Performance And Mechanism Of Microbial Fuel Cell Based On Simultaneous Congo Red Decolorization And Bioelectricity Generation

Azo dyes represent the largest class of dyes applied in textile processing. In textile dyebaths, the degree of fixation of dyes to fabrics is never complete, resulting in dye-containing effluents. The removal of dyes from these effluents is desired, not only for aesthetic reasons, but also because many azo dyes and their breakdown products are toxic to aquatic life and mutagenic to humans. Different physical, chemical and biological techniques can be applied to remove dyes from wastewater. Each technique has its technical and economical limitations. Most physicochemical dye removal methods have drawbacks because they are expensive, have limited versatility, are greatly interfered by other wastewater constituents, and/or generate waste products that must be handled. Alternatively, biological treatment may present a relatively inexpensive and environment-friendly way to remove dyes from wastewater. Generally, high rate bacterial azo dyes decolorization is usually achieved under anaerobic condition. Microbial fuel cell (MFC) is a promising environmental technology for simultaneous wastewater treatment and energy recovery in 21st century. The anaerobic anode chamber of the MFC could be employed for high rate bacterial azo dyes decolorization. MFCs may offer a new technique in enhancing azo dye decolorization while simultaneously recovering electricity in practical applications.In this study, the effect of key operation parameters on the performance of the MFC for simultaneous Congo red decolorization and electricity generation was investigated. In addition, the interaction of Congo red decolorization with electricity generation and the charge transfer mechanism were explored and the performance of the MFC was optimized. The major conclusions are given below:The effects of membrane type, biofilm growth and enrichment procedure on the performance of an air-cathode single-chamber MFC used for simultaneous Congo red decolorization and power generation were firstly investigated. Batch test results showed that the MFC using an ultrafiltration membrane (UFM) with molecular cutoff weight of 1K (UFM-1K) produced the highest power density of 324 mW/m2 coupled with an enhanced coulombic efficiency compared to microfiltration membrane (MFM). The MFC with UMF-10K achieved the fastest decolorization rate (4.77 mg/L h), followed by MFM (3.61 mg/L h), UFM-5K (2.38 mg/L h), UFM-1K (2.02 mg/L h) and Proton exchange membrane (PEM) (1.72 mg/L h). Based on the consideration of both cost and performance, UFM-1K was the best one. Biofilm growth greatly reduced the anode polarization impedance and facilitated the kinetics of the electrochemical reactions so as to enhance extracellular electron transfer from bacteria to anode electrode and increase the power generation. Higher power density of 297 mW/m2 was observed on day 30 (a maturate bioflim) compared with the power density of 33.4 mW/m2 and 8.9 mW/m2 on day 5 and day 1, respectively. Two different enrichment procedures in which glucose and Congo red were added into the MFCs sequentially or simultaneously were tested. The results showed that the enrichment procedures have a negligible effect on the dye decolorization, but significantly affected the electricity generation. 16S rRNA sequencing analysis demonstrated a phylogenetic diversity in the communities of the anode biofilm and showed clear differences between the anode-attached populations in the MFCs with a different enrichment procedure.In the study of biocathode MFC used for simultaneous Congo red decolorization and electricity generation, the pH value and the concentration of Na+ and phosphate could be maintained in a relative stable level in both anode and cathode chamber by using UFM-1K as the separator. The performance of the MFC coupled with UFM-1K was a little better than that of the MFC coupled with PEM in terms of Congo red decolorization and electricity generation. A 400% increase in maximum power density was observed in a biocathode MFC as compared with the abiotic cathode MFC. The biocathode MFC completed 90% of Congo red decolorization within 30 h while the abiotic cathode MFC required 50 h to achieve the same decolorization efficiency. These results demonstrated that the biocathode MFC could increase the electricity generation and accelerate the Congo red decolorization. However, startup period of biocathode MFC was much longer than that of the abiotic cathode MFC.Effect of the anode operation parameters (including suspended sludge in the anode, anode surface area, concentration of the co-substrate, concentration of Congo red, resistor and adding electron redox mediator) on performance of the biocathode MFC for simultaneous Congo red decolorization and electricity generation was investigated. Batch test results showed that the suspended sludge in the anode could accelerate Congo red