fabrication and characterization of a cu zn tio 2 nanotube array polymetallic nanoelectrode for electrochemically removing nitrate from groundwater

Fabrication and Characterization of a Cu-Zn-TiO2 Nanotube Array Polymetallic Nanoelectrode for Electrochemically Removing Nitrate from Groundwater Fang Liu, a,b,c Miao Li, a,c, z Hao Wan

Fabrication and Characterization of a Cu-Zn-TiO2 Nanotube Array Polymetallic Nanoelectrode for Electrochemically Removing Nitrate from Groundwater

Fang Liu, a,b,c Miao Li, a,c, z Hao Wang, d Xiaohui Lei, d Lele Wang, a and Xiang Liu a, z

a School of Environment, Tsinghua University, Beijing 100084, China

b College of Architecture & Civil Engineering, Beijing University of Technology, Beijing 100124, China

c Key Laboratory of Solid Waste Management and Environment Safety (Tsinghua University), Ministry of Education, Tsinghua University, Beijing 100084, China

d State Key Laboratory of Simulation and Regulation of Water Cycle in River Basin, China Institute of Water Resources and Hydropower Research, Beijing 100038, China

A novel Cu-Zn-TiO 2 nanotube array (TNTA) polymetallic nanoelectrode, intended to improve the electrochemical nitrate removal

efficiency, was fabricated The nanoelectrode was fabricated by plating Cu onto a Ti nanoelectrode and then plating Zn onto the

Cu/Ti bilayer electrode produced The Ti nanostructures on the Cu-Zn-TNTA nanoelectrode surface gave the nanoelectrode a large

specific surface area, and the Zn and Cu gave the nanoelectrode a high electrocatalytic activity for reducing nitrate Scanning electron

microscopy images showed that the Cu-Zn-TNTA polymetallic nanoelectrode had a honeycomb structure with spongy deposits.

X-ray diffractometry results showed that the Cu-Zn-TNTA nanoelectrode predominantly contained Cu, O, Ti, and Zn The nitrate

removal efficiency of the Cu-Zn-TNTA nanoelectrode was 345.7% of the removal efficiency for a Ti nanoelectrode The presence of

NaCl allowed both the cathodic reduction of nitrate and the anodic oxidation of the ammonia and nitrite byproducts to be achieved

with high removal efficiencies, especially using an IrO 2 anode In the present of NaCl, nitrate removal rate was 93.4% in current

density of 30 mA/cm 2 after 90 min Nitrate was completely removed using the IrO 2 anode, and little ammonia was detected in the

treated solution The reduction efficiency increased slightly as the initial nitrate concentration increased through the range 20–100

mg/L, and the pH had little effect on the nitrate reduction efficiency.

© The Author(s) 2016 Published by ECS This is an open access article distributed under the terms of the Creative Commons

Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/ ), which permits unrestricted reuse of the work in any

medium, provided the original work is properly cited [DOI: 10.1149/2.1391614jes ] All rights reserved.

Manuscript submitted August 15, 2016; revised manuscript received October 5, 2016 Published November 25, 2016.

