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Chemical Engineering Journal 183 (2012) 271– 277

Contents lists available at SciVerse ScienceDirect

Chemical Engineering Journal

j ourna l ho mepage: www.elsev ier .com/ locate /ce j

echanism of nitrate reduction by zero-valent iron: Equilibrium and kineticstudies

asuma Suzuki ∗, Mai Moribe, Yukinori Oyama, Masakazu Niinaeepartment of Environmental Science and Engineering, Graduate School of Science and Engineering, Yamaguchi University 2-16-1 Tokiwadai, Ube, Yamaguchi, 755-8611, Japan

r t i c l e i n f o

rticle history:eceived 23 September 2011eceived in revised form6 December 2011ccepted 16 December 2011

eywords:roundwater contaminationitrateero-valent ironron oxide coatinged water

a b s t r a c t

The first objective of this study was to deepen our understanding of the mechanism of nitrate (NO3−)

reduction by zero-valent iron (Fe0), especially the electron transfer process from electron donor(s) toNO3

− (i.e., an electron acceptor) under the presence of iron corrosion products. To achieve this objective,batch experiments were performed to investigate the influence of several variables, including aqueouspH, the solid–liquid ratio, the concentration of augmented ferrous ion (Fe2+), and the reaction time. Theexperimental results showed that the NO3

− reduction efficiency was enhanced by either decreasing theaqueous pH or increasing the solid–liquid ratio. Additionally, the NO3

− reduction efficiency at near neutralpH was both stoichiometrically and kinetically enhanced by augmenting the Fe2+ in the aqueous phase.These experimental data consistently indicated that NO3

− received electrons directly from Fe0 (i.e., adirect reduction mechanism) through an iron corrosion product layer (magnetite), rather than indirectlyvia H2 gas, which was produced by the reaction between Fe0 and an acid (i.e., an indirect reductionmechanism).

Based on the observation that the NO3− reduction efficiency at near neutral pH was enhanced by

augmenting Fe2+, the second objective of this study was to investigate the stoichiometric relationshipbetween the amount of augmented Fe2+ and the amount of NO3

− additionally reduced by augmenting2+ − 2+

Fe with the final goal of effectively removing NO3 at near neutral pH without leaving Fe in the treated

water. The experimental results demonstrated that Fe0 coated with an iron corrosion product (magnetite)repeatedly reduced NO3

− to ammonium ion at near neutral pH as long as the Fe2+ was augmented in theaqueous phase, and the concentrations of NO3

− and Fe2+ in the treated water were both reduced to nearzero if the proper amount of Fe2+ was augmented based on the stoichiometric relationship derived in thisstudy.

. Introduction

Groundwater is a precious source of drinking water that pro-ides relatively high quality water. In the Unites States, for example,n estimated 42% of the population uses groundwater as theirrinking water supply [1]. The corresponding value in Japan ispproximately 25%. However, contamination of groundwater isecoming a serious environmental issue worldwide.

Among a wide range of contaminants including chlorinatedydrocarbons (e.g., trichloroethylene) and pesticides, nitrateNO3

−) is the most common chemical contaminant in the world’sroundwater aquifers [2]. The National Research Council has esti-

ated that there are 300,000–400,000 NO3

− contaminated sites inhe Unites States [1]. In Japan, according to a report by the Min-stry of Environment in 2008, 4.1% of the groundwater exceeded

∗ Corresponding author. Tel.: +81 836 85 9690; fax: +81 836 85 9601.E-mail address: tsuzuki@yamaguchi-u.ac.jp (T. Suzuki).

385-8947/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.cej.2011.12.074

© 2011 Elsevier B.V. All rights reserved.

10 mgN/L, which is an environmental quality standard for totalnitrate/nitrite. Groundwater NO3

− contamination is also prevalentin other developed and developing countries where nitrogen fer-tilizers are intensively used.

