Building microbial fuel cells using Chlorella
(1 Beijing University of Technology Civil and Environmental Engineering, Beijing 100083, China; Chinese Academy of Sciences Institute of Process Engineering, State Key Laboratory of Biochemical Engineering, Beijing 100190)
Abstract: Separation of Chlorella (Chlorella vulgaris) to build a photosynthetic microbial fuel cells to study the anode and chlorella added to wastewater as substrate electrical properties and mechanism of cell production results showed that the microbial fuel cell is constructed possible, power output depends adsorbed on the electrode surface algae, with the algae suspended in basic solution has nothing to do. the light of the fuel cell voltage changes in one of the main factors in the cathode chamber with iron ions through the second and cycle between trivalent transformation to improve the electron transfer rate to speed up the reaction of protons and oxygen, the battery output power density of 11.82 mW/m2, COD removal rate reached 40%. This battery chemical energy, light energy conversion electrical energy while recycling of sewage and chlorella.
Keywords: Chlorella; microbial fuel cell; photosynthesis; sewage treatment
Microbial fuel cell (Microbial Fuel Cell, MFC) is the use of microbial catalytic chemical energy into electrical energy a device [2 associated with microorganisms, such as electrochemistry and material development of the subject, MFC continuous improvement of the structure and properties of [ 3], and gradually extended to the field of environment in the emergence of dual-chamber on the basis of a single-chamber  and the up-flow MFC , the substrate from a single small organic molecules, such as sodium acetate , glucose  , turned to mixed organic molecules, such as chlorophenol wastewater , straw waste water , brewery wastewater  Logan [10,11] developed for municipal and industrial wastewater as the substrate of the MFC in the sewage biological treatment, while access to electricity, reducing the cost of sewage treatment, waste recycling to achieve now be reported for the anaerobic treatment of MFC anodes [12? 14], and applied to practical inconvenience. chlorella (Chlorella vulgaris ) for a general class of single-cell green algae naturally, based on water as electron donor light autotrophic organisms, the process of photosynthesis, the use of simple inorganic and synthetic organic growth and reproduction of its strong environmental tolerance, degradation of pesticides can be used inorganic salts, hydrocarbons, phenols and other organic compounds , and can absorb heavy metal overload, the body is rich in protein, vitamins, minerals, dietary fiber, nucleic acids and chlorophyll, is to maintain and promote nutrients essential to human health in this work to build microbial fuel cells using chlorella. This is no mediator photosynthetic microbial fuel cell anode and cathode are in the aerobic environment to run, reducing the difficulty of manufacturing the reactor, expanded use of field In sewage treatment, while access to electricity, chlorella can also reduce production costs for the waste water treatment and energy utilization of resources to provide a new way of thinking
2.1 Experimental Materials
2.1.1 algal species and culture conditions
Micro-organisms: Chlorella vulgaris Yongding River water samples from the Beijing isolated and cultured. Were collected five water samples placed in 150 mL Erlenmeyer flask, No. 1 # 5 # # .1 # 4 in 50 mL flask followed by 4 aquatic, aquatic 105, No. 10, Yun Gee , Zarrouk medium , 5 # flask without any medium, all media at 121 autoclave sterilization 15 min. temperature 25 ~ 30 , illumination 4500 lx continuous light, 15 d after the growth of algae in bottles # 4 best. visible Zarrouk medium required for the experimental cultivation of algae.
The Institute of Hydrobiology, Chinese Academy of Sciences identified four main phytoplankton samples # Chlorophyta, Cyanophyta, Cyanophyta algae are the only seats, green algae include Scenedesmus, Chlorella is, cross Rhodophyta, One of the largest number of Chlorella, accounting for 90% of the total number of cells shown in Figure 1.
As the largest number of Chlorella, the cell density of 8 105 mL? 1, without the need for preparatory training, direct access to the water droplet separation method  on the train separation and purification of algae. After repeated separation and purification of Chlorella concentration on the graphite electrode 30 d after the scanning electron microscope shown in Figure 2.
2.1.2 Experimental conditions
Temperature 25 ~ 30 , light intensity 4500 lx continuous light, all devices are placed in the light incubator.
