Journal of Membrane Science 409–410 (2012) 16–23
Contents lists available at SciVerse ScienceDirect
Journal of Membrane Science
journal homepage: www.elsevier.com/locate/memsci
Ionic composition and transport mechanisms in microbial desalination cells
Haiping Luo a , Pei Xu b , Peter E. Jenkins c , Zhiyong Ren a,∗
a
Department of Civil Engineering, University of Colorado Denver, Denver, CO 80004, USA
Department of Civil and Environmental Engineering, Colorado School of Mines, Golden, CO 80401, USA
c
Department of Mechanical Engineering University of Colorado Denver, Denver, CO 80004, USA
b
a r t i c l e
i n f o
Article history:
Received 19 January 2012
Received in revised form 26 February 2012
Accepted 27 February 2012
Available online 5 March 2012
Keywords:
Microbial desalination
Bioelectrochemical system
Ionic transport
Membrane fouling
Energy production
a b s t r a c t
Microbial desalination cell (MDC) offers a new and sustainable approach to desalinate saltwater by
directly utilizing the electrical power generated by bacteria during organic matter oxidation. The successful MDC development relies on the fundamental understanding of the interactions and removal
mechanisms of different ion species present in saline water or wastewater, but there is limited understanding of ion transport mechanisms in MDCs and potential membrane fouling/scaling during treatment
of wastewater and saline water. In this study, we investigated the transport behavior of multiple ions in
MDCs and the effects of ionic composition on system performance and membrane scaling and fouling.
The results showed that the presence of sparingly soluble cations in saltwater negatively affected MDC
power generation and desalination. Membrane characterization revealed that the majority of such ions
precipitated on the ion exchange membrane surface and caused membrane scaling. Anions such as Br−
and SO4 2− with Na+ as counter-ion did not show significant effects on system performance. Sharp pH
changes were observed during MDC operation, which resulted in the inhibited MDC anode microbial
activity and the accelerated formation of alkaline precipitations on both sides of the cation exchange
membrane. An anode–cathode recirculation approach was proved to be effective to solve such problems
and improved the desalination rate by 152% and the electron harvest rate by 98%.
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
Microbial desalination cell (MDC) is a newly developed bioelectrochemical technology that offers a sustainable approach
for simultaneous water desalination, renewable energy production, and wastewater treatment. Compared to current desalination
processes, such as membrane-based reverse osmosis (RO) and
thermal-based distillation, which are energy intensive and may
require 2–15 kWh electricity for producing 1 m3 of fresh water from
seawater, MDC is considered an energy gaining process, because
it employs exoelectrogenic bacteria to convert the biochemical
energy stored in organic matter to electricity and use the potential gradient across the anode and cathode to drive desalination
[1–4]. Previous studies showed that MDC could recover up to 231%
of energy in the format of hydrogen gas than the external energy
used for reactor operation, and the process can either be used as
a stand-alone process or serve as a pretreatment for RO or electrodialysis (ED) to reduce salt loading, energy consumption, and
membrane fouling potential [2,4–7].
∗ Corresponding author. Tel.: +1 303 556 5287.
E-mail address: [email protected] (Z. Ren).
0376-7388/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.memsci.2012.02.059
Fig. 1 shows the schematic of a three-chamber MDC. The anode
chamber (left) is for organic removal and electron generation, the
cathode chamber (right) accepts the electrons and completes the
electric loop, and the middle chamber is for desalination. When
bacteria in the anode chamber oxidize biodegradable substrates
and produce current and protons, the anions (e.g., Cl− and SO4 2− )
migrate from the saline water in the middle chamber across an
anion exchange membrane (AEM) to the anode, and the cations
(e.g., Na+ and Ca2+ ) are drawn to cross a cation exchange membrane
(CEM) to the cathode for charge balance. The loss of ionic species
from the middle chamber results in water desalination without any
external electricity input or higher water pressure. The removed
salts can also be captured in membrane assemblies without direct
migration [8]. Same as any engineering process, MDCs are intended
to purify seawater or brackish water as well as treat wastewater
with complex composition. However, most MDC studies have so
far only used NaCl as the surrogate of saline water and neglected
the impacts of other ion species in water. The multiple ions present
in water will affect MDC performance in many aspects. For example,
sparingly soluble cations, such as Ca2+ and Mg2+ , may precipitate on
membrane surface and cause scaling that inhibits the transport of
ions through membranes and increases membrane electrical resistance. Such ions can also react with natural organic matter in water
and form compacted fouling layers on membrane surface [9,10].
