Recently, microbial electrosynthesis cells (MES) have also emerged as a promising MET, where valuable chemicals can be produced in the cathode compartment.
[8] A noteworthy addition in MFC research was made by B. Cohen in 1931,[9] 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 [10] and Geobacter sulfurreducens PCA were isolated.
In late 90s, Kim et al.[11] 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.
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.
[16] In the next years, also due to the growing concerns on greenhouse gas emissions, the field of CO2 bioelectroconversion in MES cell flourished.
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),[24][25][26][27] and an "indirect" process that is mediated by soluble redox mediators that cyclically shuttle electrons between cells and electrodes [28-30][28][29][30] (Figure 1B).
Various designs and configurations have been established to optimize the assembly of the three basic elements (anode, cathode and separator) in a functioning system.
MFCs in wastewater treatment, besides electricity generation, also help in energy savings linked to these mentioned processes which add a great advantage.
A great variety of substrates have been used in MFCs for electricity production varying from pure compounds to complex mixtures of organic matter present in wastewater.
Benthic MFCs generate power through the microbial oxidation of organic substrates in anoxic marine sediments coupled to reduction of oxygen in the overlying water column.
[44][45] The weather buoys obtained their entire power from the benthic MFC allowing them to operate continuously and independently from the need to replace batteries.
Nitrogen is conventionally removed by biological nitrification and denitrification processes which involves a very high energy and cost in wastewater treatment.
In comparison to conventional biological treatment or chemical processes, BESs employ a single or multiple electrodes which are not closed reactors for pollutants' remediation.
Solid electrodes in this system work as non-exhaustible electron acceptors/donors for stimulating microbial transformation of pollutants into non-toxic or less toxic forms.
[55][56] In other studies, reduction of perchlorate,[57] Cr(VI),[58] Cu(II), and radioactive uranium[59] have also been achieved in BESs with cathode as electron donors.
[64][65][66][67] The product spectrum in MES is largely governed by biocathodes materials (carbon- or metal-based), microorganisms involved, reduction potentials, and redox mediators activity, and operation conditions including pH, temperature and pressure.
[68][69] Potentials between -0.6 and -1.0 V vs SHE are typically applied to MES inoculated with mixed cultures to ensure production of hydrogen at the cathode, which is then uptake by acetogenic and methanogenic microorganisms to reduce CO2.
Many organic compounds such as acetate, butyrate, and lactate, largely exists in effluents of wastewater plants and fermentation units.
[82] Power-to-Gas technology potentially generates biogas with a similar grade to natural gas without the need to remove CO2 using expensive techniques, such as amine scrubbing or pressure swing adsorption.
[85] MFCs have applications in monitoring and control of biological waste treatment unit due to their correlation of coulombic yield of MFC and strength of organic matter in wastewater which serves as readings for biosensors.
[86] Systems based on the microorganism Shewanella show promise as sensors for quantifying the biological oxygen demand in sewage.
[87][88] This concept can readily be expanded to detect other compounds that can act as electron donors for electricity production, such as hydrogen or aromatic contaminants.
Anaerobic bacteria that naturally grow in the sediment produce the small current that can be used to charge a capacitor to store energy for the sensor.
Extensive research toward developing reliable MFCs to this effect, is focused mostly on selecting suitable organic and inorganic substances that could be used as sources of energy.