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Green energy generation from plant microbial fuel cells (PMFC) using compost and a novel clay separator
Sustainable Energy Technologies and Assessments 21 (2017) 59–66
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Original article
Green energy generation from plant microbial fuel cells (PMFC) using compost and a novel clay separator A. Carmalin Sophia a,⇑, S. Sreeja b a b
CSIR – National Environmental Engineering Research Institute (NEERI), CSIR Campus, Taramani, Chennai 600113, India Renewable-Energy Program, Academy of Scientific and Innovative Research (AcSIR), India
a r t i c l e
i n f o
Article history: Received 27 August 2016 Revised 27 April 2017 Accepted 4 May 2017
Keywords: Plant microbial fuel cell (PMFC) Rhizosphere Plant root exudates Soil microorganisms Green energy Bio-energy
a b s t r a c t This research study investigates the influence of three different plants (Brassica juncea, Trigonella foenumgraecum and Canna Stuttgart) and compost addition, on bioenergy generation in a PMFC. The studies revealed that Trigonella foenum-graecum and Canna stuttgart exhibit higher bio-energy generation compared to Brassica juncea. Trigonella foenum-graecum being a leguminous plant and Canna Stuttgart, tuberous plant may harbor high densities exudates and hence microorganisms. The high power density may be attributed to plant type and addition of compost to the soil- hence resulting root deposits. Further in depth research is necessary to explore the reason behind higher concentrations of exudates. Canna stuttgart (tuberous plant) showed the highest power output (power density of 222 mW m2 conversion efficiency of 0.022%) with least diurnal fluctuations. A novel clay mix separator in the place of membranes was used for the very first time in the present study. The use of laboratory materials such as artificial growth medium, ferricyanide and commercial membranes has been avoided thus simplifying the system design and cost. Development of the PMFC technology may lead to a new generation of sustainable, environment integrated energy harvesting systems. Ó 2017 Elsevier Ltd. All rights reserved.
Introduction Rapid urbanization has increased the need for clean and sustainable energy resources. The plant microbial fuel cell (PMFC) is one promising way to produce power from plants. In a PMFC, living plants are combined in the anode of the microbial fuel cell and used to generate bioenergy by the microbial action on plant root exudates [1–4]. The organic matter released through the roots is converted into electrons, protons and carbon dioxide by the electrochemically active microorganisms present in soil near the plant roots [5]. The first PMFCs were developed by Strik et al. [1] in the year 2008, establishing bioenergy generation from rhizo-deposits. Bioenergy from PMFC is a sustainable source and it produces energy without disruption of the environment. A recent study developed by Helder et al. [2] used marsh species where plants can grow in water logged conditions. PMFC systems can be combined with agricultural lands and does not contend with conventional crops for land space. It can also be integrated into areas unsuitable for
⇑ Corresponding author. E-mail address: [email protected] (A.C. Sophia). http://dx.doi.org/10.1016/j.seta.2017.05.001 2213-1388/Ó 2017 Elsevier Ltd. All rights reserved.
food production such as wetlands or even rooftops. These potential applications of the PMFC prevent deforestation [6]. Plant roots secrete innumerous compounds into the surrounding soil in an area called rhizosphere. Microbes are more copious in the rhizosphere. Root exudates initiate and control interaction between roots and soil microbes. Root exudation is part of the rhizodeposition process, a main cause for soil organic carbon released by plant roots. The characteristics of root exudates are determined by plant species, the age of the plant and external factors like biotic and abiotic stress [7]. The release of organic compounds from roots is a key factor in mineralizing acquired nutrients and in mediating plant–microbe interactions [8]. Different classes of primary and secondary compounds including amino acids, organic acids, phenolic acids, flavonoids, enzymes, fatty acids, nucleotides, tannins, steroids, terpenoids, alkaloids, polyacetylenes, and vitamins are present in the root exudates. Therefore, modulating growth and root branching in regions of nutrient-rich patches may be expected to be coincident with increased root exudation that could affect the nutrient dynamics and microbial community [9]. Higher concentration of root exudates would in-turn result in surplus substrate for the root microorganisms to metabolize, implying higher metabolic activity and hence higher bio-energy production. They mediate positive dialogues such as symbiotic associations with useful
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Fig. 1. Schematic representation of the plant microbial fuel cell system.
Fig. 2. The plant microbial fuel cell (PMFC) lab set-up.
microbes, such as mycorrhizae, rhizobia and plant growthpromoting rhizobacteria; as well as negative interactions such as association with parasitic plants, pathogenic microbes and invertebrate herbivores [10,11]. The release of organic compounds from roots is the key factor in mineralizing acquired nutrients and in mediating plant–microbe dialogues [8]. The reason behind higher concentrations of exudates may be soil nutrient content, rate of exudation, microbial population in the vicinity of roots or a combination of these and other complex factors.
This preliminary research aims to study the influence of plant types in PMFC performance using a novel clay separator in the place of PEM membrane. The diurnal variations in the three plant root types were studied to identify the system having the least fluctuations in energy production. Polarization studies were conducted under varying the external resistances. The application of a novel clay mix separator in a PMFC was explored. Their internal resistances were also compared by Electrochemical Impedance Spectroscopy (EIS) studies.
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A.C. Sophia, S. Sreeja / Sustainable Energy Technologies and Assessments 21 (2017) 59–66 Table 1 Different PMFC configurations used in the study. PMFC
Anode type
Cathode type
Cathodic condition
Separator
Brassica juncea Trigonella foenum-graecum Canna stuttgart
Brush Brush Brush
Carbon cloth Carbon cloth Carbon cloth
Air cathode Air cathode Air cathode
The novel clay separator The novel clay separator The novel clay separator
Three different plant types chosen were: (i) Brassica juncea – Mustard, is a are mesophyte (ii) Trigonella foenum-graecum – Fenugreek is a leguminous plant and (iii) Canna stuttgart is a decorative plant which is tuberous., Selection of the most suitable plant is a favorable way to enhance electricity output. All the above plants grow in normal soil conditions and can be scaled up extensively. Trigonella foenum-graecum, is a commonly growing leguminous plant in India [12]. Leguminous plants are known to harbor higher densities of bacterial populations at the rhizosphere due to their nitrogen fixing activities [13] Brassica juncea is a mesophyte. The influence of the type of plant has been explored for the very first time in this study.
