Bioresource Technology 120 (2012) 332–336
Contents lists available at SciVerse ScienceDirect
Bioresource Technology
journal homepage: www.elsevier.com/locate/biortech
Short Communication
Microbial desalination cell with capacitive adsorption for ion migration control
Casey Forrestal 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
h i g h l i g h t s
" A new microbial desalination system without releasing salts to the electrolytes.
" Integrated system for organic removal, current production, and desalination.
" Demonstrated activated carbon cloth can be an effective material for MDCs.
a r t i c l e
i n f o
Article history:
Received 31 March 2012
Received in revised form 13 June 2012
Accepted 15 June 2012
Available online 21 June 2012
Keywords:
Microbial desalination cell
Microbial fuel cell
Capacitive deionization
Salt management
a b s t r a c t
A new microbial desalination cell with capacitive adsorption capability (cMDC) was developed to solve
the ion migration problem facing current MDC systems. Traditional MDCs remove salts by transferring
ions to the anode and cathode chambers, which may prohibit wastewater beneficial reuse due to
increased salinity. The cMDC uses adsorptive activated carbon cloth (ACC) as the electrodes and utilizes
the formed capacitive double layers for electrochemical ion adsorption. The cMDC removed an average of
69.4% of the salt from the desalination chamber through electrode adsorption during one batch cycle, and
it did not add salts to the anode or cathode chamber. It was estimated that 61–82.2 mg of total dissolved
solids (TDS) was adsorbed to 1 g of ACC electrode. The cMDC provides a new approach for salt management, organic removal, and energy production. Further studies will be conducted to optimize reactor configuration and achieve in situ electrode regeneration.
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
The sustainable supply of fresh water through saltwater desalination has been developed significantly in the past century, but
one remaining challenge is high energy use during the desalination
process. Popular desalination methods include reverse osmosis
(RO) and multistage flash evaporation (MSF) are considered energy
intensive, because for treating 1 m3 of seawater, RO typically uses
3–7 kwh/m3 of electricity and MSF may require up to 68 kwh/m3
(Avlonitis et al., 2003; Xu et al., 2009). Recently, a new desalination
technology called microbial desalination cell (MDC) was developed
and demonstrated that salt water can be desalinated without using
external energy. Moreover, this process can also simultaneously
achieve wastewater treatment and energy production in the format of electricity or hydrogen gas. (Cao et al., 2009; Jacobson
et al., 2011; Kim and Logan, 2011; Luo et al., 2011, 2012a,b).
MDC reactor uses exoelectrogenic bacteria to oxidize biodegradable substrate (i.e. wastewater) in an anode chamber and transfer
⇑ Corresponding author. Tel.: +1 303 556 5287.
E-mail address: zhiyong.ren@ucdenver.edu (Z. Ren).
0960-8524/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.biortech.2012.06.044
the electrons to the anode. The electron flows through an external
circuit to a cathode, where external electron acceptors (i.e. O2) are
reduced. When a middle chamber is inserted in between the anode
and cathode chamber using a pair of ion exchange membranes,
desalination can be achieved. The potential difference between
the anode and cathode electrodes drives the migration of ions
out of the desalination chamber, with anions (Cl) migrating to
the anode chamber across an anion exchange membrane and
cations (Na+) migrating to the cathode chamber across a cation exchange membrane. The process can remove more than 99% of the
salt water and potentially produce more energy than the external
energy required for the system, making it a promising desalination
process with net energy gaining (Jacobson et al., 2011; Mehanna
et al., 2010) .
One main challenge with the MDC technology is that while the
salts are removed from the middle chamber, they become concentrated in the anode and cathode chambers, which results in salinity
increases in the anolyte and catholyte. While this ion addition is
generally acceptable for wastewater treatment and helps with
conductivity conditioning, it may cause concerns for water benefi-
C. Forrestal et al. / Bioresource Technology 120 (2012) 332–336
cial reuse, where the total dissolved solids (TDS) is regulated (Luo
et al., 2012b; Xu et al., 2008).
