Environ. Sci. Technol. 2008, 42, 8088–8094
brane (PEM)) leads to bioelectricity generation. MFC tech-
Effect of Anodic Metabolic Function nology is both interesting and more promising innovation
on Bioelectricity Generation and compared to conventional chemical fuel cells as it requires
mild reaction conditions (ambient temperature, normal
Substrate Degradation in Single pressure, and normal pH) (15). More recently, utilization of
wastewater as substrate in harnessing bioelectricity employ-
Chambered Microbial Fuel Cell ing MFC is gaining importance due to its sustainability
S. VENKATA MOHAN,* Thus far, most of the research on MFCs has been conﬁned
G. MOHANAKRISHNA, AND P. N. SARMA to the operation of anodic chamber with anaerobic metabolic
Bioengineering and Environmental Centre, Indian Institute of function. Very few reports are available in the literature
Chemical Technology Hyderabad 500 007, India pertaining to the application of aerobic metabolic function
(11, 25). Metabolism is the sum of all the biochemical
Received May 7, 2008. Revised manuscript received July processes of the cell including catabolism (oxidation of
26, 2008. Accepted August 13, 2008. substrate in order to obtain energy) and anabolism (synthesis
of cellular components from carbon source) through energy
coupling mechanism (26). Each metabolic function has its
own speciﬁc biochemical pathway involving various redox
Inﬂuence of anodic metabolic function viz., aerobic, anoxic mediators to carry protons and electrons during substrate
metabolism. Aerobic process results in rapid and near
and anaerobic on bioelectricity generation was evaluated in
complete oxidation of organic substrates, facilitates higher
single chamber mediatorless microbial fuel cells (non-catalyzed
cell growth and energy yields when compared to anaerobic
graphite electrodes; open-air cathode) during wastewater process. Activated sludge process (ASP) is the technology of
treatment under similar operating conditions (pH 7; ambient choice for wastewater treatment which is an energy intensive
temperature/pressure). Despite the ﬂuctuations observed, aerobic aerobic process. Therefore, it is imperative to reexamine the
metabolic function (379 mV; 538 mA/m2) documented higher energy cost of aerobic wastewater treatment by developing
power generation compared to anoxic (251 mV; 348 mA/m2) and more innovative and less energy consuming approaches (27).
anaerobic (265 mV; 211 mA/m2) operations. Relatively higher In this realm, MFC technology can be considered as one of
treatment efﬁciency was also evidenced in aerobic operation the alternative approaches, which provides dual beneﬁts of
(COD removal efﬁciency; 77.68% (aerobic), 56.84% (anoxic), wastewater treatment along with energy generation and has
48.68% (anaerobic)). Polarization behavior, bioelectrochemical positive inﬂuence on the economic considerations of the
treatment process. Therefore, an attempt was made in this
analysis, sustainable resistance and cell potentials also
communication to evaluate the relative efﬁciency of three
supported the aerobic operation. Aerobic metabolic function metabolic functions viz., aerobic, anaerobic, and anoxic in
showed potential to generate higher power and substrate anodic chamber of single chamber mediatorless MFC on
degradation over the corresponding anoxic and anaerobic bioelectricity generation during wastewater treatment.
metabolic functions. The relative efﬁciency of power generation
observed in aerobic microenvironment might be attributed to 2. Experimental Design
effective substrate oxidation and good bioﬁlm growth observed 2.1. Anodic Biocatalysts (Mixed Consortia). Two types of
on the anodic surface. Presence of lower dissolved oxygen metabolically diverse mixed consortia were used as parent
concentration in anodic chamber due to the establishment of inoculum (biocatalyst) in anodic chamber of MFCs. Aerobic
equilibrium between substrate oxidation and oxygen scavenging mixed consortia acquired from an operating ASP treating 10
million liters per day of composite wastewater from domestic
might also contributes positively to power generation in
and industrial processes was used in aerobic and anoxic
aerobic operation. MFCs. Anaerobic mixed consortia acquired from laboratory
scale anaerobic suspended growth reactor treating chemical-
1. Introduction based wastewater was used in anaerobic MFC. Prior to
Recently considerable interest is being generated on the inoculation, parent cultures were washed thrice in saline
recovery of energy from wastewater using biological processes buffer (5000 rpm, 20 °C) and enriched in the designed
apart from its treatment. Renewable bioenergy is viewed as synthetic wastewater (DSW) (composition provided in the
one of the ways to alleviate fuel needs of the future and the Supporting Information) (pH 6; chemical oxygen demand
crisis of current global warming. In this direction, bioelec- (COD) 3.8 g/L; biochemical oxygen demand (BOD5) 2.2 g/L;
tricity production through wastewater treatment employing total dissolved solids (TDS) 1.2 g/L) under respective mi-
microbial fuel cells (MFC) has generated considerable interest croenvironments (120 rpm; 28 °C) for 48 h.
