Treatment of nitrate contaminated water using an electrochemical method
Miao Lia, Chuanping Fenga,*, Zhenya Zhangb, Shengjiong Yanga, Norio Sugiurab
aSchool of Water Resources and Environment, China University of Geosciences (Beijing), Beijing 100083, China
bDoctoral Program in Life and Environmental Sciences, University of Tsukuba, Tsukuba 3058572, Japan
a r t i c l ei n f o
Received 2 June 2009
Received in revised form 3 February 2010
Accepted 18 March 2010
Available online 3 April 2010
Electrochemical nitrate reduction
a b s t r a c t
Treatment of nitrate contaminated water which is unsuitable for biological removal using an electro-
chemical method with Fe as a cathode and Ti/IrO2–Pt as an anode in an undivided cell was studied. In
the absence and presence of 0.50 g/L NaCl, the nitrate–N decreased from 100.0 to 7.2 and 12.9 mg/L in
180 min, respectively, and no ammonia and nitrite by-products were detected in the presence of NaCl.
The nitrate reduction rate increased with increasing current density, with the nitrate reduction rate con-
stant k1increasing from 0.008 min?1(10 mA/cm2) to 0.016 min?1(60 mA/cm2) but decreasing slightly
with increasing NaCl concentration. High temperature favoured nitrate reduction and the reaction fol-
lowed first order kinetics. The combination of the Fe cathode and Ti/IrO2–Pt anode was suitable for nitrate
reduction between initial pH values 3.0 and 11.0. e.g. k1= 0.010 min?1
k1= 0.013 min?1(initial pH 11.0). Moreover, the surface of all used cathodes appeared rougher than
unused electrodes, which may have increased the nitrate reduction rate (4–6%).
(initial pH 3.0) and
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Nitrate contamination of water resources has become an
tilizers and improper treatment of waste water from industrial sites
(Ghafari et al., 2008). Within the human body, nitrates may be re-
duced to nitrites that combine with haemoglobin to form methae-
moglobin, which can be fatal to neonates (Gupta et al., 2000).
Because of its health risks, several methods that serve to reduce ni-
trate in drinking water have been presented (Fernández-Nava et al.,
2008; Virkutyte and Jegatheesan, 2009; Wang et al., 2009). Without
the addition of a carbon source, biological denitrification is difficult
(Kesseru et al., 2003; Dhamole et al., 2009) and the problem of
contamination by dead bacteria has to be solved to make such pro-
cesses safe enough to utilize in drinking water treatment (Lei and
Maekawa, 2007). Ion exchange and reverse osmosis processes
(Samatya et al., 2006) cannot transfer nitrate into harmless com-
pounds but only concentrate nitrate from water to brine, requiring
further treatment. Recently, researchers (Petri and Safonova, 1992;
Ureta-Zanartu and Yanez, 1997; Duarte et al., 1998; Huang et al.,
1998; Vooys et al., 2000; Wang et al., 2006; Li et al., 2009a) have
focused on the electrochemical reduction of nitrate with different
cathodes (such as Cu, Ti, Rh and Cu/Zn) due to its high treatment
efficiency, the small area occupied by the plant and relatively low
The reduction of the NO?
moval from polluted waters and electrochemical reduction of ni-
Accordingly, nitrate electro-reduction is an extremely complex
process (Plieth and Bard, 1978). At the cathode, the nitrates are
mainly reduced to nitrites, ammonia and nitrogen, which is elec-
trochemically inactive. Ammonia and nitrite, in general, are the
main unfavourable reduction products and their generation limits
applications of the electrochemical process for denitrification
(Devkota et al., 2000; Lee et al., 2002; Cheng et al., 2005; Macova
and Bouzek, 2005; Katsounaros et al., 2006; Brylev et al., 2007).
However, if possible, the nitrite and ammonia products may be
oxidized to nitrate and nitrogen at the anode, respectively, before
their diffusion to the bulk. The difficulty is to find the proper con-
ditions to perform both cathodic reduction of nitrate and anodic
oxidation of the ammonia and nitrite products. Feng et al. (2003)
investigated the performance of Ti/TiO2–RuO2 for removal of
ammonia in the presence of NaCl, which has a good performance.
Vanlangendonck et al. (2005) also reported good performance for
ammonia removal in the presence chloride ions. Therefore, it is
possible to remove the unfavourable ammonia and nitrite reduc-
tion products through an anodic oxidation cycle during nitrate
Although Fe cathodes have been proven to be relatively efficient
promoters for nitrate electro-reduction, they have seldom been
investigated. Ti/IrO2–Pt has also seldom been studied as an anode
for nitrate reduction. Moreover, to the best of our knowledge, the
combination of a Ti/IrO2–Pt anode and an Fe cathode for nitrate
reduction has seldom been reported. Therefore, this combination
3ion is one of the few means for its re-
0960-8524/$ - see front matter ? 2010 Elsevier Ltd. All rights reserved.
