Mechanisms Of Nano α-Fe₂O₃ Mediated Hydrogen Autotrophic Denitrification

Three-dimensional electrode biofilm reactors（3D-BERs）is a novel technology for hydrogen autotrophic denitrification,without additional electron donors and secondary pollution.Since the formation of the biofilm of hydrogen autotrophic microorganisms on the particle electrodes requires a long time,the start-up process of the bioreactor is slow and low nitrogen removal efficiency is obtained at the initial stage.Therefore,in our previous work,we tried to dose nanoα-Fe₂O₃ into the3D-BERs.Nanoα-Fe₂O₃ was self-assembled with microorganisms at the interface of the particle electrodes to form hybrid biofilm,thereby promoting the enrichment of denitrifying microorganisms and accelerating the start-up of the denitrification process.However,there are several problems to be resolved,such as the operating conditions of 3D-BERs need to be optimized,the mechanisms of hydrogen autotrophic denitrification mediated by nanoα-Fe₂O₃ are still unclear.In this study,hybrid biofilm was cultivated in continuous-flow 3D-BERs,the denitrification performance of the hybrid biofilm under different operating conditions were explored,and the mechanisms of nanoα-Fe₂O₃ mediated hydrogen autotrophic denitrification were revealed.The main results are as follows:1.Influences of influent concentration,current and hydraulic residence time（HRT）on the denitrification performance of 3D-BERs were investigated.The results showed that the removal efficiencies of NO₃^--N and TN in 3D-BERs with hybrid biofilm（R1）were higher than those in 3D-BER with natural biofilm（R2）:The denitrification performance of 3D-BERs was influenced by the concentration of influent nitrate.Increasing the influent concentration from 50 mg/L to 100 mg/L had no significant effect on denitrification for R1 and R2.However,when the concentration was further increased to 150 mg/L,both NO₂^--N and NH₄⁺-N in R1 and R2 are significantly accumulated,and the denitrification performance decreased obviously;Increasing the current improved the denitrification performance of 3D-BERs.When the current increased from 20 m A to 40 m A,the denitrification performance of R1 and R2 was clearly improved.When the current was further increased to 60 m A,the denitrification performance of R1 and R2 slightly climbed;Prolonging HRT improved the denitrification performance of 3D-BERs.When the HRT was 17 h,the removal efficiency of TN in R1 and R2 was low,and NO₂^--N was obviously accumulated in the two reactors.When HRT was extended to 26 h,the removal efficiency of TN in R1 and R2 was elevated.Further extending HRT to 36 h,TN removal was slightly enhanced in R1 and R2.2.The operating conditions of 3D-BERs were optimized.When influent concentration was 100 mg/L,current was 40 m A,and HRT was 26 h,the removal efficiencies of NO₃^--N and TN in R1 maintained stable at 91.19%and 88.23%.And the denitrification rate was 4.2 mg TN/L/h for R2,higher than that for R2 and other biological denitrification systems;The amount of microorganisms and extracellular polymeric substances（EPS）of the hybrid biofilm in R1 were higher than those of natural biofilm in R2;The activities of nitrate/nitrite reductase and the abundance of enzyme-related genes in R1 were higher than those in R2;The microbial community analysis showed the lower microbial diversity of the hybrid biofilm in R1 than that of the natural biofilm in R2,suggesting that hybridization with nanomaterials helped to acclimate microorganisms with the function of hydrogen autotrophic denitrification;Metagenomic analysis confirmed that compared with the copper-dependent nitrite reductase Nir K,the cytochrome cd1-dependent nitrite reductase Nir S had a higher gene abundance in R1.Therefore,the reduction of nitrite through the nir S pathway might be the key process for the differences in denitrification between R1 and R2.That might be related to the high affinity ofα-Fe₂O₃ to cytochrome C.Metagenomic bins results showed that among the key genes involved in denitrification,bin.74 had the highest relative abundance for Nir S,Nos Z and dehydrogenase genes.And Zoogloeaceae,the microorganism related to bin.74,might play a key role in hydrogen autotrophic denitrification.