Water resource shortages have become increasingly serious in

re-cent years, and are aggravated by groundwater pollution Nitrate is an

important groundwater pollutant, and removing nitrate from

ground-water is attracting increasing amounts of attention.1 Major

anthro-pogenic sources of nitrate in groundwater are industrial wastewater,

domestic sewage, animal waste, nitrogen fertilizers, and other waste

liquids.2 The World Health Organization has set a guideline

max-imum nitrate concentration in drinking water of 45 mg/L, and the

European Union regulatory limit is 50 mg/L for regular potable water

and 15 mg/L for water ingested by infants.5 High nitrate

concentra-tions in drinking water can seriously harm human health, impairing

oxygen transport to the tissues The ingestion of nitrate can cause

several health problems, including congenital deformities, gastric

cancer, high blood pressure, mental decline, thyroid disorders (e.g.,

goiter), and visual and auditory problems, which develop relatively

slowly.8 13

Attention has therefore been given to developing methods for

re-moving nitrate from groundwater Various methods have been

pro-posed for the removal of nitrate such as biological, physicochemical,

chemical and electrochemical.4Biological method has various

disad-vantages e.g it need a long time to react, difficult to control, react

incomplete will release of NO2, N2O and NOx, produced a large

number of biological sludge, requires a constant supply of the organic

substrate This limits the biological removal of nitrate application.14 , 15

The physicochemical processes mainly include distillation, reverse

os-mosis, electrodialysis, ion exchange,16 , 17it would produce secondary

brine wastes, because the nitrates are merely separated but not

de-stroyed The chemical methods produce toxic byproducts, such as

nitrite and ammonia and require either large quantities of metals or

hydrogen as reducing agent.4

In recent years, attention has been particularly focused on

electro-chemical methods of removing nitrate from water Electroelectro-chemical

methods have advantages such as no demand for chemicals before

or after the treatment, no sludge produce, small area occupied by

the plant and low investment costs.4 , 18 – 24 The efficiency at which

z E-mail: watersml@126.com ; x.liu@tsinghua.edu.cn

nitrate is removed using such methods depends strongly on the ma-terial the electrode is made from The broad practical use of elec-trochemical nitrate reduction requires a material to be found with appropriate electrocatalytic properties.3 The abilities of electrodes made out of various materials, including alloys, diamonds, metals, metal complexes, and materials with adatoms, to remove nitrate have previously been studied.25 – 27Large numbers of metal electrodes, in-cluding Cu, Ni, and Zn electrodes, have been studied Ti and other substrates offer promise for use in environmental applications be-cause they are relatively resistant to corrosion Pt has a low ni-trate reduction activity, whereas Cu and Zn are much more active.3 , 4 Electrodes made of two metals often have higher catalytic activities than do electrodes made of one metal Using a Cu/Zn cathode and

a Pt anode has been found to give a high electrocatalytic activity for nitrate Using a Cu/Zn cathode, Cu catalyzes the reduction of

NO3 −to NO

2 −and Zn catalyzes the reduction of NO

2 −to the final product.28

Materials containing nanostructures are generally called nanoma-terials It has been shown that nanostructures on the surface of a metal electrode can improve the electrochemical reduction performance of the electrode.29 This could be caused by the nanostructures giving the nanoelectrode a large specific surface area Despite the good results that nanoelectrodes have given, nanoelectrodes are limited

by the poor reproducibility achieved when preparing their activated surfaces Modified nanoelectrodes that can be prepared reproducibly are therefore required To the best of our knowledge, polymetallic nanoelectrodes have not previously been used to electrochemically reduce nitrate

The purpose of this study was to develop a novel and highly efficient polymetallic nanoelectrode for electrochemically reducing nitrate We fabricated a Cu-Zn-TiO2 nanotube array (TNTA) poly-metallic nanoelectrode and investigated its nitrate reduction per-formance We developed a nanoelectrode fabrication method and optimized the method to allow a nanoelectrode to be produced that could very efficiently remove nitrate using nontoxic materi-als The mechanism involved the removal of nitrate The efficiency

of the nanoelectrode under different experimental conditions was investigated

Figure 1 Schematic diagram of the electrochemical apparatus.

Experimental

Materials and chemicals.—Two cylindrical electrolytic cells were

prepared, one with a networking volume of 100 mL and the other with

a networking volume of 150 mL Each electrolytic cell was made out of

acryl material We used an undivided electrolytic cell, Compared with

a divided cell need to add a layer of membrane between the cathode

and anode which need higher cost, more complex structure, through

the experiment we found that do not add membrane also be able to

get a better removal effect, so we used the undivided electrolytic cell

Chemicals used in this study include: NaSO4, HCl, HF, Acetic

acid, H2SO4, ZnCl2, NH2SO3H, KCl and CuSO4· 5H2O All

chemi-cals were of analytical grade and used without further purification

So-lutions were prepared with deionized water (18.2 M/cm, ELGASTAT

B124 Water Purification Unit, The Elga group, England) Solutions

and chemicals are identical in the present study Fresh electrolyte was

used for each electrolysis

Electrodes.—Each electrolytic cell (shown schematically in

Fig.1) was made out of an acrylic material Initially, a graphite plate

was used as the cathode and a Ti plate as the anode When the

Cu-Zn-TNTA was being developed, first of all, Ti nanoelectrode was used as

the cathode and a Cu plate was used as the anode, and then Cu plated

on a Ti nanoelectrode was used as the cathode and a Zn plate was

used as the anode Nitrate in an aqueous solution was reduced using

the Cu-Zn-TNTA nanoelectrode as the cathode and a Pt plate as the

anode Each electrode was 2.5 cm wide, 2 mm thick, and 10 cm high

The gap between the electrodes was 0.8 cm

Methods.—Synthesis of the Cu-Zn-TNTA

nanoelectrode.—Ac-cording to the Wang’s reports,30 Ti sheet was used to fabricate Ti

