Critical Reviews in Solid State and Materials Sciences, 31:111122, 2006Copyright c Taylor and Francis Group, LLCISSN: 1040-8436 printDOI: 10.1080/10408430601057611

Zero-Valent Iron Nanoparticles for Abatementof Environmental Pollutants: Materials andEngineering Aspects

Xiao-qin Li, Daniel W. Elliott, and Wei-xian ZhangCenter for Advanced Materials and Nanotechnology, Department of Civil and EnvironmentalEngineering, Lehigh University, Bethlehem, Pennsylvania, USA

Zero-valent iron nanoparticle technology is becoming an increasingly popular choice for treat-ment of hazardous and toxic wastes, and for remediation of contaminated sites. In the U.S.alone, more than 20 projects have been completed since 2001. More are planned or ongoingin North America, Europe, and Asia. The diminutive size of the iron nanoparticles helps tofoster effective subsurface dispersion whereas their large specific surface area corresponds toenhanced reactivity for rapid contaminant transformation. Recent innovations in nanoparticlesynthesis and production have resulted in substantial cost reductions and increased availabilityof nanoscale zero-valent iron (nZVI) for large scale applications. In this work, methods of nZVIsynthesis and characterization are highlighted. Applications of nZVI for treatment of both or-ganic and inorganic contaminants are reviewed. Key issues related to field applications such asfate/transport and potential environmental impact are also explored.

Keywords zero-valent iron, nanoparticle, remediation, environmental nanotechnoloty, synthesis,characterization, toxic waste

Table of Contents

1. INTRODUCTION ................................................................................................................................................ 111

2. IRON NANOPARTICLES FOR ENVIRONMENTAL REMEDIATION ............................................................... 1122.1 Synthesis of Iron Nanoparticles ....................................................................................................................... 1132.2 Characterization of Iron Nanoparticles ............................................................................................................. 1142.3 Degradation of Organic Contaminants .............................................................................................................. 1162.4 Remediation of Inorganic Contaminants ........................................................................................................... 1182.5 Transport of the Iron Nanoparticles .................................................................................................................. 119

3. CHALLENGES AHEAD ...................................................................................................................................... 121

ACKNOWLEDGMENTS ........................................................................................................................................... 121

REFERENCES .......................................................................................................................................................... 121

1. INTRODUCTIONDue to their subcolloidal size and unique molecular and/or

atomic structures, many nanomaterials have been shown to pos-sess distinctive mechanical, magnetic, optical, electronic, cat-alytic, and chemical properties that contribute to promising ap-

E-mail: wez3@lehigh.edu

plications in machinery, energy, optics, electronics, drug deliv-ery, and medical diagnostics.16 The large surface-to-volumeratio of nanomaterials can lead to surprising surface and quan-tum size effects. As particle size decreases, the proportion ofsurface and near surface atoms increases. Surface atoms tendto have more unsatisfied or dangling bonds with concomitantlyhigher surface energy. Thus, the surface atoms have a stronger

111

112 X.-Q. LI ET AL.

tendency to interact, adsorb, and react with other atoms ormolecules in order to achieve surface stabilization.7 For ex-ample, carbon nanotubes have been widely reported to ex-hibit an extremely high storage capacity for hydrogen and mayserve as an ideal material for fuel cells, energy storage, andtransmission.7

New discoveries on nanomaterials are being reported at anever-increasing rate. Generally speaking, gold is considered tobe a rather stable and inert substance. However, nano-gold par-ticles (

ZERO-VALENT IRON NANOPARTICLES ABATING ENVIRONMENTAL POLLUTANTS 113

installed. Relocation or major modifications to the PRB infras-tructure is often impractical.

The nanoscale iron (nZVI) technology discussed herein canbe regarded as an extension of the ZVI technology.2022 In somecases, it may serve as an alternative to the conventional ZVIPRBs. For other sites, the nZVI process can complement (orsupplement) the fixed PRBs. For example, nZVI injections canbe used to address the heavily contaminated source area or otherhot spots whereas ZVI PRB functions as barrier to contain thedispersion of contaminants. Because of their small size, nanopar-ticle slurries in water can be injected under pressure and/or evenby gravity flow to the contaminated area and under certain con-ditions remain in suspension and flow with water for extendedperiods of time. An in-situ treatment zone may thus be formed.Alternatively, iron nanoparticles also can be used in some ex-situapplications.

