Process Safety and Environmental Protection 9 1 ( 2 0 1 3 ) 304–310

Contents lists available at SciVerse ScienceDirect

Process Safety and Environmental Protection

jou rn al hom epage: www.elsev ier .com/ locate /psep

Zero valent nano-sized iron/clinoptilolite modified with zerovalent copper for reductive nitrate removal

Fatemeh Sadat Fateminiaa, Cavus Falamakib,∗

a Chemical Engineering Department, Amirkabir University of Technology, Mahshahr Branch, Mahshahr, Iranb Chemical Engineering Department, Amirkabir University of Technology, P.O. Box 15875-4413, Tehran, Iran

a b s t r a c t

Nitrates constitute one of the main toxic contaminants of groundwater. On the other hand, groundwater may be

considered anoxic (oxygen concentration less than 9 �g L−1). This fact justifies the use of nano zero valent metals for

nitrate removal. In such conditions, zero valent metals are quite stable against oxidation due to the very low level

of dissolved oxygen concentration. It has been shown that the performance of zero valent iron coated clinoptilolite

zeolite for the reduction of nitrate anion in un-buffered conditions may be enhanced by coating small amounts of

Cu0 onto the freshly prepared Fe0/zeolite composite. An optimum loading of Cu0 exists for which the rate of nitrate

removal is maximal. For this optimal composition, the nitrite anion production curve with time passes through

a maximum. Nitrite production, however, is slightly higher for the Cu modified zeolite. It has been shown that the

nitrate removal process is only slightly dependent on the initial solution pH. In the temperature range of 20–60 ◦C, the

process is controlled by both the liquid phase mass transfer and intrinsic reaction rate resistances. FESEM analysis

of the zero valent metal/zeolite composite showed that upon the metal reduction reaction, an egg-shell distribution

of zero valent metal in the zeolite agglomerate particle is produced.

© 2012 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

Keywords: Nano sized zero valent iron; Clinoptilolite; Nitrate removal; Copper; Nitrite

1. Introduction

Nitrate anions are considered as hazardous environmentalcontaminants of ground and surface water. According to theUS Environmental Protection Agency (EPA), nitrate thresholdconcentration should not exceed 10 mg L−1 (Wolfe and Patz,2002). However, this threshold level is surpassed in many casesdue to many reasons, the most important being the inten-sive and uncontrolled use of fertilizers in agriculture (Raoand Puttanna, 2000). Unfortunately, nitrates may persist fordecades in groundwater and accumulate to high levels asmore nitrogen based fertilizers are fed to the soil each year(Thomson, 2001). Various techniques have been adopted sofar for drinking water denitrification: ion-exchange, reverseosmosis, adsorption, biological denitrification and chemi-cal reduction (Bae et al., 2002; Schoeman and Steyn, 2003;Fernandez-Nava et al., 2008; Zhang and Huang, 2005). These

techniques have their advantages and disadvantages. Ion-

∗ Corresponding author. Tel.: +98 2164543160; fax: +98 2166405847.E-mail address: c.falamaki@aut.ac.ir (C. Falamaki).Received 6 April 2012; Received in revised form 16 July 2012; Accepted

0957-5820/$ – see front matter © 2012 The Institution of Chemical Engihttp://dx.doi.org/10.1016/j.psep.2012.07.005

exchange and reverse osmosis suffer from a medium to highoperating cost due to the frequent regeneration of the mediaand production of secondary brine waste. Adsorption suf-fers from strong pH and temperature dependency and spentadsorbent disposal problem (Bhatnagar and Sillanpaa, 2011).Biological denitrification has a lower operating cost and is theprevalent method used for the moment. It suffers from exces-sive biomass and soluble microbial by-products. In addition,this process is relatively slow and, compared to the chem-ical reduction method, sometimes suffer from incompleteremoval of the ion (Hwang et al., 2011).

