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New application of supported ionic liquids membranes as proton exchange membranes in microbial fuel cell for waste water treatment
Chemical Engineering Journal 279 (2015) 115–119
Contents lists available at ScienceDirect
Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Short communication
New application of supported ionic liquids membranes as proton exchange membranes in microbial fuel cell for waste water treatment F.J. Hernández-Fernández a,⇑, A. Pérez de los Ríos b, F. Mateo-Ramírez a, C. Godínez a, L.J. Lozano-Blanco a, J.I. Moreno a, F. Tomás-Alonso b a b
Department of Chemical and Environmental Engineering, Technical University of Cartagena (UPCT), Campus La Muralla, C/ Doctor Fleming S/N, E-30202 Cartagena, Murcia, Spain Department of Chemical Engineering, Faculty of Chemistry, University of Murcia (UMU), P.O. Box 4021, Campus de Espinardo, E-30100 Murcia, Spain
h i g h l i g h t s Supported ionic liquids membranes (SILMs) have been used in microbial fuel cells (MFCs). MFCs with SILMs yield similar power than with usual proton exchange membranes (PEMs). MFCs with SILMs yield similar COD reduction than with conventional PEMs. Ò
SILMs could supersede conventional PEMS such as Nafion
and UltrexÒ.
The used of SILMs could improve the efficiency of MFC and the reduction of its capital cost.
a r t i c l e
i n f o
Article history: Received 18 December 2014 Received in revised form 3 April 2015 Accepted 5 April 2015 Available online 22 April 2015 Keywords: Supported ionic liquid membrane Ionic liquid Microbial fuel cell Electricity production Waste water treatment
a b s t r a c t The present work evaluates, for the first time, the potential use of supported liquid membranes based on ionic liquids as proton exchange membranes in microbial fuel cells. Specifically, ionic liquids based on 1-n-alkyl-3-methylimidazolium (n-butyl, n-octyl) and methyl trioctylammonium cation combining with hexafluorophosphate, tetrafluoroborate, chloride and bis{(trifluoromethyl)sulfonyl}imide anions were used. NafionÒ and UltrexÒ membranes were used as reference membranes. It has been demonstrated that supported ionic liquids membranes based on ionic liquids could act as proton exchange membrane in microbial fuel cells, being power and chemical oxygen demanded removal (i.e. 103.9 mW/m3 and 89.1%, respectively for the ammonium based supported ionic liquid membrane) similar or even higher than conventional proton exchange membranes such as NafionÒ and UltrexÒ. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction In a microbial fuel cell (MFC), microbes oxidize organic matter and transfer resulting electrons directly to an electrode. Current may be produced from simple substratum (e.g. acetate, lactate or glucose). What is really innovative was the finding that current can also be generated from complex substrates like domestic and industrial wastewaters [1–3]. This biotechnology is presented as a good alternative to face both two major problems in the world: energy crisis and water availability. By using organic matter in wastewaters as a fuel, we can simultaneously produce energy and purify wastewater [4]. Over the past few years the performance of MFCs has improved almost exponentially [5]. These results are quite encouraging but a ⇑ Corresponding author. Tel.: +34 968326408. E-mail address: [email protected] (F.J. Hernández-Fernández). http://dx.doi.org/10.1016/j.cej.2015.04.036 1385-8947/Ó 2015 Elsevier B.V. All rights reserved.
