Open Journal of Applied Sciences, 2012, 2, 241-247
doi:10.4236/ojapps.2012.24036 Published Online December 2012 (http://www.SciRP.org/journal/ojapps)
Efficient Electrochemical Removal of Ammonia with
Various Cathodes and Ti/RuO2-Pt Anode
Yaning Wang, Xu Guo, Jinglu Li, Yingnan Yang, Zhongfang Lei, Zhenya Zhang*
Graduate School of Life and Environmental Sciences, University of Tsukuba, Tsukuba, Japan
Received August 31, 2012; revised September 30, 2012; accepted October 12, 2012
ABSTRACT
Electrochemical oxidation of ammonia was studied with an objective to enhance the selectivity of ammonia to nitrogen
gas and to remove the by-products in an undivided electrochemical cell, in which various cathodes and Ti/RuO2-Pt anode
were assembled. In the present study, anodic oxidation of ammonia and cathodic reduction of by-products were
achieved, especially with Cu/Zn as cathode. In the presence of 1.0 g/L NaCl the ammonia-N decreased from 100.0 to 0
after 120 min electrolysis at current density of 30 mA/cm2, and no nitrite was detected in the treated solution. The low-
est amount of nitrate was formed with Cu/Zn as cathode during electrolysis due to its high reduction ability. Initial pH
range from 7 and 9 and uncontrolled temperature were favorable for electrochemical ammonia oxidation and the am-
monia oxidation rates with Cu/Zn cathode was higher than that with Ti and Fe cathode. The reduction rate increased
with increasing current density in the range of 5 - 50 mA/cm2. As ammonia could be completely removed by the simul-
taneous oxidation and reduction in this study, it is suitable for deep treatment of ammonia polluted water.
Keywords: Electrochemical Oxidation; Ammonia; Nitrate; Cu/Zn; Sodium Chloride
1. Introduction
Nowadays, around the world most of the water bodies
are polluted and some are heavily polluted. Among the
water pollution, ammonia contamination of water bodies
is a widespread environment problem. Ammonia con-
taining wastewaters will cause eutrophication and fish kills
which disrupt aquatic ecosystems in a severe manner
[1-3]. Therefore, aim to reduce consumption and improve
treatment method efficiency for water and wastewater
should be highlighted as a main public concern in the
future [4-6]. Usually, two main groups of treatment pro-
cesses, physicochemical and biological treatment methods,
were employed for ammonia removal, which are bio-
logical process, air stripping, ion exchange, and break-
point chlorination. Although air stripping and ion ex-
change are widely used, as no destruction of the con-
taminant, it needs to be further treated. Breakpoint chlo-
rination demands a large amount of chlorine and rela-
tively low efficiency. Biological method is effective with
the addition of carbon sources, large land areas and sig-
nificantly impaired by low temperature in winter [7]. As
electrochemical method has the advantage of high treat-
ment efficiency, no sludge production, small area occu-
pied by the plant and relatively low investment costs, a
large number of researchers has focused on it recently.
In general, ammonia can be oxidized electrochemi-
cally into nitrogen gas, nitrate, or nitrite by direct or in-
direct oxidation processes.
So far several metals have been investigated [8-10],
such as Ti/IrO2-Pt, Lead Dioxide and BDD-TiO2 Anode.
It was shown that ammonia is effectively removed from
solution while active chlorine was electrogenerated on a
Ti/PtOx-IrO2 electrode. Nitrogen gaseous is postulated to
be the main by-product of ammonia electrolysis [11].
Direct (non-mediated) electrochemical oxidation of am-
monia on boron-doped diamond (BDD) electrode oc-
curred mainly when pH value above 8.0 by free ammonia
(NH3) oxidation. Otherwise when pH value was below
8.0, oxidation of ammonia was mediated by active free
chlorine. Thus, active chlorine effectively removes am-
monia from an acidic solution, while the formation of
by-products such as chlorate and possibly perchlorate is
minimized [12]. Electro-oxidation of ammonia process
inhibited the oxygen evolution reaction (OER) as ammo-
nia oxidation products was absorbed on the BDD surface.
Nitrogen, nitrous oxide, and nitrogen dioxide were de-
tected as the ammonia oxidation products. Nitrogen ga-
seous was the main product of the oxidation [13]. The
electro-activity of the Pt and DSA (Dimensionally Stable
Anode) electrodes and Ni were confirmed by cyclic
voltammogram at pH 7.0 and pH 9.4. An anodic polari-
zation showed the electro-activity of graphite. Anodized
*Corresponding author.