Chemical methods based on the reduction of the nitrateanion using nano zero valent metals (Fe, Al, Zn, Mg, etc.) haveturned out to be suitable denitrification techniques (Chenget al., 1997; Hatfield and Follett, 2001). Nano sized zero valentmetals posses a high specific surface area and a high surfacereactivity. A technical problem associated with the industrialapplication of such materials is that they cannot be used alone

24 July 2012

and should be supported. Pillared clays and zeolites have beenrecently used as low cost effective supports for nano scale zero

neers. Published by Elsevier B.V. All rights reserved.

Process Safety and Environmental Protection 9 1 ( 2 0 1 3 ) 304–310 305

vtobihemstS

tltppacvat

2

AwoFwf

utawfotsadtpmw

vtfttwatvdad(

w(

Fig. 1 – Ion-exchange isotherm for the Fe2+/clinoptilolite◦

alent iron (Zhang et al., 2011; Lee et al., 2007). Using clinop-ilolite zeolite as support has the great advantage of removalf nitrate anions without ammonium ion release under un-uffered pH, as the ammonium ion released is selectively

on-exchanged by the zeolite (Lee et al., 2007). On the otherand, the reactivity of nano zero valent metals like Fe may benhanced by coating them with small amounts of less activeetals like Pd, Pt, Ni and Cu onto the freshly prepared Fe

urface due to the promotion of iron oxidation by the poten-ial difference (Xu and Zhang, 2000; Elliott and Zhang, 2001;chrick et al., 2002).

The present work aims at presenting a new material forhe chemical reduction of NO3

−. Using an Iranian clinoptilo-ite zeolite as support for the nano zero valent iron, we triedo promote its performance in reactive nitrate removal byartly coating it with zero valent copper. In this way, the com-osite material benefits from both the advantages of leastmmonium cation release into the solution and enhancedhemical reactivity. The effects of different loadings of zeroalent Cu, solution initial pH and temperature on the nitratenion removal and nitrite anion production have been inves-igated.

. Experimental

commercial Iranian clinoptilolite zeolite (Semnan region)as used. Details about the chemical composition and purityf the zeolite may be found elsewhere (Falamaki et al., 2004).eCl2·4H2O and CuCl2·4H2O metal source materials and KNO3

ere purchased from Merck. NaBH4 (98 wt.%) was purchasedrom Sigma.

In the first stage, the ion-exchange isotherm for Fe2+ cationptake by the raw clinoptilolite zeolite was determined. Forhis means, the raw zeolite was crushed and sieved to obtain

mesh size in the range of 100–150. The zeolite was thenashed with de-ionized water, filtered and dried at 90 ◦C

or 5 h. Afterwards, it was ion-exchanged with FeCl2 aque-us solutions of different molarities (Fe2+ concentration inhe range of 0–200 ppm) using 0.25 g zeolite and 50 mL FeCl2olution. Ion-exchange was performed at 20 ◦C for 8 h undergitation (200 rpm). In continuation, the supernatants wereecanted and the remaining solids were washed three timeso remove excess Fe2+ ions in the particles’ meso and macro-ores. The amount of Fe2+ uptake by the zeolite was deter-ined by the titration of the final equilibrium liquid phaseith KMnO4 reagent (Pashmineh Azar and Falamaki, 2012).

For coating the ion-exchanged zeolite with nano zeroalent iron, 3 g of the zeolite and 50 mL of a 3000 ppm Fe2+ solu-ion were added to 50 mL screw cap-glass tubes and agitatedor 8 h at 20 ◦C. In a series of experiments, the concentra-ion of the initial Fe2+ cations was 300 ppm. After decantation,he resultant solids were washed three times with de-ionizedater to excess Fe++. The wet solids were then transferred to

beaker containing 50 mLit of 24 mM NaBH4 aqueous solu-ion and agitated for 1 h at 20 ◦C (200 rpm). The resultant zeroalent iron coated zeolite (FeCLP) was washed three times withe-ionized water. The whole process was performed undernoxic conditions using N2 gas purge flow. Measurement ofissolved oxygen in solutions was performed using a HQ30d

Hach) instrument.Recall that the threshold for boron concentration in

ater is near 1 mg B L−1 (expressed as boron equivalent)Schoderboeck et al., 2011). Therefore, the final zero valent

zeolite system at 20 C.

coated zeolite powder was tested for eventual residual boroncompounds by FTIR analysis. Comparison of the FTIR spec-tra of the raw and final zeolite sample did not show anypeak due to BO2, BO3 and BO4 structural units in the rangeof 400–1500 cm−1 (Taghavi et al., 2011).

