Microbial anode reactions

Rhoads, A., Beyenal, H., and Lewandowski, Z. (2005) Microbial fuel cell using anaerobic respiration as an anodic reaction and biomineralized manganese as a cathodic reactant. Environ. Sci. Technol, 39 (12), 4666-4671. 

A microbial electrolysis cell is based on the concept of a microbial fuel cell (MFC) and consists of an anode and cathode chamber, a membrane that electrically separates the electrodes, and an external power supply [8] (Fig. 1). The anodic reaction is the same as in a microbial fuel cell ElectrochemicaUy active microorganisms oxidize organic compounds such as acetate, generating carbon dioxide (CO2), protons (H" ), and electrons (e ), using the anode as terminal... 

Fixing of anodic reaction sites, whereby microbiological surface colonies lead to the formation of corrosion pits, driven by microbial activity and associated with the location of these colonies... 

Many of the by-products of microbial metaboHsm, including organic acids and hydrogen sulfide, are corrosive. These materials can concentrate in the biofilm, causing accelerated metal attack. Corrosion tends to be self-limiting due to the buildup of corrosion reaction products. However, microbes can absorb some of these materials in their metaboHsm, thereby removing them from the anodic or cathodic site. The removal of reaction products, termed depolari tion stimulates further corrosion. Figure 10 shows a typical result of microbial corrosion. The surface exhibits scattered areas of localized corrosion, unrelated to flow pattern. The corrosion appears to spread in a somewhat circular pattern from the site of initial colonization. 

A relatively high degree of corrosion arises from microbial reduction of sulfates in anaerobic soils [20]. Here an anodic partial reaction is stimulated and the formation of electrically conductive iron sulfide deposits also favors the cathodic partial reaction. 

The tests were conducted in an open, mixed and aerated reactor to maintain constant values of pH, DO, and temperature. Thus the difference in COD drop may not be related to pH, temperature. Aeration and mixing maintained DO around saturation in all tests, thus the effect of oxygen production at the anode is minimized. The only other process (other than microbial activity) that may relate to COD drop is abiotic transformation by electrolysis reactions at the electrodes. If abiotic redox of the organic content occurs in this study, then increasing the current density should increase the... 

Microbial electrocatalysis relies on microorganisms as catalysts for reactions occurring at electrodes. The microorganisms involved are able to transport electrons in and out of the cell, a process known as extracellular electron transfer (EET), and can catalyze both oxidation and reduction reactions [80, 81]. Their catalytic properties have been confirmed by the fact that they are able to lower the overpotentials (lower energy loss) at both anodes [82] and cathodes [56, 69], giving an increased performance of the system. Nevertheless, they cannot be considered as true catalysts since part of the substrate/electron donor is consumed for growth. 

Electrical power can also be applied to the BES to increase the electrical potential between the anode and cathode electrodes and enable cathodic reactions (reviewed in [1]) not attainable in cathode-limited systems such as MECs. Energy is added to the system with an external power source or by poising the anode potential with a potentiostat. This type of BES is often referred to as a MEC because it enables the electrochemical reduction of protons to hydrogen at the cathode (reviewed in [8]). These BESs are especially suitable for microbial studies and have been used extensively for Geohflcter bacteria. For this reason, we describe the theory and practice of MEC operation using Geobacter cultures in detail in the following sections. 

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