Role and Potential of Direct Interspecies Electron Transfer in Anaerobic Digestion
Abstract
:1. Introduction
2. Indirect Interspecies Electron Transfer
2.1. Interspecies Hydrogen Transfer
2.2. Interspecies Formate Transfer
3. Direct Interspecies Electron Transfer
3.1. Comparison between DIET and IIET
∆G°′ = 72.7 kJ/mol (37 °C, pH 7)
∆G°′ = −26.4 kJ/mol (37 °C, pH 7)
3.2. DIET Mechanisms
3.3. First Observations of Biological DIET in Anaerobic Cultures
4. Conductive Materials to Promote DIET
4.1. Iron Oxides
4.2. Biochar and Activated Carbon
4.3. Carbon Fibers
4.4. Other Conductive Materials
5. Perspectives
- Different physicochemical characteristics of conductive materials: the conductive materials introduced in this review have different characteristics except for the fact that they all have sufficient conductivity for DIET stimulation. For example, activated carbon and biochar have higher porosity than the other materials. Carbon fiber has a large surface area because of its entangled structure; therefore, it provides higher surface area for microbial attachment as well as adsorption. Magnetite is a paramagnetic iron oxide particle that is strongly attracted to magnetic force, which suggests easier separation from mixed liquor. Such unique characteristics of different conductive materials can introduce additional effects during their application to engineered processes.
- Economic feasibility of DIET promotion: when conductive materials are used to promote the DIET mechanism, one possible way to reduce cost is minimizing the consumption of materials. As most field-scale anaerobic digesters operate in a continuous mode, it is important to prevent conductive materials from being washed out of reactors. Several experiments were conducted in up-flow type reactors, and a material that has enough density to settle down can be maintained inside the reactor without getting washed-out. Sequencing batch reactors can also be used to minimize the loss of material. In addition, as introduced in this review, magnetite can be separated from effluent by magnetic force and recycled. Further study is required into methods for minimizing the consumption and loss of each material while maintaining a desired level of performance.
- Ecophysiology of electrotrophic methanogens: there is lack of knowledge about the electrotrophic methanogens compared to their syntrophic partner, the exoelectrogens. Only a few species (i.e., Methanosarcina barkeri and Methanosaeta harundinaceae) have been experimentally proven as being capable of participating in DIET in defined co-cultures. Although there are several species that putatively accept electrons directly from others, only indirect evidence is presented in most studies (e.g., increase in the relative abundance of DIET-mediated conditions). Further studies on whether the potential electrotrophic methanogens can be involved in DIET and the detailed biological mechanisms of how they can accept electrons directly are needed. The outcomes from such research would also provide valuable information on electromethanogenesis systems, which are modified microbial electrolysis systems, whose cathodic activity for converting CO2 into CH4 relies heavily on electron transfer between the electrode and attached methanogens.
- Combination with electrochemical systems: several recent studies revealed that voltage application on anaerobic digesters enhanced the organic degradation and methane production efficiencies. In this AD-microbial electrolysis cell (MEC) combined system, the exoelectrogens oxidize organic matter and release electrons to the anode, and the electrotrophic methanogens capture the electrons from the cathode to produce methane. This is a biocatalytic process in which microorganisms specialized for extracellular electron transport mechanisms participate. Further research is needed to determine whether or not there is a synergistic effect on the AD-MEC combined system when conductive materials are supplemented as DIET promoters.
Acknowledgments
Author Contributions
Conflicts of Interest
References
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Electron Carrier | Major Reactions | Reactions | ∆ G°′ (kJ/mol) |
---|---|---|---|
Hydrogen | Propionate degradation | Propionate− + 3H2O → Acetate− + HCO3− + H+ + 3H2 | +76.5 |
Methane production | 3/4HCO3− + 3/4H+ + 3H2 → 3/4CH4 + 9/4H2O | −101.7 | |
Overall reaction | Propionate− + 3/4H2O → Acetate− + 1/4HCO3− + 1/4H+ + 3/4CH4 | −25.2 | |
Butyrate degradation | Butyrate− + 2H2O → 2Acetate− + H+ + 2H2 | +48.3 | |
Methane production | 1/2HCO3− + 1/2H+ + 2H2 → 1/2CH4 + 3/2H2O | −67.8 | |
Overall reaction | Butyrate− + 1/2H2O + 1/2HCO3− → 2Acetate− + 1/2H+ + 1/2CH4 | −19.5 | |
Formate | Propionate degradation | Propionate− + 2H2O + 2CO2 → Acetate− + 3HCOO− + H+ | +65.3 |
Methane production | 3HCOO− + 3H+ → 3/4CH4 + 9/4CO2 + 3/2H2O | −144.5 | |
Overall reaction | Propionate− + 2H+ + 1/2H2O → Acetate− + 1/4CO2 + 3/4CH4 | −79.2 | |
Butyrate degradation | Butyrate− + 2H2O + 2CO2 → 2Acetate− + 2HCOO− + 2H+ | +38.5 | |
Methane production | 2HCOO− + 2H+ → 1/2CH4 + 3/2CO2 + H2O | −96.3 | |
Overall reaction | Butyrate− + H2O + 1/2CO2 → 2Acetate− + 1/2CH4 | −57.8 |
Electron-Donating Microorganism | Electron-Accepting Microorganism | Evidence | Reference |
---|---|---|---|
Geobacter metallireducens | Geobacter sulfurreducens |
| Summers et al., 2010 |
Geobacter metallireducens | Methanosarcina barkeri |
| Rotaru et al., 2014a |
Geobacter metallireducens | Methanosaeta harundinacea |
| Rotaru et al., 2014b |
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Baek, G.; Kim, J.; Kim, J.; Lee, C. Role and Potential of Direct Interspecies Electron Transfer in Anaerobic Digestion. Energies 2018, 11, 107. https://doi.org/10.3390/en11010107
Baek G, Kim J, Kim J, Lee C. Role and Potential of Direct Interspecies Electron Transfer in Anaerobic Digestion. Energies. 2018; 11(1):107. https://doi.org/10.3390/en11010107
Chicago/Turabian StyleBaek, Gahyun, Jaai Kim, Jinsu Kim, and Changsoo Lee. 2018. "Role and Potential of Direct Interspecies Electron Transfer in Anaerobic Digestion" Energies 11, no. 1: 107. https://doi.org/10.3390/en11010107
APA StyleBaek, G., Kim, J., Kim, J., & Lee, C. (2018). Role and Potential of Direct Interspecies Electron Transfer in Anaerobic Digestion. Energies, 11(1), 107. https://doi.org/10.3390/en11010107