Roland D. Cusick, Younggy Kim, Bruce E. Logan
Published 1 March 2012 on Science Express
DOI: 10.1126/science.1219330
Materials and Methods
Reactor Construction. The lab scale MRC reactor was constructed as previously
described (13) with minor modifications (Fig. 1). The 4-cm cubic anode chamber (Lexan,
30 mL empty bed volume) contained a graphite brush anode (D = 2.7 cm, L = 2.3 cm,
0.22 cm2 projected area based on all fibers in the brush; Mill-Rose Labs Inc., OH). The
brush anode was heat treated (30) before it was inoculated with the effluent from an
existing MFC and enriched in a conventional single chamber MFC prior to MRC
operation. The cathode chamber (2-cm cubic chamber, 18 mL empty bed volume)
contained a 7-cm2 (projected surface area) air cathode with a Pt catalyst (0.5 mg Pt/cm2)
applied on a carbon cloth as previously described (31), with a Nafion catalyst binder
(water side) and four layers of polytetrafluoroethylene diffusion layers (air side).
Although Pt was used as a catalyst here in order to benchmark performance against
previously tested systems using NaCl solutions (13), nearly identical cathode
performance has been obtained using activated carbon catalysts instead of Pt catalysts in
microbial fuel cells (32). The cathode chamber also served as the first flow channel of the
high concentrate salt stream to prevent the pH rise in the cathode chamber
The RED stack, assembled between the anode and cathode chambers, consists of with
6 anion- and 5 cation-exchange membranes (Selemion AMV and CMV, Asahi glass,
Japan), creating 5 pairs of alternating HC and LC chambers as previously described (13).
Inter-membrane chambers were sealed and separated by silicon gaskets, each with an 8-
cm (2 × 4 cm) rectangular cross section cut out. Inter-membrane chamber width (1.3
mm) was maintained with a 2 cm2 (0.5 × 4 cm) strip of polyethylene mesh. The total ion
exchange membrane area in the RED stack was 88 cm2. The total MRC empty bed 3
volume was 58.4 mL (RED stack + Cathode = 28.4 mL; Anode = 30 mL). The HC
solution entered the cathode chamber and flowed serially through the 5 HC cells in the
stack, exiting from the cell next to the anode chamber (Fig. 1). The LC stream entered the
RED stack near the anode and flowed serially through the 5 LC cells in the stack, exiting
from the cell next to the cathode chamber. A peristaltic pump (Cole Parmer, IL)
continuously fed the HC and LC solutions at a flow rate of 1.6 mL/min, unless specified
otherwise.
After stable performance in the MRC, the working electrodes (anode and cathode)
were transferred to a cubic 4-cm (30 mL empty bed volume) single chamber MFC
reactors to establish a performance baseline.
Peak power, maximum energy recovery, and energy efficiency of the MRC and MFC
were determined in separate experiments. During power density curve experiments fresh
HC solution was pumped through the RED stack with the effluent collected in separate
reservoirs. To maximize energy recovery and energy efficiency, 0.1-L HC and LC
solutions were recycled in airtight flow paths for the duration of anode feeding cycles
over a batch recycle experiment. Before each batch the stack and tubing were flushed
with matching solutions.
Solutions. Ammonium bicarbonate HC solutions were prepared by dissolving
ammonium bicarbonate salt (Alfa Aesar, MA) into deionized water within an airtight
vessel. The initial HC solutions tested were 1.8, 1.1, 0.95, 0.8, and 0.5 M. The LC
solutions were prepared to produce salinity ratios of 50, 100, and 200 by diluting an
aliquot of the HC solution. The anode solutions contained 1 g/L of sodium acetate
(organic substrate for exoelectrogenic bacteria growing on anode), in 50 mM carbonate.
buffer (4.2 g/L NaHCO3-) containing 0.231 g/L NH4H2PO4 and trace vitamins and
minerals (33). Domestic wastewater was collected from the primary clarifier of the Penn
State University wastewater treatment plant. The cathode contained ammoninium
bicarbonate HC solution, therefore protons for oxygen reduction at the cathode were
provided by ammonium and bicarbonate ions as well as water dissociation.
A second order relationship between ammonium bicarbonate solution concentration
and solution conductivity (determined by conducting a stepwise dilution series) was used
to estimate initial and final concentrations of HC and LC streams. Conductivity and pH
of the HC and LC streams were measured (Mettler-Toledo, OH) before and after each
batch recycle experiment.
Analysis. Power production in batch recycle experiments was determined by
measuring the potential drop across a fixed external resistance (300 Ω) for both MRC and
single chamber MFC operations. Voltage drop was recorded every 20 minutes by a
digital multimeter (Keithley Instruments, OH). Electrical current (i) was determined by
Ohm’s law. Power was calculated by multiplying the electrical current and total cell
voltage. Reported power densities were based on the cathode projected area (7 cm2). To
determine the maximum MRC power (PMRC) production at each condition the reactor was
held at open circuit voltage for one hour and then the external resistance was decreased
from 1,000 to 50 Ω every 20 minutes with the voltage recorded at each resistance. Power
contribution by the electrode reactions (PMFC) was determined by measuring the anode
potential (Ean) and cathode potential (Ecat) against Ag/AgCl reference electrodes (BASi,IN): P
MFC = (Ecat – Ean)× i. The RED stack power contribution was calculated by finding stack voltage (Vstk) with two reference electrodes located on both ends of the stack as:
PRED = Vstk × i.
