanium sheet was used as electricity-collecting material for the filtration cathode, and an external resistance of 2000 or 200 X was connected between cathode and anode to simulate an electric appliance. After assembling the MFCs, PBS-buffered acetate was used as synthetic wastewater, which contained (per liter of deionized water) 0.5 g CH3COONa (Chemical oxygen demand, COD: 390.2 mg/L; Total organic carbon, TOC: 146.3 mg/L), 0.15 g NH4Cl (NH+4–N: 39.3 mg/L), 1.04 g KH2PO4, and 2.16 g K2HPO4, all chemicals were reagent grade and purchased from Sigma-Aldrich (St. Louis, MO). During operation, one air cathode MFC was operated intermittently with synthetic wastewater replaced every 48 h as traditional batch mode MFC, therefore it was nominated as traditional MFC (T-MFC). In contrast, the other MFC was continuously operated with synthetic wastewater firstly pumped into the anode and further go through the filtration cathode at a flow rate of
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0.58 mL/h (hydraulic retention time (HRT) of 48 h), it was nominated as filtration MFC (F-MFC, Fig. S1). 2.3. Measurements and analysis Physicochemical characteristics including morphology and elemental composition were evaluated with scanning electron microscopy (SEM, TESCAN, Czech Republic) and X-ray photoelectron spectroscopy (XPS, Perkin-Elmer Phi 5300, USA) respectively. Porosity, pore size distribution, and specific surface area were completed with a mercury porosimeter (Autopore IV 9510, USA). Capacitance of the carbon filtration membrane was measured by galvanostatic charge-discharge method at both anodic and cathodic current of 10 mA in 5 M KOH solution using an electrochemical workstation (Autolab, Metrohm, Switzerland). Catalytic performance of carbon filtration membrane was evaluated by linear sweep voltammetry (LSV), with platinum wire used as control electrode and saturated calomel electrode (SCE, 0.242 V vs. standard hydrogen electrode, Leici, China) as reference electrode, and potential swept from +0.6 to 0.4 V at a scan rate of 1 mV/s. The LSV was conducted in a 20 mM PBS solution with three conditions: Ar aeration, O2 aeration, and exposed in air (Fig. S2). After the fabrication of the two MFCs, the cell voltages (U) across the external resistor (Rex) were measured automatically in 1 min interval with a data acquisition system (DAQ2213, ADLINK, China). Current (I) was determined according to ohm’s law: I = U/Rex. Power density (P) and internal resistance (Rin) were calculated according to polarization curve based on the anode chamber volume (28 mL). The coulombic efficiency (CE) was introduced to evaluate the proportion of recovered energy from organic matter, which was calculated according to the following equation:
CE ¼
R 3 Idt FVðC i C e Þ
ð1Þ
where F is Faraday constant (96,485 C/mol), V is the volume of treated wastewater (L), t is operation time (s), Ci and Ce are the TOC concentrations of influent and effluent (g/L), I is produced current (A). During operation, the quality of synthetic wastewater and effluent from the two MFCs were evaluated including TOC, NH+4, NO 2, and NO 3 . TOC was measured by a TOC analyser (Shimadzu TOC VW, Japan). NH+4, NO 2 , and NO3 were quantified by ion chromatography (ICS-1100, DIONEX, USA). Flux of the carbon filtration membrane was measured at different pressures to assess the development of membrane fouling. During the flux measurements, nitrogen gas was utilized to provide a trans-membrane pressure (TMP) for the F-MFC influent, TMP and effluent weight were continuously detected using a pressure sensor and an electric balance (Fig. S3). The calculated flux was normalized to cathode projected area (7 cm2). 3. Results and discussion 3.1. Physicochemical properties of carbon filtration membrane The filtration membrane is a carbon wafer which has a diameter of 38.0 ± 0.2 mm and thickness of 2 ± 0.1 mm (Fig. S4A). It possesses a microporous structure with porosity of 43.4% and pore size of 553 nm (Fig. S4B), and it can hold a water pressure of >50 kPa with an area of 7 cm2. The specific flux is 6.9 ± 0.5 L/(h m2 kPa), which is comparable with traditional ceramic membranes (Nandi et al., 2008). X-ray photoelectron spectroscopy (Fig. S4C) illustrated that the carbon filtration membrane is mainly comprised of C (94.14%), O (3.91%), N (1.49%) and a small amount of metals including Na (0.24%) and Ca (0.13%). The resistivity was 6.0 ± 0.4 X cm2/cm and the specific surface area was 32.2 m2/g.