H. Luo et al. / Journal of Membrane Science 409–410 (2012) 16–23
17
2.2. Reactor operating conditions
Fig. 1. Schematic of a three-chamber MDC reactor for simultaneous substrate
removal (anode), desalination (middle chamber), and energy production.
Moreover, anions like NO3 − and SO4 2− that migrated from saline
water to the anolyte will compete with the anode as the electron
acceptors and cause electron loss and reduce system performance
[11]. In addition, the charges and molecular sizes of different ions
may vary significantly, which also affect the transfer behavior of
different ions present in the saline water. It was observed that the
MDC performance decreased by 22% when artificial seawater was
used in the desalination chamber to replace pure NaCl [2]. However,
there is no reported study that characterizes membrane fouling or
multiple ion transport behavior in MDCs.
In this study, we systematically investigated the transport
of different ions during MDC operation and characterized the
effects of multiple ions on the performance of MDC reactors in
terms of desalination efficiency, power generation, and removal of
organic matter. Membrane fouling and scaling were characterized
to elucidate the associated mechanisms and impacts on MDC operation. System performance was significantly improved when an
anolyte–catholyte recirculation approach was introduced [12,13].
The findings provide quantitative information for a better understanding of MDC mechanisms and improvement of MDC research
and development.
2. Experimental
2.1. MDC construction
The MDC reactors were constructed from polycarbonate and
cut into cube shape with 7.0 cm in diameter. Each reactor consisted of three chambers that were clamped together. The anode
and middle desalination chamber were separated by a GE normal
grade AEM (AR204-SZRA-412) and the cathode and middle chamber were separated by a GE normal grade CEM (CR67-HMR-412)
[1,7]. A heat treated graphite brush (350 ◦ C, 30 min) was used as
the anode [14], and carbon cloth with 0.5 mg/cm2 Pt coating and
four diffusion layers was prepared as the air-cathode [15]. The
anode and cathode were connected with titanium wires across
an external resistance of 1.5 . The effective liquid volume of the
anode, middle, and cathode chamber was 150, 150 and 50 mL,
respectively. Cathode chamber was made thinner to minimize
the ohmic resistance created by the spacing between anode and
cathode [5].
The MDC reactors were inoculated with a mixed culture of
microbes by transferring the pre-acclimated anodes of active
acetate-fed microbial fuel cells (MFCs) into MDC anode chambers.
The anode chamber of the MDC was fed with 2 g/L sodium acetate in
a nutrient buffer solution containing (per liter in deionized water):
5.2 g KH2 PO4 , 10.7 g K2 HPO4 , 1.5 g NH4 Cl, 0.1 g MgCl2 ·6H2 O, 0.1 g
CaCl2 ·2H2 O, 0.1 g KCl, and 10 mL of trace mineral metal solution
and vitamin solution [16]. The cathode chamber was fed with
100 mM KH2 PO4 /K2 HPO4 buffer solution unless mentioned otherwise. Mixed ion solutions were used in the middle chamber to
characterize the ion transfer behavior and interactions. Specifically,
for cation characterization, 50 mM Na+ , 50 mM Mg2+ , and 50 mM
Ca2+ were added with Cl− as common counter ion (herein “cationsMDC”), and for anion characterization, 50 mM Cl− , 50 mM Br− , and
50 mM SO4 2− with Na+ as common counter-ion (herein “anionsMDC”). Such combination was chosen to represent ion composition
of brackish saline water [17]. For both cases, additional control
experiments were conducted at the same condition by using NaCl
solution. To minimize the solution-resistance effect during characterization, the initial conductivity of the feeding saltwater was
maintained at the same level of 17.5 mS/cm in anions-MDC tests or
22.4 mS/cm in cations-MDC tests.
The anion and cation tests were conducted separately in parallel in two MDC reactors. The reactors were initially operated
in fed-batch operation mode, in which the anolyte, saltwater,
and catholyte were replaced at the same time after the salt
removal reached to the maximum level. Further studies included an
anolyte–catholyte recirculation operation, where substrate solution with the same anolyte composition was recirculated from a
500 mL reservoir into the anode chamber and then to the cathode
chamber. The effluent from the cathode chamber was circulated
back into the reservoir. The flow rate was 5 mL/min, representing
a hydraulic retention time of 30 min in the anode chamber and
10 min in cathode chamber.
