Enhancement And Regulation Of Extracellular Electron Transfer In Anaerobic Biological Treatment By Iron Oxides

Anaerobic microbial treatment is one of the most effective methods to treat organic wastewater.Anaerobic microorganisms can not only degrade pollutants into volatile fatty acids,but also convert volatile fatty acids into methane through anaerobic digestion,playing a significant role in the energization of pollutants.The traditional anaerobic digestion established on the hydrolysis acidification-methane production theory is a hydrogen-producing and hydrogen-consuming process with hydrogen as electron carrier,the efficiency of which is too low to meet the requirement of a highly efficient and stable anaerobic digestion system.In reeent studies,iron oxides were found able to induce iron-reducing microorganisms(IRB)for the dissimilatory iron reduction,using organic matter as a substrate to transferr electrons to extracellular iron oxides,enhancing the degradation of organic matter.In addition,some conductive iron oxides can also directly participate in the direct interspecies electron transfer(DIET)as a conductor between IRB and methanogens,opening new channels for anaerobic digestion and methanogenesis.Obviously,the addition of iron oxides is expected to improve the anaerobic metabolism of microorganisms and increase the efficiency of extracellular electron transport of microorganisms.However,the effects of iron oxides on extracellular electron transport and the mechanism of induction in anaerobic systems are still unclear.Based on the above considerations,iron oxides were dosed in a conventional anaerobic reactor,exploring the effect of dissimilatory iron reduction and extracellular electron shuttles on extracellular electron transfer,identifying the effects of iron oxides on microbial extracellular polymers(EPS)and regulating the interspecies electron transfer,preparing iron-containing sludge-based biochar with good electron transport and electron storage capacity.The main results are as follows:(1)In order to investigate the effect of iron oxides on the electron shuttle process in anaerobic systems,Fe(OH)3 was added to the anaerobic reactor(RI)to realize the iron ion cycle process based on dissimilatory iron reduction and promote the decolorization and degradation of Acid Orange 7(AO7).In terms of reactor treatment performance,the chemical oxygen demand(COD)removal rate and decolorization rate of the R1 reactor were increased by 61.7%and 32.0%,respectively,compared with the control reactor(R2).1-amino-2-naphthol(1A2N),an intermediate of AO7 degradation,can continue to reduce AO7,which has the potential to autocatalyze the degradation of AO7 dye,but the accumulation of the reduced product 1-imino-2-naphthoquinone(1I2NQ)can make the autocatalytic reaction unsustainable.In R1 with Fe(OH)3 added,the Fe(?)produced by the dissimilatory iron reduction can spontaneously reduce 1I2NQ to form 1A2N and Fe(?).The former can continue to reduce AO7,while the latter can continue to be reduced by microorganisms.This causes the two pairs of electron shuttles,Fe(?)/Fe(?)and 1I2NQ/1A2N,to form a cycle in the reactor,continuously consuming COD,and degrading AO7,leading to better perfomances of R1 than that of R2.The cytochrome c concentration and sludge conductivity of sludge in R1 increased to 3.2 times and 2.1 times of R2,respectively,and the electron shuttle capacity of R1 microbial EPS was obtained.The humic acid content is also significantly higher than R2.In addition,the abundance of IRB and fermenting bacteria in the R1 reactor is also higher than that of R2,and the microbial community structure is optimized.In summary,the addition of iron oxide promotes and improves the electron transport process of extracellular electron shuttles,and improves the efficiency of extracellular electron transport during anaerobic biological treatment.(2)In order to investigate the effects of iron oxides on the metabolism of microorganisms in a system lacking exogenous extracellular electron shuttles,nano-ferric oxide(nFe3O4)and nano-zero-valent iron(nZVI)were added to the anaerobic reactor to compare the effects of both on microbial EPS secretion and microbial metabolic patterns.The results show that the addition of nFe3O4 significantly improves the efficiency of anaerobic treatment.The COD removal rate and methanogenesis increase by 33.1%and 403.7%,respectively,compared with the blank reactor,while the addition of LZVI led to a decrease in reactor performance.nFe3O4 stimulated the secretion of proteins and humic acids with electron transfer ability in EPS,and the sludge in the reactor also had a higher electron accepting/donating capacity(160.47 ?mol e-/gTOC vs 56.02 ?mol e-/gTOC).The extracellular electron transport capability of these electroactive substances enabled DIET between anaerobic microorganisms(iron-reducing bacteria and methanogens)to reduce CO2 and produce methane.However,in the nZVI reactor,in order to prevent the cell wall from being destroyed by the strongly reducing nZVI,the microorganism enhanced the secretion of EPS,and the concentration of the structural polysaccharide which is not conductive was significantly increased.Excessively thick EPS reduced the mass transfer efficiency of hydrogen,decreased the efficiency of interspecies hydrogen transfer(IHT),and led to the collapse of the reactor.Microbial community analysis showed that there were more electroactive microorganisms in the nFe3O4 reactor.Functional gene predictions indicated that the microorganisms in the nFe3O4 reactor have stronger ion exchange and carbohydrate metabolism capacity.Therefore,the conductive nFe3O4 and its stimulated electroactive EPS secretion enabled the microorganism to establish DIET as the main interspecies electron transfer pathway,so that the reactor can maintain efficient and stable operation.(3)According to the above research results,materials with high capacitance and conductivity may have better intervention in anaerobic metabolism.Considerign the redox property of hydroquinone/hydroquinone in biochar and the electrical conductivity of iron oxides made from high temperature burning,this study used sewage sludge as raw material,and the iron-containing sludge-based biochar is prepared by pyrolysis of Fe3+ solution adsorbed sludge.According to the optimized preparation conditions,the iron-containing sludge-based biochar synthesized by pyrolysis at 600? has excellent electrical conductivity while maintaining optimal charge and discharge characteristics.Further characterization showed that the iron oxide grown on the surface of the biochar was an anti-spinel structure of Fe3O4,which was beneficial to the microbial construction of DIET mode.In addition,the biochar has a larger specific surface area,rich surface functional groups,and a stronger electron accepting/donating capacity than the blank sludge-based biochar by 226%,which is more favorable for increasing the extracellular electron transport efficiency of the microorganism.In the anaerobic digestion and dye degradation process,the COD removal rate,methane production,and dye decolorization rate of the biochar addition reactor were compared with the blank reactor and the non-iron biochar reactor.In summary,the biochar overcomes the limitations of the conventional biochar's electrical conductivity(high temperature pyrolysis)and capacitive(low temperature pyrolysis),and enhances the microbial extracellular electron transfer efficiency from the perspectives of both charge and discharge and electron transfer,thereby improving anaerobic treatment efficiency.