Metallic iron for water treatment: A knowledge system challenges mainstream science 1 
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Chicgoua Noubactep
 
Angewandte Geologie, Universität Göttingen, Goldschmidtstraße 3, D - 37077 Göttingen, 
Germany. 
Kultur und Nachhaltige Entwicklung CDD e.V., Postfach 1502, D - 37005 Göttingen, 
Germany. 
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Tel.: +49 551 39 3191, Fax.: +49 551 399379, e-mail: cnoubac@gwdg.de
 
Abstract 
A knowledge system (KS) is a knowledge that is unique to a given group of persons. This 
form of knowledge may have a local or natural origin and is linked to the community that has 
produced it. On the contrary, the core of mainstream science (MS) is the desire to profoundly 
understand processes, through sequential studies such as hypothesis formulation, experiment 
and prediction. Thus, KS is communitarian and MS is universal. KS can be understood and 
rendered universal through MS. In general, a process discovery (know-how) may be intuitive, 
accidental, conjectural or inspirational but outcomes should be predictable and repeatable as 
soon as the know-why is achieved by MS. This paper argues that the technology of using 
metallic iron for water treatment has all the characteristics of a KS and that promoters of this 
technology have deliberately rejected scientific arguments leading to the know-why of the 
fortuitous discovery. Consequently, the technology has developed into an impasse where 
controversial discoveries are reported on all relevant aspects. It is concluded that the integrity 
of science in endangered by this communitarian behaviour. 
Keywords: False premise, Knowledge system, Mainstream science, Peer review, Zerovalent 
iron. 
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1 Introduction 27 
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The use of elemental iron (Fe0) for water treatment has attracted much attention thanks to its 
great potential for removing several classes of substances from the aqueous phase [1-9]. Fe0 
has been proven the most efficient material for subsurface reactive permeable barriers 
(reactive walls) [6, 10, 11]. Actually, there are about 180 Fe0 reactive walls installed 
worldwide [6]. The large majority of them been meeting their design goals. 
The use of Fe0 in reactive walls was discovered around 1990 as summarized in ref. [6]. 
Gillham and his colleagues [12] fortuitously found that trichloroethylene was reduced in a 
steel canister. This result was reproducible. Subsequent “laboratory testing and verification" 
experiments were performed [13-15]. To understand the importance of this discovery, one 
should consider that chlorinated organic compounds are more toxic and less biodegradable 
than their non-chlorinated reduction products. Accordingly, reducing chlorinated organic 
compounds is an ideal way to make them less toxic and more biodegradable. The discovery of 
Gillham et al. [12] coincided with the active search of appropriate materials for reactive walls 
after the concept for groundwater remediation presented by McMurty and Elton [16]. From 
1990 on, Fe0 has been tested at several scales and is now an established technology for water 
treatment (groundwater remediation, wastewater treatment, safe drinking water production) 
[4, 5, 7-9, 11]. However, despite observed efficiency, the Fe0 reactive wall technology as a 
whole is based on a false premise which should be clarified before the technology develops 
properly. 
The objective of this communication is to demonstrate that leading scientists or research 
groups working on “water treatment with Fe0” are too confident with the false premise that 
iron is a reducing agent for contaminant reductive transformations. Accordingly, each 
alternative concept is systematically disregarded, endangering the integrity of science and 
questioning the efficiency of the peer review system. For the sake of clarity, the popular state-
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of-the-art knowledge on the mechanism of contaminant removal in Fe0/H2O systems will first 
be presented. 
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2 Mechanism of contaminant removal 
Since the introduction of Fe0 bed filtration technology for groundwater remediation, several 
contaminants and contaminant groups have been reported to be removed by reductive 
transformations [17-19]. Clearly, contaminants were considered to be removed because of 
their chemical transformations possibly making them less harmful (reductive degradation) or 
less mobile (reductive precipitation). Accordingly, the case for which reduction products are 
toxic is still actively discussed [20]. Moreover, the formation of the universal oxide film on 
Fe0 (reactivity loss) and the pore filling by iron corrosion products (permeability loss) have 
been regarded as the major inhibitive factors for the process of contaminant removal [3, 21-
23].  