high power generation and less expensive material are severely required for practical implementation of MFC in wastewater treatment and energy production. In order to improve economic feasibility and current yield, new materials for the electrodes and the proton exchange membrane should be explored. The proton exchange membrane (PEM) is a critical component determining the efficiency of MFC. Perfluorinated ionomer membranes such as NafionÒ (Dupont) have been widely used as polymer electrolyte membranes of MFCs, because of their high proton conductivities in the fully hydrated state. However, these membranes are still expensive (approx. $1400 m 2), which makes their use prohibitive in large-scale applications. Furthermore, cation species also present in waste water such as K+, Na+, Ca2+, NH+4, and Mg2+ was able to cross the Nafion membrane like protons. Considering that the concentration of these cations species are higher in MFCs than the proton concentration, an accumulation of these cations are produced in the cathode chamber causing an increase in the pH
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in the previous chamber and a decrease of the pH in the anodic chamber. As consequence, MFCs efficiency is reduced by decreasing microorganism activity and decrease of the thermodynamic cell potential [6]. Different strategies have been proposed to solve the problem of the pH gradient on both sides of the membrane, such as: (i) the use of pH-static control [7], membrane-free configurations [8,9] or other membrane materials such as cation exchange membranes (CEM), anion exchange membranes (AEM), bipolar membranes (BPM), and ultracentrifugation membranes (UCM) [10–12]. The immobilization of ionic liquid (IL) in membranes could open up this field of improvement in MFCs. ILs are organic salts remaining as liquids below 100 °C. They normally consist of an organic cation and a polyatomic inorganic anion or, more and more current, an organic anion [13]. The chemical structures of common cations and anions of ILs are presented in Fig. 1. Ionic liquids possess unique properties that are interesting in the context of MFCs. For instance, they show high ion conductivity [14] and wide electrochemical windows. Furthermore, they have near-zero vapor pressure and their good chemical and thermal stabilities, high viscosity and tunable solubility which allow obtaining highly stable supported ionic liquid membranes [15]. Due to these unique properties, supported ionic liquid membranes [16] have been applied for separating organic compounds [17,18], mixed gases [19,20] and metal ions [21]. In this work, we evaluate for the first time the potential use of supported ionic liquid membranes (SILMs) as proton exchange membranes in a single-chamber MFC with cathodes of platinum sprayed on carbon cloth working with a 1 kX external resistor. SILMs were prepared by soaking up an ionic liquid in an organic porous membrane (NylonÒ membrane). NafionÒ and UltrexÒ membrane were used as reference proton exchange membranes. The effect of the ionic liquids on the power generation and water depuration was analyzed.
2. Experimental 2.1. Fuel and chemicals Substrate used was an urban waste water after the lamellar settler of the local Wastewater Treatment Plant (COD of 430 mg/L). The final COD content was adjusted to the desired starting value of 1174 mg L 1 by mixing this wastewater with high COD water from a local brewery industry.
NylonÒ HNWP an hydrophilic polyamide membranes with a pore size of 0.45 lm, a thickness of 170 lm, and 25 mm of diameter from Millipore were used as support in supported ionic liquid membranes. NafionÒ 117 cation exchange membrane (DuPont, USA; 6 cm2 area, 183 lm of thickness) was used as membrane in the MFCs after conditioning one hour at 70 °C in a 3% (v/v) solution of hydrogen peroxide and one hour at 70 °C in a 1 M sulfuric acid solution. UltrexÒ membranes CMI-7000 (Membranes International Incorp., Glen Rock, New Jersey, USA) were also used as membrane in the MFC after conditioning with a solution of potassium chloride, 5% (v/v) for two hours. 1-Octyl-3-methylimidazolium tetrafluoroborate, [omim+][BF4 ], 1-butyl-3-methylimidazolium hexafluorophosphate, [bmim+] [PF6 ], 1-octyl-3-methylimidazolium hexafluorophosphate, [omim+][PF6 ], 1-butyl-3-methylimidazolium bis{(trifluorome thyl)sulfonyl}imide, [bmim+][NTf2 ], 1-octyl-3-methylimidazolium bis{(trifluoromethyl)sulfonyl}imide, [omim+][NTf2 ] was purchased from Solvent Innovation (purity >99%) and methyl trioctyl ammonium chloride [MTOA+][Cl ] was purchased from Sigma–Al drich–Fluka (purity >97%). 2.2. Preparation of supported liquid membrane The preparation of supported ionic liquid membranes (SILMs) was achieved by passing the corresponding ionic liquid through the NylonÒ membrane by using a 10 mL AmiconTH ultrafiltration as indicated in Hernandez-Fernandez et al. [22]. 2.3. MFC studies The experiments were carried out in one-chamber MFCs. The reactors were 250 mL glass bottles with an external jackets for temperature control. The cathode consisted of platinum dispersed on Vulcan (Alfa Aesar) and sprayed onto a 4 cm diameter piece of carbon cloth being the final load of 0.3 mg Pt cm 2. The cathode was connected with the anode through 1 kX resistor. The anode consisted of 168 g of graphite granules of 2–6 mm diameter (Graphite Store, USA) and a graphite rod of 3.18 mm diameter (Graphite Store, USA) to connect to the cathode terminal through the resistor. In each experiment 105 mL of wastewater was putted in the anode chamber. Between the cathode and the anode chamber a membrane was placed. A scheme of the microbial fuel cell can be observed in Fig. 2. The fuel added to anode chambers consisted of barley processing wastewater from a brewery diluted in domestic wastewater to
Fig. 1. Structures of common cations and anions of ILs.