* Corresponding author. Tel.: +86 10 82327933.
E-mail address: email@example.com (C. Feng).
Bioresource Technology 101 (2010) 6553–6557
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was employed in the present experiments. The aim of this work is
to find a proper condition to perform both cathodic reduction of ni-
trate and anodic oxidation of the produced ammonia and nitrite in
an undivided cell, so as to completely remove nitrate and remove
its by-products. It is known that different factors such as the elec-
trolyte, current density, pH and temperature of the electrolytic
medium can affect the products and efficiency of electrochemical
nitrate reduction (Englehardt et al., 2000). The influence of several
parameters, such as chloride concentration, current density, initial
pH, temperatures and addition time of chloride ions were studied.
The corrosion and passivation of cathodes were also investigated
improve the efficiency of electrochemical denitrification.
2.1. Cyclic voltammetric experiments measurements
Cyclic voltammetric (CV) measurements were obtained in a
conventional three-electrode Pyrex glass cell using a ministat
potentiostat, a computer interface and an ALS Software (ALS Lim-
ited, Model 660), at room temperature. All potentials are quoted
against the Ag/AgCl (sat.) reference electrode. The working elec-
trodes were Fe electrodes. Ti/IrO2–Pt flag electrode was used as
counter electrodes. All glassware was cleaned in boiling H2SO4–
HNO3to remove organic contaminants. Solutions were prepared
with reagent grade chemicals (Wako) and ultra-pure water (resis-
tivity – 18.2 MX cm). The working electrode was cycled between 0
and ?1.4 V at a scan rate of 200 mV/s three times before collecting
stable polarisation data. All of the solutions studied were deaerated
by bubbling N2gas for 10 min prior to the electrochemical mea-
surements (ALS Limited, Model 660).
2.2. Batch electrolysis
A 200 mL electrolysis cell (Fig. 1) was manufactured from acryl
material. For the cell, an Fe plate of 75 cm2(15 ? 5 cm) was used as
the cathode and Ti/IrO2–Pt (TohoTech Company, Japan) with the
same area as the anode. A gap of 8 mm was set between the two
electrodes. The immersed areas of the anode and cathode in the
treated solution were equal at 40 cm2. A DC power supply with a
voltage range of 0–50 V and a current range of 0–5 A was
Synthetic nitrate solutions (NaNO3+ 0.5 g/L Na2SO4) with differ-
ent concentrations of nitrate (50–200 mg/L nitrate–N) were pre-
pared for the electrolysis experiments. Different concentrations of
NaCl were added to the nitrate solutions. A 0.50 g/L Na2SO4was
added into all experiments to enhance conductivity. About 200 mL
of the synthetic nitrate solution prepared above was poured into
the electrochemical cell and the reaction started with the applica-
tion of a specified current density. At different intervals, 1.5 mL
samples were withdrawn from the electrochemical cell for analysis.
The electrolysis was ceased when either 90% of the initial nitrate
parameters was studied by varying one parameter and keeping the
others constant. The effect of current density was investigated un-
der galvanostatic control at 10, 20, 40 and 60 mA/cm2.
All analyses were done according to standard methods (APHA,
1998). Nitrate was determined by standard colorimetric method
using spectrophotometer (DR/4000U Spectrophotometer, USA),
and nitrite was analyzed by ion chromatography (Yokogawa
IC7000, AS9-HC column). The determination of ammonia was per-
formed by Ion meter (Ti 9001, Toyo chemical laboratories Co., Ltd.).
Dissolved Fe content of the samples was detected using plasma
emission spectrophotometer equipment (ICAP-575, Jareruasshu).
Surface morphology of cathode was characterized ex situ by atom-
ic force microscopy (AFM) (Digital Instruments, Dimension™ 3000,
USA). The possible formation of hydrazine and hydroxylamine was
not investigated because the treated solutions were changed into
basic after electrolysis, in which hydrazine and hydroxylamine will
not be produced (Englehardt et al., 2000).
3. Results and discussion
3.1. Voltammetry experiments
Fig. 2 shows the cyclic voltammograms obtained for the reduc-
tion of nitrate using Fe as the working electrodes and Ti/IrO2–Pt as
the counter electrode. In the base electrolyte, the main observed
processes are the onset of water electrolysis to produce hydrogen
at about ?1.0 to ?1.2 V. Addition of nitrate gives rise to rather
broad waves that extend from about ?1.0 up to ?1.4 V, the maxi-
mum being reached at the more negative potentials. During the
bath electrolysis, the current yield at ?0.80 V and ?1.00 V is very
low and NO?
