Electron Transfer Pathway And Mechanism Of Electricity Generation In Microbial Fuel Cells

A microbial fuel cell(MFC)is a device that utilizes microbial metabolic activities to convert chemical energy stored in organic matter into electrical energy,which realizes waste water recycling with a low sludge yield.However,the output voltage(<1V)and current(<1A)of MFC are not large.So further improvement of MFC power generation efficiency is a vital problem that must be solved in practical applications.As the mechanism of MFC is not completely understood,how to improve the efficiency of MFC is lack of theoretical guidance,so it is necessary to further reveal the mechanism of MFC.In this experiment,Klebsiella oxytoca d7 and Shewanella.sp.F1 were used,and glucose was used as the fuel to start dual-chamber MFCs.The electricity generation performance of MFCs were tested.The extracellular electron transfer ways and the pathways of electron mediator transfer mechanism of K.oxytoca d7 and Shewanella.sp.F1 were studied and compared.From the perspective of energy metabolism,the significance of electricigenic respiration to the growth and reproduction of microorganisms was analyzed.We obtained the following results:(1)MFCs inoculated with K.oxytoca d7 and Shewanella.sp.F1 both had the characteristics of fast startup speed,high output of voltage,and stable performance of power generation.The removal rate of COD by non-wrapped MFC and wrapped MFC inoculated with K.oxytoca d7 were separately 57%and 42%,the coulomb efficiency was separately 3.7%and 3.1%,the internal resistance was separately 434Ωand 1457Ω,and the maximum output power densities were separately 4912m W/m~2 and1481m W/m~2.Correspondingly,the COD removal rates of non-wrapped MFC and wrapped MFC inoculated with Shewanella were separately 59%and 45%,the coulomb efficiency was separately 4.7%and 3.2%,the internal resistance was separately 317Ωand 1213Ω,and the maximum output power densities were 4686 m W/m~2 and 1596m W/m~2.Although the electricity generation performances of MFCs inoculated with the two electricigens were slightly different,in terms of the same type of electricigens,the laws of electricity generation performances caused by the different configurations were similar.(2)The two ways of extracellular electron transfer in K.oxytoca d7 are the mechanism of direct contact with the electrode and secretion of electron transfer mediators(ETCs)with redox activity.The extracellular electron transfer ways in Shewanella.sp.F1 are the mechanism of direct contact electrode,"nanowire"mechanism,and secretion of ETCs with redox activity.(3)K.oxytoca d7 can secrete the ETCs with redox potential of-0.25V during electricigenic respiration.K.oxytoca d7 could only secrete the ETCs under the condition of MFC.However,Shewanella.sp.F1 could secrete the ETCs under anaerobic and even aerobic conditions.So the production conditions of ETCs are related to the type of electricigens.The redox potentials of ETCs of K.oxytoca d7 and Shewanella.sp.F1 were between NADH and coenzyme Q.Most of the electrons transferred to outsides by the two kinds of electricigens were from NADH dehydrogenation.So sites of the ETMs intercepting the electron only depended on the redox potential of the ETMs.For the different electricigens,although the specific values were different under the condition of sufficient carbon sources,the extracellular electron transfer mainly relies on the ETCs to transfer the electrons.When the carbon source is insufficient,the contribution of ETCs to the total output also reduced to varying degrees.(4)Compared with anaerobic fermentation or substrate level phosph-orylation,electricigenic respiration is a respiratory metabolic method that more thoroughly decomposes organic matter and is more conducive to microbial proliferation.This is also the root cause of the microorganism’s choice of electricigenic respiration absence of oxygen and other soluble electrons in the environment.