NO3− reduction by zero-valent iron (Fe0), sometimes in con-

junction with surface treatment of depositing catalysts to enhancethe reduction efficiency, has been studied over the past severaldecades [3–25]. Although these studies successfully demonstratedthe effectiveness of Fe0 for NO3

− removal, there are several impor-tant aspects that still need to be investigated to fully evaluate theuse of Fe0 as a NO3

− removal technology. First, while enhancedNO3

− reduction at low pH has been reported in many studies[5–7,9,12,14,15,17,20,22], the corresponding reduction mecha-nism (i.e., electron transfer process from electron donor(s) to NO3

−)is still not elucidated [3–5,7,8,11,13,15,17,20]. Second, previous

−

studies [10,12,18,22] have reported that NO3 reduction at nearneutral pH was enhanced by augmenting ferrous ion (Fe2+) in theaqueous phase. However, it is well known that Fe2+ causes taste andodor problems when exposed to oxidants (e.g., oxygen). Therefore,

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72 T. Suzuki et al. / Chemical Engi

o avoid additional treatment for residual Fe2+ removal, it is highlyesirable to investigate the stoichiometric relationship betweenhe concentration of augmented Fe2+ and the amount of reducedO3

− by augmenting Fe2+, and then the proper amount of Fe2+

s augmented so that the concentrations of NO3− and Fe2+ in the

reated water are both near zero.Based on this information, there were two specific objectives

or this study. The first objective was to deepen our understandingf the electron transfer process from electron donor(s) to NO3

−.o achieve this objective, a series of batch experiments was per-ormed to investigate the influence of aqueous pH, the solid–liquidatio, the Fe2+ concentration in the aqueous phase, and the reactionime on the NO3

− reduction efficiency. The experimental resultsere then analyzed to examine the electron transfer process fromonor(s) to NO3

− and the role of iron corrosion product(s). Theecond objective was to remove NO3

− at near neutral pH withouteaving Fe2+ in the treated water. To achieve the second objective,he stoichiometric relationship between the amount of augmentede2+ and the amount of NO3

− additionally reduced by augment-ng Fe2+ was investigated, and the derived relationship was used toetermine the amount of augmented Fe2+ required to completelyeduce NO3

−. Subsequently, more batch experiments were per-ormed to experimentally demonstrate that the concentrations ofO3

− and Fe2+ both be reduced to near zero by augmenting theroper amount of Fe2+. The insights obtained from this study pro-ide fundamental information on the roles each treatment designariable plays and will be useful to optimize treatment conditions.

. Materials and methods

.1. Materials

The chemical reagents (guaranteed reagent grade) were pur-hased either from Nacalai Tesque, Inc. or Wako Pure Chemicalndustries, Ltd. and used without further purification. The stockolutions of ferrous ion (Fe2+) and nitrate (NO3

−) were preparedrom FeCl2·4H2O and NaNO3, respectively. The zero-valent ironFe0) purchased from Nacalai Tesque, Inc. was in the size rangef 20–60 mesh, and the measured Brunauer, Emmett and TellerBET) surface area was 1.1 ± 0.2 m2g−1 (Jemini 2375, Micromeriticsnstrument Corp., GA). Fe3O4, with the minimum purity of 95%, wasbtained from Strem Chemicals, Inc. (Newburyport, MA), and theeasured BET surface area was 9.1 m2g−1.

.2. Batch experiment procedures

All solutions used for the batch experiments were pre-ared using oxygen-saturated distilled deionized (DDI) water. Thexygen-saturated DDI water was prepared by mixing it with a resis-ivity of greater than 18 M� cm−1 (WA200, Yamato Scientific Co.td., Japan) for more than 12 h using a magnetic stirrer, and theolution was filtered through a 0.2 �m mixed cellulose ester typeembrane filter.The batch experiments were performed in 50 mL polyethylene

ottles at room temperature (24 ± 1 ◦C). Three stock solutionsNO3

− solution (60 mgN/L), Fe2+ solution (200 mgFe/L), andxygen-saturated DDI water), which were pre-adjusted to theesired pH by HCl, were mixed in the reactor to prepare 40 mLf solution containing 30 mgN/L of NO3

− and the predeterminedoncentration of Fe2+. After Fe0 was added in the bottle, the bottleap was tightly sealed immediately, and the bottle was then shaken

ith a mixing rate of 160 rpm using a horizontal shaker (NTS-4000,

okyo Rikakikai Co. Ltd., Japan). After a predetermined reactionime, the suspension was filtered through a 0.2 �m mixed cellulosester type membrane filter, and the final pH was measured. The

g Journal 183 (2012) 271– 277

filtrate was immediately acidified by HCl to avoid further reactionsbetween Fe2+ and O2 and analyzed for NO3

−, NO2−, NH4

+, Fe2+,and Fe3+.

For the experiments to investigate the reaction stoichiometry,the reaction time was determined to be 24 h according to the pre-liminary experiments, which were performed to confirm that 24 hwas long enough to finish the NO3

− reduction reaction as long as theinitial Fe2+ concentration did not exceed 50 mgFe/L. For the exper-iments to investigate the reaction kinetics, the reaction time wasset to 1, 3, and 6 h.

