Desalination 257 (2010) 158–162

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Reductive denitrification kinetics of nitrite by zero-valent iron

Zhen Zhang a, Zhiwei Hao a, Yueping Yang a,b, Jinghui Zhang a, Qian Wang a, Xinhua Xu a,⁎a Department of Environmental Engineering, Zhejiang University, Hangzhou 310027, Chinab Zhejiang Chimey Environment Science & Technology Co., Ltd, Hangzhou 310030, China

⁎ Corresponding author. Tel.: +86 571 87951239; faE-mail address: xuxinhua@zju.edu.cn (X. Xu).

0011-9164/$ – see front matter © 2010 Elsevier B.V. Adoi:10.1016/j.desal.2010.02.031

a b s t r a c t

a r t i c l e i n f o

Article history:Received 23 July 2009Received in revised form 20 February 2010Accepted 23 February 2010Available online 29 March 2010

Keywords:NitriteReductive denitrificationKineticsZero-valent iron

The objective of this current work was to investigate different factors that may affect the denitrification ofnitrite in the presence of Fe0 and the denitrification kinetics. Our results show that nitrite can be effectivelyreduced to innocuous N2 gas andNH4

+ by Fe0, no other intermediateswere generated during the denitrificationof NO2

−. The reduction efficiency of nitrite decreased quickly with increasing initial pH value, increasedconsiderably with increasing temperature, and did not vary much at initial concentrations ranging from 20 to50 mg L−1 when the excessive amount of Fe0 is utilized, but it decreased rapidly to 0.0448 min−1 when theinitial nitrite concentrationwas 100 mg L−1. The experimental data fit well to a pseudo-first-order model. Thekobs changed from0.0722 to 0.0731 min−1when the concentration of nitrite increased from20 to 50 mg L−1 inthis experiment with a Fe dosage of 10 g L−1. A larger Fe dosage led to the increase of kobs as the denitrificationof nitrite by Fe0 involved reductive reactions on metal surface. The activation energy of nitrite reductivedenitrification by Fe0 is determined to be 42.7 kJ mol−1 in the temperature range between 278 and 308 K.

x: +86 571 87952771.

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© 2010 Elsevier B.V. All rights reserved.

1. Introduction

Nitrate and nitrite contamination in groundwater and surface waterhas become an increasingly serious common environmental problem,since the plentiful use of nitrogen fertilizers, the unreasonable disposalof animal wastes and septic systems, the nitrogen contaminatedindustrial and municipal discharges, and the atmospheric deposition.The most dangerous health effect associates with nitrate and nitriteare responsible for the blue baby syndrome and acts as a precursorto carcinogenic nitrosamines as well as to other N-nitroso compounds[1–4]. The process of biological denitrification is often one of the mostwidely used techniques for the removal of nitrite andnitrate fromwater.However, the rate of denitrification in this process is relatively low, andthe addition of organic compounds is generally required for thetreatment of inorganic wastewaters [5]. Recently, chemical reductionhas been well investigated by different groups as an alternativetechnique for removing nitrite and nitrate from contaminated water[6–11]. Compared to the biological denitrification process, the chemicalreduction of nitrate and nitrite in water could also achieve highreduction rates. Recently, the use of ZVI (Fe0) has been the focus ofresearch for the treatment of contaminants such as halogenatedorganics [12–15] and inorganics [16–18].

The treatment of nitrate with Fe0 has been studied by several groups[19–23]. It has been demonstrated that nitrate can be completelyreducedbymetallic ironunder anoxic andaerobic conditions.Moreover,

it has been shown that Fe0 is also capable of reducingnitrite [2,16,24,25].Nevertheless, the reduction of nitrite by Fe0 has been studied far lessthan the reduction of either nitrate or Cr(VI). And to the best of ourknowledge, little has been reported on the reduction kinetics of thedenitrification of nitrite by Fe0. The formation of nitrite due to anincomplete reduction of nitrate, however, is a significant concernbecause of the adverse health effects associated with nitrite.

In this paper, batch tests were conducted to investigate nitritereduction in the presence of Fe0, by varying several reaction parameterssuch as initial pH value, nitrite concentration and Fe dosage. Apreliminary kinetic study has also been carried out.