The formation of the universal oxide film at the surface of Fe0 is a characteristic of iron 
corrosion at pH > 4.5 [24, 25]. The film results from the precipitation of iron (hyrdr)oxides at 
the surface or in the vicinity of Fe0. In separating Fe0 from water, the formed porous film 
necessarily lowers the kinetic of iron oxidation. On the other hand, the permeability loss 
results from the expansive nature of iron corrosion. In fact, upon corrosion, the volume of 
formed iron oxide is 2.08 to 6.40 times larger than the volume of the parent metal (V0) in the 
lattice (Fe0) [26,27]. The volumetric expansion (ΔV - Eq. 1) corresponds to the extent of 
porosity loss. 
ΔV = (η - 1)*V0     (1) 
Where 2.08 ≤ η ≤ 6.40. 
The hitherto presentation shows that three major opened questions of the Fe0 technology are: 
(i) how can harmful reaction products be removed? (Question 1) (ii) how can reactivity loss 
be prevented? (Question 2) and how can permeability loss be properly considered? (Question 
3) Answering these three questions will accelerate technology development and could open 
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new perspectives. However, it is not likely that the popular state-of-the-art knowledge 
presented above will enable the adequate search of answers because it has neglected several 
key aspects. It should be emphasized that Question 2 and Question 3 have been recently 
theoretically addressed. It was shown that mixing Fe
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0 (< 52 vol-%) with inert or reactive but 
non expansive additive will sustain long term corrosion and prolong Fe0 bed’s lifespan [28, 
29] 
 3 Fundamental flaws of the current accepted mechanism 
The premise that aqueous contaminants are reduced by Fe0 is founded on the thermodynamic 
instability of Fe0 in aqueous solutions (immersed iron corrosion). Accordingly, all oxidized 
contaminants which electrode potential is larger than that of Fe0 (E0Fe2+/Fe0 = -0.44 V) should 
be reduced by Fe0. However, water (H2O or H+ - E0H+/H2 = 0.0 V) as solvent is also corrosive 
for Fe0 and is present in stoichiometric abundance. Accordingly, irrespective from the 
presence of any reducible species (including dissolved oxygen and contaminants), water will 
oxidize Fe0. This reaction can not be considered a side reaction as has been mistakenly done 
[17] (Mistake 1). Moreover, it is the oxidation of Fe0 by water that produces the oxide scale 
responsible for the reported reactivity loss. Accordingly, reactivity loss is an inherent property 
of Fe0 and should properly considered if Fe0 is to be used in aqueous solutions (at pH > 4.5). 
In this respect, enhancing the reactivity of Fe0 by reducing its particle size (μm and nm) for 
example will not avoid the formation of the universal oxide scale. The oxide scale is 
necessarily a diffusion barrier for all dissolved species as its shields the Fe0 surface [24, 25]. 
Note that the initial oxide scale is porous alloying retarded transport of water and dissolved 
species including FeII, H/H2 and eventually molecular O2. The initial porous film may develop 
to an impervious one which stops corrosion. The process of film formation is rigorously the 
same under anoxic and oxic conditions. The major difference being the kinetics. According to 
Cohen [25] the film forms 65 times slower under anoxic conditions. Given that adsorbed FeII 
and H/H2 are powerful reducing agents, they will more likely contribute to contaminant 
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reduction (if applicable) than the surface of Fe0 [30]. Accordingly, even when contaminant 
reduction is quantitative, effective reducing agents are likely primary iron corrosion products 
(indirect reduction by Fe
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II and H/H2) (Mistake 2). 
The paramount mistake (Mistake 3) has been to confound “contaminant removal” and 
“contaminant reduction”. The goal of water treatment is contaminant removal and it is very 
difficult to say whether the reduction of a species has been achieved by electrons from Fe0, 
FeII or H/H2 (direct or indirect reduction). It is clear that a reduced contaminant should be 
removed from water to obtain clean water. The five major contaminant removal mechanisms 
in Fe0 beds are: adsorption, co-precipitation, precipitation, size-exclusion and volatilization. 