Materials and methods
Table 2 Soil and compost characteristics. Parameter
Soil
Soil:Compost (1:1)
pH Electrical Conductivity, (mS cm1) Moisture (%) Organic Matter% Nitrate (mg L1) Sulphates (mg L1) Available Phosphorous (mg L1) Exchangeable Potassium (mg L1) Exchangeable Calcium (mg L1) Exchangeable Magnesium (mg L1) Exchangeable Sodium (mg L1) Available Zinc (mg L1) Available Manganese (mg L1) Available Iron (mg L1) Available Copper (mg L1) Available Boron (mg L1)
7.10 ± 0.4 0.474 ± 0.052 5.20 ± 0.5 2.03 ± 0.22 16.80 ± 1.50 35.30 ± 2.8 522.06 ± 20.5 457 ± 25 1375 ± 50 528 ± 20 88 ± 8 6.33 ± 0.4 11.7 ± 0.5 23.98 ± 1.2 1.82 ± 0.05 0.9 ± 0.1
7.68 ± 1.0 2.55 ± 0.081 24.6 ± 1.6 6.86 ± 0.56 33.8± 128.1 ± 10.6 958.32 ± 55 1934 ± 75 2620 ± 95 896 ± 35 758 ± 29 10.47 ± 0.96 8.9 ± 0.5 17.65 ± 1.4 2.9 ± 0.08 1.2 ± 0.11
Experimental set-up The experimental set-up of the PMFC is given in Fig. 1. For the experiment a cylindrical PMFC was constructed using an anode chamber made of a novel clay-mix. The clay-mix container (dimensions: height 10 cm dia 5 cm) was fabricated using pre-weighed mix of potter’s clay with fixed percentage of kaolinite and montmorillonite, fired to an elevated temperature. The clay wall acts as separator, replacing the conventionally used proton exchange membrane [2,6]. This study eliminates the use of expensive commercial membranes, significantly reducing the cost and complexity of the systems – making it economically feasible for real time applications. The role of clay in PMFC needs in depth scientific research and the study is in progress. Carbon brush anodes 8 cm length (locally purchased in Chennai, India) and hydrated carbon cloth (purchased from ebay, India) was used as air cathode. The anode chamber was covered along its sides with hydrated carbon cloth as air cathode. The plants were directly planted in the anode chamber of the PMFC with their root-system close [14] to the anode (Fig. 2). The connections were made by using copper wires purchased from local electrical outlet in Chennai. The current collectors were Cu wires placed in the anode and the cathode. Carbon brush Cu metal core was used as the anodic current collector. The system was operated in continuous mode at open circuit voltage (OCV). The artificial growth medium and ferricyanide, used by previous studies [1,2,8,15], was avoided in this study. Brassica juncea, Trigonella foenum-graecum (leguminous) and Canna stuttgart (tuberous) were grown in the anode chambers of the PMFCs containing soil and electrodes. The PMFCs were placed under natural light conditions with an average light intensity of 700 W m2, illuminating for 12 h a day. Solar irradiation data for the period of study was obtained from weather station at CSIR-NEERI Chennai Laboratory. The plants were allowed to grow from seeds (Trigonella foenum-graecum and Canna stuttgart) and root cuttings (Brassica juncea). The voltages of the PMFCs were monitored from day one. After start-up of the PMFCs, the anode and cathode wires were connected to the data acquisition system for monitoring of the voltage, performance polarization studies and for electrochemical impedance spectroscopy studies. Table 1 shows the different PMFCs investigated in this study. The experiments were conducted in the open terrace
of the laboratory. All the experiments were conducted in triplicates and the average values are presented in the study. Polarization studies The voltage and current were measured using a digital multimeter (Mastech M3900). Maximum power was determined via polarization curves using Biologic Data Acquisition System. The external resistance was changed every 20 min from 500 X through 20 X periodically. An equilibration time of 30 min. was given between each measurement. The polarization studies were performed in triplicate to check for precision. Electrochemical impedance studies (EIS) EIS measurements were recorded using Frequency Response Analyzer coupled to a potentiostat. The frequency range was 100 kHz–100 mHz and a small AC signal of amplitude 10 mV was applied to analyze the current response of PMFC without disturbing its operation [16]. The PMFC was connected to the potentiostat in two-electrode mode. The PMFC at the time of EIS measurements showed a voltage of 370 mV. Analyzing impedance elements Impedance data was fitted in ZSimpWin 3.1. circuit modelling software to establish the Chi-squared value and in turn the error percentage. Low Chi squared value indicated low error percentage. Analysis of soil Compost was prepared in the laboratory using the campus garden waste. The garden waste was accumulated in a pit (0.5 m deep) until it became full. The same was covered with soil and left to decay and form compost. After 45 days the pit was dug open and the compost was stored for PMFC use. Soil was mixed thoroughly with organic compost in a ratio of 1:1 in triplicates, prior to planting. Physico-chemical analysis [17] of soil and soil mixed with compost was done in triplicate and the average results are
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The soil bacteria living in the rhizosphere were calculated at 5 day intervals by determining the colony forming units/ml during the experiment by pour plate’s method. The typical soil microorganism culture medium was prepared, as it would automatically select the microorganisms of importance in the context of the PMFC [17]. Soil microorganism morphology was studied using Carl Zeiss optical microscope, model no: Stereo Discovery V20.
Culture medium
Fig. 3. Effect of compost addition on bio-energy production.
presented in Table 2. The organic content of the soil increased three fold after the addition of compost. The nitrate and the phosphorus content also increased almost approximately two folds.
Tryptone – 5 g, Yeast Extract – 2.5 g, Dextrose 1.0 g, Agar – 9.0 g in 1 L of distilled water. It was autoclaved at 121 °C, 15psi for 15 min. The media was allowed to cool down to 45 °C by placing in a water bath. Pour plates were prepared by serially diluting aliquots of the collected sample containing soil microorganisms belonging to the rhizosphere was added to the petri plate by holding it close to the flame in a laminar air flow (all the glassware was autoclaved at 121 °C at 15psi for 15 min prior to use). The media was poured onto plate and lid was closed immediately and it was allowed to solidify. Then it was sealed along the edges using
Fig. 4. (a) Growth curve of soil microflora in soil-compost PMFC systems [Inset picture: pour plate of soil microflora] (b) Optical microscopy images of the soil microflora in soil-compost PMFC system of Brassica juncea.
