The Suitability of Metallic Iron for Environmental Remediation 1 
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Noubactep C. 
Angewandte Geologie, Universität Göttingen, Goldschmidtstraße 3, D - 37077 Göttingen, Germany. 
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e-mail: cnoubac@gwdg.de; Tel. +49 551 39 3191, Fax: +49 551 399379 
(for Environmental Progress – Accepted 05-Sep-2009) 
Abstract 
Aqueous contaminant removal in the presence of metallic iron is often regarded as a reductive 
transformation mediated by the Fe0 surface. However, successful removal of theoretically 
non-reducible contaminants has been largely reported. This paper presents a rebuttal of the 
concept of contaminant reductive transformation. It is argued through a careful examination 
of the evolution of the volume and adsorptive properties of iron and its corrosion products 
that contaminants are primarily adsorbed and co-precipitated with iron corrosion products. 
One may wonder how the Fe0 technology will develop with the new concept. 
Keywords: Adsorption, Co-precipitation, Contaminant, Removal, Zerovalent iron. 
Introduction 
In 1990 Canadian hydrogeologists have rediscovered iron corrosion as metallic iron (Fe0) 
became a remediation agent for contaminated aquifers, soils and waters. It was fortuitously 
found that Fe0 eliminated trichloroethylene from aqueous solutions (1-3). Since then intensive 
efforts have been devoted to remediation with Fe0 materials. As result Fe0 is now regarded as 
a very competent reactive agent for remediation of systems that are contaminated with 
reducible substances (including chlorinated hydrocarbons, nitrate, nitro aromatics, chromium, 
uranium) (4-8). 
The rediscovery of iron corrosion was followed by a seminal work on the mechanism of 
aqueous contaminant removal in the presence of Fe0 (e.g. in Fe0/H2O systems) (9). These 
authors proposed three possible mechanisms for contaminant removal: (i) contaminant 
adsorption onto the surface of in situ formed corrosion products, (ii) contaminant reduction by 
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Fe0 (direct reduction), (iii) contaminant reduction by FeII or H2/H (indirect reduction). The 
work of Matheson and Tratnyek (9) was re-evaluated by Weber (10) and the results indicate 
that (i) direct reduction (electrons from Fe
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0) is the major reaction pathway, and (ii) reductive 
transformation by Fe0 is a surface-mediated process. Accordingly, the involved contaminant 
must contact the Fe0 surface for electron transfer to take place. Alternatively, the oxide film 
on Fe0 must be electronic conductive or the system must contain appropriate electron 
mediators (so-called “electron shuttles”). Since the work of Weber (10) the know-why of 
contaminant removal in Fe0/H2O systems was considered to be achieved and the iron/sand 
mixture lost his nickname of “magic sand” (2). However, the acceptance of the concept of 
reductive transformation was primarily a consensus (4) as this concept failed to consistently 
explained many experimental facts (11-14). For example, while using differential pulse 
polarography to investigate the reduction of nitrobenzene in Fe0/H2O systems, Lavine et al. 
(11) concluded that their studies were very informative but they couldn't evidence reduction 
of organic compounds as mediated by the Fe0 surface. Similarly, a very recent work of Jiao et 
al. (14) has shown that the reduction of carbon tetrachloride in the presence of Fe0 is primarily 
mediated by H2 from iron corrosion (indirect reduction). Moreover, the quantitative removal 
for non-reducible species as methylene blue (15), triazoles (16) and zinc (17) in Fe0/H2O 
systems has been reported. Because of the inconsistency of the consensus on the mechanism 
of contaminant removal in Fe0/H2O systems, it was pertinent to reconsider the Fe0/H2O 
system as a whole. For this purpose it is necessary to go back to the literature on metal 
corrosion. 