For the production of zero valent Cu coated FeCLP (CuFe-CLP), the freshly prepared FeCLP composite particles were putin contact with aqueous CuCl2 solutions of different concen-trations for 20 min. Based on atomic absorption analysis of thefinal liquid solution, the amount of metallic copper in the finalsamples were 0.00, 0.18, 1.94, 9.21 and 36.86 mg/g clinoptilolitezeolite. The corresponding weight ratio percent (Cu/(Cu + Fe))in the resultant samples was calculated as 0.0, 0.5, 5.0, 20.0and 50.0. The corresponding solid samples were nominatedaccordingly 0CuFeCLP (or FeCLP), 0.5CuFeCLP, 20CuFeCLP and50CuFeCLP, respectively. Afterwards, the CuFeCLP particleswere washed twice with de-ionized water. Again, every actionwas performed under anoxic conditions. The iron content ofthe FeCLP sample was measured to be 36.86 mg/g zeolite.

Nitrate experiments were performed in batch mode andunder anoxic conditions. Nitrate solutions with initial con-centration of 30 mg L−1 were prepared by dissolving KNO3

in de-ionized water. 50 mL of 30 mg L−1 nitrate solution wasinjected into a series of 50 mL amber glass bottles contain-ing 3 g of CuFeCLP without headspace. Experiments wereperformed in un-buffered conditions. Samples were mixedon a shaker with an agitation speed of 200 rpm at 20 ◦C atpredetermined time intervals. Afterwards, each sample wascentrifuged, filtered and the supernatant was analyzed for themeasurement of nitrate and nitrite anions concentration. AUV–Vis spectrophotometer (Lovibond Co.) was used.

It should be noted that the nitrate removal method usedin this study is accompanied with the release of Fe2+ cations,and this obviously changes the color of the water. However,from an industrial point of view, this causes no problem asthe released iron may be simply precipitated in the form ofFe(OH)3 using an MnO2 coated/clinoptilolite catalyst at roomtemperature. The interested reader may refer to the recentwork of Pashmineh Azar and Falamaki (2012) for more details.

FESEM analysis was performed using a S-4160 Hitachiinstrument. ICP analysis of the raw zeolite was performedusing an ICP-AES ARL-3410 instrument.

3. Results and discussion

Fig. 1 shows the ion-exchange isotherm at 20 ◦C for clinop-tilolite at different Fe2+ solution equilibrium concentrations

306 Process Safety and Environmental Protection 9 1 ( 2 0 1 3 ) 304–310

Fig. 2 – FESEM picture of FeCLP zeolite: (a) picture showing the inner and outer surface of an intentionally broken zeoliteagglomerate particle, (b) outer surface, (c) inner core (depth from the outer surface ca. 1.5 �m) and (d) outer surface using a

NaBH4 concentration two times the stoichiometric value.

(0–135 ppm). An asymptote is observed at solution concen-trations higher than 50 ppm corresponding to a Fe2+ cationexchange capacity of 0.42 mmol (g zeolite)−1. This is in accor-dance with the exchange capacity calculated from the Na, K,Ca and Mg content of the raw zeolite based on ICP analysis,i.e. 0.44 meq (g zeolite)−1. The ion exchange capacity for theFe2+ of the Iranian clinoptilolite zeolite used in this study isapproximately twice the capacity reported by Lee et al. (2007)for a Korean clinoptilolite zeolite, i.e. 0.17 meq (g zeolite)−1.Determination of the NaBH4 solution concentration was basedon the total ion-exchangeable Fe2+ cation capacity of thezeolite (stoichiometric equivalent). However, as it will be men-tioned later in the text, only a small percentage of the totalcovalently bond Fe2+ cation reacts and transforms into zerovalent iron. Accordingly, using a 0.054 M solution, as explainedin Section 2, we expect to obtain an Fe2+ content of ca.0.42 mmol (g zeolite)−1 for the zeolite prior to reaction withNaBH4.