NaCl, while the nitrate is converted mainly to NH3 at ?1.00 V
and ?1.20 V in the absence of NaCl.
3ions are converted mainly to NO?
2in the absence of
3.2. Electrochemical nitrate reduction with no NaCl addition
Fig. 3 shows the variation of total nitrogen (TN), nitrate–N, ni-
trite–N and ammonia–N during electrolysis in the absence of NaCl.
Fig. 1. Schematic of the electrochemical apparatus.
Na2SO4+ 100 mg/L NO?
Ti/IrO2–Pt as the counter electrode.
3–N (NaNO3) with Fe used as the working electrodes and
of(????) 0.50 g/LNa2SO4, (––)0.50 g/L
M. Li et al./Bioresource Technology 101 (2010) 6553–6557
A current density of 20 mA/cm2was used in most nitrate reduction
electrolysis as it exhibited a relatively high reduction rate. The con-
centration of nitrate–N decreased with respect to treatment time,
from 100.0 to 7.2 mg/L in 180 min. On the other hand, the ammo-
nia–N increased from 0 to 51.1 mg/L. The nitrite-N increased to
1.56 mg/L during the first 30 min and then decreased. The results
agree with a previous study (Dash and Chaudhari, 2005), which re-
ported that the selectivity for nitrate reduction to ammonia was
very high using an Fe cathode. There was also a very low concen-
tration of nitrite, which proved to be an intermediate product,
and this was probably further reduced into nitrogen gas or ammo-
nia or oxidized into nitrate again at the anode. The total nitrogen
decreased from 100.0 mg/L to 58.2 mg/L. It has been reported that
using an Fe cathode, no decrease in total nitrogen concentration
was detected (Dash and Chaudhari, 2005). The selectivity of nitrate
reduction to nitrogen was relatively high in the present experi-
ment probably because of the utilization of the Ti/IrO2–Pt anode.
As the Ti/IrO2–Pt anode has a relatively high oxidation ability (Li
et al., 2009b), a certain amount of nitrite and ammonia, before their
diffusion to the bulk, are oxidized to the original nitrate and nitro-
gen, respectively, during the anodic cycle.
Previous studies for Pb (Bockris and Kim, 1997), Cu (Bouzek and
Paidar, 2001) and bronze (Polatides et al., 2005) indicated a similar
shape of the nitrate reduction curves, which indicates that nitrate
reduction on the Fe electrode also occurs via a consecutive-reac-
tion mechanism (Scott, 1985; Katsounaros et al., 2006). This can
be described as follows:
Therefore, the corresponding differential equations are:
3? ? k21½NO?
2? ? k22½NO?
2? ¼ þk1½NO?
3? ? k2½NO?
where k2= k21+ k22.
The solutions of the differential equations using Laplace trans-
forms gives the concentrations versus time:
Thus, the nitrate concentration follows a first-order exponential
decay and in logarithmic coordinates its variation with electrolysis
time. Non-linear regression of the experimental results according
to the above equations gives k1= 0.017 min?1and k2= 0.637 min?1
at 20 mA/cm2. The k2value is 37 times higher than k1for the reduc-
tion of nitrate to nitrite. This confirms good capability for nitrite
reduction at the Fe cathode. In contrast, the k1value was about
40 times higher than k2during the NO?
electrode in 0.1 M K2SO4+ 0.05 M KNO3solution (Polatides et al.,
3reduction on a Sn85Cu15
3.3. Influence of NaCl addition
The combination of the Ti/IrO2–Pt anode and Fe cathode en-
abled good performance for electrochemical nitrate reduction
and different effects on reduction products in the presence of NaCl
(data not shown). With the addition of 0.25, 0.50, 0.75 and 1.00 g/L
NaCl, the nitrate–N decreased from 100.0 to 8.8, 12.9, 15.1 and
18.8 mg/L in 180 min; no nitrite was detected in any of the treated
solutions. On the other hand, with the addition of 0.50–1.00 g/L
NaCl, no ammonia was detected in the treated solutions; and with
the addition of 0.25 g/L NaCl, a smaller amount of ammonia
(11.6 mg/L ammonia–N) occurred than that (51.1 mg/L ammo-
nia–N) in the absence of NaCl. It is clear that the total nitrogen
sharply decreased compared with that without the addition of
NaCl, as only nitrate was present in the solution in most cases. In
the presence of chloride ions, oxidizing hypochlorite acid will be
formed during electrolysis (Rajeswar and Ibanez, 1997). Hypochlo-
rite acid formed during the electrolysis would oxidize the by-prod-
ucts of ammonia and nitrite, which were assumed to be oxidized
into nitrogen gas and nitrate, respectively (Pressley et al., 1972).
