Effect And Mechanism Of Direct Interspecies Electron Transfer(DIET) Mediated By Coffee Ground Biochar

The organic substrates can be degrade by direct interspecies electron transfer(DIET)of methane-producing microorganisms in anaerobic digestion system independently and efficiently.During the process of anaerobic digestion,biochar can effectively enrich the microorganisms involved in the DIET and strengthen the DIET.The process was influenced by the complexity of the system and DIET between the syntrophic bacteria and methanogens was not clearly illuminated.How biochar participates in the DIET process(the main factors of biochar affecting the DIET)has not been explored clearly.In this study,hydrothermal carbonization and low-speed pyrolysis under different temperature were used for biochar preparation.Oxidation modification,reduction modification,acid modification,alkali modification and pore-forming modification of biochar were carried out.Those kinds of biochar were added into the anaerobic digestion system.Modern catalytic characterization and 16 s RNA gene sequencing method were conducted to find out the influence of different properties of biochar to microbial community and alcohol anaerobic digestion,in order to get the mechanism of DIET process under the influence of biochar.The main conclusions are as follows:(1)First,the influence of biochar(pyrolysis under 700 ℃,named T700)and granular activated carbon(GAC)in ethanol anaerobic digestion was compared.The results showed that the addition of biochar and GAC could effectively promote the methane production.The cumulative methane production of the experiment group added with biochar increased by about 23% compared with the group added with GAC.Microbial analysis showed that Clostridiaceae and Pseudomonadaceae,which were considered as microorganisms that might participate in the DIET were enriched in the experiment group added with biochar and GAC.The results of material characterization showed that biochar and GAC influenced the microorganisms in anaerobic digestion system differently due to the vary physical and chemical properties.The large number of aromatic hydrocarbons on the surface of biochar may be an important factor in its participation in DIET.At the same time,the specific surface area of the material and the dispersion state in the liquid phase may also affect the system.(2)Then,the influence of biochar aromatization on DIET was studied.The results showed that the cumulative methane production could be increased by 15% to 29%.In the anaerobic digestion system added with oxidants modified biochar,cumulative methane production increased by 12% to 13%,which was lower than that of the experiment group added with unmodified biochar.The detection of Zeta potential indicates that the stability of biochar dispersion system may play an important role in the electron transfer process of microorganisms.The results of Fourier transform infrared spectroscopy(FTIR),X-ray photoelectron spectroscopy(XPS)and 16 s RNA gene sequencing showed that the aromatic hydrocarbons on the surface of biochar mainly affected the relative content of microorganisms participating in DIET(e.g.in the experimental group with the influence of T700 biochar,the total relative content of Pseudomonadaceae,Bacillus and Clostridiaceae was 65.71%,while that of Methanosarcina was 97.97%).(3)Last but not the least,the effects of biochar specific surface area and surface acidity and alkalinity on the process of DIET were studied.The results showed that the acid modification biochar and alkali modification biochar inhibited methane production during the anaerobic digestion,which may be related to the surface acidity and alkalinity of the material.The 16 s RNA gene sequencing results showed that the effects of unmodified biochar,oxidants modified biochar,reduction modified biochar and poreforming modified biochar on different microbial communities were different.Acid modification was not beneficial to the growth of Clostridiaceae,Pseudomonadaceae and Methanosaetaceae,while alkali modification was not beneficial to the growth of Clostridiaceae and Pseudomonadaceae.The content of Pseudomonadaceae in the experimental group with oxidized modified biochar was relatively low(e.g.34.13% in the experimental group with oxidants modified biochar(O700)and 41.33% in the T700 experimental group in chapter 4 while in chapter 5 it was 15.21% in O700 group and33.63% in T700 group).Pore-forming modified biochar was not conducive to the growth of Pseudomonadaceae and Methanosaetaceae.There was not multiple functional groups(especially aromatic functional groups)in the surfance of modified biocha or the surface physical and chemical properties are not conducive to the growth and reproduction of microorganisms participating in DIET.