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Cheng, K.Y., Kaksonen, A.H. and Cord-Ruwisch, R. (2013)
Ammonia recycling enables sustainable operation of bioelectrochemical systems. Bioresource Technology,
143 . pp. 25-31.
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Ammonia recycling enables sustainable operation of bioelectrochemical sys‐
tems
Ka Yu Cheng, Anna H. Kaksonen, Ralf Cord-Ruwisch
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DOI: http://dx.doi.org/10.1016/j.biortech.2013.05.108
Reference: BITE 11895
To appear in: Bioresource Technology
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Please cite this article as: Cheng, K.Y., Kaksonen, A.H., Cord-Ruwisch, R., Ammonia recycling enables sustainable
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1
Bioresource Technology
Ammonia recycling enables sustainable operation of
bioelectrochemical systems
Ka Yu Cheng1*, Anna H. Kaksonen1, Ralf Cord-Ruwisch2
1 CSIRO Land and Water, Floreat, WA 6014, Australia
2 School of Biological Science and Biotechnology, Murdoch University, WA 6150,
Australia
* Correspondence: +61(8) 9333 6158; fax: +61(8) 9333 6211; email:
2
Abstract
Ammonium (NH4+) migration across a cation exchange membrane is
commonly observed during the operation of bioelectrochemical systems (BES). This
often leads to anolyte acidification (< pH 5.5) and complete inactivation of biofilm
electroactivity. Without using conventional pH controls (dosage of alkali or pH
buffers), the present study revealed that anodic biofilm activity (current) could be
sustained if recycling of ammonia (NH3) was implemented. A simple gas-exchange
apparatus was designed to enable continuous recycling of NH3 (released from the
catholyte at pH > 10) from the cathodic headspace to the acidified anolyte. Results
indicated that current (110 mA or 688 A m-3 net anodic chamber volume) was
sustained as long as the NH3 recycling path was enabled, facilitating continuous
anolyte neutralization with the recycled NH3. Since the microbial current enabled
NH4+ migration against a strong concentration gradient (~10 fold), a novel way of
ammonia recovery from wastewaters could be envisaged.
Keywords: microbial fuel cells, microbial electrolysis cells, proton gradient, pH split,
cation exchange membrane
3
1 Introduction
Bioelectrochemical systems (BES) have shown promise for the conversion of
organic compounds in wastewaters into valuables (e.g. electricity, fuel gases,
chemicals, etc) (Logan et al. 2008; Rabaey and Verstraete 2005; Rittmann 2008).
Arguably, the microbial catalysed anodic reaction is the most critical reaction in a
BES process as it transforms the chemical energy captured in the organics directly
into electrical energy. Typically, this reaction not only produces electrons, but also
liberates protons (H+). For instance, anodic oxidation of one mole acetate liberates
nine mole H+ and eight moles electrons according to the following equation (Schroder
2007):
CH3COO-+4H2O → 2HCO3- + 9H+ + 8e-
Since the electrons are continuously scavenged by the anode, the liberated H+
accumulate and lead to anolyte acidification (Marcus et al. 2011). This has been
shown to severely inhibit the catalytic activity of anodic biofilms and consequently
leading to a complete shutdown of the BES (Cheng et al. 2010; Clauwaert et al. 2007;
2008). In general, a pH neutral condition (pH 7, i.e. [H+]= 10-7 mol L-1) is essential to
sustain optimal anodic microbial activities (He et al. 2008; Patil et al. 2011).
In theory, pH neutral anodic condition can be sustained if the requirement of
cation migration (to establish charge balance) is satisfied exclusively by proton
migration, resulting in the proton flux from anode to cathode being equal to the
electron flow and the proton generation at the anode. However, in reality cations other
than protons (Na+, K+ , Ca2+, Mg2+, NH4+) are available in much higher concentrations
and will hence tend to migrate instead of protons. The higher the concentration of
other cations is relative to protons, the lower is the likelihood of proton migration.
Under typical experimental conditions the concentration of alkali cations such as Na+
4
is orders of magnitudes higher than that of protons resulting in the migration of Na+
from anode to cathode (Harnisch et al. 2008; Rozendal et al. 2006a). This effect has
been suggested to be industrially exploited for the production of caustic soda (Rabaey
et al. 2010).
In laboratory trials, the problem of anolyte acidification is often masked by
dosage of pH buffer or alkali (Cheng et al. 2010; Rozendal et al. 2008a). However,
these pH control strategies are not sustainable as pH controlling chemicals must be
externally added to the process. To approach self-sustaining pH control for CEM-BES
without using external chemicals, the migrating cation species across the CEM should
posses four properties: (1) it must be an alkaline species neutralizing the excess
protons in the anolyte; (2) upon reacting with proton it becomes a cation that migrates
across the CEM to the catholyte to maintain charge balance; (3) in the catholyte, it
readily dissociates and releases the proton and hence replenishes the proton consumed
in the cathodic reaction; (4) upon releasing the proton in the catholyte, the species can
be recycled for neutralizing the anolyte again (Cord-Ruwisch et al. 2011).
