Effect of electron transfer mediator on reductive degradation and current generation by Shewanella putrefaciens

Abstract

Dissimilatory metal reducing bacteria (DMRB) such as Shewanella spp. or Geobacter spp, are known to transfer electron to the solid state metal or electrode, oxidizing organic or inorganic electron donor. They can transfer the electron to solid surface, directly contacting with membrane bound c-type cytochromes and microbial conductive nanowires. However, the rate and efficiency of direct electron transfer is not enough to draw a significant influence on the possibilities for the application of these microbes to biotechnology processes. Artificially supplemented ETMs can create new routes of electron flow in the microbial energy metabolism, thereby enhancing the electron transfer from DMRB to metal or electrode. Despite the interest in the role of ETMs in microbial metal reduction in natural and engineered systems, there has not been an extensive investigation of the relative effectiveness of a broad range of natural and synthetic ETMs for enhancing the bioreduction of contaminants and current generation. We chose cyclic nitramine explosive, hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) as a representative contaminant because it is a powerful and highly energetic chemical and hard to degrade due to its complex molecular structure. In the batch test result, RDX reduction by CN32 alone was insignificant, while 110 ㎛ RDX was completely reduced by CN32 with riboflavin (100㎛) in 78 h. Through the parametric experiments and spectroscopic analysis, we confirmed that reduced riboflavin was responsible for the enhanced microbial reduction of RDX by Shewanella. Also, in this study, we suggest that ETMs such as flavins and quinones play a significant role in enhanced electricity generation by Shewanella. Depending on their physical structure (i.e. redox active elements) and chemical properties (i.e. theoretical redox potential), their effect on the potential or power generation of MFC showed distinguishable differences. And by the addition of conductive iron minerals, ...