Study of Pb(Ⅱ) Removal of Microbial Electrolysis Cells Derived by Cr(Ⅵ)-reduced Microbial Fuel Cells

Author:Li Meng

Supervisor:zhou shao qi


Degree Year:2019





Pollution of heavy metal wastewater and energy crisis are important problems in the rapid development of society and economy.MFCS are emerging bio-electrochemical technology,which can oxidize organic matters by electro-active bacteria in the anode electrode and reduce heavy metal in the cathode,and then achieve bioelectricity.Based on the development of MFCS,in order to enhance the ability of MFCS to remove heavy metal and generate electricity,effects of different environmental conditions,different cathodic catalysts and electronic intermediaries on the bioelectricity generation and heavy metals reduction by MFCS are studied.Finally,the MFCS-MECS coupling system is used to synchronously remove heavy metal Cr(VI)and Pb(II)by using Cr(VI)reduced-MFCS to drive MECS in-situ.The main study results are as follows:The catholyte pH,cathode materials and concentration of metal ions played important roles in reducing of Cr(VI)and generating bioelectricity simultaneously.When the pH of cathode solution was 2.0,MFCS achieved the best electrochemical performance,and too high or too low pH of cathode solution could decrease the ability of MFCS to reduce Cr(VI)and produce electricity by affecting the activity of electro-active bacteria.Compared with carbon brush and carbon felt electrode.MFCS with carbon cloth cathode produced the maximum Cr(VI)removal and the highest power production due to low impedance.Besides,with the increase of initial metal ion concentration,the electrochemical reaction rate for Cr(VI)reduction and output power in MFCS increased gradually.In addition,MFCS can completely reduce Cr(VI)within 72 h during continuous operation.However,Cr(VI)was mainly attached to the electrode surface in the form of Cr(OH)3 after reduction,and the conductivity of the cathode would be affected and the output power of MFCS would be reduced.The electrochemical performance of MFCS cathode modified with poly-aniline,α-Fe2O3and complex was studied and compared with that of MFCS cathode not modified with catalyst.These results showed that the power density of the MFCS modified with catalyst was 1502.78mW/m2,which was higher than those of MFCS modified with poly-aniline andα-Fe2O3.And the power density was much higher than that of the MFCS unmodified with catalyst.The modification ofα-Fe2O3/PANI resulted in the formation of positively charged nitrogen and imide on the electrode surface,and the network structure with more electrochemical active area on the surface of the carbon cloth electrode appeared.The interfacial contact between the cathode surface and electrolyte was reduced,and the electron transfer was enhanced,and the diffusion length of ions in solution was also reduced.Hence,the kinetic reaction of Cr(VI)was enhanced,and the reduction of Cr(VI)was accelerated.Use of electronic-shuttle mediator Cu(II)can improve the performance of MFCS Cr(VI)reduction and simultaneous bio-electricity generation.In MFCS without Cu(II),Cr(VI)was directly reduced to Cr(III)in MFCS.In the presence of Cu(II),Cu(II)was reduced to Cu(I)or Cu(0)in MFCS,and then the reduced Cu products further reacted with Cr(VI)to form Cr(III).The maximum power density of MFCS increased from 1009.42 mW/m2 to 1235.53 mW/m2with the increase of the concentration of electron-shuttle mediator,which was 1.18 times and1.34 times higher than that of MFCS without electron-shuttle mediator,respectively.With the increase of Cu(II)concentration from 10 mg/L to 50 mg/L,the reduction rate of Cr(VI)increased from 0.868 g?m-3?h-1 to 1.130 g?m-3?h-1.After consecutive 20-cycle operations,the reduction rate of Cr(VI)decreased by 11.5%due to the depostion of Cr(OH)3.It was also found that Cr(VI)and Cu(II)by electrochemical reduction in MFCS mainly existed in the form of Cr(OH)3 and Cu(0).A large number of Cr(III)precipitate and a small amount of Cu(0)precipitation were found on the MFCS cathode in the presence of Cr(VI)and Cu(II)in catholyte.Cu(II)used as the electron-shuttle mediator can decrease the ohmic internal resistance and charge transfer resistance.Besides,the electron-shuttle mediator can further reduce the diffusion resistance of the electrode by increasing the ion diffusion path.The synchronous removal of Cr(VI)and Pb(II)was realized by bioelectricity driven MECS generated by Cr(VI)reducing-MFCS.It was noted that the final Cr(VI)and Pb(II)existed in the reactor in the form of Cr(III)and Pb(0).Linear sweep voltammetry(LSV)analysis showed that the NF electrode with excellent electrochemical performance had low internal resistance,and the power density of 702.86 mW/m2 could be obtained when NF electrode was used in MFCS,which was 1.65 times and 1.47 times higher than that of MFCS with CC electrode(420.65 mW/m2)and MFCS with stainless steel mesh electrode(477.84mW/m2),respectively.The Cr(VI)reduction rate of MFCS with NF electrode was 1.72g?m-3?h-1,which was significantly higher than those of MFCS with CC electrode and SSM electrode.In addition,the removal rate and reduction rate of Pb(II)was increased from 98.22%to 99.92%and from 4.09 g?m-3?h-1 to 4.16 g?m-3?h-1,respectively with the increase of initial Pb(II)concentration from 50 mg/L to 150 mg/L,which was significantly higher than those of CC electrode and SSM electrode.When the initial concentration of Pb(II)was 150 mg/L,the maximum lead yield and the system efficiency were obtained by coupling system.Because the presence of a large amount of acetate in the solution,more hydroxide ions can be produced and the cathode pH value would be increased,further hindering the formation of lead metal and making the Pb(II)to exist in the form of complex(PbOH+)finally.Compared with using(CH3COO)2Pb as cathodic solution,Cr(VI)removal,Pb(II)removal,Cr(VI)reduction rate and Pb(II)reduction rate was increased by 19.22%,10.73%,0.402 g?m-3?h-1,and 0.402 g?m-3?h-1,respectively.For the long term operation of the coupled system,it was found that the coupling system with NF electrode had the minimum decrease of Pb(II)reduction rate.Consequently,NF electrode was more stable than the CC and SSM electrode.To sum up,it is theoretically feasible to reduce Cr(VI)by using MFCS or to reduce Cr(VI)and Pb(II)by MFCS-MECS coupling system,and the effective results can be obtained under laboratory conditions.Microbial electrochemical technology will play an important role in reducing of single heavy metal or several heavy metals in the future provided that low-cost electrode materials with excellent performance under suitable experimental conditions.