Environmental Engineering Reference
In-Depth Information
18.6 Electron Transfer Mechanism of Exoelectrogens
The electron transfer mechanism is a key issue to understand the theory of how
MFCs work. Numerous investigations were conducted to study how electrons were
transferred from microbial cells to anode surface in the MFCs. There are generally
two main mechanisms that are direct or mediator-less and indirect or mediated
electron transfer (MET).
18.6.1 Direct or Mediator-Less Electron Transfer
Direct electron transfer (DET) requires a physical contact between the microbial
cell membrane or a membrane organelle and the electrode surface. Shewanella
putrefaciens (Kim et al. 2002 ), Geobacter sulferreducens (Bond and Lovley 2003 ),
and Geobacter metallireducens (Min et al. 2005 ) can effectively transfer electrons
directly to an electrode across the membrane. Some of DET bacteria transfer
electrons through direct attachment of cell membrane to anode (Fig. 18.4 a), while
the rest use their pili or nanowires to transfer electrons to anode (Fig. 18.4 b).
Generally c-type cytochromes associated with bacterial outer membrane and
conductive nanowires or pili can be used for DET (Peng et al. 2010 ).
18.6.2 Indirect or Mediated Electron Transfer
Although some bacteria can transfer electrons directly, many other microbes need
redox-active chemical species or mediators to carry out electron transfer to anode;
this type of mechanism is known as indirect or MET. In MET, direct contact
between the bacterial cell membrane and the electrode surface is not required, but a
redox mediator is essential. An electron mediator is a molecule that functions as an
electron shuttle between microbes and an electrode. Mediators in the oxidized state
are easily reduced by capturing electrons from within the bacterial cell membrane
or the cytoplasm (Fig. 18.4 c). The reduced mediators after passing across the
membrane release their electrons to the electrode and become oxidized again in
anode chamber and thus are reutilized. Generally chemical mediators are supplied
from outside into the anode chamber of a MFC. Apart from externally provided
mediators, some microorganisms are able to excrete their own mediators such as
phenazine, 2-amino-3-carboxy-1,4-naphthoquinone and 2,6-di-tert-butyl-p-benzo-
quinone (Rabaey et al. 2005a ; Freguia et al. 2009 ; Deng et al. 2010 ) that are used to
transfer electron from cytoplasm to anode (Fig. 18.4 d). In addition, there is another
way by which some bacteria, especially fermentative bacteria, produce energy rich
reduced metabolites such as H 2 , ethanol or formate, which can be subsequently
oxidized to provide electron to anode (Schroder 2007 ) (Fig. 18.4 e). Furthermore, in
Search WWH ::




Custom Search