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Fig. 5. (a) Diurnal variation of the PMFCs (b) Relationship between irradiance and voltage for Brassica juncea.
organic residues, cycling of nutrients, and formation of organic matter and soil structure [20,21]. Among the three PMFC systems studied Canna Stuttgart exhibited the highest voltage readings. Tuberous roots of Canna Stuttgart may be one of the reasons for higher root exudates, which increase the availability of organic matter for microorganism. Root exudates are comprised of carbohydrates, carboxylic acids, and amino acids that are highly degradable by microorganisms and it causes electron donation and in turn voltage [22]. This may be one main reason for increasing voltage, however further research is needed to explore the same. Fig. 3 shows the increase in bio-energy production. The effect of organic compost on electricity production was studied. Studies revealed that the added organic compost enhanced the growth rate of the plants and bio-energy production. Organic compost will not only have more nutrients that enable faster plant growth, it also harbours hotspots with high density of soil microflora, which means increased action in the PMFC. The control pots (with and without compost) without plants did not show any bio energy generation. Since soil-compost mix showed better performance than soil only systems, soil-compost PMFC systems were chosen to be further investigated. Earlier studies [23] had indicated that addition of compost enhances the microbial hotspots in soil, thus enhancing performance of the PMFC. The study illustrated the comparison for changes in bio-energy with paddy length. Fig. 4a and b shows the increase in microflora with time in soilcompost mix systems. The picture shows the bacterial colonies formed after incubation. It was observed that Canna Stuttgart exhibited the higher rhizosphere microbial populations which may be attributed to the root exudates and in-turn the tuberous root structure. However, since the rhizosphere is intriguingly complex and dynamic, and understanding its ecology and evolution requires further research to support this.
Diurnal changes and photosynthesis para-film. The plates were inverted and incubated at 37 °C for 24 h in the incubator.
Results and discussion Role of compost and bio-energy production It was observed that the voltage produced was higher in compost added systems and it increased with time during the experiment. This study was stopped at 35 days as the plants specially Trigonella foenum-graecum over grew the pots. Long term experiments are in progress and the results will be published in future research. Hydrolase enzyme activities in the rhizosphere soil and their changes is important since they indicate the potential of a soil to carry out specific biochemical reactions, and these hydrolytic enzymes are important in maintaining soil fertility and plant productivity [18]. Because plant nutrient uptake occurs through the rhizosphere, the activity of rhizosphere microbial community is of great importance for plant growth [19]. Soil enzymes are involved in the catalysis of a large number of reactions necessary for life processes of microorganisms in soils, decomposition of
Day length was found to be one of the most important factors modifying potential photosynthesis. Diurnal pattern of bioenergy production was observed for the PMFCs during the experimental period of 30 days (Fig. 5a). This diurnal variation can be attributed to photosynthesis in the plants investigated. The control pots were not studied for diurnal changes as they did not have any plants for photosynthesis. Brassica juncea was observed to have the highest diurnal variation. This may be due to lesser concentration of released exudates as compared to the leguminous Trigonella foenum-graecum and tuberous Canna stuttgart. Further in depth research is required to prove the above hypothesis. Leguminous Trigonella foenum-graecum plant showed lesser variation than the tuberous Canna Stuttgart. Lowest diurnal variation was observed in Canna Stuttgart. The rate of photosynthesis in tea leaves (Camellia sinensis (L.) O. Kuntze) grown at high elevation in the fields in Sri Lanka was seen to be affected by diurnal changes [24]. Photosynthesizing plants in a plant microbial fuel cell produce organic materials and release them as rhizodeposits into the surrounding soil via their roots; thus producing energy was reported by David et al. [25] Diurnal variation is further related to the variation in voltage observed in the plot of voltage versus irradiance for the Brassica juncea plant
Table 3 Comparison of PMFC performances (30th day). PMFC
Voc (mV)
Isc (mA)
Vw (mV)
Iw (mA)
R (O)
Power density (mW m2)
Efficiency (%)
Brassica juncea Trigonella foenum-graecum Canna stuttgart
523 485 505
0.19 0.22 0.56
203 309 459
16 12 224
1.27 2.58 2.05
69.32 80.26 222.54
0.007 0.008 0.022
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Fig. 6. Performance of PMFC, current versus voltage.
as presented in Fig. 5b. Kaku et al. [26] observed that electricity generation was dependent on sunlight and artificially shading plants in the daytime, reduced electricity generation because of the reduction in photosynthesis. Moqsud et al. [23] has reported that the paddy based cell, in which plant photosynthesis is combined with microbial conversion of organics into energy. Their studies have proved that solar radiation affects the paddy system. Voltage generation was found to be high when the solar radiation was high. Performance of PMFC PMFCs convert solar energy into electrical power. PMFCs are sustainable, as the substrate flow is achieved via continuous input by roots. The reason for 20% of photosynthesized carbohydrates is root exudates [27]. The performance of PMFC systems in terms of power density, using three different plants were tested for the first time. Power density (mW m2) and conversion efficiency of the PMFC systems were calculated using the Eqs. (1) and (2)
Power density ðmW m2 Þ ¼
Conversion Efficiency ¼
2
ðVoltageÞ Resistance Area of the anode
ð1Þ
ðPower Density Area of the AnodeÞ ðAvg: Incident Solar RadiationÞ 100 ð2Þ
Stable power output was observed in all the three plants. However, Canna stuttgart exhibited the highest power density of 222.54 mW m2, compared to Trigonella foenum-graecum plants (80.26 mW m2) and Brassica juncea (69.32 mW m2) (as seen in Table 3 and Fig. 6). The peak voltage produced in our study was approximately 350 mV and 278 mV in Canna stuttgart and Trigonella foenum-graecum PMFCs respectively (Fig. 7) on the 7th day after plantation. This implied that the root systems were giving comparatively stable performance. This further proves that root exudates were available to be converted into energy by the soil micro-organisms. Photosynthesizing plants in a plant microbial fuel cell produce organic materials and release them as rhizodeposits into the surrounding soil via their roots; thus producing energy was reported by David et al. [25]. Kaku et al. [26] tried to develop rice PMFCs, where the voltage generation was minimum and the system faced problems of growth. Reed manna grass had been used to generate bio-energy by Timmers et al. [28]. The utility of one of the well investigated photosynthetic plants Glyceria max-
Fig. 7. Voltage vs time for the different types of plants (7th day after germination/plantation).