Amongst all cation species typically found in wastewaters (e.g. Na+, K+ , Ca2+,
Mg2+, NH4+), ammonium (NH4
+) is the only species that fulfils all the above criteria.
It has a characteristic acid-dissociation constant (pKa value) of 9.25 (25oC). Hence,
once it has migrated across a CEM to the catholyte, where the localized pH exceeds
9.25, it dissociates predominately as free volatile ammonia (NH3) which can be
recovered as a gas (Figure 1). The concept of using NH4+ as a proton shuttle in a
CEM-equipped BES (CEM-BES) has been evaluated in our recent work (Cord-
Ruwisch et al. 2011). The ammonia recycling was achieved by continuously stripping
the catholyte with nitrogen (N2), which was directed through the acidified anolyte to
close the loop. N2 was used to maintain anaerobic condition required for the microbial
5
anodic reaction. However, N2 stripping is impractical as it incurs substantial energy
input and creates large volume of low-value off-gas. In order to use the very effective
principle of ammonia recycling for the sustainable operation of BES, a simple low
energy input approach is required.
This work examined a new approach to sustain current generation in a CEM-
BES by internal ammonia recycling without using N2 stripping. The idea is based on
the well-known phenomenon where under anaerobic and highly reducing conditions
(e.g. ≤ -500 mV vs. Ag/AgCl), a BES cathode produces hydrogen gas. This gas
stream is in principle a vector that could help driving the volatilized ammonia out of
the cathodic half cell (Liu et al. 2005; Logan et al. 2008; Rozendal et al. 2006b;
Rozendal et al. 2008b). The aim of this study was to develop an effective way of
sustaining BES operation by maintaining suitable anodic pH levels using ammonia
from the cathode as the alkalinity carrier. In contrast to our previous work (Cord-
Ruwisch et al., 2011), a more efficient (10 fold higher current density) anodic biofilm
was used to demonstrate the concept of ammonia recycling in a BES.
6
2 Materials and methods
2.1 Bioelectrochemical system configuration and process monitoring
A two-chamber CEM-BES was used in this study. It was made of transparent
Perspex and has a similar configuration as the one described in an earlier work
(Cheng et al. 2008). The two half cells were of equal volume and dimension (316 mL,
14 cm 12 cm 1.88 cm). They were physically separated by a cation exchange
membrane (CMI-7000, Membrane International Inc.) which has a surface area of 168
cm2. Both chambers were filled with conductive granular graphite (3-6 mm diameter),
which reduced the void volume of the working chamber from 316 to 160 mL. A
graphite rod (diameter 5 mm) was inserted into each half cell to allow electric contact
between the graphite granules and the external circuit. In this study, only one half cell
was inoculated with bacteria and was operated as an anodic half cell, which is termed
here as the working chamber. The other half cell (cathodic) is termed as the counter
chamber.
The graphite granules inside the working chamber (i.e. working electrode) was
polarized against a silver-silver chloride (Ag/AgCl) reference electrode (saturated
KCl) at a potential of -300 mV by using a potentiostat (Model no. 362, EG&G,
Princeton Applied Research, Instruments Pty. Ltd.). This potential was selected as it
facilitated effective anodic acetate oxidation that could significantly acidify the
anolyte if no effective pH control was implemented. The Ag/AgCl electrode was
mounted inside the working chamber at a distance of less than one cm away from the
working electrode. All electrode potentials (mV) in this article refer to values against
the Ag/AgCl reference (ca. +197 mV vs. standard hydrogen electrode, (Bard and
Faulkner 2001)). The working electrolyte redox potential and the pH of both working
and counter electrolyte were continuously monitored. A computer program
7
(LabVIEW™, National Instrument) was developed to continuously control and
monitor the bioprocess. An analog input/ output data acquisition card (National
InstrumentTM) was used to interface between the computer and the potentiostat. The
BES current was monitored directly from the potentiostat. The electrode potentials,
redox potentials and pH voltage signals were recorded at fixed time intervals and all
data were regularly logged into an Excel spreadsheet.
