Facet-dependent Extracellular Electron Transport Mechanism Mediated By Iron Oxide

Extracellular electron transport is a novel metabolic form of microorganism,which is the nature of extracellular respiration.In the surface environment,iron oxide minerals can not only provide essential nutrients for microbial growth and metabolism,but also promote microbial metabolism as a terminal electron donor/acceptor and electron mediator.In recent years,the study about exposed facets of iron oxide has aroused great interest.Different exposed facets possess different atomic arrangements,which show obvious differences in interface chemistry properties.However,there are few studies on facet-dependent extracellular electron transport mechanism mediated by iron oxide.In this thesis,iron oxides with different exposed facets were synthesized and used as electron acceptors and electron donors,respectively,to explore how different exposed facets affect the interaction between iron oxides and microorganisms,and the potential mechanism of iron oxides with different exposed facets mediating the extracellular electron transfer of microorganisms.The main conclusions are as follows:When iron oxide with different exposed facets were used as electron acceptors,the exposed facets not only significantly affected the reduction dissolution of iron oxides by microorganisms,but also the biological mineralization process driven by dissimilated iron reduction.Specifically,hematite with predominantly exposed{001}facet(Hem?{001})and goethite with high content of exposed{110}facet(Goe?H{110})were more easily reduced by Geobacter sulfurreducens.The initial reduction rate and final Fe(II)content of Hem?{001}were about 2.25 and 1.43 times of Hem?{100},respectively,while the initial reduction rate and final Fe(II)content of Goe?H{110}were about 1.50 and 1.43 times of Goe?L{110},respectively.In addition,in the presence of sufficient PO₄^3-,Fe(II)would combine with PO₄^3-to form phosphorus crystal products,and the final phosphorus removal efficiency of Hem?{001}system was about 5.2%,1.3 times that of Hem?{100},while the final phosphorus removal efficiency of Goe?H{110}system was about 13.6%,1.6 times that of Goe?L{110}system.The results of X-ray diffraction(XRD)?scanning electron microscope(SEM)and energy dispersive spectroscopy(EDS)analysis confirmed that the phosphorus crystal product was vivianite,and the size of vivianite crystal was significantly affected by the surface structure of iron oxides.The formation process of vivianite driven by DIR showed obvious facet dependence,which may be due to the difference in the type and abundance of active groups on the surface of iron oxide.Compared with Hem?{100}and Goe?L{110},Hem?{001}and Goe?H{110}may have a higher density of active groups,such as-Fe OH group,which has a stronger affinity for PO₄^3-adsorption and promotes the biogenesis vivianite.When Iron oxide acted as electron mediator,the ability of electroactive bacteria to produce electricity was significantly affected by different exposed facets.The results showed that Hem?{001}electrode had higher electrochemical activity and lower resistance than other electrodes,indicating that Hem?{001}was more conducive to the formation of electroactive biofilm,thus improving the electron transfer efficiency.The maximum biocurrent density of Hem?{001}electrode reached 287.0±19.9?A/cm~2,which was 2.2,3.9 and 199.3 times that of Hem?{100},Hem?NPs and blank ITO electrode,respectively.In addition,Extracellular polymeric substances(EPS)component analysis showed that Hem?{001}significantly promoted the secretion of EPS,especially the protein content of EPS,which is closely related to electron transport.Based on metagenomic analysis,we further explored the potential mechanisms by which different exposed facets affected EET efficiency,and found that Hem?{001}promoted the microorganisms extracellular electron transfer process by regulating the microbial community structure and the expression of nanowires and cytochrome c related genes,so as to improve the efficiency of biological electrochemical system.In conclusion,this thesis proved that different exposed facets could affect microbial reduction of iron oxides and biomineralization driven by dissimilatory iron reduction,as well as affect the process of extracellular electron transfer mediated by iron oxides.