Over the past decade, extensive studies have demonstratedthat ZVI nanoparticles are effective for the treatment of manypollutants commonly identified in groundwater, including per-chloroethene (PCE) and trichloroethene (TCE), carbon tetra-chloride (CT), nitrate, energetic munitions such as TNT andRDX, legacy organohalogen pesticides such as lindane andDDT, as well as heavy metals like chromium and lead.10,2135Dozens of pilot and large-scale in-situ applications have alsobeen conducted and demonstrated that rapid in-situ remedia-tion with nZVI can be achieved.21,3637 Examples of some re-cent field applications using ZVI nanoparticles are presented inTable 1.

2.1 Synthesis of Iron NanoparticlesRobust methods for large-scale and cost-effective production

of nanomaterials are essential to the growth of nanotechnology.Environmental applications often require usage of considerablequantities of treatment reagents and/or amendments for the re-mediation of huge volumes of contaminated water and soil. Un-like many industrial applications, environmental technologiesoften exhibit relatively low market values. Therefore, the appli-cation of these technologies can be particularly sensitive to thecosts of nanomaterials. This may have been the main factor forthe relatively slow adaptation of some environmental nanotech-nologies.

There are two general strategies in terms of nanoparticle syn-thesis: top-down and bottom-up approaches. The former startswith large size (i.e., granular or microscale) materials with thegeneration of nanoparticles by mechanical and/or chemical stepsincluding milling, etching, and/or machining. The latter ap-proach entails the growth of nanostructures atom-by-atom ormolecule-by-molecule via chemical synthesis, self-assembling,positional assembling, and so on.

Both approaches have been successfully applied in the prepa-ration of nZVI nanoparticles. For example, ZVI nanoparticleshave been fabricated by vacuum sputtering,38 synthesized fromthe reduction of goethite and hematite particles with hydrogengas at elevated temperatures (e.g., 200600C),39 by decompo-

TABLE 1ZVI nanoparticles for site remediationA list of recent

projectsSite Location

PhoenixGoodyear Airport(Unidynamics)

Phoenix, AZ

Defense Contractor Site CAJacksonville dry cleaner sites, pilot

tests using nZVI (several sites)FL

State Lead Site IDGroveland Wells Superfund Site Groveland, MAAberdeen site MDSierra Army Depot NVPharmaceutical plant. Pilot test Research Triangle

Park, NCIndustrial Site Edison, NJPicattiny Arsenal Dover, NJShieldalloy plant NJManufacturing Site Passaic, NJKlockner Road Site Hamilton Township, NJManufacturing Plant Trenton, NJNaval Air Engineering Station Lakehurst, NJConfidential site. Pilot test Winslow Township, NJConfidential site. Pilot test Rochester, NYNease superfund site, Pilot test OHFormer Electronics Manufacturing

PlantPA

Rock Hill, pharmaceutical plant, fullscale using nZVI

SC

Memphis Defense Depot TNGrand Plaza Drycleaning Site Dallas, TXIndustrial plant, pilot tests using

nZVIOntario, Canada

Public domain, pilot test using nZVI Quebec, Canada

Solvent manufacturing plant, pilottest using nZVI

Czech Republic

Industrial plant, pilot test using nZVI Czech RepublicIndustrial plant, pilot test using nZVI GermanyIndustrial plant, pilot test using nZVI ItalyBrownfields, pilot test using nZVI Slovakia

This table is partially based information provided by Marti Otto (U.S.EPA).

sition of iron pentacarbonyl (Fe(CO)5) in organic solvents or inargon,4042 and by electrodeposition of ferrous salts. The gen-eration of nZVI by the bottom-up reduction of ferric (Fe(III))or ferrous (Fe(II)) salts with sodium borohydride has been usedby many research groups:10

4Fe3+ + 3BH4 + 9H2O 4Fe0 +3H2BO3 + 12H++6H2 [1]

