Mechanism Of Ferric Citrate/Ferric Chloride Coupled With Nitrite Reduction Synergistically Driving Anaerobic Oxidation Of Methane

Anaerobic oxidation of methane（AOM）is very important for the reduction of greenhouse gases such as methane and carbon dioxide.However,the enrichment and cultivation of the AOM-related functional microorganisms is very difficult,and the efficiency of AOM driven by single acceptor is low,which limits the engineering application of AOM.In this paper,the landfill cover soil was used as the substrate for reactor operation and microbial enrichment culture,and stable isotope tracing and metagenomic methods were used to monitor the synergy of the AOM driven by ferric citrate/ferric chloride coupled nitrite reduction.The microbial community structure and metabolic characteristics in reactors were explored the process and mechanism of the synergy of the AOM by ferric citrate/ferric chloride coupled with nitrite reduction.The results showed that:（1）According to the efficiency of AOM in different reactors,adding ferric citrate or ferric chloride could promote the reduction of nitrite and the AOM,and adding ferric citrate had the best effect.The methane consumption rate,nitrogen consumption rate and nitrite consumption rate of the reactors added ferric citrate/ferric chloride were ordered as ferric citrate group>ferric chloride group>control group.It was preliminarily inferred that ferric citrate could be introduced as an external carbon source,and the enrichment functional microorganisms and life activities could be more active.（2）The results of stable isotope tracing experiments showed that the addition of ferric citrate and ferric chloride promoted the AOM driven by ferric citrate/ferric chloride coupled with nitrite reduction,and the addition of ferric citrate had the best effect.The rate of methane consumption in the ferric citrate group was about 2.71 times that of the control group,and that in the ferric chloride group was about 2.17 times that of the control group.The denitrification rate of the ferric citrate group was about 1.67times that of the control group,and the ferric chloride group was about 1.48 times that of the control group.The ¹³C conversion process was that ¹³CH₄ was mostly converted into ¹³C-TIC through microorganisms,and a small amount may be used by microorganisms as energy and carbon sources to supply their own life activities.At the same time,the ¹³C-TIC conversion rate was the highest,and carbon dioxide generated least in the ferric citrate group,which was the most environmentally friendly.The addition of ferric citrate and ferric chloride could promote the anaerobic oxidation of methane through Fe³⁺and nitrite reduction.The addition of ferric citrate and ferric chloride also promoted the transformation of nitrite into ammonia and organic nitrogen,and the transformation into organic nitrogen was more obvious.The green rust reaction in ferric chloride group kept the nitrate and ammonia nitrogen at a high concentration level.The nitrogen conversion rate of the ferric citrate group was the lowest,and nitrite was converted more into organic nitrogen,which was very likely to be used as a nitrogen source for the growth and development of microorganisms and enhance the activity of microorganisms.The ferric citrate group and the ferric chloride group had their own special transformation.（3）The results of metagenomic sequencing showed that the microbial diversity in the ferric citrate group and the ferric chloride group was lower than that in the control group.And the dominant species were not the same.unclassified＿p＿Chloroflexi,Methylocaldum,Methylocystis,Hyphomicrobium,Lautropia and Methylobacillus contributed significantly to the addition of ferric citrate and ferric chloride promoted the AOM driven by ferric citrate/ferric chloride coupled with nitrite reduction and its associated metabolic processes.Unclassified＿p＿Chloroflexi was the most dominant genus and it could oxidize methane and fix carbon dioxide.The influence of environmental factors on the microbial community in all groups was ranked as NH₄⁺-N>Fe²⁺>DIC>Fe³⁺>NO₃^--N>Methane>NO₂^--N>TOC.According to the metabolic pathway analysis,the addition of ferric citrate and ferric chloride could enhance the methane metabolism and nitrogen metabolism activities and accelerate electron transfer,thereby the rate of AOM driven by iron-coupled nitrite was increased.