nanoelectrode Before fabrication, the sheet was polished with

sand-paper until the surface showed no scratches, followed by rinsing in

deionized water and drying in air In the process of nanoelectrode

fabrication Ti foil as the cathode and an iridium dioxide foil as the

anode In our study, each electrode was 2.5 cm wide, 10 cm high and

2 mm thick The gap between the electrodes was 0.8 cm In the

pro-cess of nanoelectrode fabrication Ti foil (99.6% purity) as the anode

and an graphite foil as the cathode The surface of the anode was

mechanically polished with 800–1200 SiC metallographic paper and

washed several times with deionized water The fabrication procedure

had three steps, described below

Step A A 150 mL aliquot of a 1:10 mixture of acetic acid and water

containing 0.05 wt% HF was poured into the 150 mL electrolysis cell

The process was performed at room temperature A voltage of 25 V

was applied for 2 h After 2 hours, turned off the dc power supply,

removed the Ti sheet, rinsed the plate surface with deionized water and then dried the Ti sheet with blower At the end of the process, a

Ti nanoelectrode had been fabricated

Step B The Cu catalyst middle layer was deposited on the surface

of the Ti nanoelectrode fabricated in step A It was necessary to ensure that only Cu was present on the nanoelectrode surface Cu was electrodeposited from a 180 g/L CuSO4 solution in 60 g/L H2SO4 using a deposition current of 0.15 A for 10 s At the end of the process

a Cu/Ti bilayer nanoelectrode had been fabricated

Step C A Zn catalyst layer was deposited on the surface of the

Cu/Ti bilayer nanoelectrode produced in step B It was necessary to ensure that only Zn was present on the nanoelectrode surface Zn was electrodeposited from 160 g/L KCl and 80 g/L ZnCl2solution in 500

μL of a HCl using a deposition current of 0.25 A for 10 s At the end

of this process a Cu-Zn-TNTA polymetallic nanoelectrode had been fabricated

Nitrate removal.—A solution with an initial nitrate concentration

of 50 mg/L was prepared A 100 mL aliquot of the solution was poured into the electrolytic cell, then the reaction was started by applying a current density of 30 mA/cm2 The solution used in each experiment contained 0.5 g/L Na2SO4to increase the conductivity of the solution

A 1.0 mL sample was removed from the electrolytic cell at specified intervals Each sample was analyzed to allow temporal changes in the nitrate concentration in the solution to be monitored

The effect of the current density was investigated by perform-ing experiments at current densities of 10, 20, 30, and 50 mA/cm2 (controlled using a galvanostat) The effect of the temperature was in-vestigated by performing experiments at 0◦C (keeping the apparatus

in an ice bath) and at 25, 40, and 60◦C (keeping the apparatus in a water bath)

Analysis method.—All analyses were carried out according to

Stan-dard Methods (APHA et al., 1998) Nitrate and nitrite were detected by

a standard colorimetric method using a spectrophotometer (DR/6000 Spectrophotometer, HACH Company, Loveland, Colorado) Ammo-nia was measured by Nesslerization reagent spectrophotometry The determination of nitrate was based on its absorption spectra at 220 and

275 nm Nitrite and ammonia were based on its light absorption at 540 and 420 nm, respectively The microstructure and morphology of elec-trodes are analyzed by field-emission scanning electron microscopy (SEM) (Hitachi S5500, Japan) and energy dispersive spectroscopy (EDS) (Hitachi S5500, Japan)

Results and Discussion

Characterization and the electrochemical denitrification activity

of the Cu-Zn-TNTA nanoelectrode.—All experiments for nitrate

re-moval were at room temperature with current density of 30 mA/cm2,

Figure 2 Removal rates achieved using (A) the Ti electrode, (B) the Ti

na-noelectrode, and (C) the Cu-Zn-TNTA nanoelectrode (plating Cu: 0.15 A for

15 s, plating Zn 0.2 A for 10 s), (D) the Cu-Zn-TNTA nanoelectrode (plating Cu: 0.15 A for 10 s, plating Zn 0.15 A for 5 s), and (E) the Cu-Zn-TNTA nanoelectrode (plating Cu: 0.15 A for 10 s, plating Zn 0.25 A for 10 s).