114 X.-Q. LI ET AL.

A major advantage of this method is its relative simplicity withthe need of only two common reagents and no need for any spe-cial equipment/instrument, it can been done in almost any wetchemistry lab. In our laboratory at Lehigh University, we haveused this approach to produce high-quality iron nanoparticlesfor over ten years. Typically it is achieved by slowly adding 1:1volume ratio of 0.25 M sodium borohydride into 0.045 M ferricchloride solution. Nevertheless, there are important health andsafety considerations associated with the borohydride reductionapproach. The synthesis needs to be conducted in a fume hoodas the chemical reactions produce hydrogen gas as a byproduct.Moreover, explosion-resistant mixers should be used to mini-mize the possibility of sparks, and so on. The jet-black nanopar-ticle aggregates can be collected by vacuum filtration. Selectedimages of nZVI are given in Figure 2.

Bimetallic iron nanoparticles, in which a second and oftenless reactive metal such as Pd, Ni, Pt, or Ag can be preparedsimply by soaking the freshly prepared nZVI in a solution of thesecond metal salt.23 It is believed that the second noble metalpromotes iron oxidation and may act as a catalyst for electrontransfer and hydrogenation. Several studies have demonstratedthat bimetallic iron nanoparticles (Pd-Fe, Pt-Fe, Ni-Fe, Ag-Fe)can achieve significantly higher degradation rates and preventor reduce the formation of toxic byproducts.20,43

The iron nanoparticles are colloidal in nature and exhibita strong tendency to aggregate as well as adhere to the sur-faces of natural materials such soil and sediment. Increasingefforts have been directed on the dispersion of iron nanopar-

FIG. 2. TEM images of zero-valent iron nanoparticles synthe-sized by the reduction of FeCl3 with NaBH4.

ticles. For example, He and Zhao used water-soluble starchas a potential nanoparticle stabilizer.27 The average size ofiron nanoparticles was reduced to 14.1 nm in diameter. Inanother work, Mallouk and co-workers have synthesized car-bon platelets with 50200 nm in diameter and water solublepolyelectrolyte like poly(acrylic acid) (PAA) as supports anddispersants.34 Saleh et al. used poly(methacrylic acid)-block-ploy(methyl methacrylate)-block-poly(styrensulfonate) to mod-ify the iron nanoparticle surface. Although the hydrodynamicdiameter of the modified particles increased 3050 nm, the col-loidal stability of the modified nZVI was enhanced.44

Over the last decade, the costs for ZVI nanoparticles havebeen reduced dramatically (e.g., from>$500/kg to $50100/kg).At the time of the initial field demonstration of the nZVI technol-ogy in 2001, there were no commercial suppliers of iron nanopar-ticles while presently multiple vendor options exist. Still, pricesremain too high for many applications. Compared to laboratoryprepared ZVI particles, the quality and efficacy of commerciallyavailable nZVI products can be highly variable as inadequatequality assurance and quality control and characterization pro-tocols exist to support the products.

2.2 Characterization of Iron NanoparticlesOnly limited work has been published thus far on the surface

characterization of ZVI nanoparticles. A detailed knowledge ofthe surface properties is vital for understanding the salient re-action mechanisms, kinetics, and intermediate/product profiles.The transport, distribution, and fate of nanoparticles in the envi-ronment also depend on these surface properties. However, it isoften not practical to define an average or typical iron nanopar-ticle because ZVI nanoparticles produced with different meth-ods may exhibit widely varying properties. Fundamentally, ironnanoparticles are reactive species and their surface propertieschange rapidly and profoundly over time and solution chem-istry and with environmental conditions. In practice, the termaging has often been used to describe the observed changesof iron nanoparticles.

Figure 2 presents the Transmission Electron Microscopy(TEM) images of iron nanoparticles synthesized using thesodium borohydride method. The nanoparticles are mostlyspherical in shape and exist as chain-like aggregates. An ac-cumulative size distribution survey of over 400 nanoparticlesfrom TEM images suggests that over 80% of the nanoparti-cles have diameters of less than 100 nm whereas 50% are lessthan 60 nm (Figure 3).45 The average BET surface area is about30,00035,000 m2/kg.

