Cathodic biofilm

Second, biofllm thickness, structure, composition, and density affect the flux of substrates and products within the biofllm. The latter can result in large overpotentials, which have a negative impact on the performance of the system. In the case of bioanodes, higher power production was observed from thicker anodic biofilms [120]. Strikingly, the reverse effect has been observed in cathodic biofilms [35]. 

Babauta JT, Hsu L, Atci E, Kagan J, Chadwick B, Beyenal H. Multiple cathodic reaction mechanisms in seawater cathodic biofilms operating in sediment microbial fuel cells. ChemSusChem 2014 7 2898-2906. 

The following sections on anodic and cathodic biofilms provide a more detailed discussion of the electrochemical reactions occurring in EABs. Unlike Case... 

Cathodic biofilms are biofilms that accept electrons from an electrode, as opposed to anodic biofilms, such as the G. sulfurreducens biofilms discussed previously, which deliver electrons to an electrode [34]. Cathodic biofilms are less well understood than their anodic counterparts despite being well documented in biocathode studies. Often, a more operational definition is given to cathodic biofilms. For example, biofilms that... 

Microsensors and CV can be coupled to assess ORR in cathodic biofilms operating in aerobic environments and to investigate cathodic reaction mechanisms operating in biocathodes in SMFCs and other bioelectrochemical systems. 

Babauta JT, Nguyen HD, Istanbullu O, Beyenal H. Microscale gradients of oxygen, hydrogen peroxide, and pH in freshwater cathodic biofilms. ChemSusChem 2013 6(7) 1252-1261. 

From Equations (11.3) and (11.4), we expect that ORR at cathodes, in the absence of cathodic biofilms, will tend to accumulate hydrogen peroxide and decrease the pH near the cathode surface. Babauta et al. demonstrated that when graphite electrodes were used. Equation 11.3 controls ORR at the electrode when a large offset potential (relative to open circuit) is applied to the cathode [16]. However, when we operate... 

Babanta JT et al. Microscale Gradients of Oxygen, Hydrogen Peroxide, and pH in Freshwater Cathodic Biofilms. Chemsuschem 2013 6(7) 1252-1261. 

Bergel et al. (2005) discovered that the formation of seawater improved the performance of stainless steel cathodes used in seawater applications of hydrogen fuel cells. The amount of improvement depended on the electrode sizes and pH. The fuel cell produced 41 mWW in the presence of the biofilm, but only 1.4 mW/m when the biofilm was removed, an increase of 30x (power normalized to cathode projected surface area). Increasing the pH of the anode compartment from 8.2 to 12.5 increased power to 270 mWW, which was almost lOOx larger than that with the cathode biofilm removed (2.8 mWW). The highest power output was 325 mW/m, which was obtained by decreasing the cathode surface area from 9 to 1.8 cm. ... 

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. 

SRB, a diverse group of anaerobic bacteria isolated from a variety of environments, use sulfate in the absence of oxygen as the terminal electron acceptor in respiration. During biofilm formation, if the aerobic respiration rate within a biofilm is greater than the oxygen diffusion rate, the metal/biofilm interface can become anaerobic and provide a niche for sulfide production by SRB. The critical thickness of the biofilm required to produce anaerobie conditions depends on the availability of oxygen and the rate of respiration. The corrosion rate of iron and copper alloys in the presence of hydrogen sulfide is accelerated by the formation of iron sulfide minerals that stimulate the cathodic reaction. 

Production of differential aeration cell. A scatter of individual barnacles on a stainless steel surface creates oxygen concentration cells. The formation of biofilm generates several critical conditions for corrosion initiation. Uncovered areas will have free access to oxygen and act as cathodes, while the covered zones act as anodes. Underdeposit corrosion (crevice corrosion) or pitting can occur. Depending on the oxidizing capacity of the bacteria and the chloride ion concentration, the corrosion rate can be accelerated. However, the presence of a biofilm does not necessarily mean that there will always be a significant effect on corrosion. (Dexter)5... 

