Optimization of Soil Microbial Fuel Cell for Sustainable Bioelectricity Generation and Bioremediation.

Abstract

The focus of energy strategies of all countries has shifted from traditional fossil fuels to environmentally friendly and renewable energy sources out of concern for environmental protection and sustainability. Microbial fuel cell (MFC) technology is one of the leading research topics as a potential renewable energy source due to its environmental friendliness and versatility. Among MFC concepts, soil MFC (SMFC) is very attractive for bioremediation of polluted environments while generating bioelectricity, which has applications in sensor development. However, the low energy yield and associated performance instability require optimization of some key factors to develop SMFC beyond laboratory experiments. While many of these factors have generally been investigated in single-factor experiments, knowledge of the interactive effects of key variables on SMFC performance is limited. Therefore, this study aims to determine the individual and interactive effects of selected variables on SMFC performance by integrating the variables into a single experimental design. The objective is to find a solution for the optimal performance of SMFC for sustainable bioelectricity generation and application in bioremediation. Design Expert software was used to obtain an optimal user-defined design for the optimization study. In this regard, two electrode materials, namely carbon felt (CF) and surface-modified stainless-steel mesh (SM), were tested at three different electrode spacings (ES) and three levels of substrate feeding interval or duration (FD). Using a new electrode fabrication method, the effect of the binder component of the electrode material was further investigated by comparing the performance of four different polymer binders, namely epoxy, polyvinyl alcohol, polytetrafluoroethylene, and polyvinylidene fluoride, in the SMFC. 16S rDNA gene amplicon sequencing was performed to gain insight into the effects of the experimental treatments on the microbial community diversity and its effect on the overall performance of the SMFC. Finally, a customized multicell SMFC was developed to study the bio-electrochemical stimulation of indigenous soil microbes for the biodegradation of petroleum hydrocarbon-contaminated soils. The results of the study based on the influences of the individual factors show that the SM electrode produced an average 4-fold higher power density compared to the CF electrode at all experimental treatment levels. The result of the combined optimization including the interactive effects of the variables showed that the optimum stable performance of the SMFC could be achieved at an ES of about 4 cm and a FD of 8 days, although the electrode material proved to be the most influential factor. By further optimizing the binder component of the SM, a maximum power density of 515.4 mW/m2 was achieved with epoxy, which is 2.3 times higher than the maximum power achieved in the first optimization. Application of the optimized electrode in the SMFC designed for bioremediation resulted in biodegradation of up to 24.26 % of total petroleum hydrocarbons in the contaminated soil within 24 days with concurrent power generation, compared to 11.04 % and 5.34 % removal in open-circuit and under natural attenuation, respectively. Microbial community analysis revealed that both the contaminated and uncontaminated soils were enriched in a variety of microbes, including common electroactive bacteria such as Shewanella oneidensis and Lactococcus lactis. While the microbial community was dominated by a group of bacteria belonging to the Proteobacteria phylum, the diversity in composition and abundance in the SMFC was associated with the cathode and anode and the time of sampling, but not with the different electrode materials. This result indicated that the difference in electrode performance was due to the electrochemical properties of the electrodes, but not to the diversity in microbial composition and abundance. This study contributes to the body of scientific knowledge by determining the best combination of electrode material, electrode spacing, and substrate loading interval to achieve the optimal performance of SMFC for stable bioelectricity generation. Among others, the study demonstrated the potential practical implementation of the optimized SMFC system for lighting, biosensor development, and bioremediation of polluted soil. Further studies could focus on improving the SMFC design for continuous substrate utilization and moisture availability during bioremediation, as well as developing an efficient power management system to scale up the output of a single SMFC without stacking many units.