According to the core-shell model, the mixed valence ironoxide shell is largely insoluble under neutral pH conditions andmay protect the ZVI core from rapid oxidation. The speciationof the actual oxide layer is complicated due to the lack of long-range order and its amorphous structures. The composition ofthe oxide shell may also depend on the fabrication processesand environmental conditions. For example, the oxide shell of-Fe nanoparticles generated by sputtering consists mainly of

ZERO-VALENT IRON NANOPARTICLES ABATING ENVIRONMENTAL POLLUTANTS 115

FIG. 3. An accumulative size distribution of zero-valent ironnanoparticles. More than 420 nanoparticles were measured fromTEM images.

maghaemite ( -Fe2O3) or partially oxidized magnetite (Fe3O4).Nanoparticles formed via nucleation of metallic vapor also con-tain -Fe2O3 and Fe3O4, with richer -Fe2O3 for smaller parti-cles due to the higher surface-to-volume ratio and rapid surfaceoxidation.38 On the other hand, particles produced by hydro-gen reduction of goethite and hematite particles reportedly haveonly Fe3O4 in the shell.39,46 The presence of wustite (FeO) hasalso been noted.38 It is not clear from the existing literaturewhether variations in the shell structure and composition haveany effect on the iron nanoparticle reactivity, aggregation, andtransport.

Detailed X-ray Photoelectron Spectroscopy (XPS) studies onthe ZVI nanoparticles have been performed in our laboratory.Figure 4 shows the XPS survey on the Fe2p3/2 and O1s regions.For the Fe2p3/2 spectrum, the binding energy of the main peakwas located at ca. 711 eV, which is attributable to ferric iron[Fe(III)]. The smaller peak at ca. 707 eV suggests the presenceof the elemental metallic iron. The photoelectron peak of O1s inFigure 4b can be decomposed into three separate peaks at 529.9eV, 531.2 eV, and 532.5 eV, representing the binding energiesof oxygen in O2, OH, and chemically or physically adsorbedwater, respectively. The oxygen species are similar to those onthe surface of iron oxides in water.

Further examination of the peak area ratios of Fe/OH andOH/O2 suggest that the oxide shell is composed of mainly ironhydroxides or iron oxyhydroxide. As a result of iron oxidation,Fe2+ is first formed on the surface:

2Fe + O2 + 2H2O 2Fe2+ + 4OH [2]Fe + 2H2O Fe2+ + H2 + 2OH [3]

Fe2+ can be further oxidized to Fe3+:

4Fe2+ + 4H+ + O2 4Fe3+ + 2H2O [4]

FIG. 4. (a) XPS survey on Fe 2p3/2 and (b) XPS survey on O 1sof zero-valent iron nanoparticles. Results suggest that the ironon the particles surface is extensively oxidized.

Fe3+ reacts with OH or H2O and to yield hydroxide oroxyhydroxide:

Fe3+ + 3OH Fe(OH)3 [5]Fe3+ + 2H2O FeOOH + 3H+ [6]

Fe(OH)3 can also dehydrate to form FeOOH:Fe(OH)3 + 3H+ FeOOH + H2O [7]

The results constitute the foundation of the conceptual modelof the nZVI nanoparticle as illustrated in Figure 1. In water,ZVI nanoparticles can exhibit metal-like or ligand-like coordi-nation properties depending on the solution chemistry. At lowpH (

116 X.-Q. LI ET AL.

reduction potential (E) of 0.44 V, which is lower than manymetals such as Pb, Cd, Ni, and Cr, as well as many organiccompounds like chlorinated hydrocarbons. These compoundsare thus susceptible to the reduction by ZVI nanoparticles.