Fundamental aspects of aqueous metal corrosion 
Most metals (M) in their natural state are not pure metals (M0), but are in the form of metallic 
salts (mostly oxides - MxOy, sulphides - MxSy, and carbonates - Mx(CO3)y). When these metal 
ores are refined or smelted, a losing battle with thermodynamics begins with the metal 
tending toward formation of metallic oxides, sulphides or carbonates depending on the 
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working environment (18-20). The rate and the extent at which a metal dissolves in an 
aqueous environment (immersed metal corrosion) depends on many inter-dependant factors 
(18, 21, 22) including: (i) the chemistry of water (pH, salinity, concentration and 
concentration of chelating agents), (ii) the nature of the oxide layer formed by initial metal 
corrosion (composition, electronic conductivity, porosity and thickness), (iii) the 
manufacturing history of the metal (e.g. whether the metal been cast, forged, wrought or 
welded), and (iv) the metal thermodynamic susceptibility to oxidation (position on the 
reduction-potential scale). It has been established that the most important factor responsible 
for immersed metal corrosion under conditions pertinent to natural waters (4.5 ≤ pH ≤ 9.5) is 
the electronic conductivity and the porosity of the oxide layer (23). This statement will be 
supported by a classical example. 
Aluminium (E0 = -1.71 V) is more susceptible to oxidation than iron (E0 = -0.44 V) but Al is 
known to be relatively inert to atmospheric and aqueous corrosion, whereas Fe is very 
corrosive. There are two main reasons for this. 
First, the oxidation of Al0 exclusively yields a non-conductive layer of Al2O3 whereas the 
oxidation of Fe0 may yield a conductive layer of FeII/FeIII species (e.g. Fe3O4, green rust) (24). 
Accordingly an oxide film on Fe0 may act as a semiconductor and mediate electron transfer 
from Fe0 (25-27). In this situation Fe0 corrosion continues despite the presence of the oxide 
film. It is evident, that this behaviour can not be observed under oxic conditions where FeII is 
instable and (at least) the outer layer of the oxide film will exclusively consist of non-
conductive FeIII oxides (FeOOH or Fe2O3). 
Second, the unit-cell in Al and Al2O3 are very similar to one another; thus the aluminium 
oxide can adhere tightly to the metallic aluminium beneath it (23). The oxidized surface 
provides a protective layer that prevents oxygen from getting to the underlying Al surface. In 
contrast, the packing dimensions of Fe0 and Fe oxides are not particularly close; thus there is 
no tendency for an iron oxide layer to adhere to metallic iron. Therefore, regarding its 
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protective properties 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
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0 for environmental remediation. 
Metallic iron for environmental remediation 
Fe-based alloys (Fe0 materials, mostly cast iron and steel) are certainly suitable for 
environmental remediation because of their low tendency to passivity due to the porosity and 
the instability of generated oxide layers. Instead of this trivial reason, Fe0 has been considered 
as a strong reducing agent for the reductive transformation of several species in natural waters 
(3-10). This consideration is not acceptable even from a pure thermodynamic perspective as 
the electrode potential of iron is almost the same (about -0.44 V) in low alloyed and stainless 
steels (Fe-based alloys). Accordingly, various Fe0 materials should have exhibited similar 
behaviour for the removal of the same species. This has not been the case as for example 
Miehr et al. (28) reported variation of rate constants for contaminant removal varying over up 
to four orders of magnitude due to differences in Fe0 “type”. 
Because Fe0 materials for environmental remediation are primarily susceptible to oxidation, 
complete passivation can only result from the transformation of initially non-protective films 
to impervious layers under specific environmental conditions. Therefore, a sake for an 
overview of factors likely to influence film formation and transformation (stability and 
breakdown) under environmental conditions should be undertaken. This could be a very 
difficult task because chemical breakdown occurs when the film is dissolved (e.g. by a 
chelating agent) or penetrated chemically (e.g. by a contaminant or Cl ions). 
Fundamental aspects of contaminant removal in Fe0/H2O systems 
While considering Fe0 as a reducing agent for contaminant reductive transformation it has 
been impossible to explain several experimental and field observations as recalled above (also 
see refs. 29 and 30). The main reason for this is that iron corrosion products (oxide film) have 
been regarded as simple coatings, mediating at most electron transfer from Fe0 to the 
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contaminant. However, the oxide film formation and transformation (recrystallization, 
dissolution, precipitation) is a dynamic process occurring in the presence of contaminants. 