The MRC anode was transferred to a single chamber MFC to determine baseline
power production in fed-batch experiments. In the single chamber MFC, same substrate
solutions (sodium acetate in carbonate buffer solution and domestic wastewater) were
provided to determine peak power production.
Coulombic efficiency was determined as previously described (34). Energy recovery
(rE) is defined by the ratio of energy produced by the MRC reactor and the energy input
as substrate and salinity gradient as written in Eq. (1). Energy efficiency (ηE) was
calculated as the ratio of energy produced to the energy consumed based on the substrate
used and the salinity gradient, according to (13):
where R is the ideal gas constant (8.314 J/mol-K), T is solution temperature, V is the
volume of solution, c is the molar concentration of ionic species i in the solution, and a is the activity of species i in the solution.
At a neutral pH, concentrated ammonium bicarbonate is dominated by ammonium (NH4+) and bicarbonate (HCO3-) ions, but significant amounts of carbamate (NH4CO3-) and carbonate (CO32-) also contribute to ionic strength. Species specific concentrations and activities were estimated with OLI Stream Analysis software (OLI Systems, Inc., Morris Plains, NJ) at a pH of 7 and temperature of 25 °C.
To determine ammonia transport into the anode, total ammonia nitrogen (TAN = NH3+ NH4+) concentration in the substrate was estimated before and after each fed-batch cycle (HACH, Loveland, CO) (31). Based on observed pH, corresponding free ammonia
concentration (NH3) was calculated by:
Polarization curves of the MRC using different HC salt solutions, compared to that of an MFC
MRC energy input (acetate and salinity energy) and output at different HC concentrations.
a) Peak power density and b) anode (A) and cathode potentials (C) of MRC and single chamber
MFC fed domestic wastewater. Notice that the anode and cathode potentials remained relatively
constant over the range of current densities. The relatively constant potential indicates that the
power performance is stable, suggesting that the system could easily sustain higher power
densities with higher organic matter concentrations in the wastewater.
Published 1 March 2012 on Science Express
DOI: 10.1126/science.1219330
Materials and Methods
Reactor Construction. The lab scale MRC reactor was constructed as previously
described (13) with minor modifications (Fig. 1). The 4-cm cubic anode chamber (Lexan,
30 mL empty bed volume) contained a graphite brush anode (D = 2.7 cm, L = 2.3 cm,
0.22 cm2 projected area based on all fibers in the brush; Mill-Rose Labs Inc., OH). The
brush anode was heat treated (30) before it was inoculated with the effluent from an
existing MFC and enriched in a conventional single chamber MFC prior to MRC
operation. The cathode chamber (2-cm cubic chamber, 18 mL empty bed volume)
contained a 7-cm2 (projected surface area) air cathode with a Pt catalyst (0.5 mg Pt/cm2)
applied on a carbon cloth as previously described (31), with a Nafion catalyst binder
(water side) and four layers of polytetrafluoroethylene diffusion layers (air side).
Although Pt was used as a catalyst here in order to benchmark performance against
previously tested systems using NaCl solutions (13), nearly identical cathode
performance has been obtained using activated carbon catalysts instead of Pt catalysts in
microbial fuel cells (32). The cathode chamber also served as the first flow channel of the
high concentrate salt stream to prevent the pH rise in the cathode chamber
The RED stack, assembled between the anode and cathode chambers, consists of with
6 anion- and 5 cation-exchange membranes (Selemion AMV and CMV, Asahi glass,
Japan), creating 5 pairs of alternating HC and LC chambers as previously described (13).
Inter-membrane chambers were sealed and separated by silicon gaskets, each with an 8-
cm (2 × 4 cm) rectangular cross section cut out. Inter-membrane chamber width (1.3
mm) was maintained with a 2 cm2 (0.5 × 4 cm) strip of polyethylene mesh. The total ion
exchange membrane area in the RED stack was 88 cm2. The total MRC empty bed 3
volume was 58.4 mL (RED stack + Cathode = 28.4 mL; Anode = 30 mL). The HC
solution entered the cathode chamber and flowed serially through the 5 HC cells in the
stack, exiting from the cell next to the anode chamber (Fig. 1). The LC stream entered the
RED stack near the anode and flowed serially through the 5 LC cells in the stack, exiting
from the cell next to the cathode chamber. A peristaltic pump (Cole Parmer, IL)
continuously fed the HC and LC solutions at a flow rate of 1.6 mL/min, unless specified
otherwise.
After stable performance in the MRC, the working electrodes (anode and cathode)
were transferred to a cubic 4-cm (30 mL empty bed volume) single chamber MFC
reactors to establish a performance baseline.