Forms of corrosion of metals and alloys. It should be noted that organisms are more likely to cause localized than general corrosion because of the differential oxygen cell. In each case, the localized attack was found beneath macrofouling layers. Corrosion of copper, steel, and aluminum anodes was significantly higher when connected to cathodes on which the biofilm was allowed to grow naturally Unexpectedly rapid localized... 

The increase in cathodic kinetics due to the action of biofilms on passive alloy surfaces can also increase the propagation rate of galvanic corrosion. Potentiodynamic polarization studies show that cathodic kinetics are increased during biofilm formation on passive alloy surfaces. Tests on crevice corrosion samples of passive alloys S30400 and S31600 revealed that crevice initiation times were reduced when natural marine biofilms were allowed to form on the exposed external cathode surface. (Dexter)5... 

Some researchers [121, 126, 139] have demonstrated that intermittent application of AC or DC electric fields across MF or UF membranes during filtration can often promote displacement of polarization- or adherent fouling layers, with significant improvements in sustained permeation flux. The electrode is installed on either side of the membrane with the cathode on the permeate side and the anode on the feed side. Usually, the membrane support is made of stainless steel or the membrane itself is made of conductive materials to form the cathode. Titanium coated with a thin layer of a noble metal such as platinum could be one of the best anode materials [140], The electromagnetic field reduces fouling and biofilm development. 

Biffinger JC, Ketron J, Ray R, Little B, Ringeisen BR (2007) A biofilm enhanced miniature microbial fuel cell using ShewaneUa oneidensis DSPIO and oxygen reduction cathodes. Biosens. Bioelectron. 22 1672-1679. 

FIGURE 4.11 Differential aeration cell and ion concentration ceU mechanisms as operative on a biofihn-metal system. Relative size difference of the arrows represents the corresponding oxygen partial pressures. When via cathodic reactions, atomic metal (M) is oxidized, it will liberate enongh positively charged ions (cations, M ) to shift the charge balance to yield an ion concentration difference. In either case, pitting under the biofilm is inevitable. 

Another model is that put forward by Gu and Xu. They claim that this model can be used in explaining many MIC mysteries such as, but not limited to, the relationship between biofilm thickness and MIC-induced severe pitting, efficiency of pigging in always reducing MIC likelihood, and the often-observed lack of an existing link that correlates planktonic SRB cell counts and MIC pit-ting. The heart of this model is a new theory about MIC that has been branded by these authors as biocatalytic cathodic sulfate reduction (BCSR) theory. 

Both mixed and pure cultures can be used for the establishment of biological anodes or cathodes. From a practical point of view, mixed-culture electrodes seem to be more robust and resilient. Several groups have observed a higher power output when comparing mixed and pure culture systems [56]. This could be explained by either synergistic effects or the presence of a more productive exoelectrotroph in the mixed culture biofilms [56, 117]. However, several other scenarios could have occurred, such as pH effects and underdeveloped biofilms of the pure cultures [56]. 

Cheng, K.Y., Ho, G., and Cord-Ruwisch, R. (2010) Anodophilic biofilm catalyzes cathodic oxygen reduction. Environ. Sci. Technol, 44 (1), 518-525. 

More recently, attention has been directed to the "ninth form of corrosion, biologically influenced corrosion, which includes studies on an area referred to as "ennoblement. The presence of biofilms on metals and alloys immersed in natural seawater produces a complex, heterogeneous chemistry along the metallic surface. It has usually been observed that passive alloys such as aluminum, stainless steels, nickel-base alloys, or titanium show an increase to more noble (electropositive) potentials or ennoblement of several hundred millivolts with exposure time in natural seawater, thus magnifying the potential differences that may exist between dissimilar alloys [26,55-64]. Ennoblement is likely caused by the formation of microbiological films, which increase the kinetics of the cathodic reaction [55-63],... 

Patchy biofilms and localized colonization give rise to localized corrosion reactions and to anodes and cathodes that are fixed in space and stable in time [6S] as opposed to the randomly spaced smd mobile oxidation and reduction reactions required by uniform corrosion. Under these conditions, the cedcirlated polarization resistance vsdue is correct but the anode and cathode aresrs are unknown, so that one does not know how to determine the cirrrent density (i.e., the penetration rate). Adding to the level of... 

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