The detection of ammonia in the treated solution with 0.25 g/L
NaCl addition was presumably because insufficient hypochlorite
acid was formed to oxidize the ammonia. The nitrate reduction
rate was slightly decreased with the increasing concentration of
NaCl, in agreement with previous studies (Pletchek and Pwrabeui,
1979), which reported the effectiveness of the halide in poisoning
nitrate reduction, due to the tendency of these anions to be spe-
cifically adsorbed on the electrode surface. It could be considered
that the optimum NaCl addition in the present experiment was
3.4. Influence of current density
The combination of the Ti/IrO2–Pt anode and Fe cathode gave
good performance for electrochemical nitrate reduction at current
densities ranging from 10 to 60 mA/cm2. At current densities of 10,
20, 40 and 60 mA/cm2, the nitrate–N decreased from 100.0 to 22.0,
12.9, 9.6 and 5.9 mg/L in 180 min and no nitrite was detected in
any of the treated solutions. At a current density of 10 mA/cm2, a
small amount of ammonia (2.1 mg/L ammonia–N) was detected
after 180 min electrolysis, while at a current density range of 20–
60 mA/cm2, no ammonia was detected. The nitrate reduction rate
increased with increasing current density, especially from 10 to
20 mA/cm2. Vanlangendonck et al. (2005) found that the ammonia
oxidation rate was linearly related to current density. Therefore, at
a lower current density of 10 mA/cm2, only a small amount of
hypochlorite acid was produced, which was not enough to oxidize
all of the ammonia. The nitrate reduction rate constant k1 in-
creased from 0.008 min?1(10 mA/cm2, R2= 0.999) to 0.016 min?1
(60 mA/cm2, R2= 0.997). However, the nitrate reduction rate did
not linearly increase with increasing current density, due to the
large power consumption by the side reaction of hydrogen evolu-
tion. It could be concluded that an optimum current density in
the present experiment was 20 mA/cm2.
Fig. 3. Nitrate converted to nitrite, ammonia and nitrogen by an Fe cathode and a
Ti/IrO2–Pt anode, 20 mA/cm2, with no NaCl addition.
M. Li et al./Bioresource Technology 101 (2010) 6553–6557
3.5. Influence of initial pH
The tendencies for electrochemical reduction of nitrate were
similar at different initial pHs. The nitrate–N at initial pH values
of 3.0, 5.0, 7.0, 9.0 and 11.0 decreased from 100.0 to 19.2, 15.6,
12.9, 11.6 and 9.8 mg/L in 180 min, respectively and no ammonia
and nitrite was detected in the treated solutions. The total nitrogen
was the same as the nitrate, as only nitrate was present in the trea-
ted solution. The pHs of all treated solutions increased to 11.2–11.3
in 180 min (data not shown), mainly due to the reactions forming
hydroxyl ions and nitrogen during the electrochemical reduction of
nitrate. The increments in the final pH value suggests that the pro-
duction of H+did not match the production of OH?ions. It is well
known a decrease in the pH of solution promotes hydrogen forma-
tion. More alkaline media suppressed hydrogen evolution during
nitrate reduction (Ludtke et al., 1998). At a lower pH, besides
nitrate reduction, a stronger competitive hydrogen evolution reac-
tion would occur at the electrode surface. The nitrate reduction
rate constant k1 slightly increased as the initial pH became
more alkaline at initial pH values 3.0–11.0, e.g. 0.010 min?1
(R2= 0.995) at initial pH 3.0; 0.013 min?1(R2= 0.999) at initial
pH 11.0, which verifies that hydrogen evolution has a negative
influence on nitrate reduction. It could be concluded that the elec-
trochemical method has good performance at an initial pH range
3.0–11.0 in the present experiments.
Fig. 4 shows the surface of the cathode appeared to be rougher
than initially after electrolysis (only shown at initial pH 7.0). Com-
pared with an unused cathode, a 4–6% increase in nitrate reduction
rate was found for a used Fe cathode (30 h) (data not shown), for
which the greater surface roughness of the used electrode leads
to an increase of its specific area and that could promote the
adsorption of nitrate on the cathode. Analysis of the electrolyte
after electrolysis for 3 h at initial pH 3.0–11.0 indicated that the
concentrations of Fe in the electrolytes were all less than
0.16 mg/L, which is the allowed limit for drinking water. Overall,
electrochemical nitrate reduction with a Fe cathode displayed good
performance between initial pH 3.0 and 11.0.
