| Anaerobic digestion(AD)technology has been widely used in the"double-carbon"background,but the low methane yield is still the main bottleneck for its large-scale application.The main reason is that the treatment of high-concentration organic wastewater is volatile fatty acid The accumulation of volatile fatty acids(VFA),the occurrence of the electron transfer process between VFA-oxidizing bacteria and methanogenic archaea with hydrogen or formic acid as the mediator,has a thermodynamic energy barrier,and the reaction efficiency is low.The study found that by adding biochar to the anaerobic digestion system,the extracellular electron transfer(EET)process of microorganisms can be stimulated,and the direct interspecies electron transfer(DIET)metabolic pathway between the mutualistic microorganisms can be established,accelerate VFA oxidation,improve AD system performance.However,biochar usually has low electrical conductivity,and its characteristics and mechanism of enhancing the extracellular electron transfer of microorganisms are still unclear.Based on this,this study systematically studied the characteristics of biochar in enhancing volatile fatty acid microbial extracellular electron transfer metabolism by constructing a Bio-Electrochemical System(BES),using typical VFA as electron donor,and analyzed its characteristics.potential mechanism of action.The main research contents and conclusions are as follows:(1)The enhanced effect of suspended biochar on the EET metabolic pathway during VFA oxidation was investigated.The results show that when VFA is used as the electron donor,adding 10 g/L of biochar particles to the BES anode chamber can increase the maximum current density of the system by 83.3%-220.0%.Meanwhile,suspended biochar reduced anode resistance by 66.6%～16.6%.Suspended biochar promoted the enrichment of typical electrogenic bacteria(family Geobacteraceae)on the anode biofilm surface compared with the control group.At the same time,the type of electron donor has a certain influence on the type of dominant electrogenic bacteria in the anode biofilm.When acetic acid was used as the electron donor,the dominant genus was Desulfuromonas,and its relative abundance increased from 1.1%in the control group to25.0%in the biochar group.When propionic acid was the electron donor,Geobacter was the dominant genus,and its relative abundance increased from 4.6%in the control group to 31.7%in the biochar group.The analysis of the electrochemical characteristics of the system found that the suspended biochar hardly improved the conductivity of the sludge in the anode chamber,but the biochar had a strong redox electron transfer ability,which made it possible to act as an electron transfer mediator and accelerate the production of electricity-producing microorganisms.enrichment.Meanwhile,the biochar has abundant porous structure,which may provide a place for electrogenic bacteria to attach and grow at the initial stage of the reaction,and promote its enrichment to the anode surface,thereby enhancing the VFA oxidation process based on the EET pathway.(2)The efficiency of biochar anode-mediated EET oxidation of VFA was investigated.The results show that compared with the addition of suspended biochar to the BES anode,the introduction of biochar anode can increase the maximum current density by 300%,and the Coulombic efficiency can be increased from 19.4%to 42.2%.The biochar anode transfers the electrons released by substrate oxidation to the anode more efficiently for system energy production rather than self-growth.Cyclic voltammetry analysis found that the peak-to-peak currents of the oxidation peak and reduction peak of the suspended biochar group were proportional to the 1/2 square of the scan rate(R~2=0.94/0.96),and the electrode reaction was controlled by diffusion.The redox peak-to-peak current of the biochar anode group is proportional to the scan rate(R~2=0.93/0.95),indicating that the electrode reaction is kinetically controlled and the system electron transfer efficiency is higher.Observing the microscopic morphology of the electrode,it was found that the microorganisms on the biochar anode were more likely to be enriched on the biochar particles rather than the carbon felt surface.Compared with suspended biochar,the biochar anode optimizes the electrochemical properties of the anode,enhances the enrichment of microorganisms on the anode surface,and achieves efficient mediation of EET to accelerate VFA oxidation.(3)The timeliness and mechanism of biochar anode-mediated EET oxidation of VFA were studied.After the first cycle of operation,the biochar anode was split into two parts:biochar particles and carbon felt,and the biochar particles were reloaded into the biochar anode,and the seed sludge was excluded to run the second cycle.The results show that it is difficult for carbon felt to oxidize the substrate to release electrons under the condition of no seed sludge,and the current density is maintained at about 0.03 A/m2.The biochar anode achieves rapid startup,the highest current density reaches 0.96 A/m2,the actual power production can be increased from 70C of carbon felt to 1578C,and the acetic acid degradation rate is increased from 20%to 86.2%,indicating that in the biochar anode,the main Biochar particles enhanced EET to accelerate VFA oxidation while carbon felt only played the role of collecting current.Under the same conditions,the actual electricity production of the biochar anode in the second cycle was 323C higher than that in the first cycle,and the start-up cycle was shorter,indicating that the biochar anode could achieve continuous EET-mediated oxidation of VFA.At the same time,microbial community analysis found that the biochar anode was metabolized by Hydrogenophaga and G.sulfurreducens,which could continuously and efficiently mediate the EET-accelerated VFA oxidation process. |
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