MET THROUGH EXOGENOUS REDOX MEDIATORS

Some microbes such as Escherichia coli, Pseudomonas sp., Proteus and Bacillus (Lovley, 2006a) cannot directly trans­fer electrons to the anode and must rely on mediators (Lovley, 2006a). When the oxidized mediators reach the surface of the microbes, they penetrate the cell mem­brane of the microbes, and they are reduced by electrons. The reduced mediators pass through the cell membrane again and reach the anode surface where then they are reoxidized (losing the electrons). In this fashion, electrons are transferred to the anode while the oxidized media­tors enter the microbes again, thereby continuing the redox cycle (Figure 9.3(a)) (Neto et al., 2010; Rabaey et al., 2005b).

Properties of good exogenous mediators should be the ability to (a) cross cell membranes with ease; (b) receive electrons from electron donors without interfering with other metabolic processes; (c) deliver electrons inside the cytoplasm for oxidation reactions and regenerate at rapid rates; (d) have good solubility and stability in both oxidized and reduced forms; (e) have no cytotoxicity; and (f) not be consumed by microbes in the biofilm as a nutrient (Bao and Wu,

2004) . These mediators include thionine, neutral red,

2- hydroxy-1,4-naphthoquinone, phenazines, quinines,

Fe(III) ethylenediaminetetraacetic acid, methylene blue, phenothiazines, phenoxazines and others (Choi et al., 2003b; Lovley, 2006a; McKinlay and Zeikus, 2004; Newman and Kolter, 2000; Osman et al., 2010; Park and Zeikus, 2000). However, these mediators are unsuitable for practical applications because they are costly and most of them are toxic and recalcitrant, harmful to the environment (Erable et al., 2010a; Lovley, 2006a).

MET THROUGH THE SECONDARY METABOLITES

Researchers have found that some microbes can trans­fer electrons without DET in the absence of exogenous redox mediators. These microbes such as S. putrefaciens, S. oneidensis, G. sulfurreducens, Pseudomonas aeruginosa, and Clostridium butyricum can produce their own media­tors (Angenent et al., 2004; Erable et al., 2010a; Fitzgerald et al., 2012; Newman and Kolter, 2000; Rabaey et al., 2005a). The presence of these microbes in the mixed cultures enhances electron transfer. These mediators mainly include phenazine derivatives like pyocyanine and 2-amino-3-carboxy-1,4-naphthoquinone (Osman et al., 2010).

In practical applications, the secondary metabolites (endogenous redox mediators) may be very important to MFCs because they can transfer the electron without the exogenous redox mediators (Schroder, 2007). The mechanism of electron transfer by the secondary metab­olites is similar to that of the exogenous electrochemical redox mediators (Figure 9.3(a)). The secondary metabo­lites can be reused, and one metabolite molecule can transfer thousands of electrons (Schroder, 2007). So a small amount of the secondary metabolites can single — handedly enhance the rate of electron transfer and thus increasing power density and improve the MFC performance without introducing costly exogenous mediators.

In batch-mode operations, these microbes are very suitable because the mediators will accumulate in the anodic chamber, thus improving the MFC performance (Osman et al., 2010). However, in continuous flow MFCs for wastewater treatment, the secondary metabo­lites can be insufficient due to diluted concentrations as a result of flow (Lee et al., 2003; Rabaey et al., 2005c), thus resulting in the decline of the performance after the flow starts (Lovley, 2006a; Osman et al., 2010).

MET THROUGH PRIMARY METABOLITES

The other endogenous redox mediators are primary metabolites. Some microbes can produce fermentation products such as hydrogen (H2), hydrogen sulfide (H2S), alcohols and ammonia (Erable et al., 2010a). When these primary metabolites reach the surface of the anode, they are oxidized, and the released electrons will be further transferred to the anode surface.

There are two types of anaerobic metabolism that can produce primary redox metabolites: one is anaerobic respiration, and the other is fermentation. Some microbes such as Proteus vulgaris, E. coli, P. aeruginosa and Desul — fovibrio desulfuricans can produce sulfide which may serve as the mediator to transfer electrons (Bullen et al., 2006; Schroder, 2007):

Cytoplasm : SO|~ + 9H+ + 8e~ / HS~ + 4H2O (9.3) Anode : HS~ + 4H2O/SO4~ + 9H+ + 8e~ (9.4)

This process relies on sulfate reducing bacteria (SRB) that cannot metabolize carbohydrates. A fermentation process can produce small organic acids and alcohols that can be used in anaerobic respiration (Schroder,

2007) . Many SRB degrade the substrates incompletely and this lowers the MFC power output. Electrode poisoning by sulfide due to its easy absorption on the electrode surface is also a major drawback (Reimers et al., 2006; Ryckelynck et al., 2005).

Fermentation also produces primary metabolites such as hydrogen, ethanol and formate. They can be oxidized directly by electrolysis on an anode such as platinum or tungsten carbide (Rosenbaum et al., 2006). For example, through electrocatalysis, the molecular hydrogen near and on an anode surface would be oxidized to H+, accompanying the electron transfer (Figure 9.3(b)). Molecular hydrogen is known to be used as an electron carrier used by hydrogenase — positive microbes such as some SRB in microbiologically influenced corrosion (Gu, 2012). Thus, it contributes to power generation.