Fig. 2a–c shows the FESEM pictures of the clinoptilolitezeolite after the reaction with NaBH4. Fig. 2a shows a zeo-lite particle which has been intentionally broken such thatthe outer and inner surfaces are observable. Fig. 2b shows theouter surface of the zeolite agglomerate particles with a highermagnification. It is observed that zero valent iron particleswith a size in the range 50–100 nm with a quasi-spherical mor-phology have been created. Fig. 2c shows the inner core of thezeolite particle. The depth (distance from the outer surface),as indicated in Fig. 2a, is around 1.5 �m. In other words, Fig. 2cconsiders a zone very near to the outer surface. No zero valentiron particles could be detected on the individual zeolite crys-tals outer surface in Fig. 2c. Further investigation of more inner

depths gave the same result. This is an important finding, asit shows that the Fe2+ cations covalently present in the inner

layers of the agglomerate particle do not take part in the reduc-tion reaction with NaBH4, despite the abundance of NaBH4 inthe liquid phase. It should be reminded that the content ofNaBH4 in the liquid phase is the stoichiometric equivalent ofthe total Fe2+ cation content of the zeolite. To elucidate themechanism behind this phenomenon, we performed an extraexperiment using a solution with a NaBH4 concentration two-fold that of the conventional method. Fig. 2d shows the FESEMpicture of the outer surface of the zeolite after reduction withNaBH4. The size, spatial distribution and morphology of thezero valent iron nano-agglomerates differ slightly from thatobserved in Fig. 2b. In addition, no zero valent iron could beseen on the outer surface of individual crystals in the innerlayers. It should be reminded that the reaction duration was30 min. The same result was obtained also using a four-foldconcentration of NaBH4. The main deduction is that NaBH4

diffusion within the macro-pores of the zeolite agglomerate isnot the reason for the absence of reaction in the inner layers ofthe particles. A rough estimate of this penetration thicknessconsidering a relatively high value for the diffusion coefficientas 10−5 cm2 s−1 for a 30 min period of time is 2000 �m. Thisestimation excludes the role of diffusion resistance on thecreation of zero valent iron only on the very outer surface ofthe particles. The authors of the present work attribute thisphenomenon to the creation of H2 gas bubbles on the outerlayers in contact with BH4

+ ions, which impede further pene-tration of the reactant into the zeolite agglomerates. Hydrogenbubbles also tend to adhere to the solid surface. The gas pro-duction is relatively voluminous as 7 mol H2 are produced per1 mol Fe0 created. Accordingly, by the chemical reduction pro-cedure used in this work, an egg-shell distribution of the zero

valent metal in the zeolite agglomerate particle is produced(see Fig. 3).

Process Safety and Environmental Protection 9 1 ( 2 0 1 3 ) 304–310 307

Fig. 3 – Scheme proposed for the creation of an egg-shell distribution of Fe0 nano particles within the zeolite agglomerateparticle. Hydrogen bubbles prevent further reaction of NaBH4 with the inner agglomerate core.

Fig. 4 – Effect of Cu concentration on the nitrate removalk

urlanvaAtoCttdrtfsmirrt

Fig. 5 – Concentration of nitrite anion as a function of timeusing FeCLP and 5CuFeCLP composites.

inetics.

Fig. 4 shows the effect of Cu2+ concentration in the liq-id phase in contact with the FeCLP sample on the nitrateemoval kinetics. Compared with the FeCLP sample (no Cuoading), the 50CuFeCLP exhibits a poor nitrate separationbility. The initial separation rate is relatively slow and theitrate removal reaches an asymptote of low yield. This obser-ation is attributed to the probable production of un-reactivegglomerate zero valent Cu particles of relatively large sizes.s the Cu2+ concentration is reduced to 20% (20CuFeCLP),

he initial nitrate reaction rate is enhanced and the extentf asymptotic removal increases up to 20%. Decreasing theu2+ concentration down to 5% (5CuFeCLP) further increases

he initial removal rate and the final removal extent. Fur-her decrease of the initial Cu2+ concentration results in theecrease of the initial rate but does not change the final nitrateemoval extent. The existence of an optimum Cu2+ concen-ration for the nitrate removal has been previously reportedor ‘un-supported’ nano sized Fe particles (Liou et al., 2005). Ithould be noted that the stoichiometric available zero valentetal (Fe0, Cu0 or both) is constant for the different Cu0 load-

ngs and, as far as the metal particles are nano-sized, they areeactive and can totally transform into cationic form through

eaction with NO3

− anions. This explains the equal ‘satura-ion’ nitrate removal extent for low and zero Cu0 loading of the

zeolite particles. An important observation regarding usingzeolites as supports is that for any extent of nitrate removal,no detectable NH4