Y. N. WANG ET AL. 242
Al electrodes showed no electro-oxidation activity. The
electrochemical activity of Ni electrodes demonstrated
this material for ammonia and ammonium ion electro-
oxidation at both pH values investigated [14]. A disad-
vantage of the platinum electrode is that their activities
are disrupted by the formation of a platinum oxide (PtO)
layer at the cathodic electrode surface [15]. Combination
of graphite and titanium dioxide electrode considerably
enhance the removal efficiency for the present study [16].
In general, the anode is believed to be the key factor that
affects the ammonia removal, and nitrate and nitrite were
found to be the side products during the electrochemical
ammonia removal. With our best knowledge, most re-
searchers focused on the effect of anode on ammonia
oxidation, seldom has focused on the cathode, which
may play an important role on the side products gene-
rated during electrochemical ammonia oxidation [17]. On
the other hand, researchers found that the cathodes such
as Cu/Zn and Fe had high efficiency on nitrate and nitrite
removal [18]. Therefore, it is possible to decrease the
production of side products by employing cathode with
high nitrate and nitrite reduction ability.
In this work, in order to completely electrochemical
oxidize ammonia and reduce the formation of by-pro-
ducts, cathodic reduction of by-products and anodic oxi-
dation of ammonia were investigated in an undivided cell.
As only a few researchers focus on the effect of Fe,
Cu/Zn cathode on ammonia removal, and previous re-
search had suggested that cathode has significant effect
on reduction product. The effect of ammonia removal by
varies cathodes (Fe, Cu/Zn, Ti) was investigated, with
aim to completely destroy ammonia, to reduce the for-
mation of by-products and enhance current efficiency
simultaneously. On the other hand, Ti/RuO2-Pt anode
was reported to have good oxidation ability, therefore it
is employed as anode in the present study.
2. Materials and Methods
2.1. Electrochemical Apparatus
Batch experiments were conducted in an apparatus at
room temperature (about 23.0˚C - 26.0˚C). A continuous
electrochemical cell was designed in our lab with a net
working volume of 400.0 ml. The 400-mL electrolysis
cell was made of acryl plates with four outer spots for the
electrodes assembled. Three metal plates include Cu/Zn
(Cu: 62.2 wt%; Zn: 37.8 wt%), Ti and Fe plate of 75.0
cm2 (15.0 cm × 5.0 cm) were used as the cathode more-
over Ti/RuO2-Pt (TohoTech company, Japan) was used
as the anode with the same area with a distance of 8.0
mm between the two electrodes respectively. A DC power
with a voltage range of 0 - 50.0 V and a current range of
0 - 5.0 A was employed as power supply. Test synthetic
ammonia solutions were prepared using (NH4)2SO4 and
distilled water to give a final concentration of 100.0
mg· L –1, with no Cl−. In the solution, free ammonia con-
centration is only 0.0198 mg·L–1. The NaCl dosage of 0
g·L–1, 0.5 g·L–1, 1.0 g·L–1, 2.0 g·L–1 (w/v) were added
into the synthetic ammonia solutions to investigate the
effect of influencing factors (sodium chloride dosage,
current density, temperature, initial pH value; respec-
tively). While current density was varied between 5.0
mA/cm2 and 50.0 mA/cm2. The initial pH value was ad-
justed from 3.0 to 11.0 by NaOH (0.1 M) or H2SO4 (0.1
M). 0.50 g·L–1 Na2SO4 was added in all solution as sup-
porting electrolyte. At different intervals, 1.5 mL of
sample was drawn from the electrochemical cell for
analysis. The electrolysis was ceased when either 90.0%
of initial ammonia was converted or 2 h elapsed.
2.2. Analysis
All analyses were done according to standard methods
(APHA, 1998). The determination of ammonia was per-
formed by Ion meter (Ti 9001, Toyo chemical laborato-
ries Co., Ltd.). Nitrate was determined by standard co-
lorimetric method using spectrophotometer (DR/4000U
Spectrophotometer, USA), and nitrite was analyzed by
ion chromatography (Yokogawa IC7000, AS9-HC column).
3. Results and Discussion
3.1. Influence of NaCl Dosages
In the present experiment, as NaCl plays an important
role on not only the ammonia oxidation rate, but also the
formation of the by-product nitrate, the performance of
electrochemical ammonia oxidation with different NaCl
dosages was investigated.
Figures 1 and 2 were concentrations of ammonia-N
and nitrate-N with respect of time.
It confirmed that without NaCl addition, ammonia re-
moval rate showed almost same results with different
cathode. Opposite the different amounts of nitrate forma-
tion during the electrochemical oxidation of ammonia
can be attributed to the different of the cathodes in the
absence of NaCl, in which Cu/Zn cathode showed lowest
by-product formation. Cupro-zinc materials were well-
known as their good corrosion resistance. Moreover, zinc
has a good electroactivity while copper displays a good
activity of electroreduction. As a result, a synergistic
effect of alloying Zn with Cu could be expected.
From the observed results, it can be concluded that
Cu/Zn cathode is more suitable for ammonia removal
than Ti and Fe cathodes in the absence of NaCl because
of its high selectivity for ammonia oxidation to nitrogen
gas. The effect of varying Cl− concentration on ammo-
nia-N and nitrate-N removal was also shown. It can be
concluded that these three cathodes showed almost same

