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Journal of Hazardous Materials 182 (2010) 923–927

Contents lists available at ScienceDirect

Journal of Hazardous Materials

journa l homepage: www.e lsev ier .com/ locate / jhazmat

hort communication

n nanoscale metallic iron for groundwater remediation�

. Noubactepa,c,∗, S. Caréb

Angewandte Geologie, Universität Göttingen, Goldschmidtstraße 3, D-37077 Göttingen, GermanyUniversité Paris-Est, Laboratoire Navier, Ecole des Ponts – ParisTech, LCPC, CNRS, 2 allée Kepler, F-77420 Champs sur Marne, FranceKultur und Nachhaltige Entwicklung CDD e.V., Postfach 1502, D-37005 Göttingen, Germany

r t i c l e i n f o

rticle history:eceived 5 February 2010eceived in revised form 1 June 2010ccepted 2 June 2010vailable online 10 June 2010

a b s t r a c t

This communication challenges the concept that nanoscale metallic iron (nano-Fe0) is a strong reducingagents for contaminant reductive transformation. It is shown that the inherent relationship betweencontaminant removal and Fe0 oxidative dissolution which is conventionally attributed to contaminantreduction by nano-Fe0 (direct reduction) could equally be attributed to contaminant removal by adsorp-

II

eywords:dsorptiono-precipitationanoscale ironeduction

tion and co-precipitation. For reducible contaminants, indirect reduction by adsorbed Fe or adsorbed Hproduced by corroding iron (indirect reduction) is even a more probable reaction path. As a result, thecontaminant removal efficiency is strongly dependent on the extent of iron corrosion which is larger fornano-Fe0 than for micro-Fe0 in the short term. However, because of the increased reactivity, nano-Fe0

will deplete in the short term. No more source of reducing agents (FeII, H and H2) will be available in thesystem. Therefore, the efficiency of nano-Fe0 as a reducing agent for environmental remediation is yet to

erovalent iron be demonstrated.

. Introduction

Increased soil and groundwater contamination has promptedesearchers to investigate affordable and efficient strategies fornvironmental protection. One recent innovative possibility haseen the use of microscale granular metallic iron (micro-Fe0 ore0), initially for reductive dechlorination of halogenated carbons1–4]. The success of Fe0 for reductive dechlorination has encour-ged researchers to test its applicability to several other classesf substances [4–10]. In the meantime Fe0 is considered as a uni-ersal material for unspecific contaminant removal from aqueousystems [6,8,11,12]. It is essential to note that neither isolated Fe0

or its individual corrosion products are responsible for quanti-ative contaminant removal. The whole dynamic process of Fe0

xidative dissolution coupled to iron hydroxide precipitation andrystallization at pH > 4.5 is responsible of contaminant removalnd sequestration [12,13]. In this dynamic process, contaminant

eduction via surface-mediated reaction (by Fe0, adsorbed FeII ordsorbed H) is likely to occur but the discussion of its extent is aomplex issue. It is the intension of this communication to drawhe attention of the scientific community on this key issue for

� Capsule: The scientific basis for the observed efficiency of nanoscale metallicron for environmental remediation is yet to be systematically addressed.∗ Corresponding author at: Angewandte Geologie, Universität Göttingen, Gold-

chmidtstraße 3, D-37077 Göttingen, Germany. Tel.: +49 551 39 3191;ax: +49 551 39 9379.

E-mail address: cnoubac@gwdg.de (C. Noubactep).

304-3894/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.jhazmat.2010.06.009

© 2010 Elsevier B.V. All rights reserved.

nanoscale Fe0 injected into the subsurface for groundwater reme-diation.

2. Nanoscale iron for groundwater remediation

Reducing the particle size of granular Fe0 materials (mm) to10–100 nm (nanoscale Fe0 or nano-Fe0) increases surface area andthus chemical reactivity. Potentially, nano-Fe0 can be readily placedin the subsurface in slurry form via injection to contaminated zoneslocated well below the ground surface [14,15]. The creation of reac-tive zones by nano-Fe0 injection, e.g. via monitoring wells extentsthe applicability of Fe0 technology to depths for which engineeredFe0 walls may be prohibitively expensive [15]. Currently nano-Fe0

is mostly synthesized via the borohydride-catalyzed reduction ofdissolved FeII or FeIII [16–18]. The reactivity of nano-Fe0 is some-times further enhanced by plating Fe0 by more electropositivemetals (e.g. Pd and Ni) to form so-called nanoscale bimetallic par-ticles [19].