One solution to manage salt removal from MDC is to incorporating capacitive deionization (CDI) concept into MDC systems
(Forrestal et al., 2012; Yuan et al., 2012). When a saline solution
flows between two charged electrodes, the ions can be adsorbed
by the double layer capacitor formed on the high surface electrodes. When the potential is removed the ions can be released
back into the liquid to form a concentrate for salt recovery. Using
this integrated deionization approach in combination with the traditional MDC, salt water can be deionized through electrochemical
salt adsorption on the electrodes, so no ions will migrate into the
electrolyte solutions. In this study, the integration of an MDC and
electrochemical adsorption was developed into a single reactor
for capacitive microbial desalination (cMDC). This system demonstrates the feasibility of a new process for concurrent power production and saltwater desalination without contaminating the
anode or cathode chambers.
2. Methods
333
2011). Prior to use, the membranes were pretreated in 10 g/L NaCl
for 24 h to remove impurities and maximize ion exchange capacity.
The Ni/Cu current collectors were connected to a titanium wire that
connected the anode and cathode across a 1 X resistance (Fig. 1).
2.2. Reactor inoculum and growth media
The reactor was initially inoculated with anaerobic sludge and
operated in fed-batch microbial fuel cell mode by using a CEM to
separate the anode and cathode chambers (Luo et al., 2012b).
The anolyte contained per liter: 1.6 g NaCH3COO, 0.62 g NH4Cl,
4.9 g NaH2PO4H2O, 9.2 g Na2HPO4, 0.3 g KCL, and 10 mL trace metals and 10 mL vitamin solution (Ren et al., 2011). The catholyte
contained per liter: 100 mM KFe(CN)6, 5 mM KH2PO4, and 5 mM
K2HPO4. Ferricyanide solution was used as the catholyte to minimize cathode mass transfer effects. The salt solution for desalination contained 10 g/L NaCl. When the repeatable voltage profile
was obtained in three consecutive batches from the MFC, the middle chamber was inserted to the reactor to form a cMDC as
described previously (Forrestal et al., 2012).
2.1. cMDC reactor design
2.3. Reactor operation, analysis and calculations
Each cMDC reactor consisted of three polycarbonate cubeshaped blocks. Each block has a 3-cm diameter hole, and three
blocks were clamped together to form one anode chamber, one cathode chamber, and one desalination chamber, with the volume of 23,
27, and 10 mL, respectively (Fig. 1). ZorflexÒ Activated Carbon Cloth
(ACC) (Chemviron Carbon, UK) was used as the electrode material
and was pretreated by washing in acetone overnight and heating
to 350 °C for 30 min (Wang et al., 2009). The project anode or cathode surface area was approximately 7.1 cm2. An anode assembly
consisted of one layer of AEM (AMX, Astom Corporation, Japan),
one Ni/Cu mesh current collector (Grade 400, McMaster Carr, IL),
and an ACC anode was formed to separate the anode chamber and
the desalination chamber. Similarly, a cathode assembly was formed
by pressing one piece of CEM (CMX, Astom Corporation, Japan), one
layer of Ni/Cu mesh, and an ACC cathode together to separate the
cathode chamber and the desalination chamber. Similar as previous
MDC studies, the ion exchange membranes serve as barriers to allow
selective ion transfer, and the metal current collectors in the assemblies will greatly reduce ohmic resistance and facilitate electron
transfer. The total surface area of the ACC electrodes were 18 cm2
with the weight of 1 gram. The specific surface area of the ACC is
1020 m2/g, determined by the Brunauer-Emmet-Teller (BET)
method (ASAP 2020, Micromeritics, Norcross, GA) (Wang et al.,
The active cMDC reactor was operated in fed-batch mode. Electrolyte conductivity was determined by a conductivity meter (Sension 156, HACH Co., CO), and pH was determined with a pH meter
(Sension 4, HACH Co., CO). Before the reactor was connected for
each batch, the anode and cathode chambers were allowed to
reach the maximum open circuit potential (OCP), which was determined using a potentiostat with a saturated Ag/AgCl reference
electrode (G 300, Gamry Instruments Inc. NJ). Using a data acquisition system (model 2700, Keithley Instruments, Inc. OH), the voltage across the external resistor was recorded every 1 min.