the reaction time is 90 minutes The nitrate removal rates for

differ-ent electrodes are shown in Fig.2 It can be seen that, when plating

Cu: 0.15 A for 10 s, plating Zn 0.25 A for 10 s, the highest nitrate

removal rate was obtained using the Cu-Zn-TNTA nanoelectrode and

the lowest using the Ti cathode The nitrate removal rate increased

as the treatment time increased for all of the electrodes, and reached

21.3%, 28.3%, 65.1%, 73.3%, and 97.5% at 90 min using the Ti

elec-trode, Ti nanoelecelec-trode, and three Cu-Zn-TNTA nanoelectrode with

different current and time, respectively

We attempted to determine why the polymetallic nanoelectrode had

a good effect on nitrate removal by analyzing the surface morphologies

and elemental compositions of the Ti electrode, Ti nanoelectrode, and

Cu-Zn-TNTA nanoelectrode The surface morphologies and

elemen-tal compositions were analyzed by field-emission scanning electron

microscopy and energy dispersive spectroscopy, respectively It can

be seen from Fig.3that the Ti electrode surface was smooth, the Ti

nanoelectrode surface had a loose honeycomb configuration with

nu-merous micropores, and the Cu-Zn-TNTA nanoelectrode surface had

a honeycomb structure with spongy deposits on it

Energy dispersive spectra of the Ti electrode, Ti nanoelectrode,

and Cu-Zn-TNTA nanoelectrode are shown in Fig.4 The Ti

elec-trode contained 100 wt% Ti, the Ti nanoelecelec-trode contained 65.73

wt% Ti and 34.27 wt% O, and the Cu-Zn-TNTA nanoelectrode

con-tained 43.04 wt% Ti, 28.97 wt% O, 15.99 wt% Zn, and 5.95 wt%

Cu M´acov´a found that Zn plus Cu contents of 30 wt% to 41 wt%

significantly affect the current density kinetics, increasing the

electro-catalytic activity for the reduction of nitrate Our results did not agree

with the results found by M´acov´a, possibly because the copper layer

on our nanoelectrode was uneven The energy dispersive spectrum

acquired where less copper plating had occurred, shown in Fig.5,

confirmed this conclusion

It can be seen from Fig 5 that O, Ti, and Zn were basically

evenly distributed but that a small proportion of the Ti was completely

covered by Cu (from the Ti elemental distribution) However, Cu was

concentrated in certain areas, possible because the Cu particles were

large and a short plating time was used, meaning that it would be

difficult to achieve a uniform distribution of Cu

The Cu-Zn-TNTA nanoelectrode gave a higher nitrate removal

rate when plating Cu with 0.15 A was applied for 10 s and plating

Zn with 0.25 A was applied for 10 s than the other electrodes gave,

and we assumed that this was because the nanostructure of the Ti

surface in the Cu-Zn-TNTA nanoelectrode improved the

electroana-lytical performance This could have been caused by the presence of

more reactive sites on the Cu-Zn-TNTA nanoelectrode than on the

other electrodes and possibly by the effect of the nanostructure of the

surface of the Cu-Zn-TNTA nanoelectrode on diffusion Adding Cu

enhanced the catalysis of the nitrate reduction reaction In a previous

study, a Zn cathode was found to have a higher electrocatalytic

ac-tivity for the reduction of NO3 −than did other cathodes Keita found

that adding Zn2 +to the electrolyte positively affected cathodic

reduc-tion of NO2 −, indicating that Zn is a suitable complementary cathode

metal to Cu in terms of reaction kinetics Cu catalyzed the reduction

of NO3−to NO2−and Zn catalyzed the reduction of NO2−to the final

product The current applied and the plating time strongly affected the

effectiveness of the fabricated nanoelectrode Cu and Zn may not have

adhered to the electrode surface if a low current and short plating time

were used However, too much Cu and Zn may have adhered to the

electrode surface if a high current and long plating time were used

This could have caused the nanoholes in the Ti surface to become

filled with Cu and Zn

In Z MACOVA and K BOUZEK’s28study, they provide a

compar-ative study of the electrocatalytic activity of electrodes with different

Cu:Zn ratios for NO3 −reduction In their research, using an electrode

containing 30, 35, 41 wt% Zn, the kinetic current densities for NO3 −

reaction determined by Koutecky–Levich analysis were−125, −140,

−185 A/m2, respectively It shows that by increasing the content of

Zn in the alloy its electrocatalytic activity increased So they

con-cluded that the highest electrocatalytic activity was obtained using an

electrode containing 41 wt% Zn

Figure 3 Field-emission scanning electron microscopy images of the (A) Ti

electrode surface, (B) Ti nanoelectrode surface, and (C) Cu-Zn-TNTA nano-electrode surface.