Reactions at the nZVI surface involve many steps, for ex-ample, mass transport of molecules to the surface and electrontransfer (ET) from the ZVI to the surface adsorbed molecules.Kinetic analysis suggests that for many organic compounds suchas PCE and TCE, the surface reaction or electron transfer is thedominant factor or the rate limiting step. There are several po-tential pathways for the electron transfer (ET) to occur: (1) directET from Fe(0) through defects such as pits or pinholes, wherethe oxide layer acts as a physical barrier; (2) indirect ET fromFe(0) through the oxide layer via the oxide conduction band,impurity bands, or localized bands; or (3) ET from sorbed orlattice Fe(II) surface site.48,49 In this regard, the iron oxide shellcan appropriately be considered as n-type semiconductor. Onthe other hand, we are not aware of direct evidence on the ETmechanisms at this time.

2.3 Degradation of Organic ContaminantsA substantial portion of the peer-reviewed nZVI literature has

been limited to the degradation of various organic contaminantssuch as chlorinated organic solvents, organochlorine pesticides,polychlorinated biphenyls (PCBs), and organic dyes. Selectedcompounds studied in our laboratory at Lehigh University arelisted in Table 2.

The chemical principles underlying the transformation ofhalogenated hydrocarbons have been particularly well docu-mented as those compounds are among the most commonlydetected soil and groundwater pollutants. Metallic iron (Fe0)serves effectively as an electron donor:

Fe0 Fe2+ + 2e [8]Chlorinated hydrocarbons on the other hand accept the elec-

trons and undergo reductive dechlorination:50,51

RCl + H+ + 2e RH + Cl [9]From a thermodynamic perspective, the coupling of the re-

actions [8] and [9] is often energetically highly favorable:RCl + Fe0 + H+ RH + Fe2+ + Cl [10]

For example, tetrachloroethene (C2Cl4), a common solvent,can be completely reduced to ethane by nZVI in accordance withthe following overall equation:

C2Cl4 + 5Fe0 + 6H+ C2H6 + 5Fe2+ + 4Cl [11]Figure 5 presents an example of a laboratory study on a

mixture of chlorinated hydrocarbons. Six common compoundsincluding trans-dichloroethene (t-DCE), cis-dichloroethene (c-DCE), 1,1,1- trichloroethane (1,1,1-TCA), tetrachloroethylene(PCE), trichloroethylene (TCE), and tetrachloromethane wereinvestigated. The initial concentration was 10 mg/L for each

TABLE 2Common contaminants that can be remediated by iron

nanoparticles20

Chlorinated Methanes TrihalomethanesCarbon tetrachloride (CCl4) Bromoform (CHBr3)Chloroform (CHCl3) Dibromochloromethane

(CHBr2Cl)Dichloromethane (CH2Cl2) Dichlorobromomethane

(CHBrCl2)Chloromethane (CH3Cl)Chlorinated Benzenes Chlorinated Ethenes

Hexachlorobenzene (C6Cl6) Tetrachloroethene (C2Cl4)Pentachlorobenzene

(C6HCl5)Trichloroethene (C2HCl3)

Tetrachlorobenzenes(C6H2Cl4)

cis-Dichloroethene(C2H2Cl2)

Trichlorobenzenes(C6H3Cl3)

trans-Dichloroethene(C2H2Cl2)

Dichlorobenzenes(C6H4Cl2)

1,1-Dichloroethene(C2H2Cl2)

Chlorobenzene (C6H5Cl) Vinyl Chloride (C2H3Cl)Pesticides Other Polychlorinated

DDT (C14H9Cl5) HydrocarbonsLindane (C6H6Cl6) PCBs

Pentachlorophenol1,1,1-trichloroethane

Organic Dyes Other Organic ContaminantsOrange II (C16H11N2NaO4S) N-nitrosodiumethylamine

(NDMA) (C4H10N2O)Chrysoidin (C12H13ClN4) TNT (C7H5N3O6)Tropaeolin O

(C12H9N2NaO5S)Heavy Metals Inorganic Anions

Mercury (Hg2+) Perchlorate (ClO4 )Nickel (Ni2+) Nitrate (NO3 )Cadium (Cd2+)Lead (Pb2+)Chromium (Cr(VI))

of the six compounds and the total nZVI loading was 5 g/L.As illustrated by the trend illustrated in the gas chromatogramsover the reaction time course, all of the six compounds werereduced by the ZVI nanoparticles. Within one hour, over 99%removal of tetrachloroethylene was observed. Greater than 95%removal efficiency of the 6 compounds was achieved within120 hours.

