MFC desulfurization

1. Simultaneous nitrate and sulfide removal using a
bio-electrochemical system
Alper Bayrakdar a,b,
⁎, Ebrahim Tilahun c
, Bariş Çalli c
a
Department of Environmental Resources Engineering, College of Environmental Science and Forestry, State University of New York, 1 Forestry Drive, Syracuse, NY 13210, USA
b
Necmettin Erbakan University, Environmental Engineering Department, 42090 Meram, Konya, Turkey
c
Marmara University, Environmental Engineering Department, 34722 Kadikoy, Istanbul, Turkey
a b s t r a c t
a r t i c l e i n f o
Article history:
Received 16 April 2019
Received in revised form 5 June 2019
Accepted 5 June 2019
Available online 07 June 2019
This study addresses the applicability of simultaneous nitrate and sulfide removal using two-chamber bio-
electrochemical systems (BES). The anode and cathode chambers of a BES were fed with the effluent of a sulfate
reducing reactor and a nitrate-rich groundwater as an electron donor and acceptor sources, respectively. BES has
been found to be effective for simultaneous removal of sulfide and nitrate coming from different sources and
without mixing them. As a result, 10 gS/m3
/d of sulfide oxidation and 7.26 gN/m3
/d of nitrate reduction rates
were achieved. The number of electrons used for denitrification was more than that of delivered from the
anode, especially when the anode chamber was fed with the SRR effluent and operated at pH 7–7.5. It was sup-
posed that H2S was used for denitrification in the cathode by passing through the membrane. Another reason for
this might be the electrons released from the corroding steel mesh current collector.
© 2019 Elsevier B.V. All rights reserved.
Keywords:
Sulfide
Autotrophic denitrification
Bio-electrochemical systems
Groundwater
1. Introduction
Nowadays, increasing energy demand and environmental pollution
are the main concern in the world. Bio-electrochemical system (BES),
which provides energy recovery during waste or wastewater treatment,
has been one of the promising approach recently [1]. Typically, BES,
which consists of anode and cathode electrode connected to external re-
sistance or a power source to complete the electrical circuit, is the com-
bination of biological and electrochemical processes to produce
electricity, hydrogen and certain valuable chemicals by means of mi-
crobes as a bio-catalyst [2]. Pollutants are oxidized and electrons are re-
leased by electrogenic bacteria in the anode chamber. Electrons released
are then flow to cathode side through an external circuit and used to re-
duce an oxidizing agent or pollutant [3].
Although bio-electrochemical systems (BESs) have developed rap-
idly over the past two decades, they still have very limited power gen-
eration capacity. Due to the high cost of the materials used, limited
sources of suitable substrates and low columbic efficiencies, they are
not expected to be used for electricity generation in the near future.
Recent research has therefore shifted towards alternative uses of BES,
such as the recovery of value-added chemicals from waste streams
and the reclamation of challenging pollutants [4–6].
Although there are some structural and management problems, BES
technology has showed a very rapid development recently [7]. How-
ever, further research is required for the development of BES for the
commercial applications. Generation of electricity from wastewater
using BES technology is not yet economically feasible.
BES have been mostly reported for the removal of organic pollutants
coupled with electricity generation from different types of wastewater
[8]. In addition to organic materials, some inorganic pollutants such as
nitrate, nitrite and sulfide could be removed by BES [9–11]. It was also
reported that sulfide and organic matter containing wastewater, like an-
aerobic digester effluent, could be simultaneously removed in the anode
of BES [11–13]. In most of the literature studies, the removal of the sul-
furous and nitrogenous compounds in the BES were performed sepa-
rately [13–15]. There are also studies focused on the treatment of
wastewater containing sulfide, nitrate and organic matter in BES. How-
ever, these studies showed when the electron donor and acceptor
existed in the same chamber, power generation decreased much, as ex-
pected [16].
Simultaneous removal of nitrate and sulfide in separate chambers of
the BES was investigated in only a few studies [3,17]. Theoretically, de-
nitrification can be performed in cathode chamber with the electrons
released by sulfide oxidation in anode chamber coupled with power
Bioelectrochemistry 129 (2019) 228–234
⁎ Corresponding author at: Department of Environmental Resources Engineering,
College of Environmental Science and Forestry, State University of New York, 1 Forestry
Drive, Syracuse, NY 13210, USA.
