The Study Of Congo Red Decolorization By Microbial Fuel Cell With Redox Mediator Modified Anode And Membrane-free Microbial Electrolysis Cell With Bio-cathode

Azo dye wastewater has the characteristics of high color,high COD,large amount,complex composition,poor biodegradability and environmental damage.It is recognized as one of the most recalcitrant industrial wastewater at home and abroad.In previous studies,our group found that the microbial fuel cell could effectively decolor azo dyes in its anode.However,there is still a long distance from the practical application of the bioelectrochemical system,for which the efficiency of electrogenesis and pollutant treatment must be further improved.Meanwhile,it is aslo necessary to reduce the construction cost and operating cost of the reactor.In this study,we aimed to improve BESs,a immobilized redox mediator modified anode microbial fuel cell and a membrane-free bio-cathodic microbial electrolytic cell were constructed.The electricity generation and azo dye decolorization was enhanced by the improvement of electrode and configuration,and the mechanism of strengthening system performance was analyzed by combining to experimental data.It provided data support and theoretical basis for BES treatment of azo dyes,and paved the way for BES to practical applications.Although the electrogenesis and decolorization performance can be effectively improved by adding redox mediator in MFC azo dye decolorization process,redox mediators are lost in vain when replacing substrates for batch run.To address this issue,redox mediator modified anodes are prepared by electrodepositing riboflavin?RF?and humic acid?HA?on the surface of graphite felt,and air-cathode single-chamber MFCs with different modified anodes?0.5C RF,0.5C HA,1.25 C RF and 2.5C HA?are built to decolor Congo red and generate electricity.Compared to the bare anode MFC,MFC with 0.5C RF,0.5C HA,1.25 C RF and 2.5C HA modified anode exhibite excellent electrocatalysis activity,EIS showed that the charge transfer impedances of 0.5C RF and 0.5C HA anode MFC were 425.3? and 323?,respectively.When the mediator concentration was increased to 1.25 C RF and 2.5C HA,the electron transfer impedances were further reduced to 258.5? and 155.5?,much lower than control group?452??.By analyzing and calculating power density and polarization curves showed 31%,34%,44% and 49% decrease in internal resistance along with 20%,21%,40% and 66% increase in maximum power density.MFCs with 2.5C HA and 1.25 C RF modified anode result in high decolorization efficiency of Congo red to 86% and 75% in 16 h,which are higher than that of the bare controlled group.There are significant redox mediator crystals on the modified anodes,and bacterial colonies on the anode surface after MFC running.The redox mediator crystals on the modified anodes can accelerate electron transfer,which benefit both bioelectricity generation and decolorization.A membrane-free microbial electrolysis cells?MFMEC?with bio-cathode running in batch model was constructed to decolor azo dye Congo red.The decolorization performance of MFMEC was examined by switching the external voltage from 0 V to 0.9 V.The role of microorganism in decolorization process was judged by comparing MFMEC with membrane-free electrolysis cell?MFEC?.The electrochemical characteristics of MFMEC and its relation with decolorization were analyzed by measuring cathode potential,EIS and current change.The results showed that MFMEC with bio-cathode will acquire lower cathode potential than MFEC.The charge transfer resistance of MFMEC was 5.21 ? which was lower than MEC?43.59??.The decolorization efficiency of MFMEC with voltages of 0.3 V,0.6 V and 0.9 V were nearly identical and stable at 87.9%,85.1%,and 86.7% respectively in 24 h,and it had a remarkable increasing in circle 2 and 3,which was 25% higher than that in circle 1 in 12 h,then had declined since circle 4 in batch tests without renewal solution.The decolorization efficiency was only 69.6% when no voltage was applied,and the decolorization rate of MFEC under the same conditions were all less than 10%.In MEC,the decolorization rate of the cathode fed with glucose was 6.67 mg·L^-1·h^-1,which was 1.2-fold than that of the cathode fed with NaHCO₃(5.56 mg·L^-1·h^-1)and 1.3-fold than that of the anode fed with glucose(5.17 mg·L^-1·h^-1).The main degradation products were benzidines which was produced by azo bond cleavage.More CH4 were produced with 0.9 V as a side reaction that restricted further increase of decolorization rate.Both action of co-substrates and accepting electron from cathode were the main decolorization mechanism of MFMEC.The results proved that the bio-cathodes can accelerate the electron transfer and take part of the role of decolorization.By removing the membrane,the electrochemical performance of the reactor had been improved,and it was conducive to the circulation of solution in anode and cathode chambers.