Efficient Electrochemical Removal of Ammonia with Various Cathodes and Ti/RuO<sub>2</sub>-Pt Anode

doi:10.4236/ojapps.2012.24036

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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

Email: *[email protected]

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).

2ClCl 2e







(1)

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

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

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



 



(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

HNHOH NO



 (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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