2. Experiments and methods

2.1. Chemicals

All chemicals, such as sodium nitrate, sodium nitrite, sulfuric acidand acetone were used as received without further purification. Thesereagents except iron powderwere purchased from Shanghai ChemicalReagents Company. Iron powder (N200mesh (b0.07 mm), N98%) wasobtained from Jinshan Metallurgical Factory. The measured specificsurface area of the Fe0, determined by a BET N2 adsorption analyzer(Micromeritics ASAP 2020), was 0.50 m2 g−1, while its carbon contentwas not detectable. Before each experiment, iron powderwasfirst pre-treated with diluted H2SO4 (pH=2) and acetone, and then rinsed bydeionized water several times. After these procedures, impurities andorganic compounds on the surface of the iron powder were removed,which resulted in a fresh surface where the nitrite reduction reactioncould take place.

Fig. 1. Profile of nitrite reduction by Fe0 at initial pH 2.5, T=25 °C, C0=50 mg L−1,CFe=10 g L−1.

159Z. Zhang et al. / Desalination 257 (2010) 158–162

2.2. Batch experimental procedures

Batch experiments for the reduction of nitrite were conducted inthe same three-necked flask with a total volume of 500 mL solution.The temperature of the reactor was kept constant using a water bath.500 mL of an aqueous nitrite solution was added to the flask con-taining Fe0. The reactant solution was stirred by a mixer at a speed of100–200 rpm at a definite initial solution pH. The pH of the reactionsolution was adjusted to the desired level using sulfuric acid beforereaction. To simulate an anaerobic environment in groundwater, thedeionized water used for the preparation of the nitrite solution wasboiled and the reactor was purged with nitrogen gas to removedissolved oxygen before and during the reaction. 5-ml aliquots ofsamples were withdrawn by glass syringes at various reaction times,and then were filtered twice through 0.22 µm membrane filters, andsubjected to analysis in 4 h.

2.3. Analytic methods

The concentrations of nitrate and nitrite were measured witha Mentrohm 792 Basic Ion Chromatograph IC equipped with aMetrosep A Supp 4 column (250⁎4.0 mm), a Metrosep A Supp 4/5guard column, and a conductivity detector. The eluent (flowrate=1.0 ml min−1) was a standard Metrohm mixture of 2.0 mMsodium carbonate and 1.0 mM sodium bicarbonate. The concentra-tions of ferrous ions and total iron ions were determined by aspectrometric method involving 1,10-phenanthroline. pH value wasmeasured with a pH meter (PHS-25C pH). Ammonia and totalnitrogen were determined by an electrode (an ammonia gas-sensingconnected PHS-25C pH meter, Shanghai). The morphology of theparticles was photographed by a XL30ESEM (Philips, Netherlands)Scanning ElectronMicroscope (SEM). The outlet gas concentrations ofNO were measured with a ThermoElectron Model 42C NO-NO2-NOx

Analyzer.

2.4. Model application

Previous studies have shown that the reaction of nitrite reductionby Fe0 follows a pseudo-first-order reaction, with the assumption thatthe reaction will not be limited by the mass transfer with lowconcentrations of reactants (lower than about 50 mg L−1 with a Fedosage of 10 g L−1 in this experiment), and the dosage of Fe0 isexcessive in the reaction [2,26].

The reduction rate of nitrite can be expressed as Eq. (1)

v =−dcdt

= kobsc ð1Þ

where c is the concentration of nitrite in the reaction solution at timet, v is the reaction rate and kobs is the observed first-order reaction rateconstant, which is the slope of the regression lines by plotting anatural log graph with respect to nitrite concentration and thereaction time according to Eq. (2)

lncc0

� �= −kobst ð2Þ

c0 is the initial nitrite concentration in the reaction solution.

3. Results and discussion

3.1. Reduction of nitrite by Fe0 in low pH conditions

An example of the reduction characteristics of nitrite was shown inFig. 1. Under the condition of initial pH=2.5, the concentration ofnitrite decreased rapidly from 50 to 0.801 mg L−1 in 60 min.

Meanwhile, a control experiment showed that nitrite could hardlybe reduced spontaneously. It clearly indicates that Fe0 couldeffectively reduce nitrite. Few amount of nitrate (about 0.1 mg L−1)was detected during the reaction, probably due to the instability ofnitrite, according to the Eq. (3) that nitrous acid is not stable and itmay decompose into NO3

− and NO. But the self-decomposition ofnitrous acid to NO3

− and NO under the experimental conditions mightbe neglected, as and the outlet gas concentration of NO wasundetectable in our experiment.

