| At present, global energy crisis and environmental pollution problems is becoming serious increasingly. Looking for green, sustainable alternative sources of energy is a priority.The MFC is a special electrochemical device that can use microorganism as biocatalyst to produce an electrical current from many organic substrates. It is a typical interdisciplinary subject which mainly relates to microbiology, environment, electrochemistry, etc. It is a promising yet challenging technology with many advantages, such as a wide range of sources of raw materials, mild operating conditions, safety, continuity. Especially they are emerging as a novel technology with a great potential to treat wastewater. Increasing attention has been paid to MFCs.The study on MFCs is still in experimental stage currently, there is a lack of systematic and profound theoretical foundation. MFC is a special type of electrochemical reaction system, electrochemical reaction occurs on the biofilm adhering on the electrode.It is important to explore the electrode kinetics to the development and application of MFC; In addition, application of MFC in the field of wastewater treatment mostly limited to less toxic or non-toxic sewage.The study on highly toxic and refractory wastewater is rare. In view of the problem existing in the MFC technology, This thesis focuses on two aspects: 1. Electrochemical test methods including CV and EIS were used to explore the electrode kinetics of domesticating, the influence of mediator concentration,the influence of temperature, comparision of different kinds of substrates,the long running of MFC, the relationship of electrochemical activity of biofilm,charge transfer resistance,anode electrode, capacity of producing electricity.Through the analysis of polarization curves, intrinsic kinetics of was studied, kinetic parameters-exchange current density i0 was calculated.Based on the theory of enzyme kinetics,the relation of power output and substrate concentration was analyzed; Based on the electrochemical reaction kinetics, the law of the variation of substrate concentration with time was analyzed.2. Using coking wastewater from wastewater treatment plant of coking plant as inoculum,using coking wastewater,ammonium chloride,phenol as substrate,a MFC was built, toxic pollutants was treated and electricity was produced synchronously. Electrochemical test methods including CV and EIS were used to explore the electrode kinetics of influence of various COD loads, the running process of MFC. Electron transfer mechanism was studied.A dual-chamber microbiolfuel cell was built, using glucose as substrate, methylene blue (MB) as mediator, E. coli as biological catalyst. The main conclusions of basic research are summarized as follows.(1) In MFC, Electricity was produced through a thin layer of biofilm adhering on the electrode surface, the electrochemical activity of biofilm directly affect the electrochemical performance of MFC. To form a mature biofilm, five fed-batch cycles were repeated with every period of 1 day. Electroactive biofilms were formed on carbon paper under constant external resistance of 1000Ω With the formation of the mature anode biofilm, the anode polarization resistance decreased by 66.5%, the anode potential gradually decreased also, while peak output power density was enhanced over 260%. It was evident that the enhanced MFC performance was associated with the development of the biofilm. It took 4 days to form a mature biofilm.lt marked that the biofuel cell had been successfully started up.(2) Electronic was transmitted through mediator between E.coli cell and anode, the electronic generated from decomposition of organic fuels was transmitted to the cell wall,then to the electrode surface. The electronic passed through an external circuit to the cathode and generates current.The impact of methylene blue (MB) concentrations on MFC performance was examined.At lower MB concentration between 9.98×10-3mmol/L and 1.66×10-1mmol/L, the charge transfer impedance decreased by 79.9%,the anode potential is gradually reduced,the increased power density is enabled by using higher mediator concentrations, the peak power density enhanced by 343%. But MB at too high concentration is disadvantageous to the perform of MFC. At the MB concentration of 2.50×10-1mmol/L, the charge transfer impedance increased, anode potential increased conversly, the peak power output decreased. The glucose concentration being 2000 mg/L, the best mediator concentration of 1.66×10-1mmol/L. There exists the most appropriate ratio between the substrate concentration and mediator concentration.(3) Influences of temperature on the electrochemical properties of MFC was studied, the electrode kinetics was discussed.In a certain temperature range, with temperature increasing,the lag phase of bacterial growth shortened, its growth accelerated,its electrochemical activity enhanced.Due to too high temperature^heating made the enzyme inactivate in the MFC, bacterial degenerate or coagulate, bacterial activity reduce or disappear. The microbial fuel cells(MFCs) were tested in batch mode at different temperatures of 20°C, 37°C,45°C. The charge transfer resistance was 741.9Ω,42.4Ω,416.1Ω, respectively.The peak output power density was 58.1 mW/m2,610.5 mW/mf, 171 mW/m2.Higher electrochemical activity of the bacteria may strengthen the electronic conduction process, improve the efficiency of electron transfer, reduce the polarization effect, decrease the anode electrode potential. At 37°C, MFC displayed a optimum electrochemical performance.The steady-state polarization curves was analyzed, the electrode kinetics was studied.