ima, as a model, in photo MFC achieved a maximum power production of 67 mW m2. An average power density of 50 mW m2 was accomplished by Spartina anglica for 33 days among the various PMFCs investigated by Timmers et al. [28]. Moqsud et al. [23] used 1 and 3% of compost on paddy based PMFC. The maximum power density of around obtained in the study was 23 mW m2. In the present study we have observed a much higher power density (10 times) than that obtained in a recent study [23]. A maximum power density of 15.73 mW m2 was obtained for a canna indicaMFC system [29], and it was about 15 times lesser than that generated in this study. The high power density may be attributed to plant type and addition of compost to the soil- hence resulting root deposits degradable by microorganisms and therefore accountable for electron donation [22]. Electrochemical impedance spectroscopy The significance of impedance is important for a heterogeneous PMFC system. The electrochemical interface of PMFC is highly complex. The microbial activity is expected near the electrode, making the electrochemical reaction intricate. This method investigates the dynamics of the mobile or bound charges in the bulk or interfacial region of such system. An electrode surface with a smooth and undamaged coating usually has high impedance. The equivalent circuit for such an undamaged metal surface consists of a resistance (R) in series with a simple capacitance (C) and is called purely capacitive coating. The corresponding Nyquist plot for such an ideally polarized electrode will have a straight vertical intersecting the Z’-axis and the X-intercept gives the value of resistance. The internal resistances of the cell were obtained from Nyquist plots (Fig. 8), where the intercept of the curve with the Ze axis is defined as the ohmic resistance. Table 4 shows the individual internal resistances the PMFC systems obtained by fitting the EIS Data in ZSimpWin. These values are derived by using the potentiostat readings; plotting the spectrum in Nyquist plot and later the fitting the same in equivalent circuit models. A circuit modeling software was used to fit the EIS data acquired from biologic data acquisition system. The model exhibited a good fit, having the least percentage error (less than 1%) and very low Chi-squared value (of the order of 104 or lesser). Low Chi squared indicated low error percentage. Comparing the three PMFC systems which had similar conditions except for the type of plant; it was observed that the electrolyte
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Fig. 8. Nyquist plots.
Table 4 Internal resistances of the PMFC systems. PMFC configuration
Electrolyte resistance (O)
Brassica juncea Trigonella foenum-graecum Canna stuttgart
137.7 ± 5.6 32.57 ± 3.1 22.57 ± 3.0
resistance was the highest in the case PMFC triplicates of Brassica juncea and lowest in the case of PMFC triplicates with Trigonella foenum-graecum and Canna stuttgart. This may be due to the differences in root exudates, rate of exudation, soil nutrients, microbial population in the vicinity of roots or a combination of these and other complex factors present in the heterogenous bio-electrochemical system. Electrochemical impedance spectroscopy has not been studied for PMFC as yet. Variation in ohmic
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resistance over a desalination cycle have been investigated [30], where the voltage quickly attained an initial peak, and then unceasingly decreased over the course of the cycle, consistent with the observed increase in ohmic resistance. A previous study reported that, a higher electrolyte concentration reduced the membrane solution interface resistance due to the compression of electrical double layer [31]. This enhancement in membrane resistance over a cycle is also consistent with research results obtained in a MFC tests with Nafion 117 at low electrolyte concentrations [32]. Economics Most MFCs use NafionÒ membranes that cost approximately $2500 m2, Agar ($165 m2) and Gore-Tex ($82.5 m2). Use of expensive Nafion has been eliminated, significantly reducing the cost and complexity of the systems – making it easier for scaling up. Cost of clay is 1$ kg1. Therefore approximate cost per meter square with thickness of 0.8 cm was calculated to be $135 m2 (including cost of fabrication and transport) as compared to $2500 m2 of NafionÒ. Conclusions In this study, compost: soil mix (1:1) was used in PMFCs for bioelectricity generation by using three different plant systems namely, Brassica juncea, Trigonella foenum-graecum and Canna stuttgart. Addition of compost increased the bio-energy output nearly 2.5 times. Bio-energy harvesting did not disturb the growth of the plant. Among the systems studied, Canna stuttgart and Trigonella foenum-graecum exhibited higher bio-energy output than Brassica juncea. Canna stuttgart plants were observed to show the most stable output with very little diurnal fluctuations. The system exhibited a power density of 222 mW m2 and a conversion efficiency of 0.022%. The high power density may be attributed to plant root type (tuberous in the case of Canna stuttgart), presence of compost/nutrients in soil and hence higher root deposits which are degradable by microorganisms and responsible for electron donation. A reduction in cost of 94.6% is estimated to be achieved by replacing Nafion PEM with the novel clay mix separator. Acknowledgements The authors wish to acknowledge the financial support by SERB Science & Engineering Research Board (SERB) Fast Track Young Scientist Award Scheme; Financial order no. SR/FT/LS-14/2011. Special thanks to the Director, CSIR – NEERI for providing permission and lab facilities to perform experiments at CSIR-NEERI, Chennai Zonal Laboratory. The authors also thank Dr. K. Ramesha, Senior Scientist, CSIR-CECRI for providing VMP3 Biologic multi-channel potentiostat/galvanostat facility of CSIR-Innovation Complex, Chennai. References [1] Strik DPBTB, Terlouw H, Hamelers HVM, Buisman CJN. Renewable sustainable biocatalyzed energy production in a photosynthetic algal microbial fuel cell (PAMFC). Appl Microbiol Biotechnol 2008;81:659–68. [2] Helder M, Strik DPBTB, Hamelers HVM, Kuhn AJ, Blok C, Buisman CJN. Concurrent bio-energy and biomass production in three plant-microbial fuel cells using Spartina anglica, Arundinella anomala and Arundo donax. Bioresour Technol 2010;101:3541–7. [3] Strik DPBTB, Timmers RA, Helder M, Steinbusch KJJ, Hamelers HVM. Microbial solar cells: applying photosynthetic and electrochemically active organisms. Trends Biotechnol 2011;29:41–9.