E-mail addresses: alperbayrakdar@gmail.com (A. Bayrakdar),
Baris.calli@marmara.edu.tr (B. Çalli).
https://doi.org/10.1016/j.bioelechem.2019.06.003
1567-5394/© 2019 Elsevier B.V. All rights reserved.
Contents lists available at ScienceDirect
Bioelectrochemistry
journal homepage: www.elsevier.com/locate/bioelechem

2. generation according to Eqs. (1) and (2). This is a promising alternative
to remove pollutants coming from different streams without mixing.
S2−
þ 4H2O➔SO4
2−
þ 8e−
þ 8Hþ
ð1Þ
NO3
−
þ 6Hþ
➔0:5 N2 þ 5e−
þ 3H2O ð2Þ
The objective of this study was to investigate the applicability of
using BES in the simultaneous treatment of two different streams con-
taining sulfide and nitrate without contaminating each other. The sul-
fide driven denitrification of a nitrate-rich groundwater was for the
first time investigated using a two-chamber BES. The BES was operated
with low loading rates because the main purpose was the simultaneous
treatment of sulfide and nitrate, and therefore the power generation po-
tential was not addressed in this study.
2. Material and methods
2.1. Configuration of BES
Two identical two-chambered BES (175 ml of each chamber) made
of transparent acrylic plates were used in the study. While both cham-
bers of BES-B were seeded with the inoculum taken from a lab-scale el-
emental sulfur oxidizing and nitrate reducing long-term operated
autotrophic denitrification reactor [18], BES-A was operated abiotically
as control experiment. Anode and cathode chambers were separated
with a 6 cm × 6 cm cation exchange membrane (FumaTech,
Germany). Stainless steel mesh and carbon cloth (AvCarb 1071 HCB,
USA) were used as current collector and electrode, respectively. Stain-
less steel mesh, carbon cloth electrode and the membrane were placed
in contact with each other to minimize the distance between anode and
cathode electrodes.
2.2. Operation of BES
Both chambers were fed with the medium consisted of KH2PO4
(56 mg/l), NH4Cl (110 mg/l), yeast extract (3 mg/l), ascorbic acid
(5 mg/l), CaCl2.2H2O (10 mg/l), MgCl2.6H2O (10 mg/l) and NaHCO3
(150 mg/l). In addition, Na2S.9H2O and NaNO3 were supplied as sulfide
and nitrate sources to the anode and cathode chambers, respectively.
Medium was prepared daily and flushed with N2 to maintain anaerobic
conditions. The amount of sulfide and nitrate fed to the chambers was
determined according to theoretical sulfide/nitrate (HS−
/NO3
−
) molar
ratio of 2.5 according to Eq. (3). The pH of the anode feed solution was
adjusted to 8 with 1 N HCl.
S2−
þ 0:4 NO3
−
þ 2:4Hþ
➔S0
þ 0:2 N2 þ 1:2 H2O ð3Þ
Starting from Period 3, instead of the sulfide containing medium, the
effluent of a lab-scale sulfate reducing reactor (SRR) was fed to the
anode chamber of BES
B. The sulfide rich effluent was drawn directly
from the upper part of the upflow SRR and delivered to the anode cham-
ber with a peristaltic pump. In Period 4 and 5, the cathode chamber of
BES-B was fed with a nitrate containing groundwater. Characteristic of
the groundwater is given in Table 1.
Both BES were placed in a temperature-controlled cabinet (WTW,
TS606-G/2i) and operated at 25 ± 0.5 °C. Each chamber was stirred con-
tinuously using a dual-magnetic stirrer. Before feeding and sampling, a
gas bag filled with N2 was connected to the head space of each chambers
to maintain anaerobic conditions. BES-B was operated for 142 days and
the experiments were performed in five periods according to conditions
listed in Table 2. Except in Period 1, the anode chamber was fed contin-
uously using a peristaltic pump (Watson Marlow, 323Du) at a flow rate
of 200 ml/d which corresponds to hydraulic retention time of 21 h. In
the last period, the groundwater was fed to the cathode chamber
using a syringe pump (NE-1000) at a flow rate of 24 ml/d. The opera-
tional conditions are summarized in Table 2.