An inherent problem of nanoscale particles in aqueous solu-tions is the formation of aggregates (particle aggregation). Theaggregation of single nanoscale particles to micrometer size aggre-gates is coupled with reduced availability and transport limitations[20–23]. Accordingly, nano-Fe0 must be readily dispersible in water

such that they can migrate through water saturated porous mediato the domain where the reactive zone is to be built. Clearly, forin situ environmental remediation, colloidal stability of aqueousnano-Fe0 dispersions is a critical property [24,25]. To enhancenano-Fe0 colloidal stability several tools have been developed in

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24 C. Noubactep, S. Caré / Journal of H

ecent years [23–25]. For example, Phenrat et al. [24] could stabi-ize nano-Fe0 dispersions against aggregation and sedimentationy adsorbing anionic polyelectrolytes on their surface. Using a sup-ort material for nano-Fe0 is another way to solve the aggregationroblem. Due to their inexpensiveness, availability, environmen-al stability, abundant natural resources (e.g. clay minerals andeolites) are suitable candidates to work as supporting materials23,26]. As an example, Frost et al. [23] reported increased methy-ene blue discoloration by a natural palygorskite impregnated withe0.

. Rationale for enhanced reactivity of nano-Fe0

Increased nano-Fe0 efficiency for contaminant removal relativeo granular Fe0 can be mathematically demonstrated on the basis ofhe ratio surface to diameter of the particles. The relation betweenhe specific surface area (SSA in m2/g) of a spherical particle and itsiameter is given by Eq. (1) [15]

SA = 6� × d

(1)

here d is the diameter of the spherical Fe0 particle and= 7800 kg/m3 the specific weight of iron. Eq. (1) shows that the

maller the particle size (d), the larger the specific surface areaSSA). For example, a granular Fe0 having a mean diameter of 50 �mheoretically has a SSA of about 15 m2/kg while a nano-Fe0 with a

ean particle diameter of 50 nm has a SSA of about 15,000 m2/kg.hat is a reactivity ratio of 1000 (103). This large reactivity ratioxplains why nano-Fe0 is much more reactive than granular Fe0.ctually, what is the meaning of “more reactive”? The next sec-

ion will attempt to answer this question while considering thathe same material is available in micro and nano-size.

. Significance of increased reactivity of nano-Fe0

.1. Modeling iron dissolution

Assuming the same material, available in two fractions: micro-e0 (material 1, d1 = 50 �m) and nano-Fe0 (material 2, d2 = 50 nm),he material reactivity is solely a function of the particle size. Ifarallel experiments are performed with 0.25 g (2.5 × 10−4 kg or50 mg) of each material, the number of Fe0 particles at the begin-ing of the experiment can be calculated using Eq. (2):

= M

�Fe · 4/3� · R30

(2)

here M is the mass of Fe0 (2.5 × 10−4 kg), �Fe is the specific weightf Fe (7800 kg/m3) and R0 is the initial radius of the Fe particleR1 = 25 �m or R2 = 25 nm).

Calculations (Table 1) show that N1 = 4.90 × 105and2 = 4.90 × 1014 particles are equivalent to 2.5 × 10−4 kg of Fe0. The

orresponding ratio N2/N1 (= 109 = [(103)3]) gives a more realisticxplanation of the reactivity difference than the ratio of SSA (103).he number of Fe0 atoms per particle (N′) and the number of Fe0

toms at the surface of each particle (N′′) are evaluated in bothases. Calculations are made by considering the following formula

able 1verview on the differential reactivity of microscale (d1 = 50 �m) and nanoscale (d2 = 50

urface of individual particles. N is the total number of particles in 250 mg of the materialhe surface of the individual spheres. N1 = N × N′′ is the total number of Fe0 which is exposniform corrosion (first stage of monolayer dissolution).