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Fig. 2. Schematic of the device used for these experiments.
give a chemical oxygen demand (COD) of 1174 mg L 1. All experiments have run in batch mode using wastewater as a unique source of microorganisms and fuel. The experiment was run at 28 °C and the duration was 170 h. COD was monitored by taken regular water samples (5 mL). After centrifugation (5000 rpm for 5 min) and filtration (0.45 lm pore diameter membrane filter), 2 mL aliquot was used for COD analysis. Chemical oxygen demand (COD) was measured using the methods described in APHA (Larrosa et al. [4]). Voltage was also monitored with a continuous voltage recorder Pico log ADC-16 and ADC-24. 3. Results and discussion In this work, supported liquid membranes based on ionic liquids were tested for using as proton exchange membranes in microbial fuel cell. For that, four supported ionic liquids membranes were prepared. Because of the water nature of the microbial fuel system, in order to get stable supported ionic liquids membranes, the ionic liquid immobilized must be insoluble in water. Therefore, for the present study, ionic liquids were selected on the basis of their low solubility in water: [MTOA+][Cl ] < 0.02% (v/v), [omim+][NTf2] < 0.04% (v/v), [omim+][PF6 ] < 0.1% (v/v) and [omim+][BF4 ] < 1.4% (v/v). Those ionic liquids are composed of different anions and cations in order to study the effect of the ionic liquid composition on the performance of microbial fuel cell. NafionÒ and UltrexÒ (CMI-7000) membranes were used as reference membranes in microbial fuel cell. The MFC voltage with different supported ionic liquid membranes was continuously monitored during more than 160 h. Fig. 3 shows the voltage profile of the assayed microbial fuel cells. As can be seen from Fig. 3, firstly, in the first hour, the voltage increased due to the biofilm was formed on the graphite granules. After the maximum, the voltage was continuously reduced due to the reduction of the organic matter on the anode chamber. The highest voltages during the full experiment were reached by using the supported liquid membrane based on [OMIM+][PF6 ] ionic liquid, being the maximum voltage around 140 mV. Membranes based on [MTOA+][Cl ] followed a similar tendency to that of NafionÒ membrane and the UltrexÒ membranes. Initially the
membrane based on the ionic liquid [omim+][NTf2 ] showed a little voltage, however this voltage was kept constant during more time. Regarding to voltage, the membranes based on the ionic liquid [omim+][BF4 ] showed the worst behavior, however high initial voltage, which decreased dramatically until 30 mV after one day working. It is important to point out that the membrane based on the ionic liquid [omim+][BF4 ] after one day working started losing water due to the instability of the membrane against water. As we commented above, this ionic liquid presents the higher water solubility. Table 1 shows the COD removal for the MFCs with different supported ionic liquid membranes. Similar COD removal (around 90%) was reached with the NafionÒ, UltrexÒ and the supported liquid membranes based on [MTOA+][Cl ]. Lower reduction (around 80%) was reached with the ionic liquids [omim+][NTf2 ] and [omim+][BF4 ]. Supported ionic liquid membrane based on [omim+][PF6 ] involved the lowest COD reduction (30%). The lowest COD reduction reached in the case of [omim+][PF6 ] could be explained based on the ecotoxicity and solubility of ionic liquids used. The ecotoxicity values of the ionic liquid using MicrotoxÒ assay were the following [23]: log EC50(lM) (15 min, [omim+] [PF6 ]) = 0.82, log EC50(lM) (15 min, [omim+][NTf2 ]) = 0.83, log EC50(lM) (15 min, [omim+][BF4 ]) = 1.40. None ecotoxicity value for MicrotoxÒ toxicity test is found for [MTOA+][Cl ] on literature. Due to ionic liquid solubility in the aqueous phase some of ionic liquid can be released and affect to microorganism population. The low COD reduction in the case of [omim+][PF6 ] could be explained due to it is the highest toxic ionic liquid. Similar toxicity were found in the case of [omim+][NTf2 ], however the lower water solubility of [omim+][NTf2 ] could explain the higher COD reduction for [omim+][NTf2 ] than for [omim+][PF6 ]. The higher columbic efficiency was reached for [omim+][PF6 ] since high voltage was reached and COD reduction was low. After that, the highest columbic efficiency value was obtained for NafionÒ. Similar values were reached by the membranes based on [MTOA+][Cl ], [omim+][NTf2 ], [omim+][PF6 ] and UltrexÒ membranes. Since the ionic liquids [omim+][BF4 ] was the most water-soluble, the membrane based on this ionic liquid suffered from instability, even leakage of water through the membrane
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Fig. 3. Voltage output of single chamber MFCs with carbon cloth cathodes, under 1 kO working in batch mode with different supported ionic liquids membranes based on the ionic liquids [MTOA+][Cl ], [omim+][NTf2 ], [omim+][PF6 ] and [omim+][BF4 ] and NafionÒ and UltrexÒ (CMI-7000) membranes as reference membranes.