The batch experiment data shown in the figures are the mean ofat least two independent experiments performed in duplicate. Thereproducibility (expressed in relative duplicate error) was within12% for all data points. The error bars in the figures were omittedfor graphic simplicity unless necessary.

2.3. Analytical methods

The pH of the solution was measured using a pH meter (HM-30P, DKK-TOA Corp., Japan). NO3

−, NO2−, and NH4

+ were quantifiedusing a HPLC system with conductivity detection (Prominenceseries, Shimadzu Corp., Japan). Fe2+ and Fe3+ were quantifiedbased on the 1,10-phenanthroline colorimetric method accord-ing to the Japanese Industrial Standard (JIS) method K0102 usinga UV–Vis spectrophotometer (UV-2550, Shimadzu Corp., Japan).Dissolved oxygen (DO) was measured using a potable O2 meter(SG6-ELK, Mettler Toledo Inc., OH) with a polarographic DO elec-trode (InLab605, Mettler Toledo Inc., OH). The black coating thatformed on the Fe0 particles as a result of the reaction betweenNO3

− and Fe0 was analyzed by X-ray diffraction (XRD, Ultima IVProtectus, Rigaku Corp., Japan) using CuKa radiation at 40 kV and30 mA. The black coating on the Fe0 particles was also analyzed byfield emissive scanning electron microscope (FE-SEM, JSM-7000F,JEOL Ltd., Japan) with add-on for energy-dispersive spectrometry(EDS, JED-2300F, JEOL Ltd., Japan).

3. Results and discussion

3.1. Reaction end products

The nitrogen mass balance calculation for NO3−, NO2

−, andNH4

+ showed that NH4+ accounted for >93% recovery of NO3

− forall experiments. Therefore, consistent with many previous studies[4–6,9–15,18,20,22,24], the major product of NO3

− reduction in theFe0–H2O system was NH4

+.The reaction end products of Fe species were somewhat more

complicated. Regardless of the initial Fe2+ concentration, in thecases where NO3

− was completely reduced, Fe2+ remained in theaqueous phase (see Fig. 6). On the contrary, in the cases where NO3

−

was not completely reduced, the final aqueous Fe2+ concentrationwas negligible, meaning the reaction end products of Fe specieswere only undissolved species. The representative XRD spectrumfor the residue remaining on a 0.2 �m membrane filter (black-coated Fe0 particles) is shown in Fig. 1. The XRD pattern in Fig. 1indicates that the chemical composition of the residue was predom-inantly crystalline magnetite (Fe3O4). Other relatively insoluble Fespecies, such as ferrous hydroxide (Fe(OH)2) and ferric hydroxide(Fe(OH)3), were not detected.

Fig. 2 shows the representative SEM images and EDS spectrumfor the black-coated Fe0 particles. The analysis of EDS spectrumshowed that Fe and O were predominant elements of the black

coating and the Fe/O ratio was 1.7 ± 0.4 (n = 4). This observed Fe/Oratio was similar with the corresponding ratio obtained for Fe3O4(1.8 ± 0.3 (n = 4)). Note that EDS analysis provided poor sensitiv-ity for light element (e.g., oxygen) and therefore, the theoretical

T. Suzuki et al. / Chemical Engineering Journal 183 (2012) 271– 277 273

(a) Black coated sample

(b) Magnetite (Library)

(c) Fe0 (Library)

2 Theta (degree)80706050403020100

Fig. 1. (a) The representative XRD spectroscopy of the black coating formed aroundFt(

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0

5

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30

4035302520151050

Initial p H = 2.5Initial p H = 4.5

NO

3- (m

gN/L

)

Fe0 dosage (g)

0 −

Fc

e0 after reacting with NO3− . Experimental conditions: Fe0 = 12 g, initial pH 4.5, ini-

ial Fe2+ concentration = 0 mgFe/L, reaction time = 24 h. (b) XRD peaks of magnetite.c) XRD peaks of Fe0.

e/O ratio for Fe3O4 (i.e., 0.75) was not obtained for Fe3O4. Thisxperimental result obtained from EDS was consistent with theRD results, supporting the previous conclusion that Fe3O4 was

he predominant iron reaction product.