2.3. Analyses and calculations
The voltage (E, volts) across the external resistor (Re ) was
recorded continuously using a data acquisition system (Model
2700, Keithley Instruments, Inc., OH). The current density (I, amps)
was calculated according to I = E/Re . Salt concentrations were measured by conductivity using a conductivity meter (Sension 156,
HACH Co., USA). The concentrations of cations, such as Na+ , Ca2+
and Mg2+ , were determined by an Optima 3000 Inductive Coupled Plasma (ICP) Spectrometer (Perkin Elmer, Norwalk, CT). Prior
to analysis, samples were acidified to pH 2 using nitric acid concentrate. The concentrations of anions such as Cl− , Br− and SO4 2−
were determined using a Dionex DC80 ion chromatography system
(IC) (Dionex, Sunnyvale, CA). The concentration of chemical oxygen
demand (COD) was measured using a colorimeter according to the
manufacturer’s procedure (Hach DR/890, Hach Campany, Loveland,
CO). The pH values of electrolytes were monitored using a pH meter
(Sension 156, HACH Co., USA). Electron harvest rates (v, coulomb
per hour) were calculated by
v=
n
Ui ti
ni=1
3600
R
t
i=1 i
where Ui (V) is the output voltage of MFC at time ti , R () is the
external resistance, ti (s) is the interval over which data are collected [7]. The salt removal rate (g TDS/L d) was calculated by the
conductivity removal per day (g TDS/d) based on the volume (L) of
salt solution in middle chamber.
18
H. Luo et al. / Journal of Membrane Science 409–410 (2012) 16–23
Electrochemical impedance spectroscopy (EIS) was conducted
by a potentiostat (G300, Gamry Instruments Inc., NJ) to determine
the internal resistance change of each MDC system. Whole cell
scan was conducted by a two-electrode configuration, in which the
cathode served as the working electrode and the anode was the
reference and counter electrode. To focus on membrane impedance
changes, the anode chamber + AEM (or cathode chamber + CEM) EIS
measurements were conducted using a three-electrode configuration, with a Ag/AgCl reference electrode in the middle chamber
and the anode (or cathode) serving as the working electrode and
the cathode (or anode) as the counter electrode [16]. All EIS measurements were performed at open circuit voltage condition. The
internal resistances of the cell were obtained from Nyquist plots,
where the intercept of the curve with the Zre axis was defined as
the ohmic resistance [16,18].
After the experiment, the MDC reactors were dissembled for
membrane characterization using environmental scanning electron microscopy (ESEM) and energy-dispersive spectroscopy (EDS).
The membrane samples were cut carefully using a sterilized scissor
to get the center samples of the AEM/CEM membrane and then airdried overnight before analyses. ESEM (Quanta 600, FEI Company,
Hillsboro, OR) was used to examine the surface structure of both
sides of the membranes and analyze the structure of fouling layer
at nano/micro-meter scale. Elemental compositions of fresh and
fouled membrane specimens were quantified by the EDS equipped
in the ESEM.
3. Results and discussion
3.1. Effects of ion composition on MDC performance
When the desalination chamber was fed with saltwater containing a mixture of Na+ , Mg2+ and Ca2+ (50 mM each) with Cl−
as a common counter ion (cations-MDC), both power outputs and
desalination efficiencies of the batches declined gradually during
the long-term batch operation. The maximum current density produced on the first batch cycle was 660 mA/m2 (cathode), but it
decreased to 210 mA/m2 on the fifth cycle (Fig. 2A). The corresponding desalination efficiency on the first and fifth cycle was 29% and
13%, respectively. The duration of each cycle ranged from 166 to
217 h. Such decline was presumably due to membrane scaling during the desalination process and will be discussed in the following
sections.