Conductivity and pH measurements for all three chambers were
taken at the beginning and the end of each desalination cycle. Conductivity was converted to TDS (mg/L) using the HACH Co. general
calculation equation presented below.
mS=cm 500 ¼ mg=L
The ACC assemblies were manually regenerated by removing
the ACC from the reactor and rinsed in 1 L of deionized (DI) water
for 30 min. The DI rinse was repeated a total of three times till zero
conductivity was shown in rinse solution. Then the electrodes were
added back to the reactor for additional experiments. Negative
control experiments were conducted by removing the acclimated
anode and replacing it with an unacclimated piece of ACC and performed as described previously for desalination experiments
(Forrestal et al., 2012).
R
3. Results and discussion
Microbes
3.1. Removal and adsorption of desalination chamber salts
Cl-
Na+
Acetate
Cl-
Na+
CO2
Cl-
Na+
Ferricyanide
Ferrocyanide
ACC
Ni/Cu
AEM
NaCl
Feed
ACC
CEM
Ni/Cu
Fig. 1. cMDC reactor configuration and operation with desalinated ions being
physically and electrically adsorbed by the ACC.
During cMDC operation an electrical potential is formed on the
ACC across the anode and the cathode due to microbial oxidation of
substrate and electron transfer to the cathode. This potential forms
a double layer capacitor on the ACC electrodes, which adsorbs ions
to achieve water deionization. At the start of the desalination cycle
the cMDC reactor had an OCP of 712 mV. When a 1-ohm resistor
was used in the circuit, a maximum current of 2.5 mA was generated. The reactor was operated for 3 months, and Fig. 2 shows the
production of electric current across the resistor is proportional to
the percent removal of NaCl in the desalination chamber for three
consecutive batch cycles. The substrate consumption and ion loss
in the desalination chamber caused an increase of internal resis-
334
C. Forrestal et al. / Bioresource Technology 120 (2012) 332–336
2.5
80
70
2
Current
(mA)
50
1.5
40
1
30
%NaCl Removed
Current (mA)
60
% NaCl
Removed
20
0.5
10
0
0
0
50
Time (hr)
100
150
Fig. 2. Three typical cycles of cMDC current generation and NaCl removal profile. The arrows indicate the manual washing and regenerating the ACC electrodes.
20
Conductivity (mS/cm)
18
16
Initial
14
Final
12
Negative
Control
10
8
6
4
2
0
Desalination
Anode
Cathode
Fig. 3. The initial and final conductivity for the anode, cathode, and desalination chamber for the active cMDC system, as well as the negative control reactor.
9
8
Initial
pH
7
6
Final
5
Negative
Control
4
3
2
1
0
Desalination
Anode
Cathode
Fig. 4. The initial and final pH for the anode, cathode, and desalination chamber for the cMDC, as well as the negative control reactor.
tance, which resulted in a decrease of current along with one batch
cycle. Such phenomenon is consistent with other MDC studies
using similar reactor configurations (Cao et al., 2009; Luo et al.,
2012b). Unlike traditional MDC batch operations, which migrate
salts from one chamber to another, the cMDC stores the removed
salts in the electrode assemblies. Therefore, after a period of desalination operation, the salts need to be removed from the assemblies during regeneration, which will shortly interrupt
desalination operation. Traditional 3-chamber cube MDC configuration was used in the study to demonstrate the feasibility of the
process, and system optimization such as using stack configura-
tions with narrow chambers and/or operating the reactor in a continuous mode have been shown effective to address the increased
resistance problem (Chen et al., 2011; Kim and Logan, 2011; Qu
et al., 2012).
Fig. 3 shows the salt removal from three chambers as represented by conductivity changes. The average salt removal in desalination chamber during one batch cycle was 69.4%, which
correlates to the removal of 69.4 mg (1.19 mM) of NaCl from the
10 mL desalination chamber. The 1.19 mM of salt removed from
the desalination chamber correlates to 42.2 mg of Cl migrated
to the anode and 27.2 mg of Na+ migrated to the cathode. The an-
C. Forrestal et al. / Bioresource Technology 120 (2012) 332–336
ode chamber conductivity in average increased slightly by 0.7% or
1.1 mg presumably due to the limited diffusive ion release from
the anode. Such ion balance suggests that 41.1 mg of chloride
was electrochemically adsorbed on the anode, which represents
97.3% of the desalinated chloride from the desalination chamber.