in both basic and applied research (1–14). MFC is a hybrid 2.2. MFC Conﬁguration. Three single chambered MFCs
bioelectrochemical system which directly transforms chemi- were operated separately under three diverse anodic mi-
cal energy to electrical energy via electrochemical reactions croenvironment/metabolic functions viz., anaerobic (MFCAn),
involving biochemical pathways. Here, microorganisms serve aerobic (MFCA), and anoxic (MFCAx) keeping all other
as biocatalysts and convert energy stored in chemical bonds operating conditions constant. MFCs were fabricated in the
of bioconvertible substrate into electrical energy. The bio- laboratory using “perplex” material. Anode compartment
potential developed between the metabolic activity (leads to (diameter 8.9 cm; height 11.4 cm) was designed to have a
generation of electrons (e-) and protons (H+)) and electron total/working volume of 550/500 mL. Non-catalyzed graphite
acceptor conditions (separated by a proton exchange mem- plates (5 × 5 cm; 10 mm thick; surface area 70 cm2 (plain
anode) and 83.5 cm2 (perforated open-air cathode)) were
* Corresponding author phone: 0091-40-27163159; fax: 0091-40- used as electrodes. Proton exchange membrane (PEM;
27163159; e-mail: email@example.com. NAFION 117; Sigma-Aldrich) was sandwiched between anode
8088 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 42, NO. 21, 2008 10.1021/es8012529 CCC: $40.75 2008 American Chemical Society
Published on Web 10/08/2008
FIGURE 1. a. Voltage (open circuit) (•) and current generation (2) during the operation of MFCA with the function of time (six
cycles); b. Voltage (open circuit) generation during the sixth cycle operation of MFCA, MFCAx, and MFCAn with the function of time; c.
Current density during the sixth cycle operation of MFCA, MFCAx, and MFCAn with the function of time; d. Substrate degradation rate
(SDR), speciﬁc power yield (SPY), and Coulombic efﬁciency (CE) during the operation of MFCA, MFCAx, and MFCAn with the function
of time (sixth cycle).
and cathode after pretreatment (14). Anode was ﬁxed over workers (28). Potential difference/open circuit voltage (OCV)
the liquid layer where the bottom surface remained sub- and current (I) (in series; 100 Ω) measurements were recorded
merged in the wastewater. Top portion of the cathode using digital multimeter. Bioelectrochemical behavior of
(aerated) was exposed to air while bottom portion was mixed consortia during operation in respective microenvi-
covered with PEM and exposed to the wastewater. Copper ronments was studied employing cyclic voltammetry (CV)
wires sealed with epoxy sealant, were used for contact with using potentiostat-glavanostat system (Autolab, PGSTAT12,
electrodes. Provisions were made in the design to have Ecochemie) linked to a microcomputer data acquisition
sampling ports, wire input points (top), inlet and outlet ports, system during stabilized phase of operation. All electro-
etc. chemical assays were performed in MFCs (in situ) at a
2.3. Operation. MFCA and MFCAx were inoculated with predeﬁned time intervals by considering anode (graphite)
aerobic mixed consortia, while MFCAn was inoculated with as working electrode and cathode (graphite) as counter
anaerobic mixed consortia. MFCA was operated in aerobic electrode against Ag-AgCl reference electrode. Voltammo-
microenvironment by supplying O2 through continuous grams were recorded by applying a potential ramp at a scan
aeration (dissolved oxygen (DO) varied between 0.2 to 0.4 rate of 10 mV/s over the range from +0.5 to -0.5 V to working
mg/L), while anodic chamber of MFCAx was aerated inter- electrode (anode). Polarization behavior of MFCs was
mittently (once in 6 hours for a period of 1 hour). MFCAn was recorded by linear sweep voltammetry by polarizing between
operated under anaerobic microenvironment. Prior to star- 0.5 and 0.1 V. Cell potentials of anode and cathode against
tup, anodic compartments were ﬁlled with DSW (400 mL) saturated Ag/AgCl electrode. Treatment efﬁciency of MFCs
after inoculating with respective mixed consortia (100 mL). was monitored by analyzing the samples collected periodi-
MFCs were operated at room temperature (29 ( 2 °C) in cally from anodic chamber for COD, BOD5, TDS and pH, as
fed-batch mode after adjusting pH of DSW to 7 (concentrated per the standard methods (29).
