Effects And Mechanisms Of Modified Biochars On Mic Robial Iron Reduction Of Geobacter Sulfurreducens

Fe(?)minerals,as vital redox activity substances on earth,have played an important role in geochemical cycling processes.The redox process of iron is determined by the respiratory metabolism of iron reducing bacteria(IRB)and iron oxidizing bacteria(IOB).Among them,Geobacter,a typical IRB,uses minerals(such as hematite)as electron acceptor to gain energy,which affects the geochemical cycle of iron.Therefore,microbial iron reduction have attracted scientific attention.In addition,it has been found that biochar can act as an electron shuttle,accepting electrons from IRB and donating them to iron minerals,to speed up the process of microbial iron reduction.Biochar is a solid phase material produced by pyrolysis of waste biomass at high temperature under anaerobic conditions.It has abundant functional groups and developed pore structure.At the same time,biochar has redox-active groups,such as quinone/hydroquinone and aromatic structures,enabling it to accept electrons and donate electrons reversibly,thus acting as electron shuttle to affect the microbial iron reduction process.Although it has been proved that the electron shuttle action of biochar is mainly related to the surface functional groups of biochar,the research mechanism is not fully understood.Therefore,in this study,two kinds of modified biochar were prepared and their effects on the iron reduction process of Geobacter were studied.From this to further explore how the regulation of functional groups of biochar affects the iron reduction process of IRB.Including the following two experiments:1)Preparation of modified biocharBiochar was prepared via the pyrolysis of rice straw at 600°C for 1 h,labelled as BC.Oxidized biochar(NBC)and reduced biochar(RBC)were prepared by adding HNO₃ and Na BH₄ to BC respectively.Through scanning electron microscopy(SEM),fourier infrared spectroscopy(FTIR),X-ray photoelectron spectroscopy(XPS),electron paramagnetic resonance(ESR),electrochemical measurements,x-ray diffraction(XRD)and compared with the biochar which washed with deionized water(DIW)(WBC)(a control biochar),the changes of physicochemical properties of the modified biochar were explored.This laid a foundation for the subsequent experiment,which explored the effects and mechanisms of modified biochar on microbial iron reduction.2)Effects of and mechanisms of modified biochar on microbial iron reduction of G.sulfurreducens PCAAfter the addition of three different biochar(NBC,WBC and RBC),the changes of soluble(and insoluble)Fe(?),Fe(?)and total iron concentration in the medium were determined by ultraviolet-visible spectrophotometer.From this way,the effect of biochar on microbial iron reduction was studied.SEM,EDS,XPS,FTIR and XRD were analyzed to indicate the changes of biochars'physicochemical properties and the formation of iron minerals after microculture.In this way,the mechanism of biochar affected microbial iron reduction was revealed.Results showed that WBC and RBC significantly(p<0.05)accelerated microbial iron reduction of G.sulfurreducens PCA.During the initial culture process(0-3 days),Fe(?)concentration in the experimental group treated with WBC and RBC was significantly(p<0.05)higher than that in the pure PCA group.The production rate of Fe(?)and the consumption rate of Fe(?)in the experimental group after WBC and RBC treatment were also significantly(p<0.05)higher than those in the pure PCA group.In addition,the addition of WBC and RBC significantly(p<0.05)accelerated the formation process of iron min erals,but had no significant effect on the final Fe(?)minerals,such as vivianite and green rust(CO₃^2-)).However,NBC completely inhibited the Fe(?)reduction of G.sulfurreducens PCA.In the experimental group added with NBC,the iron concentration hardly changed,and the production rate of Fe(?)and the consumption rate of Fe(?)were almost 0.Brunauer-Emmett-Teller(BET)surface area,electron spin resonance(ESR)and electrochemical measurements showed that larger surface area,lower redox potential,and more redox-active groups(e.g.,aromatic structures and quinone/hydroquinone moieties)in RBC explained its better electron transfer performance comparing to WBC.Interestingly,NBC completely suppressed the Fe(?)reduction process,mainly due to the production of reactive oxygen species which inhibited the growth of G.sulfurreducens PCA.Overall,this work paves a feasible way to regulate the surface functional groups for biochar,and comprehensively revealed its effect on extracellular electron transfer(EET)process of microorganisms.