Understanding Biochemical Interactions and Optimization of Electrochemical Activities in Microbial Fuel Cell System for Dichlorophenol Degradation and Electricity Generation using Microbial Consortia

Abstract

Toxic pollutants such as phenols and dyes in industrial wastewater have raised increasing environmental and human health concerns in many industrialized countries around the world. There is an ongoing need to develop sustainable and cost-effective technologies to remove these pollutants. Bioelectrochemical technology such as microbial fuel cell (MFC) system has been approved as promising and sustainable treatment process for removing of these organic toxic pollutants, while generating electricity by exoelectrogenic bacteria. The exceptional MFC ability to degrade chlorophenol as one of the recalcitrant pollutants in industrial wastewater has been explored in this study. Three functional bacteria: pure culture Bacillus subtilis, and two mixed microbial consortia derived from domestic and industrial petrochemical wastewaters have been utilized for electricity generation and 2,4-dichlorophenol (2,4-DCP) degradation in a double chamber MFC system. The selection of microorganisms is based on their ability to degrade phenols and its derivatives so as to discover good exoelectrogenic bacteria to drive MFC system for degradation of 2,4-DCP and production of electricity. The industrial petrochemical wastewater as a highly phenolic-contaminated wastewater is expected to provide its mixed consortium better acclimatization in 2,4-DCP-mediated MFC environment as compared to typical mixed consortium from domestic wastewater. Bacillus subtilis has been approved to show its great ability to generate maximum current density of 64 mA/m² in persulfate-based catholyte MFC system while degrading up to 60% of 2,4-DCP. Chemical properties of catholytes, for instance, oxidizing and buffering abilities, could improve the MFC performance through well controlled pH and electron transfer mechanism. The experimental results revealed that low-cost and low-toxicity catholytes, such as potassium persulfate, M9 and phosphate buffer could amplify the electricity generation with simultaneous 2,4-DCP degradation in double chamber MFC system. Like B. subtilis, mixed consortia from both domestic and industrial wastewaters have demonstrated high performance in electricity generation and 2,4-DCP degradation using Pt/Ti electrode in MFC systems. The important bacteria in domestic mixed consortium for 2,4-DCP degradation have been identified as Arcobacter and Cloacibacterium which showed positive response towards the toxic pollutants in anodic MFC. Industrial mixed consortium in which Bacillus dominated the cultures, performed well in generating 156 current density, with 41% phenolic degradation as compared to domestic consortium with 123 mA/m² and 62% phenolic degradation. This study proved that Bacillus sp. from petrochemical wastewater could have high adaptation in chlorophenol containing medium through its high current generation profiles in MFC despite of its relatively low 2,4-DCP degradation capability. The performance of both mixed consortia was further investigated for its growth kinetics and 2,4-DCP biodegradation pathways in double chamber MFC systems. Domestic consortia biofilm was found to have higher phenol degradation ability with a high growth constant of 27 mg/L and specific biodegradation rate of 0.32 mg/L/h. Although industrial consortia yielded lower apparent kinetic parameters, its growth profile at low 2,4-DCP concentration implied its excellent bacterial acclimatization with higher voltage outputs. The toxicology analysis of the final metabolites of 3-oxoadipate and acetate produced by industrial and domestic consortia suggests that both consortia could degrade 2,4-DCP into very much less hazardous and simpler compounds in the MFC system. Finally, an optimization study on MFC performance by B. subtilis was investigated using statistical central composite design (CCD) of response surface methodology (RSM). The optimized parameters, including anode pH, cathode pH and inoculum size, were found to have good interaction to generate optimal current density of 106 mA/m² with over 70% 2,4-DCP degradation rate through the developed quadratic models. Analysis of variance revealed that the optimum current density could be achieved at anode pH 7.5, cathode pH 6.3-6.6 and 21 – 28% inoculum size. Only inoculum size cathode pH interaction appeared to be significant for phenolic degradation response where the optimum predicted phenolic degradation could be attained at cathode pH 7.6 and 29.6 % inoculum size. This project therefore has contributed several valuable fundamental and biosystem outcomes in terms of systematic biochemistry, microbiology electrochemistry and optimization in MFC system. Pure culture Bacillus subtilis, Arcobacter- and Bacillus- dominated domestic and industrial microbial consortia have been discovered their special capabilities in generating electricity and 2,4-DCP degradation, thus could be promising bacterial cultures in forthcoming MFC studies.