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Linear sweep voltammetry (LSV) displayed an increase of reduction current at 0.04 V (Fig. S4D) when one side of the carbon membrane was exposed in air (Fig. S2C), 0.2 V higher than the position of reduction current increase (0.16 V) in O2 saturated solution (Fig. S2B). And the reduction current at both conditions (with presence of O2) were much higher than that in Ar aerated solution (Figs. S4D and S2A), indicating excellent oxygen reduction catalysis of the carbon filtration membrane. In addition, the carbon filtration membrane exhibited excellent capacitance (7.0 F/cm2, Fig. S5), which was even higher than previous capacitive electrode material (3.47 (Lv et al., 2014), 0.804 (Zhang et al., 2015) F/cm2) and may promote power production by facilitating electron transfer (Zhang et al., 2015). As compared to previous Pt air/bio cathode which had a reduction current of only 5 mA (Malaeb et al., 2013) and 0.7 mA (Zhang et al., 2013) in LSV measurements at potential of 0.4 V, the better performance of the carbon filtration membrane was mainly due to its porous structure and large specific surface (32.2 m2/g), which provided high encountering probability for three phase reaction of solid cathode, gaseous oxygen and liquid water. Another factor may be the high content of nitrogen (1.49%), which was proved having a good catalysis for oxygen reduction (Feng et al., 2011), indicating an excellent material for carbon membrane as a filtration cathode.
Their maximum power density reached 581.5 and 443.5 mW/m2, and their internal resistance were 210.5 X (F-MFC) and 253.8 X (T-MFC, Fig. 1A), which were comparable with previous Pt air (820 mW/m2) and bio- cathode (380 mW/m2) MFCs (Malaeb et al., 2013). To further evaluate the performance of carbon filtration cathode, the T-MFC and F-MFC were operated for 20 days at external resistance of 2000 X and 200 X (Fig. 1B). F-MFC achieved a voltage of about 508 mV and kept almost stable at this level during the first 10 days of operation at external resistance of 2000 X. And a voltage profile from 101 to 79 mV was observed during the next 10 days operation (external resistance of 200 X). As a contrast, the T-MFC presented a periodic decrease due to the fed-batch operation, and its peak voltage were 421 and 61 mV at 2000 and 200 X respectively, much lower than that of F-MFC. The better performance of F-MFC than T-MFC was attributed to two main reasons: (I) enhanced mass transfer between anode and cathode due to continuous flow, and (II) decreased diffusion distance for O2 in cathode reaction. As shown in Fig. S7, the cathodic three phase (oxygen, water and cathode) reaction in T-MFC happened on the interface adjacent to the inner side of the cathode. However an interface shifting to the outer side of the filtration cathode would happen in F-MFC mode, which decreased the distance for oxygen passive diffusion and may improve power generation.
3.2. Power production of F-MFC and T-MFC
3.3. Organics and nutrient removal during 20 days operation
After fabrication, the carbon filtration membranes were assembled as air cathodes of single-chamber F-MFC and T-MFC, before which the two pre-cultivated anodes were selected from six parallel operated MFCs (with 50 mM potassium ferricyanide as catholyte) and they possessed the most similar performance (Fig. S6). After acclimated for one day at 2000 X resistance, the open circuit voltage of F-MFC and T-MFC reached 605 and 590 mV respectively.
The cathode permeate was also evaluated in terms of TOC and nitrogen during the 20 days operation. As shown in Fig. 1C, the TOC removal efficiency of F-MFC reached 74.2% and 93.7% at external resistance of 2000 and 200 X, the effluent TOC were 37.7 and 9.2 mg/L respectively, which were comparable with traditional MBRs (Sun et al., 2014). In T-MFC, the TOC removal efficiency was only 55.3% and 72.4%, much lower than that of F-MFC. The
Fig. 1. Power generation and organics removal in F-MFC and T-MFC. (A) Polarization and power density curves of traditional MFC (T-MFC) and filtration MFC (F-MFC) after 1 day stabilization at external resistance of 2000 X, (B) voltage generation during long term operation at various external resistance, (C) effluent TOC concentration and TOC removal efficiency, and (D) nitrogen removal efficiency.
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be due to (I) direct utilization of microorganisms, (II) simultaneous nitrification–denitrification in anode anoxic zone, (III) electrochemical denitrification by cathode (as electron donor) (Liang et al., 2013), and (IV) passing through the filtration cathode in the form of NH3. Although the dominant process was still unknown, nitrification did happen in the F-MFCs and T-MFCs, as NO 2 –N and NO3 –N were detected in the effluent (Fig. S9). 3.4. Performance decline due to cathode fouling
Fig. 2. Performance of F-MFC and T-MFC after 20 days operation. (A) Polarization and power density curves, and (B) cathode membrane flux.