3HNO2→Hþ þ NO

−3 þ 2NO þ H2O ð3Þ

With the reduction of nitrite, H+ was consumed according to thefollowing equation mentioned by Hu [2], so the pH value quicklyincreased to 6.3 during the following reactions (Eqs. (4)–(9)).

3Fe0→3Fe

2þ þ 6e− ð4Þ

2Fe0→2Fe

3þ þ 6e− ð5Þ

2NO−2 þ 8H

þ þ 6e−→N2 þ 4H2O ð6Þ

NO−2 þ 8H

þ þ 6e−→NH

þ4 þ 2H2O ð7Þ

3H2 þ 2NO−2 þ 2H

þ→4H2O þ N2 ð8Þ

3H2 þ NO−2 þ 2H

þ→2H2O þ NHþ4 ð9Þ

At low pH, Fe2+ formed as a result of the corrosion on the ironsurface, when the reaction proceeds, the solution pH quickly becomesneutral or weak acidic. And it is possible that the aqueous protons aredirectly reduced by Fe0 and form hydrogen species, such as hydrogenatoms (H), which react with nitrite or evolve as H2. The increase in thegeneration rate of hydrogen will also enhance the indirect reductionof nitrite. When the reaction proceeds, the pH rose to 6.3 quickly, thereduction rate of nitrite slowed down greatly with the formation ofpassivation layer Fe(OH)2 and Fe(OH)3 that led to precipitation on thesurface of Fe0 powder, so the reduction of nitrite by zero-valent ironmay be also considered as an acid-induced process [2].

Because the outlet gas concentrations of NO were undetectable inour experiment, according to the N mass balance, the outlet N2

concentration can be estimated, about 64% of nitrite can be convertedinto N2.

The electrons required to reduce nitrite must come from Fe0 eitherdirectly or indirectly through the corrosion products, Fe2+ andhydrogen. It is believed that the mechanism responsible for the rapidreduction of nitrite observed at low pH involves either hydrogen or Fe0

Fig. 2. pε-pH diagram for Fe–N–H2O system. Solid lines are the boundaries for Fe speciesand dashed lines for N species; assuming CT,Fe=0.01 mol L−1.

Fig. 3. Effects of nitrite concentration on nitrite reduction by Fe0, T=25 °C, pHin=2.5,C0=20–100 mg L−1, CFe=10 g L−1.

160 Z. Zhang et al. / Desalination 257 (2010) 158–162

[2]. Fig. 2 shows the relationship between pH and pε for Fe–N–H2Oredox system at 25 °C. Fe2+ and NH4

+ existed steadily under ourexperiment conditions (anoxic and pH lower than 7). Fig. 1 also showedclearly there was no more than 0.3 mM NH4

+–N generated towards1.1 mMNO2

−–N degradation, and this result agrees well with that of Huet al. [2], who found during the nitrite reduction when pH values were2.0–3.0, there were 37–26%molar ratios of nitrogen element convertedto NH4

+–N. So we could deduce that a part of ferrous and ammoniumions might be formed, ammonium ion and N2 is the principal and finalproducts of nitrite reduction under our experimental conditions.

Ferrous ions and total iron ions can also be detected since thereduction reaction took place in an aqueous solution. Considering themass balance, ferrous ions and total iron ions in solution must comefrom the corrosion of Fe0. During the reaction the concentrations offerrous ions and total iron ions were no less than 3 mg L−1 and2 mg L−1 respectively, and the concentration of ferrous ions weremuch higher than those of ferric irons under our experimentalconditions. It indicates that the corrosion of Fe0 indeed existed duringthe reduction course of nitrite by Fe0.

In the presence of dissolved oxygen under weak acidic or neutralconditions, the following reactions also occur.

4Fe2þ þ 10H2O þ O2→4FeðOHÞ2↓ þ 8H

þ ð10Þ

2Fe3þ þ 6H2O→2FeðOHÞ3↓ þ 6H

þ ð11Þ

The formation of Fe(OH)2 and Fe(OH)3 precipitates on the surfaceof Fe0 powder slows down the reduction rate of nitrite [2]. After thereaction, the surface passivating layers formed due to the precipita-tion of metal hydroxides and green rust on the surface of Fe0.