2.2 Equipment and instruments
Devices used in the experiment shown in Figure 3, the cathode, the anode electrode chamber 2 form, through the proton exchange membrane (Nafion-117, American Dupont Company) connected with bipolar proton exchange membrane sandwiched between the vacuum chamber to maintain the sealing pad, Each electrode chamber can be put into 250 mL solution, all devices are placed in the light incubator experiment.
Proton exchange membrane in order before use in 30% H2O2 and 0.5 mol / L H2SO4, deionized water, boiled 1 h , and then stored in deionized water for use. Electrodes were not polished high-purity graphite electrodes, the physical surface area of 65 cm2, before use with 1.0 mol / L HCl soaked place impurity ions, and then used with 1.0 mol / LNaOH soaked to remove the surface adsorbed cells . graphite electrodes placed in the treated vessel in the culture of Chlorella algae enrichment.
2.3 Experimental Methods
Voltage acquisition: Rui Bohua AD8201H data acquisition card, 16-bit, 32 channel, programmed double-ended and acquisition accuracy 0.1 mV. COD test uses dichromate method (GB11914-89) determination.
3 Results and discussion
3.1 Maximum electric power production
Select the appropriate external resistance in order to facilitate cell to produce the best output power, the normal operation when the battery voltage platform to change the external resistor (0 to 5000?), Get resistance? Output power and resistance? Voltage (Figure 4). With the external resistance increases, the battery voltage increases, but the growth rate is slowing down; battery power is increased first and then smaller in the 510? near the battery output power (0.077 mW), the output power density of 11.82 mW/m2 this experiment to determine the resistance of the battery is 510 outside?.
3.2 chlorella and its role in the MFC anode power law for the study of Chlorella produced at the anode effect on the production of electricity carried out control experiments. Anodes access Chlorella device I, II do not access, the cathode by adding 50 mmol / L potassium ferricyanide solution , external resistance was 510?, experimental results shown in Figure 5. I microbial fuel cell voltage is significantly higher than II. I microbial fuel cell reaction occurs as follows:
I have battery voltage consists of two parts, part of Chlorella photosynthesis photolysis of H2O, the process produced some of the cathode electron and proton transfer to [(2)]; the other is outside the cell metabolism photosynthesis carbohydrates in the process of accumulation outside the cell membrane cytochrome lose electrons to the anode [(3)], electronic and through the external circuit, the protons through the proton exchange membrane to the cathode. chlorella battery without the voltage values from 0.07 V II down to 0.01 ~ 0.02 V, then add in the cathode 50 mmol / L potassium ferricyanide, 0.07 V voltage to rise rapidly around the battery voltage with the potassium ferricyanide solution II the consumption decline, potassium ferricyanide solution can accelerate the electron transfer rate, improve the battery voltage.
To examine the cell suspension on the impact of power output, battery run time, it will replace the battery electrodes is not I access Chlorella electrode, the results shown in Figure 6 experiments show that after replacing the battery voltage to keep the electrode in the background voltage After a period of time has increased, and when re-replacement of the original electrode, the voltage quickly rises to about 0.09 V, which shows the adsorption removal of the battery cells, battery electric performance of a significant decline in production, the replacement anode adsorption Chlorella electrical properties of the battery capacity after recovery can be seen suspended cells contribute to the power output is very small, power output due to adsorption on the electrode on the chlorella, the biofilm formed on the electrode the electrode into the dish to pre-enrichment the electrodes into the room when a very mature biofilm formation when the battery I run stable, the cathode and then adding 50 mmol / L potassium ferricyanide solution, a rapid increase in voltage, which is due to Fe3 + in potassium ferricyanide
Reduction rate faster than the O2 [(4), (5)], can accelerate the electron transfer rate, power output increase the reaction period of time, consume potassium ferricyanide, stabilize the battery voltage drop. This When injected into 50 mmol / L potassium ferricyanide solution, a rapid increase in voltage.
When the battery when I run stable again, the abolition of the light, the voltage has dropped to the background voltage (Figure 7) in the absence of light conditions, photosynthesis of Chlorella can not continue to produce carbohydrates and hydrolysis, only decomposition carbohydrates has been formed, until the complete decomposition of the voltage gradually decreased to the background current, and the experiments show that illumination of photosynthetic microbial fuel cell voltage changes in one of the main factors is the battery I can be stabilized at 0.09 V voltage around the main reason.
