Characteristics And Mechanism Of Using Microbial Fuel Cell For Simultaneousazo Dye Degradation And Bioelectricity Generation

Sustainable energy production and wastewater treatment are a top priority in the developing global community. Microbial fuel cells (MFCs) can recover renewable energy from waste organic sources and facilitate energy production while simultaneously accomplishing wastewater treatment. It is becoming a rapid evolving field in abroad and be given much attention by researchers as a novel notion for wastewater treatment.Azo compounds constitute the largest group of synthetic dyes (over 80%) and are widespread used in the dye-manufacturing and dye-consuming industries. The discharge of azo dye-containing wastewater represents a serious environmental problem and a public health concern not only because of their intense color, but also because most of them and their breakdown products are toxic or mutagenic to life and resist to further biodegrade. The azo dye-containing wastewaters are among the most recalcitrant wastewaters due to its high content and unbiodegradable nature.The treatment of dye-containing wastewater still presents a technical challenge. Most physicochemical methods can removal dye efficiently but not feasible due to their expensive cost, limited versatility, sensitive to other wastewater constituents. Alternatively, biological treatment may present a less expensive and environment-friendly way to remove dyes from wastewater. The most logical method for removal of azo dyes in biological wastewater treatment systems is based on anaerobic treatment for the reductive cleavage of the azo linkages in the dye in combination with aerobic treatment for further degradation of the products from azo dye cleavage, which are aromatic amines. MFC technologies also involve in anaerobic, but they are very different from traditional anaerobic wastewater treatment systems. In most cases, the bacteria in anode must be grown in an anaerobic environment in order to produce higher power output. It is also an aerobic system, however, because oxygen is used at the cathode and the use of oxygen is not coupled to microbial respiration. Moreover,a novel MFC incorporating a recently developed biocathode was also devoploved. It can be explored for removal pollutants (inorganic or organic) from water phase due to its variety of terminal electron acceptors. For these novel systems, there are many research aspects that remain to be explored. High and direct energy recovery from various readily biodegradeable organic compounds is another benefit of MFC over traditional anaerobic process. MFCs may offer a new technique in enhancing degradation of azo dye while at the same time recoveringelectricity from a readily biodegradable organic carbon source in practical applications.In this study, an air-cathode single-chamber microbial fuel cell was constructed and its performance on simultaneously wastewater treatment and electricity generation was investigated and improved. The MFC with inproved performance was used in consecutive study for simultaneously decolorization of azo dye and production of electricity. Active brilliant red X-3B (ABRX3) and Congo red were selected as model azo dyes. The effects ofdifferent parameters on the decolorization and elceticity generation process were systemic studied. Detailed mechanistic involves in this process was also elucidated. Considering of the low mineralization efficiency of the decolorization products of ABRX3 in the anaerobicbioanode of MFC and re-colored decolorization products due to autoxidiaztion reaction after exposure to air, a novel aerobic biocathode was introduced for further treatment of the decolorization effluent of the ABRX3 from the anode of MFC coupled with electricity generation. The feasibility was demonstrated and underlying mechanism was clarified. The major conclusions are given below:The performance improvement of an air-cathode single-chamber MFC during confectionery wastewater treatment was firstly demonstrated by using a microfiltration membrane (MFM) on water-facing side of the cathode and multiple aerobic sludge (AES), anaerobic sludge (ANS), and wetland sediment (WLS) as anodic inoculums. Batch test results show that the MFC with an MFM resulted in an approximately two-fold increase in maximum power density compared to the MFC with a proton exchange membrane (PEM). The Coulombic efficiency increased from 4.17% to 5.16% in comparison with the membrane-less MFC, without a significant negative effect on power generation and internal resistor. Overall performance of the MFC was also improved by using multiple sludge inoculums in the anode. The MFC inoculated with ANS+WLS produced the greatest maximal power density of 373 mW/m2 with a substantially low internal resistor of 38 . Higher power density with a decreased internal resistor was also achieved in MFC inoculated with AES+ANS and AES+ANS+WLS in comparison with those inoculated with only one sludge. The MFCs inoculated with AES+ANS achieved the highest Coulombic efficiency. Over 92% COD was removed from confectionery wastewater in all tested MFCs, regardless of the membrane or inoculum used.Electricity generation from readily biodegradable organic substrates accompanied bydecolorization of ABRX3, a representative azo dye, was demonstrated and investigated using a microfiltration membrane air-cathode single-chamber MFC. Batch experiment results showed that accelerated decolorization of ABRX3 can be achieved in the MFC as compared to traditional anaerobic bioreactor using glucose as co-substrate. The