Table 1 Comparison of the percentage reduction of COD (initial COD 1174 mg/L) in MFC with different supported ionic liquids membranes based on [MTOA+][Cl ], [omim+][NTf2 ], [omim+][PF6 ] and [omim+][BF4 ] and NafionÒ and UltrexÒ membranes as reference membranes. Membrane
COD removal (%)
Coulombic efficiency (%)
Maximum power (mW/m2)
NafionÒ UltrexÒ [MTOA+][Cl ] [omim+][NTf2 ] [omim+][BF4 ] [omim+][PF6 ]
90.7 88.3 89.1 81.3 80.3 27.3
4.44 2.50 2.06 2.74 1.31 18.60
157.9 102.2 103.9 72.1 147.1 215.0
was observed. Consequently, the latter membrane involved the lower value of the columbic efficiency. As can been seen from Table 1, similar power were reached with microbial fuel cell based on supported ionic liquid membranes than conventional membranes. The higher value was reached for [omim+][PF6 ]. The maximum power of the membranes based on [omim+][BF4 ] was similar than that obtained with Nafion membrane and the maximum power reaches with [MTOA+][Cl ] was similar than that reaches with UltrexÒ. The ionic nature of ionic liquids assures the transport of proton through the membrane. The solubility of ionic liquid in the aqueous phase and the ecotoxicity of ionic liquid are found to be key factors to assure the feasibility of SILMs application in MFCs, since could affect to membrane stability and microorganism activity in the anode chamber, respectively.
4. Conclusions In this study we outlined, for the first time, the possibility of using SLMs based on ionic liquids as proton exchange membranes in microbial fuel cell. For Instance, the SILM based on the ionic liquid [MTOA+][Cl ] involves similar maximum power, COD reduction and coulombic efficiency that the UltrexÒ membrane used as reference membrane. The results of this study are quite encouraging and suggest that SLMs based on ionic liquids might be incorporated in microbial fuel cells as substituent of conventional and expensive proton exchange membranes such as NafionÒ or UltrexÒ.
Acknowledgments This work was partially supported by the Spanish Ministry of Science and Innovation (MICINN) and by the FEDER (FondoEuropeo de Desarrollo Regional), ref.CICYT ENE201125188 and by the Seneca Foundation 18975/JLI/2013 grants.
References [1] B.E. Logan, B. Hamelers, R. Rozendal, U. Schröder, J. Keller, S. Freguia, P. Aelterman, W. Verstraete, K. Rabaey, Microbial fuel cells: methodology and technology, Environ. Sci. Technol. 40 (2006) 5181–5192. [2] Z. Du, H. Li, T. Gu, A state of the art review on microbial fuel cells: a promising technology for wastewater treatment and bioenergy, Biotechnol. Adv. 25 (2007) 464–482. [3] F.J. Hernández Fernández, A. Pérez de los Ríos, M.J. Salar, V. Martínez Ortiz, L.J. Lozano-Blanco, C. Godínez, F. Tomás-Alonso, J. Quesada-Medina, Recent progress and perspectives on microbial fuel cells for bioenergy generation and wastewater treatment, Fuel Process Technol. (2015) (in press). [4] A. Larrosa, K. Scott, I.M. Head, K. Katuri, L.J. Lozano, C. Godínez, On the repeatability and reproducibility of two-chambered microbial fuel cells, Fuel 88 (2009) 1852–1857. [5] B.E. Logan, J.M. Regan, Electricity-producing bacterial communities in microbial fuel cells, Trends Microbiol. 14 (2006) 512–518. [6] G.C. Gil, I.S. Chang, B.H. Kim, M. Kim, J.K. Jang, H.S. Park, H.J. Kim, Operational parameters affecting the performance of a mediatorless microbial fuel cell, Biosens. Bioelectron. 18 (2003) 327–334. [7] R.A. Rozendal, H.V.M. Hamelers, C.J.N. Buisman, Effects of membrane cation transport on pH and microbial fuel cell performance, Environ. Sci. Technol. 40 (2006) 5206–5211. [8] H. Liu, B.E. Logan, Electricity generation using an air-cathode single chamber microbial fuel cell (MFC) in the absence of a proton exchange membrane, ACS, Division of Environmental Chemistry—Preprints of Extended Abstracts 44 (2) (2004) 1485–1488. [9] J.C. Biffinger, R. Ray, B. Little, B.R. Ringeisen, Diversifying biological fuel cell designs by use of nanoporous filters, Environ. Sci. Technol. 41 (2007) 1444– 1449. [10] R.A. Rozendal, H.V.M. Hamelers, R.J. Molenkamp, C.J.N. Buisman, Performance of single chamber biocatalyzed electrolysis with different types of ion exchange membranes, Water Res. 41 (2007) 1984–1994. [11] J.R.C. Kim, S.