| Microbial electrochemical system(MES)is a new sewage treatment technology that simultaneously realizes sewage treatment and energy recovery.It can generate clean energy such as electricity and hydrogen while degrading organic matter in sewage.Improving the electron transfer of MES and its operational stability are still the bottlenecks in MES applications.In order to enhance the electron transfer of MES and conduct its strengthening mechanism,this study constructed a magnetic MES.Magnetic nitrogen-doped carbon material was synthesized,and the influence of the composition and structure of the catalyst on the catalytic activity of oxygen reduction reaction(ORR)and the cathode microbial community were investigated;Magnetic materials and magnetic fields were introduced into the anode,and the influence of magnetic effects on MES electricity generation and anode microbial community were investigated;RNA sequencing technology was used to analyze the mechanism of response of electroactive bacterial transcriptome to magnetic field.Magnetic nitrogen-doped carbon material(Fe3O4@N-m C)was synthesized as an efficient catalyst for oxygen reduction reaction(ORR)in the cathode.Scanning and transmission electron microscopy analysis of the prepared Fe3O4@N-m C revealed that it had obvious core-shell structure.X-ray diffraction(XRD),fourier transform infrared(FTIR)and X-ray photoelectron spectroscopy(XPS)confirmed the presence of Fe3O4 and nitrogen-doped carbon in the material.The synthesized Fe3O4@N-m C material had an electron transfer number of 3.76 during the oxygen reduction reaction,which was higher than that of N-C(3.55),Fe3O4(2.96),PANI(2.45),and Fe3O4@m PANI(2.74).The microbial fuel cell(MFC)with Fe3O4@N-m C as the cathode promoted the enrichment of electroactive bacteria Dietzia.To further increase the active site of the material,the active center Fe3O4 was changed to nanoscale.The improved Fe3O4@N-m C exhibited better oxygen reduction catalytic activity and electrochemical stability than Pt/C catalyst.MFC(1141 m W/m2)using Fe3O4@N-m C cathode had higher power density than MFC using Pt/C cathode(1022 m W/m2),indicating that Fe3O4@N-m C catalyst can replace the commonly used Pt/C catalyst.The above studies confirmed that various nitrogen functional groups such as Fe-N,core-shell structure and nano-scale Fe3O4 core are the main enhancement mechanisms for improving the ORR catalytic performance of Fe3O4@N-m C catalyst and the enrichment of electroactive bacteria.The effects of static magnetic field on MES performances were explored.The magnet was directly used as the electrode of MFCs.The effects of different magnetic field strengths and magnetic field directions on the electricity generation of MFCs were investigated in situ.It was found that the output voltage of MFC with the magnetic anode increased by 71.0%-105%,and the power density increased by 42.9%-104% compared to the MFC with non-magnetic anode.When the magnetic field strength of anode was 80 m T,MFCs obtained the best power generation.Electrochemical impedance spectroscopy(EIS)showed that the magnetic field reduced the diffusion and activation resistances of MFCs.It was confirmed t hat the constant magnetic field can also increase the voltage output,methane production rate and energy efficiency of the MEC,indicating that the influence of the magnetic field on the MES is universal.When the applied voltage was 0.4 V,the improvement effect was most significant,the methane yield was increased by 26.2%,and the energy efficiency was increased by 19.7%.16 S r RNA sequencing showed that the relative abundance of Geobacter in MFC with the magnetic anode was 32.5% higher than that of the MFC with non-magnetic anode.The above results indicated that the magnetic field reduced the total internal resistance of the MES and promoted the enrichment of electroactive bacteria at the anode,thereby improving the performance of the system.Based on above research,the system that magnetic microbial fuel cell assisted by pulsed magnetic field(PEMF-MMFC)was constructed by combining magnetic nitrogen doping carbon material with magnetic field.Results showed that the power density of MFCs under PEMF was 25.3%-36.0% higher than that of MFCs without PEMF.When PEMF was removed,the power density dropped by 25.7%,while when PEMF was reintroduced,the value was restored to the previous level,indicating that the PEMF improved the electricity production of MFCs,and the effect was instantaneous and reversible.At the same time,the PEMF also promoted the enrichment of electroactive bacteria at the anode.The relative abundance of Geobacter in PEMF-MMFCs anode biofilms(86.1-90.0%)were higher than that in PEMF-OFF-MMFCs(82.5-82.7%).In order to further explore the promotion mechanism of the magnetic field on the extracellular electron transfer of electroactive bacteria,the transcriptomics analysis of the electroactive bacteria Geobacter sulfurreducens PCA under the magnetic field was conducted.The results showed that the magnetic field up-regulated the expression of the electron transfer related genes such as periplasmic c-type cytochromes(Ppc A and Ppc D),outer membrane cytochrome(Omc F,Omc Z,and Omc B),and pili(Pil A-C,Pil M,and Pil V-2).The above results show that the magnetic field promotes the enrichment of electroactive bacteria at the anode and the expression of genes related to electron transfer(Ppc A,Ppc D,Pil A-C,etc.),thereby improving the performance of the system.This research clarified the promotion mechanism of magnetic effect enhanced extracellular electron transfer of microorganisms,and proposed a control strategy based on magnetic electrode,magnetic catalyst and magnetic field to strengthen MES electron transfer,which provided a theoretical basis for improving the efficiency and operating stability of MES. |
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