2.3. Chemical control experiments
To investigate the contribution of synthetic feed solution and/or
metal mesh on denitrification, three chemical control experiments
were carried out. Control experiments were performed in a 300 ml-
test bottles having PTFE necks-caps with multiple distributors with
stopcocks (GL 3 × 14 BOLA). A N2 filled gas bag connected to one of
the distributors of the cap to prevent oxygen entrance during sampling.
In the first control experiment, only nitrate containing synthetic feed so-
lution filled in the test bottle and a piece of metal mesh put inside. In the
second experiment, nitrate and sulfide containing feed solution was
filled to the test bottle. Nitrate and sulfide containing feed solution
and metal mesh were put in the test bottle in the third control experi-
ment. In the control experiments Nitrate, nitrite, sulfide and ammonia
analyses were performed daily.
2.4. Analytical methods and electrochemical monitoring of BES
Every day 3.5 ml of sample was taken from each chamber under an-
aerobic conditions, centrifuged and filtered with 0.45 μm syringe filters
before sulfate, nitrate, nitrite and ammonium analyses. Sulfide was de-
termined spectrometrically (WTW PhotoLab 6100VIS) according to
the method described by Cord-Ruwisch [19]. Sulfate was analyzed ac-
cording to standard methods [20]. Nitrate and nitrite were determined
with an HPLC (Shimadzu Prominence LC-20A) equipped with UV detec-
tor [21]. Ammonia was determined colorimetrically by direct
nesslerization (DR2500, Hach Lange GmbH, Germany). The pH was
measured with a pH meter (Eutech Insturments, CyberScan PCD 6500).
Cell voltage (mV) of BES was measured on-line by 24-bit analog
input data acquisition module (NI, USB9239) over a 10 Ohm external re-
sistance (Rex). Ag/AgCl reference electrodes (+0.197 V according to
SHE) which were connected to the same module used to determine
the cell voltages (Vcell) [22]. The current (I, mA) and power (P, mW)
were calculated according to Ohm's Law: I=Vcell/Rex, P=I × Vcell.
Table 1
Characteristics of the groundwater fed to the cathode.
Parameter
Nitrate, mg/l 302 ± 5
Sulfate, mg/l 210 ± 3
NH3-N, mg/l b0.015
pH 7.1–7.2
Total alkalinity, mgCaCO3/l 390 ± 10
Table 2
Operational conditions of BES.
Period Day Sulfide
source
in anode
Sulfide
feeding
mode
Nitrate source
in cathode
Nitrate
feeding
mode
1 0–38 Na2S
solution
Batch NaNO3 solution Batch
2 38–82 Na2S
solution
Continuous NaNO3 solution Batch
3 82–100 SRR
effluenta
Continuous NaNO3 solution Batch
4 100–118 SRR effluent Continuous NO3
−
rich
groundwater
Batch
5 118–142 SRR effluent Continuous NO3
−
rich
groundwater
Continuous
a
SRR effluent: effluent of a lab-scale sulfate reducing reactor.
229
A. Bayrakdar et al. / Bioelectrochemistry 129 (2019) 228–234

3. 3. Results and discussion
3.1. Bio-electrochemical sulfide and nitrate removal
In Period 1 (Day 0–38), anode and cathode chambers of BES were fed
with 100 mg/l sulfide and 15.5 mg/l nitrate‑nitrogen containing syn-
thetic feed solutions, respectively to keep the S/N molar ratio appropri-
ate for partial oxidation of sulfide to elemental sulfur. After the start-up
period, for few days no electrical current was detected in the system.
However, all nitrate was denitrified within 4 days. It is thought that
elemental sulfur, which came together with the inoculum, served as
an alternative electron donor in the cathode chamber. Considering the
increasing sulfate concentration in the cathode chamber, it is supposed
that the elemental sulfur existing in the inoculum was oxidized to sul-
fate in relation to denitrification and depleted in 5 days (Data not
shown).