Material N (–) N′ (–)

Micro-Fe0 4.90 × 105 5.56 × 1015

Nano-Fe0 4.90 × 1014 5.56 × 106

ous Materials 182 (2010) 923–927

(Eqs. (3) and (4)) for the spheres:

V = 43

�R3 (3)

S = 4�R2 (4)

Fe (�-Fe) is body-centered cubic (bcc) in structure. The bcc sys-tem has one atom in the center of the unit cell in addition to theeight corner atoms. A unit cell thus has a net total of two atoms perunit cell (2 Fe per cube). The lattice parameter of �-Fe is a = 2.866 Å(1 Å = 10−10 m). The corresponding volume is given by V = a3 andthe cross section by S = a2. A monolayer is supposed to be madeup of individual unit cells of Fe0 (cubes). Accordingly, each layerhas a thickness of a (lattice parameter) and its cross section S isthe area occupied by 2 Fe0. Calculations attested that there are 109

times more Fe0 in and at the surface of micro-Fe0 than in nano-Fe0

(Table 1).

4.2. Discussion of modeling results

Assuming uniform corrosion for spherical particles, the corro-sion process of concentric layers of Fe0 yields concentric layers ofiron oxides (iron corrosion products) (Fig. 1). The number of Fe atomper particle can be calculated using Eq. (5).

N′ = 2[(4/3)�R3]a3

(5)

Similarly, the number of Fe atom at the surface of each particlecan be calculated using Eq. (6).

N′′ = 2 · [4�R2]a2

(6)

The factor “2” in Eqs. (5) and (6) accounts for the fact that each unitcell contains 2 Fe atoms.

Calculations shown that 9.36 × 1016 atoms (1.55 × 10−7 mol or0.155 �mol) of Fe0 is dissolved in the first stage of micro-Fe0 cor-rosion versus 9.36 × 1019 atoms (1.55 × 10−4 mol or 155 �mol) fornano-Fe0. This corresponds to a molar ratio of 103 and is the mostelegant way to explain increased reactivity of nano-Fe0 relativeto micro-Fe0. The reactivity ratio based on the specific surfacearea was also 103 and that based on the number of particles 109.Although all three approaches exhibit the same trend, Fe0 oxidativedissolution is a chemical process, which is discussed the best on thebasis of molar ratios (from the surface). Additionally, the amount ofavailable contaminant has to be considered as well. In other wordsthe molar ratio of dissolved Fe to available contaminants should beused to discuss difference in reactivity.

For example, complete dissolution of 250 mg Fe0 yields 4.47 mMFeII. Nano-Fe0 (88 layers per particle) is depleted when only 0.3%of micro-Fe0 is dissolved (see also Fig. 2). Assuming that the ini-tial contaminant concentration is 84 �M (e.g. 20 mg/L uranium),the molar ratio FeII/contaminant is larger than 53 in the nano-Fe0 system at Fe0 depletion. In the system with the micro-iron

(0.3% consumption at 88 layers), the molar ratio FeII/contaminantis only 0.16. Actually, the dissolution of the first layer of nano-Fe0

yields 152 �M FeII which corresponds to 1.8 times the stoichiomet-ric amount of Fe0 necessary for the reduction of the contaminant(84 �M). On the other hand, to reduce the 84 �M of contaminant

nm) metallic iron as reflected by the difference of the number of Fe atom at the. N′ is the number of Fe atoms per particle (sphere). N′′ is the number of Fe atoms ated to the aqueous solution and could be oxidized to FeII in the first stage assuming

N′′ (–) N1 (atoms) N1 (mol)

1.91 × 1011 9.36 × 1016 1.55 × 10−7

1.91 × 105 9.36 × 1019 1.55 × 10−4

C. Noubactep, S. Caré / Journal of Hazardous Materials 182 (2010) 923–927 925

Fig. 1. Time dependence consumption of Fe0 from a spherical material assuming uniformconcentric layers of iron hydroxides, which are further transformed to iron oxides.