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The influence of H2SO4 electrolyte concentration on proton transfer resistance of membrane/solution interface. Eur Pol J 2006;42:2894–8. [32] Harnisch F, Schroder U, Scholz F. The suitability of monopolar and bipolar ion exchange membranes as separators for biological fuel cells. Environ Sci Technol 2008;42:1740–6.
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Sustainable Energy Technologies and Assessments journal homepage: www.elsevier.com/locate/seta
Original article
Green energy generation from plant microbial fuel cells (PMFC) using compost and a novel clay separator A. Carmalin Sophia a,⇑, S. Sreeja b a b
CSIR – National Environmental Engineering Research Institute (NEERI), CSIR Campus, Taramani, Chennai 600113, India Renewable-Energy Program, Academy of Scientific and Innovative Research (AcSIR), India
a r t i c l e
i n f o
Article history: Received 27 August 2016 Revised 27 April 2017 Accepted 4 May 2017
Keywords: Plant microbial fuel cell (PMFC) Rhizosphere Plant root exudates Soil microorganisms Green energy Bio-energy
a b s t r a c t This research study investigates the influence of three different plants (Brassica juncea, Trigonella foenumgraecum and Canna Stuttgart) and compost addition, on bioenergy generation in a PMFC. The studies revealed that Trigonella foenum-graecum and Canna stuttgart exhibit higher bio-energy generation compared to Brassica juncea. Trigonella foenum-graecum being a leguminous plant and Canna Stuttgart, tuberous plant may harbor high densities exudates and hence microorganisms. The high power density may be attributed to plant type and addition of compost to the soil- hence resulting root deposits. Further in depth research is necessary to explore the reason behind higher concentrations of exudates. Canna stuttgart (tuberous plant) showed the highest power output (power density of 222 mW m2 conversion efficiency of 0.022%) with least diurnal fluctuations. A novel clay mix separator in the place of membranes was used for the very first time in the present study. The use of laboratory materials such as artificial growth medium, ferricyanide and commercial membranes has been avoided thus simplifying the system design and cost. Development of the PMFC technology may lead to a new generation of sustainable, environment integrated energy harvesting systems. Ó 2017 Elsevier Ltd. All rights reserved.
Introduction Rapid urbanization has increased the need for clean and sustainable energy resources. The plant microbial fuel cell (PMFC) is one promising way to produce power from plants. In a PMFC, living plants are combined in the anode of the microbial fuel cell and used to generate bioenergy by the microbial action on plant root exudates [1–4]. The organic matter released through the roots is converted into electrons, protons and carbon dioxide by the electrochemically active microorganisms present in soil near the plant roots [5]. The first PMFCs were developed by Strik et al. [1] in the year 2008, establishing bioenergy generation from rhizo-deposits. Bioenergy from PMFC is a sustainable source and it produces energy without disruption of the environment. A recent study developed by Helder et al. [2] used marsh species where plants can grow in water logged conditions. PMFC systems can be combined with agricultural lands and does not contend with conventional crops for land space. It can also be integrated into areas unsuitable for
⇑ Corresponding author. E-mail address: [email protected] (A.C. Sophia). http://dx.doi.org/10.1016/j.seta.2017.05.001 2213-1388/Ó 2017 Elsevier Ltd. All rights reserved.
food production such as wetlands or even rooftops. These potential applications of the PMFC prevent deforestation [6]. Plant roots secrete innumerous compounds into the surrounding soil in an area called rhizosphere. Microbes are more copious in the rhizosphere. Root exudates initiate and control interaction between roots and soil microbes. Root exudation is part of the rhizodeposition process, a main cause for soil organic carbon released by plant roots. The characteristics of root exudates are determined by plant species, the age of the plant and external factors like biotic and abiotic stress [7]. The release of organic compounds from roots is a key factor in mineralizing acquired nutrients and in mediating plant–microbe interactions [8]. Different classes of primary and secondary compounds including amino acids, organic acids, phenolic acids, flavonoids, enzymes, fatty acids, nucleotides, tannins, steroids, terpenoids, alkaloids, polyacetylenes, and vitamins are present in the root exudates. Therefore, modulating growth and root branching in regions of nutrient-rich patches may be expected to be coincident with increased root exudation that could affect the nutrient dynamics and microbial community [9]. Higher concentration of root exudates would in-turn result in surplus substrate for the root microorganisms to metabolize, implying higher metabolic activity and hence higher bio-energy production. They mediate positive dialogues such as symbiotic associations with useful
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Fig. 1. Schematic representation of the plant microbial fuel cell system.
Fig. 2. The plant microbial fuel cell (PMFC) lab set-up.
microbes, such as mycorrhizae, rhizobia and plant growthpromoting rhizobacteria; as well as negative interactions such as association with parasitic plants, pathogenic microbes and invertebrate herbivores [10,11]. The release of organic compounds from roots is the key factor in mineralizing acquired nutrients and in mediating plant–microbe dialogues [8]. The reason behind higher concentrations of exudates may be soil nutrient content, rate of exudation, microbial population in the vicinity of roots or a combination of these and other complex factors.
This preliminary research aims to study the influence of plant types in PMFC performance using a novel clay separator in the place of PEM membrane. The diurnal variations in the three plant root types were studied to identify the system having the least fluctuations in energy production. Polarization studies were conducted under varying the external resistances. The application of a novel clay mix separator in a PMFC was explored. Their internal resistances were also compared by Electrochemical Impedance Spectroscopy (EIS) studies.
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Anode type
Cathode type
Cathodic condition
Separator
Brassica juncea Trigonella foenum-graecum Canna stuttgart
Brush Brush Brush
Carbon cloth Carbon cloth Carbon cloth
Air cathode Air cathode Air cathode
The novel clay separator The novel clay separator The novel clay separator
Three different plant types chosen were: (i) Brassica juncea – Mustard, is a are mesophyte (ii) Trigonella foenum-graecum – Fenugreek is a leguminous plant and (iii) Canna stuttgart is a decorative plant which is tuberous., Selection of the most suitable plant is a favorable way to enhance electricity output. All the above plants grow in normal soil conditions and can be scaled up extensively. Trigonella foenum-graecum, is a commonly growing leguminous plant in India [12]. Leguminous plants are known to harbor higher densities of bacterial populations at the rhizosphere due to their nitrogen fixing activities [13] Brassica juncea is a mesophyte. The influence of the type of plant has been explored for the very first time in this study.
