the suitability of metallic iron

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The Suitability of Metallic Iron

for Environmental RemediationChicgoua Noubactep

Angewandte Geologie, Universitat Gottingen, Goldschmidtstrae 3, D-37077 Gottingen, Germany; [email protected](for correspondence)

Published online 19 November 2009 in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/ep.10406

Aqueous contaminant removal in the presence ofmetallic iron is often regarded as a reductive trans-

formation mediated by the Fe0surface. However, suc-cessful removal of theoretically nonreducible contam-inants has been largely reported. This article presentsa rebuttal of the concept of contaminant reductivetransformation. It is argued through a careful exami-nation of the evolution of the volume and adsorptive

properties of iron and its corrosion products that con-taminants are primarily adsorbed and coprecipitatedwith iron corrosion products. One may wonder howthe Fe0 technology will develop with the new concept. 2009 American Institute of Chemical Engineers Environ

Prog, 29: 286291, 2010Keywords: adsorption, co-precipitation, contami-

nant, removal, zerovalent iron

INTRODUCTION

In 1990 Canadian hydrogeologists have rediscov-ered iron corrosion as metallic iron (Fe0) became aremediation agent for contaminated aquifers, soils,and waters. It was fortuitously found that Fe0 elimi-nated trichloroethylene from aqueous solutions [13].Since then intensive efforts have been devoted toremediation with Fe0 materials. As result Fe0 is nowregarded as a very competent reactive agent for reme-

diation of systems that are contaminated with reduci-ble substances (including chlorinated hydrocarbons,nitrate, nitro aromatics, chromium, uranium) [48].

The rediscovery of iron corrosion was followed bya seminal work on the mechanism of aqueous con-taminant removal in the presence of Fe0 (e.g. in Fe0/H2O systems) [9]. These authors proposed three possi-ble mechanisms for contaminant removal: (1) contami-nant adsorption onto the surface of in situ formed cor-rosion products, (2) contaminant reduction by Fe0

(direct reduction), (3) contaminant reduction by FeII

or H2/H (indirect reduction). The work of Mathesonand Tratnyek [9] was reevaluated by Weber [10] andthe results indicate that (1) direct reduction (electronsfrom Fe0) is the major reaction pathway, and (2)reductive transformation by Fe0 is a surface-mediatedprocess. Accordingly, the involved contaminant mustcontact the Fe0 surface for electron transfer to takeplace. Alternatively, the oxide film on Fe0 must beelectronic conductive or the system must containappropriate electron mediators (so-called electronshuttles). Since the work of Weber [10] the know-whyof contaminant removal in Fe0/H2O systems was con-sidered to be achieved and the iron/sand mixture lost

his nickname of magic sand [2]. However, the accep-tance of the concept of reductive transformation wasprimarily a consensus [4] as this concept failed to con-sistently explained many experimental facts [1114].For example, while using differential pulse polarogra-phy to investigate the reduction of nitrobenzene inFe0/H2O systems, Lavine et al. [11] concluded thattheir studies were very informative but they couldntevidence reduction of organic compounds as medi-ated by the Fe0 surface. Similarly, a very recent workof Jiao et al. [14] has shown that the reduction of car-bon tetrachloride in the presence of Fe0 is primarilymediated by adsorbed H from iron corrosion (indirectreduction). Moreover, the quantitative removal for

nonreducible species as methylene blue [15], triazoles[16] and zinc [17] in Fe0/H2O systems has beenreported. Because of the inconsistency of the consen-sus on the mechanism of contaminant removal in Fe0/H2O systems, it was pertinent to reconsider the Fe

0/H2O system as a whole. For this purpose it is neces-sary to go back to the literature on metal corrosion.

FUNDAMENTAL ASPECTS OF AQUEOUS METAL CORROSION

Most metals (M) in their natural state are not puremetals (M0), but are in the form of metallic salts(mostly oxidesMxOy, sulphidesMxSy, and carbo- 2009 American Institute of Chemical Engineers

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natesMx(CO3)y). When these metal ores are refinedor smelted, a losing battle with thermodynamicsbegins with the metal tending toward formation ofmetallic oxides, sulphides, or carbonates dependingon the working environment [1820]. The rate andthe extent at which a metal dissolves in an aqueousenvironment (immersed metal corrosion) depends onmany interdependant factors [18, 21, 22] including:

(1) the chemistry of water (pH, salinity, and concen-tration of chelating agents), (2) the nature of the ox-ide layer formed by initial metal corrosion (composi-tion, electronic conductivity, porosity, and thickness),(3) the manufacturing history of the metal (e.g.

whether the metal been cast, forged, wrought, orwelded), and (4) the metal thermodynamic suscepti-bility to oxidation (position on the reduction-potentialscale). It has been established that the most importantfactor responsible for immersed metal corrosionunder conditions pertinent to natural waters (4.5 pH 9.5) is the electronic conductivity and the po-rosity of the oxide layer [23]. This statement will besupported by a classical example.