.2. Influence of aqueous pH and the solid–liquid ratio (withoutugmenting Fe2+)

To demonstrate the significance of aqueous pH and theolid–liquid ratio for NO3

− reduction, two sets of experiments, eachaving a variable solid–liquid ratio, were conducted at two different

nitial pH values of 2.5 and 4.5. As shown in Fig. 3, when the initialH was adjusted to 2.5, nearly 100% of the NO3

− was reduced withhe addition of 4.0 g or more of Fe0. In contrast, when the initial pHas adjusted to 4.5, only 50% of the NO3

− was reduced even though0 g of Fe0 was added. Therefore, consistent with previous studies5–7,9,12,14,15,17,20,22], NO3

− reduction by Fe0 was significantlynhanced by lowering the aqueous pH.

Fig. 3 also shows the non-linear relationship between the NO3−

eduction efficiency and the Fe0 dosage at an initial pH of 4.5, mean-ng that each Fe0 particle was more effectively used to reduce NO3

−

hen a smaller amount of Fe0 was dosed. This result makes sense

onsidering the pH shift during the 24 h experiments. The final pHfter 24 h increased from 4.5 to 6.3–8.4 (the higher the Fe0 dosage,he higher the final pH), meaning that acid was consumed in theeaction. Here, the acid consumption and the corresponding pH

ig. 2. (a) The representative SEM images of the black coating formed around Fe0 after

oncentration = 0 mgFe/L, reaction time = 24 h. (b) The EDS spectrum obtained from the sq

Fig. 3. The influence of initial pH and Fe dosage on NO3 reduction. Initial concen-trations of NO3

− and Fe2+ were 30 mgN/L and 0 mgFe/L, respectively. The reactiontime was 24 h.

increase were supposed to be slower when a smaller amount ofFe0 was dosed because the smaller amount of Fe0 resulted in fewerreaction sites. Consequently, when a smaller amount of Fe0 wasdosed, the Fe0 particles were exposed to a lower pH solution for alonger time compared to the case where a larger amount of Fe0 wasdosed, and each Fe0 particle was more effectively used to reduceNO3

−.

3.3. The influence and the role of aqueous Fe2+ (with augmentingFe2+)

As demonstrated in Fig. 3, decreasing the aqueous pH is requiredto effectively reduce NO3

− by Fe0. However, several previousstudies have shown that NO3

− reduction in a Fe0–H2O sys-tem at near neutral pH was enhanced by augmenting aqueousFe2+ [10,12,18,22]. Based on this observation, another series ofexperiments were performed to comprehensively investigate theinfluence of aqueous Fe2+ on the NO3

− reduction efficiency. Thereaction time was held constant at 24 h based on the prelimi-nary experiments which showed that 24 h was long enough tofinish the reactions if the initial Fe2+ concentration did not exceed

−

50 mgFe/L. The results are shown in Fig. 4. As expected, the NO3reduction efficiency was improved by augmenting aqueous Fe2+.The results indicated that either a) Fe2+ augmented in the aque-ous phase provided electrons to NO3

−, 2) Fe2+ somehow facilitated

reacting with NO3− . Experimental conditions: Fe0 = 12 g, initial pH 4.5, initial Fe2+

uare shown in (a).

274 T. Suzuki et al. / Chemical Engineering Journal 183 (2012) 271– 277

0

5

10

15

20

25

30

4035302520151050

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10 mgFe/L

25 mgFe/L

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Fig. 4. Enhancement of NO3− reduction by augmenting Fe2+ (as mgFe/L) in the

F 0 −

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Fig. 5. The increment in NO3− reduction by augmenting aqueous Fe2+. Each data

equation proposed by Huang et al. (Eq. (5)) [12].

e –H2O system. The initial NO3 concentration and initial pH were 30 mgN/L and.5, respectively. The reaction time was 24 h.

lectron transfer from the Fe0 core to NO3−, or 3) the Cl− added to

he solution as a counter-ion of Fe2+ hindered the formation of theassive iron oxide coating by enhancing the water solubility of ironpecies.

Additional experiments and data analyses were performed tolucidate the mechanism behind the enhanced NO3

− reduction byugmenting aqueous Fe2+. First, the third hypothesis was tested bydding Cl− into the Fe0–H2O system as NaCl rather than FeCl2. Theesults obtained from this experiment, however, successfully dis-issed the hypothesis because adding Cl− as NaCl did not enhance

he NO3− reduction (see Fig. S.1 in the Supporting information).

herefore, it is clear that the augmented Fe2+ was responsible forhe enhanced NO3

− reduction.Next, using the data shown in Fig. 4, the change in reduced NO3

−

y augmenting aqueous Fe2+ was calculated for each Fe2+ dosage.f Fe2+ is the sole electron source to NO3

− (this is the case for therst hypothesis), the reaction can be summarized in Eq. (1) basedn the results of the XRD and EDS analyses showing that Fe3O4 washe end product of Fe species (Figs. 1 and 2).

