Exogenous electron transfer mediator enhancing gaseous toluene degradation in a microbial fuel cell: Performance and electron transfer mechanism

Abstract

Effective electron transfer (ET) between microorganisms and electrodes is essential for the toluene degradation and power generation in a microbial fuel cell (MFC). In this work, the neutral red, with excellent electrochemical reversibility and compatible redox potential as NADH/NAD+, was selected as electron mediator to boost the performance of the MFC. Experimental results revealed that, with the 0.5 μM neutral red, the removal efficiency and coulombic efficiency of the gaseous toluene powered MFC was increased by ~19% and ~400%, respectively. However, further increase in neutral red concentration resulted in a decreased in removal efficiency and coulombic efficiency, which was attributed by the toxicity of neutral red to the microbes. The microbial community analysis indicated that, with the dosage of the neutral red, the dominated bacteria shifted from Geobacter to Ignavibacteriales, resulting in a high coulombic efficiency. With the further increase in the neutral red, the amount of Ignavibacteriales gradually decreased and thus the coulombic efficiency declined at a high neutral red concentration. Based on the cyclic voltammetry analysis, an electron transport pathway involving neutral red, cytochromes, and OMCs in neutral red mediated MFC was proposed. Overall, the dosage of neutral not only enhanced the electron transfer but also induced the growth of the exoelectrogens, and thus significantly improve the MFC performance.