Aluminium (E

0

5 21.71 V) is more susceptible tooxidation than iron (E0 5 20.44 V) but Al is knownto be relatively inert to atmospheric and aqueous cor-rosion, whereas Fe is very corrosive. There are twomain reasons for this.

First, the oxidation of Al0 exclusively yields a non-conductive layer of Al2O3 whereas the oxidation ofFe0 may yield a conductive layer of FeII/FeIII species(e.g. Fe3O4, green rust) [24]. Accordingly an oxidefilm on Fe0 may act as a semiconductor and mediateelectron transfer from Fe0 [2527]. In this situation Fe0

corrosion continues despite the presence of the oxidefilm. It is evident, that this behaviour can not beobserved under oxic conditions where FeII is instable

and (at least) the outer layer of the oxide film willexclusively consist of nonconductive FeIII oxides(FeOOH or Fe2O3).

Second, the unit-cell in Al and Al2O3 are very simi-lar to one another; thus the aluminium oxide canadhere tightly to the metallic aluminium beneath it[23]. The oxidized surface provides a protective layerthat prevents oxygen from getting to the underlying

Al surface. In contrast, the packing dimensions of FeFe0 and Fe oxides are not particularly close; thusthere is no tendency for an iron oxide layer to adhereto metallic iron. Therefore, regarding its protectiveproperties for iron corrosion, the curse of rust [23]is not that it forms, but that it constantly flakes off

and exposes fresh iron surface for attack [23, 24].This curse of rust became a blessing in using Fe 0

for environmental remediation.

METALLIC IRON FOR ENVIRONMENTAL REMEDIATION

Fe-based alloys (Fe0 materials, mostly cast iron andsteel) are certainly suitable for environmental reme-diation because of their low tendency to passivitydue to the porosity and the instability of generatedoxide layers. Instead of this trivial reason, Fe0 hasbeen considered as a strong reducing agent for thereductive transformation of several species in natural

waters [310]. This consideration is not acceptableeven from a pure thermodynamic perspective as theelectrode potential of iron is almost the same (about20.44 V) in low alloyed and stainless steels (Fe-basedalloys). Accordingly, various Fe0 materials shouldhave exhibited similar behaviour for the removal ofthe same species. This has not been the case as forexample Miehr et al. [28] reported variation of rate

constants for contaminant removal varying over up tofour orders of magnitude due to differences in Fe0

type.Fe0 materials for environmental remediation are

primarily susceptible to oxidation, complete passiva-tion can only result from the transformation of ini-tially nonprotective films to impervious layers underspecific environmental conditions. Therefore, a sakefor an overview of factors likely to influence film for-mation and transformation (stability and breakdown)under environmental conditions should be under-taken. This could be a very difficult task becausechemical breakdown occurs when the film is dis-solved (e.g. by a chelating agent) or penetrated

chemically (e.g. by a contaminant or Cl ions).

FUNDAMENTAL ASPECTS OF CONTAMINANT REMOVAL INFe0/H2O SYSTEMS

While considering Fe0 as a reducing agent for con-taminant reductive transformation it has been impos-sible to explain several experimental and field obser-

vations as recalled above (also see Refs. 29 and 30).The main reason for this is that iron corrosion prod-ucts (oxide film) have been regarded as simple coat-ings, mediating at most electron transfer from Fe0 tothe contaminant. However, the oxide film formationand transformation (recrystallization, dissolution, andprecipitation) is a dynamic process occurring in the

presence of contaminants. Moreover, the oxide filmformation can be regarded as the process of iron pre-cipitation [3133]. Here, iron precipitation occurs inthe presence of small amounts of foreign species(including contaminants). These foreign species arenecessarily sequestrated within the oxide film as dis-cussed in the next section [34]. In this manner con-taminants are primarily removed from the aqueousphase by a nonspecific mechanism as they are justsequestrated in the matrix of precipitating iron oxides[33]. This process is widely used in water treatmentby electrocoagulation using Fe electrodes (Fe0 EC)[35, 36] and explains why bacteria, viruses, and ther-modynamically nonreducible substances (e.g. Zn)

have been quantitatively removed in Fe0

/H2O sys-tems. There are two main differences between pas-sive Fe0/H2O systems and iron electrocoagulation: (1)Fe0 oxidation is electrically accelerated in Fe0 EC asthe aim is to quantitatively produce iron hydroxidesfor contaminant removal by flocculation, and (2)

while contaminants are flocculated from the bulk so-lution in Fe0 EC, they are precipitated in the vicinityof Fe0 in passive Fe0/H2O systems. Accordingly a pas-sive Fe0/H2O system could be regarded as a filter(working on the principle of size exclusion) in whichspecies are additionally trapped by in situ generatediron hydroxides (and oxides).