-E. Oh, B.E. Logan, Power generation using different cation, anion, and ultrafiltration membranes in microbial fuel cells, Environ. Sci. Technol. 41 (2007) 1004–1009. [12] A.T. Heijne, H.V.M. Hamelers, V. de Wilde, R.A. Rozendal, C.J.N. Buisman, A bipolar membrane combined with ferric iron reduction as an efficient cathode system in microbial fuel cells, Environ. Sci. Technol. 40 (2006) 5200–5205. [13] J.F. Brennecke, E.J. Maginn, Ionic liquids: innovative fluids for chemical processing, AIChE J. 47 (2001) 2384–2389. [14] P. Bonhôte, A.-P. Dias, N. Papageourgio, K. Kalayanasundaram, M. Grätzel, Hydrophobic, highly conductive ambient-temperature molten salts, Inorg. Chem. 35 (1996) 1168–1178.
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Contents lists available at ScienceDirect
Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Short communication
New application of supported ionic liquids membranes as proton exchange membranes in microbial fuel cell for waste water treatment F.J. Hernández-Fernández a,⇑, A. Pérez de los Ríos b, F. Mateo-Ramírez a, C. Godínez a, L.J. Lozano-Blanco a, J.I. Moreno a, F. Tomás-Alonso b a b
Department of Chemical and Environmental Engineering, Technical University of Cartagena (UPCT), Campus La Muralla, C/ Doctor Fleming S/N, E-30202 Cartagena, Murcia, Spain Department of Chemical Engineering, Faculty of Chemistry, University of Murcia (UMU), P.O. Box 4021, Campus de Espinardo, E-30100 Murcia, Spain
h i g h l i g h t s Supported ionic liquids membranes (SILMs) have been used in microbial fuel cells (MFCs). MFCs with SILMs yield similar power than with usual proton exchange membranes (PEMs). MFCs with SILMs yield similar COD reduction than with conventional PEMs. Ò
SILMs could supersede conventional PEMS such as Nafion
and UltrexÒ.
The used of SILMs could improve the efficiency of MFC and the reduction of its capital cost.
a r t i c l e
i n f o
Article history: Received 18 December 2014 Received in revised form 3 April 2015 Accepted 5 April 2015 Available online 22 April 2015 Keywords: Supported ionic liquid membrane Ionic liquid Microbial fuel cell Electricity production Waste water treatment
a b s t r a c t The present work evaluates, for the first time, the potential use of supported liquid membranes based on ionic liquids as proton exchange membranes in microbial fuel cells. Specifically, ionic liquids based on 1-n-alkyl-3-methylimidazolium (n-butyl, n-octyl) and methyl trioctylammonium cation combining with hexafluorophosphate, tetrafluoroborate, chloride and bis{(trifluoromethyl)sulfonyl}imide anions were used. NafionÒ and UltrexÒ membranes were used as reference membranes. It has been demonstrated that supported ionic liquids membranes based on ionic liquids could act as proton exchange membrane in microbial fuel cells, being power and chemical oxygen demanded removal (i.e. 103.9 mW/m3 and 89.1%, respectively for the ammonium based supported ionic liquid membrane) similar or even higher than conventional proton exchange membranes such as NafionÒ and UltrexÒ. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction In a microbial fuel cell (MFC), microbes oxidize organic matter and transfer resulting electrons directly to an electrode. Current may be produced from simple substratum (e.g. acetate, lactate or glucose). What is really innovative was the finding that current can also be generated from complex substrates like domestic and industrial wastewaters [1–3]. This biotechnology is presented as a good alternative to face both two major problems in the world: energy crisis and water availability. By using organic matter in wastewaters as a fuel, we can simultaneously produce energy and purify wastewater [4]. Over the past few years the performance of MFCs has improved almost exponentially [5]. These results are quite encouraging but a ⇑ Corresponding author. Tel.: +34 968326408. E-mail address: [email protected] (F.J. Hernández-Fernández). http://dx.doi.org/10.1016/j.cej.2015.04.036 1385-8947/Ó 2015 Elsevier B.V. All rights reserved.
