Enhanced Degradation Of Recalcitrant Contaminants In Microbial Fuel Cells

Energy shortage and environment pollution are two of the largest challenges around the world. Microbial fuel cells (MFCs), a recently developed technology which simultaneously generate electricity and reduce pollution, might provide an alternative solution to these problems. In this work, we studied the enhanced degradation of recalcitrant contaminants, especially azo dyes and nitroaromatic compounds, in MFCs. Also, cathode modification or biocathode was introduced to improve the degradation efficiency.Firstly, methyl orange (MO), a typical azo dye, was decolorized at the cathode of MFCs. With in situ utilization of the electrons derived from the anaerobic conversion of organics, MO was steadily reduced in the abiotic cathode. The MO reduction could be described by a pseudo first-order kinetic model with a rate constant of1.29day-1. Electrochemical impedance spectroscopic analysis shows the cathode had a high polarization resistance, which could decrease the reaction rate and limit the electron transfer. To enhance the electron transfer from cathode to MO for its reduction, the cathode was modified with a redox mediator, thionine. Then the polarization resistance significantly decreased by over50%and the MO decolorization efficiency increased by about20%. These results provide useful information about the key factors limiting the azo dye reduction in the MFCs and measures to improve such a process. However, the reduced products of azo dyes-aromatic amines, are reported to be toxic and carcinogenic, and should be further treated.Secondly, a novel approach was developed to completely mineralize azobenzene, a typical azo dye with simple structure, through coupling cathodic reduction and anodic oxidation processes in MFCs. Azobenzene was first electrochemically reduced at the abiotic cathode of MFCs to aniline, which was then further oxidized and degraded by the microorganisms at the anodic chamber of the same MFCs. Thus, the electrons present in azobenzene were fully utilized and no supply of external power or additional organic carbon source was needed in this process. Meanwhile, electricity was simultaneously produced. Compared with open circuit control, degradation rate of aniline was two-times higher. A characterization of the anode by cyclic voltammetry, electrochemical impedance spectroscopy and scanning electron microscopy indicate that both the electrochemically active microorganisms on the anode and the redox mediators contributed to the electron transfer in enhanced aniline oxidation. Furthermore, the aniline oxidation was favored by a higher applied anodic potential. This method provides a promising energy-efficient way to treat wastewater containing electron-withdrawing substances and may offer valuable reference for bioremediation of these pollutants.Lastly, biocathode was adopted to enhance the reduction rate of nitrobezene and reduced product was further degraded in situ the biocathode of MFCs. Nitrobenzene, a representative nitroaromtaic pollutant, was first reduced to aniline and then oxidized to inorganic carbons in the biocathode of MFCs with aeration. At initial concentration of40mg/1, over90%nitrobenzene was reduced to aniline in4days while97%aniline was further in situ degraded with aeration within1day. Electricity was generated in the whole reduction and oxidation processes with nitrobenzene and oxygen as electron acceptor respectively. Compared with abiotic cathode, biocathode obviously enhanced the reduction of nitrobenzene through catalysis by microorganisms, which greatly decreased the charge transfer resistance and then promoted electron transfer from biocathode to nitrobenzene. This strategy might provide another novel approach not only for treatment of industry wastewater containing nitrobenzene or other electron-withdrawing structure contaminants, but also for in situ bioelectrochemical remediation of groundwater and sediment contaminated by these pollutants. Energy shortage and environment pollution are two of the largest challenges around the world. Microbial fuel cells (MFCs), a recently developed technology which simultaneously generate electricity and reduce pollution, might provide an alternative solution to these problems. In this work, we studied the enhanced degradation of recalcitrant contaminants, especially azo dyes and nitroaromatic compounds, in MFCs. Also, cathode modification or biocathode was introduced to improve the degradation efficiency.Firstly, methyl orange (MO), a typical azo dye, was decolorized at the cathode of MFCs. With in situ utilization of the electrons derived from the anaerobic conversion of organics, MO was steadily reduced in the abiotic cathode. The MO reduction could be described by a pseudo first-order kinetic model with a rate constant of1.29day-1. Electrochemical impedance spectroscopic analysis shows the cathode had a high polarization resistance, which could decrease the reaction rate and limit the electron transfer. To enhance the electron transfer from cathode to MO for its reduction, the cathode was modified with a redox mediator, thionine. Then the polarization resistance significantly decreased by over50%and the MO decolorization efficiency increased by about20%. These results provide useful information about the key factors limiting the azo dye reduction in the MFCs and measures to improve such a process. However, the reduced products of azo dyes-aromatic amines, are reported to be toxic and carcinogenic, and should be further treated. Secondly, a novel approach was developed to completely mineralize azobenzene, a typical azo dye with simple structure, through coupling cathodic reduction and anodic oxidation processes in MFCs. Azobenzene was first electrochemically reduced at the abiotic cathode of MFCs to aniline, which was then further oxidized and degraded by the microorganisms at the anodic chamber of the same MFCs. Thus, the electrons present in azobenzene were fully utilized and no supply of external power or additional organic carbon source was needed in this process. Meanwhile, electricity was simultaneously produced. Compared with open circuit control, degradation rate of aniline was two-times higher. A characterization of the anode by cyclic voltammetry, electrochemical impedance spectroscopy and scanning electron microscopy indicate that both the electrochemically active microorganisms on the anode and the redox mediators contributed to the electron transfer in enhanced aniline oxidation. Furthermore, the aniline oxidation was favored by a higher applied anodic potential. This method provides a promising energy-efficient way to treat wastewater containing electron-withdrawing substances and may offer valuable reference for bioremediation of these pollutants.Lastly, biocathode was adopted to enhance the reduction rate of nitrobezene and reduced product was further degraded in situ the biocathode of MFCs. Nitrobenzene, a representative nitroaromtaic pollutant, was first reduced to aniline and then oxidized to inorganic carbons in the biocathode of MFCs with aeration. At initial concentration of40mg/1, over90%nitrobenzene was reduced to aniline in4days while97%aniline was further in situ degraded with aeration within1day. Electricity was generated in the whole reduction and oxidation processes with nitrobenzene and oxygen as electron acceptor respectively. Compared with abiotic cathode, biocathode obviously enhanced the reduction of nitrobenzene through catalysis by microorganisms, which greatly decreased the charge transfer resistance and then promoted electron transfer from biocathode to nitrobenzene. This strategy might provide another novel approach not only for treatment of industry wastewater containing nitrobenzene or other electron-withdrawing structure contaminants, but also for in situ bioelectrochemical remediation of groundwater and sediment contaminated by these pollutants.