Biological and chemical transformations could influence the removal capability of a 
substance but are not removal mechanisms. Accordingly, Question 1 (how can harmful 
reaction products be removed? – Section 2) should be revisited because contaminants and 
transformation products should be removed from the aqueous phase. Moreover, contaminant 
transformation products have been removed from the aqueous phase, unless the technology 
would have not been efficient. The problem is that “contaminant removal” and “contaminant 
reduction” were randomly interchanged. 
The hitherto presentation has unmasked the popular state-of-the-art knowledge on the process 
of aqueous contaminant removal by Fe0 as non accurate. This false premise is well-accepted 
and repeated in the introduction of many articles and books. The question arises why the 
technology is efficient. 
4 The real mechanism of contaminant removal in Fe0/H2O systems 
The real mechanism of aqueous contaminant removal in the presence of Fe0 (e.g. Fe0/H2O 
systems) can be derived from the pH dependence of the solubility of iron (FeII, FeIII) in 
aqueous solutions (Fig. 1) [31-33]. In other words, Fe0 can be regarded as generator for Fe 
species and the behaviour of these species used to discuss the efficiency of contaminant 
removal. In this case, the major question for Fe0/H2O sustainability is how to maintain the 
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corrosion process? (Question 4). Answering Question 4 is over the scope of this 
communication. However, is should be noticed that the continuation of the corrosion process 
depends on the porosity of the initial oxide scale on Fe
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0 [34]. In other words, answering 
Question 4 will consist in investigating the effects of operational conditions on the porosity of 
the oxide scale in long-term experiments. In such experiments, the oxide scale should allow to 
form and transform on the surface of Fe0 (no or slow agitation, shaking or stirring). 
Figure 1 shows clearly, that for pH > 5 the solubility of Fe under oxic conditions is smaller 
than 10–5 M (0.56 mg). Under anoxic conditions, FeII has a relatively high solubility. The 
saturation concentration of FeII species approaches 0.5 M (28 g/L) at room temperature. 
However, dissolved Fe concentrations drop below 10-12 M (5.6*10-11 g/L) if FeII is 
polymerized e.g. [Fe(OH)2]n or oxidized to FeIII species [35]. This reaction results in the 
precipitation of Fe phases. FeIII species can also form under anoxic conditions, for example, 
when water contents oxidizing species like NO3-, MnO2 or MnOOH. FeII oxidation and Fe 
phase formation is often catalyzed by microorganisms. FeIII phases which form from solution 
begin as small clusters that evolve into larger polymeric units with time, eventually reaching 
colloidal sizes [36]. Contaminants are necessarily enmeshed in the matrix of corrosion Fe 
phases and/or adsorbed at their surface [37, 38]. Adsorbed or enmeshed contaminants can be 
further reduced by diffusing FeII and H/H2. Reduced contaminants will be 
adsorbed/enmeshed. That is the way contaminants have been successfully removed in 
Fe0/H2O systems [39, 40]. This statement is supported by results of some few authors (e.g. 
ref. [41-45], but the validity of the reductive transformation concept has never been 
systematically questioned. For example, Furukawa et al. [43] stated that “the ubiquitous 
presence of ferrihydrite suggests that the use of Fe0-PRBs may be extended to applications 
that require contaminant adsorption rather than, or in addition to, redox-promoted 
contaminant degradation”. Jia et al. [44] used Fe0 as adsorbent generator of the removal of 
non-reducible organic species (triazoles). The new concept stipulates that adsorption and co-
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precipitation are the fundamental mechanism of contaminant removal, regardless of the redox 
properties. 
It is very important to note that contaminants are not removed by Fe0 or individual Fe phases 
but by the whole process of aqueous iron corrosion in a porous system (e.g. bed, wall) [39, 
40]. Although this plausible explanation is compatible with the open literature on the 
interaction of iron oxides with contaminants in the hydrosphere as recently demonstrated [7, 
40], some leading scientists [46-48] are continuing to defend the false premise with 
supposedly scientific arguments [49, 50]. This attitude has degraded the research community 
on Fe0 to a sort of modern communitarian knowledge system. 