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It should be explicitly stated that adsorption,coprecipitation and redox transformations are noteach other exclusive as adsorbed or co-precipitatedcontaminants may be further (1) reduced by Fe0,adsorbed or soluble FeII species or (2) oxidized byHO(/H2O2 species (Fenton-like reactions). It is how-

ever certain, that the extend of reduction is difficultto evaluate. This assertion is supported by the factthat to date, no carbon balances between reactantsand supposedly reaction products have ever beensuccessfully done for many chlorinated hydrocarbons[3]. However, one should no care about the fate ofcoprecipitated contaminants as they will remainsequestrated so far iron oxides are not dissolved.

Alternatively and complementary, the possibility ofiron oxide dissolution and its consequence for copre-cipitated contaminants should be discussed at eachrelevant site. The next section will discuss on the suit-ability of the Fe0/H2O system for contaminant re-moval.

PECULIARITY OF THE Fe0/H2O SYSTEM

The singularity of the Fe0/H2O system is the in situgeneration of soluble FeII species and their furthertransformations to crystalline iron oxides and hydrox-ides (Fe(OH)2, Fe(OH)3, Fe3O4, Fe2O3, FeOOH,Fe5HO84H2O). The various forms of iron oxides,oxyhydroxides, and hydroxides are called ironoxides through the end of this article. Thus, in anaqueous solution, Fe0 ideally converts to crystallineiron oxides via a sequence of oxidation/hydrolysis/precipitation/dehydration reactions (Table 1). Theconversion of Fe0 to crystalline iron oxides goes

towards several intermediate stages of amorphousand poorly crystalline precipitates, including greenrust formation and transformation. Intermediatestages also include solid-state transformation ofoxides (recrystallization). For example, in aqueoussolution crystalline Fe(OH)2 may convert to other

iron oxides via oxidation/hydrolysis/dehydration.In essence, iron oxide formation involves two ba-sic mechanisms: (1) direct precipitation from Fe21/Fe31-containing solutions, and (2) transformation ofan Fe oxide precursor. Both mechanisms may occurin natural Fe0/H2O systems, even though direct pre-cipitation is dominant. Due to the diversity of ironcorrosion products (CP), a common problem facedby studies on Fe0/H2O systems is the proper charac-terization of available iron oxides [3739]. A commonprocedure is the use of synthetic iron oxides to simu-late natural CP. However, in natural Fe0/H2O systems,CP are composed primarily of different iron oxides.Individual iron oxides possess different chemical

properties such as crystal structure, morphology, andadsorptive properties (Table 2). Accordingly, no syn-thetic iron oxides (or oxide mixtures) can rigorouslysimulate natural CP with regard to transformationoccurring within.

TRANSFORMATIONS WITHIN Fe0/H2O SYSTEMS

Synthetic iron oxides as simulates for natural ironoxides are prepared in the pure phase while naturaloxides are precipitated and further transformed in thepresence of foreign species (including contaminants).Thus a synthetic oxide can remove contaminantsolely by adsorption while natural oxides incorporate

Table 1. Relevant reactions for the process of aqueous Fe0 dissolution, iron corrosion products formation andcontaminant (Ox) removal in Fe0/H2O system.

Process Reaction Eq.

Fe0 dissolution Fe0 , Fe21 1 2e2 1Fe0 passivation Fe0 1 H2O ) Fe(O)ads 1 2H

11 2e2 2

Fe0 depassivation Fe(O)ads 1 2H1) Fe21 1 H2O 3

Fe(O)ads 1 H2O ) Fe(OH)2 4

Fe(O)ads 1 OH2

) HFeO22

5H2 evolution 2H

11 2e2 ) H2: 6

O2 reduction O2 1 2H2O 1 4e2) 4OH2 7

Fe21 oxidation Fe21 ) Fe31 1 e2 8Fe21 1 2 OH2 ) Fe(OH)2 9Fe31 1 3 OH2 ) Fe(OH)3 10

Scale formation Fe(OH)2 ) FeO 1 H2O 112 Fe(OH)3 ) Fe2O3 1 3 H2O 124 Fe(OH)3 ) Fe(OH)2 1 Fe3O4 1 5 H2O 1 O2 13Fe(OH)3 ) FeOOH 1 H2O 14

Ox reduction Fe0 1 Ox(aq) ) Fe21 1 Red (s or aq) 15

FeII(aq) 1 Ox(aq) ) FeIII

1 Red (s or aq) 16FeII(s) 1 Ox(aq or aq) ) Fe

III 1 Red (s or aq) 17H2 1 Ox(aq or aq) ) H

11 Red (s or aq) 18

Ox adsorption FeOOH 1 Ox(aq) , FeOOH2

Ox 19Ox coprecipitation Ox(aq) 1 n Fex(OH)y

(3x 2 y), Ox[Fex(OH)y

(3x 2 y)]n 20

Red is a reduced form of Ox. FeOOH is a proxy of corrosion products and Fex(OH)y(3x 2 y) is an iron hydrox-

ide.