2.2. Process start-up and general operation
To start up the BES process, the working chamber was inoculated with returned
activated sludge collected from a municipal sewage treatment plant (Subiaco, Perth,
WA) (final mixed liquor suspended solid concentration was ca. 2 g L-1). A synthetic
wastewater medium with a limited pH buffering capacity was used as both the
working and counter electrolyte throughout the study. It consisted of (mg L-1): NH4Cl
125, NaHCO3 125, MgSO4·7H2O 51, CaCl2·2H2O 300, FeSO4·7H2O 6.25, and 1.25
mL L-1 of trace element solution, which contained (g L-1): ethylene-diamine tetra-
acetic acid (EDTA) 15, ZnSO4·7H2O 0.43, CoCl2·6H2O 0.24, MnCl2·4H2O 0.99,
CuSO4·5H2O 0.25, NaMoO4·2H2O 0.22, NiCl2·6H2O 0.19, NaSeO4·10H2O 0.21,
H3BO4 0.014, and NaWO4·2H2O 0.050 (Cheng et al. 2010). The working electrolyte
was supplemented with yeast extract (50 mg L-1 final concentration) as bacterial
growth supplement during the initial start up period (ca. 2 weeks). During this period,
the entire medium was refreshed once every 4 days.
Acetate was used as the sole electron donor substrate for the anodic biofilm. A
known amount of acetate standard solution (1 M) was added to the working
electrolyte to obtain a desired acetate concentration (ranged from 1 to 50 mM). Unless
stated otherwise, each half cell was hydraulically linked to a separate glass
8
recirculation bottle, which accommodated the extra volume of electrolyte; 0.5 and 0.7
L of anolyte and catholyte were continuously recirculating through the anodic and
catholyte half cell, respectively at a rate of 6 L h-1. The CEM-BES was operated in
batch mode at ambient temperature (25 ± 3oC).
2.3. Ammonia recycling apparatus and its integration with the CEM-BES
An ammonia-recycling apparatus was designed to enable recycling of ammonia
from the cathodic headspace to the anolyte (Figure 2). It was constructed by gluing
two 390-mL modified polyethylene terephthalate-made plastic containers together
side by side with a common open window between them (5 cm × 4 cm). The two
containers were hydraulically separated from each other but were hydraulically
connected with the anodic and cathodic half cell of the BES, respectively. As such,
one container received the anolyte recirculation stream and the other the catholyte
recirculation stream. The common window was sealed with a gas permeable fibre
cloth (6 cm × 5 cm), which was mounted at the anodic side and was continuously
trickled with the anolyte. After trickling through the cloth, the anolyte was
recirculated back to the BES anode.
As both the influx and out flux rate of anolyte were identical (6 L h-1), a fixed
volume of anolyte (ca. 50 mL) was always retained in the apparatus. At the cathodic
side of the apparatus, the catholyte from the BES cathode was continuously sprinkled
over the catholyte reservoir (ca. 50 mL) to facilitate ammonia volatilization within the
cathodic container. Since the cathodic BES half cell was gas-tight, any build up of gas
pressure would help drive the ammonia-containing gas in the cathodic container
through the gas permeable cloth and the laminar flow of anolyte. This facilitated the
dissolution of ammonia gas into the acidified anolyte. After retrofitted with the gas
9
exchange apparatus, the total volume of both the working and counter electrolyte was
reduced to about 220 mL. The recirculation rates of both the anolyte and catholyte
were kept at 6 L/h, corresponding to a hydraulic retention time of about 30 seconds
for both anolyte and catholyte in the apparatus.
2.3 Experimental procedures
Experiments were conducted to first investigate the effect of anolyte
acidification on the anodic biofilm activity (current production). No ammonia
recycling was implemented during this period. The initial biofilm establishment was
facilitated by actively controlling the anolyte pH at around 7 using feedback dosing of
NH4OH (1M). The dosage was computer recorded to obtain the NH4OH application
rate. The catholyte was not pH controlled and hence it would become significantly
alkaline, facilitating the NH3 volatilization at the cathodic half cell. No aeration was
provided to the catholyte. At the beginning of each batch run, the ammonia recycling
apparatus was flushed with N2 to obtain anaerobic condition. The effect of anolyte
acidification was examined by comparing the current obtained with or without active
dosing of NH4OH. Coulombic efficiencies of the anodic acetate oxidation (5mM)
under pH neutral and acidified conditions were also compared (Logan et al. 2006). At
selected time intervals, aliquots (1 mL) of anolyte and catholyte were sampled for
chemical analysis.
To demonstrate the effect of recycling the ammonia containing off-gas from
the cathode to the anodic half cell, the catholyte in the recirculating bottle was
continuously stripped with a pure nitrogen gas stream (~1 L min-1) which was then
introduced back into the anolyte, completing an NH3 recycling loop. Current, anolyte
pH, acetate, ammonium, sodium and potassium concentrations were recorded over a
10
period of two days with or without NH3 looping. To verify whether the ammonia
transfer from the cathodic headspace to the anolyte could sustain the anodic biofilm
activity, the BES was retrofitted with the ammonia recycling apparatus as described
above. The effect of looping (i.e. gas exchanged was enabled via gas permeable cloth)
on electrolyte pH and current was evaluated over an operational period of about 100
hours.