coulombic efficiency of T-MFC was 24.0% (2000 X) and 25.6% (200 X), and that for F-MFC was 46.4% (2000 X) and 63.9% (200 X), much higher than previous filtration MFC systems (8.5% (Wang et al., 2011), 19.1% (Wang et al., 2013)). A shaking bottle adsorption experiment showed that the adsorption capacity of activated carbon was 1.7 mg/g at TOC concentration of 146 mg/L (Fig. S8), which means that the 2.5 g carbon filtration membrane can only adsorb 4.2 mg TOC of NaAc (10% of influent TOC during 20 days operation), indicating the major TOC removed by biodegradation. The influent NH4Cl is 0.15 g/L (NH+4–N is 39.3 mg/L). After 48 h hydraulic retention, the NH+4–N removal efficiency of F-MFC reached 83.2% (2000 X, Fig. 1D) and 96.2% (200 X), and the total nitrogen removal efficiency is 91.6% at 200 X, higher than that of T-MFC (91.6% for NH+4–N and 83.7% for TN at external resistance of 200 X). In previous studies, the NH+4 removal was proposed to
After 20 days of operation, the power generation in both two types of MFCs exhibited a decline. The maximum power densities of the F-MFC and T-MFC decreased by 16.8% (483.8 mW/m2) and 8.8% (404.3 mW/m2), and internal resistance increased to 268.8 and 307.1 X respectively (Fig. 2A). LSV profile also illustrated a current decline for the used filtration membranes (Fig. S10), and the performance of used F-MFC cathode was a little bit worse than T-MFC cathode, indicating more serious fouling of the carbon membrane in F-MFC. The specific flux of the used carbon filtration cathode in F-MFC and T-MFC were 5.16 and 5.77 L/(h m2 kPa) (Fig. 2B), which decreased 25.5% and 16.7% respectively compared to the pristine carbon filtration membrane (6.93 L/(h m2 kPa)), despite that they were still on the same level with traditional micro filtration membranes (Liang et al., 2012). Biofilm was also observed on the used F-MFC and T-MFC cathode (Fig. 2B), and that on F-MFC was much thicker, mainly due to the dead end filtering operation pattern. It was interesting to observe that the decline of power generation due to cathode fouling did not influence the effluent quality, which was even enhanced during the 20 days operation (Fig. 1). Previous study showed that the fouling of filtration membrane may also facilitate the removal of COD and nutrients (Ng and Elimelech, 2004) by interception of gel layer and degradation of the microorganisms. Future improvements on fouling control could be conducted from optimization of filtration material and operation condition. For example, nano material may provide an excellent candidate for antifouling, production of oxidizing substances (such as H2O2 and so on) can remove contaminants, operation in side filtration mode or backwashing could also ameliorate fouling in filtration system. 3.5. Comparison with previous studies Table 1 summarized the power generation and wastewater treatment performance of various filtration MFC systems. In
Table 1 Comparison of power generation and wastewater treatment of various filtration MFC systems. V (mL)
28 40 210 350U-0.7 350b 254 14,600
Pore size (lm)
Material
Pmax
Imax@Pmax 2
2
W/m
mA/m
14.6 14.5 4.35
1671.4 2400* 77.9*
Organics removal A/m
Anode
Cathode
Carbon brush Graphite brush Granular graphite Graphite brush Carbon felt
AC air MCNT Pt air SS mesh biocat
0.55 0.07 40
Ni-HFM Pt air
1 N/A
Graphite felt biocat Polyesterc biocat
50
32.1*
7.6
156.1*
37
20*
2.6
N/A
13*
N/A
Graphite felt Iron
mW/m
3
581.5 820 18.5* 1656* 53.5
6.6* 0.4*
2700 280
3
41.8 42* 18.32 11.1 2*
TOC/ COD 93.7% 96.9% 91.4% >95% >90% 88.7% 100%
NH+4–N
TN
96.2% 95.2% 99.6%
91.6% 96.0% 60.7%
N/A N/A
N/A N/A
89.3%
55.0%
83.3%*
N/A
Filter costa (USD/m2)
Ref.
7.1 >484* 135.6*
This study Malaeb et al. (2013) Wang et al. (2011)
11.7* 50* 27.8* 1.9*
Katuri et al. (2014) Kyoung Yeol et al. (2013) Wang et al. (2013) Liu et al. (2013)
V: reactor volume; Pmax: maximum power density; Imax@Pmax : maximum current density or current density at maximum power density; Ref.: reference; AC: Activated carbon; SS mesh: stainless steel mesh; biocat: biocathode; U-0.7: applied voltage of 0.7 V; Ni-HFM: nickel-based hollow fiber membranes; a The filter costs were estimated mainly according to the material price with the same thickness applied in corresponding articles. b Two chamber MFC with filtration membrane as separator. c With polypyrrole (PPy) modification. * Data calculated or estimated according to figures and profiles presented in corresponding articles; N/A: data not available.