Table 1Kobs values in different experimental conditions.

Reaction conditions Kobs, min−1

Initial concentration of nitrite 20 mg L−1 0.072530 mg L−1 0.072250 mg L−1 0.0731100 mg L−1 0.0448

Fe dosage 5 g L−1 0.032210 g L−1 0.073120 g L−1 0.0812

Temperature 5 °C 0.018115 °C 0.029625 °C 0.073135 °C 0.0984

pH values 2.5 0.07313.0 0.02014.0 0.00985.0 0.0048

3.2. Effect of initial concentration of nitrite on nitrite reduction by Fe0

The effect of initial nitrite concentration on nitrite reduction wasshown in Fig. 3a. After 60 min reaction, the reduction rates of nitritewere 92.3, 98.4, 98.9 and 99.3% when the initial nitrite concentrationswere 100, 50, 30 and 20 mg L−1, respectively. The results indicate thatthe removal efficiencies of nitrite were close to each other at lowinitial concentration range (20–50 mg L−1), and there was an obviousdecrease in nitrite removal efficiency when the initial nitriteconcentration increased to 100 mg L−1, most likely due to the factthat 10 g L−1 Fe0 was not enough for the nitrite reduction when theinitial nitrite concentration increased to 100 mg L−1.

The linear relationship between ln(C0/C) and the reaction time fordifferent initial nitrite concentrations was shown in Fig. 3b. Theexperimental data agrees well with the model. The values of kobswere found to be close to each other when the initial nitrite

concentrations were 20, 30 and 50 mg L−1 (Table 1). An average kP

obs of0.0726 min−1 was obtained according to this study, and the reactionwould not be limited by the mass transfer with low concentrations ofreactants (lower than about 50 mg L−1 with a Fe dosage of 10 g L−1 inthis experiment). But the kobs decreased rapidly to 0.0448 min−1 whenthe initial nitrite concentration was 100 mg L−1, this further proved thenitrite reductionwould be slowed downwhen the supply of Fe0 was notenough. Comparing k

Pobs with the kobs calculated at different initial nitrite

concentrations, the maximum of relative deviation was less than 0.7%. Itstrongly indicates that there is no apparent relationship between k

Pobs

and initial nitrite concentration, it could further be deduced that thereduction of nitrite was a pseudo-first order reaction under theconditions of this experiment. This is in good agreement with a previousstudy on nitrate reduction [16,22,24].

3.3. Effect of initial pH value on the reduction of nitrite

Fig. 4a showed the effect of initial pH value on the reduction ofnitrite. The removal efficiencies of nitrite decreased with theincreasing initial pH. When the initial pH was 2.5, about 98% of

Fig. 4. Effects of initial pH on nitrite reduction by Fe0, T=25 °C, pHin=2.5–5.0,C0=50 mg L−1, CFe=10 g L−1.

Fig. 5. Effects of Fe dosage on nitrite reduction by Fe0, T=25 °C, pHin=2.5,C0=50 mg L−1, CFe=5–20 g L−1.

161Z. Zhang et al. / Desalination 257 (2010) 158–162

nitrite was reduced in 60 min, while the removal efficienciesdecreased to 70, 54 and 32% when the initial pH values were 3.0,4.0 and 5.0, respectively. This suggests that the reduction of nitritecould be well performed in acidic conditions. In fact, the reduction ofnitrite proceeded on the surface of iron particles, and at low pHs, ironparticles avoid forming oxides which would otherwise reduce theactivity of iron particles.

The linear relationship between ln(C0/C) and the reaction time fordifferent initial pH values was shown in Fig. 4b. It could be found thatthe value of kobs at pH 2.5 was about 15 times of that at pH 5.0(Table 1), which suggests an acidic condition is necessary in order toachieve a satisfactory nitrite removal efficiency.