In our study we adopted the similar proportion with Z MACOVA and K BOUZEK’s, and we controlled the proportion through control

of electroplating time and electroplating current

After many experiments, we found that performing the Cu plating procedure by applying 0.15 A for 10 s and performing the Zn plating procedure by applying 0.25 A for 10 s gave the best nitrate removal effect

Figure 4 Energy dispersive spectra showing the elemental

compositions of the (A) Ti electrode, (B) Ti nanoelectrode, and (C) Cu-Zn-TNTA nanoelectrode.

Nitrate reduction performance of the Cu-Zn-TNTA

nanoelectrode.—Influence of the NaCl concentration.—As

men-tioned above, Cu has a good electrochemical activity for the reduction

of nitrate.31 However, the main product of the reduction process

is NH3 This would be problematic when treating drinking water

Using other methods, nitrate would be mainly reduced to nitrite,

ammonia, and nitrogen (which is electrochemically inactive) The

main reactions occurring during the electrochemical reduction of

nitrate ions to give nitrogen and ammonia are discussed below.32

NO3 −+ 3H2O+ 5e−= (1/2) N2+ 6OH− [2]

2H2O+ 2e−= H2+ 2OH−(sidereaction) [9]

Figure 5 Energy dispersive spectra of the elemental distributions.

In the present of NaCl, the effects of different anode on nitrate

removal, nitrite and ammonia generation are shown in Fig.6 We

performed experiments using Pt or Ir as an anode and a NaCl

concen-tration of 0.1, 0.3, or 1.0 g/L to identify the conditions under which

the least nitrite and ammonia were produced

After many experiments, we found that using an IrO2anode and a

NaCl concentration of 0.3 g/L gave a nitrate removal rate of 93.44%

and generated the smallest amounts of byproducts This may have

been because IrO2 has a stronger oxidizing effect than does Pt, so

less ammonia and nitrite will be produced during electrolysis using

an IrO2electrode than using a Pt electrode NaCl was added because

the chloride ions would form hypochlorite and hypochlorous acid

Hypochlorite and hypochlorous acid have strong oxidizing properties,

and will cause nitrite and ammonia to be oxidized and produce nitrate

or nitrogen In the present of 0.3 g/l of NaCl, whether using Pt anode

or IrO2 anode, nitrite generation are almost zero However, in the

present of the 0.3 g/l of NaCl, using Pt anode, ammonia generation

is 0, 8.5, 19.4, 29.1, 28.9, 1.235 mg/l respectively in 0, 10, 30, 60,

90, 120 minutes; While using IrO2anodes, ammonia generation is 0,

0.15, 2.75, 2.95, 1, 0.04 mg/l respectively in 0, 10, 30, 60, 90, 120

minutes It can be seen that the IrO2anodes have always maintained

a low ammonia production, while Pt anodes have higher ammonia

production in the middle of the reaction So we deem IrO2is a better

oxidizing agent for NH4 +and NO

2 −than Pt.

Adding too little NaCl would cause too little hypochlorite to be

produced, meaning that all of the nitrite and ammonia would not be

removed Adding excess NaCl would cause competitive adsorption to

occur, inhibiting nitrate removal

Influences of the initial nitrate concentration and the

temperature.—The effects of different initial nitrate concentrations

on nitrate removal are shown in Fig.7A The selectivity of the

ni-trate removal process was approximately constant over the nini-trate

concentration range that was tested At each concentration, the nitrate removal rate rapidly decreased at the beginning of the electrolysis process However, the nitrate removal rate was almost the same at different initial nitrate concentrations The removal rate when the ini-tial nitrate concentration was 20 mg/L was 75.8% (i.e., the nitrate concentration decreased from 20.00 to 4.83 mg/L) The removal rates when the initial nitrate concentrations were 30, 50, and 100 mg/L were 74.4%, 74.6%, and 72.6%, respectively This could have been because a thin sponge-like Cu layer formed over each Ti nanopore and a granular Zn layer formed, protecting the nanopores