2.4 Chemical analysis
Liquid samples taken from the BES were immediately filtered through a 0.2
µm filter (0.8/0.2 µm Supor® Membrane, PALL® Life Sciences) and were stored at
4oC prior analysis. Acetate in the samples was analyzed using a Dionex ICS-3000
reagent free ion chromatography (RFIC) system equipped with an IonPac® AS18 4 x
250 mm column (Cheng et al., 2012). 10 µl of the sample was injected into the
column. Potassium hydroxide was used as an eluent at a flow rate of 1 mL min−1. The
eluent concentration was 12-45 mM from 0-5 min, 45 mM from 5-8 min, 45-60 mM
from 8-10 min and 60-12 mM from 10-13 min. Ammonium (NH4+ -N), potassium and
sodium in the filtered samples were measured with the same RFIC with a IonPac®
CG16, CS16, 5 mm column. Methansulfonic acid was used as an eluent with a flow
rate of 1 mL min-1. The eluent concentration was 30 mM for 29 min. The temperature
of the two columns was maintained at 30°C. Suppressed conductivity was used as the
detection signal (ASRS ULTRA II 4 mm, 150 mA, AutoSuppressioin® recycle
mode).
11
3 Results and discussion
3.1 Anolyte acidification severely limited current generation
The reactor described above was started up and operated to quantify the effect
of pH drifts typically observed in CEM-equipped bioelectrochemical systems. After
approximately two days of incubation, anodic current evolved gradually indicating
that the microbial inoculum in the anodic chamber became electrochemically active.
As expected, the current generation coincided with an acidification of the anolyte (pH
5.5) and an alkalization of the catholyte (pH 13), respectively (Figure 3A).
As the working electrode (biofilm anode) was maintained at a constant
potential (-300 mV) and the substrate was not limiting (acetate, > 10 mM), the
observed current decline which followed the anodic acidification indicated that the
low pH had suppressed the anodic activity of the biofilm. Similar observation has
been reported by others (Cheng et al. 2010; Harnisch and Schroder 2009; He et al.
2008; Sleutels et al. 2010).
3.2 Effect of anolyte neutralisation on current production
The pH neutralisation by the traditional NaOH addition was replaced by using
an ammonium hydroxide solution. Neutralizing the anolyte acidity to pH 7 by dosing
ammonium hydroxide immediately resumed current production (at 110 h, Figure 3).
Prolonged current generation necessitated the demand of ammonium hydroxide,
suggesting that this pH control approach could effectively sustain the anodic activity
of the biofilm. As long as the anolyte pH was maintained neutral, the established
12
biofilm could effectively catalyze acetate oxidation with a good coulombic recovery
(>80%) (Figure 4).
The intermittent dosing of ammonium hydroxide into the anolyte resulted in a
gradual increase of ammonium concentration in the catholyte, reaching a level of
about 90 mM NH4+-N (Figure 3B). Although ammonium hydroxide was continuously
added to the anolyte, its concentration stayed approximately 10-fold lower than in the
catholyte (Figure 3B). Clearly, the ammonium kept migrating against its
concentration gradient from the anolyte to the catholyte across the cation exchange
membrane. Of the amount of ammonium migrated from the anolyte to the catholyte,
88.7% of the ammonium was lost from the catholyte (data not shown). Such a loss
was most likely due to the high catholyte pH (>11) that favours the dissociation of
ammonium as free ammonia and its volatilization from the catholyte.
3.3 Effect of NH3 recycle from cathode to anolyte via N2 gas stream
To quantify to what extent recycling the migrated ammonium from catholyte
back to the anolyte could overcome the detrimental anolyte acidification, N2 gas was
purged in series through the catholyte and then through the anolyte (Figure 5). Before
allowing the N2 flow, the CEM-BES responded to the addition of an acetate spike (10
mM) not only by producing an anodic current (Figure 5A) and degrading acetate, but
also, as in Figure 4, by an associated pH drop in the anolyte (Figure 5B). As expected,
the anolyte acidification had slowed down the anodic activity of the biofilm and the
current declined significantly even though acetate was still present in excess (>3 mM).
When switching on the N2 flow (~21-27 h, Figure 5B), the current and acetate
degradation resumed as explained the rise in pH caused by the alkalinity transfer from
13
the catholyte (as ammonia) to the anolyte. The BES now operated sustainably without
a marked drift in anolyte pH. Repeating the stopping and starting of ammonia
exchange by N2 transfer showed the same principal effect.