Different from the cations-MDC, MDC performance was more
stable and repeatable between the cycles in the anions-MDC, which
contains Cl− , Br− , and SO4 2− (50 mM each) with Na+ as a common counter ion. Fig. 2B shows the current density obtained by
the anions-MDC which ranged from 2200 to 2800 mA/m2 over the
duration of four cycles without performance decline. The corresponding desalination efficiency was comparable, approximately
24% for each cycle. A typical cycle of anions-MDC was about 71 h,
much shorter than the cations-MDC, indicating a more efficient
ion transfer process. However, back-diffusion of salt to the middle
chamber based on the conductivity measurements was observed
at the end of each cycle, as indicated by the desalination efficiency
drop at the end of each cycle (Fig. 2B). This may be due to the decline
of potential gradient as a result of anode substrate consumption
during each batch, and the low potential/current at the batch end
may be not high enough to counter the osmosis pressure generated across the different chambers. Such phenomenon has been
reported in a reverse electrodialysis system when it was operated
under open-circuit condition [19]. Continuous-flow will mitigate
such problems, as demonstrated later in the recirculation test.
The remarkable difference in the current density and cycle duration between the cations-MDC and anions-MDC was likely due to
membrane scaling and the transport behavior differences between
the divalent cations and anions will be further characterized and
discussed in the following sections. The other possible reason may
come from materials used in the MDC reactors, such as membranes
and electrodes. To potentially correlate such effects, control experiments were applied to each MDC system before the cation/anion
study using pure NaCl solution in the middle chamber. The initial feeding solution conductivity was adjusted to the same level
as the working MDCs to obtain comparable solution resistance [2].
Results showed when using NaCl to replace the multiple cations
in the same cations-MDC reactor, the current density increased by
82%, to 1200 mA/m2 . A slight increase (100 mA/m2 ) was observed
when using NaCl to replace the mixed-anion solution in the anionsMDC. These results are consistent with the findings of Jacobson et al.
that the salt removal efficiency of pure NaCl solution was higher
than artificial seawater, which is made from sea salt and contained
multiple ions [2]. The findings suggest that the presence of divalent cations in the middle chamber has negative effects on MDC
power generation and salt removal, and further characterizations
are needed to understand their transport mechanisms in order to
facilitate MDC development and application.
3.2. Competitive ion transport behavior in MDCs
In conjunction with conductivity measurements, the ion transport and mass balance of multivalent ions were examined based on
the mass decrements in the middle chamber and the corresponding mass increments in the catholyte (cations-MDC) and anolyte
(anions-MDC), respectively.
Fig. 3 shows the mass change of different cations in the cationsMDC. In the middle chamber, the concentrations of all cations
including Na+ , Ca2+ and Mg2+ decreased over time. However, the
mass increments of the corresponding cations exhibited different
trends in the cathode chamber (Fig. 3). Further calculations showed
that up to 84% of the Na+ ions that removed from the middle
chamber was recovered in the catholyte, but only 0.4% and 0.1%
of the removed Ca2+ and Mg2+ were found in the cathode solution,
respectively. Such low recovery of divalent cations in the cathode
chamber should mainly attribute to the calcium and magnesium
precipitations on the membranes, although other factors such as
ion adsorption on the cathode and ion diffusion to the anode chamber due to concentration gradients could also play certain roles.
The mass variations of different anions in the anions-MDC are
illustrated in Fig. 4. Within 72 h of operation, the decrement of Cl− ,
Br− and SO4 2− in the middle chamber was 1.89, 4.61 and 4.34 mmol,
respectively. Meanwhile, the mass increment of the respective ions
in the anolyte reached up to 2.95, 4.54, and 2.75 mmol. The corresponding recovery efficiency was 156%, 98%, and 63% for the Cl− ,
Br− , and SO4 2− ions, respectively. Because the high-surface carbon
brush anode was transferred from a long-term operated MFC reactor and mass balance was conducted at the end of the experiment,
the extra amount of Cl− recovered in the anolyte was considered
due to the potential release of Cl− from the anode, which may accumulatively absorb ions from the anolyte (which contained chloride
during inoculation of the cell) and long-term operation.
In order to compare the transport rates of various ions in the
MDC system, the changes of normalized ion concentration (the
ratio of residual concentration over the initial concentration in the
middle chamber) as a function of time are illustrated in Fig. 5.
The normalized concentrations decrease due to the ions transfer
out of the middle chamber. It was found that Ca2+ concentration
decreased slightly faster than Na+ , while the removal of Mg2+ was
the slowest. The transport competition and selectivity of monovalent and multivalent ions in the cations-MDCs may be ascribed to
the size and charge effects, because larger ions were reported sterically hindered when transporting through the membranes [20].
