Construction And Mechanisms Of Microbial Electrosynthesis For CO₂ Reduction Integrating Activated Sludge

The rapid development of industrialization has led to the increasing consumption of traditional fossil fuels,and the sharp rise in greenhouse gas emissions,mainly CO₂,has caused serious environmental pollution and global warming.Due to the dependence of economic development on carbon based compounds and the need to mitigate climate damage caused by fossil carbon emissions,how to use carbon dioxide to produce value-added chemicals has become a research hotspot.Microbial electrosynthesis system（MES）is a novel bio-cathode-driven production technology that uses electroactive microorganisms as catalysts to convert carbon dioxide into valuable chemicals and fuels within the cathode chamber.During wastewater treatment,the activated sludge system is rich in microbial populations with high electroactivity and tolerance of bacterial populations,and its carbon sequestration potential can be realized by coupling with the MES system.However,the research on this system is still at the initial research stage,and there are problems such as low reduction performance,unknown limiting factors and unclear reduction mechanism,which restrict the promotion and application of this system.Therefore,this article has conducted systematic experiments and analyses to construct a microbial electro-synthetic carbon sequestration system coupled with activated sludge,investigated the effects of cathode potential and cathode initial pH on the carbon sequestration performance of the MES system from multiple perspectives,analyzed the structure,differences and changes of microbial communities in both cathode electrode and suspension environments,and analysed the functional metabolism associated with the cathode community using PICRUSt2 functional prediction method.Microbial community-related functional metabolic activities were analysed using PICRUSt2functional prediction method.The main research results are as follows:（1）The constructed MES carbon sequestration system has the ability to reduce CO₂to volatile fatty acids（mainly acetic acid）,and the low and medium potentials are favorable to obtain higher amounts of reduction products.When powered by a constant potential meter and a DC regulated power supply,The MES system reached the highest yields of 328.7 mg/L and 424.7 mg/L at-0.8 V（vs Ag/Ag Cl）and 3.0 V,respectively.The applied cathode potential had a significant effect on the electrochemical activity and impedance of the biocathode,and the MES system showed the strongest electrochemical activity at 3.0 V potential.Under different potential conditions,biofilms were formed on the surface and inside the cathodic carbon felt electrode,and the morphological structure was similar but the activity varied greatly,with the highest biofilm activity at 3.0 V potential and the worst activity of microorganisms in the electrode and suspension at 4.0 V potential.（2）The initial pH of the cathode has an important effect on the reduction products of the system.At pH 5.6,a small amount of formic acid was produced（81.6 mg/L）,and only acetic acid was produced at other pH conditions,with the lowest acetic acid yield at pH 7.2（174.1mg/L）.The proton consumption patterns were different under different conditions,and the pH of the cathode chamber decreased only at pH 7.2.The suitable pH environment was favorable for the formation of electroactive biofilm with low internal resistance and high electron transfer efficiency,and the MES system had the strongest electrochemical activity and lowest impedance at pH 6.0.The morphological structure of cathodic biofilm was less affected by pH,mostly rod,filamentous and agglomerated,but there were differences in biofilm activity and electron transfer activity.pH 7.2 had more dead cells and relatively higher cathodic resistance,resulting in poor reduction performance of the system at this pH.（3）Electric potential and pH conditions have a significant impact on the microbial community diversity in the biocathode and suspension,and the microbial response patterns in the two environments are different,with more stable microbial growth and more regular changes in characteristics in the biocathode.The low and medium potential conditions（2.0V～3.0V）and optimal pH conditions（pH6.0）were favorable for microorganisms to attach and grow at the carbon felt electrode,and the higher species abundance,homogeneity and diversity were favorable for CO₂reduction in the MES system.in both environments,the phylum Anaplasma,Phylum Green Curvilinearis,Phylum Thickwalled and Campylobacter were the dominant phylum,and the genera Helicobacter,Acetobacter,Desulfovibrio and Hydrogenophagus were the dominant genera.（4）The results of PICRUSt2 function prediction shows that the applied potential and cathode pH can impact the metabolic activities of cathode microorganisms in the MES system,and Carbon metabolism and Carbon fixation pathways in prokaryotes are the two important metabolic activities in the system constructed in this experiment.The metabolic function abundance of microbial communities was larger under the medium-low potential and suitable pH conditions,and the abundance of metabolic functions of the biocathode colony was larger than that of the suspension,indicating that the biocathode was the main site of the CO₂ reduction reaction.