Influence Mechanism Of Fe(?) On The Electricity Generation Of Microbial Fuel Cell

Sediment microbial fuel cell(SMFC)with the anode electrode buried into the anaerobic sediments is an emerging processing technology.And the way to remove the pollutants and recycle electrical energy in a SMFC is using the microbial metabolism on the anode surface and in the sediment.Trace metal elements play an important role in the process of SMFC operation.The lack of necessary trace metal elements can lead to the decrease of microbial activity,which affects the stability of the whole system and the organic matter removal efficiency.It has been found that the electroactive microbes are mostly iron-reducing bacteria,and the iron concentration in the system will affect the abundance of iron-reducing bacteria,thus affecting the performance of the SMFC.However,in the SMFC system for the treatment of residual sludge,iron on the relationship between the sludge hydrolysis and production performance remains to be studied.In the process of SMFC electron transfer,cytochrome c in electroactive microorganisms plays a key role.The activity center of cytochrome c is iron.The addition of Fe(?)can cause the change of cytochrome c.But the relationship between iron and cytochrome c content and activity remains to be explored.In this study,Fe(?)was the main research object.The operating characteristics of the SMFC system with Fe(?)were systematically explored.And in order to further improve the electricity production characteristics of the MFC system with Fe(?),the feasibility of the static magnetic field was applied.The main research contents were as follows:(1)The effect of Fe(?)on the electrical performance of the SMFC system has been investigated.The addition of Fe(?)could improved the production performance of SMFC.When Fe(?)concentration was below 500 mg/L,the electricity production performance of SMFC system increased with the increase of Fe(?)concentration.The SMFC system achieved the best production performance at the Fe(?)concentration of 500 mg/L.At this concentration,the electrochemical activity of the electrode surface was the strongest and the internal resistance was the lowest.The maximum output power density,the maximum output voltage,the coulomb efficiency and the apparent internal resistance were 597.2 mW/m2,509.7 mV,37.7% and 101.1 ?,respectively.(2)The effects of Fe(?)on the anode sludge properties has been investigated.The removal rate of VSS and TSS of anode sludge was first increased then decreased with the increase of Fe(?)concentration.The removal rates of VSS and TSS were the highest when the Fe(?)concentration was 500 mg/L,which were 37.2% and 26.4%,respectively.During one cycle of the operation,the SCOD,the dissolved polysaccharides and proteins in the supernatant of the sludge first increased and then decreased.With the increase of Fe(?)concentration,the dehydrogenase activity of the anode sludge was increased and then decreased,while the methane production activity was continuously decreased.When the concentration of Fe(?)was 500 mg/L,the activity of dehydrogenase was the highest,reaching 2569.4 ?gTF/gVSS.When Fe(?)was added to the SMFC system,Fe(?)was combined with the substances in the excess sludge rapidly,so that its bioavailability was reduced.The contents of the carbonate-bound,organic-bound and sulfate-bound iron,which were not easily used by the microorganisms,were almost unchanged before and after the operation.The exchangeable and adsorbed states were mostly converted by microorganisms into the remaining state.The Fe(?)concentration of 500 mg/L could meet the needs of microbial growth,and the concentration of the initial bio-availability was 100 mg/L.(3)The influence mechanisms of Fe(?)on the anode biofilm of SMFC and the relationship between iron and cytochrome c have been studied.When the Fe(?)concentration is 500 mg/L,the biomass of the anode biofilm of the SMFC system was about 30% higher than that of the control group without Fe(?),while the content of extracellular polymer is decreased by 14%.The addition of Fe(?)could increased the abundance of Proteobacteria on the anode biofilm.When the Fe(?)concentration was 500 mg/L,the abundance of Proteobacteria in the anode biofilm increased from 47% to 80% compared with the control.The addition of Fe(?)did not altered the type of anodic biofilm electroactive species,but increased the electrochemical activity of the anode surface.The electron transfer efficiency of microbial cytochrome c was increased by 2.4 times,and the electron transport efficiency per unit mass biofilm was increased by 1.1 times with 500 mg/L Fe(?)concentration compared with the control.However,the number of cytochrome c transfer per unit mass decreased due to the lack of the substrate.Purification of cytochrome c from the anode biofilm showed that 1 mol of cytochrome c contained 5 mol of iron.Iron in the anode could also formed Fe(II)/Fe(?)electrons to improve the electron transfer rate,while iron in the cathode would been as a temporary capacitor to reduce the cathodic polarization resistance and improved the cathode performance.And the optimum Fe(?)concentration for cathode was 500 mg/L.(4)The feasibility of applying static magnetic field(SMF)to further promote the system performance under the optimal Fe(?)concentration has been verified.The system performances,the changes of the anode biofilm and the activities of the cytochrome c under different SMF intensities were studied.Appling SMF could further improved the system performance and effectively shorten the system startup time.The operating characteristics of the MFC system were enhanced with the increase of the low-intensity SMF(?220 mT).Under the condition of high-intensity SMF(?370 mT),the SMF would cause irreversible damage to the system and the performance would decreased sharply.When the SMF intensity was 220 mT,the MFC system exhibited the best performance,the output peak voltage,the highest power density and the apparent internal resistance of the MFC were 756.1 mV?1.5 W/m2 and 66.5 ?,respectively.Under low-intensity SMF,the biomass,extracellular polymer content and the dehydrogenase activity in the biofilm increased with the increase of the SMF intensity.When the SMF intensity was 220 mT,the biomass,extracellular polymer content and dehydrogenase activity in biofilms were hightest,reaching 60.31 ?gP/cm3,0.81 ?g/?gP and 2.45 ?gTF/(?gP·h),respectively.The addition of low-intensity SMF could increase the efficiency of electrons transfer from cytochrome c to the anode.Under the condition of 220 mT,the content of cytochrome c and the electron transfer rate were increased by 36% and 47%,respectively,compared with the control group without SMF.(5)In order to further analyze the influence mechanism of SMF on the performance of MFC,the flow of carbon and nitrogen in the system under different SMF intensities have been analyzed and a mathematical model which could been used to describe the operation process of MFC has been established.The results shown that the removal of organic matter,ammonia nitrogen and the efficiency of the coulombic efficiency increased with the increase of low-intensity SMF.When the SMF intensity was 220 mT,the COD removal rate,total nitrogen removal rate,ammonia nitrogen removal rate and the coulombic efficiency were the highest,reaching 67.4%,63.6%,13.6% and 60.5%,respectively.And nitrogen balance analysis revealed that about 2% of the ammonia in the system was removed by volatilization,and the remaining was removed by biochemical action.Under the applied SMF,more ammonia nitrogen in the MFC system was converted to nitrate,and the nitrification of the system was more thorough.Through the analysis of carbon balance,the process of production of methane in the magnetic field would has been inhibited,more carbon flow would flow to the electricity generation process.By establishing the mathematical model,we found that the methanogenic bacteria in the MFC system decreased with the increase of the SMF strength,and the concentration of the electroactive microbes increased first and then decreased with the increase of the SMF intensity.The SMF almost had no influence to the concentration of the sulfate-reducing bacteria.The effect of anode polarization potential,substrate utilization and cell thickness increased first and then decreased with the increase of SMF intensity,while the change trend of activation potential loss was opposite.