Y. N. WANG ET AL. 243
Figure 1. Concentration of ammonia with respect of time. Anode: Ti/RuO2-Pt, cathode: Ti, Fe, Cu/Zn, 30 mA/cm2, 0 (a), 0.5
(b), 1.0 (c), 2.0 (d) g/l NaCl respectively.
Figure 2. Concentration of nitrate with respect of time. Anode: Ti/RuO2-Pt, cathode: Ti, Fe, Cu/Zn, 30 mA/cm2, 0 (a), 0.5 (b),
.0 (c), 2.0 (d) g/l NaCl respect ively. 1

Y. N. WANG ET AL. 244
results on ammonia removal with the same dosage of
NaCl. In the presence of 0.5 g/L NaCl after 120 min
electrolysis, ammonia-N decreased from 100 mg/L to 1.8
mg/L, 0.4 mg/L and 0.4 mg/L respectively with Ti,
Cu/Zn and Fe cathodes.
It is clear that the ammonia-N was significantly de-
creased compared with that in the absence of NaCl,
which proved the key role of indirect oxidation during
electrochemical oxidation of ammonia [19]. Meanwhile
nitrate-N increased from 0 to 6.4 mg/L, 5.2 mg/L and 6.0
mg/L respectively. In the presence of 1.0 g/L and 2.0 g/L
NaCl, the ammonia decreased sharply, which conform
that in the presence of chloride ions, hypochlorite ions
will be formed and then oxidize the ammonia and by-
product presumably to nitrogen gas [20] as listed in Equa-
tions (1)-(5).
2
2ClCl 2e