H. Luo et al. / Journal of Membrane Science 409–410 (2012) 16–23
19
600
30
500
25
400
20
300
15
200
10
100
5
0
0
100
200
300
400
500
600
700
800
900
0
1000
3000
2
Current density (mA/m )
Desalination Efficiency
(%)
35
(A)
30
(B)
2500
25
2000
20
1500
15
1000
10
500
5
0
Desalination Efficiency
(%)
2
Current density (mA/m )
700
0
0
50
100
150
200
250
300
Time (h)
Fig. 2. MDC current density (, circle) and desalination efficiency (, triangle) change over batch cycle operation. (A) Cations-MDC (middle chamber contains 0.05 M NaCl,
0.05 M MgCl2 , and 0.05 M CaCl2 ) and (B) anions-MDC (middle chamber contains 0.05 M NaCl, 0.05 M NaBr, and 0.05 M Na2 SO4 ). Arrows indicate the electrolyte changes in all
three chambers.
Fig. 3. Cation concentrations decrease in the middle chamber (top) and increase in the cathode chamber (bottom) in a typical batch of cations-MDC operation. Inserted graph
indicates the concentrations increase of Mg2+ and Ca2+ in the cathode chamber, which is significantly less than the respective ion loss from the middle chamber.
20
H. Luo et al. / Journal of Membrane Science 409–410 (2012) 16–23
Fig. 4. Anion concentrations decrease in the middle chamber (top) and increase in the cathode chamber (bottom) in a typical batch of anions-MDC operation.
The hydrated Mg2+ ion has a radius of 0.429 nm, which is the
largest among the three cations, and hydrated Na+ ion (0.365 nm) is
slightly larger than Ca2+ ion (0.349 nm) [21]. It should be noted that
the precipitation of Ca2+ and Mg2+ from the solution could interfere the transport behavior of the divalent ions estimated based
Normalized concentration (-)
1
Na +
0.95
Mg 2+
0.9
3.3. Characterization of membrane fouling and scaling in the
cations-MDC
Ca 2+
0.85
0.8
0.75
0.7
0.65
0.6
0
50
100
150
200
250
Normalized concentration (-)
1
Cl -
0.9
Br2-
0.8
SO4
0.7
0.6
0.5
0.4
on the hydrated radius. In the anions-MDC, because the similar
radii among the tested anions (SO4 2− 0.300 nm, Br− 0.330 nm, Cl−
0.332 nm), the relationship between ion size and transfer rate was
not evident (Fig. 5B), likely due to the pore size of the normal grade
membranes is larger than the studied ions. The slowest removal of
Cl− could be attributed to the low driving force of concentration
gradient between the anode and middle chamber.
0
10
20
30
40
50
60
70
80
Time (h)
Fig. 5. Normalized concentration changes of various ions (the ratio of residue concentration over the initial concentration) in the middle chamber of MDCs indicate
the different transport rates among the different cations (top) and anions (bottom).
After five batches of operation, scaling layers were visible on
the surfaces of both AEM and CEM membranes in the cations-MDC.
Correlated with the declined desalination performance and small
Mg2+ /Ca2+ transfer, EIS measurements also indicated the increase
of ohmic resistance in the system, especially the part of cathode + CEM. Results showed that ohmic resistance of anode + AEM,
cathode + CEM, and the whole reactor increased from 25, 10, and
37 at the beginning of the experiment to 45, 90, and 140 respectively at the end of the fifth cycle (Fig. 6). Therefore, the
cations-MDC was dissembled for membrane autopsy using ESEM
and X-ray EDS.
The ESEM micrographs of the new membranes and both sides
of the used AEM and CEM membranes after five cycles of operation displayed distinctive structure and morphology as a result of
different fouling characteristics (Fig. 7A–F). The used CEM membrane facing the cathode chamber was completely covered by
aggregation-like spherical crystals with a flaky texture (Fig. 7B). In
contrast, a smooth scaling layer was observed on the other side of
the CEM facing the desalination chamber. For the used AEM membrane, flaky crystals were distributed sparsely over the surface in
contact with the middle chamber (Fig. 7F). The AEM facing the
anode chamber was covered by a porous biofouling layer which is
H. Luo et al. / Journal of Membrane Science 409–410 (2012) 16–23
35
Whole MDC
30
-Zimag (ohm)
Anode
25
Cathode
20
15
10
(A)
5
0
0
5
10
15
20
25
30
35
40
60
(B)
-Zimag (ohm)
50
40
30
20
Whole MDC
Anode
10
Cathode
0
0
50
100
150
200
250
Zreal (ohm)
Fig. 6. Nyquist plots used to determine Ohmic resistance of the whole system, the
anode + AEM, and the cathode + CEM, at (A) the beginning and (B) the end of the
experiment.
similar to our previously reported gel-like layer caused by bacteria
embedded in the extracellular matrix (Fig. 7E) [22].
