Microbial Fuel Cell For Gaseous BTEX Degradation:Mechanism And Power Generation

Volatile organic pollutants（VOCs）are one of the important precursors for the combined air pollution.Among various VOCs removal technologies,the bioreactors have attracted more and more attentions for their advantages of environment friendly,non-secondary polluted,low-cost,safe and so on.However,the degradation efficiency of bioreactors for O₂-free waste gas is still low due to the limitation of the electron transfer rate and the types of the end electron acceptor.In the microbial fuel cell（MFC）,the produced electrons could be transferred to the electrode surface in situ,improving the electron transfer rate,thereby greatly enhancing the degradation of VOCs while converting the chemical energy into electrical energy.At present,the researches on VOCs treating by MFC are rare and most of them only focus on the removal efficiency,more in-depth mechanism researches for microbial and electrochemical characteristics are still insufficient.Therefore,a continuous aeration dual-chambered MFC was constructed in this work,the dynamic changes of microbial community,the spatial distribution of the biofilm and the electron transfer pathway under different pollutant conditions were revealed by examining the electrochemical characteristics and degradation performance in the individual and mixed BTEX powered MFC,and,the regulation mechanisms for improving BTEX degradation in the MFC were further clarified.The feasibility of gaseous VOCs degradation in the MFC was confirmed.The results indicated that the toluene removal efficiency on the MFC was comparable to the traditional biofilter even though the residence time was reduced by half.The closed-circuit MFC exhibited 1.4～3.5 times higher toluene removal efficiency compared with the open-circuit MFC,indicating that the electrons transferring between electrodes had enhanced the degradation of toluene in MFC.A modified Monod model was constructed to predict the power generation of the MFC at various toluene concentrations.The microbial community analyzing indicated that the toluene degraders of Chryseobacterium sp.and Zoogloea sp.（accounting for 58%）were dominant in the toluene powered MFC.However,the exoelectrogens of Arcobacter sp.and Geobacter sp.were less than 7%,thus,the coulomb efficiency was as low as 1.6%.The effects of different anode potentials（-0.52 V,-0.35 V,-0.16 V,and 0.17 V vs.Ag/Ag Cl）on the power generation and the VOCs degradation mechanism were investigated.It was indicated that the microbial community was almost unaffected by the applied anodic potentials,the significantly increasement of power generation and removal efficiency may be achieved by shifting its own electron transfer pathway.Assuming electrons were transferred through the Dms ABC complex when the biofilms were tuned by potentials of-0.52 V and-0.35 V,and the electrons were transferred through Mtr ABC complex or the NADH-quinone-hydroquinone respectively when the biofilms were tuned by potentials of-0.16 V or 0.17 V.The toluene removal efficiencies of biofilm tuned by different anode potentials were all improved,the one turned by the potential of-0.35 V showed the best performance because it was closer to the normal growth environment of exoelectrogens（-0.3～-0.4 V）.The biofilm tuned by anodic potential of-0.35 V exhibited a removal efficiency of 99%～78%（500～2000mg/m³）,the internal resistance was reduced by 24%,and the maximum power density was as higher as 8.97 mw/m².The coulomb efficiency of the biofilm turned by-0.35V was 2.06%,which was 29%higher compared with the control,and was the highest among all the gaseous toluene powered MFC among literatures.The internal relationship between microbial community regulation and MFC performance was revealed by adding the specific BTEX degrader and exoelectrogens.Experimental results illustrated that the removal efficiency of the MFC was not improved obviously by adding the specific o-xylene degrader Rhodococcus sp.ZJUT312.In contrast,the exoelectrogen Shewanella oneidensis MR-1 introduced the production of nanowires between microorganisms,which was conductive to construct a uniform biofilm.The present of S.oneidensis MR-1 significantly increased the proportion of o-xylene degrader Hydrogenophaga sp.and Sedimentibacter sp.,hence the o-xylene removal efficiencies with concentrations of 300～2000 mg/m³ were improved by 1.4～1.7 times.The electron transfer rate between cells-electrode was improved by the conductive nanowires,so,the maximum power density was increased by 77%,and the coulomb efficiency was increased from 2.18%to 3.56%.The interaction and degradation mechanism of gaseous BTEX in the MFC were clarified.It was clearly found that the BTEX can be effectively removed when they were supplied in the form of individual,dual,and ternary mixture.The removal efficiencies of individual toluene,ethylbenzene and o-xylene were decreased by1%～8%,16%～40%and 9%～33%respectively when they were mixture.The presence of toluene and o-xylene exhibited an adverse influence on the degradation of other mixed components.The inhabitation of toluene to other pollutants may be caused by its higher cell membrane affinity to Zoogloea sp.,Pseudomonas sp.and Stenotrophomonas sp..The biological toxicity of o-xylene caused deaths for the exoelectrogens Hydrogenophaga sp.,Chryseobacterium sp.and Zoogloea sp.,so,the maximum power density of the MFC was as low as 2.26 m W/m² when individual o-xylene was supplied as fuel.But,with the assistance of toluene or ethylbenzene,the maximum power density of MFC was increased by 39%～166%via producing a large number of nanowires due to the enhancement of the direct electron transfer between cells or between cells and anode.Changing the component of individual gaseous BTEX to the mixture,the electron transfer pathway of MFC may be adjusted from the between a-type and d-type cytochrome to among the a-type cytochrome,quinone,and d-type cytochrome.In summary,this work confirmed that the constructed gaseous BTEX could be effectively degraded by the MFC while recovering part of the electrical energy.Modulation of anode potential and microbial community were two important methods to enhance the performance of removal efficiency and power generation of MFC.The generated electrons were transferred similarly through membrane-bound cytochrome protein,NADH,hydroquinone,menaquinone and so on whether the BTEX were introduced individually or mixed.Therefore,the enhancement methods of setting the electrode potential and adding specific microorganisms during individual gaseous BTEX removal process are also available to the mixed gaseous BTEX.