3.4. Effect of Fe dosage on nitrite reduction by Fe0

Since the denitrification of nitrite by Fe0 involves reaction at a metalsurface, the effect of various Fe dosages on nitrite reduction wasinvestigated. Our results showed that the removal efficiency of nitriteincreasedwith increasing Fe dosage (see Fig. 5a). The removal efficiencywas about 97% at the time of 40 min for a Fe dosage of 20 g L−1, andthen reached 99% 20min later. When a dosage of 10 g L−1 Fe was used,the removal efficiency could exceed 98% after 60 min, while reachedonly 86% when the Fe dosage was 5 g L−1. Based on this study, a Fe0

dosage of 10 g L−1 Fe0 was selected under this experimental conditionfor efficient reductive denitrification of nitrite and yet minimal Fe0

usage.A pseudo-first-order reaction model was used to fit the experi-

mental data, as the metal surface area should strongly influence thekinetics of nitrite reduction. The available surface area is one of themost significant experimental variables affecting contaminant reduc-tion rates. Increasing the concentration of iron particles in thesolution, although little difference was found in the overall removalefficiency with excess iron dosage, would speed up the initial reaction,hence provide more active sites of iron particles for collision of nitriteions and reduction. The nitrite reduction rate profiles with differentiron contents in terms of kobs, and the observed first-order reactionrate constant kobs with different iron concentrations are shown inFig. 5b. It is shown that kobs increased from 0.0322 to 0.0731 min−1

when the Fe dosage increased from 5 to 10 g L−1, although acontinued increase of Fe dosage to 20 g L−1 made kobs increase to0.0812 min−1, compared to the extent of the increase in terms of Fedosage, the change of kobs is muted.

3.5. Effect of temperature on nitrite reduction by Fe0

The temperature of the reactor was kept constant using a waterbath, and the reaction solutionwas stirred by amixer at 100–200 rpm.Fig. 6a showed the effect of temperature on the reduction of nitrite,and the experiment was conducted in a temperature range between 5and 35 °C. The nitrite removal efficiency considerably increased withthe increasing temperature. And it increased from less than 68.2 toover 99.7% after 60 minwhen the reaction temperature increased from5 to 35 °C. Following this procedure, a linear decrease of ln(C/Co) withtime is obtained as shown in Fig. 6b. kobs under four differenttemperatures (5, 15, 25 and 35 °C) were determined as 0.0181,0.0296, 0.0731 and 0.0984 min−1 (Table 1), respectively. It is observedthat an increase in the reaction temperature could significantlyaccelerate the reaction rates.

The integrated form of the proposed pseudo-first-order kineticequation (Arrhenius equation) is:

ln kobs = A− EaRT

ð12Þ

where Ea is the activation energy (kJ mol−1), with A the pre-exponentialfactor, R refers to the universal gas constant (8.314 J mol−1 K−1), and Trepresents the reaction temperature (K).

The kobs values of various temperatures were correlated by theArrhenius equation, which gives rise to an estimated activationenergy of 42.7 kJ mol−1.

4. Conclusions

The experimental results have shown that nitrite can be effectivelyreduced by Fe0 at low pH values. Nitritewas almost reduced completelyin 60 min under our experimental conditions. Few nitrates could bedetected probably because of the decomposition of nitrous acid. The

Fig. 6. Effects of temperature on nitrite reduction by Fe0, T=5−35 °C, pHin=2.5,C0=50 mg L−1, CFe=10 g L−1.

162 Z. Zhang et al. / Desalination 257 (2010) 158–162

removal efficiency of nitrite decreased quickly with an increasing pHvalue possibly due to the formationof a passivation layer on ironparticlesurface caused by Fe(OH)2 and Fe(OH)3 precipitation.. The reduction ofnitrite by Fe0 follows pseudo-first order (PFO) kinetics when anexcessive dosage of Fe0 is used, and the observed rate constants (kobs)varied slightlywith different concentrations of nitrite. The experimentaldata fit very well to this model. The denitrification of nitrite by Fe0

involved reactions at themetal surface, and the increasing Fe dosage ledto the increasing of kobs and the reduction rate of nitrite. It isdemonstrated that the reduction of nitrite by Fe0 is a pseudo-firstorder reaction with the activation energy of 42.7 kJ mol−1 in thetemperature range between 278 and 308 K.

Acknowledgements

The authors are grateful for the financial support of the ZhejiangProvincial Natural Science Foundation of China (No. R5090033),Zhejiang Provincial Water Pollution Control and Management Projectof China (2008C13007-1) and the National Water Pollution Controland Management Project of China (2008ZX07101-006).

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