Further, the result also suggested that cations other than ammonium (sodium
and potassium) also migrated across the CEM from the anolyte to the catholyte, but
unlike ammonium which could be removed from the catholyte as a gas, both sodium
and potassium accumulated in the catholyte during current production (Figure 5 C and
D). This observation clearly highlights the uniqueness of ammonium as both the
charge-balancing species and recyclable alkalinity carrier in a BES system.
3.4 Diffusional ammonia transfer from cathode to anode
Using nitrogen to strip off the ammonia gas from the catholyte and recycle the
ammonia containing nitrogen gas stream into the anolyte incurs extra energy to the
extent where the energetic sustainability of a BES would become questionable.
Further, in BES where H2 is produced at the cathode, this valuable fuel gas would be
lost via dilution with N2. Instead of continuously purging N2 through the cell, the
recycling of ammonia could in theory be done by allowing the ammonia gas to vent
from the catholyte and diffuse back to the anolyte. For this purpose a sufficiently
large “gas exchange window” between the gas space of anodic and cathodic chamber
would need to be used. This could be accomplished in a number of ways, including
the use of a gas permeable membrane.
To test whether diffusional gas exchange between the gas spaces of anodic and
cathodic chamber enable adequate ammonia transfer, a separate apparatus was
14
designed to act as the extended gas space of the two chambers (Figure 2). The anolyte
recycle was used to wet a vertically mounted gas permeable cloth that acted as the gas
diffusion window, while the catholyte recycle was via a spray to facilitate NH3
transfer from the alkaline catholyte to the gas phase (Figure 2). The vented NH3 in the
gas phase of the apparatus is expected to readily dissolve in the water saturated cloth
which served as an ammonia scrubber. The anolyte acidity was thus neutralized with
the aim of sustaining the anodic microbial activity. Below the effectiveness of this
ammonia looping technique for sustaining the microbe-driven current of a CEM-BES
is described (Figure 6).
With the ammonia recycling apparatus (Figure 2) disconnected, the CEM-BES
responded to the addition of an acetate spike (10 mmols) by instantly producing an
anodic current and a decrease in anolyte pH to about 5.8. At this stage, another acetate
addition (Ac 2 in Figure 6) did not resume the current demonstrating again that the
CEM-BES had become inactive due to acidification of the anolyte. As soon as the gas
exchange by the described apparatus was enabled, the anolyte pH was neutralised
almost immediately (Figure 6B) allowing sustained current close to the maximum rate
of this particular anodic biofilm as long as acetate was available. Renewed acetate
addition at 55 h resumed the current. When the gas exchange between anolyte and
catholyte was interrupted (~78 h) the anodic current could not be sustained again due
to the anolyte acidification. By merely allowing the gas diffusion window between
anodic and cathodic chamber (at ~96 h), continued current production could be
sustained for several days as long as the anodic substrate (acetate) was not limiting
(data not shown).
15
It was noticed that a net gas pressure build-up in the system occurred,
presumably due to cathodic hydrogen production as the system was anaerobic and the
cathode was maintained at highly reducing (negative) potentials (< -2V vs. Ag/AgCl,
Figure 6C). This gas was released from the system via the vent (Figure 2), and may
have participated in stripping of the NH3 from the catholyte to the gaseous phase.
Hydrogen transfer from the cathodic chamber to the ammonia recycling apparatus can
be explained by minute bubbles visible in the catholyte. However, due to its poor
solubility compared to ammonia the transfer of hydrogen to anodic chamber is
expected to be minimal.
3.5 Practical considerations
The experiments suggest that sustained current production in a CEM-BES is
possible only if the anodic biofilm was operated under a neutral pH condition.
Maintaining such a condition is difficult particularly for systems designed for high
current output (Marcus et al. 2011; Picioreanu et al. 2010). This study demonstrated
that without using conventional active pH control methods (e.g. regular dosage of
alkali hydroxide or pH buffering chemicals), a relatively high anodic current (110 mA;
688 A m-3 net anodic chamber volume; 500 A m-3 total anolyte volume) could be
sustained if a simple ammonia recycle via gas diffusion was implemented.
Although the diffusivity of protons is about five times higher than that of
ammonium (9.31 × 10-5 vs. 1.96 × 10-5 cm2 s-1 (Vanysek 2000)), in the absence of
ammonium the current is limited by the slow proton flux because of the low proton
concentrations (10-6 M at pH 6). The presence of about 10 mM (10-2 M) of
ammonium to the anode can enable an up to 104 times higher cation flux from anode
16
to cathode and hence a higher current. While high currents can also be guaranteed
with the addition of other cations such as sodium, as is the case when using traditional
sodium hydroxide based pH control of the anolyte (Cheng et al. 2008; Cheng et al.