ABRX3 with an initialconenctration of 300 mg/L was removed almost completely within 48 h. Biodegradation was the dominant mechanism of the ABRX3 decolorization other than the absorption by biomass. Reductive cleavage of–N N– bond resulted in the ABRX3 decolorization but the decolorization liquid of the ABRX3 (DL) from the anode compartment of MFC was soonautoxidized and was re-colored as dark brown color after exposed to air.The MFC can be repeatly used for simultaneously ABRX3 decolorization and electricity generation without performance deterioration. Stable voltage output of around 0.5 V was obsevered accompanied by ABRX3 decolorization. The bacterial consortia in the MFC were capable of utilizing ABRX3 as a sole metabolite to transfer electrons to the anode. However,the voltage developed remarkably lower than the MFCs fed with readily biodegradableorganic carbons, such as glucose.Effect of operation parameters on performance of the air-cathode single-chamber MFC for simultaneously ABRX3 decolorization and electricity generation was investigated. Electricity generation was not significantly affected by the ABRX3 at 300 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 not containing ABRX3. Glucose was the optimal co-substrate for ABRX3 decolorization while acetate was the worst one. Confectionery wastewater was also shown to be a good co-substrate for ABRX3 decolorization and a cheap fuel source for electricity generation in the MFC. Low resistor was more favorable for dye decolorization than high resistor. Suspended sludge should be retained in the MFC to achieve accelerated decolorization of ABRX3 and higher power output.The interaction of a representative azo dye - Congo red with anode was investigated in an air-cathode single-chamber MFC using glucose as co-substrate. The maximum voltage output of the MFC was not significantly affected during decolorization of 300 mg/L Congo red, but the time needed to reach the stable voltage plateau was prolonged, indicating that Congo red will compete electrons with anode and Congo red decolorization is prior to the electricity generation in the MFC. Five main degradation products of Congo red were detected in the anode solution and were identified as benzene and naphthalene derivatives by liquid chromatography– mass spectrometry (LC-MS). Decolorization of different concentration of Congo red has negligible effects on the Ohmic resistor (Rohm) of the anode, but the charge-transfer resistor (Rc) and the diffusion resistor (Rd) were significantly influenced. The Rc and Rd firstly decreased then increased with increasing of Congo red concentration, possibly due to the fact that Congo red can be served as electron shuttle for conveniently electrons transfer from bacteria to the anode at low concentration, but results in accelerated consumption of electrons at high concentration. The cathode impedance was totally not affected by the Congo red addition. Addition of Congo red did not result in any noticeable decrease in the peak catalytic current until Congo red concentration up to 900 mg/L. Longterm decolorization of Congo red resulted in the change in the catalytic active site and the bacterial cellular morphology of the anode biofilm.Denaturing gradient gel electrophoresis (DGGE) and 16S-rRNA gene analysis was performed to investigate the bacterial diversity in the MFC used and not used for Congo red decolorization. The results revealed that the anode biofilm of the two MFCs were all dominated by bacteria which were phylogenetically very closely related to Proteobacteria.AS the most representative electrochemically active bacteria, Geobacter-like species were found to be integral members of the bacterial community in the two MFCs. There are some unique bacterial species in the Congo red–fed MFC compared to that in the glucose-fed MFC, mainly including of Azospirillum and Methylobacterium-like species ( -Proteobacteria), sulfate reducing bacteria ( -Proteobacteria) and some uncultured bacterial species. These bacteria may responsible for the effectively Congo red decolorization or further degradation of the decolorization products in the anode of the MFC .A MFC incorporating a recently developed aerobic biocathode was able to further treat the liquid containing decolorization products of ABRX3, a respective azo dye, and also provides increased power production. Batch test results showed that 24.8% of COD was removed from the DL by the biocathode within 12 h. Metabolism-dependent biodegradation of aniline-like compounds might be mainly responsible for the decrease of overall COD. Glucose is not necessary in this process and contributes little to the COD removal of the DL. The similar COD removal rate observed under closed circuit condition (500 ?) and opened circuit condition indicated that the current had an insignificant effect on the degradation of the DL. Addition of the DL to the biocathode resulted in an almost 150% increase in the open cycle potential of the cathode accompanied by a 73% increase in stable voltage output from 0.33 V to 0.57 V and a 300% increase in maximum power density from 50.74 mW/m2 to 213.93 mW/m~2. Cyclic voltammetry indicated that the decolorization products of the ABRX3 contained in the DL play a role as redox mediator for facilitating electron transfer from the cathode to the oxygen. SEM images revealed that a thick, homogeneous bioflim was formed on the surface of the cathode. The bacteria were clustered in aggregates and produced a large number of nanowires-like long thin filaments that connected to different bacteria aggregates.