+ ion is released in the resulting liquid solu-tion (Lee et al., 2007). This is due to the high selectivity ofthe ‘inner’ zeolite core for NH4

+ cations, which is readily ion-exchanged with covalently bond Fe2+ or other cations like Na+,K+, Ca2+ or Mg2+ of the zeolite. This is an important findingand highlights the role of the inner core as an ion-exchangerfor un-wanted cationic products of the nitrate reaction withzero valent Fe or Cu. The synergetic effect in nitrate removalactivity observed for the bimetallic Fe0/Cu0 metallic particlessupported by clinoptilolite zeolite is due to the promotion ofFe0 oxidation by the potential difference established betweenthe two different metals (Liou et al., 2005). Cu0 is a less activemetal with respect to Fe0. Their couple produces a galvaniccell in which iron acts as anode and copper acts as cathode(Elliott and Zhang, 2001), thus enhancing iron oxidation. Thelatter, as a corollary, increases the rate of nitrate reaction withmetallic iron producing the mentioned synergetic effect.

Fig. 5 shows the concentration of nitrite ion in the solution

as a function of reaction time. For both FeCLP and 5CuFeCLPsamples, it is observed that at early times the concentration

308 Process Safety and Environmental Protection 9 1 ( 2 0 1 3 ) 304–310

Fig. 6 – Variation of solution pH with time as a function ofinitial solution pH.

Fig. 7 – NO3− removal kinetics as a function of initial

solution pH.

of NO2− anion increases sharply and afterwards undergoes

a gradual decay. Such a behavior for FeCLP was not reportedin the work of Lee et al. (2007). The production of nitriteanion using FeCLP needs further explanation. Using unsup-ported zero valent iron nitrite generation is usually reportedto be near zero (Hwang et al., 2011; Zhang et al., 2010). Thisis attributed to the strong catalytic activity of zero valent irontowards converting nitrite species to ammonium and nitrogenspecies via the following reactions (Zhang et al., 2011):

3Fe0 + NO2− + 8H+ → 3Fe2+ + 2H2O + NH4

+ (1)

NO2− + 3Fe0 + 8H+ → 3Fe2+ + NH4

+ + 2H2O (2)

On the other hand, it is well known that the presenceof zero valent metals like Mg0, Cu0 near Fe0 result in theunwanted generation of nitrite anions. The reason for sucha phenomenon has been reported to be the weak adsor-bance of nitrite ions on such bimetallic compounds whichresults in their release in the solution. Clinoptilolite supportedFe0 is inherently prone to the presence of bimetallic zerovalent compounds. In other words, FeCLP cannot be reallyconsidered as sole Fe0 supported by the zeolite. Recall thatthe raw clinoptilolite used has an approximate chemical for-mula as (Ca1.41,Mg0.96, Na1.66,K1.01)(Al7.40Si42.78O72.00)23·00H2O(Falamaki et al., 2004). On the other hand, the so-called fullyexchanged clinoptilolite zeolite is never devoid of cations likeCa,2+, Mg2+, K+ and even Na+ (Falamaki et al., 2004). This factis responsible for the production of zero valent metals likeMg0 upon contacting the Fe-exchanged zeolite with NaBH4

solutions. Accordingly, the FeCLP composite may contain zerovalent metallic species like Mg0, although in very low concen-trations. Such species may enhance nitrite anion generation(Kumar and Chakraborti, 2006).