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contaminant in their structure while precipitating(adsorption and coprecipitation). At any moment af-ter implementation of a Fe0 reactive wall, a natural

Fe

0

/H2O system is made up of Fe

0

and various ironoxides, possibly including transforming phases likeFeO which are not stable under natural (sub)surfaceconditions. Given that the iron oxides are of variousreactivity toward contaminant removal, the contribu-tion of individual removal mechanisms to decontami-nation is difficult to access. However, it is the goal ofthis article to demonstrate that, beside adsorption,contaminant sequestration (coprecipitation) is thesole certain removal mechanism. The occurrence andthe extent of all other processes can be discussed ona contaminant-specific basis. Therefore adsorptionand co-precipitation are the fundamental mechanismsof contaminant removal in Fe0/H2O system. To illus-

trate the transformations yielding to contaminantsequestration, the evolution of three iron atoms fromthe Fe0 material will be discussed.

The three atoms (3 Fe0) will be first oxidized to 3FeII species and may further be oxidized (e.g. by O2)to 3 FeIII species. Then they will be transformed tocolloidal species partly having specific surface areas(SSA) higher than 500 m2/g [43, 44] before they ag-gregate and crystallize to one Fe3O4, 1.5 Fe2O3 or 3Fe(OH)2, Fe(OH)3 or FeOOH. The relative variationof the volume of resulted crystalline oxides is repre-sented in Figure 1 using values from Table 2. Figure1 clearly shows volume expansion relative to 3 Fe0

for all iron oxides except magnetite for which a vol-

ume reduction of 30% was noticed. However, itshould be kept in mine that even magnetite is a finalstate of a transformation going through even more

voluminous colloidal, amorphous and highly adsorp-tive species (SSA values in Table 2). Although volumeexpansion is discussed here on the basis of the vol-ume of crystallized iron oxides, this process is a rulein the process iron corrosion, irrespective from thenature and the crystallinity of the final products. Thusiron oxidative dissolution and iron oxide precipitationshould be regarded a cycle of volume expansion/contraction in the course of which available contami-nants are adsorbed and sequestrated. Sequestrated

contaminants could be further transformed (oxidizedor reduced).

CONCLUSIONSThis article has contributed to open a new avenue

for the scientific understanding of processes of con-taminant removal in Fe0/H2O systems. Researchersand practitioners have long recognized the limits ofthe reductive transformation concept [1114]. How-ever, the view that contaminants are primarilyadsorbed and coprecipitated with iron corrosionproducts [29, 30, 33] has partly faced with very scep-tic views [45, 46] or is just degraded to an alternativehypothesis to be considered [47]. Fortunately, sceptic

views are based on the large acceptability of the con-cept of reductive transformation which was a consen-

Table 2. Some relevant characteristics of metallic iron and its main corrosion products.

Species Formula Symmetry Density SSA (m 2/g) Vrust/VFe

Iron Fe bcc 7.86


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sus [4] and not the results of hard experimental facts.While former attempts to disprove the reductivetransformation concept were based on extensive liter-ature review [29, 30] and a munitions examination ofthe abundance of reactive species in Fe0/H2O systems[33], the present study is based on a simple analysisof processes occurring in a Fe0/H2O system. Themajor output is that Fe0 oxidative dissolution and

iron oxide precipitation should by regarded as a cycleof volume expansion/contraction in the course ofwhich chemical contaminants and pathogens areadsorbed and sequestrated. Because this argumentdoes not care about the nature of a contaminant, itshould be definitively clear that specific interactionsare an exception and not the rule in a Fe 0/H2O sys-tem (statement 1). Statement 1 is the cornerstone on

which comprehensive knowledge on the behavior ofFe0/H2O systems under natural conditions should beacquired. In this effort, it is almost impossible totransfer good results from the open aqueous iron cor-rosion. For example, corrosion inhibition of mildsteel by methylene blue (MB) has been reported [48].

However, corrosion inhibition is effective with 5.0mM (1595 mg/L) MB in HCl at temperatures varyingfrom 30 to 608C. The used temperatures are not rele-

vant for groundwater remediation and the neededconcentration (1595 mg/L) is even non relevant for

wastewater situations. Additionally the reaction tookplace in a strong acidic solution (2 M HCl).

ACKNOWLEDGMENTS

The work was supported by the Deutsche For-schungsgemeinschaft (DFG-No. 626/2-2).

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the suitability of metallic iron

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