It is clearly seen in Fig. 1a, b and c that when the anode and cathode
chambers were fed with sulfide and nitrate, respectively, a net current
generation was observed. In parallel with the decreasing electron
donor and/or acceptor concentrations, it was observed that the current
generation reduced and ceased finally. On day 10, although 43 mg/l of
HS−
was available in anode chamber, the electrical current decreased
to zero because the nitrate in the cathode chamber was exhausted. It
shows that nitrate was reduced bio-electrochemically on the cathode
with the electrons delivered from anode over the external circuit. Ni-
trate was first reduced to nitrite and then to nitrogen gas, if there was
no electron donor limitation (Fig. 1b). Besides, the stable total ammonia
concentrations and steadily increasing pH of cathode chamber proved
that the nitrate was removed by denitrification. It is known that oxygen
is more favorable than nitrate as electron acceptor and sulfur is easily
oxidized under aerobic conditions. Therefore, during the experiments,
extra care was taken to ensure the strict anaerobic conditions in the
anode and cathode chambers. The cause of the decrease in denitrifica-
tion efficiency and the consequent irregular current production was
the introduction of air into the cathode chamber (Fig. 1a). On day 30,
the cathode was fed with almost 50% more nitrate by mistake. As a re-
sult, nitrite accumulation was observed in the cathode chamber because
the number of electrons supplied by the anode was not sufficient for
complete denitrification. To reduce the accumulated nitrite and the re-
maining nitrate, the anode chamber was fed with sulfide again on day
35. Accordingly, although the sulfide was utilized and oxidized
completely to sulfate or elemental sulfur, which was previously pro-
duced and accumulated in the anode, was used as extra electron source
and oxidized to sulfate, the nitrite was not reduced and continued to in-
crease. (Fig. 1b and c). The results showed that the extra sulfide fed to
the anode was not sufficient to reduce the nitrite accumulated in the
cathode. The lower electrical current obtained after sulfide feeding on
day 35 supports this finding (Fig. 1a).
Considering the amount of sulfide consumed and sulfate produced
in anode, nitrate removed in cathode and the electrical current gener-
ated, it was calculated that in average 60% of the electrons released
from anodic reactions were used for nitrate reduction in the cathode
in Period 1.
The average current density obtained was not comparable with the
values achieved in other similar studies, because the BES was not oper-
ated with an objective to maximize the current generation. It is known
that to maximize the power output of a BES, the anode and cathode
chamber should be fed with high concentrations of electron donor
and acceptor, respectively [23]. In this study, the anode chamber was
loaded lightly with sulfide to test the applicability of using low
sulfide-containing waste(water) as electron source rather than maxi-
mizing the power output, therefore the electrical current obtained
was not comparable to the values reported by other similar studies in
the literature [3,17]. We investigated the simultaneous treatment of
two different pollutants; sulfide and nitrate coming from different
sources, in separate chambers of a BES without mixing them with
each other. Average sulfide oxidation and nitrate (as NO3-N) reduction
rates achieved in this period were 30.8 g/m3
/d and 3.2 g/m3
/d,
respectively.
In Period 2, between days 39 and 79, the anode chamber was contin-
uously fed with the sulfide solution at a flow rate of 200 ml/d. The cath-
ode chamber continued to be operated in batch mode. Between days 39
and 44, the cathode chamber was not fed until the accumulated nitrite
was completely consumed. Like in Period 1, in the cathode chamber ni-
trate is first reduced to nitrite and then nitrite to N2 gas (Fig. 1f). The
regular nitrate and nitrite reduction in Period 2 were reflected in the
current output in the same way (Fig. 1e). Autotrophic denitrification is
a proton consuming process [21]. Therefore, the increase in pH observed
after feeding the cathode chamber was one of the evidences of denitri-
fication (Fig. 1h). Fluctuations were observed in sulfide removal in the
anode chamber (Fig. 1g). As the electron acceptor concentration de-
creased in the cathode chamber, which was fed in batch, the sulfide re-
moval rate decreased as well. In period 2, the rate of sulfide removal and
denitrification were calculated as 8.1 gS/m3
/d and 3.3 gN/m3
/d, respec-
tively. In a similar study, maximum sulfide removal rate and accompa-
nying nitrate removal rate were 1.96 kgS/m3
/d and 12.6 gN/m3
/d,
respectively [3]. Additionally, it was estimated that in average 59% of
the electrons released from the sulfide oxidation in the anode were
transferred through the external circuit and used in the cathode for
denitrification.