The influence of the reaction temperature on nitrate reduction is shown in Fig 7B The earlier experiments were performed under isothermal conditions at 25◦C The experiments at different temper-atures were performed by placing the reactor in an ice bath (i.e.,

at 0◦C) or in a water bath at 25, 40, or 60◦C Most of the experi-ments were performed without stirring, although mixing eventually occurred because hydrogen and oxygen bubbles formed at the cath-ode and ancath-ode, respectively When the temperature was uncontrolled, the temperature of the treated solution increased from 25.0◦C at the beginning of the electrolysis process to 78◦C after 60 min of electrol-ysis Increasing the temperature could affect the removal of nitrate in several ways, including by increasing the diffusion rate and increas-ing the strength of the adsorption that occurred The nitrate removal rates were slightly higher at 40 and 60◦C than at 25◦C (the initial nitrate concentration of 50.00 mg/L decreased to 15.0 mg/L at 25◦C,

to 13.2 mg/L at 40◦C, and to 8.3 mg/L at 60◦C) This was because in-creasing the temperature will have favored the formation of ammonia and decreased the amount of nitrogen formed, as has been reported previously

Influence of current density and pH.—The nitrate concentrations at

different times when the Cu-Zn-TNTA nanoelectrode was used with different current densities are shown in Fig.8A Increasing the current

Figure 6 (A) Temporal changes in nitrate, nitrite, and ammonia

concentra-tions using an IrO 2 anode and a NaCl concentration of 0.3 g/L with current

density of 30 mA/cm 2 and (B) Temporal changes in nitrate, nitrite, and

am-monia concentrations using an Pt anode and a NaCl concentration of 0.3 g/L

with current density of 30 mA/cm 2

density between 10 and 40 mA/cm2increased the removal rate, and

the removal rate was particularly high at current densities of 30 and 40

mA/cm2 Increasing the current density is a simple way of accelerating

the reduction reaction but usually decreases both current efficiency

and selectivity We found that the current efficiency decreased as the

current density increased The current efficiency probably decreased

because more hydrogen was produced in the electrode structure The

potentials were 10.8–9.1, 17.3–11.5, 26.9–14.2, and 30.4–14.2 V at

current densities of 10, 20, 30, and 40 mA/cm2, respectively The

current efficiency is 2.71, 1.54, 1.16, 1.08 mg/L∗(mA/cm2 ∗h)−1in the

current densities of 10, 20, 30, and 40 mA/cm2, respectively A low

current density will be economically favorable However, in practical

use, selecting the most suitable current density will depend on the

treatment conditions

The effect of the initial pH on nitrate removal is shown in Fig.8B

The solution was brought to the desired pH by adding dilute NaOH

or dilute H2SO4 The nitrate removal trends were similar at different

initial pH values, and different initial pH values gave almost the same

nitrate removal rates This may have been because a thin

sponge-like Cu layer became attached to each Ti nanopore and a granular

Zn layer formed during the preparation of the nanoelectrode These

Figure 7 Effects of the (A) initial nitrate concentration and (B) temperature

on temporal changes in the nitrate concentration (I = 30 mA/cm 2 , 0.50 g/L

Na 2 SO 4 ).

would have prevented adhesion to the nanopores, meaning that the initial pH would have hardly affected the nitrate removal rate

Conclusions

A novel Cu-Zn-TNTA nanoelectrode was fabricated to improve the electrochemical nitrate removal rate The nitrate removal rates achieved using the Cu-Zn-TNTA nanoelectrode in an undivided and unbuffered cell under different conditions were determined The re-sults showed that the Cu-Zn-TNTA nanoelectrode could be used to remove nitrate from an aqueous solution The conclusions shown be-low were drawn

(1) The electrode played an important role in nitrate removal The nitrate removal rate reached 97.5% using the Cu-Zn-TNTA na-noelectrode in the presence of 0.50 mg/L Na2SO4after 90 min

of electrolysis at a current density of 30 mA/cm2 (2) In the presence of 0.3 g/L NaCl, nitrate was completely re-moved using an IrO2 anode, and little ammonia was detected

in the treated solution At the same initial nitrate concentration, the nitrate removal efficiency increased as the current density increased At the same current density, the nitrate removal rate decreased slightly as the initial nitrate concentration increased The temperature strongly affected nitrate removal Changing the

pH had little effect on nitrate removal

Figure 8 Effects of the (A) current density and (B) pH on changes in the

nitrate concentration over time (I= 30 mA/cm 2 , 0.50 g/L Na 2 SO 4 ).

Acknowledgments

The authors thank the National Natural Science Foundation of

China (No 51408335) and Beijing Natural Science Foundation

(J150004) for the financial support of this work

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