orthophosphoric acid (88%)/2 N NaOH) at organic loading
rate (OLR) of 0.95 Kg COD/m3-day for six cycles. Anolyte was 3. Results
continuously recirculated to keep anodic phase in suspen- 3.1. Performance of MFC under Diverse Anodic Metabolic
sion. Fresh wastewater was fed immediately after the voltage Functions. 3.1.1. Production of Bioelectricity. Experimental
drop was observed. Prior to feed change, suspended culture data depicted the feasibility of bioelectricity generation under
in anodic chamber was allowed to settle (settling; 30 min) diverse anodic microenvironments by utilizing wastewater
and treated wastewater (0.4 L) was removed (decant; 15 min). as substrate (Figure 1). However, the efﬁcacy of MFC viz.,
Settled culture (∼100 mL by volume) was used for subsequent power generation and substrate removal was found to be
operations. inﬂuenced by the type of anodic metabolic function main-
2.4. Bioelectrochemical Analysis. The current output tained. It was observed that the aerobic operation showed
parameters and substrate degradation rate (SDR) were the highest power generation efﬁciency over the corre-
considered as the two key parameters to evaluate the sponding anoxic and anaerobic metabolic functions. Figure
performance of MFCs. Electrochemical calculations were 1a illustrates MFCA performance under aerobic microenvi-
done based on the procedure outlined by Logan and co- ronment operated for six consecutive cycles. A consistent
VOL. 42, NO. 21, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 8089
improvement in open circuit voltage (OCV) was observed
from ﬁrst cycle (54 mV) to third cycle (350 mV) and
approached maximum during fourth cycle (385 mV) opera-
tion which stabilized thereafter. Visible difference in current
(100 Ω; series) output was observed between fourth and ﬁfth
cycles despite uniform potential output (3rd cycle, 262 mA/
m2 (362 mV); fourth cycle, 388 mA/m2 (385 mV)). After four
cycles of operation, signiﬁcant improvement in current (100
Ω; series) production was observed without much change in
voltage generation (5th cycle, 527 mA/m2 (378 mV); sixth
cycle, 534 mA/m2 (379 mV)). Similarly a marked improvement
in OCV and current were noticed with every additional feeding
event from ﬁrst cycle (129 mV, 80 mA/m2 and 136 mV, 97
mA/m2) to sixth cycle (348 mV, 348 mA/m2 and 336 mV, 205
mA/m2) under anoxic and anaerobic operations, respectively.
Consistent improvement in power output was noticed with
every feeding cycle irrespective of the applied metabolic
function prior to stabilization after the fourth (MFCA and
MFCAx) and ﬁfth (MFCAn) cycles of operation which might be
attributed to adaptation tendency of microbial consortia.
Immediately after feeding with fresh wastewater, marked
increase in OCV was noticed within a short period of time.
Among the three anodic metabolic functions studied, aerobic
microenvironment registered higher current outputs (MFCA,
534 mA/m2; 379 mV) followed by anoxic (MFCAx, 348 mA/m2;
251 mV) and anaerobic (MFCAn, 211 mA/m2; 265 mV)
microenvironments compared during sixth cycle operation
(Figure 1b and c). Similarly, the highest power outputs were
also observed in aerobic operation (0.204 W/m2; m539 A/m2)
followed by anoxic (0.087 W/m2; 348 mA/m2), and anaerobic
(0.056 W/m2; 211 mA/m2) microenvironments. Higher volu-
metric power production was also visualized in aerobic
operation (0.204 W/m3) when compared to anoxic (0.087
W/m3) and anaerobic (0.056 W/m3) microenvironments.