O3− + 12Fe2+ + 13H2O → NH4

+ + 4Fe3O4 + 22H+ (1)

According to Eq. (1), the amount of NO3− reduced by 1 mg of

e2+ was 0.021 mgN/mg Fe2+ (=14,000/(55,800 × 12)). However, ashown in Fig. 5, the change in NO3

− reduction by adding aqueouse2+ was 0.31 mgN/mgFe2+ in the range investigated in this study.his large discrepancy between the theoretical value calculatedrom the hypothesized reaction equation (0.021 mgN/mgFe2+) andhe experimentally obtained value (0.31 mgN/mgFe2+) revealedhat the dominant electron source for NO3

− was not Fe2+ but Fe0.n other words, the major role of Fe2+ in the reaction was not torovide electrons to NO3

− but to facilitate electron transfer fromhe Fe0 core to NO3

−.Unfortunately, it was beyond the scope of this study to char-

cterize the chemical transformation of the iron oxide coatingy surface-bound Fe2+, which is why the mechanism behind thenhanced electron transfer from the Fe0 core through the iron oxideoating in the presence of Fe2+ was not fully elucidated. However,

two-layer semiconductor model proposed by Huang et al. [12]eems to be plausible to explain the experimental results. Accord-ng to a two-layer semiconductor model, the iron oxide coatingonsists of an inner Fe3O4 layer (which was detected by XRD and

point was calculated using the data in Fig. 3 by taking the average for different Fe0

dosages. Error bars represent the standard deviation.

EDS) and an outer thin layer of iron oxides, such as lepidocrocite(�-FeOOH) (which were not detected by XRD and EDS because thethickness is a few Angstroms). The inner Fe3O4 layer did not hinderthe electron migration from the Fe0 core because Fe3O4 is a highlyconductive material. However, the electron transfer from the Fe0

core to the NO3− was hindered by the outer �-FeOOH layer because

�-FeOOH is not a conductive material. Nonetheless, in the presenceof Fe2+, the semiconducting �-FeOOH layer was transformed to themore conductive Fe3O4 according to Eq. (2) [26].

2�-FeOOH + Fe2+ → Fe3O4 + 2H+ (2)

As a result, as long as enough Fe2+ is present in the aqueousphase, electron transfer from the Fe0 core to NO3

− proceeds.Given the stoichiometric relationship obtained from Fig. 5, Eqs.

(1) and (3) (the direct reduction of NO3− by Fe0), the results were

used to propose the overall reaction representing the enhancedNO3

− reduction in the Fe0–H2O system by augmenting aqueousFe2+ (Eq. (4)).

NO3− + 3Fe0 + H2O + 2H+ → NH4

+ + Fe3O4 (3)

NO3− + 2.80Fe0 + 0.80Fe2+ + 2.20H2O → NH4

+

+ 1.20Fe3O4 + 0.40OH− (4)

It is important to note that Eq. (4) shows only the starting mate-rials and reaction end products and does not provide any detailedinformation on the reaction mechanism. Therefore, it is incorrectto state that Fe2+ directly donates electrons to NO3

− as apparentlyshown in Eq. (4). In fact, it was experimentally confirmed that Fe2+

and NO3− did not react with each other (even though Fe3O4 parti-

cles were present) under the experimental conditions in this study(see Fig. S.2 in the Supporting information). Nonetheless, Eq. (4) isstill valid from a stoichiometric point of view because our exper-imental results showed that the reaction end products were onlyNH4

+ and Fe3O4.The equation derived in this study (Eq. (4)) nicely matched the

NO3− + 2.82Fe0 + 0.75Fe2+ + 2.25H2O → NH4

+

+ 1.19Fe3O4 + 0.50OH− (5)

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T. Suzuki et al. / Chemical Engi

It should be noted that there were two fundamental differencesn the experimental procedures in this study and the ones used toerive Eq. (5). First, while oxygen-saturated water was used in thistudy, Huang et al. used deoxidized water. Second, while bare Fe0

articles were used in this study and the Fe3O4 layer was graduallyormed as the chemical reactions proceeded, Huang et al. used Fe0

articles that were precoated by Fe3O4. Regardless of these differ-nces in experimental procedures, however, the similarity betweenhe stoichiometric relationship in Eqs. (4) and (5) was not surpris-ng because Eq. (4) considers only the change in NO3

− reduction byugmenting Fe2+. As experimentally confirmed, O2 was depletednd the black Fe3O4 layer was formed even for the samples wheree2+ was not externally augmented (data expressed as 0 mgFe/L inig. 4). Therefore, when the externally augmented Fe2+ began tolay the roles which were previously discussed, the inside of theatch reactors was already an anoxic environment, and Fe0 parti-les were already coated by the black Fe3O4 layer, which representhe identical conditions used by Huang et al. to obtain Eq. (5).