Moreover, the oxide film formation can be regarded as the process of iron precipitation (31-
33). Here, iron precipitation occurs in the presence of small amounts of foreign species 
(including contaminants). These foreign species are necessarily sequestrated within the oxide 
film as discussed in the next section (34). In this manner contaminants are primarily removed 
from the aqueous phase by a non-specific mechanism as they are just sequestrated in the 
matrix of precipitating iron oxides (33). This process is widely used in water treatment by 
electrocoagulation using Fe electrodes (Fe
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0 EC) (35, 36) and explains why bacteria, viruses 
and thermodynamically non-reducible substances (e.g. Zn) have been quantitatively removed 
in Fe0/H2O systems. There are two main differences between passive Fe0/H2O systems and 
iron electrocoagulation: (i) Fe0 oxidation is electrically accelerated in Fe0 EC as the aim is to 
quantitatively produce iron hydroxides for contaminant removal by flocculation, and (ii) 
while contaminants are flocculated from the bulk solution in Fe0 EC, they are precipitated in 
the vicinity of Fe0 in passive Fe0/H2O systems. Accordingly a passive Fe0/H2O system could 
be regarded as a filter (working on the principle of size exclusion) in which species are 
additionally trapped by in situ generated iron hydroxides (and oxides).  
It should be explicitly stated that adsorption, co-precipitation and redox transformations are 
not each other exclusive as adsorbed or co-precipitated contaminants may be further (i) 
reduced by Fe0, adsorbed or soluble FeII species or (ii) oxidized by HO•/H2O2 species 
(Fenton-like reactions). It is however certain, that the extend of reduction is difficult to 
evaluate. This assertion is supported by the fact that to date, no carbon balances between 
reactants and supposedly reaction products have ever been successfully done for many 
chlorinated hydrocarbons (3). However, one should no care about the fate of co-precipitated 
contaminants as they will remain sequestrated so far iron oxides are not dissolved. 
Alternatively and complementary, the possibility of iron oxide dissolution and its 
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consequence for co-precipitated contaminants should be discussed at each relevant site. The 
next section will discuss on the suitability of the Fe
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0/H2O system for contaminant removal. 
Peculiarity of the Fe0/H2O system 
The singularity of the Fe0/H2O system is the in situ generation of soluble FeII species and their 
further transformations to crystalline iron oxides and hydroxides (Fe(OH)2, Fe(OH)3, Fe3O4, 
Fe2O3, FeOOH, Fe5HO8·4H2O). The various forms of iron oxides, oxyhydroxides, and 
hydroxides are called “iron oxides” through the end of this paper. Thus, in an aqueous 
solution, Fe0 ideally converts to crystalline iron oxides via a sequence of oxidation/ 
hydrolysis/precipitation/dehydration reactions (Tab. 1). The conversion of Fe0 to crystalline 
iron oxides goes towards several intermediate stages of amorphous and poorly crystalline 
precipitates, including green rust formation and transformation. Intermediate stages also 
include solid-state transformation of oxides (recrystallisation). For example, in aqueous 
solution crystalline Fe(OH)2 may convert to other iron oxides via 
oxidation/hydrolysis/dehydration.  
In essence, iron oxide formation involves two basic mechanisms: (i) direct precipitation from 
Fe2+/Fe3+-containing solutions, and (ii) transformation of an Fe oxide precursor. Both 
mechanisms may occur in natural Fe0/H2O systems, even though direct precipitation is 
dominant. Due to the diversity of iron corrosion products (CP), a common problem faced by 
studies on Fe0/H2O systems is the proper characterization of available iron oxides (37-39). A 
common procedure is the use of synthetic iron oxides to simulate 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, and 
adsorptive properties (Tab. 2). Accordingly, no synthetic iron oxides (or oxide mixtures) can 
rigorously simulate natural CP with regard to transformation occurring within. 
Transformations within Fe0/H2O systems 
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Synthetic iron oxides as simulates for natural iron oxides are prepared in the pure phase while 
natural oxides are precipitated and further transformed in the presence of foreign species 
(including contaminants). Thus a synthetic oxide can remove contaminant solely by 
adsorption while natural oxides incorporate contaminant in their structure while precipitating 
(adsorption and co-precipitation). At any moment after implementation of a Fe
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0 reactive wall, 
a natural Fe0/H2O system is made up of Fe0 and various iron oxides, possibly including 
transforming phases like FeO which are not stable under natural (sub)surface conditions. 
Given that the iron oxides are of various reactivity toward contaminant removal, the 
contribution of individual removal mechanisms to decontamination is difficult to access. 