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previous studies, combination of filtration technology and MFC or MEC have been realized with bio- or air- cathode, and excellent wastewater treatment performance have been achieved with COD removed >90%. However, these filtration cathodes were mostly composed of (I) modified MF or UF (with polypyrrole (Liu et al., 2013)); (II) metal mesh (stainless steel mesh (Wang et al., 2011)); and (III) combined electrode of nonwoven fabrics with carbon materials (CNT (Malaeb et al., 2013), carbon felt (Wang et al., 2013)), resulting in a high electric resistance, big pore size, or high material cost. In this study, a homogeneous carbon based membrane with multi-functions of microfiltration, electron conduction, and oxygen reduction catalysis was fabricated without using noble metals. It has a uniform pore size (553 nm), a high resistance (6.0 ± 0.4 X cm2/cm) and specific surface area of (32.2 m2/g). After it was assembled in MFC as a filtration air cathode, a power density of 581.5 mW/m2 and current density of 1671.4 mA/m2 were achieved, which was almost the highest in previous studies except a Pt air cathode fabricated by Malaeb et al in 2013 (Malaeb et al., 2013). It also achieved an excellent wastewater treatment performance with TOC, NH+4–N, and TN removed 93.6%, 97.2% and 91.6%, comparable with practical MBR systems (Sun et al., 2014). In addition, as it is made of activated carbon powder, the carbon membrane is almost the cheapest (7.1 $/m2) compared to other conductive filtration electrode (Katuri et al., 2014; Malaeb et al., 2013; Wang et al., 2011), and it is much cheaper than commercial microfiltration membrane (100 $/m2). Moreover, it is easily controllable for the pore size and shape by changing powder size and suppressing intensity, and its catalytic property may also be optimized by adding or coating metal particles such as Fe, Co, and Ni in the carbon membrane during or after its fabrication. 4. Conclusion The main contribution of this study is the fabrication of a homogeneous conductive filtration membrane by re-carbonization of activated carbon without using noble metal catalyst. It possessed excellent surface area, porosity, conductivity, capacitance, and catalytic performance. When it was used as filtration cathode, the resulted F-MFC achieved considerable power generation (581.5 mW/m2) during 20 days of operation, and effluent quality was greatly improved with removal of TOC (93.6%) and nitrogen (>90%). In addition, the carbon filtration membrane was cheap and controllable in shape and performance, suggesting a promising multi-functional material in filtration-bio-electrochemical system in wastewater treatment field. Acknowledgements This research was financially supported by the Key Program of the National Natural Science Foundation of China (No. 51238004), the International Program of MOST (No. 2013DFG92240), and Tsinghua University Initiative Scientific Research Program (No. 20121087922).
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Short Communication
Carbon filtration cathode in microbial fuel cell to enhance wastewater treatment Kuichang Zuo, Shuai Liang, Peng Liang 1, Xuechen Zhou, Dongya Sun, Xiaoyuan Zhang, Xia Huang ⇑ State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing 100084, PR China
h i g h l i g h t s A homogeneous carbon filtration membrane was fabricated without using noble metals. Excellent effluent quality and power generation were achieved in the filtration MFC. The performance of filtration MFC kept relative stable during 20 days operation.
a r t i c l e
i n f o
Article history: Received 3 January 2015 Received in revised form 25 February 2015 Accepted 27 February 2015 Available online 6 March 2015 Keywords: Carbon filtration membrane Microbial fuel cell Advanced wastewater treatment Energy recovery
a b s t r a c t A homogeneous carbon membrane with multi-functions of microfiltration, electron conduction, and oxygen reduction catalysis was fabricated without using noble metals. The produced carbon membrane has a pore size of 553 nm, a resistance of 6.0 ± 0.4 X cm2/cm, and a specific surface area of 32.2 m2/g. After it was assembled in microbial fuel cell (MFC) as filtration air cathode, a power density of 581.5 mW/m2 and a current density of 1671.4 mA/m2 were achieved, comparable with previous Pt air cathode MFCs. The filtration MFC was continuously operated for 20 days and excellent wastewater treatment performance was also achieved with removal efficiencies of TOC (93.6%), NH+4–N (97.2%), and total nitrogen (91.6%). In addition, the carbon membrane was much cheaper than traditional microfiltration membrane, suggesting a promising multi-functional material in wastewater treatment field. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Microbial fuel cell (MFC) is an electrochemical device that utilizes electroactive bacteria to recover energy in the form of electricity through oxidization of organic matter. As it couples electricity generation to wastewater treatment, MFC has drawn a great attention and received significant improvement in structure and performance in recent years. During the past decade, MFC has achieved a 105 times increase in power density from 0.01 mW/m2 in 1999 (Kim et al., 1999) to 2.7 W/m2 in 2007 (Fan et al., 2007) and various configuration with reactor size ranged from mL to >50 L (Liang et al., 2013) were reported. However, the present MFC is still impotent in achieving high quality effluent (Ge et al., 2013), due to the limited biomass retention (Logan, 2008) and low reaction rate under anaerobic condition in anode.
⇑ Corresponding author. Tel.: +86 10 62772324. E-mail addresses: [email protected] (P. Liang), [email protected]. cn (X. Huang). 1 Co-corresponding author. Tel.: +86 10 62796790. http://dx.doi.org/10.1016/j.biortech.2015.02.108 0960-8524/Ó 2015 Elsevier Ltd. All rights reserved.
