INTRODUCTION
Microalgae usage in the microbial fuel cells has picked up particular attention because of their ability to accumulate lipids, proteins, and carbohydrates in their biomass; in addition to mitigating the atmospheric wastes such as CO2 emission.[] In principle, algal fuel cells are consisted of two electrodes (an anode and a cathode) connected by an external electric circuit and separated internally by a proton exchange membrane, or in some case, there is no membrane in which the biological activities of algae are harvested.[] Depending on the microbial catalytic activity of exoelectrogens, oxidation of organic waste occurs at the anode producing electrons that flow through an external circuit to the cathode where the process of the bioelectricity generation is completed.[] Commonly, with different roles, microalgae can be used as a biocatalyst in the anode or cathode compartments of the MFC.[] In the anodic chamber, microalgae exhibit the exoelectrogenic activity to oxidize the organic wastes, liberating free electrons that are pumped directly to the anode surface. In such a process, algal-anode communication is taking place directly without accessing artificial redox electron mediators.[] However, the direct electron transfer from algal cells to anode surfaces is not common as in bacterial systems. Therefore, most of the designed anode-based algal fuel cells are relying on the use of artificial electron shuttles to wire enable the accessibility between the living algae and conductive anode surface.[] However, in some recent studies living microalgae was used as bio-anode in a mediator less MFC. In one of these studies, blue-green alga Oscillatoria agardhii was used to construct MFC for electricity production and wastewater treatment. The reported power output reached 26.8 mW/m2 with a very high efficiency of COD removal (almost 85%).[] Nevertheless, there are three advantages for using alive algal cells in the anode of MFC: (1) Sustainable energy production without the need for adding the substrate like glucose; (2) More clean than other microbes used in MFC that using wastewater as organic substrate; (3) An easy technique to produce bioelectricity from algae without various pretreatments of algae biomass.[] Besides, the role of cathodic microalgae is different as they can supply active electron acceptors for the cathode in a form of dissolved oxygen for the redox reaction and produce biomass that can be used as a substrate for the MFC anode.[] High photosynthetic activity, lipid content, and biomass productivity make microalgae an appropriate source for oxygen supply to the cathodic chamber.[] In this case, bacteria oxidize the organic substrate under anaerobic conditions in the anolyte of bacterio-algal MFCs.[] There were several previous reports on the role of microalgae as oxygen producer at the cathode, One of these researches used green algae as an oxygen supplier in the cathode chamber of MFC, which produced a power density of 68 mW/m2.[] Meanwhile, Chlorella vulgaris was introduced into the illuminated cathode chamber, the maximum power density of 146 mW/m2 is achieved.[] Apart from using microalgae as bio-anode or bio-cathode in MFC, microalgal biomass can act as an alternative to the substrate in the MFCs owing to its plentiful biochemical profile.[] Nanostructured materials exhibited various appreciable properties including good conductivity, large specific surface area, and strong catalytic activity.[] Furthermore, nanomaterials with unique electrochemical properties provide strong charge interactions with organic compounds and the direct electrochemistry process between bacteria and the anode.[] Moreover, the decoration with nanoscale materials efficiently enables the extracellular electron transfer process from microbes to anode surface in the nanostructured MFC compared to the conventional MFC.[] Various researchers have successfully enhanced the electricity output in MFC by incorporating different nanomaterials during electrodes fabrication.[] Huge quantities of algal biomass are produced as a secondary product of wastewater treatment in high rate algal ponds (HRAPs).[] HRAPs are shallow, open raceway ponds and have been used for the treatment of municipal, industrial, and agricultural wastewaters.[] Hence, in the present study, a number of trials were carried out to get the maximum power and wastewater treatment percent by utilizing algal biomass from high rate algal pond (HRAP) using nanostructured MFCs alternatively on either side of the electrolytic compartments and also as an organic substrate for bacterial mixed culture bio-anode.