In the cathode chamber the conductivity decreased in average by
1.4% or by 3.2 mg. Therefore the cathode chamber with the ACC adsorbed 30.4 mg of salt or 100% of the desalinated salt plus an additional 12% from the cathode chamber. These results indicate that
the electrical adsorption capacity of the ACC assembly was between 61–82.2 mg TDS/g ACC. Such results are comparable to the
findings from a previous study, which showed the physical and
electrical adsorption was 72.7 mg/g ACC (Forrestal et al., 2012).
This salt removal profile from the desalination chamber is different
from traditional MDC systems, because the removed salts didn’t
transfer from the desalination chamber to the anode and cathode
chamber but rather got adsorbed onto the ACC electrodes. The conductivity of the anolyte and catholyte were kept quite stable,
which prevented significant salinity changes that might affect
effluent reuse. The abiotic control reactor without microbial activities showed no current generation or desalination performance, as
shown in Fig. 3. This finding confirms that the potential generated
by the microbial exoelectrogenic activities was the driving force of
desalination.
3.2. The change in pH over the course of desalination
While the initial pH of all three chambers was measured at 6.8,
the final pH after a batch cycle varied between chambers. As
shown in Fig. 4, the desalination chamber pH slightly decreased
to 6.4 after a batch cycle. The anolyte pH dropped to 5.9, while
the catholyte pH increased to an average of 7.9. Such pH variations
have been reported by previous MDC studies, because the accumulation of protons in the anode chamber caused pH drop, while the
loss of protons in the cathode chamber due to water formation led
to pH increase. To alleviate pH fluctuation, the anion exchange
membrane maybe replaced by a cation exchange membrane to allow free proton transfer, or electrolyte recirculation can be implemented to neutralize the anolyte and catholyte (Forrestal et al.,
2012; Luo et al., 2012a; Qu et al., 2012). The negative control without microbial activities showed no pH changes, confirming no electrons or protons were transferred in the system.
3.3. The potential and challenge of the cMDC configuration
Compared with traditional MDC systems, where the salt removal from the middle chamber is accompanied by the salinity increase in the anode and cathode chambers, this cMDC
configuration is able to incorporate capacitive deionization with
microbial desalination and captures salts on electrodes without
releasing the salts to the electrolytes. This integrated process addressed the concerns of increased salinity on cMDC effluent reuse
and provides a new approach for more complete salt management.
Moreover, compared with a recent study conducted by our group,
which used separated carbon brush anode and carbon cloth cathode (Forrestal et al., 2012), this cMDC configuration significantly
simplifies the system design, because it eliminates additional electrodes and directly uses the activated carbon cloth assembly as the
electrodes. This study, however, adopted traditional MDC approaches by using one anion and one cation exchange membrane
rather than two cation membranes, which showed more pH variations in the anode and cathode chamber.
By directly using high surface activated carbon electrodes, the
cMDC was able to adsorb ions on the double layer capacitor formed
on the electrode surface. However, it is not clear how the increased
ion concentration might affect anode biofilm activity and commu-
335
nity on the electrode. During this study, manual cleaning and
regeneration of electrodes was performed after reactor operation,
which prevented a mass balance or salt recovery calculation. It also
impacted the microbial community evident in the last cycle of
Fig. 2, which showed lower current production. Further studies will
be conducted to develop in situ electrode regeneration methods.
One possible method could be to develop a reactor that can switch
the ACC electrodes in situ once they have become fully adsorbed.
Switching the electrodes would cause the adsorbed salts to desorb
due to the reverse potential, which would solve the problem of
having to manually regenerate the electrodes (Forrestal et al.,
2012).
4. Conclusion
This study presents a step forward in sustainably desalinating
salt water with a capacitive microbial desalination cell. The cMDC
reactor was capable of removing an average of 69.4% of the salt
from the desalination chamber through electrochemical ion
adsorption on the electrodes without adding salinity to the anode
or cathode chamber. The physical and electrical adsorption capacity of the ACC electrodes was between 61 and 82.2 mg/g ACC. Further studies are needed to improve system efficiency and develop
in situ ACC regeneration process. By combining this process with
optimized reactor configurations a sustainable method of desalination can be obtained.
Acknowledgements
This work was partially supported by the National Science
Foundation under Award CBET-1235848.
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