high power generation and less expensive material are severely required for practical implementation of MFC in wastewater treatment and energy production. In order to improve economic feasibility and current yield, new materials for the electrodes and the proton exchange membrane should be explored. The proton exchange membrane (PEM) is a critical component determining the efficiency of MFC. Perfluorinated ionomer membranes such as NafionÒ (Dupont) have been widely used as polymer electrolyte membranes of MFCs, because of their high proton conductivities in the fully hydrated state. However, these membranes are still expensive (approx. $1400 m 2), which makes their use prohibitive in large-scale applications. Furthermore, cation species also present in waste water such as K+, Na+, Ca2+, NH+4, and Mg2+ was able to cross the Nafion membrane like protons. Considering that the concentration of these cations species are higher in MFCs than the proton concentration, an accumulation of these cations are produced in the cathode chamber causing an increase in the pH
116
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in the previous chamber and a decrease of the pH in the anodic chamber. As consequence, MFCs efficiency is reduced by decreasing microorganism activity and decrease of the thermodynamic cell potential [6]. Different strategies have been proposed to solve the problem of the pH gradient on both sides of the membrane, such as: (i) the use of pH-static control [7], membrane-free configurations [8,9] or other membrane materials such as cation exchange membranes (CEM), anion exchange membranes (AEM), bipolar membranes (BPM), and ultracentrifugation membranes (UCM) [10–12]. The immobilization of ionic liquid (IL) in membranes could open up this field of improvement in MFCs. ILs are organic salts remaining as liquids below 100 °C. They normally consist of an organic cation and a polyatomic inorganic anion or, more and more current, an organic anion [13]. The chemical structures of common cations and anions of ILs are presented in Fig. 1. Ionic liquids possess unique properties that are interesting in the context of MFCs. For instance, they show high ion conductivity [14] and wide electrochemical windows. Furthermore, they have near-zero vapor pressure and their good chemical and thermal stabilities, high viscosity and tunable solubility which allow obtaining highly stable supported ionic liquid membranes [15]. Due to these unique properties, supported ionic liquid membranes [16] have been applied for separating organic compounds [17,18], mixed gases [19,20] and metal ions [21]. In this work, we evaluate for the first time the potential use of supported ionic liquid membranes (SILMs) as proton exchange membranes in a single-chamber MFC with cathodes of platinum sprayed on carbon cloth working with a 1 kX external resistor. SILMs were prepared by soaking up an ionic liquid in an organic porous membrane (NylonÒ membrane). NafionÒ and UltrexÒ membrane were used as reference proton exchange membranes. The effect of the ionic liquids on the power generation and water depuration was analyzed.
2. Experimental 2.1. Fuel and chemicals Substrate used was an urban waste water after the lamellar settler of the local Wastewater Treatment Plant (COD of 430 mg/L). The final COD content was adjusted to the desired starting value of 1174 mg L 1 by mixing this wastewater with high COD water from a local brewery industry.