However, it is the goal of this paper to demonstrate that, beside adsorption, contaminant 
sequestration (co-precipitation) is the sole certain removal mechanism. The occurrence and 
the extent of all other processes can be discussed on a contaminant-specific basis. Therefore 
adsorption and co-precipitation are the fundamental mechanisms of contaminant removal in 
Fe0/H2O system. To illustrate the transformations yielding to contaminant sequestration, the 
evolution of three iron atoms from the Fe0 material will be discussed. 
The three atoms (3 Fe0) will be first oxidized to 3 FeII species and may further be oxidized 
(e.g. by O2) to 3 FeIII species. Then they will be transformed to colloidal species partly having 
specific surface areas (SSA) higher than 500 m2/g (43, 44) before they aggregate and 
crystallize to one Fe3O4, 1.5 Fe2O3 or 3 Fe(OH)2, Fe(OH)3 or FeOOH. The relative variation 
of the volume of resulted crystalline oxides is represented in Fig. 1 using values from Tab. 2. 
Fig. 1 clearly shows volume expansion relative to 3 Fe0 for all iron oxides except magnetite 
for which a volume reduction of 30 % was noticed. However, it should be kept in mine that 
even magnetite is a final state of a transformation going through even more voluminous 
colloidal, amorphous and highly adsorptive species (SSA values in Tab. 2). Although volume 
expansion is discussed here on the basis of the volume of crystallized iron oxides, this process 
is a rule in the process iron corrosion, irrespective from the nature and the crystallinity of the 
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final products. Thus iron oxidative dissolution and iron oxide precipitation should by regarded 
a cycle of volume expansion/contraction in the course of which available contaminants are 
adsorbed and sequestrated. Sequestrated contaminants could be further transformed (oxidized 
or reduced). 
Conclusions 
This paper has contributed to open a new avenue for the scientific understanding of processes 
of contaminant removal in Fe0/H2O systems. Researchers and practitioners have long 
recognized the limits of the reductive transformation concept (11-14). However, the view that 
contaminants are primarily adsorbed and co-precipitated with iron corrosion products (29, 30, 
33) has partly faced with very sceptic views (45, 46) or is just degraded to an “alternative 
hypothesis to be considered” (47). Fortunately, sceptic views are based on the large 
acceptability of the concept of reductive transformation which was a consensus (4) and not of 
hard experimental facts. While former attempts to disprove the reductive transformation 
concept were based on extensive literature review (29, 30) and a munitions examination of the 
abundance of reactive species in Fe0/H2O systems (33), the present study is based on a simple 
analysis of processes occurring in a Fe0/H2O system. The major output is that Fe0 oxidative 
dissolution and iron oxide precipitation should by regarded as a cycle of volume 
expansion/contraction in the course of which chemical contaminants and pathogens are 
adsorbed and sequestrated. Because this argument does not care about the nature of a 
contaminant, it should be definitively clear that specific interactions are an exception and not 
the rule in a Fe0/H2O system (statement 1). Statement 1 is the cornerstone on which 
comprehensive knowledge on the behaviour of Fe0/H2O systems under natural conditions 
should be acquired. In this effort, it is almost impossible to transfer good results from the 
open aqueous iron corrosion. For example, corrosion inhibition of mild steel by methylene 
blue (MB) has been reported (48). However, corrosion inhibition is effective with 5.0 mM 
(1595 mg/L) MB in HCl at temperatures varying from 30 to 60 °C. The used temperatures are 
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not relevant for groundwater remediation and the needed concentration (1595 mg/L) is even 
non relevant for wastewater situations. Additionally the reaction took place in a strong acidic 
solution (2 M HCl). 
Acknowledgments 
The work was supported by the Deutsche Forschungsgemeinschaft (DFG-No 626 / 2-2). 
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Serving as an Electron Shuttle”. Environ. Sci. Technol., 43, 3966-3967. 
47. Gui, L., Jeen, S-W., Blowes, D.W., Gillham, R.W., Yang, Y., Reply to the Comment on 
“Reduction of Chromate by Granular Iron in the Presence of Dissolved CaCO3 ” by C. 
Noubactep, Appl. Geochem. (2009), doi: 10.1016/j.apgeochem.2009.08.006. 