Integration of MFC with filtration technology may provide a potential solution to the existing problems. In a filtration bioelectrochemical system, energy could be ideally extracted in the form of electricity through oxidation of organic matter, while a high quality permeate could be obtained due to the filtration of membrane, achieving simultaneous energy recovery and advanced wastewater treatment. In previous studies, microfiltration (MF) or ultrafiltration (UF) membranes have been used as separation materials instead of expensive ion-exchange membranes in MFCs to intercept organic matter and biomass in anode (Kyoung Yeol et al., 2013). However, these traditional MF or UF membranes presented low mass transfer due to their low ion conductivity, resulting in a high internal resistance (Kim et al., 2007) of the integrated filtration MFC system. Another combination approach could be realized by using a conductive filtration membrane as air- or biocathode of a single-chamber MFC, thereby avoiding the requirements for separator or aeration in catholyte. Stainless steel mesh (Wang et al., 2011) and carbon-nanotube-deposited UF membrane (Malaeb et al., 2013) were employed as such conductive membranes to constitute a filtration cathode, and the feasibility and capacity of this configuration for simultaneous wastewater
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treatment and energy recovery had been confirmed. Nevertheless, these fabricated filtration cathode materials had the following shortages: (I) nonuniform or much bigger pore size compared to MF or UF membrane (Wang et al., 2011, 2013); (II) utilization of noble metal (Pt) as catalyst (Malaeb et al., 2013); (III) relative high cost for large scale application (Malaeb et al., 2013; Wang et al., 2013). In this study, conductive activated carbon powder, which possessed high specific surface area, was used as raw material for preparation of a homogeneous MF membrane. The obtained carbon MF membrane was then set as an air cathode to establish a filtration microbial fuel cell (F-MFC), which exhibited combined functions of MFC and membrane bioreactor (MBR). The physicochemical characteristics of the carbon filtration membrane were investigated before assembled in MFC. The power production and pollutants removal were evaluated during 20 days operation. And the performance of polluted filtration cathode in terms of power generation and flux were further investigated. In addition, the state-of-the-art progress in filtration MFC fabrication and performance was also summarized.
2. Methods 2.1. Fabrication of carbon filtration membrane Carbon powder, originated from milling of activated carbon granule (Weishimei Environmental Technology Co., Ltd., Beijing, China), was screened with a 70-mesh sieve. Then 2.4 g screenthroughs were adequately mixed with 0.3 g sodium carboxymethyl cellulose (binder) and 1.2 mL H2O (solvent) by stirring with a glass rod until a fluffy mixture was obtained. After that the fluffy mixture was transferred into a mould and pressed at 25 MPa for 2 min using a press machine (FW-4, Tianjin Optical Instrument Factory, China). The resulted carbon wafer was dewatered in a drying oven and at last carbonized in a vacuum tube furnace (GSL1700X, Heifeikejing, China) at 900 °C for 90 min under argon protection. After cooling down, the obtained carbon membrane was stored at room temperature before use.
2.2. MFC assembly and operation As illustrated in Fig. S1, the filtration MFC is a cylindrical bioreactor (total volume: 28 mL) comprising a carbon brush anode and a carbon filtration air cathode. The anode brushes are 3 cm in length and 3 cm in diameter, which were inoculated with anolyte of a mature MFC that has been operated for several years in our laboratory. The effective area of the cathode is 7 cm2 and the shortest distance between cathode and anode carbon is 1 cm. An O-ring titanium sheet was used as electricity-collecting material for the filtration cathode, and an external resistance of 2000 or 200 X was connected between cathode and anode to simulate an electric appliance. After assembling the MFCs, PBS-buffered acetate was used as synthetic wastewater, which contained (per liter of deionized water) 0.5 g CH3COONa (Chemical oxygen demand, COD: 390.2 mg/L; Total organic carbon, TOC: 146.3 mg/L), 0.15 g NH4Cl (NH+4–N: 39.3 mg/L), 1.04 g KH2PO4, and 2.16 g K2HPO4, all chemicals were reagent grade and purchased from Sigma-Aldrich (St. Louis, MO). During operation, one air cathode MFC was operated intermittently with synthetic wastewater replaced every 48 h as traditional batch mode MFC, therefore it was nominated as traditional MFC (T-MFC). In contrast, the other MFC was continuously operated with synthetic wastewater firstly pumped into the anode and further go through the filtration cathode at a flow rate of
