Microbial electrochemical technologies (METs) use microorganisms as electrochemical catalyst, merging the microbial metabolism with electrochemical processes for the production of bioelectricity, biofuels, H2 and other valuable chemicals. Microbial fuel cells (MFC) and microbial electrolysis cells (MEC) are prominent examples of METs. While MFC is used to generate electricity from organic matter typically associated with wastewater treatment, MEC use electricity to drive chemical reactions such as the production of H2 or methane. Recently, microbial electrosynthesis cells (MES) have also emerged as a promising MET, where valuable chemicals can be produced in the cathode compartment. Other MET applications include microbial remediation cell, microbial desalination cell, microbial solar cell, microbial chemical cell, etc.,.
History The use of microbial cells to produce electricity was perceived by M.C. Potter in 1911 with the finding that "The disintegration of organic compounds by microorganisms is accompanied by the liberation of electrical energy". A noteworthy addition in MFC research was made by B. Cohen in 1931, when microbial half fuel cells stack connected in series was created, capable of producing over 35 V with a current of 0.2 mA. Two breakthroughs were made in the late 1980s when two of the first known bacteria capable of transporting electron from the cell interior to the extracellular metal oxides without artificial redox mediators: Shewanella (formerly Alteromonas) oneidensis MR-1 and Geobacter sulfurreducens PCA were isolated. In late 90s, Kim et al. showed that the Fe(III)-reducing bacterium, S. oneidensis MR-1 was electrochemically active and can generate electricity in a MFC without any added electron mediators. These findings set basis for the development of electromicrobiology, and the field of MFC started. However, due to low power generation, it was also doubtful whether the MFC can be practical application on wastewater organics reduction. This view was changed when it was established that domestic wastewater could be treated to practical limits while simultaneously producing power. Furthermore, power densities two orders of magnitude higher was demonstrated in an MFC using glucose, without the need for exogenous chemical mediators. Building upon these works, a race to develop practical applications of MFCs initiated to emerge at a very fast pace, with the major goals being development of a large scale technology for the treatment of domestic, industrial, and other types of wastewaters. In 2004, extracellular electron uptake (EEU) from cathodes to microbes (Geobacter spp.) was established with attached biofilm, where fumarate was reduced to succinate. This reverse reaction for electron transport generated the research field of MES. In 2010, Nevin et al. discovered that the acetogenic microorganism Sporomusa ovata can convert CO2 to acetic acid in MES cells by uptaking electrons from the cathode electrode. In the next years, also due to the growing concerns on greenhouse gas emissions, the field of CO2 bioelectroconversion in MES cell flourished. Several autotrophic microorganisms showed ability of capturing electrons from the cathode, either directly or through mediators. Besides specific microbial species, it was shown that CO2 reducing communities can be enriched in MES cells from inoculum sources such as sewage sludge, digester sludge or marine/river sediments. In the following decade, technical improvements led to an increase of acetate production rate from few to hundreds g/m2cathode/d. MES cells demonstrated also a promising technology for converting CO2 into biomethane, with production rates up to 200 L CH4/m2cathode/d. Furthermore, the MES scope was expanded to target more valuable products, including ethanol and caproate.
Principles
Microbial extracellular electron transfer There are various mechanisms for bacteria to electrons with an electrode. These include a "direct" process, where redox components located on the cell surface, that can be multiheme cytochromes or nanofilaments, contact directly with the solid surfaces (Figure 1A, C and D), and an "indirect" process that is mediated by soluble redox mediators that cyclically shuttle electrons between cells and electrodes [28-30] (Figure 1B). Electron shuttles can be humic substances that are not produced by the cells, or secondary metabolites that are produced by the organisms including phenazines [32, 33] and flavins [34, 35]. In addition, some primary metabolites of bacteria, such as sulphur species and H2, can convey electrons towards extracellular electron acceptors. In addition to heme cofactors in multiheme cytochromes, flavin mononucleotide also were shown to enhance the rate of electron transfer in some outer membrane cytochrome as redox cofactors [27]. Because electrons are transferred from the interior to the exterior of microbial cells across the cellular membrane during EET, ions with positive charge need to simultaneously move in the same direction as the electron flow to maintain charge neutrality (Figure 1A).
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