3.1.2. Wastewater Treatment. In addition to power gen-
eration, MFCs also showed signiﬁcant removal of substrate FIGURE 2. a. Cyclic voltammetry curves of anode generated
(COD) during operation (Figure 1d). Substrate degradation from sixth cycle operation of MFCA, MFCAx, and MFCAn. b.
efﬁciency was also found to be inﬂuenced by the anodic Polarization curves measured at various applied voltages
microenvironment. Aerobic metabolic function (εCOD, 77.68%; generated during stabilized performance (sixth cycle) in MFCA,
SDR, 0.74 kg CODR/m3-day) documented highest substrate MFCAx, and MFCAn.
degradation potential compared to anoxic (εCOD, 56.84%; SDR,
0.54 kg CODR/m3-day) and anaerobic (εCOD, 48.68%; SDR, to anoxic and anaerobic conditions, suggesting a higher
0.46 kg CODR/m3-day) conditions. A gradual improvement electrochemical activity. Aerobic metabolic function visual-
in the substrate removal was observed with time along with ized maximum current of 0.032 A in the forward scan (0.41
current generation irrespective of the metabolic function V) and -0.017 A in the reverse scan (-0.41 V) followed by
applied, prior to stabilization after the fourth cycle of anoxic (forward, 0.026 A (0.41 V); reverse, -0.011 A (-0.41
operation. Relatively higher speciﬁc power yield (SPY) was V)) and anaerobic (forward, 0.019 A (0.5 V); reverse, -0.016
registered under aerobic microenvironment (0.484 W/kg A (-0.5 V)) microenvironments during the sixth cycle of
CODR) compared to anoxic (0.283 W/kg CODR) and anaerobic operation. Maximum power was observed at the lowest
(0.212 W/kg CODR) conditions. Aerobic cultures were capable applied voltage (-0.41 V) and a consistent improvement in
of generating current from a much wider variety of nutrients power was recorded up to the applied maximum voltage
than those using anaerobic cultures (30). Organic substrate (0.41 V) suggesting effective function of aerobic metabolic
present in the wastewater acted as an electron donor during function. CV proﬁles showed an improved electrochemical
the metabolic activity pertaining to the applied microenvi- behavior with every additional feeding event. Electrochemical
ronment. analysis illustrated positive inﬂuence of aerobic metabolic
3.2. Characterization of the Performance of MFCs. function on power output.
3.2.1. Bioelectrochemical Evaluation. Bioelectrochemical Electrons generated from substrate metabolism and their
behavior of MFCs was evaluated employing CV. It permits reduction by protons at cathode eventually governs the power
direct electrochemical detection of redox signals and helps generation efﬁcacy in MFC operation. Substrate conversion
to elucidate the electrochemical reactions occurring on the (energy) to the metabolic intermediates associated with
electrode surface (16, 30–34). It also measures potential electron discharge provided good evidence for the presence
difference across the interface and redox activities of the of a charge carrier moving between the anode and the cathode
components involved in the biochemical system both in in the anolyte mixture to complete the redox reactions (35).
solution and components bound to the bacteria. Voltam- Electron discharge can be directly correlated to energy during
mogram proﬁles (vs Ag/AgCl) proﬁles evidenced visible and MFC operation and provides information regarding the
signiﬁcant variations in the electron discharge and energy metabolic activity occurring in the system. The energy
generation patterns as a function of applied anodic mi- generated during potential sweep was calculated based on
croenvironment (Figure 2a; Supporting Information Figure 1). maximum voltage applied and charge obtained from the
Relatively higher current output was recorded in the forward respective voltammogram. Aerobic metabolic function
scan of the voltammogram in aerobic operation compared showed higher energy and energy yield (0.452 J; 145.88 J/Kg
8090 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 42, NO. 21, 2008
CODR) compared to anoxic (0.256 J; 118.63 J/Kg CODR) and
anaerobic (0.235 J; 127.02 J/Kg CODR) conditions.