5 A modern knowledge system 
Currently the expression knowledge system is used in the scientific literature to describe 
efficient knowledge that has been used by small communities in the third world and by 
indigenous peoples in North America and Australia [51-55]. According to Mapara [55], 
indigenous knowledge systems (IKS) are bodies of knowledge of the indigenous people of 
particular geographical areas that have survived on for a very long time. They are knowledge 
forms that have failed to die despite colonial onslaught and scholar arrogance [55]. In social 
science, it is commonplace to scientifically investigate a IKS in order to extent its 
applicability [55, 56]. Sometimes, despite proven amelioration of IKS by mainstream science, 
indigenous people are not ready to modify their original knowledge unless its holistic socio-
cultural and spiritual dimensions are conserved. 
Presently, the same trend is observed in using Fe0 for water treatment as demonstrated above. 
Whatever modern experimental tools have demonstrated to sustain the false premise, the 
objective of investigations were falsified and/or good results were misinterpreted. There is no 
need to multiply examples. Interested readers are referred to two recent articles [57, 58]. The 
sole example will be that of efforts to establish a mass balance to demonstrate the efficiency 
of Fe0 to reduce contaminants [59]. In real systems contaminants are removed within the 
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oxide scale on Fe0. It is not likely that water will release enmeshed contaminants because 
water can not dissolve iron oxides [60]. Accordingly, researchers reporting on mass balance 
without dissolving corrosion products have necessarily done something wrong like 
mechanically stirring the solution [59].  
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The fact that many researchers are confident to this modern knowledge system is related by 
the huge number of peer review articles, that have been published on this topic during the last 
17 years (from 1994 on). Peer review is the process that requires experts in a given field to 
evaluate an author’s work and ideas in that same field, usually for the purpose of publishing a 
paper or awarding a grant [61]. Considering this definition, it is fair to say that experts in the 
field of water treatment with Fe0 have built a “modern knowledge system” which is now 
challenging mainstream science. It is superfluous to recall that good manuscripts and 
proposals have been rejected, the curse of their authors being to think outside of the box. 
6 Concluding remarks 
The present communication has recalled the danger of the expert feedback system when peer 
reviewers are not competent. The situation is exacerbated when a college of experts has been 
built around a false premise for years (yet two decades) [62]. During this time at least four 
generations of PhD students have spent their youth working on an idea defying mainstream 
science. This harsh criticism is not to make colleagues look bad, but rather to question the 
efficiency of the interdisciplinary approach. Iron corrosion is an old research field, where 
chemists, electrochemists, physicists have controversially discussed for decades [34, 63-65]. 
It is not likely that researchers without good chemical background will understand the 
important features of iron corrosion without guidance. 
The process of discovery may be intuitive, accidental, conjectural or inspirational but 
outcomes should be developed to predictable and repeatable issues. Despite 20 years of 
intensive research, no repeatable or predictable results have been achieved while using 
metallic iron for water treatment [57, 58]. Accordingly the whole achieved results is highly 
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qualitative. Furthermore, all existing models for the process of contaminant removal in 
Fe
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0/H2O systems are faulty as none up to date properly considers the fundamental process of 
co-precipitation. To exploit the huge potential of Fe0 for environmental remediation and safe 
drinking water provision, a concerted research scientific effort is needed [39, 67-70]. The first 
step to this great future will be to abandon false premises and develop the actual 
communitarian knowledge system to a real scientific community. 
Acknowledgments 
The original manuscript was improved by the insightful comments from Daniel Fotio 
(University of Maroua – Cameroon). The manuscript was further improved by the insightful 
comments of anonymous reviewers from Fresenius Environmental Bulletin. 
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 15
Fig. 1: Solubility data of FeII in 0.1 M NaCl (25 °C) and FeIII in 0.01 M NaCl (25 °C) as a 
function of pH. Data for Fe
380 
381 
382 
II are from ref. [33] and data for FeIII from ref. [31]. 
 
2 4 6 8 10 12
-10
-8
-6
-4
-2
0
[Fe] = 10-5 M
 FeII solubility
 FeIII solubility
lo
g 
[F
e]
pH value
 383 
384  
 16