Fig. 2. Residual mass of metallic iron (Fe0) as function of consumed layers fromi 0 0

FcF

5tpodc

these corrosion products FeII (dissolved or adsorbed) and hydro-

TCsf

ndividual Fe particles. It is evident that nano-Fe is depleted as only 0.30% of micro-e0 is consumed. Uniform corrosion is assumed and material is supposed to be idealoncentric layers of Fe0. Accordingly a nanoscale particle is made up of 88 layers ofe0.

52 layers of the micro-Fe0 should be dissolved. All this calcula-ions have intentionally neglected (i) Fe0 oxidation by water, (ii)

recipitation of FeII/FeIII species, (iii) contaminant reduction by FeII

r H/H2, and (iv) the differential dissolution kinetics of particle ofifferent sizes. Nevertheless, it is clear that the current approach ofomparing Fe0 material of different size should be improved.

able 2omparison of the experimental conditions of Katsenovich and Miralles-Wilhelm [27] andurface area. The large variability of used experimental conditions evidences the difficultyrom 26 to 11161.

Contaminant (X) [X] (mg/L) Material Fe0 size (nm)

Trichloroethene 9.0 Fe0 1209.0 Fe0 1209.0 Fe0 120

Trichloroethylene 10.0 Fe0 1–200Chloroform 10.0 Fe0 1–200Nitrobenzene 10.0 Fe0 1–200Nitrotoluene 10.0 Fe0 1–200Dinitrobenzene 10.0 Fe0 1–200Dinitrotoluene 10.0 Pd0/Fe0 1–200

Tetrachloroethene 20.0 Pd0/Fe0 10–100Trichloroethene 20.0 Pd0/Fe0 10–1001,1-Dichloroethene 20.0 Pd0/Fe0 10–100Cis-1,2-dichloroethene 20.0 Pd0/Fe0 10–100Trans-1,2-dichloroethene 20.0 Pd0/Fe0 10–100Vinyl chloride 20.0 Pd0/Fe0 10–100

Carbon tetrachloride 0.6 Fe0 70Carbon tetrachloride 0.6 Fe0 10–100Carbon tetrachloride 132.6 Fe0 70Carbon tetrachloride 132.6 Fe0 10–100

corrosion. Initial concentric layers of Fe0 atoms are progressively transformed to

The relevance of such pre-experimental theoretical calculationsfor future works is depicted in 2. Table 2 compares some importantissues of the experimental conditions of Katsenovich and Miralles-Wilhelm [27] and three of the references therein. The four worksobviously employed varying experimental procedures to charac-terize the efficiency of nano-Fe0 for 12 different compounds. Theseprocedures differ in initial contaminant concentration, Fe0 dosage,Fe0 preparation, particle size (10–200 nm), volume of experimentalbottles, volume of added solution (50–150 mL), experimental time,mixing type and mixing intensities. The lack of systematic studydesigned to elucidate the effects of operational conditions on theefficiency of nano-Fe0 has certainly complicated the elucidation ofthe intrinsic reactivity of these materials.

5. Mechanism of contaminant removal by nano-Fe0

The hart fact that nano-Fe0 is more reactive and extremely effi-cient for contaminant removal compared to granular Fe0 (mm and�m) is certainly due to their increased surface area as confirmedabove by calculations. However, this evidence tells little about thereal mechanism of contaminant removal. In fact, increased Fe0 reac-tivity also means increased iron corrosion product generation. From

gen (atomic or molecular) are also reducing agents and shouldbe regarded as co-reductants rather that iron corrosion prod-ucts (ICPs). These co-reductants do compete with non-corrodedFe0 for contaminant reductive transformation. On the other hand,

three therein referenced articles on remediation with nano-Fe0. SSA is the specificof comparing achieved results. In particular, the molar ratio Fe0/contaminant varies

SSA (m2/g) [Fe0] (mg) V (L) [Fe0]/[X] (–) Ref.