2010), dosage of sodium hydroxide requires substantial energy costs (Cord-Ruwisch
et al 2011) and intermittent renewal of the catholyte due to sodium accumulation
(Rabaey et al. 2010).
Nevertheless, the proposed concept has several constraints that may limit its
practical use. For instance, this approach may only be applied in a microbial
electrolysis mode as the production of hydrogen is essential to create a carrier stream
to bring the ammonia from the catholyte to the anolyte. The high pH environment
(>pH 9.25) required for the shift of NH4+/NH3 equilibrium in favour of ammonia
stripping may also limit the use of this approach for (cathodic) bioelectrosynthesis,
which typically occurs under neutral pH condition. Further, the use of ammonia
recycling in a BES can be unacceptable if the presence of ammonia in the anolyte is
undesirable (e.g. wastewater treatment).
If relying on ammonia as the proton carrier in a BES, the use of N2 as the
recycling agent of ammonia has been described as a proof of concept (Cord-Ruwisch
et al 2011). However, due to costs and dilution of cathodic hydrogen the use of
external N2 is likely to be impractical for most applications. The gas exchange
apparatus described in the present study showed immediate effects on anolyte
neutralization and the production of anodic current, suggesting that the current was
not limited by the rate of ammonia recycle but by the rate of microbial electron
delivery to the anode. Accordingly, simpler and smaller gas exchange devices could
be designed. Further research is warranted to explore how efficient a BES process
17
with the proposed NH3 recycling mechanism could sustain cathodic hydrogen
production.
A further step towards implementing low energy ammonia recycle as a build-
in proton carrier in BES could be the use of a gas permeable membrane between
anolyte and catholyte or to develop and use a gas permeable cation exchange
membrane. Such a system would not depend on the physical movement of anolyte and
catholyte also should enable a more effective low-energy NH3 recycle. To what extent
membrane-free BES could profit from ammonium as proton carriers is yet to be
investigated. It is conceivable that by using gas permeable membranes as part of the
ion exchange separator (e.g. a CEM that is also gas permeable) the ammonia recycle
could become a seamless feature of a BES.
The presented work shows that ammonia readily migrates from the anode to
the cathode against a rather strong diffusion gradient (~10 fold, Figure 3B). In
principle that would mean that ammonium containing wastewaters treated in the BES
anode would be likely to lose ammonia by migration to the catholyte. If this effect can
be used to concentrate up ammonia by 10 or even 100 fold a novel way of ammonia
recovery from wastewater could be envisaged.
4 Conclusions
Overall, this study demonstrates that by continuously recycling ammonia from
the catholyte to the anolyte in a CEM-BES, neutral pH condition desirable for the
anodic biofilm could be maintained without using conventional pH control methods
(e.g. regular dosage of alkaline hydroxide or pH buffering chemicals). The use of the
18
proposed gas exchange device was effective to recycle the ammonia without using
energy-intensive N2 stripping of the catholyte. This approach of pH control in BES
systems has not been reported elsewhere, and it has the potential to be further
developed to achieve recovery of ammonia from wastewaters.
5. Acknowledgement
This work was funded by the CSIRO Water for a Healthy Country Flagship.
6. References
1. Bard AJ, Faulkner LR. 2001. Electrochemical methods: fundamentals and
applications. New York, USA: John Wiley & Sons, Inc.
2. Cheng KY, Ho G, Cord-Ruwisch R. 2008. Affinity of microbial fuel cell biofilm
for the anodic potential. Environ. Sci. Technol. 42, 3828-3834.
3. Cheng KY, Ho G, Cord-Ruwisch R. 2010. Anodophilic biofilm catalyzes
cathodic oxygen reduction. Environ. Sci. Technol. 44, 518-525.
4. Cheng KY, Ginige MP, Kaksonen, AH. 2012. Ano-cathodophilic biofilm
catalyzes both anodic carbon oxidation and cathodic denitrification. Environ. Sci.
Technol. 46, 10372-10378.
5. Clauwaert P, Van Der Ha D, Boon N, Verbeken K, Verhaege M, Rabaey K,
Verstraete W. 2007. Open air biocathode enables effective electricity generation
with microbial fuel cells. Environ. Sci. Technol. 41, 7564-7569.
6. Cord-Ruwisch R, Law Y, Cheng KY. 2011. Ammonium as a sustainable proton
shuttle in bioelectrochemical systems. Bioresource Technology 102, 9691-9696.
19
7. Harnisch F, Schroder U. 2009. Selectivity versus mobility: How to separate
anode and cathode in microbial bioelectrochemical systems? Chem. Sus. Chem.
2, 921-926.