In Period 3 and 4, between days 82 and 118, the anode chamber was
continuously fed with the sulfide-rich effluent of a lab-scale sulfate-
reducing reactor (SRR). The cathode chamber was operated in batch
mode and fed with the synthetic nitrate solution until day 100 (Period
3) and then (Period 4) with the groundwater containing 300 mg/l of ni-
trate (67.7 mg/l NO3-N). During this time interval, significant fluctua-
tions in sulfide removal were observed in the anode chamber (Fig. 2c).
Besides, the removal rates were lower compared to the values achieved
in former periods. It is known that free H2S becomes dominant sulfide
specie as pH decreases [24]. Therefore, the reason of the unstable sulfide
removal was supposed to be the spontaneous release of H2S gas from
the anode chamber operating at pH of about 7.5 due to the neutral pH
of feed (SRR effluent). The synthetic sulfide solution used in Period 1
and 2 had a higher buffering capacity due to its alkaline pH (~8.5). Ac-
cordingly, a significant portion of the hydrogen sulfide was present in
the solution in ionic form and consumed more uniformly. Despite de-
creasing sulfide oxidation rates in the anode chamber, cathodic denitri-
fication rates increased to 7.0 gN/m3
/d.
Dissimilar to the previous periods, the number of electrons used in
the cathode for denitrification was 140% more than the number of elec-
trons obtained from sulfide oxidation in the anode. It was explained
with the presence of additional electron donors in the anode or cathode
chambers. In average, the SRR effluent was containing 30–40 mg/l of
COD (Data not shown). However, the COD analysis results showed
that almost no COD was removed in the anode chamber. In addition,
no noticeable increase in the current output indicating an extra electron
donor consumption in the anode was observed. At pH 7.5, about 50% of
the total sulfide exists in the gas form (H2S) which can pass from anode
to cathode through the membrane and be directly used as electron
donor for nitrate reduction.
The extra electron donors in the cathode were supposed to be the
main reason of the imbalance between the amount of electron released
and consumed. The elemental sulfur particles attached to the walls of
the cathode chamber were an evidence supporting our assumption.
In Period 5, between days 118 and 142, the nitrate-containing
groundwater was continuously fed to the cathode chamber with a
flow rate of 22 ml/d (HRT 8 days). Average NO3-N load was 37 gN/
m3
/d. Because both chambers were operated with continuous feed-
ing, a very stable current output of about 0.15 mA was obtained
(Fig. 2e). The average nitrate removal efficiency was 85%. Based on
the electron balance of the system, the nitrate removal rate was cal-
culated as 7.27 g/m3
/d instead of 3.31 g/m3
/d. It is speculated that
230 A. Bayrakdar et al. / Bioelectrochemistry 129 (2019) 228–234

4. Fig. 1. a&e: Current; b&f: Nitrate (▲), Nitrite (o); c&g: Sulfide (o), Sulfate (□); d&h: anode pH (×) and cathode pH (o) values of Period 1 and 2.
231
A. Bayrakdar et al. / Bioelectrochemistry 129 (2019) 228–234

5. like in Period 3 and 4, the H2S instantaneously diffusing through the
membrane and/or the elemental sulfur accumulated in the cathode
chamber were used as extra electron donor in addition to electrons
flowing through the external circuit. In order to clarify how the ex-
cess nitrate removal was achieved in the cathode, a number of con-
trol experiments have been performed and the alternative chemical
Fig. 2. a&e: Current; b&f: Nitrate (▲), Nitrite (o), Nitrate influent (●); c&g: Sulfide (o), Sulfate (□); d&h: anode pH (×) and cathode pH values of Period 3, 4 and 5.
