Enhanced Mechanism Of Energy And Mass Transfer In Carbon Dioxide And Hydrogen Biomethanation

The development of green and sustainable technology to achieve CO2 conversion and utilization is of great significance for promoting the goals of carbon emissions peak and carbon neutrality.It is a hot research topic at home and abroad to reduce CO2 in flue gas to produce methane by biological method with H2 from water electrolysis.In the process of CO2 biomethanation,there are technical bottlenecks such as poor H2 dissolved mass transfer leading to limited methane production rate,low hydrogenase activity of methanogenic archaea leading to slow electron transfer process,and multi-pathway competition of biological reaction in the flora leading to non-directional electron transfer,which restrict the industrialization development of methane production technology by biological reduction of CO2 and H2.In this paper,perfluorocarbon nanoemulsions have been proposed as hydrogen carriers to promote H2 dissolution and transport to methanogenic archaea,and then iron-based nanoparticles have been used to enhance the cell activity and extracellular electron transport of methanogenic archaea.At last,the synergistic pathway of stepwise conversion of H2 and CO2 into methane by Geobacter and methanogenic archaea have been explored,which significantly improved the CO2 biomethane ability.In order to solve the problem of limited CO2/H2 biomethanation by methanogenic archaea due to poor H2 gas-liquid mass transfer,perfluorocarbon nanoemulsions have been proposed as hydrogen carriers to promote H2 dissolution and increase the rate of CO2-to-methane conversion.When the concentration of perfluorocarbon nanoemulsions was 1 vol.%,the H2 solubility and adsorption capacity increased by 34.5%.Perfluorocarbon nanoemulsions composed of perfluorocarbon molecules and surfactants could effectively transport H2 molecules in the solution and alleviate the restriction of H2 gas-liquid mass transfer.The non-specific binding between lipophilic ether bond groups of the perfluorocarbon nanoemulsions and the cell membrane of Methonasarcina barkeri was beneficial to the transport of H2 molecules from the nanoemulsions to the cell.When 1.5 vol.%of perfluorocarbon nanoemulsions was added,the peak rate of methane production was increased by 30.6%and the lag time was shortened by 84.5%,which effectively accelerated the process of CO2 biomethanation.In order to simulate the molecular structure of the[NiFe]hydrogenase active center on the cell membrane of methanogenic archaea and promote the catalytic decomposition of H2 into protons and electrons,the nickel-iron bimetallic sulfide([Ni,Fe]S2)has been synthesized to enhance the cell activity and extracellular electron transport capacity of methanogenic archaea.Nickel-iron bimetallic sulfide had a lower overpotential in the H2 decomposition reverse reaction,and could increase the peak current in electrochemical cyclic voltammetry curve,which was more conducive to the electron transfer process in CO2 biomethanation.The content of tryptophan-like and fulvic acid-like substances in extracellular polymer substance of M.barkeri increased,which enhanced the cell activity and extracellular electron transport ability,respectively.When 200 mg/LR of nickel-iron bimetallic sulfide was added,the H2 utilization rate was accelerated to enhance the biomethanation process,and the conversion efficiency of CO2 to methane was increased from 63.6%to 91.6%.In order to improve the electron storage and transfer ability of biomethanation system,zero-valent iron nanoparticles have been used to improve CO2 biomethanation by enrichment the dominant hydrotrophic methanogens.The electric capacity and electron transfer constant(kapp)of CO2 biomethanation system were increased by 2 times and 32.8 times,respectively.The high-throughput gene sequencing showed that the dominant methanogenic archaea were Methanoculleus,Methanothrix and Methanothermobacter.It was worth noting that the relative abundance of Methanothermobacter increased from 7%to 16%,which indicated the dominant hydrotrophic methanogens were optimized and enriched.when 1.5 g/LR of zero-valent iron nanoparticles was added,the peak rate of methane production increased by 80%,the methane yield reached 0.186 L/LR(increased by 66.1%)and the carbon conversion efficiency increased to 92.9%.In order to solve the problem of non-directional electron transport caused by the competition of multiple microbial reactions during biomethanation,the synergistic pathway of stepwise conversion of H2 and CO2 into methane by G.sulfurreducens and M.barkeri has been explored:the electrons generated from H2 utilization by G sulfurreducens were directed to M.barkeri for the reduction of CO2.Proteomics analysis revealed that membrane-bound[NiFe]hydrogenase Hyb in G sulfurreducens accelerated H2 consumption during methanogenesis,while methyl-coenzyme M reductase,ferredoxin,flavodoxin,membrane-associated ATP synthase,and the membrane-associated proton pump were significantly up-regulated.As a consequence,carbon fixation,energy conversion,and electron transfer in the metabolic pathways of M.barkeri cells were all improved.Interspecies electron transport mediated by fulvic acid-like substances occurred between G sulfurreducens and M.barkeri,which promoted CO2-to-CH4 conversion.The maximum CH4 production rate increased by 2.1-fold to 0.158 mL/mL-H2/d,in addition,the lag time and peak rate time were shortened by 24.6%and 45.7%,respectively.The addition of G.sulfurreducens to the mixed methanogens enhanced the process of CO2 biomethanation.The increase in the relative abundance of G.sulfurreducens enhanced the decomposition of H2 into protons and electrons,and promoted the electron storage and transfer ability of mixed methanogens.The increased in the relative abundance of Methanoculleus promoted the process of CO2 conversion to methane.The increase in the content of fulvic acid-like substances in extracellular polymer substance effectively mediated the interspecies electron transfer to reduce CO2 to methane.When the volume ratio of G sulfurreducens to mixed methanogens was 2:4,the methane yield was increased by 33.3%to 0.24 mL/mL-H2(close to the theoretical maximum yield of 0.25 mL/mL-H2),and carbon conversion efficiency increased to 96%.