Environmental Engineering Reference
In-Depth Information
External Circuit
e
-
Electron
donor
(fuel)
e
-
Oxidant (O
2
)
ions
Reduced
Oxidants (H
2
O)
Oxidized
electron
donor
Biofilm
Anode
Electrolyte
Cathode
Fig. 1.3 Schematic illustration of the essential components of a MFC, particularly the
separate flows of electrons (e
-
) and ions from the anode to the cathode
anode and transfer electrons from the donor to the anode. For example, when
acetate is the fuel, its oxidation reaction is CH
3
COO
+2H
2
O CO
2
+
HCO
3
-
+8H
+
+8e
-
. In a unique form of respiratory metabolism, the anode-
respiring bacteria (ARB) transfer electrons to the anode electrode, and the
difference in the electrochemical potential between the anode and cathode
electrodes drives the flow of electrons through an external circuit, generating
electrical current and electrical energy. At the cathode, the oxidant is reduced,
and the reduced oxidant carries the electrons out of the reactor. O
2
is the typical
oxidant: O
2
+4H
+
+4e
-
2H
2
O.
Since the goal of an MFC is to circuit electrons to generate electricity,
completely removing electrons from organic compounds to produce CO
2
is
desirable. Hence, in contrast to biofuel generation, an MFC makes the oxida-
tion state of carbon more positive than biomass.
Dramatic advancements in MFC technology are possible today because of
the recent discovery that many bacterial species are ARB [10, 11, 12]. The
discovery of ARB eliminates the need for externally supplied electron shuttles,
which are expensive and often toxic. Provided with suitable environmental
conditions, ARB naturally establish a biofilm community on the anode [13,
14, 15]. The community structure of ARB-containing biofilm often is diverse
[13, 16, 17], and the diversity is expected to be enhanced by the wide range of
available fuels, which include agricultural residues, animal manures, and waste-
water [14, 18, 19].
Although the novelty of an MFC is in the anode respiration by ARB, other
microbial metabolic processes, such as hydrolysis and fermentation, can play
significant roles in an MFC too. As shown in Fig. 1.2, hydrolysis and fermenta-
tion break down complex polymeric materials to simpler byproducts like acetate,
which is a favorable electron donor for ARB. An MFC fed with fermentative
substrates, such as glucose, becomes enriched with fermentative bacteria such as
Clostridia, Fermicutes,andBacterioides [13, 17, 20]. Fermentative products, such