MATERIALS AND METHODS
Algal culture collection and preparation
Algal mixed culture collected from HRAP which is constructed in Zenin wastewater treatment plant, Giza, Egypt. The collected biomass is washed with phosphate buffer saline (pH 7.0) several times and centrifuged (5000 rpm for 15 min). The washed cells were then resuspended in a phosphate buffer (pH 7.0). Microalgae species were identified microscopically (using Olympus CX41 microscope, equipped with an Olympus digital camera (Olympus SC30, Japan)). to species level, where possible, based on the taxonomic descriptions.[]
Construction of algal fuel cells
Two different types of fuel cells (as the so-called single chamber, and double chamber) were constructed and fueled with Algae. Both cells have been prepared in the same way, where they have filled with the same concentration of organic substrate, and they have the same construction bases from the electrode types and dimensions. In both dual and single MFCs, the anode material is multi-walled carbon nanotubes (MWCNTs)/MnO2/graphite (2.5 %, 2.5%, and 95% w/w) and the cathode material is Pt/rGO/carbon that were prepared according to our previous studies.[] An external resistor of 150 KΩ was applied to connect the anode electrode and cathode electrode.[]
Home-made design of the dual-chamber MFC
Double chamber MFCs were H-type Plexiglas identical bottles with a height of 137 mm and an outer diameter of 70 mm (250 mL total volume, 200 mL working volume). A proton exchange membrane (PEM, Polyethersulfone (PES) 18% treated with Nafion/MWCNTs) was used to separate anode and cathode, and allow proton transfer from anode to cathode chamber.[]
Home-made design of the single chamber MFC
Single chamber MFCs were fabricated with a transparent polyacrylic material and each MFC was 200 ml working volume membrane-less assembled with anode and cathode electrodes in the same room.[]
Application of algae mixed culture as bio-anode
Both cathode and anode were positioned 16 cm apart and connected with a copper wire and a variable resistor. Anodic and cathodic chambers of the DCMFC were inoculated with 200 mL of autoclaved domestic raw wastewater (from Zenin wastewater treatment plant, Giza, Egypt). Algal mixed culture from the HRAP (0.4OD600) was suspended into the anodic chamber under complete dark and anaerobic conditions. The air was pumping gently in the cathodic chamber (using a sterile silicone hose connected to an air pump) to complete the oxygen reduction by the cathode materials (Figure ). The generated voltage was monitored as a function of time through the electrochemical working station (Gamry Potentiostat/Galvanostat/ZRA G750).
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Application of wastewater grown microalgae as bio-cathode
The growth of microalgae at the cathode side was tested through two different approaches. In the first approach, microalgae grown in a high rate algal pond were subjected in a cathodic chamber, and the anode side was inoculated with (0.4 OD600) of the Enterobacter sp. consortium (Figure ) that were previously isolated from active biofilm and showed exoelectrogenic activity.[]
The second trial was to use microalgae attached cathode as oxygen producers for SCMFC operation (Figure ). First of all, the microalgae attached cathode was prepared by soaking the cathode (1% Pt/rGO) in microalgae mixed culture from HRAP for 3 weeks. The prepared cathode was positioned on one side of the SCMFC as the microalgae-coated layer facing the solution. The anode (50% MWCNTs/MnO2) was placed horizontally on the other side of the MFC chamber without any membrane used. The SCMFC was inoculated with the Enterobacter cloacae consortium in raw wastewater (0.4 OD600). The output voltage of MFCs was recorded by a data acquisition system (Adam -4017+, China). Variable external resistances (from 550K Ω to 256 Ω) are suggested to obtain current density and power density curves.
Application of microalgae in MFC as a substrate for bacteria
A dried form of algal biomass collected from HRAP was fed into a single chamber microbial fuel cell (SCMFC) with a concentration of 5 g/L as the total dry biomass (Figure ). The SCMFC is powered by the electroactive bacterial strain, Enterobacter cloacae, with optical density of 0.4 OD600.
Measuring the rate of organic waste removal
The efficiency of each algal fuel cell was tested for organic wastes removal, where the COD concentration was tracked during MFCs operation. The chemical oxygen demand (COD) was measured according to standard methods.[]
Statistics
The experiments were repeated at least three times. Statistical significance was determined by Student's t-test and statistical significance was taken at P < 0.05. All figures included in this manuscript are presented by taking the average of repeated experiments three times; the SD did not exceed ±10% for each experiment.
RESULTS AND DISCUSSION
According to our previous report,[] the dual-chamber MFCs operated with algal strains under dark conditions, and in the presence of degradable organic substrate were found to be effective in biodegradation of the organic wastes when the anode assisted algal biofilms were formed. Thus, in the present study, algal mixed culture collected from HRAP was identified as (Figure ). The microscopic examination of the culture showed the predominance of Microsyctis Species, Microcystis viridis, M. aeruginosa, and M. flosaquae (Figure ).
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Revealing the electron transfer mechanism in microalgae, cyclic voltammetry (CV) was performed for three algal pure strains in a three-electrode system with platinum wire as counter electrode, modified carbon (C) paste with multi-walled carbon nanotubes (MWCNTs)/MnO2 as a working electrode, and Ag/AgCl (KCl, sat.) as reference electrode. After a period of adaptation under dark anaerobic conditions, remarkable increases in the bioelectrochemical signals were obtained. In addition, the detected anodic and cathodic peaks at the early electrical potential were the characteristic peaks of viable formed biofilms (Figure ).
The Output voltage of different MFCs over 6 days is shown in Figure . Rapid changes in the fuel cell voltages was observed, followed by a stable platform over the incubation time, and sharp decrease in the generated potential was attained by the end of the cycle which is resulted from the depletion of the organic matters (degradable organic substrates around the microalgae in the anode chamber). Anode-microalgae are responsible for the biodegradation, and the direct electron transfer that have been achieved in this case.
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On the other hand, double chamber microbial fuel cells supported by a mixed culture of algal suspension in the cathodic chamber generated higher potential (0.72 V), when compared with that potential obtained by the single chamber cathode-algal fuel cells. Worth mentioning here that the maximum fuel cell voltage that is created by the anode-algal fuel cells was 0.245 V.