The EDS spectra revealed the major elements of the scaling
layer were calcium, magnesium and phosphate on both sides of
21
the used CEM membrane (Fig. 8). This finding is consistent with
the ion mass balance that the majority of the Ca2+ and Mg2+ were
removed from the middle chamber but was not detected in the
catholyte solution. Carbon, oxygen and sulfur are intrinsic elements
of the CEM ion exchange polymers. The ESEM and EDS results indicate that the CEM membrane was scaled more severely on the side
facing cathode than the side facing desalination chamber. This is
due to the higher pH and salt concentrations of calcium and magnesium as well as phosphate as buffer solution in the cathode
chamber, which enhanced the scaling on the cathode side of the
membrane. It was also observed that phosphate diffused from the
anode and cathode chambers to middle chamber and caused calcium phosphate scaling on the desalination side of the CEM and
AEM membranes. The EDS spectra of the used AEM membrane
facing the anode chamber exhibited the carbon, oxygen, zinc, phosphate, sulfur, chloride and potassium in the fouling layer, of which
carbon, oxygen, and chloride were intrinsic elements in the AEM
ion exchange membrane polymer. The detection of high amount
of carbon, oxygen, phosphate as well as sulfur, zinc and potassium
indicated biofouling and such elements were induced by bacterial
growth.
Based on the EDS analysis, it is confirmed that significant
amounts of calcium, magnesium and phosphate were deposited
on the surface of membranes, especially the CEM membrane. The
presence of membranes in MDC systems prevents free H+ transfer
to the cathode chamber and causes highly alkaline environment of
the catholyte. For example, although buffered with 100 mM phosphate buffer solution (PBS), the pH of catholyte in the anions-MDC
still increased from 7 to above 12 within 24 h of operation (data
not shown). Therefore, calcium and magnesium hydroxide could
be formed on the CEM membrane during the desalination of Ca2+
or Mg2+ . In addition, calcium phosphate (Ksp = 2.07 × 10−33 ) and
magnesium phosphate (Ksp = 1.04 × 10−24 ) may also be formed
Fig. 7. ESEM micrographs showing the surfaces of the membranes before and after the operation of cations-MDC: (A) fresh CEM; (B) cathode side of the CEM; (C) desalination
side of the CEM; (D) fresh AEM; (E) anode side of the AEM; (F) desalination side of the AEM.
22
H. Luo et al. / Journal of Membrane Science 409–410 (2012) 16–23
Fig. 8. Elemental map of the surface of the membranes in the cations-MDC: (A) fresh CEM; (B) cathode side of the CEM; (C) desalination side of the CEM; (D) fresh AEM; (E)
anode side of the AEM; (F) desalinate side of the AEM.
and precipitated on membrane surfaces. The formation of these
precipitations resulted in an increase of membrane resistance
which was confirmed by the EIS results.
3.4. System improvement using anolyte–catholyte recirculation
In the anions-MDC, the current outputs increased linearly during the first 8–10 h and then decreased slowly till the end of
each batch (Fig. 2B). Measured COD value of the anode effluent
was 1200 ± 90 mg/L, with a removal rate of 25%, suggesting that
substrate depletion was not a limiting factor. A further pH characterization showed that the catholyte pH sharply increased from 7
to above 12 within 24 h, while the anolyte pH decreased from 7 to
around 5.6 at the end of a batch. Therefore, it was assumed that
the lack of free H+ ion transfer in both anode and cathode chambers limited the reactor performance in batch operation. Similar
results were reported in previous MDC studies, and high strength
buffer solution or sulfuric acid was used to address the problem
[6,23]. Low buffer capacity has been a major concern in bioelectrochemical system operation, and it was estimated that for each
mole of oxidized substrate an additional amount of 7 mol of buffer
is required to maintain a neutral pH at the anode chamber [4,24,25].