.4. Kinetics of NO3− reduction by Fe0 with or without

ugmenting Fe2+

The experiments to this point were designed to investigatehe reaction stoichiometry under various initial conditions. It wasound that NO3

− was more effectively reduced by Fe0 at an initialH of 2.5 compared to 4.5. It was also found that NO3

− reduction atn initial pH of 4.5 was significantly enhanced by adding Fe2+ in thequeous phase because surface-bound Fe2+ facilitated the electronransfer from the Fe0 core to NO3

−. However, these results did notrovide any information on the reaction rate, which is an essen-ial aspect to comprehensively evaluate the use of Fe0 as a NO3

−

emoval technology.With the specific objective of comparing the NO3

− reductionate between an initial pH of 2.5 and 4.5 with or without augment-ng Fe2+, samples were taken at different reaction times. The resultsre shown in Fig. 6. As illustrated in Fig. 6(a), external Fe2+ augmen-ation had a negligible influence on the NO3

− reduction rate at annitial pH of 2.5. In contrast, at an initial pH of 4.5, the NO3

− reduc-ion rate steadily accelerated by augmenting more Fe2+, and when00 mgFe/L of Fe2+ was augmented, the NO3

− reduction rate even-ually became equal to that at an initial pH of 2.5, (Fig. 6(b)). It is alsomportant to note that the time course of the Fe2+ concentrationnder these conditions were somewhat close to that at an initial pHf 2.5 without Fe2+ augmentation (i.e., the Fe2+ concentrations after

h were 159 mgFe/L and 209 mgFe/L, respectively). These experi-ental results showed that Fe2+ augmentation improves the Fe0

erformance at near neutral pH from both a thermodynamic and ainetic point of view. Additionally, the NO3

− reduction rate was notontrolled by the initial aqueous pH but by the Fe2+ concentrationn the aqueous phase.

.5. NO3− reduction mechanism by Fe0

The enhanced NO3− reduction at a lower pH has been reported

y many researchers [5–7,9,12,14,15,17,20,22]. It was believed thatcid dissolves the iron oxide passive layers at low pH, and as a result,he regenerated Fe0 surface effectively reduces NO3

−. Regardinghe electron transfer from the Fe0 core to NO3

−, two mechanismsave been proposed. The first proposed mechanism involves theirect electron transfer from the Fe0 core to the NO3

− (i.e., Eq.3)), whereas the second mechanism involves indirect electronransfer via H gas produced according to Eq. (6). However, there

2s still controversy about the dominant electron transfer process3–5,7,8,11,13,15,17,20].

e0 + 2H+ → Fe2+ + H2 ↑ (6)

g Journal 183 (2012) 271– 277 275

The experimental results obtained in this study consistentlyindicated that direct electron transfer from the Fe0 core to NO3

−

was the dominant NO3− reduction mechanism under the exper-

imental conditions investigated. First, the stoichiometric studiesshowed that the NO3

− reduction efficiency was enhanced by aug-menting Fe2+ in the water (Fig. 4). Here, if indirect reduction wasthe dominant electron transfer mechanism, the NO3

− reductionefficiency should be controlled only by the amount of H2 gas gen-erated according to Eq. (6), which was in turn controlled only bythe initial pH. Therefore, the observed enhancement in the NO3

−

reduction efficiency by augmenting Fe2+ reasonably dismissed thepossibility of an indirect reduction mechanism. Second, based onthe kinetics studies shown in Fig. 6, the NO3

− reduction rate at aninitial pH of 4.5 could be as fast as the rate at an initial pH of 2.5 if100 mgFe/L of Fe2+ was augmented. Here again, if indirect reductionwas the dominant electron transfer mechanism, considering thatthe reaction between H2 and NO3

− is presumably fast because H2is a strong reducing agent, the NO3

− reduction rate at an initial pHof 2.5 should always be faster compared to a pH of 4.5 even thoughenough Fe2+ was augmented. Therefore, the enhanced NO3

− reduc-tion rate by augmenting Fe2+ (Fig. 6(b)) supported our previousconclusion that direct electron transfer from the Fe0 core to NO3