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0.58 mL/h (hydraulic retention time (HRT) of 48 h), it was nominated as filtration MFC (F-MFC, Fig. S1). 2.3. Measurements and analysis Physicochemical characteristics including morphology and elemental composition were evaluated with scanning electron microscopy (SEM, TESCAN, Czech Republic) and X-ray photoelectron spectroscopy (XPS, Perkin-Elmer Phi 5300, USA) respectively. Porosity, pore size distribution, and specific surface area were completed with a mercury porosimeter (Autopore IV 9510, USA). Capacitance of the carbon filtration membrane was measured by galvanostatic charge-discharge method at both anodic and cathodic current of 10 mA in 5 M KOH solution using an electrochemical workstation (Autolab, Metrohm, Switzerland). Catalytic performance of carbon filtration membrane was evaluated by linear sweep voltammetry (LSV), with platinum wire used as control electrode and saturated calomel electrode (SCE, 0.242 V vs. standard hydrogen electrode, Leici, China) as reference electrode, and potential swept from +0.6 to 0.4 V at a scan rate of 1 mV/s. The LSV was conducted in a 20 mM PBS solution with three conditions: Ar aeration, O2 aeration, and exposed in air (Fig. S2). After the fabrication of the two MFCs, the cell voltages (U) across the external resistor (Rex) were measured automatically in 1 min interval with a data acquisition system (DAQ2213, ADLINK, China). Current (I) was determined according to ohm’s law: I = U/Rex. Power density (P) and internal resistance (Rin) were calculated according to polarization curve based on the anode chamber volume (28 mL). The coulombic efficiency (CE) was introduced to evaluate the proportion of recovered energy from organic matter, which was calculated according to the following equation:
CE ¼
R 3 Idt FVðC i C e Þ
ð1Þ
where F is Faraday constant (96,485 C/mol), V is the volume of treated wastewater (L), t is operation time (s), Ci and Ce are the TOC concentrations of influent and effluent (g/L), I is produced current (A). During operation, the quality of synthetic wastewater and effluent from the two MFCs were evaluated including TOC, NH+4, NO 2, and NO 3 . TOC was measured by a TOC analyser (Shimadzu TOC VW, Japan). NH+4, NO 2 , and NO3 were quantified by ion chromatography (ICS-1100, DIONEX, USA). Flux of the carbon filtration membrane was measured at different pressures to assess the development of membrane fouling. During the flux measurements, nitrogen gas was utilized to provide a trans-membrane pressure (TMP) for the F-MFC influent, TMP and effluent weight were continuously detected using a pressure sensor and an electric balance (Fig. S3). The calculated flux was normalized to cathode projected area (7 cm2). 3. Results and discussion 3.1. Physicochemical properties of carbon filtration membrane The filtration membrane is a carbon wafer which has a diameter of 38.0 ± 0.2 mm and thickness of 2 ± 0.1 mm (Fig. S4A). It possesses a microporous structure with porosity of 43.4% and pore size of 553 nm (Fig. S4B), and it can hold a water pressure of >50 kPa with an area of 7 cm2. The specific flux is 6.9 ± 0.5 L/(h m2 kPa), which is comparable with traditional ceramic membranes (Nandi et al., 2008). X-ray photoelectron spectroscopy (Fig. S4C) illustrated that the carbon filtration membrane is mainly comprised of C (94.14%), O (3.91%), N (1.49%) and a small amount of metals including Na (0.24%) and Ca (0.13%). The resistivity was 6.0 ± 0.4 X cm2/cm and the specific surface area was 32.2 m2/g.
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Linear sweep voltammetry (LSV) displayed an increase of reduction current at 0.04 V (Fig. S4D) when one side of the carbon membrane was exposed in air (Fig. S2C), 0.2 V higher than the position of reduction current increase (0.16 V) in O2 saturated solution (Fig. S2B). And the reduction current at both conditions (with presence of O2) were much higher than that in Ar aerated solution (Figs. S4D and S2A), indicating excellent oxygen reduction catalysis of the carbon filtration membrane. In addition, the carbon filtration membrane exhibited excellent capacitance (7.0 F/cm2, Fig. S5), which was even higher than previous capacitive electrode material (3.47 (Lv et al., 2014), 0.804 (Zhang et al., 2015) F/cm2) and may promote power production by facilitating electron transfer (Zhang et al., 2015). As compared to previous Pt air/bio cathode which had a reduction current of only 5 mA (Malaeb et al., 2013) and 0.7 mA (Zhang et al., 2013) in LSV measurements at potential of 0.4 V, the better performance of the carbon filtration membrane was mainly due to its porous structure and large specific surface (32.2 m2/g), which provided high encountering probability for three phase reaction of solid cathode, gaseous oxygen and liquid water. Another factor may be the high content of nitrogen (1.49%), which was proved having a good catalysis for oxygen reduction (Feng et al., 2011), indicating an excellent material for carbon membrane as a filtration cathode.
Their maximum power density reached 581.5 and 443.5 mW/m2, and their internal resistance were 210.5 X (F-MFC) and 253.8 X (T-MFC, Fig. 1A), which were comparable with previous Pt air (820 mW/m2) and bio- cathode (380 mW/m2) MFCs (Malaeb et al., 2013). To further evaluate the performance of carbon filtration cathode, the T-MFC and F-MFC were operated for 20 days at external resistance of 2000 X and 200 X (Fig. 1B). F-MFC achieved a voltage of about 508 mV and kept almost stable at this level during the first 10 days of operation at external resistance of 2000 X. And a voltage profile from 101 to 79 mV was observed during the next 10 days operation (external resistance of 200 X). As a contrast, the T-MFC presented a periodic decrease due to the fed-batch operation, and its peak voltage were 421 and 61 mV at 2000 and 200 X respectively, much lower than that of F-MFC. The better performance of F-MFC than T-MFC was attributed to two main reasons: (I) enhanced mass transfer between anode and cathode due to continuous flow, and (II) decreased diffusion distance for O2 in cathode reaction. As shown in Fig. S7, the cathodic three phase (oxygen, water and cathode) reaction in T-MFC happened on the interface adjacent to the inner side of the cathode. However an interface shifting to the outer side of the filtration cathode would happen in F-MFC mode, which decreased the distance for oxygen passive diffusion and may improve power generation.