To address the pH imbalance problem in MDC anode and cathode chambers, an anolyte–catholyte recirculation operation was
performed in the anions-MDC. The substrate solution was recirculated continuously and sequentially among the anode chamber, the
cathode chamber, and a 500 mL reservoir. At a flow rate of 5 mL/min
and hydraulic retention time of 30 min (anode chamber) or 10 min
(cathode chamber), the MDC removed up to 90% of mixed salt from
the middle chamber within 110 h of the operation. The desalination
rate reached up to 21.1 g TDS/L d, which was 152% improvement
as compared to the fed-batch operation (8.4 TDS/L d) (Fig. 9). The
electron harvest rate was increased to 18.8 C/h, representing a 98%
increase compared to the non-recirculation mode. In contrast to the
significant pH changes in batch operations as mentioned above,
the pH of the anolyte/catholyte solution only increased from 7.0
to 8.0 after 110 h of operation under the recirculation operation.
These findings are consistent with a parallel study that recirculation is an effective way to balance the pH variation in MDC
reactors and improve system performance [12]. Other approaches
for alleviating membrane scaling may include antiscalant application, modification of membrane surface, and periodic cleaning.
Further studies are underway to optimize materials and reactor
operation for reducing long-term membrane scaling and fouling
potential.
H. Luo et al. / Journal of Membrane Science 409–410 (2012) 16–23
Fig. 9. Comparison of MDC current density and desalination efficiency under
anolyte–catholyte recirculation condition (e.g., Current-R and Desalination-R) and
nonrecirculation condition.
4. Conclusions
This study demonstrated that the MDC performance is sensitive to the composition and different transport mechanisms of the
ion species present in the saline water. Specifically, the presence
of multivalent cations such as Ca2+ and Mg2+ could significantly
decrease MDC power generation and desalination efficiency, while
anions such as Br− and SO4 2− (with Na+ as counter ion) did not
show considerable effects on system performance. The precipitation of Ca2+ and Mg2+ on ion exchange membrane surfaces
was discovered to be the main cause of performance drop. Ionic
mass balance showed that only a small fraction (0.1–0.4%) of the
removed Ca2+ and Mg2+ from the middle chamber could be recovered in the catholyte. ESEM, EDS, and EIS characterizations all
confirmed significant CEM scaling and AEM fouling caused by calcium and magnesium precipitation. Although biofouling occurred
on the AEM membrane facing the anode side, the biofouling did
not exhibit immediate and radical impact on MDC performance as
opposed to membrane scaling. A new operation approach through
anolyte–catholyte recirculation significantly improved system performance, as it stabilized the pH in both anode and cathode
chambers and increased desalination rate by 152%, and electron
harvest rate by 98%, respectively. These findings provide quantitative information for identifying main challenges facing MDC
systems and directing the development of the technology.
Acknowledgement
This work was supported by the Office of Naval Research (ONR)
under Awards N000140910944 and N0001410M0232.
References
[1] X.X. Cao, X. Huang, P. Liang, K. Xiao, Y.J. Zhou, X.Y. Zhang, B.E. Logan, A new
method for water desalination using microbial desalination cells, Environ. Sci.
Technol. 43 (2009) 7148–7152.
[2] K.S. Jacobson, D.M. Drew, Z. He, Use of a liter-scale microbial desalination cell
as a platform to study bioelectrochemical desalination with salt solution or
artificial seawater, Environ. Sci. Technol. 45 (2011) 4652–4657.
23
[3] M. Mehanna, P.D. Kiely, D.F. Call, B.E. Logan, Microbial electrodialysis cell for
simultaneous water desalination and hydrogen gas production, Environ. Sci.
Technol. 44 (2010) 9578–9583.
[4] H.P. Luo, P.E. Jenkins, Z.Y. Ren, Concurrent desalination hydrogen generation
using microbial electrolysis and desalination cells, Environ. Sci. Technol. 45
(2011) 340–344.
[5] Y. Kim, B.E. Logan, Series assembly of microbial desalination cells containing stacked electrodialysis cells for partial or complete seawater desalination,
Environ. Sci. Technol. 45 (2011) 5840–5845.
[6] X. Chen, X. Xia, P. Liang, X.X. Cao, H.T. Sun, X. Huang, Stacked microbial desalination cells to enhance water desalination efficiency, Environ. Sci. Technol. 45
(2011) 2465–2470.
[7] H. Luo, P. Xu, T.M. Roane, P.E. Jenkins, Z. Ren, Microbial desalination cells for
improved performance in wastewater treatment, electricity production, and
desalination, Bioresour. Technol. 105 (2012) 60–66.
