Study On Mechanism Of Enhancing Methane Bio-conversion And Process Of Carbon Sequestration By Microbial Fuel Cells

It is of great practical significance to convert methane（CH₄）into treasure on account of its dual identity of renewable energy material and greenhouse gas.As a waste treatment and energy recovery device with microorganisms as a catalyst,anaerobic oxidation of methane-microbial fuel cells（AOM-MFCs）can convert the chemical energy contained in methane into electricity and produce other value-added products at room temperature,so as to strengthen the methane bio-conversion process.Nevertheless,there still exist some urgent problems in related research,such as the controversial main intermediate of anode,the long start-up time of the reactor,the unclear conversion mechanism of methane and the important greenhouse gas-carbon dioxide（CO₂）as the final product.In view of the above shortcomings of correlational researches on AOM-MFCs,in this study,acetate-induced and formate-induced electroactive culture reactors（MFC 1 and MFC 2）were designed respectively.The start time and voltage output of the two reactors were evaluated accordingly and the performance and mechanism of methane conversion and power generation of microbial fuel cells were analyzed and explored.The Chlorella-cathode was added in the reactor which took on relatively excellent performance to deeply utilized the carbon dioxide produced form the anode.The carbon sequestration and electricity generation performance of the Chlorella-cathode reactor in the process of methane conversion were investigated and the specific process of carbon sequestration was described.The main conclusions are as follows:（1）The effects of electroactive cultures pre-enriched by different induced precursors on the start-up and operating performance of the reactors were different:The methane utilization rate of MFC 2 was higher and the start-up time was shorter（51 d）while MFC 1had a relatively low charge transfer resistance and a relatively high output voltage（0.526±0.001V）.In addition,the methane utilization rate of the two reactors was much higher than that of the methane bio-conversion process outside of the microbial fuel cell system,so the two systems could enhance the bio-conversion process of methane.（2）The composition and distribution of microorganisms in MFC 1 and MFC 2favored anaerobic oxidation of methane（AOM）and electron transport processes.The mechanism of methane conversion in the two reactors was respectively:in MFC 1,acetylmethanogens（e.g.,Methanosaeta）converted methane into acetate and other intermediates by reversing the methanogenesis pathway and carried out the direct interspecific electron transfer（DIET）with Geobacter-predominated electricigens;whereas,in MFC 2,hydrogen methanogens（e.g.,Methanobacterium）were able to faster metabolize methane-producing formate-dominated intermediates and intermediate-dependent extracellular electron transfer between electroactive bacteria（e.g.,Geothrix）.Among which,the electroactive bacteria were responsible for oxidizing the intermediate to carbon dioxide and transferring electrons to the electrode.（3）MFC 2 performed relatively excellently in startup and operation,so the Chlorella-cathode was introduced for carbon fixation.The optimal operation conditions of the cathode reactor were an inoculation amount of 25%v/v,an inoculation age of 24 h,a light intensity of 8000 lx,and a photoperiod of 12 h.Under the conditions,the maximum peak voltage and maximum biomass concentrations of the reactor were 0.385±0.001 V and576.33±1.53 mg·L^-1.In addition,the anode effluent is suitable for the cathode liquid of the Chlorella-cathode reactor,and the reactor was provided with a certain wastewater treatment capacity.（4）The Chlorella-cathode reactor could convert the chemical energy contained in methane into electrical energy and biomass energy to achieve the efficient clean conversion and fixation of carbon elements in methane.The specific process of carbon fixation in the reactor consisted of three parts:the microbial alliance composed of methane oxidation archaea and electroactive bacteria used methane to produce metabolites dominated by carbon dioxide and produce electrons;the Chlorella in the cathode captured and immobilized carbon dioxide through photosynthesis to produce oxygen（O₂）;the produced oxygen acted as a terminal electron acceptor to receive electrons transferred to the cathode and combined with protons to produce water.In summary,this study explored the mechanism of microbial fuel cells to enhance methane bio-conversion,analyzed the specific process of microbial fuel cells to convert methane and sequester carbon at the same time,and realized the efficient and clean conversion and utilization of methane in microbial fuel cells,providing a theoretical basis for the practical application of microbial fuel cell system.It is of great practical significance to promote the process of"converting carbon into treasure"of methane and to alleviate energy and environmental problems.