232 A. Bayrakdar et al. / Bioelectrochemistry 129 (2019) 228–234

6. and/or electrochemical nitrate reduction potentials were discussed
in detail in the following sections.
3.2. Abiotic electrochemical control experiment
To determine whether electrochemical nitrate reduction is possible
or not, an identical BES (BES- A) was operated abiotically. Anode and
cathode chambers were fed with 95 mg/l of HS−
and 16 mg/l of NO3-
N containing synthetic solutions, respectively. The other operation con-
ditions were similar to the ones tested with biotically BES-B in Period 1.
As seen in Fig. 3a, almost no electrical current was obtained with BES-A
in this control experiment. However, despite no current output, a ni-
trate removal rate of 1.2 gN/m3
/d was achieved (Fig. 3b). This denitrifi-
cation rate is about 35% of the rate obtained with BES-B in Period 1. To
clarify this unexpected result, some chemical control experiments
were also performed.
3.3. Chemical control experiments
In the first chemical control experiment, the metal mesh used as cur-
rent collector was placed in a test bottle filled with only nitrate contain-
ing synthetic feed solution. After 10 days, no nitrate removal was
observed (Data not shown). In the second chemical control experiment,
the nitrate and sulfide containing feed solution was filled to the test bot-
tle. Similar to the abiotic electrochemical control test result, the sulfide
concentration decreased during the experiment as a result of chemical
oxidation, but no nitrate removal was determined (Data not shown).
In the last chemical control experiment, the metal mesh was immersed
into the synthetic solution containing both nitrate and sulfide. The ex-
periment lasted 10 days. The sulfide, nitrate and nitrite profiles are
shown in Fig. 4. The reduction of nitrate to nitrite and nitrite to N2 gas
followed a profile similar to the one obtained with BES-A in the abiotic
electrochemical control test. In the literature [25], there are some stud-
ies showing that nitrate can be reduced biologically and/or chemically
using zero valent iron as electron donor (Eq. (3)).
5Fe0
þ 2NO3
−
þ 6H2O➔5Fe2þ
þ N2 þ 12OH−
ð4Þ
Suzuki et al. [26] reported that nitrate directly received electrons
from zero valent iron through an iron corrosion product layer. In the
third control test, the pH increased steadily from 8 to 9.7 through the
experiments. Similarly, Hao et al. [27] reported an increasing pH during
reduction of nitrate by scrap iron filings. Suzuki et al. [26] also reported
that the pH increased from 4.5 to 8.4 during the 24-h nitrate reduction
experiments by using zero-valent iron. In some of our former studies,
we experienced corrosion problem with the same steel mesh. However,
further studies are still needed to determine how much the steel mesh
contributes to nitrate removal.
4. Conclusions
The two-chamber BES used was found to be applicable for the simul-
taneous treatment of sulfide and nitrate in separate chambers without
contaminating each other. 10 gS/m3
/d of sulfide oxidation and 7.26
gN/m3
/d of nitrate reduction rates were achieved with the effluent of
a sulfate reducing anaerobic reactor and a nitrate-rich groundwater, re-
spectively. Unexpectedly, when the anode chamber was operated at
neutral pH, the number electrons used for denitrification was more
than that of released by sulfide oxidation. It was supposed that H2S,
which is the dominant sulfide form at neutral pH, passed through the
membrane and served as extra electron donor in denitrification. Control
experiments have also shown that the corrosion of the steel mesh
Fig. 3. a: Current; b: Nitrate (▲), Nitrite (o); c: Sulfide (o), Sulfate (□); d: anode pH (×) and cathode pH values of abiotic electrochemical control experiment.
233
A. Bayrakdar et al. / Bioelectrochemistry 129 (2019) 228–234

7. current collector in the cathode may be another reason. The current
density obtained were quite low, because the BES was preferably used
for treatment with low loading rates.
Acknowledgements
The authors would like to acknowledge the financial support by
TUBITAK (Project No. 112Y390).
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Fig. 4. Changes in concentration of sulfide (o), nitrate (△) and nitrite (□) during chemical control experiment.
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