8. Harnisch F, Schroder U, Scholz F. 2008. The suitability of monopolar and bipolar
ion exchange membranes as separators for biological fuel cells. Environ. Sci.
Technol. 42, 1740-1746.
9. He Z, Huang Y, Manohar AK, Mansfeld F. 2008. Effect of electrolyte pH on the
rate of the anodic and cathodic reactions in an air-cathode microbial fuel cell.
Bioelectrochemistry 74, 78-82.
10. Liu H, Grot S, Logan BE. 2005. Electrochemically assisted microbial production
of hydrogen from acetate. Environ. Sci. Technol. 39, 4317-4320.
11. Logan BE, Call D, Cheng S, Hamelers HVM, Sleutels THJA, Jeremiasse AW,
Rozendal RA. 2008. Microbial electrolysis cells for high yield hydrogen gas
production from organic matter. Environ. Sci. Technol. 42, 8630-8640.
12. Logan BE, Hamelers B, Rozendal RA, Schroder U, Keller J, Freguia S,
Aelterman P, Verstraete W, Rabaey K. 2006. Microbial fuel cells: methodology
and technology. Environ. Sci. Technol. 40, 5181 -5192.
13. Marcus AK, Torres CI, Rittmann BE. 2011. Analysis of a microbial
electrochemical cell using the proton condition in biofilm (PCBIOFILM) model.
Bioresource Technol. 102, 253-262.
14. Patil SA, Harnisch F, Koch C, Hubschmann T, Fetzer I, Carmona-Martinez AA,
Muller S, Schröder U. 2011. Electroactive mixed culture derived biofilms in
microbial bioelectrochemical systems: the role of pH on biofilm formation,
performance and composition. Bioresource Technol. 102, 9683-9690.
20
15. Picioreanu C, Loosdrecht MCM, Cirtis TP, Scott K. 2010. Model based
evaluation of the effect of pH and electrode geometry on microbial fuel cell
performance. Bioelectrochemistry 78, 8-24.
16. Rabaey K, Butzer S, Brown S, Keller J, Rozendal RA. 2010. High current
generation coupled to caustic production using a lamellar bioelectrochemical
system. Environ. Sci. Technol. 44, 4315-4321.
17. Rabaey K, Verstraete W. 2005. Microbial fuel cells: novel biotechnology for
energy generation. Trends Biotechnol. 23, 291-298.
18. Rittmann BE. 2008. Opportunities for renewable bioenergy using
microorganisms. Biotechnol. Bioeng. 100, 203-212.
19. Rozendal RA, Hamelers HVM, Buisman CJN. 2006a. Effects of membrane
cation transport on pH and microbial fuel cell performance. Environ. Sci.
Technol. 40, 5206-5211.
20. Rozendal RA, Hamelers HVM, Euverink GJW, Metz SJ, Buisman CJN. 2006b.
Principle and perspectives of hydrogen production through biocatalyzed
electrolysis. Int. J. Hydrogen Energy 31, 1632-1640.
21. Rozendal RA, Hamelers HVM, Rabaey K, Keller J, Buisman CJN. 2008a.
Towards practical implementation of bioelectrochemical wastewater treatment.
Trends Biotechnol. 26, 450-459.
22. Rozendal RA, Jeremiasse AW, Hamelers HVM, Buisman CJN. 2008b. Hydrogen
production with a microbial biocathode. Environ. Sci. Technol. 42, 629-634.
23. Schroder U. 2007. Anodic electron transfer mechanisms in microbial fuel cells
and their energy efficiency. Phys. Chem. Chem. Phys. 9, 2619-2629.
21
24. Sleutels THJA, Hamelers HV, Buisman CJ. 2010. Reduction of pH buffer
requirement in bioelectrochemical systems. Environ. Sci. Technol. 44, 8259-
8263.
25. Vanysek P. 2000. Ionic conductivity and diffusion at infinite dilution. In: Lide
DR, editor. CRC Handbook of chemistry and physics. 81 ed. Boca Raton: CRC
Press. p 2556.
22
Figure Captions
Figure 1. The principle of using ammonium/ ammonia as a proton shuttle in a CEM-
BES. Here, the cathode is anaerobically operated enabling abiotic hydrogen
gas formation. At neutral anolyte pH (pH 6.5-7.5), ammonia predominately
exists as NH4+, whereas free volatile NH3 dominates in the catholyte
(pH>10).
Figure 2. Schematic of the proposed ammonia recycling apparatus connected in-series
with a CEM-BES. The process is anaerobic and hence favoring cathodic
hydrogen production. Diagram not drawn to scale.