−

was the dominant NO3− reduction mechanism under the experi-

mental conditions investigated in this study. In fact, regardless ofthe amount of Fe2+ augmented in the aqueous phase, the DO con-centration dropped by approximately 70% (for an initial pH of 2.5)and 25% (for an initial pH of 4.5) after a 1 h reaction time, while thecorresponding reduction of NO3

− was approximately 10% and <5%,respectively (Fig. 6). This observation further supported the impor-tance of the direct electron transfer process in the experimentalconditions investigated in this study. The H2 gas produced accord-ing to Eq. (6) reacted quickly and preferentially with DO and wasdepleted. Thus, the electrons for NO3

− reduction were provideddirectly from Fe0.

It is important to note that it was very unlikely that a fresh Fe0

surface remained exposed to the solution for a long time, even forthe cases where the initial pH was adjusted to 2.5. As shown inFig. 6(a), the aqueous pH sharply increased from 2.5 to the neutralrange, and a black coating was visually confirmed on the surface ofFe0 particles after a 1 h reaction time. These observations indicatedthat Fe2+ might play an important role in the enhanced NO3

− reduc-tion at lower pH. In other words, although H2 produced throughEq. (6) was not a dominant electron donor to NO3

−, Eq. (6) stillplayed an important role in the reduction of NO3

− by producingFe2+, which is an essential agent to transform the passive iron oxidelayer into Fe3O4 (according to Eq. (2)) and allow more efficientelectron transfer from the Fe0 core to the NO3

−. More Fe2+ wasreleased at a lower pH, and as a result, more NO3

− was reducedbecause the electron transfer from the Fe0 core to NO3

− proceededas long as Fe2+ transformed the passive layer into Fe3O4. In termsof reaction kinetics, as shown in Fig. 6(a), Fe2+ augmentation didnot enhance the NO3

− reduction rate at an initial pH of 2.5. This isbecause enough Fe2+ was released through the acidic corrosion ofFe0 particles, and external Fe2+ augmentation was not required toeffectively transform the passive iron oxide layer into Fe3O4. How-ever, at an initial pH of 4.5, less Fe2+ was released (Fig. 6(b)), andtherefore augmenting Fe2+ in the aqueous phase was required toeffectively transform the passive iron oxide layer into Fe3O4.

3.6. Removal of NO3− at near neutral pH without leaving Fe2+ in

the treated water

The experimental results (Figs. 4 and 5) and the previous dis-cussion indicated that NO3

− could be repeatedly reduced to NH4+

by Fe3O4-coated Fe0 at near neutral pH, as long as Fe2+ was

276 T. Suzuki et al. / Chemical Engineering Journal 183 (2012) 271– 277

0

50

100

150

200

250

300

350

0 1 2 3 4 5 6 7 23 24

0 mgFe/ L25 mgFe/ L50 mgFe/ L100 mgFe/ L

Fe2+

(mgF

e/L)

Time (hours )

(a) Initial pH 2.5 (b)In itial pH 4.5

0

5

10

15

20

25

30

0 1 2 3 4 5 6 7 23 24

NO

3- (mgN

/L)

Time (hours )

0

50

100

150

200

0 1 2 3 4 5 6 7 23 24

Fe2+

(mgF

e/L)

Time (h ours)

(6.0)

(5.8)

(5.5)

(5.7)

(6.4)

(6.2)

(6.3)

(6.7)

(6.9)

(6.8) (7.0) (7.2)

(6.5)

(6.8)

(6.8)

(6.7)

(6.4)

(6.0)

(6.1)

(6.2)

(6.8) (6.8)

(6.8-7.9)

(6.7) (7.2 )

(8.1) (6.3)

(6.7)

(7.3)

(6.9 )

0

5

10

15

20

25

30

0 1 2 3 4 5 6 7 23 24

0 mgFe/L25 mgFe /L50 mgFe /L100 mgFe/L

NO

3- (mgN

/L)

Time (hours)

Fig. 6. Influence of Fe2+ augmentation on the NO3− reduction rate. (a) Initial pH of 2.5, (b) initial pH of 4.5. The Fe0 dosage was 12 g for all experiments. The aqueous pHs are

shown beside the data points. Note (a) and (b) have different scales for the Fe2+ concentration.

