Mechanism Elucidation And Functionization Of Microbial Fuel Cells

Microbial fuel cell(MFC) is an electrochemical cell that directly captures the energy contained in bio-convertible substrates in the form of electricity.It has attracted increasing interests to employ MFCs to produce "green" energy from wastes. In an MFC,the microorganisms that completely oxidize organic compounds with an electrode as the sole electron acceptor are so-called exoelectrogens.The exoelectrogens play a key role in the anode oxidation by facilitating electron transfer between the electron donor and the electrode.In this thesis,the electron transfer mechanisms,substrate metabolism and exoelectrogens behavior in MFCs were investigated.Also,the functions of MFCs were further extended for their utilization in sulfide removal and biohydrogen production.In chapter 2,the influence of a transient external electric field on the performance of an MFC inoculated with mixed cultures was investigated.Different positive and negative electric fields were applied to a set of anodes.During the start-up period,the MFCs imposed by the electric fields of +1 V,-1 V,and -5 V obtained higher current densities than the control.The MFC exposed to the electric field of +1 V had the highest maximum power density of 73.5 mW m^-2 after 96-hr operation.On the other hand,the electric fields of +5 and +10 V delayed and even destroyed the MFC start-up.The electric field of-10 V initially induced a higher power output,but later had a detrimental effect on the MFC performance.An electrochemical quartz crystal microbalance test demonstrates that the electrophoresis was involved in the attachment/detachment of exoelectrogens on the electrode during the electric field application period.Furthermore,the application of negative electric field was proven to enhance the catalytic activity of the exoelectrogens,which was partially responsible for its influence on the MFC performance.In chapter 3,the sulfide oxidation process in MFCs was explored through identifying the sulfur species evolution and microbial communities.The results demonstrate that both electrochemical reactions and microbial catalysis were involved in such a complex sulfide oxidation process in the MFC anode.The microbe-assisted sulfide oxidation generated a higher persistent current density than the sulfide oxidation via single electrochemical reactions only.Three valence states of S(-â…¡),S (0) and S(+â…¥) were discovered from the sulfide oxidation,and S₀,S_x^2-,S₄O₆^2-, S₂O₃^2-,and SO₄^2- were detected as the intermediates.The sulfur-oxidizing bacteria and sulfate-reducing bacteria were found in the MFC anode.Based on the sulfur speciation and microbial community analysis,the sulfide oxidation pathways in the MFC were proposed.The oxidation of sulfide to S₀/S_x^2- and further to S₄O₆^2-/S₂O₃^2- occurred spontaneously as electrochemical reactions,and electricity was generated. The formation of S₀/S_x^2- and S₂O₃^2- was accelerated by the bacteria in the MFC anode, and SO₄^2- was generated because of a microbial catalysis.The microbe-assisted production of S₂O₃^2- and SO₄^2- resulted in a persistent current from the MFC.Molecular biological techniques were applied to analyze and compare the microbial diversity on the electrode and in the sediment of the anode of the sulfide-fed MFC.The microbial community in the sediment exhibited higher diversity than that on the electrode.The population on the electrode consisted mainly ofα-Proteobacteria,β-Proteobacteria,γ-Proteobacteria and Firmicutes.In addition to four phyla above,δ-Proteobacteria,Firmicutes and Gemmatimonadetes were also found in the sediment.The sulfur-oxidizing bacteria were in much more abundance on the electrode than in the sediment.On the contrary,the sulfate-reducing bacteria preferably grew in the sediments.Besides the sulfur-related bacteria,Acinebobacter sp.was found to be rich in the MFC anode.The exoelectrogens-containing species Pseudomonas sp.,Clostridium sp and Ochrobactrum sp.were found both on the electrode and in the sediments.The microbial diversity in the MFC anode further reveals the complexity of the anodic reactions in the sulfide-fed MFC.There might be a syntrophic association among the various bacteria in the MFC anode.In Chapter 4,an MEC-MFC-coupled system was constructed for biohydrogen production,in which hydrogen was produced in an MEC and the extra power was supplied by an MFC.This MEC-MFC-coupled system has a potential for biohydrogen production from wastes,and provides an effective way for in situ utilization of the power generated from MFCs.In this coupled system,hydrogen was successfully produced from acetate and propionate without external electric power supply.The performance of the MEC and the MFC was influenced by each other.A stable system requires the high efficiencies of four half-reactions in both MEC and MFC,and that any reaction at a low efficiency will have a negative effect on the overall system performance.It was demonstrated that the hydrogen production in such an MEC-MFC-coupled system could be manipulated through adjusting the power input on the MEC.The power input of the MEC was regulated by applying different loading resistors connected into the circuit in series.All circuit current,volumetric hydrogen production rate,hydrogen recovery,Coulombic efficiency,and hydrogen yield decreased with the increase in loading resistance.Thereafter,in order to add power supply for hydrogen production in the MEC,additional one or two MFCs were introduced into this coupled system.When the MFCs were connected in series,the hydrogen production was significantly enhanced.In comparison,the parallel connection slightly reduced the hydrogen production.Connecting several MFCs in series was able to effectively increase power supply for hydrogen production,and had a potential to be used as a strategy to enhance hydrogen production in the MEC-MFC-coupled system from wastes.In chapter 5,experiments were conducted to elucidate the electron transfer mechanisms and glucose metabolism of Shewanella oneidensis MR-1 in an MFC.The results show that the S.oneidensis MR-1 secreted flavins into the culture as electron shuttles.The flavins were also secreted in the absence of the anode electrode, implying that they were not deliberately produced for electron transfer.A omcA/mtrC-deficient mutant,which has a poor electricity-producing ability,was observed to secrete a similar level of flavins as the wide type.This suggests that the electron transfer by flavins was not the key mechanisms in the electricity production of MR-1.During the glucose metabolism of MR-1 in an MFC,formate,acetate, propionate,butyrate,pyruvate,and ethanol were identified as the main metabolic products.In this MFC,the glucose was initially metabolized to pyruvate via ED pathway,and then to formate,acetate,propionate,butyrate,and ethanol.The electricity was not produced directly from the glucose decomposition,but was generated from the metabolism of the intermediate products such as formate and butyrate.