3.2. Power production of F-MFC and T-MFC
3.3. Organics and nutrient removal during 20 days operation
After fabrication, the carbon filtration membranes were assembled as air cathodes of single-chamber F-MFC and T-MFC, before which the two pre-cultivated anodes were selected from six parallel operated MFCs (with 50 mM potassium ferricyanide as catholyte) and they possessed the most similar performance (Fig. S6). After acclimated for one day at 2000 X resistance, the open circuit voltage of F-MFC and T-MFC reached 605 and 590 mV respectively.
The cathode permeate was also evaluated in terms of TOC and nitrogen during the 20 days operation. As shown in Fig. 1C, the TOC removal efficiency of F-MFC reached 74.2% and 93.7% at external resistance of 2000 and 200 X, the effluent TOC were 37.7 and 9.2 mg/L respectively, which were comparable with traditional MBRs (Sun et al., 2014). In T-MFC, the TOC removal efficiency was only 55.3% and 72.4%, much lower than that of F-MFC. The
Fig. 1. Power generation and organics removal in F-MFC and T-MFC. (A) Polarization and power density curves of traditional MFC (T-MFC) and filtration MFC (F-MFC) after 1 day stabilization at external resistance of 2000 X, (B) voltage generation during long term operation at various external resistance, (C) effluent TOC concentration and TOC removal efficiency, and (D) nitrogen removal efficiency.
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be due to (I) direct utilization of microorganisms, (II) simultaneous nitrification–denitrification in anode anoxic zone, (III) electrochemical denitrification by cathode (as electron donor) (Liang et al., 2013), and (IV) passing through the filtration cathode in the form of NH3. Although the dominant process was still unknown, nitrification did happen in the F-MFCs and T-MFCs, as NO 2 –N and NO3 –N were detected in the effluent (Fig. S9). 3.4. Performance decline due to cathode fouling
Fig. 2. Performance of F-MFC and T-MFC after 20 days operation. (A) Polarization and power density curves, and (B) cathode membrane flux.
coulombic efficiency of T-MFC was 24.0% (2000 X) and 25.6% (200 X), and that for F-MFC was 46.4% (2000 X) and 63.9% (200 X), much higher than previous filtration MFC systems (8.5% (Wang et al., 2011), 19.1% (Wang et al., 2013)). A shaking bottle adsorption experiment showed that the adsorption capacity of activated carbon was 1.7 mg/g at TOC concentration of 146 mg/L (Fig. S8), which means that the 2.5 g carbon filtration membrane can only adsorb 4.2 mg TOC of NaAc (10% of influent TOC during 20 days operation), indicating the major TOC removed by biodegradation. The influent NH4Cl is 0.15 g/L (NH+4–N is 39.3 mg/L). After 48 h hydraulic retention, the NH+4–N removal efficiency of F-MFC reached 83.2% (2000 X, Fig. 1D) and 96.2% (200 X), and the total nitrogen removal efficiency is 91.6% at 200 X, higher than that of T-MFC (91.6% for NH+4–N and 83.7% for TN at external resistance of 200 X). In previous studies, the NH+4 removal was proposed to
After 20 days of operation, the power generation in both two types of MFCs exhibited a decline. The maximum power densities of the F-MFC and T-MFC decreased by 16.8% (483.8 mW/m2) and 8.8% (404.3 mW/m2), and internal resistance increased to 268.8 and 307.1 X respectively (Fig. 2A). LSV profile also illustrated a current decline for the used filtration membranes (Fig. S10), and the performance of used F-MFC cathode was a little bit worse than T-MFC cathode, indicating more serious fouling of the carbon membrane in F-MFC. The specific flux of the used carbon filtration cathode in F-MFC and T-MFC were 5.16 and 5.77 L/(h m2 kPa) (Fig. 2B), which decreased 25.5% and 16.7% respectively compared to the pristine carbon filtration membrane (6.93 L/(h m2 kPa)), despite that they were still on the same level with traditional micro filtration membranes (Liang et al., 2012). Biofilm was also observed on the used F-MFC and T-MFC cathode (Fig. 2B), and that on F-MFC was much thicker, mainly due to the dead end filtering operation pattern. It was interesting to observe that the decline of power generation due to cathode fouling did not influence the effluent quality, which was even enhanced during the 20 days operation (Fig. 1). Previous study showed that the fouling of filtration membrane may also facilitate the removal of COD and nutrients (Ng and Elimelech, 2004) by interception of gel layer and degradation of the microorganisms. Future improvements on fouling control could be conducted from optimization of filtration material and operation condition. For example, nano material may provide an excellent candidate for antifouling, production of oxidizing substances (such as H2O2 and so on) can remove contaminants, operation in side filtration mode or backwashing could also ameliorate fouling in filtration system. 3.5. Comparison with previous studies Table 1 summarized the power generation and wastewater treatment performance of various filtration MFC systems. In
Table 1 Comparison of power generation and wastewater treatment of various filtration MFC systems. V (mL)
28 40 210 350U-0.7 350b 254 14,600
Pore size (lm)
Material
Pmax
Imax@Pmax 2
2
W/m
mA/m
14.6 14.5 4.35
1671.4 2400* 77.9*
Organics removal A/m
Anode
Cathode
Carbon brush Graphite brush Granular graphite Graphite brush Carbon felt
AC air MCNT Pt air SS mesh biocat
0.55 0.07 40
Ni-HFM Pt air
1 N/A
Graphite felt biocat Polyesterc biocat
50
32.1*
7.6
156.1*
37
20*
2.6
N/A
13*
N/A
Graphite felt Iron
mW/m
3
581.5 820 18.5* 1656* 53.5
6.6* 0.4*
2700 280
3
41.8 42* 18.32 11.1 2*
TOC/ COD 93.7% 96.9% 91.4% >95% >90% 88.7% 100%
NH+4–N
TN
96.2% 95.2% 99.6%
91.6% 96.0% 60.7%
N/A N/A
N/A N/A
89.3%
55.0%
83.3%*
N/A
Filter costa (USD/m2)
Ref.