NylonÒ HNWP an hydrophilic polyamide membranes with a pore size of 0.45 lm, a thickness of 170 lm, and 25 mm of diameter from Millipore were used as support in supported ionic liquid membranes. NafionÒ 117 cation exchange membrane (DuPont, USA; 6 cm2 area, 183 lm of thickness) was used as membrane in the MFCs after conditioning one hour at 70 °C in a 3% (v/v) solution of hydrogen peroxide and one hour at 70 °C in a 1 M sulfuric acid solution. UltrexÒ membranes CMI-7000 (Membranes International Incorp., Glen Rock, New Jersey, USA) were also used as membrane in the MFC after conditioning with a solution of potassium chloride, 5% (v/v) for two hours. 1-Octyl-3-methylimidazolium tetrafluoroborate, [omim+][BF4 ], 1-butyl-3-methylimidazolium hexafluorophosphate, [bmim+] [PF6 ], 1-octyl-3-methylimidazolium hexafluorophosphate, [omim+][PF6 ], 1-butyl-3-methylimidazolium bis{(trifluorome thyl)sulfonyl}imide, [bmim+][NTf2 ], 1-octyl-3-methylimidazolium bis{(trifluoromethyl)sulfonyl}imide, [omim+][NTf2 ] was purchased from Solvent Innovation (purity >99%) and methyl trioctyl ammonium chloride [MTOA+][Cl ] was purchased from Sigma–Al drich–Fluka (purity >97%). 2.2. Preparation of supported liquid membrane The preparation of supported ionic liquid membranes (SILMs) was achieved by passing the corresponding ionic liquid through the NylonÒ membrane by using a 10 mL AmiconTH ultrafiltration as indicated in Hernandez-Fernandez et al. [22]. 2.3. MFC studies The experiments were carried out in one-chamber MFCs. The reactors were 250 mL glass bottles with an external jackets for temperature control. The cathode consisted of platinum dispersed on Vulcan (Alfa Aesar) and sprayed onto a 4 cm diameter piece of carbon cloth being the final load of 0.3 mg Pt cm 2. The cathode was connected with the anode through 1 kX resistor. The anode consisted of 168 g of graphite granules of 2–6 mm diameter (Graphite Store, USA) and a graphite rod of 3.18 mm diameter (Graphite Store, USA) to connect to the cathode terminal through the resistor. In each experiment 105 mL of wastewater was putted in the anode chamber. Between the cathode and the anode chamber a membrane was placed. A scheme of the microbial fuel cell can be observed in Fig. 2. The fuel added to anode chambers consisted of barley processing wastewater from a brewery diluted in domestic wastewater to
Fig. 1. Structures of common cations and anions of ILs.
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Fig. 2. Schematic of the device used for these experiments.
give a chemical oxygen demand (COD) of 1174 mg L 1. All experiments have run in batch mode using wastewater as a unique source of microorganisms and fuel. The experiment was run at 28 °C and the duration was 170 h. COD was monitored by taken regular water samples (5 mL). After centrifugation (5000 rpm for 5 min) and filtration (0.45 lm pore diameter membrane filter), 2 mL aliquot was used for COD analysis. Chemical oxygen demand (COD) was measured using the methods described in APHA (Larrosa et al. [4]). Voltage was also monitored with a continuous voltage recorder Pico log ADC-16 and ADC-24. 3. Results and discussion In this work, supported liquid membranes based on ionic liquids were tested for using as proton exchange membranes in microbial fuel cell. For that, four supported ionic liquids membranes were prepared. Because of the water nature of the microbial fuel system, in order to get stable supported ionic liquids membranes, the ionic liquid immobilized must be insoluble in water. Therefore, for the present study, ionic liquids were selected on the basis of their low solubility in water: [MTOA+][Cl ] < 0.02% (v/v), [omim+][NTf2] < 0.04% (v/v), [omim+][PF6 ] < 0.1% (v/v) and [omim+][BF4 ] < 1.4% (v/v). Those ionic liquids are composed of different anions and cations in order to study the effect of the ionic liquid composition on the performance of microbial fuel cell. NafionÒ and UltrexÒ (CMI-7000) membranes were used as reference membranes in microbial fuel cell. The MFC voltage with different supported ionic liquid membranes was continuously monitored during more than 160 h. Fig. 3 shows the voltage profile of the assayed microbial fuel cells. As can be seen from Fig. 3, firstly, in the first hour, the voltage increased due to the biofilm was formed on the graphite granules. After the maximum, the voltage was continuously reduced due to the reduction of the organic matter on the anode chamber. The highest voltages during the full experiment were reached by using the supported liquid membrane based on [OMIM+][PF6 ] ionic liquid, being the maximum voltage around 140 mV. Membranes based on [MTOA+][Cl ] followed a similar tendency to that of NafionÒ membrane and the UltrexÒ membranes. Initially the