The importance of electron donors in the reduction of chlo-rinated hydrocarbons has received significant attention in theresearch and environmental remediation communities. Exten-sive research on both biological and chemical dechlorination

ZERO-VALENT IRON NANOPARTICLES ABATING ENVIRONMENTAL POLLUTANTS 117

FIG. 5. Reactions of iron nanoparticles (5 g/L) with a mixture of chlorinated aliphatic hydrocarbons. Gas chromatograms are shownin this figure. Six compounds with initial concentration at 10 mg/L are presented: Trans-dichloroethene (t-DCE), cis-dichloroethene(c-DCE), 1,1,1- trichloroethane (1,1,1-TCA), tetrachloroethylene (PCE), trichloroethylene (TCE), and tetrachloromethane.

has been published over the last two decades. In general, therate of dechlorination increases with the number of chlorinesubstituents. For dechlorination with ZVI, three potential trans-formation mechanisms have been proposed: (1) direct reductionat the metal surface; (2) reduction by ferrous iron at the surface;and (3) reduction by hydrogen.51 Ferrous iron, in concert with

certain ligands, can slowly reduce the chlorinated hydrocarbons.However, electron transfer from ligand ferrous ions through thenZVI oxide shell is thought to be relatively slow and is proba-bly not of great consequence. Dissolved hydrogen gas has beenshown of little reactivity in the absence of a suitable catalyticsurface (e.g., Pd).

118 X.-Q. LI ET AL.

The incorporation of a noble or catalytic metal such as Pd,Ni, Pt, or Ag, can substantially enhance the overall nZVI reac-tion rate. It has been observed that the surface area normalizedrate constant (KSA) of Fe/Pd nanoparticles for the degradationof tetrachloromethane is over two orders of magnitude higherthan that of microscale iron particles. The activation energy ofFe/Pd nanoparticles in the transformation of tetrachloroethy-lene (PCE) was calculated to be 31.1 kJ/mole, compared to44.9 kJ/mole for iron nanoparticles.52 The fast reactions gen-erated by the bimetallic nanoparticles also reduce possibility oftoxic byproduct formation and accumulation.

Although chlorinated aliphatic compounds with 1 or 2 car-bons have been studied extensively, chlorinated alicyclic andaromatic compounds have received far less attention. Withouta doubt, chlorinated alicyclic and aromatic compounds featuremore complicated chemical structures, are less aqueous solu-ble, react more slowly with nZVI, and often generate moreintermediates and byproducts. Limited research indicates thatiron nanoparticles can exhibit fairly high reactivity toward thesecompounds even though some researches with micro- and milli-meter iron particles reported little reactions. Xu et al. evalu-ated the degradability of hexachlorobenzene (HCB) by Fe/Agbimetallic nanoparticles.43 With an initial HCB concentrationof 4 mg/L and an applied loading of 25 g/L nZVI, over 50%of HCB was reduced from aqueous solution within 30 min-utes. HCB concentrations were reduced below the detectionlimit (

ZERO-VALENT IRON NANOPARTICLES ABATING ENVIRONMENTAL POLLUTANTS 119

FIG. 7. Reactions of Ni(II) with iron nanoparticles. This fig-ure shows high resolution XPS survey on Ni 2p3/2. Both Ni(II)and Ni(0) were observed on the nanoparticle surface. Ni initialconcentration was 1,000 mg/L. Iron nanoparticle loading was5 g/L.

follows pseudo first-order kinetics with ammonia being the endproduct:30,54

NO3 + 4Fe0 + 10H+ 4 Fe2+ + NH+4 + 3H2O [13]

The reduction of nitrate by iron nanoparticles is fast even atrelatively high pH (e.g., 810), a range that is known to be in-hibitory to the iron oxidation reactions. For example, Sohn etal.33 designed repetitive experiments and observed that reac-tion rates remained about the same even after exposed to newnitrate solutions for 6 times. It is speculated that in alkaline so-lutions, the anionic hydroxo species (Fe(OH)2yy or Fe(OH)3xx )are dissolved and then precipitated yielding different phase ofiron oxide, most likely magnetite (Fe3O4). The phase transfor-mation may expose fresh ZVI iron surface to nitrate, resultingin a relatively stable reaction rate.