0

10

20

30

NO

3- (mgN

/L)

0

10

20

30

NH

4+ (mgN

/L)

0102030405060

Fe2+

(mgF

e/L)

02468

10

0 1 2 3 4 5

pH

Cycle nu mbe r

0

10

20

30

NO

3- (mgN

/L)

0

10

20

30

NH

4+ (mgN

/L)

0102030405060

Fe2+

(mgF

e/L)

02468

10

0 1 2 3 4 5

pH

Cycle nu mber

(a)no Fe2+ augment ation (b) 50 mgFe/ L Fe2+ augme ntation

Initial conditions at each cycle

Fina l conditions afte r 24 hours

Fig. 7. Repeated NO3− reduction to NH4

+ by Fe3O4-coated Fe0. (a) In the absence of Fe2+, (b) in the presence of 50 mgFe/L Fe2+. The Fe3O4-coated Fe0 particles were preparedby reacting 12 g Fe0 with 40 mL of 30 mgN/L NO3

− at an initial pH of 4.5 for 24 h.

neerin

acnwsvfs(rFmtdpaficfcsr5aoFeltzs

4

tdaadfcHaduptbms

A

ffd

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

T. Suzuki et al. / Chemical Engi

ugmented in the aqueous phase. The results also indicated that theoncentrations of NO3

− and Fe2+ in the treated water could both beear zero if the amount of Fe2+ augmented in the aqueous phaseas properly determined based on the stoichiometric relationship

hown in Eq. (4). Therefore, with the specific objective of testing thealidity of these indications, another set of experiments was per-ormed according to the following procedures. First, the experimenthown in Fig. 3 was repeated to prepare Fe3O4-coated Fe0 particlesexperimental conditions: initial pH 4.5, Fe0 = 12 g, Fe2+ = 0 mgFe/L,eaction time = 24 h). After 24 h reaction time, the Fe3O4-coatede0 particles were collected by filtering the solution with a 0.2 �membrane filter, and the collected Fe3O4-coated Fe0 particles were

hen transferred into another reactor containing 40 mL of deoxi-ized solution (deoxidized by purging with argon gas) adjusted toH 6.0 by HCl. The deoxidized solution contained 15 mgN/L of NO3

−

nd 50 mgFe/L of Fe2+. After 24 h reaction time, these processes ofltering the solution, analyzing the filtrate, transferring the Fe3O4-oated Fe0 particles, and shaking the reactor for 24 h were repeatedour more times to confirm the repeated NO3

− reduction by Fe3O4-oated Fe0 in the presence of Fe2+. The experimental results arehown in Fig. 7. As shown in Fig. 7, in the absence of Fe2+, NO3

−

eduction by Fe3O4-coated Fe0 was negligible. In contrast, when0 mgFe/L of Fe2+ was externally provided in the aqueous phase,s predicted from Fig. 5 or Eq. (4), practically all of the 15 mgN/Lf NO3

− was repeatedly reduced to NH4+ without leaving residual

e2+. Therefore, it was experimentally proven that NO3− is repeat-

dly reduced to NH4+ at near neutral pH by Fe3O4-coated Fe0 as

ong as Fe2+ was augmented in the aqueous phase, and the concen-ration of NO3

− and Fe2+ in the treated water could both be nearero if the proper amount of Fe2+ was augmented based on thetoichiometric relationship shown in Eq. (4).

. Conclusions

A series of batch experiments were performed with the objec-ive of (i) elucidating the electron transfer process from electrononor(s) to NO3

− and (ii) reducing the concentrations of NO3−

nd Fe2+ to near zero simultaneously by augmenting the propermount of Fe2+. The experimental results and the subsequentata analysis showed that the NO3

− received electrons directlyrom the Fe0 (i.e., a direct reduction mechanism) through an ironorrosion product layer (magnetite), rather than indirectly via2 gas produced by the reaction between Fe0 and an acid (i.e.,n indirect reduction mechanism). The experimental results alsoemonstrated that the Fe0 coated with an iron corrosion prod-ct (magnetite) repeatedly reduced NO3

− to NH4+ at near neutral

H as long as Fe2+ was augmented in the aqueous phase, andhe concentrations of NO3

− and Fe2+ in the treated water wereoth reduced to near zero if the proper amount of Fe2+ was aug-ented based on the stoichiometric relationship derived in this

tudy.

cknowledgments

The authors gratefully acknowledge the partial financial supportrom the Japan Society for the Promotion of Science (Grant-in-Aidor Scientific Research (B), no. 22360380, 2010) and the Steel Foun-ation for Environmental Protection Technology (no. 28, 2011).

[

g Journal 183 (2012) 271– 277 277

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.cej.2011.12.074.

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