Thus, the use of microalgae in the cathode side (i.e., the cathode-algal model), either through a single or double chamber fuel cells, is necessary for increasing the fuel cell voltage due to the algal photosynthetic activity and oxygen production that facilitate the oxygen reduction reaction (ORR) in the cathode chamber.[] However, the rate of organic waste biodegradation or removal by microalgae is recognized only by the algal-anode fuel cell model.
Regarding the power density generated by each model of the algal fuel cells, a comparison is made between all designed algal fuel cells to show the efficiency of bioenergy generation along with the main role of microalgae in each model. As a result, after applying various external resistances raged from 256 Ω to 550 KΩ, the polarization curves that are plotted in Figure demonstrated the highest power density 250 mW/m2 is generated by the cathode-based double chamber algal fuel cells, while the cathode-based single chamber fuel cell was lower with the power density of 34.9 mW/m2. The mentioned values of powered densities (250 mW/m2 and 34.9 mW/m2) represent the performance of double chamber vs single chamber cathode-based algal fuel cells. Therefore, to explain the reason behind this large gap between the bioenergy generation efficacy of cathode-based microalgae, in the double chamber, microalgae are allocated in a separate chamber (room), and its photosynthetic productivity and oxygen production is not affecting the anode-bacterial interaction. However, in the essence of applying a suspension of microalgae in the single-chamber fuel cells, the released oxygen is interfering as a final electron acceptor instead of the anode. Therefore, much lower energy was generated by this model.[]
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In terms of measuring the efficiency of each model to remove organic wastes, monitoring the decrease in the concentration of COD is used as an indicator of organic pollutants removal from water. In this regard, COD removal by E. cloacae reached 44.8% and 47.7% for SCMFC and DCMFC, respectively, Figure . Besides, wastewater-grown sun-dried algal biomass from HRAP was used as a substrate for SCMFC operating by E. cloacae consortium. During SCMFC operation, the highest power density of 14.1 mW/m2 accompanied with COD removal of 24.3% is obtained. It was also noticeable in our previous study[] when the raw wastewater is a sole substrate under the same condition in SCMFC, the maximum power density was 326 mW/m2 accompanied with COD removal of 49.7%. This big difference in the power output is due to the difficulty of bacterial digestion to the algae biomass was difficult owing to the strength and complexity of the algal cell wall and COD measurements confirmed these findings.[]
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CONCLUSION
In this study, single- and double-chamber microbial fuel cells were designed to study the effective role of microalgae in bioenergy production as well as their efficiency of COD removal from wastewater. In these designs, algae were applied as a cathode-based or anode-based fuel cell driver. The cathode-based algal double chamber fuel cells generated the highest power density along with the best removal of COD content. Thus, this model is suggested/recommended for exploiting the high algal biomass produced from high rate algal ponds in the treatment of wastewater and energy production.
FUNDING INFORMATION
This work is supported by the research project funded by the National Research Centre (NRC, Cairo, Egypt) entitles” Production of bioelectricity and biofuel associated with wastewater treatment using High Rate Algal Pond (Project Code: 11050110).
CONFLICT OF INTEREST
The authors declare no conflict of interest.
DATA AVAILABILITY STATEMENT
The data that support the findings of this study are available from the corresponding author upon reasonable request.
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Abstract
Increasing environmental pollution along with the depletion of energy resources are critical challenges. Microbial fuel cells (MFCs), a simple fuel cell that converts chemical energy into bioelectricity by the catalytic activities of living microbes, are evolving as a multipurpose renewable energy technology. The power generation via the MFCs depends on harvesting free electrons from the electroactive microorganisms, popularly known as exoelectrogens, to simultaneously produce electricity and treat wastewater. In the present work, nanostructured bio‐electrochemical systems are designed to exploit the usability of algae biomass collected from high rate algal pond system (HRAP) either as algal‐living cells or as dry biomass for bioelectricity production via several approaches (i.e., the algae will act as bioanode, biocathode, or nutrient substrate). The obtained results indicated that a higher electric current is produced when microalgae living cells are exploited as bio‐cathode in a double‐chamber‐microbial fuel cell (DCMFC) with a net power density (250 mW/m2) combined with high‐efficiency removal of COD reached 44.8%. Overall, the study's findings suggested that living algal cells from HRAP support high power output from a DMFC when they are exploited as biocathode, hence, the system introduced the opportunity to redesign the high rate algal ponds in the wastewater treatment plants in a way to produce energy plus the main assigned tasks which are the removal of wastes.
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Details
; Samhan, Farag A. 1 ; Ibrahim, Mohamed K. 2 ; Ali, Gamila H. 1 ; Hassan, Rabeay Y. A. 3 1 Water Pollution Research Department, National Research Centre (NRC), Giza, Egypt
2 Faculty of Science, Ain Shams University, Cairo, Egypt
3 Nanoscience Program, University of Science and Technology (UST), Zewail City of Science and Technology, Giza, Egypt