Materials and methods
Table 2 Soil and compost characteristics. Parameter
Soil
Soil:Compost (1:1)
pH Electrical Conductivity, (mS cm1) Moisture (%) Organic Matter% Nitrate (mg L1) Sulphates (mg L1) Available Phosphorous (mg L1) Exchangeable Potassium (mg L1) Exchangeable Calcium (mg L1) Exchangeable Magnesium (mg L1) Exchangeable Sodium (mg L1) Available Zinc (mg L1) Available Manganese (mg L1) Available Iron (mg L1) Available Copper (mg L1) Available Boron (mg L1)
7.10 ± 0.4 0.474 ± 0.052 5.20 ± 0.5 2.03 ± 0.22 16.80 ± 1.50 35.30 ± 2.8 522.06 ± 20.5 457 ± 25 1375 ± 50 528 ± 20 88 ± 8 6.33 ± 0.4 11.7 ± 0.5 23.98 ± 1.2 1.82 ± 0.05 0.9 ± 0.1
7.68 ± 1.0 2.55 ± 0.081 24.6 ± 1.6 6.86 ± 0.56 33.8± 128.1 ± 10.6 958.32 ± 55 1934 ± 75 2620 ± 95 896 ± 35 758 ± 29 10.47 ± 0.96 8.9 ± 0.5 17.65 ± 1.4 2.9 ± 0.08 1.2 ± 0.11
Experimental set-up The experimental set-up of the PMFC is given in Fig. 1. For the experiment a cylindrical PMFC was constructed using an anode chamber made of a novel clay-mix. The clay-mix container (dimensions: height 10 cm dia 5 cm) was fabricated using pre-weighed mix of potter’s clay with fixed percentage of kaolinite and montmorillonite, fired to an elevated temperature. The clay wall acts as separator, replacing the conventionally used proton exchange membrane [2,6]. This study eliminates the use of expensive commercial membranes, significantly reducing the cost and complexity of the systems – making it economically feasible for real time applications. The role of clay in PMFC needs in depth scientific research and the study is in progress. Carbon brush anodes 8 cm length (locally purchased in Chennai, India) and hydrated carbon cloth (purchased from ebay, India) was used as air cathode. The anode chamber was covered along its sides with hydrated carbon cloth as air cathode. The plants were directly planted in the anode chamber of the PMFC with their root-system close [14] to the anode (Fig. 2). The connections were made by using copper wires purchased from local electrical outlet in Chennai. The current collectors were Cu wires placed in the anode and the cathode. Carbon brush Cu metal core was used as the anodic current collector. The system was operated in continuous mode at open circuit voltage (OCV). The artificial growth medium and ferricyanide, used by previous studies [1,2,8,15], was avoided in this study. Brassica juncea, Trigonella foenum-graecum (leguminous) and Canna stuttgart (tuberous) were grown in the anode chambers of the PMFCs containing soil and electrodes. The PMFCs were placed under natural light conditions with an average light intensity of 700 W m2, illuminating for 12 h a day. Solar irradiation data for the period of study was obtained from weather station at CSIR-NEERI Chennai Laboratory. The plants were allowed to grow from seeds (Trigonella foenum-graecum and Canna stuttgart) and root cuttings (Brassica juncea). The voltages of the PMFCs were monitored from day one. After start-up of the PMFCs, the anode and cathode wires were connected to the data acquisition system for monitoring of the voltage, performance polarization studies and for electrochemical impedance spectroscopy studies. Table 1 shows the different PMFCs investigated in this study. The experiments were conducted in the open terrace
of the laboratory. All the experiments were conducted in triplicates and the average values are presented in the study. Polarization studies The voltage and current were measured using a digital multimeter (Mastech M3900). Maximum power was determined via polarization curves using Biologic Data Acquisition System. The external resistance was changed every 20 min from 500 X through 20 X periodically. An equilibration time of 30 min. was given between each measurement. The polarization studies were performed in triplicate to check for precision. Electrochemical impedance studies (EIS) EIS measurements were recorded using Frequency Response Analyzer coupled to a potentiostat. The frequency range was 100 kHz–100 mHz and a small AC signal of amplitude 10 mV was applied to analyze the current response of PMFC without disturbing its operation [16]. The PMFC was connected to the potentiostat in two-electrode mode. The PMFC at the time of EIS measurements showed a voltage of 370 mV. Analyzing impedance elements Impedance data was fitted in ZSimpWin 3.1. circuit modelling software to establish the Chi-squared value and in turn the error percentage. Low Chi squared value indicated low error percentage. Analysis of soil Compost was prepared in the laboratory using the campus garden waste. The garden waste was accumulated in a pit (0.5 m deep) until it became full. The same was covered with soil and left to decay and form compost. After 45 days the pit was dug open and the compost was stored for PMFC use. Soil was mixed thoroughly with organic compost in a ratio of 1:1 in triplicates, prior to planting. Physico-chemical analysis [17] of soil and soil mixed with compost was done in triplicate and the average results are
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The soil bacteria living in the rhizosphere were calculated at 5 day intervals by determining the colony forming units/ml during the experiment by pour plate’s method. The typical soil microorganism culture medium was prepared, as it would automatically select the microorganisms of importance in the context of the PMFC [17]. Soil microorganism morphology was studied using Carl Zeiss optical microscope, model no: Stereo Discovery V20.
Culture medium
Fig. 3. Effect of compost addition on bio-energy production.
presented in Table 2. The organic content of the soil increased three fold after the addition of compost. The nitrate and the phosphorus content also increased almost approximately two folds.
Tryptone – 5 g, Yeast Extract – 2.5 g, Dextrose 1.0 g, Agar – 9.0 g in 1 L of distilled water. It was autoclaved at 121 °C, 15psi for 15 min. The media was allowed to cool down to 45 °C by placing in a water bath. Pour plates were prepared by serially diluting aliquots of the collected sample containing soil microorganisms belonging to the rhizosphere was added to the petri plate by holding it close to the flame in a laminar air flow (all the glassware was autoclaved at 121 °C at 15psi for 15 min prior to use). The media was poured onto plate and lid was closed immediately and it was allowed to solidify. Then it was sealed along the edges using
Fig. 4. (a) Growth curve of soil microflora in soil-compost PMFC systems [Inset picture: pour plate of soil microflora] (b) Optical microscopy images of the soil microflora in soil-compost PMFC system of Brassica juncea.