3.2.2. Polarization Behavior. Polarization behavior of
MFCs was evaluated in situ by polarizing the electrodes
between 0.5 and 0.1 V using linear sweep voltammetry during
stabilized phase of operation (sixth cycle) (Figure 2b). Increase
in power density (PD) was observed with decrease in applied
voltage. Higher PD of 0.192 W/m2 was observed during
aerobic operation at 0.27 V. It is always preferred to operate
MFC to the right side of PD peak, and the MFCA could be
operated effectively in the voltage below 0.27 V. The next
higher PD was observed in anoxic microenvironment (0.206
W/m2; 0.26 V), and MFCAx could be operated effectively in
the voltage below 0.26 V. MFCAn showed maximum PD of
0.0114 W/m2 at 0.253 V and can be effectively operated in
the voltage below 0.25 V. Aerobic metabolic function
documented a relatively wide range of applied voltages for
effective electron discharge compared to the anaerobic and
3.2.3. Cell Potentials and Sustainable Resistance. Cell
potential is one of the important parameters used to trace
fuel cell efﬁciency. Decrease in cell potential (emf) is directly
proportional to the increased electron discharge and the
system is said to be efﬁcient when the system discharges
electrons at higher applied external resistance (RE) (Figure
3a). In aerobic operation, a signiﬁcant decrease in cell
potential (-0.194 to -0.184 V) was observed starting at
applied RE of 15 KΩ leading to electron discharge. In the case
of anoxic and anaerobic operations, effective electron
discharge started at relatively lower applied RE of 10 KΩ
(MFCA: -0.204 to -0.182 V; MFCAn: -0.218 to -0.202 V)
showing lower stability. Cathode potentials (aerobic, 0.055
V; anoxic; 0.046mV; anaerobic, 0.052 V) were observed to be
almost constant in all the anodic metabolic functions studied
in the particular range of applied RE. This phenomenon
suggests that the current generation in MFC was limited by
anode microenvironment. Generally, anode potential con-
trols the electron transfer kinetics from microorganism to
the anode and was observed to vary with respect to the
applied anodic microenvironment. Despite the low variation
observed, anaerobic metabolic function recorded higher
anodic potential compared to the other two anodic mi-
croenvironments at RE of 30 KΩ (MFCA: -0.194 V; MFCAx:
-0.204 V; MFCAn: -0.218 V) (Figure 3b). A well developed
bioﬁlm growth observed on the anodic surface in aerobic
operation could be the reason for the recorded lower anodic
potential compared to corresponding anaerobic operation
Relative decrease in anodic potential (RDAP) with the
function of applied RE was used to evaluate the maximum
sustainable power (Figure 3c). The linear ﬁt at high RE
represents a region in which RE controls power (37). At applied FIGURE 3. a. Effect of external resistance on total cell potential
low RE, the electron delivery to the cathode was limited due with respect to applied external resistance during the sixth
to kinetic and/or mass transfer (or internal resistance (RI)), cycle of operation of MFCA, MFCAx and MFCAn. b. Effect of
and the RDAP increased linearly with decreasing RE. The external resistance on the anodic potential with respect to
conditions where external and internal resistance limitations applied external resistance during the operation of MFCA,
were equal between these two lines, a horizontal line from MFCAx and MFCAn (sixth cycle). c. Effect of external resistance
the intersection was drawn to estimate the value of RE for on the variation of percent deviation of anodic potential (RDAP)
sustainable resistance. Aerobic metabolic function showed with respect to applied external resistance during the sixth
effective performance at higher resistance (11.5 KΩ) com- cycle of operation of MFCA, MFCAx, and MFCAn.
pared to anoxic (9.0 KΩ) and anaerobic (7.4 KΩ) operations. viz., respiration and fermentation depending on the involve-
Sustainable resistance enumerated the effective function of ment of exogenous oxidants (external terminal electron
MFC where stable power generation could be achieved. acceptors) (10). Each metabolic function is governed by
Function at higher resistance indicates the stability of system speciﬁc biochemical pathways and triggers corresponding
in discharging the electrons. redox reactions. Extracted free energy is required to build
biomass (the anabolic process) from redox reactions (ca-
4. Discussion tabolism) involving an electron donor/electron acceptor
Heterotrophic organisms obtain energy required for their couple (38). The energy yielding substrates are usually
survival from the oxidation of organic compounds (from the oxidized stepwise during catabolism, through metabolic
Gibbs free energy) based on two major metabolic pathways intermediates before the formation of end products (26).