Perchlorate is another high profile contaminant in the U.S.,particularly in the western states. It is persistent in the environ-ment due to its intrinsic chemical stability. Consequently, ratesof natural attention under most cases are very slow. Becauseof its stability in water, only few reductants, such as complexesof tin(II), vanadium(II, III), molybdenum(III), titanium(III), andruthenium(III, IV), have been identified as being capable of re-ducing perchlorate. Still, the half-lives of perchlorate may rangefrom 0.83 years to up to 11.3 years for these transition metalreductants.55 In a study by Cao et al.,31 ZVI nanoparticles wereobserved to reduce perchlorate to chloride with no observanceof the sequential degradation products. The nZVI-mediated re-

duction of perchlorate proceeds according to Equation 14:

ClO4 + 4Fe0 + 8H+ Cl + 4Fe2+ + 4H2O [14]In contrast, the reduction by microscale iron particles is negli-

gibly slow. Not surprisingly, the activation energy was calculatedto be 79 kJ/mole, indicative that the reduction is limited by theslow kinetics.

2.5 Transport of the Iron NanoparticlesBecause ZVI nanoparticles are increasingly being used in site

remediation, a critical issue for the future development of thetechnology involves nZVI transport, dispersion, and fate in thesubsurface environment. At present, little information is avail-able on the transport and fate of ZVI nanoparticles in the en-vironment. In laboratory soil column experiments, the mobilityof pure iron nanoparticles has been observed to be limited dueto the colloidal nature and efficient filtration mechanisms ofaquifer materials. Data from some recent field tests indicate thatthe iron nanoparticles may migrate only a few inches to a fewfeet from the point of injection. The mobility of nanoparticles inthe subsurface environment depends on many factors includingthe particle size, solution pH, ionic strength, soil composition,ground water flow velocity, and so on27,34,37,44,5658 For ex-ample, groundwater usually has relatively high values of ionicstrength, which results in a reduction of the electrostatic repul-sion among particles and an increase in particle aggregation.

Recent research indicates that promising new synthetic meth-ods are being developed to produce more mobile ZVI nanopar-ticles without sacrificing significant surface reactivity. Malloukand his group at Penn State57 have successfully synthesized ZVInanoparticles on supports, which feature a high density of nega-tive charges (dubbed as delivery vehicles) to aid in electrostaticrepulsion between nZVI particles and with the predominantlynegatively surface aquifer materials. These nZVI delivery vehi-cles, including anionic hydrophilic carbon and poly(acrylic acid)(PAA), bind strongly to nZVI, create highly negative surfacesthus effectively reducing the aggregation among ZVI particlesand reduce the filtration removal by aquifer materials. Labora-tory soil column tests with Fe/C, Fe/PAA, and unsupported ironnanoparticles suggest that the anionic surface charges can en-hance the transport of iron nanoparticle through soil- and sand-packed columns, where as the unsupported iron nanoparticlesaggregate and impede the flow of water through the column.Further analysis show that the Fe/C and Fe/PAA nanoparticleshave lower sticking coefficients.

Work by Y. P. Sun at Lehigh University on the use ofpoly(vinyl alcohol-co-vinyl acetate-co-itaconic acid) (PV3A) asa dispersant also helped to generate a new class of nZVI with sub-stantially better subsurface mobility potential.58 As illustrated inFigure 8, the iron nanoparticles were well dispersed with mostof the resulted particles less than 2030 nm. Further analysisusing acoustic spectrometry indicated that the dispersed ironnanoparticles have a mean size at 15.5 nm with 90% of the par-ticles smaller than 33.2 nm. The stabilized particles also have

120 X.-Q. LI ET AL.

FIG. 8. Characterization of iron nanoparticles (ad) dispersed with poly(vinyl alcohol-co-vinyl acetate-co-itaconic acid) (PV3A),and (e) to (h): pure nanoparticles.