Chlorella in MFC 3.3 and its production of cathode power law in order to investigate the chlorella cell voltage cathode to improve the role of preparing two sets of experimental apparatus, numbered III and IV. III cathode battery access a small isolated training ball algae, chlorella cell IV is not access, adding activated sludge and anode are 0.2 mol / L sodium acetate 10 mL, add 510? fixed value resistor before the anode through N2/CO2 adding activated sludge mixture (80 / 20?) divisible medium of oxygen, sealed after inoculation or slowly through the mixture , the cathode without ventilation.
Shown in Figure 8, III cathode microbial cells generated by the battery voltage slightly higher than the IV. This is because the battery's cathode III in Chlorella photosynthesis to produce oxygen for microbial cells to provide more adequate electron acceptor, to speed up cathode oxygen reduction rate, accelerate the electron transfer rate, thereby reducing the oxygen in the cathode reaction with H + overpotential, increase the battery output voltage.
Battery III running for some time, microorganisms adsorbed on the electrode gradually formed biofilm, then the voltage stabilized at between 35 ~ 40 mV, but prolonged reaction, sodium acetate consumed, the voltage dropped to the background voltage, and then add the acetic acid sodium, voltage rise rapidly (Figure 8). photosynthetic Chlorella cathode microbial fuel cell adsorption reaction is as follows:
Anode inside the activated sludge process of cell metabolism acetate [equation (6)], accumulated outside the cell membrane to the cathode cytochrome lose electrons, e and then through the external circuit to the cathode, the anode chamber generated by H + through the proton exchange membrane to the cathode chamber , dissolved oxygen in the cathode chamber the electron is eventually restored, and then the reaction of water with H + [(9)] at the same time, photosynthesis of Chlorella in the cathode water decomposition [equation (7)], cell metabolism and decomposition of photosynthetic role of carbohydrates produced [equation (8)] will increase the oxygen in the cathode reaction with H + super potential, which led to the battery voltage IV III than a small increase, an increase of only 0.01 V or so.
3.4 the use of photosynthetic micro-battery sewage
Beijing will be taken from the settling tank Gaobeidian sewage treatment plant effluent (COD = 321.2 mg / L) to join the stable operation of the microbial fuel cell anode chamber I, while the treatment of organic waste generation, respectively, after dealing with different time sampling measured COD, the results shown in Figure 9. can be seen by adding organic waste, the microbial cell voltage from 0.09 V I rose to about 0.12 V, stabilized after a period of decline, remained at around 0.1 V, which is due to join in the anode organic waste provides nutrients for the chlorella, the voltage increased rapidly to a peak when the battery is running for some time, the battery voltage is stabilized due to the organic wastewater containing organic nutrients, so the stable voltage value slightly higher than 0.09 V.
Chlorella handle the sewage, the sewage in the 21 d of COD values from 321.2 mg / L reduced to 189.6 mg / L, removal rate of 40% proved, the use of chlorella wastewater treatment can effectively remove waste water organic matter, phosphorus, nitrogen, significantly reducing wastewater COD. So, the use of chlorella to build microbial fuel cell, producing electricity at the same time in the wastewater, while the use of waste water and solar energy into chemical energy to electrical energy is feasible.
This work was built using microbial fuel cell chlorella, chlorella examined were added to the anode and cathode and wastewater as substrate cell production of electricity and electrical principles of production, sewage treatment and the use of the battery feasibility study draw the following conclusions:
(1) the use of chlorella is the feasibility of building a microbial fuel cell, this battery can be chemical energy, light energy into electrical energy, while sewage treatment and recycling chlorella, chlorella to reduce production costs for wastewater treatment resources to provide a new way of thinking. battery output power density of 11.82 mV/m2, the COD removal rate of 40%.
(2) The power output depends on the adsorption on the electrode surface of the algae, with the algae suspended in basic solution has nothing to do. Anode and cathode are aerobic running, compared with the MFC has been making simple, easy to operate and expand the practical application of the field.
(3) Chlorella access to the cathode, produce oxygen through photosynthesis, reducing the oxygen in the cathode reaction with H + super potential to improve the electron transfer rate to speed up the protons and oxygen in the cathode reaction, increasing the battery output voltage.
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