Figure 3. Effect of continuous anodic pH control by ammonium hydroxide addition
on the current of the BES and ammonium migration to the cathode. At time
zero, the anodic chamber was inoculated with biofilm fragments from a
microbial fuel cell operated with acetate. Arrow 1 and 2 indicate additions of
sodium acetate: 0.5 mmole (1 mM) and 2 mmole (4 mM), respectively.
Figure 4. Current production from a 5 mM acetate spike (dotted vertical lines) in the
presence (0-22h) and absence (22-40h) of anodic pH control by automated
ammonium hydroxide addition. The anode was potentiostatically controlled
at -300 mV vs. Ag/AgCl.
Figure 5. Effect on current generation of the CEM-BES of recycling ammonia from
the cathode to the anolyte via a gas stream. Legend: (1) acetate was added to
the anolyte; (2) purged N2 through the catholyte and the off-gas was
introduced directly into the anolyte (N2 flow rate was about 1 L/min); (3)
23
sodium acetate was added to the anolyte; (4) nitrogen purging was
terminated; (5) acetate was added to the anolyte.
Figure 6. Effect of looping the gas-permeable path in the ammonia recycling
apparatus on current production by the CEM-BES. Anodic potential was
controlled at -300 mV vs. Ag/AgCl throughout the experiment. The BES was
operated under anaerobic condition. Ac 1-4 represent additions of 10 (45
mM), 5 (23 mM), 9 (41 mM) and 9 (41 mM) mmole of sodium acetate to the
anolyte, respectively.
24
Figure 1
NH4+B
acte
rium
Bio-anode Cathode
1e-
CEM
H+
NH3
H+
GasLiquid
Electron flow
0.25 CH2O
0.25CO2
1e-
0.25 H2O
0.5H2
0 25 50 75
100
5 7 9 11 13
% fo
rmat
ion
pH
>98% as NH4
+>84% as freeNH3
NH4+
NH3
25
Figure 2
NH4+
(aq)
H+(aq)
NH3 Scrubber (Fibre Cloth)
H2 (g) + CO2 (g)
pH>10pH7
NH3 (g)+H2 (g)
Cation Exchange Membrane
NH4+
(aq)
H+ (aq)
H2 (g)
NH3 (aq)
Electrical current
COD
CO2 + H+
Ano
de
Cat
hode
Anodic Chamber Cathodic Chamber
Aci
difie
d S
tream
Neu
traliz
ed S
tream
Ammonia RecyclingApparatus
BioelectrochemicalSystem
26
Figure 3
No pH Control pH Controlled with NH4OH
(B)
(A)
(1)
(2)
0
50
100
150
200
24 48 7296
120 144 168 192Time (h)
Cum
mul
ativ
eN
H4O
H A
dded
(mm
ole)
0
20
40
60
80
NH
4+ -N
(mM
)
NH4OH AddedNH4
+-N AnolyteNH4
+-N Catholyte
96
0
50
100
150
Cur
rent
(mA)
0
4
8
12
pH/ A
ceta
te C
onc.
(mM
)
CurrentAnolyte pHCatholyte pHAcetate
27
Figure 4
0
50
100
150
200C
urre
nt (m
A)
7
9
11
13
pH
Catholyte pH
Anolyte pH
(A)
(B)
(C)
Acetate Acetate
CoulombicRecovery= 81 %
CoulombicRecovery= 36 %
Neutral Anolyte pH Anolyte Acidification
0
20
40
0 1020
30 40Time (h)Cum
ulat
ed N
H4O
H(m
mol
e)
28
Figure 5
0
50
100
150
200
5
7
9
11
13
0
5
10pH
AcetateCatholyte pHAnolyte pH
020
40
60
80 CatholyteAnolyte
NH
4+ -N
(mM
)N
a+(m
M)
K+
(mM
)
Ace
tate
(mM
)C
urre
nt (m
A)
[1] [2] [3] [4] [5]
No NH3 RecycleNH3 Recycled No NH3 Recycle
A
B
C
D
0
1
2
3
4
0
20
40
60
80
0 10 30Time (h)
Na+(Catholyte)
Na+(Anolyte)
K+ (Catholyte)
K+(Anolyte)
30
Graphical abstract
NH4+B
acte
rium
Bio-anode Cathode
1e-
CEM
H+
NH3
H+
GasLiquid
Electron flow
0.25 CH2O
0.25CO2
1e-
0.25 H2O
0.5H2
0 25 50 75
100
5 7 9 11 13
% fo
rmat
ion
pH
>98% as NH4
+>84% as freeNH3
NH4+
NH3
31
Highlights Anolyte acidification (pH<5.5) severely inhibited current generation Ammonium is selectively migrated across a cation exchange membrane to the
catholyte Recycling the ammonia to anolyte neutralized the acidity and sustained current Ammonia recycling was achieved without using external carrier gases