(4) Enzyme kinetics and its chemical kinetics of MFC were studied.The maximum power density as a function of glucose concentration was modeled according to Michaelis-Menten enzyme kinetics equation,with a maximum power density of Pmax= 320.2m W/m2 (1000Ω) and half-saturation concentration ofKs= 138.5mg/L.The initial glucose concentration being 400 mg/L, the reaction follows first order kinetics. The initial glucose concentration being 2000 mg/L, the reaction follows zero-order kinetics.(5) Taking the glucose concentration of 2000mg/L, the mediator concentration of 1.66×10-1mmol/L, the MFC was operated for up 12 days. It took 1.5h for starting up, attained its maximum output voltage,then entered plateaus and operated steadily. The charge transfer resistance decreased from 216Ω to 76.52Ω, manifesting the biofilm had become immature completely.To the 12th day, the charge transfer resistance has increased by up to 1657Ω, the long time operation leaded to the greatly decrease of biofilm electrochemical activity.(6)There are a wide variety of substrates in MFC.Glucose is fermented substrate, sodium acetate is non-fermented substrate. We selected two kinds of representative substances as substrates. The species of substrates on MFC performance was studied. In sodium acetate-fuel MFC, the higher concentration of ions in the anolyte accelerated electron transfer rate. Sodium acetate-fuel MFC attained its peak voltage faster than glucose-fuel MFC, but its maximum output voltage was less than the latter.The MFC of the two kinds of substrates run 20h respectively. COD removal efficiency of glucose-fuel MFC and sodium acetate-fuel MFC were 50.6% and 55.8%, Coulombic efficiency of them were 42.1% and 46.2% respectively.The coking wastewater-fuel MFC, ammonium chloride-fuel MFC phenol-fuel MFC were built for the first time using activated sludge as the inoculum, using carbon felt as anode,which came from wastewater treatment workshop of coking plant. It was used to deal with the actual coking waste water, ammonia, phenol, respectively. The change laws of electrochemical properties, the biofilm domestication steps and manner, the influence of COD load on the MFC performance, the influence of co-substrate on electricity producing performance were studied.The electricity production mechanism was studied, the mechanism of electron transmition was explained, The basic information of producing electricity bacteria on anode was got. The pollutants degradation rate and COD removal efficiency were studied. The main conclusions about real wastewater are summarized as follows.(1) Coking wastewater degradation and electricity production was achieved synchronously in MFC. Once anolyte was inoculated with activated sludge, the output current increased constantly, it rose to 0.052mA. It was shown from cyclic voltammetry that the mechanism of producing electricity may be conducting electron through mediator generated by electricity producing bacteria. Using 160mg/L,560mg/L COD load of coking wastewater as fuel, the impact of the COD load on the electrochemical performance of MFC was examined. COD load should not be too large, otherwise it may interfere with normal metabolic pathways of bacteria, weak discharge activity of current producing microorganisms. Taking the sodium chloride concentration of 100mmol/L,500mmol/L, the influence of different electrolyte concentration on MFC performance was studied. Too high electrolyte concentration is disadvantageous to capacity of current producing microorganisms.(2) Ammonium chloride degradation and electricity production was achieved synchronously in MFC. The ammonium chloride concentration being 0.01mol/L, MFC attained its peak current density of 0.052 mA. The mechanism of conducting electron was probably due to mediator generated from current producing microorganisms. COD load of ammonium chloride being 350mg/L 650mg/L,950mg/L, the impact of the COD load on the electricity production performance was studied. The COD load being 650mg/L, the maximum the current output, the lowest the potential of the anode electrode, the minimum the anode charge transfer impedance. Too high COD and too low COD are all adverse to electricity generation performance of MFC.(3) Phenol degradation and electricity production was achieved synchronously in MFC.In the process of MFC starting up, it was very important to select a appropriate biofilm acclimation program and steps.In the whole process of domestication, co-substrate of phenol and glucose were used at first, the concentration of glucose was reduced gradually, then a only phenol substrate was used, Finally, the concentration of phenol was increased gradually.The entire acclimation process lasted 72 days. By reducing the concentration of glucose gradually, microorganisms gradually adapt to phenol.Reasonable domestication process is very important to the electrochemical activity and anti-toxicity of biofilm. The mechanism of conducting electron was probably due to close contaction between producing electricity bacteria and electrode or through nanowires. The maximum output power density is 42.9mW/mz: (300mA/W) at phenol concentration of 0.2g/L. Phenol degradation rate attained 31% for ldays, phenol degradation rate attained 100% for 4days, the rate of COD removal attained 40.9%, the coulomb efficiency is 18.5%. The MFC performance was contrasted at phenol COD load of 500mg/L and 1500mg/L, Its maximum output power density were 42.9mW/m2,67.9 mW/m2 respectively. With COD load increasing, the maximum output power increased, but at the same time the power output decreased substantially. The resistance to toxicity of biofilm had enhanced, but its stability needed to be further improved. Using 0.2g/L phenol and co-substrate of 0.2g/L phenol, Ig/L glucose as substrates, the electrochemical performance of MFC was contrasted. Its maximum output power density was 42.9mW/m,64.0mW/m2 respectively; the internal resistance of the battery was 1200Ω,800Ω respectively. |
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