3.6. Influence of temperature and nitrate concentration
Fig. 5 shows the influence of reaction temperature on nitrate
reduction. As maintenance of high temperature is difficult in prac-
tice, the temperatures were only set to be at uncontrolled and at
25 ?C. Under the condition of uncontrolled temperature, the tem-
perature of the treated solution increased from 25.0 to 46.8 ?C after
180 min electrolysis. The nitrate reduction rate constant at 25 ?C
(0.007 min?1) was lower than that at uncontrolled temperature
(0.012 min?1). This was due to different pH changes in the electro-
lyte at different temperatures during the electrolysis, e.g. at
180 min, the treated solution pH changed from 6.5 to 10.9 at
25 ?C and to 11.3 at the uncontrolled temperature. The pH change
was a natural result of the various reactions during nitrate reduc-
tion (Eqs. (1)–(3)). As previously mentioned, increasing pH was
favourable for nitrate reduction; moreover, increasing temperature
could increase the rate of diffusion and the strength of adsorption.
Consequently, the nitrate reduction rate increased when the tem-
perature was increased from 25.0 to 48.5 ?C. It could be considered
that the uncontrolled temperature was favourable for electro-
chemical nitrate reduction.
The effect of different concentrations (50.0, 100.0 and 200.0 mg/
L nitrate–N) on nitrate reduction at a current density of 20 mA/cm2
in the presence of 0.50 g/L NaCl was investigated. No nitrite was
detected in any treated solution and only ammonia was detected
in 200 mg/L nitrate, which was due to the amount of hypochlorite
acid produced being insufficient to oxidize all of the ammonia. In
50.0, 100.0 and 200.0 mg/L nitrate, the nitrate reduction rate con-
stant k1was 0.011 (R2= 0.995), 0.012 (R2= 0.999) and 0.011 min?1
(R2= 0.999), respectively, which suggests that the reduction fol-
lowed a first order reaction. The slightly higher value of k1 in
100.0 mg/L nitrate was probably because the addition of 0.50 g/L
NaCl was appropriate for oxidizing the ammonia and nitrite. The
addition 0.50 g/L NaCl could either produce too much (in
50.0 mg/L nitrate) hypochlorite acid or not enough (in 200.0 mg/L
nitrate), which interferes with the reaction (Eqs. (3)–(5)), resulting
in the slighter lower nitrate reduction rate constant.
Electrochemical denitrification of nitrate contaminated water
using an Fe cathode and a Ti/IrO2–Pt anode in an undivided cell
Fig. 4. AFM photograph of (A) an unused, (B) a used (30 h) for electrolysis with a Fe cathode at initial pH 7.0.
0 3060 90120 150180
Fig. 5. Variation of concentration of nitrate with time at different temperatures,
I = 20 mA/cm2, 0.50 g/L NaCl.
M. Li et al./Bioresource Technology 101 (2010) 6553–6557
was studied. In the presence of 0.50 g/L NaCl, the nitrate–N de- Download full-text
creased from 100.0 to 12.9 mg/L in 180 min, and no ammonia
and nitrite by-products were detected. The influences of several
parameters were studied and conclusions are drawn as follows.
(1) The nitrate reduction rate increased with increasing current
(2) Although a high initial pH was favourable for nitrate reduc-
tion, between initial pH 3.0 and 11.0, the combination of Fe
cathode and Ti/IrO2–Pt anode was suitable for nitrate reduc-
tion. Moreover, the surface of all used cathodes appeared
rougher than unused electrodes, which increased the nitrate
reduction rate (3–5%).
(3) The nitrate reduction rate slightly decreased with increasing
NaCl concentration, while in the presence of lower concen-
trations of NaCl, the ammonia by-product could not be
(4) High temperature was favourable for nitrate reduction. In
the present experiments, the nitrate reduction followed a
first order reaction.
Overall, it can be suggested that the optimum conditions for ni-
trate reduction were NaCl addition, 0.50 g/L; current density,
20 mA/cm2; pH range, 3.0–11.0; and uncontrolled temperature.
The combination of the Fe cathode and Ti/IrO2–Pt anode was suit-
able for nitrate reduction, especially in the presence of NaCl, and
shows high potential for practical use.
The authors thank the National Key Technology R&D Program in
the 11th Five year Plan of China (2006BAJ08B04, 2006BAD01B03),
863 Project (2007AA06Z351), the Key Science and Technology Pro-
gram (No. 108027) of Ministry of Education of the People’s Repub-
lic of China for the financial support of this work.
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