[8] C. Forrestal, P. Xu, Z. Ren, Sustainable desalination using a microbial capacitive
desalination cell, Energy Environ. Sci. (2012), doi:10.1039/c2ee21121a.
[9] C. Casademont, G. Pourcelly, L. Bazinet, Effect of magnesium/calcium ratio
in solutions subjected to electrodialysis: characterization of cation-exchange
membrane fouling, J. Colloid Interface Sci. 315 (2007) 544–554.
[10] L. Bazinet, M. Araya-Farias, Effect of calcium and carbonate concentrations on
cationic membrane fouling during electrodialysis, J. Colloid Interface Sci. 281
(2005) 188–196.
[11] J.M. Morris, S. Jin, Influence of NO(3) and SO(4) on power generation from
microbial fuel cells, Chem. Eng. J. 153 (2009) 127–130.
[12] Y. Qu, Y. Feng, X. Wang, J. Liu, J. Lv, W. He, B.E. Logan, Simultaneous water
desalination and electricity generation in a microbial desalination cell with
electrolyte recirculation for pH control, Bioresour. Technol. 106 (2012) 89–94.
[13] F. Zhang, K. Jacobson, P. Torres, Z. He, Effects of anolyte recirculation rates and
catholytes on electricity generation in a liter-scale upflow microbial fuel cell,
Energy Environ. Sci. (2010) 1347–1352.
[14] X. Wang, S.A. Cheng, Y.J. Feng, M.D. Merrill, T. Saito, B.E. Logan, Use of
carbon mesh anodes and the effect of different pretreatment methods on
power production in microbial fuel cells, Environ. Sci. Technol. 43 (2009)
6870–6874.
[15] S. Cheng, H. Liu, B.E. Logan, Increased performance of single-chamber microbial
fuel cells using an improved cathode structure, Electrochem. Commun. 8 (2006)
489–494.
[16] Z. Ren, H. Yan, W. Wang, M. Mench, J. Regan, Characterization of microbial fuel
cells at microbially and electrochemically meaningful timescales, Environ. Sci.
Technol. 45 (2011) 2435–2441.
[17] P. Xu, D. Heil, G. Wang, J.E. Drewes, Treatment of brackish produced water using
carbon aerogel-based capacitive deionization technology, Water Res. 42 (2008)
2605–2617.
[18] Z. Ren, R.P. Ramasamy, S.R. Cloud-Owen, H. Yan, M.M. Mench, J.M. Regan, Timecourse correlation of biofilm properties and electrochemical performance in
single-chamber microbial fuel cells, Bioresour. Technol. 102 (2011) 416–421.
[19] J.W. Post, H.V.M. Hamelers, C.J.N. Buisman, Influence of multivalent ions on
power production from mixing salt and fresh water with a reverse electrodialysis system, J. Membr. Sci. 330 (2009) 65–72.
[20] B. Van der Bruggen, A. Koninckx, C. Vandecasteele, Separation of monovalent
and divalent ions from aqueous solution by electrodialysis and nanofiltration,
Water Res. 38 (2004) 1347–1353.
[21] L. Firdaous, F. Quemeneur, J.P. Schlumpf, J.P. Maleriat, Modification of the
ionic composition of salt solutions by electrodialysis, Desalination 167 (2004)
397–402.
[22] P. Xu, C. Bellona, J.E. Drewes, Fouling of nanofiltration and reverse osmosis
membranes during municipal wastewater reclamation: membrane autopsy
results from pilot-scale investigations, J. Membr. Sci. 353 (2010) 111–121.
[23] K.S. Jacobson, D.M. Drew, Z. He, Efficient salt removal in a continuously operated
upflow microbial desalination cell with an air cathode, Bioresour. Technol. 102
(2011) 376–380.
[24] T.H. Sleutels, H.V. Hamelers, C.J. Buisman, Reduction of pH buffer requirement in bioelectrochemical systems, Environ. Sci. Technol. 44 (2010)
8259–8263.
[25] H.S. Lee, P. Parameswaran, A. Kato-Marcus, C.I. Torres, B.E. Rittmann, Evaluation of energy-conversion efficiencies in microbial fuel cells (MFCs)
utilizing fermentable and non-fermentable substrates, Water Res. 42 (2008)
1501–1510.