(1)
22
ClH OHClOHCl


  (2)
HClOClO H





(3)
422
NHHClONH OHCl

 (4)
+
432
NHHClONOH OCl

 (5)
None nitrite was detected throughout the experiment.
Overall, about 90% of removed ammonia was changed
into N2 gas with NaCl addition, which was similar with
other report [21].
The production of N2 could be calculated as equal to
the loss of N element according to the N conservation
law [22]. Simultaneously, intermediate nitrate ions were
formed in the water.
On the contrary the by-product formation had different
results. The nitrate formation could come from not only
the indirect oxidation of ammonia by HClO but also by
hydroxyl radicals. The by-product formation generated
by Ti and Fe was increased during electrolysis while us-
ing Cu/Zn cathode, it increased for the first period of
time and then decreased finally. The reason why the
3
N
O concentration was the lowest with Cu/Zn cathode
was that because of its high reduction ability. Ma’cova’
and Bouzek found that brass containing Zn higher than
35 wt%, lower than 41 wt% will significantly influence
the kinetic of current density, and higher electrocatalytic
activity of Cu/Zn alloy containing Zn of 35 - 41 wt%
than both Cu and Zn was observed
It can be concluded that ammonia oxidation rate was
increased with the increasing dosage of NaCl, while ni-
trate formation was different with various cathodes,
which suggested that with an appropriate cathode and an
appropriate concentration of chloride ion in the ammonia
solution during the electrolysis process, the ammonia can
be efficiently removed and relatively low amount of by-
products will be formed. Therefore, the electrochemical
process in the presence of chlorides showed a higher ca-
pacity and selectivity in ammonia transformation into
nitrogen gas [22]. It could be considered that an optimum
NaCl addition in the present experiment is 1.0 g/L, the
best cathode is Cu/Zn cathode. As the combination of
Ti/RuO2-Pt anode and Cu/Zn cathode showed a good
performance for ammonia oxidation, the influence of
several parameters, such as, current density, initial pH,
temperatures were studied.
3.2. Influence of Current Density
The rate of an electrochemical reaction is measured as
current density, current per area. As current density con-
trols the reaction rate that may be the most frequently
referred term in an electrochemical process [23]. In this
work, current densities ranged from 5.0 mA/cm2 as it is
the minimum required to achieve an effective oxidation
of ammonia. The effect of applied current density on
ammonia removal during the electrochemical reduction
was shown in Figure 3(a ).
As seen, the ammonia oxidation rate almost increased
with increasing current density in the range of 5.0
mA/cm2 to 50.0 mA/cm2. Ammonia-N decreased from
100.0 mg/L to 66.9 mg/L, 28.9 mg/L, 0.9 mg/L, 0 mg/L
in 120 min respectively. That conformed to the previous
report, an increase in current density improves ammo-
nia-N treatment efficiencies under the same charge load-
ing. The possible reason is that the increased current
density during electrochemical oxidation could enhance
chlorine generation, which was responsible for subse-
quent removal of pollutions [19].
Figure 3(b) showed the variation of nitrate-N during
electrolysis at different current densities. The final con-
centration of nitrate-N decreased with the increasing of
current density. The nitrate-N increased from 0 to 1.8
mg/L, 1.7 mg/L, 2.4 mg/L, 3.0 mg/L, 0.7 mg/L and 0.8
mg/L in 120 min respectively. As it has been reported
that the ammonia oxidation rate was linearly in accor-
dance with current density, which is in agreement with
the present experiments [24]. At lower current density,
less amount of hypochlorite acid was produced, which
was not enough to oxidize all of the ammonia. Therefore,
in the present experiments, in order to remove all of the
ammonia, the current density should higher than 30.0
mA/cm2.
3.3. Influence of Initial pH
The effect of different initial pH values on ammonia re-
moval and by-product formation was shown in Figure 4.
It can be seen that the tendencies of reduction of ammo-
nia were similar both at initial pH 3.0, 5.0 and 7.0. The
ammonia-N decreased from 100.0 mg/L to 3.0 mg/L, 0.8

Y. N. WANG ET AL. 245
Figure 3. Concentration of ammonia and nitrate with respect of time at different current density. Anode: Ti/RuO2-Pt, cathode:
Cu/Zn, 1.0 g/l NaCl.
Figure 4. Concentration of ammonia and nitrate with respect of time at different initial pH. Anode: Ti/RuO2-Pt, cathode:
Cu/Zn, 1.0 g/l NaCl, 30 mA/cm2.
mg/L and 0 mg/L in 120 min, respectively, while at ini-
tial pH 9.0 and 11.0, the ammonia-N decreased from
100.0 mg/L to 0 mg/L in only 60 min. As free ammonia
concentration is about 35.95 and 98.25 mg/L respectively,
according to previous research, solubility of free ammo-
nia in water at room temperature is about 3 mol/kg [25],
it can be considered that electrochemical method was
contributed to ammonia removal mainly. The finial con-
centration of by-product with initial pH 9 was the lowest
throughout the experiment. It can be conclude from the
present experiment that the optimum initial pH range for
ammonia removal was pH 7 - pH 9. This result is agree
with Lin and Wu [16], but different from Vlyssides et al.
[26]. Similar results were also obtained by Chiang et al.
[27] and Li and Liu [28]. The reason for that is because
of the formation of HOCl was affected disadvanta-
geously by low pH, as shown in Equation (2). Oppositely,
in strongly alkaline conditions, HOCl, a strongly oxida-
tion agent to ammonia, is transformed into 3
ClO

