Anodes microbial

Kiely PD, Cusick R, Call DF, Selembo PA, Regan JM, Lxjgan BE. Anode microbial communities produced by changing from microbial fuel cell to microbial electrolysis cell operation using two different wastewaters. Bioresour Technol 2011 102(l) 388-394. 

The anodic microbial catalysts are expected to play a key role for the power density and efficiency of an MFC. The highest power generation of MFCs seems to be produced by those operating with a mixed culture, or a microbial community, rather than those operating with a pure culture. The structure and activity of the bacterial community are sensitive to various environmental conditions, such as solution pH, electrode potential, ionic strength, and temperature. Since these environmental parameters can also affect other processes (e.g. the proton transfer efficiency, and cathode performance), it is now more difficult to conclude a quantified relationship between the microbial community and these parameters. Moreover, the configuration and operating mode of MFCs also appear important for the composition and activity of anodic biofilms. 

The inoculum source is another factor affecting the anodic microbial community and activity. Kim et al. [67] suggested that start-up of an MFC is most successful with biofilm harvested from the anode of an existing MFC. In another study, enrichment of the bacteria on the anode of an MFC resulted in increased power output and a change in the bacterial community [17]. 

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 origin of the observed correlation was not established, and the relation was not interpreted as causal. It could be argued that a sustained elevated potential due to as-yet unknown microbial processes altered the passive film characteristics, as is known to occur for metals polarized at anodic potentials. If these conditions thickened the oxide film or decreased the dielectric constant to the point where passive film capacitance was on the order of double-layer capacitance (Cji), the series equivalent oxide would have begun to reflect the contribution from the oxide. In this scenario, decreased C would have appeared as a consequence of sustained elevated potential. 

A schematic diagram of the microbial sensor is illustrated in Figure 1. The sensor consisted of double membranes of which one layer was the bacteria-collagen membrane (thickness 40jam), the other an oxygen permeable Teflon membrane (thickness 27jam), an alkaline electrolyte, a platinum cathode, and a lead anode. 

A diagram of the microbial sensor is illustrated in Figure 2. When the sensor was inserted into a sample solution containing formic acid, formic acid permeated through the porous Teflon membrane. Hydrogen, produced from formic acid by butyricum, penetrated through the Teflon membrane, and was oxidized on the platinum anode. As a result, the current increased until it reached a steady state. The steady state current depended on the concentration of formic acid. The steady state current was obtained within 20 min. 

Figure 2. Schematic diagram of the microbial sensor for formic acid. 1. Pt anode 2. Teflon membrane 3. Immobilized C. butyricum ...

Figure 6. The microbial sensor system for ammonia. 1. Electrolyte (NaOH) 2. Cathode (Ft) 3. Immobilized cells 4. Magnetic stirrer 5. Gas permeable Teflon membrane 6. Teflon membrane 7. Anode (Pb)...

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... 

Biofuel cells — also referred to as biochemical, or bio-electrochemical fuel cells, exploit biocatalysts for the direct conversion of chemical energy to electrical energy. Based on the nature of the biocatalyst, biofuel cells are generally classified as enzymatic fuel cells and microbial fuel cells [i]. Enzymatic fuel cells use purified enzymes to catalyze the oxidation of substrates at the - anode and... 

The cathodes are similarly corroded (chemically or electrochemically) as the anodes but metallurgical factors affect to a lesser degree, that is, the transformation is gradual [69]. However selective attack, such as from electrolyte impurities, often takes place and the production of a homogeneous surface layer of sub-products or crystal metallic impurities produce micro-cracks reducing the durability [70]. In addition, microbial factors produce cathode failure especially in the electrochemical treatment of wastewaters [71]. 

The use of tannin is not commonly practised and nitrite and silicate treatments are less popular than orthophosphates for anodic protection because of their technical limitations, and the need for constant supervision to maintain effectiveness. Among the shortcomings are the potential for the formation of deposits, and the encouragement of microbial activity. 

Figure 7. Scheme of the microbial electrode sensor for glucose. 1. Sample solutions. 2. Bacteria-collagen membrane. 3. Teflon membrane. 4. Cathode (Ft). 5. Anode (Fb). 6. Electrolyte (KOH). 7. Air pump. 8. Amplifier. 9. Recorder. 

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