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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,

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Ammonia recycling enables sustainable operation of bioelectrochemical sys‐

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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

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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:

kayu.cheng@csiro.au

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.

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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)

29

Figure 6

Ac 1

Ac 2Ac 3 Ac 4

Time (h)

Un-Loop NH3-Loop Un-Loop NH3-Loop

B

A

C

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