VOL. 42, NO. 21, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 8091
FIGURE 4. Proposed proton and electron generation and transfer mechanism during substrate degradation under diverse metabolic
The chemical energy released during the oxidation is phosphorylation, whereas anaerobic process continues with
conserved by transfer of electrons to electron carriers/ interconversion (dehydrogenation), decarboxylation, and
reduced coenzymes (such as NADH/FADH2) and in the methanogenesis. Electrons derived from substrate oxidation
formation of energy rich phosphate-to-phosphate bonds are donated either to organic electron acceptors/carriers (E+/
(such as ATP). The electrons and the phosphate bond energy E-H) (fermentation) or to O2 by way of an ETC (aerobic
may then be transferred to other parts of the cell where they respiration). Under anoxic microenvironment the facultative
are required for cell synthesis, maintenance or mobility. The aerobic/anaerobic bacteria which survive either in oxygen-
transfer of energy between catabolism and anabolism is ated or deoxygenated microenvironments can switch be-
termed as energy coupling. Under aerobic metabolic func- tween aerobic oxidation and fermentation processes. In
tion, electrons are liberated during substrate (R) oxidation anoxic operation, the proton will not be neutralized due to
(respiratory) and are transferred via a redox cascade called the absence of O2. In the absence of O2, reduced coenzymes
as respiratory/electron-transport chain (ETC) where their are not oxidized by ETC because of nonavailability of terminal
energy is gradually decreased due to transfer to an externally electron acceptor. Other possible routes by which proton
available terminal electron acceptor (10). Aerobic respiration reduction takes place is through H2 formation. Due to the
path facilitates highest energy gain but depends on the oxygen less positive redox potentials of oxidants, the energy gain for
availability. In presence of oxygen (under prevailing oxygen- the anaerobic metabolism is considerably low compared to
ated microenvironment), the protons (H+) get reduced to the aerobic respiration (10).
form water (Figure 4). Here, higher energy gains are possible Aerobic metabolic function showed higher efﬁcacy than
with positive redox potential of a terminal electron acceptor. anaerobic and anoxic operations with respect to power output
But in the presence of low oxygen (dissolved) concentration parameters (current, voltage), wastewater treatment ef-
it may not be able to reduce all the protons released. The ﬁciency (COD removal), and MFC characterization (polar-
remaining protons cross the PEM and form a gradient against ization, electrochemical behavior, sustainable resistance, and
which the electrons will ﬂow through the circuit in MFC. cell potential). Columbic efﬁciency (CE) and anode potentials
Under anaerobic metabolism (anaerobiosis), electrons are were observed to be comparatively low in aerobic operation.
derived from substrate (R) oxidation in the absence of oxygen. Observed lower CE might be attributed to the result of rapid
Both pathways share the initial glycolysis but aerobic and effective substrate degradation due to aerobic oxidation.
metabolism proceeds with the Krebs cycle and oxidative Anoxic operation showed higher power yield and voltage
8092 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 42, NO. 21, 2008
output compared to the corresponding anaerobic operation. mediator dependent (by soluble redox mediators such as
Anaerobic operation yielded the least energy among the three NAD+/NADH, FAD+/FADH2, etc.) or independent (by cellular
metabolic functions studied. components such as cytochromes) during fuel cell operation
Electron transport to anode is more crucial in aerobic (10). Especially with mixed bacterial suspensions, the CV
operation due to thermodynamically favorable O2 reaction peaks could appear for both cellular components and
with protons and electrons to form water (25). Power excreted redox mediators (34). In aerobic operation, a clear
generation efﬁciency was a result of successful scrubbing of redox peaks was observed during the sixth cycle in the reverse
O2 within the MFC. Higher power output efﬁciency observed scan (-0.210 V vs Ag/AgCl) along with a poorly deﬁned peak
in the aerobic microenvironment might also be attributed in the forward scan (0.125 V vs Ag/AgCl) which might be
to the near-elimination of electron scavenging by O2 in the related to NAD+/NADH (E0, -0.335 V) (Figure 2). However,
anode chamber, thereby allowing more electrons to anode in the case of anoxic and anaerobic conditions redox peaks
(11). This corroborated well with the observed low DO were not observed in voltammogram in the range of applied
concentration (0.2-0.4 mg/L) in anodic chamber of MFCA voltage. The redox peak obtained in the aerobic condition
during operation. Marked ﬂuctuations in power output might also be attributed to the observed relatively rapid and
parameters were observed speciﬁcally in aerobic operation. good bioﬁlm growth on the anode surface. The secondary
OCV and current did not show good correlation and this metabolites (endogenous redox mediators) serve as a revers-
might be due to the prevailing oxygenated microenvironment. ible terminal electron acceptors, transferring electrons from
Rapid potential drop observed immediately after feeding the bacterial cell either to a solid oxidant or into aerobic
might be attributed to the presence of higher concentration layers of the bioﬁlm, where it becomes reoxidized and is
of O2 (2.0 mg DO/L) in anolyte. With time O2 got consumed again available for subsequent redox processes (10). Con-
in the substrate oxidation and stabilized (equilibrium) in the sequently, the production of small amounts of these com-
range of 0.2-0.4 mg DO/L in subsequent operations. pounds (directly in the anodic bioﬁlm) enables the organism
Disturbance caused in the system equilibrium pertaining to to dispose off electrons at sufﬁciently high rates.