48. Oguzie, E.E. (2005). Corrosion inhibition of mild steel in hydrochloric acid solution by 
methylene blue dye. Mater. Lett., 59, 1076-1079. 
 13
Table 1: Relevant reactions for the process of aqueous Fe0 dissolution, iron corrosion 
products formation and contaminant (Ox) removal in Fe
330 
331 
332 
333 
0/H2O system. Red is a 
reduced form of Ox. FeOOH is a proxy of corrosion products and Fex(OH)y(3x-y) is 
an iron hydroxide.  
Process Reaction Eq. 
Fe0 dissolution Fe0 ⇔ Fe2+ + 2 e- 1 
Fe0 passivation Fe0 + H2O ⇒ Fe(O)ads + 2 H+ + 2e- 2 
Fe0 depassivation Fe(O)ads + 2H+ ⇒ Fe2+ + H2O 3 
 Fe(O)ads + H2O ⇒ Fe(OH)2 4 
 Fe(O)ads + OH- ⇒ HFeO2- 5 
H2 evolution 2 H+ + 2e- ⇒ H2↑ 6 
O2 reduction O2 + 2 H2O + 4 e- ⇒  4 OH- 7 
Fe2+ oxidation Fe2+ ⇒  Fe3+ + e- 8 
 Fe2+ + 2 OH- ⇒  Fe(OH)2  9 
 Fe3+ + 3 OH- ⇒  Fe(OH)3 10 
Scale formation Fe(OH)2 ⇒   FeO + H2O 11 
 2 Fe(OH)3 ⇒   Fe2O3 + 3 H2O 12 
 4 Fe(OH)3 ⇒   Fe(OH)2 + Fe3O4 + 5 H2O + ½ O2 13 
 Fe(OH)3 ⇒   FeOOH + H2O 14 
Ox reduction Fe0 + Ox(aq) ⇒ Fe2+ + Red (s or aq) 15 
 FeII(aq) + Ox(aq) ⇒ FeIII + Red (s or aq) 16 
 FeII(s) + Ox(aq or aq) ⇒ FeIII + Red (s or aq) 17 
 H2 + Ox(aq or aq) ⇒ H+ + Red (s or aq) 18 
Ox adsorption FeOOH + Ox(aq) ⇔ FeOOH-Ox 19 
Ox co-precipitation Ox(aq) + n Fex(OH)y(3x-y) ⇔ Ox[Fex(OH)y(3x-y)]n 20 
334 
335 
 
 14
Table 2: Some relevant characteristics of metallic iron and its main corrosion products. SSA 
is the specific surface area. V
335 
336 
337 
338 
rust/VFe is theoretical ratio between the volume of 
expansive corrosion products and the volume of iron in the Fe0 material. Data from 
refs. 40-42. 
Species Formula Symmetry Density SSA Vrust/VFe
    (m2/g)  
Iron Fe bcc 7.86 <1 - 
Magnetite Fe3O4 Cubic 5.18 6.0 2.08 
Hematite α-Fe2O3 Rhombohedral 5.26 64 2.12 
Maghemite γ-Fe2O3 Cubic 4.69 30 n.a. 
Goethite α-FeOOH Orthorhombic 4.28 82 2.91 
Akageneite β-FeOOH Tetragonal 3.55 41 3.48 
Lepidocrocite γ-FeOOH Orthorhombic 4.09 221 3.03 
 Fe(OH)2 Trigonal 3.4 n.a. 3.75 
Bernalite Fe(OH)3 Orthorhombic 3.35 328 4.2 
339 
340 
341 
n.a. = not available. 
 
 15
341  
Fe Fe(OH)2 Fe(OH)3 Fe2O3 Fe3O4
0
1
2
3
4
1.0
0.7
1.1
4.2
3.8
vo
lu
m
e
Fe species
 342 
343 
344 
345 
346 
347 
348 
Figure 1: Relative volumes of iron and selected crystalline corrosion reaction products. The 
values is the bar represent the expansion coefficient. The calculations were made 
for three iron atoms and show that a volume compression will occur upon 
formation of Fe3O4. Strictly any crystallization goes through dissolution, 
nucleation and aggregation. Intermediate species are of high specific area and even 
more voluminous than crystalline Fe(OH)3. 
 16