ZERO-VALENT IRON NANOPARTICLES ABATING ENVIRONMENTAL POLLUTANTS 121

high reactivity and high stability in terms of suspension (over6 months). Measurements of -potential suggest that dispersediron nanoparticles have net negative charge at pH 4.5, likely theresult of the dissociation of the carboxylic acid groups withinthe PV3A molecules. In comparison, the original or bare ironnanoparticles have much larger particle size and settle out of so-lution in less than 10 minutes. The bare iron actually has slightpositive charges in the neutral pH range. This may help to explainthe low mobility of ZVI nanoparticles.

3. CHALLENGES AHEADSince the initial field demonstration of the nZVI technol-

ogy in 2001, significant progress has been made in researchand development of iron nanoparticles for soil and groundwatertreatment. New research and development efforts should be di-rected toward enhancing real-world performance and minimiz-ing potential economic and environmental risks. The followingsections present some questions, problem, and/or challenges wehave experienced in our laboratory and field work.

Materials chemistry. The synthesis of iron nanoparticles rep-resents the foundation of this technology. Several nZVI syntheticmethods have been identified but the economics of each is con-siderably different. The challenge is to scale up the processesand produce nano iron in large enough quantity such that furtherprice reductions into the $1025 per pound range become feasi-ble. Methods for quality control and assurance (e.g., particle size,reactivity, surface charge, and mobility) should be established.Other issues requiring resolution might include optimization orcustomization of the ZVI surface properties (hydrophobicity,charge, functional group, etc.) for efficient subsurface transportunder site-specific conditions and for degradation of targetedcontaminants (e.g., perchlorate, PCBs, DNAPL).

Environmental Chemistry. This work includes the study ofreaction rate, mechanism, and effect of environmental factors(e.g., pH, ionic strength, competing contaminants) on the trans-formation of targeted environmental contaminants. As discussedearlier, the published work so far has been largely limited to 1-and 2-carbon chlorinated aliphatic compounds. Details on thereaction mechanisms at the nanoparticle-water interface are stillscarce. More work needs to be focused on exploring the feasi-bility of using nZVI for the treatment of halogenated aromaticcompounds, PCBs, dioxins, pesticides, and other organic com-plex pollutants. Remediation of metal contaminated sites alsopresents interesting opportunities and challenges.

Geochemistry. This entails the nZVI reactions with both sur-face and ground water, interactions (sorption, desorption) withsoil and sediment, settling, aggregation, and transport phenom-ena in porous media. Evaluating the long-term fate of ironnanoparticles represents another topic worthy of more focusedattention. To date, virtually no studies in this key area have beenpublished.

Environmental Impact. Thus far, no reports on the ecotoxicityof low-level ZVI in soil and water have been published in thepeer-reviewed literature. However, it is the authors view that

systematic research on the environmental transport, fate, andecotoxicity is needed to overcome increasing concerns and fearin the environmental use of nanomaterials, and minimize anyunintended impact. To date, the overwhelming proportion ofnZVI research deals with the applications of the technology andwork is needed to adequately assess its implications.

Iron nanoparticles may actually provide a valuable opportu-nity to demonstrate the positive effect on environmental qual-ity. Iron is the fifth most used element; only hydrogen, carbon,oxygen, and calcium were consumed in greater quantities. It hasbeen found at the active center of many biological molecules andlikely plays an important role in the chemistry of living organ-isms. It is well documented that iron is an essential constituentof the blood and tissues. Iron in the body is mostly present asiron porphyrin or heme proteins, which include hemoglobin inthe blood, myoglobin, and the heme enzymes. The challengeis to determine eco-and human toxicity of highly reactive ZVInanoparticles.

ACKNOWLEDGMENTSResearch described in this work has been supported by Penn-

sylvania Infrastructure Technology Alliance (PITA), the U.S.National Science Foundation (NSF), and by the U.S. Environ-mental Protection Agency (U.S. EPA). The authors thank Mr.Sisheng Zhong for his able assistance in the preparation of thismanuscript and Marti Otto (U.S. EPA) for information presentedin Table 1.

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