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Fig. 5. (a) Diurnal variation of the PMFCs (b) Relationship between irradiance and voltage for Brassica juncea.
organic residues, cycling of nutrients, and formation of organic matter and soil structure [20,21]. Among the three PMFC systems studied Canna Stuttgart exhibited the highest voltage readings. Tuberous roots of Canna Stuttgart may be one of the reasons for higher root exudates, which increase the availability of organic matter for microorganism. Root exudates are comprised of carbohydrates, carboxylic acids, and amino acids that are highly degradable by microorganisms and it causes electron donation and in turn voltage [22]. This may be one main reason for increasing voltage, however further research is needed to explore the same. Fig. 3 shows the increase in bio-energy production. The effect of organic compost on electricity production was studied. Studies revealed that the added organic compost enhanced the growth rate of the plants and bio-energy production. Organic compost will not only have more nutrients that enable faster plant growth, it also harbours hotspots with high density of soil microflora, which means increased action in the PMFC. The control pots (with and without compost) without plants did not show any bio energy generation. Since soil-compost mix showed better performance than soil only systems, soil-compost PMFC systems were chosen to be further investigated. Earlier studies [23] had indicated that addition of compost enhances the microbial hotspots in soil, thus enhancing performance of the PMFC. The study illustrated the comparison for changes in bio-energy with paddy length. Fig. 4a and b shows the increase in microflora with time in soilcompost mix systems. The picture shows the bacterial colonies formed after incubation. It was observed that Canna Stuttgart exhibited the higher rhizosphere microbial populations which may be attributed to the root exudates and in-turn the tuberous root structure. However, since the rhizosphere is intriguingly complex and dynamic, and understanding its ecology and evolution requires further research to support this.
Diurnal changes and photosynthesis para-film. The plates were inverted and incubated at 37 °C for 24 h in the incubator.
Results and discussion Role of compost and bio-energy production It was observed that the voltage produced was higher in compost added systems and it increased with time during the experiment. This study was stopped at 35 days as the plants specially Trigonella foenum-graecum over grew the pots. Long term experiments are in progress and the results will be published in future research. Hydrolase enzyme activities in the rhizosphere soil and their changes is important since they indicate the potential of a soil to carry out specific biochemical reactions, and these hydrolytic enzymes are important in maintaining soil fertility and plant productivity [18]. Because plant nutrient uptake occurs through the rhizosphere, the activity of rhizosphere microbial community is of great importance for plant growth [19]. Soil enzymes are involved in the catalysis of a large number of reactions necessary for life processes of microorganisms in soils, decomposition of
Day length was found to be one of the most important factors modifying potential photosynthesis. Diurnal pattern of bioenergy production was observed for the PMFCs during the experimental period of 30 days (Fig. 5a). This diurnal variation can be attributed to photosynthesis in the plants investigated. The control pots were not studied for diurnal changes as they did not have any plants for photosynthesis. Brassica juncea was observed to have the highest diurnal variation. This may be due to lesser concentration of released exudates as compared to the leguminous Trigonella foenum-graecum and tuberous Canna stuttgart. Further in depth research is required to prove the above hypothesis. Leguminous Trigonella foenum-graecum plant showed lesser variation than the tuberous Canna Stuttgart. Lowest diurnal variation was observed in Canna Stuttgart. The rate of photosynthesis in tea leaves (Camellia sinensis (L.) O. Kuntze) grown at high elevation in the fields in Sri Lanka was seen to be affected by diurnal changes [24]. Photosynthesizing plants in a plant microbial fuel cell produce organic materials and release them as rhizodeposits into the surrounding soil via their roots; thus producing energy was reported by David et al. [25] Diurnal variation is further related to the variation in voltage observed in the plot of voltage versus irradiance for the Brassica juncea plant
Table 3 Comparison of PMFC performances (30th day). PMFC
Voc (mV)
Isc (mA)
Vw (mV)
Iw (mA)
R (O)
Power density (mW m2)
Efficiency (%)
Brassica juncea Trigonella foenum-graecum Canna stuttgart
523 485 505
0.19 0.22 0.56
203 309 459
16 12 224
1.27 2.58 2.05
69.32 80.26 222.54
0.007 0.008 0.022
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Fig. 6. Performance of PMFC, current versus voltage.
as presented in Fig. 5b. Kaku et al. [26] observed that electricity generation was dependent on sunlight and artificially shading plants in the daytime, reduced electricity generation because of the reduction in photosynthesis. Moqsud et al. [23] has reported that the paddy based cell, in which plant photosynthesis is combined with microbial conversion of organics into energy. Their studies have proved that solar radiation affects the paddy system. Voltage generation was found to be high when the solar radiation was high. Performance of PMFC PMFCs convert solar energy into electrical power. PMFCs are sustainable, as the substrate flow is achieved via continuous input by roots. The reason for 20% of photosynthesized carbohydrates is root exudates [27]. The performance of PMFC systems in terms of power density, using three different plants were tested for the first time. Power density (mW m2) and conversion efficiency of the PMFC systems were calculated using the Eqs. (1) and (2)
Power density ðmW m2 Þ ¼
Conversion Efficiency ¼
2
ðVoltageÞ Resistance Area of the anode
ð1Þ
ðPower Density Area of the AnodeÞ ðAvg: Incident Solar RadiationÞ 100 ð2Þ
Stable power output was observed in all the three plants. However, Canna stuttgart exhibited the highest power density of 222.54 mW m2, compared to Trigonella foenum-graecum plants (80.26 mW m2) and Brassica juncea (69.32 mW m2) (as seen in Table 3 and Fig. 6). The peak voltage produced in our study was approximately 350 mV and 278 mV in Canna stuttgart and Trigonella foenum-graecum PMFCs respectively (Fig. 7) on the 7th day after plantation. This implied that the root systems were giving comparatively stable performance. This further proves that root exudates were available to be converted into energy by the soil micro-organisms. Photosynthesizing plants in a plant microbial fuel cell produce organic materials and release them as rhizodeposits into the surrounding soil via their roots; thus producing energy was reported by David et al. [25]. Kaku et al. [26] tried to develop rice PMFCs, where the voltage generation was minimum and the system faced problems of growth. Reed manna grass had been used to generate bio-energy by Timmers et al. [28]. The utility of one of the well investigated photosynthetic plants Glyceria max-
Fig. 7. Voltage vs time for the different types of plants (7th day after germination/plantation).