decolorization and increase electricity output. The Congo red decolorization increased with the anode surface area increased. However, the power density of the MFC increased firstly and then decreased. Thus, an appropriated anode surface area should be selected. The low concentration of the co-substrate (glucose) could significantly decrease the performance of the MFC in terms Congo red decolorization and electricity generation. The optimum concentration of co-substrate was 300 mg COD/L. The increased concentration of Congo red could decrease the decolorization rate. Electricity generation was not significantly affected by Congo red at 900 mg/L, while higher concentrations inhibited electricity generation due to accumulation of decolorization products. However, voltage can be recovered to the original level after replacement with anodic medium without Congo red. Thus, the preferable concentration of Congo red was not higher than 900 mg/L. Low resistor was more helpful for Congo red decolorization as compared with high resistor. The electron redox mediator in the anode was more favorable for accelerating Congo red decolorization. A thick and dense biofilm was obtained on the anode of a biocathode MFC and many spherical microorganisms on the anode surface formed chain-like colonies.Effect of the cathode operation parameters (including suspended sludge in the cathode, cathode aeration rate and adding Fe-Mn mixed solution) on performance of the biocathode MFC for simultaneous Congo red decolorization and electricity generation was also investigated. The suspended sludge in the cathode could improve the performance of the MFC in electricity generation, but had a negligible effect on the Congo red decolorization. The maximum voltage increased as the aeration rate was increased up to 100 ml/min. At the aeration rate of 200 ml /min, the maximum voltage was lower than that at 100 ml /min. In the meantime, Congo red decolorization rate decreased with the increasing of the cathode aeration rate. These results showed that excessive aeration was not favorable in a biocathode MFC used for simultaneous Congo red decolorization and electricity generation. A thick and dense biofilm was obtained on the cathode in the MFC without Fe-Mn mixed solution and many spherical microorganisms on the cathode surface formed chain-like colonies. However, a sparse biofilm was observed on the cathode in the MFC with Fe-Mn mixed solution and SEM images revealed that the microorganisms were wrapped by a lot of porous iron and manganese oxides, indicating that Fe2+ and Mn2+ were involed in the cathode reaction. Therefore, addition of the Fe-Mn mixed solution to the biocathode resulted in a significant decrease in startup period accompanied by a 40% increase in maximum voltage output from 0.25 V to 0.35 V and a 74% increase in maximum power density from 122 mW/m2 to 212 mW/m2,but had no effect on the Congo red decolorization.Denaturing gradient gel electrophoresis (DGGE) and 16S rRNA gene analysis were performed to explore the bacterial diversity in the anode and cathode of a aerobic biocathode MFC. The results revealed that the microbial communities in the anode biofilm of the aerobic biocathode MFC were dominate by fermentative Bacteroidetes and a lot of methanogenic bacteria and autotrophic denitrifying bacteria were obtained simultaneously. In addition, the observed sulfate-reducing bacteria which belonged to the phylumδ-Proteobacteria were responsible for the Congo red decolorization. There are some unique bacterial species in the cathode of the MFC with Fe-Mn mixed solution compared to that in the cathode of the MFC without Fe-Mn mixed solution, mainly including of Leptothrix discophora and Uncultured Chlorobi bacterium-like species.Congo red was decolorized in the MFC through two different pathways simultaneously. In one pathway, 4 electrons were assigned averagely to the two azo linkages in the Congo red at the same time. In another pathway, all of the 4 electrons were given to one of the azo linkages for complete cleavage. Benzidine was identified as the main decolorization products and can not be further mineralized in the anode.The reduction of Congo red would consume some electrons produced from co-substrate degradation, thus there is a competition between decolorizing microorganisms and electricity-producing bacteria. Congo red decolorization would inhibit electricity output whereas the presence of the anode in the MFC could accelerate the decolorization rate of Congo red.The target microorganisms, such as electricity-producing bacteria with high tolerability and highly efficient decolorization bacteria, could be obtained by regulating the factors which could affect the metabolism pathway of the microorganisms during the startup period of MFC. The performance of the MFC used for Congo red decolorization and electricity generation could be optimized by regulating the working conditions of the anode and the cathode simultaneously.