membrane based on the ionic liquid [omim+][NTf2 ] showed a little voltage, however this voltage was kept constant during more time. Regarding to voltage, the membranes based on the ionic liquid [omim+][BF4 ] showed the worst behavior, however high initial voltage, which decreased dramatically until 30 mV after one day working. It is important to point out that the membrane based on the ionic liquid [omim+][BF4 ] after one day working started losing water due to the instability of the membrane against water. As we commented above, this ionic liquid presents the higher water solubility. Table 1 shows the COD removal for the MFCs with different supported ionic liquid membranes. Similar COD removal (around 90%) was reached with the NafionÒ, UltrexÒ and the supported liquid membranes based on [MTOA+][Cl ]. Lower reduction (around 80%) was reached with the ionic liquids [omim+][NTf2 ] and [omim+][BF4 ]. Supported ionic liquid membrane based on [omim+][PF6 ] involved the lowest COD reduction (30%). The lowest COD reduction reached in the case of [omim+][PF6 ] could be explained based on the ecotoxicity and solubility of ionic liquids used. The ecotoxicity values of the ionic liquid using MicrotoxÒ assay were the following [23]: log EC50(lM) (15 min, [omim+] [PF6 ]) = 0.82, log EC50(lM) (15 min, [omim+][NTf2 ]) = 0.83, log EC50(lM) (15 min, [omim+][BF4 ]) = 1.40. None ecotoxicity value for MicrotoxÒ toxicity test is found for [MTOA+][Cl ] on literature. Due to ionic liquid solubility in the aqueous phase some of ionic liquid can be released and affect to microorganism population. The low COD reduction in the case of [omim+][PF6 ] could be explained due to it is the highest toxic ionic liquid. Similar toxicity were found in the case of [omim+][NTf2 ], however the lower water solubility of [omim+][NTf2 ] could explain the higher COD reduction for [omim+][NTf2 ] than for [omim+][PF6 ]. The higher columbic efficiency was reached for [omim+][PF6 ] since high voltage was reached and COD reduction was low. After that, the highest columbic efficiency value was obtained for NafionÒ. Similar values were reached by the membranes based on [MTOA+][Cl ], [omim+][NTf2 ], [omim+][PF6 ] and UltrexÒ membranes. Since the ionic liquids [omim+][BF4 ] was the most water-soluble, the membrane based on this ionic liquid suffered from instability, even leakage of water through the membrane
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Fig. 3. Voltage output of single chamber MFCs with carbon cloth cathodes, under 1 kO working in batch mode with different supported ionic liquids membranes based on the ionic liquids [MTOA+][Cl ], [omim+][NTf2 ], [omim+][PF6 ] and [omim+][BF4 ] and NafionÒ and UltrexÒ (CMI-7000) membranes as reference membranes.
Table 1 Comparison of the percentage reduction of COD (initial COD 1174 mg/L) in MFC with different supported ionic liquids membranes based on [MTOA+][Cl ], [omim+][NTf2 ], [omim+][PF6 ] and [omim+][BF4 ] and NafionÒ and UltrexÒ membranes as reference membranes. Membrane
COD removal (%)
Coulombic efficiency (%)
Maximum power (mW/m2)
NafionÒ UltrexÒ [MTOA+][Cl ] [omim+][NTf2 ] [omim+][BF4 ] [omim+][PF6 ]
90.7 88.3 89.1 81.3 80.3 27.3
4.44 2.50 2.06 2.74 1.31 18.60
157.9 102.2 103.9 72.1 147.1 215.0
was observed. Consequently, the latter membrane involved the lower value of the columbic efficiency. As can been seen from Table 1, similar power were reached with microbial fuel cell based on supported ionic liquid membranes than conventional membranes. The higher value was reached for [omim+][PF6 ]. The maximum power of the membranes based on [omim+][BF4 ] was similar than that obtained with Nafion membrane and the maximum power reaches with [MTOA+][Cl ] was similar than that reaches with UltrexÒ. The ionic nature of ionic liquids assures the transport of proton through the membrane. The solubility of ionic liquid in the aqueous phase and the ecotoxicity of ionic liquid are found to be key factors to assure the feasibility of SILMs application in MFCs, since could affect to membrane stability and microorganism activity in the anode chamber, respectively.
4. Conclusions In this study we outlined, for the first time, the possibility of using SLMs based on ionic liquids as proton exchange membranes in microbial fuel cell. For Instance, the SILM based on the ionic liquid [MTOA+][Cl ] involves similar maximum power, COD reduction and coulombic efficiency that the UltrexÒ membrane used as reference membrane. The results of this study are quite encouraging and suggest that SLMs based on ionic liquids might be incorporated in microbial fuel cells as substituent of conventional and expensive proton exchange membranes such as NafionÒ or UltrexÒ.
Acknowledgments This work was partially supported by the Spanish Ministry of Science and Innovation (MICINN) and by the FEDER (FondoEuropeo de Desarrollo Regional), ref.CICYT ENE201125188 and by the Seneca Foundation 18975/JLI/2013 grants.
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