ima, as a model, in photo MFC achieved a maximum power production of 67 mW m2. An average power density of 50 mW m2 was accomplished by Spartina anglica for 33 days among the various PMFCs investigated by Timmers et al. [28]. Moqsud et al. [23] used 1 and 3% of compost on paddy based PMFC. The maximum power density of around obtained in the study was 23 mW m2. In the present study we have observed a much higher power density (10 times) than that obtained in a recent study [23]. A maximum power density of 15.73 mW m2 was obtained for a canna indicaMFC system [29], and it was about 15 times lesser than that generated in this study. The high power density may be attributed to plant type and addition of compost to the soil- hence resulting root deposits degradable by microorganisms and therefore accountable for electron donation [22]. Electrochemical impedance spectroscopy The significance of impedance is important for a heterogeneous PMFC system. The electrochemical interface of PMFC is highly complex. The microbial activity is expected near the electrode, making the electrochemical reaction intricate. This method investigates the dynamics of the mobile or bound charges in the bulk or interfacial region of such system. An electrode surface with a smooth and undamaged coating usually has high impedance. The equivalent circuit for such an undamaged metal surface consists of a resistance (R) in series with a simple capacitance (C) and is called purely capacitive coating. The corresponding Nyquist plot for such an ideally polarized electrode will have a straight vertical intersecting the Z’-axis and the X-intercept gives the value of resistance. The internal resistances of the cell were obtained from Nyquist plots (Fig. 8), where the intercept of the curve with the Ze axis is defined as the ohmic resistance. Table 4 shows the individual internal resistances the PMFC systems obtained by fitting the EIS Data in ZSimpWin. These values are derived by using the potentiostat readings; plotting the spectrum in Nyquist plot and later the fitting the same in equivalent circuit models. A circuit modeling software was used to fit the EIS data acquired from biologic data acquisition system. The model exhibited a good fit, having the least percentage error (less than 1%) and very low Chi-squared value (of the order of 104 or lesser). Low Chi squared indicated low error percentage. Comparing the three PMFC systems which had similar conditions except for the type of plant; it was observed that the electrolyte
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Fig. 8. Nyquist plots.
Table 4 Internal resistances of the PMFC systems. PMFC configuration
Electrolyte resistance (O)
Brassica juncea Trigonella foenum-graecum Canna stuttgart
137.7 ± 5.6 32.57 ± 3.1 22.57 ± 3.0
resistance was the highest in the case PMFC triplicates of Brassica juncea and lowest in the case of PMFC triplicates with Trigonella foenum-graecum and Canna stuttgart. This may be due to the differences in root exudates, rate of exudation, soil nutrients, microbial population in the vicinity of roots or a combination of these and other complex factors present in the heterogenous bio-electrochemical system. Electrochemical impedance spectroscopy has not been studied for PMFC as yet. Variation in ohmic
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resistance over a desalination cycle have been investigated [30], where the voltage quickly attained an initial peak, and then unceasingly decreased over the course of the cycle, consistent with the observed increase in ohmic resistance. A previous study reported that, a higher electrolyte concentration reduced the membrane solution interface resistance due to the compression of electrical double layer [31]. This enhancement in membrane resistance over a cycle is also consistent with research results obtained in a MFC tests with Nafion 117 at low electrolyte concentrations [32]. Economics Most MFCs use NafionÒ membranes that cost approximately $2500 m2, Agar ($165 m2) and Gore-Tex ($82.5 m2). Use of expensive Nafion has been eliminated, significantly reducing the cost and complexity of the systems – making it easier for scaling up. Cost of clay is 1$ kg1. Therefore approximate cost per meter square with thickness of 0.8 cm was calculated to be $135 m2 (including cost of fabrication and transport) as compared to $2500 m2 of NafionÒ. Conclusions In this study, compost: soil mix (1:1) was used in PMFCs for bioelectricity generation by using three different plant systems namely, Brassica juncea, Trigonella foenum-graecum and Canna stuttgart. Addition of compost increased the bio-energy output nearly 2.5 times. Bio-energy harvesting did not disturb the growth of the plant. Among the systems studied, Canna stuttgart and Trigonella foenum-graecum exhibited higher bio-energy output than Brassica juncea. Canna stuttgart plants were observed to show the most stable output with very little diurnal fluctuations. The system exhibited a power density of 222 mW m2 and a conversion efficiency of 0.022%. The high power density may be attributed to plant root type (tuberous in the case of Canna stuttgart), presence of compost/nutrients in soil and hence higher root deposits which are degradable by microorganisms and responsible for electron donation. A reduction in cost of 94.6% is estimated to be achieved by replacing Nafion PEM with the novel clay mix separator. Acknowledgements The authors wish to acknowledge the financial support by SERB Science & Engineering Research Board (SERB) Fast Track Young Scientist Award Scheme; Financial order no. SR/FT/LS-14/2011. Special thanks to the Director, CSIR – NEERI for providing permission and lab facilities to perform experiments at CSIR-NEERI, Chennai Zonal Laboratory. The authors also thank Dr. K. Ramesha, Senior Scientist, CSIR-CECRI for providing VMP3 Biologic multi-channel potentiostat/galvanostat facility of CSIR-Innovation Complex, Chennai. References [1] Strik DPBTB, Terlouw H, Hamelers HVM, Buisman CJN. Renewable sustainable biocatalyzed energy production in a photosynthetic algal microbial fuel cell (PAMFC). Appl Microbiol Biotechnol 2008;81:659–68. [2] Helder M, Strik DPBTB, Hamelers HVM, Kuhn AJ, Blok C, Buisman CJN. Concurrent bio-energy and biomass production in three plant-microbial fuel cells using Spartina anglica, Arundinella anomala and Arundo donax. Bioresour Technol 2010;101:3541–7. [3] Strik DPBTB, Timmers RA, Helder M, Steinbusch KJJ, Hamelers HVM. Microbial solar cells: applying photosynthetic and electrochemically active organisms. Trends Biotechnol 2011;29:41–9.
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