6 50 150 86 [27]6 125 150 2156 200 150 344

31.4 625 125 1170 [28]31.4 625 125 106531.4 625 125 110031.4 625 125 122531.4 625 125 151331.4 625 125 1653

35 250 50 740 [29]35 250 50 58735 250 50 43335 250 50 43335 250 50 43335 250 50 279

29 150 120 5580 [30]33.5 300 120 1116129 150 120 2633.5 300 120 52

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26 C. Noubactep, S. Caré / Journal of H

recipitated and precipitating ICPs are contaminant scavengers.ontaminants enmeshed in the structure of precipitating ICPs coulde further reduced by co-reductants as demonstrated for micro-cale Fe0 [6,8]. In other words, it is not likely, that increasedeactivity changes the fundamental mechanism of contaminantemoval by Fe0. This statement is supported by a recent reportrom Nawrocki et al. [31] showing that “steady water” has reductiveroperties in a Fe0-based water pipe. The reductive characteristicf corrosion scales on iron is well-documented [6,32]. The so-alled “steady water” is an electrolyte and can dissolve reducinggents and facilitate the transport of reactive species. As discussedbove for 250 mg of Fe0, Fe0 depletion occurs in the nano-Fe0 sys-em as only 0.3% is consumed in the micro-Fe0 system (Fig. 2). Inroundwater, anoxic conditions prevail and iron oxidation is cou-led with water electrolysis resulting in the production of hydrogenmolecular and atomic). Adsorbed FeII, dissolved FeII, atomic (H)nd molecular (H2) hydrogen all served as an electron donoror contaminant reduction. Therefore, the reported high-effectiveeduction by nano-Fe0 results from reducing conditions createdy providing the system in the short term with elevated amountf electron donors. Accordingly, contaminant reduction inducedy nano-Fe0 is efficient for the short term. The question is how

ong these reductive conditions will prevail or whether they couldanage to reductively transformed the whole contamination.

. Efficiency of nano-Fe0 reactive zones

The first problem of reactive zones is inherent to the higheactivity of nano-Fe0 [33]. Unlike inert adsorbents (e.g. activatedarbons) having an adsorbing area (adsorption capacity) which isnly “reserved” to contaminants, nano-Fe0 are readily oxidized byater and the primary products (FeII, H/H2) are transported byater or are adsorbed on Fe0, solid corrosion products or geo-aterials. Unless nano-Fe0, is added to sustain a process in the

ubsurface its efficiency in the short and middle term is question-ble. In fact, whether nano-Fe0 is transported to the reactive zone orot, it will be oxidized by water. Nano-Fe0 oxidation can be accel-rated by contaminants, which are then reduced more likely byo-reductants [6,8]. But once Fe0 is depleted, no subsequent sup-ly of co-reductants is possible. From this time on, quantitativeontaminant removal can only be attributed to adsorption on ironorrosion products and available biomaterials. The long-term sta-ility of adsorbed contaminants is uncertain. On the other hand,he long-term stability of contaminant removal in micro-Fe0 sys-ems is mainly due to their continuous enmeshment in the matrixf in situ generated corrosion products in the inter-particular spacepores) within the Fe0 wall [12]. Beside the side exclusion favour-ng this dynamic process, the long-term availability of Fe0 was thether warrant for this mechanism. The efficiency of size exclusionn reactive zone is uncertain and Fe0 depletion is rapid. Therefore,he stability of removed contaminants in reactive zone created byano-Fe0 injection is uncertain.

. Concluding remarks

This communication has complained that the reaction kineticsn systems with nano-Fe0 is not properly considered in the currentiscussion of the efficiency of reactive zones. More reactive mate-ials (smaller particle size) may rapidly reduced contaminants butlso rapidly produced co-reductants. Accordingly, it is still not cer-

ain whether the Fe0 surface (direct reduction) plays any importantole in the process of contaminant removal in Fe0/H2O systems.ortunately, for micro-Fe0 removed contaminants are enmeshedn the matrix of iron corrosion products and are stable under nat-ral conditions [34]. For nano-Fe0 however, upon Fe0 depletion

[

ous Materials 182 (2010) 923–927

contaminants will be simply adsorbed on aged iron corrosion prod-ucts. In other words, although nano-Fe0 is currently regarded asan established remediation technology, its efficiency is still to bedemonstrated.

Acknowledgments

Sven Hellbach (student research assistant) is acknowledged fortechnical assistance. The manuscript was improved by the insight-ful comments of anonymous reviewers from Journal of HazardousMaterials.

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