O2 concentration due to continuous metabolic activity and Despite the ﬂuctuations observed in power output
electron transfer mechanism might be the possible reason parameters, aerobic metabolic function showed potential to
for the observed inconsistency in power production. On the generate higher power along with effective substrate deg-
contrary, in anoxic condition a relatively good correlation radation over the corresponding anoxic and anaerobic
(R2- 0.5673) was observed between current and OCV com- operations if optimum conditions are provided. The positive
pared to aerobic operation indicating comparatively stable function of aerobic metabolic role in anodic chamber of MFC
performance. Anaerobic operation documented good cor- on power generation efﬁciency might be attributed to rapid
relation (R2-0.9364) between current and OCV indicating most and effective substrate oxidation (results in higher generation
stable performance among the three metabolic functions. of electron and proton), good bioﬁlm growth on anodic
Internal resistance (RI) plays a major role in electron transfer surface due to effective aerobic oxidation of substrate (direct
mechanism in fuel cell operations. Higher RI was observed anodic electron transfer) and low dissolved oxygen concen-
in anaerobic operation (156.03-126.35 KΩ) compared to tration (0.2-0.4 mg DO/L) due to the equilibrium established
aerobic (7.6-0.5 KΩ) and anoxic (52.29-19.00 KΩ) conditions. between substrate oxidation and oxygen scavenging. Need
One of the important aspects in aerobic operation was for low DO concentration and survival of aerobic culture
effective proton transport (to the PEM) and electron transport under lower O2 concentrations warrants the application of
(to the anode) (11). Rapid and good bioﬁlm growth observed aerobic metabolic function in the anodic chamber of MFC.
on the anode surface (self-immobilization) in aerobic mi- However, further studies are required to enumerate the role
croenvironment might be one of the reasons for the resulting of individual metabolic functions integrating physical, chemi-
effective power generation compared to anoxic and anaerobic cal, and biochemical components of the MFC on bioelectricity
operations. Bioﬁlm growth relates to rapid microbial colo- generation.
nization due to higher oxidative metabolic activity and
triggers effective transfer of electrons through bacterial Acknowledgments
contact and signaling with the electrode (2, 14). Bioﬁlm We acknowledge ﬁnancial support from the Department of
growth on anode surface also mediates direct electron transfer Biotechnology (DBT), Government of India, New Delhi in the
(10, 14). Microbial growth rate was relatively rapid in aerobic form of research grant (Project No. BT/PR8972/GBD/27/56/
oxidation (10-20 min) compared to fermentation (>90 min) 2006). We also thank Dr. J. S. Yadav, Director, IICT for his support
(39, 40). Bioﬁlm also results in higher substrate degradation and encouragement in carrying out this work. G.M. also
due to hyper metabolic activity and tends to liberate more acknowledges the Council for Scientiﬁc and Industrial Research
number of protons and electrons as evidenced by the current (CSIR), New Delhi, for providing research fellowship.
in CV especially in aerobic operation. Moreover, in the
internal layers of the bioﬁlm matrix, induced anoxic zone Supporting Information Available
persists due to presence of relatively low O2 concentration Composition of designed synthetic wastewater (DSW),
(41, 42). Hence, there was a possibility of forming anaerobic calculations and Sﬁg I. This material is available free of charge
microenvironment (precisely anoxic) at the interface of via the Internet at http://pubs.acs.org.
anode-bioﬁlm which also facilitates effective electron trans-
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