Peak power, maximum energy recovery, and energy efficiency of the MRC and MFC
were determined in separate experiments. During power density curve experiments fresh
HC solution was pumped through the RED stack with the effluent collected in separate
reservoirs. To maximize energy recovery and energy efficiency, 0.1-L HC and LC
solutions were recycled in airtight flow paths for the duration of anode feeding cycles
over a batch recycle experiment. Before each batch the stack and tubing were flushed
with matching solutions.
Solutions. Ammonium bicarbonate HC solutions were prepared by dissolving
ammonium bicarbonate salt (Alfa Aesar, MA) into deionized water within an airtight
vessel. The initial HC solutions tested were 1.8, 1.1, 0.95, 0.8, and 0.5 M. The LC
solutions were prepared to produce salinity ratios of 50, 100, and 200 by diluting an
aliquot of the HC solution. The anode solutions contained 1 g/L of sodium acetate
(organic substrate for exoelectrogenic bacteria growing on anode), in 50 mM carbonate.
buffer (4.2 g/L NaHCO3-) containing 0.231 g/L NH4H2PO4 and trace vitamins and
minerals (33). Domestic wastewater was collected from the primary clarifier of the Penn
State University wastewater treatment plant. The cathode contained ammoninium
bicarbonate HC solution, therefore protons for oxygen reduction at the cathode were
provided by ammonium and bicarbonate ions as well as water dissociation.
A second order relationship between ammonium bicarbonate solution concentration
and solution conductivity (determined by conducting a stepwise dilution series) was used
to estimate initial and final concentrations of HC and LC streams. Conductivity and pH
of the HC and LC streams were measured (Mettler-Toledo, OH) before and after each
batch recycle experiment.
Analysis. Power production in batch recycle experiments was determined by
measuring the potential drop across a fixed external resistance (300 Ω) for both MRC and
single chamber MFC operations. Voltage drop was recorded every 20 minutes by a
digital multimeter (Keithley Instruments, OH). Electrical current (i) was determined by
Ohm’s law. Power was calculated by multiplying the electrical current and total cell
voltage. Reported power densities were based on the cathode projected area (7 cm2). To
determine the maximum MRC power (PMRC) production at each condition the reactor was
held at open circuit voltage for one hour and then the external resistance was decreased
from 1,000 to 50 Ω every 20 minutes with the voltage recorded at each resistance. Power
contribution by the electrode reactions (PMFC) was determined by measuring the anode
potential (Ean) and cathode potential (Ecat) against Ag/AgCl reference electrodes (BASi,IN): P
MFC = (Ecat – Ean)× i. The RED stack power contribution was calculated by finding stack voltage (Vstk) with two reference electrodes located on both ends of the stack as:
PRED = Vstk × i.
The MRC anode was transferred to a single chamber MFC to determine baseline
power production in fed-batch experiments. In the single chamber MFC, same substrate
solutions (sodium acetate in carbonate buffer solution and domestic wastewater) were
provided to determine peak power production.
Coulombic efficiency was determined as previously described (34). Energy recovery
(rE) is defined by the ratio of energy produced by the MRC reactor and the energy input
as substrate and salinity gradient as written in Eq. (1). Energy efficiency (ηE) was
calculated as the ratio of energy produced to the energy consumed based on the substrate
used and the salinity gradient, according to (13):
where EMRC is the energy produced per batch (kJ), ns is the moles of substrate (acetate)
initially fed to the anode (0) and at the end of the batch cycle (f), and ∆Gs is the Gibb’s
free energy of substrate [acetate = –846.6 kJ/mol (35), domestic wastewater = 17.8 kJ/gCOD (36)]. ∆Gmix is the free energy that can be created by mixing of HC and LC
solutions until the two solutions reach equilibrium concentration, calculated as:
where R is the ideal gas constant (8.314 J/mol-K), T is solution temperature, V is the
volume of solution, c is the molar concentration of ionic species i in the solution, and a is the activity of species i in the solution.
At a neutral pH, concentrated ammonium bicarbonate is dominated by ammonium (NH4+) and bicarbonate (HCO3-) ions, but significant amounts of carbamate (NH4CO3-) and carbonate (CO32-) also contribute to ionic strength. Species specific concentrations and activities were estimated with OLI Stream Analysis software (OLI Systems, Inc., Morris Plains, NJ) at a pH of 7 and temperature of 25 °C.
To determine ammonia transport into the anode, total ammonia nitrogen (TAN = NH3+ NH4+) concentration in the substrate was estimated before and after each fed-batch cycle (HACH, Loveland, CO) (31). Based on observed pH, corresponding free ammonia
concentration (NH3) was calculated by:
Labeled picture of an MRC reactor showing positions of the working (anode and cathode) and
reference (Ag/AgCl) electrodes, the RED membrane stack, as well as high (HC) and low
concentrate (LC) salt solution influent and effluent ports.
Power density curves of the MRC (HC = 0.95 M, SR = 100) at different salt solution flow rates
a) Peak power density and b) anode (A) and cathode potentials (C) of MRC and single chamber
MFC fed domestic wastewater. Notice that the anode and cathode potentials remained relatively
constant over the range of current densities. The relatively constant potential indicates that the
power performance is stable, suggesting that the system could easily sustain higher power
densities with higher organic matter concentrations in the wastewater.
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