7.1 >484* 135.6*
This study Malaeb et al. (2013) Wang et al. (2011)
11.7* 50* 27.8* 1.9*
Katuri et al. (2014) Kyoung Yeol et al. (2013) Wang et al. (2013) Liu et al. (2013)
V: reactor volume; Pmax: maximum power density; Imax@Pmax : maximum current density or current density at maximum power density; Ref.: reference; AC: Activated carbon; SS mesh: stainless steel mesh; biocat: biocathode; U-0.7: applied voltage of 0.7 V; Ni-HFM: nickel-based hollow fiber membranes; a The filter costs were estimated mainly according to the material price with the same thickness applied in corresponding articles. b Two chamber MFC with filtration membrane as separator. c With polypyrrole (PPy) modification. * Data calculated or estimated according to figures and profiles presented in corresponding articles; N/A: data not available.
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previous studies, combination of filtration technology and MFC or MEC have been realized with bio- or air- cathode, and excellent wastewater treatment performance have been achieved with COD removed >90%. However, these filtration cathodes were mostly composed of (I) modified MF or UF (with polypyrrole (Liu et al., 2013)); (II) metal mesh (stainless steel mesh (Wang et al., 2011)); and (III) combined electrode of nonwoven fabrics with carbon materials (CNT (Malaeb et al., 2013), carbon felt (Wang et al., 2013)), resulting in a high electric resistance, big pore size, or high material cost. In this study, a homogeneous carbon based membrane with multi-functions of microfiltration, electron conduction, and oxygen reduction catalysis was fabricated without using noble metals. It has a uniform pore size (553 nm), a high resistance (6.0 ± 0.4 X cm2/cm) and specific surface area of (32.2 m2/g). After it was assembled in MFC as a filtration air cathode, a power density of 581.5 mW/m2 and current density of 1671.4 mA/m2 were achieved, which was almost the highest in previous studies except a Pt air cathode fabricated by Malaeb et al in 2013 (Malaeb et al., 2013). It also achieved an excellent wastewater treatment performance with TOC, NH+4–N, and TN removed 93.6%, 97.2% and 91.6%, comparable with practical MBR systems (Sun et al., 2014). In addition, as it is made of activated carbon powder, the carbon membrane is almost the cheapest (7.1 $/m2) compared to other conductive filtration electrode (Katuri et al., 2014; Malaeb et al., 2013; Wang et al., 2011), and it is much cheaper than commercial microfiltration membrane (100 $/m2). Moreover, it is easily controllable for the pore size and shape by changing powder size and suppressing intensity, and its catalytic property may also be optimized by adding or coating metal particles such as Fe, Co, and Ni in the carbon membrane during or after its fabrication. 4. Conclusion The main contribution of this study is the fabrication of a homogeneous conductive filtration membrane by re-carbonization of activated carbon without using noble metal catalyst. It possessed excellent surface area, porosity, conductivity, capacitance, and catalytic performance. When it was used as filtration cathode, the resulted F-MFC achieved considerable power generation (581.5 mW/m2) during 20 days of operation, and effluent quality was greatly improved with removal of TOC (93.6%) and nitrogen (>90%). In addition, the carbon filtration membrane was cheap and controllable in shape and performance, suggesting a promising multi-functional material in filtration-bio-electrochemical system in wastewater treatment field. Acknowledgements This research was financially supported by the Key Program of the National Natural Science Foundation of China (No. 51238004), the International Program of MOST (No. 2013DFG92240), and Tsinghua University Initiative Scientific Research Program (No. 20121087922).
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