, which
has lower oxidability than HOCl [29]. Therefore,
strongly alkaline conditions decrease ammonia removal
efficiency. Therefore, in this study initial pH ranges from
7 to 9 was suggested for ammonia removal due to the
presence of high concentration of HOCl.
3.4. Influence of Temperature
Figure 5 showed the variation of ammonia-N and ni-
trate-N during electrolysis at different temperatures. As it
is difficult to maintenance high temperature throughout
experiments, the temperatures was only set to be at un-
controlled and at 25˚C. Under the condition of uncon-
trolled temperature, the temperature of the treated solu-
tion increased from 25.0˚C to 40.3˚C after 120 min elec-
trolysis. Meanwhile all of the ammonia was destroyed in
60 mins. Under the controlled temperature (25˚C) am-
monia oxidation was a little slower. This was mainly due
to different pH changes in the electrolyte at different
temperatures during the electrolysis (date did not show-
ed). The pH change was caused by the various reactions
during ammonia oxidation process (Equations (1)-(5)).
As previously mentioned, increasing pH was favorable
for ammonia oxidation; moreover, increasing tempera-
ture could increase the rate of diffusion and the strength
of adsorption. Consequently, the ammonia oxidation rate
increased when the temperature was increased from
25.0˚C to 40.3˚C.
In addition, according to previous study, ammonia in
aqueous solution can exist in two forms: un-ionized form
(NH3) and/or ionized form (4
N
H) [30]. Compare these

Y. N. WANG ET AL. 246
Figure 5. Concentration of ammonia and nitrate with respect of time at different temperatures. Anode: Ti/RuO2-Pt, cathode:
Cu/Zn, 1.0 g/l NaCl, 30 mA/cm2.
two forms of ammonia, the un-ionized one is much easier
to be oxidized. These two forms of ammonia established
an equilibrium following the Equation (6) [16].
32 4
NHH ONHOH

 

x
(6)
Under uncontrolled condition, with increasing tempe-
rature larger amount of NH3 formed subsequent oxidized on
electrode, which also increased ammonia oxidation rate.
In general, increasing temperature could affect ammonia
oxidation rate in several ways, the uncontrolled temperature
was favorable for electrochemical oxidation of ammonia.
From above, the mechanism of ammonia electro-oxi-
dation on RuO2 electrodes and formation of N2 as a final
product was indicated, that ammonia is oxidized through
several steps to various nitrogen compounds, which was
accord with previous reports, as Equation (7).
32ads
2
N
HNHOH NO
N

 (7)
4. Conclusions
In order to complete electrochemical oxidize of ammonia
and reduce the formation of by-products, cathodic reduc-
tion of by-products, and anodic oxidation of ammonia
were investigated in an undivided cell using Ti/RuO2-Pt
plate as anode and three plates as cathode for treatment
of the synthetic ammonia solution. It can be concluded: 1)
the ammonia-N decrease from 100.0 mg/L to 0 mg/L in
the presence of 1.0 g/L NaCl after 120 min electrolysis at
the current density of 30 mA/cm2 with Cu/Zn cathode
and Ti/RuO2-Pt anode. Throughout experiment none ni-
trite was detected in the treated solution; 2) Initial pH
range from 7 and 9 was favorable for electrochemical
ammonia oxidation; 3) the ammonia oxidation rates with
Cu/Zn cathode was higher than that with Ti and Fe cathode.
The nitrate formed during electrolysis with Cu/Zn was
the lowest than that with other cathodes. The reason why
the nitrate-N concentration was the lowest with Cu/Zn
cathode was that because of its high reduction ability;
and 4) as increasing temperature could affect ammonia
oxidation rate in several ways, the uncontrolled tempera-
ture was favorable for electrochemical oxidation of am-
monia.
In the present study, the ammonia was removed com-
pletely by electrochemical approach using Cu/Zn cathode
and Ti/RuO2-Pt anode with the addition of NaCl, it is a
worthy method for treatment of ammonia polluted water.
5. Acknowledgements
This work was supported by Grant-in-Aid for Research
Activity Start-up 22880007 from Japan Society for the
Promotion of Science (JSPS).
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