The main reactions involved in the production and con-sumption of NO2

− anion are the following (Zhang et al., 2011):

Fe0 + NO3− + 2H+ → Fe2+ + H2O + NO2

− (3)

NO2− + 3Fe0 + 8H+ → 3Fe2+ + NH4

+ + 2H2O (2)

Presumably, at early times (t < 2 h) and at low pH values,reaction (3) is dominant. After this period, due to the reductionof NO3

− concentration and increase of NO2− concentration

in the liquid phase, reaction (2) prevails. The presence of thezeolite support is very effective in the progression of bothreactions (3) and (2). As the inner zeolite agglomerate core isprone to any kind of ion-exchange, and due to the great affin-ity of clinoptilolite versus NH4

+ compared to Fe2+, releasedammonium cations undergo immediate ion-exchange withthe inner core. Consequently, reaction (2) proceeds further dueto the physical elimination of one of its products (NH4

+). Inthe absence of the zeolite support, Liou et al. (2005) reporteda steady increase in the concentration of NO2

− concentrationwith reaction time. This is in clear contrast with our results,were the concentration of the nitrite anion initially rises andundergoes a constant further decay. This last behavior isdesired, as also the nitrite anion concentration should notexceed a threshold value. Summing up, we suppose that thepresence of the zeolite support has two main effects: (1) Nearcomplete elimination of ammonium cations from the solutionand (2) damping the production of nitrite anions. However,

referring to Fig. 5, it is observed that the zeolite supported5CuFeCLP sample releases more nitrite anion throughout the

reaction period in comparison to FeCLP. Such a behavior hasbeen reported for unsupported Cu0/Fe0 nano particles mixtureby Liou et al. (2005). They attributed the latter phenomenon tothe higher adsorption affinity of Cu0 particles with respect toFe0 nano-particles for the NO2

− anions.In continuation, we consider only the CuFeCLP sample.

Fig. 6 shows the variation of solution pH with time as a func-tion of initial solution pH. A sharp initial increase up to ca. 1 hand up to a pH around 9.2 is observed for three different initialpH values of 2, 3 and 6, respectively and independent of initialpH. After 1 h, the pH undergoes a gradual decrease, reachingan asymptotic value ca. 8.5 after 30 h, again independent ofthe initial pH. Fig. 7 shows the corresponding NO3

− removalkinetics for different initial pH’s. No significant dependence ofremoval rate on initial solution pH is observed. There exists astrong dependence between the nitrate removal kinetics andthe initial pH of the solution when using sole zero valent iron(Choe et al., 2004). Based on reaction (1), nitrate removal isaccompanied with the increase of pH. This is clearly observedin Fig. 6 during the first hour. The slow pH decrease afterwardsis due to the consumption of hydroxyl ions due to the precip-itation of metal hydroxides. The pH variation with time hasa slight dependence on the initial pH. Referring to Fig. 6, itmay be observed that higher initial pH’s results in slightlyhigher pH’s for equal reaction times. Referring to Fig. 7, a

slight decrease in nitrate removal kinetics may be observed

Process Safety and Environmental Protection 9 1 ( 2 0 1 3 ) 304–310 309

Fig. 8 – Effect of temperature in the range of 20–60 ◦C onn

waoa2mclaitlrastpdFis7sbtAoac

nv2vmwo5s

rstloe

Fig. 9 – Arrhenius plot of the nitrate removal kinetics using5CuFeCLP.

Fig. 10 – Nitrate removal kinetics using an initial loading of2+ −1

itrate removal kinetics using 5CuFeCLP.

ith increasing the initial pH. Such a weak dependence of pHnd nitrate removal kinetics on the initial pH has been previ-usly reported for zero valent iron coated clinoptilolite (no Cuddition), although without any due explanation (Lee et al.,007). According to Choe et al. (2004), the nitrate removal rateay undergo sever reduction if any iron hydroxide formed pre-

