MET THROUGH EXOGENOUS REDOX MEDIATORS

Similar to bioanodes, the same exogenous mediators including neutral red, methyl viologen and the anthra — quinone-2,6-disulfonate can be used for biocathodes (Hatch and Finneran, 2008; Park and Zeikus, 1999; Stein — busch et al., 2010) to enhance MFC performance signifi­cantly. When mediators are added into the cathode chamber, they are reduced by the electrons donated by the cathode. The reduced mediators reach the microbial cell wall and then transfer the electrons through the wall while the mediators are oxidized. Subsequently, the oxidized mediators diffuse back to the cathodic sur­face for reuse. This cyclic process is illustrated in Figure 9.4(b). Usually one mediator molecule can accom­plish thousands of cycles. These mediators are relatively short-lived and costly, making their use unsustainable. Just like their use for bioanodes, these exogenous media­tors are used only in laboratory investigations of MFC mechanisms for academic purposes. Pili can also be used by microbes to transfer extracellular electrons to the cytoplasm (Zhou et al., 2013).

In manganese-oxidizing bacteria, manganese (IV) plays an important role in the electron transfer. This mechanism is similar to the exogenous mediator MnO2 on the biocathode surface. It is first reduced to MnOOH by the electrons donated from the cathode and then Mn2+ is released. Finally, with the help of manganese — oxidizing bacteria, Mn2+ was oxidized by dissolved ox­ygen to regenerate MnO2 (Nguyen et al., 2007). The power density can be improved by two orders of magni­tude, compared with the abiotic cathode (Rhoads et al.,

2005) , making it attractive for potential practical applications.

MET THROUGH SELF-EXCRETED REDOX MEDIATORS

Apart from exogenous redox mediators, some microbes can excrete metabolites that are redox active. For example, Pseudomonas spp. can produce phenazines (Venkataraman et al., 2010) and S. oneidensis can produce flavins (Marsili et al., 2008). These mediators can be used by biocathodes indirectly. In the presence of these medi­ators, the rate of electron transfer is enhanced. These mediators are more easily utilized by other microbes than their producers (Rosenbaum et al., 2011). Therefore, in biocathodes, the self-excreted mediators play an important role in a synergistic biofilm consortium cove­ring a cathode. Their mechanism of electron transfer is

similar to that used by exogenous redox mediators. Table 9.2 shows some reported microbial species for biocathodes.