ipitates on the zero valent iron particles, creating a passiveayer on them. Such a phenomenon is promoted when theverage pH during the reaction is higher (i.e. using a highernitial pH). The question is why the effect of initial pH onhe removal rate is slightly observable when using clinoptilo-ite as a support. We propose the following explanation. Theemoval rate is strongly dependent on the exposed and avail-ble area of Fe0 or Fe0/Cu0 particles. Referring to the schemehown in Fig. 3, using clinoptilolite as support, a shell aroundhe zeolite agglomerate is formed where metallic iron or cop-er is deposited on the zeolite crystals. The average poreiameter of the shell is around 20 nm (Pashmineh Azar andalamaki, 2012). We presume that any metal hydroxide precip-tate formed in the liquid phase cover mainly the outer mosturface of the shell. Recall that rust particles may as large as00 nm (Choe et al., 2004). However, due to the mesoporoustructure of the shell, the inner metallic particles will stille at disposition and nitrate anion may reach them throughhe connected mesopores (average diameter 20 nm) available.ccordingly, any ‘rust’ formation will take place mainly on theuter surface while the inner active sites will still be avail-ble. Thus, the reduction of nitrate removal will be less whenompared to unsupported zero valent iron.

Fig. 8 shows the effect of reaction temperature on theitrate removal kinetics. The process is thermally acti-ated and the activation energy has been calculated to be1.7 kJ mol−1 (see the Arrhenius plot, Fig. 9). The relatively lowalue obtained highlights the presence of the liquid phaseass transfer and intrinsic reaction rate resistances. It is note-orthy to mention that Liou et al. (2005) reported a valuef 20 kJ mol−1 for the activation energy using un-supportedCuFe. The latter value is close to our calculated value for theupported 5CuFeCLP sample.

At this stage, we aim at presenting an important aspectegarding the synthesis of Cu0/Fe0 nano particles on zeoliticupports. Using an initial Fe2+ concentration of 300 ppm (1/10he corresponding value for the standard procedure), an initialoading of ca. 0.9 mmol Fe2+ (g zeolite)−1 was obtained (instead

f 0.44 mmol Fe2+ (g zeolite)−1 for the standard procedure). Asxplained in Section 2, the zeolite was transformed into Fe

0.9 mmol Fe (g zeolite) in the synthesis of 5CuFeCLP.

form using an excess amount of NaBH4 in order to completelytransform covalently bond Fe2+ into zero valent iron nanoparticles. Using the optimum procedure for inclusion of Cu0

particles, the zeolite was transformed into the corresponding5CuFeCLP form. This time, however, with half of the standardinitial Fe2+ loading. The effect of the initial solution pH on thenitrate removal kinetics is shown in Fig. 10. Interestingly, theextent of nitrate removal at long times is ca. half of the cor-responding value for the 5CuFeCLP sample produced with thedouble loading of Fe0. As discussed before, only the Fe0/Cu0

particles residing in the very outer shell of the zeolite agglom-erate is active in the denitrification process. Accordingly, itmay be stated that there exists a linear relationship betweenthe Fe0 content of the zeolite agglomerate in its outer shelland the amount of nitrate removal.

4. Conclusions

Nitrates constitute one of the main toxic contaminants ofgroundwater. On the other hand, groundwater may be con-sidered anoxic (oxygen concentration less than 9 �g L−1). Thisfact justifies the use of nano zero valent metals for nitrateremoval. In such conditions, zero valent metals are quite sta-ble against oxidation due to the very low level of dissolvedoxygen concentration.

It has been shown that the performance of zero valent ironcoated clinoptilolite zeolite for the reduction of nitrate anion

310 Process Safety and Environmental Protection 9 1 ( 2 0 1 3 ) 304–310

in un-buffered conditions may be enhanced by coating smallamounts of Cu0 onto the freshly prepared Fe0/zeolite com-posite. An optimum loading of Cu0 exists for which the rateof nitrate removal is maximal. For this optimal composition,the nitrite anion production curve with time passes through amaximum. Nitrite production, however, is slightly higher forthe Cu modified zeolite.

It has been shown that the nitrate removal process is onlyslightly dependent on the initial solution pH. In the temper-ature range of 20–60 ◦C, the process is controlled by both theliquid phase mass transfer and intrinsic reaction rate resis-tances.

FESEM analysis of the zero valent metal/zeolite compositeshowed that upon the metal reduction reaction, an egg-shelldistribution of zero valent metal in the zeolite agglomerateparticle is produced.

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