The Fe⁰ technology was born with the premise that contaminants are reduced as fortuitously observed by Reynolds et al. (1990). ²¹From that time on, efforts were directed at identifying the reduction mechanism and the impact of relevant operational parameters thereon (Matheson and Tratnyek, 1994; Ruhl et al., 2012b)^{12, 23} Moreover, any likely argument was suggested to justify how electron transfer occurs despite the presence of a (non conductive) diffusion layer (Noubactep, 2011).¹⁵ Any critical view was systematically ignored as presented in details elsewhere (Noubactep, 2011).¹⁵ This attitude has not changed despite the presentation of an alternative concept rationalizing the removal of non reducible species and microbial contamination. As an example, Chen et al. (2012) ³maintained that “although there are other mechanisms that likely contribute to organic contaminant removal by Fe⁰ (3 references including Noubactep (2011)),¹⁵ there is substantial evidence from multiple investigators that the abiotic removal of TCE by Fe⁰ largely follows the -elimination pathway”. It is interesting to point out that Noubactep (2011) ¹⁵is entitled “Aqueous contaminant removal by metallic iron: is the paradigm shifting?” Actually, what is the relevance of the ‘substantial evidence from multiple investigators’ if the paradigm is shifting? This example alone evidences that some working researchers on Fe⁰ technology are not willing to test new ideas. Fortunately, some other researchers have positively tested the new concept (e.g. Ghauch et al., 2011; ⁶Gheju and Balcu, 2011).⁷

It is frustrating to notice that equations similar to Eq. 1 are still written to rationalize contaminant reductive transformation.

Fe⁰ + RX + H⁺ Fe²⁺ + RH + X^-(1)

Where RX is a reducible alkyl halide and RH its reduced form. RH is less toxic than RX as a rule. RH is more biodegradable.

From the open literature on iron corrosion however, it is known that Fe⁰ is permanently covered by an oxide scale (Stratmann and Müller, 1994²⁴; Cole and Marney 2012⁴; Wang et al. 2013)²⁶. Even dissolved oxygen (O₂) can not quantitatively reach the iron surface such that iron is essentially corrodes by water (H⁺, Eq. 2) and O₂ reduced by Fe²⁺ (Eq. 3) (Stratmann and Müller, 1994).²⁴

Fe⁰ + 2 H⁺ Fe²⁺ + H₂(2)

Fe²⁺ + 1/4 O₂ + H⁺ Fe³⁺ + 1/2 H₂O(3)

Disregarding the relative affinity of species of concern to iron oxides, the question arises why a RX, that is necessarily larger in size than O₂ should diffuse through the oxide film. This question suggests that equations like Eq. 4 should be routinely used to model processes in Fe⁰/H₂O systems.

2 Fe²⁺ + RX + H⁺ 2 Fe³⁺ + RH + X^-(4)

Next to Fe^II species as relevant reducing agent, future research should properly consider the volumetric expansive nature of iron oxidation in discussing the evolution of the porosity of Fe⁰ filtration systems (Caré et al., 2013).² It may be difficult to admit, that the volumetric expansive nature of metal corrosion, that was presented 90 years ago (Pilling and Bedworth, 1923; Caré et al., 2008)^{20, 1}, has not been properly considered in discussing the decrease of the hydraulic conductivity of Fe⁰/H₂O systems (Henderson et al., 2011⁸; Ruhl et al., 2012; ²²Jeen et al., 2013)¹⁰. The ongoing discussion considers H₂ and foreign precipitates including CaCO₃ and FeCO₃ (Henderson et al., 2011;⁸ Henderson et al., 2013⁹; Jeen et al., 2013¹⁰) but not properly hydroxides and oxides. Overseeing the importance of volumetric expansion has let to various explanations of the fact that a pure Fe⁰ system is not sustainable. The best illustration is perhaps a recent paper by Ruhl et al. (2012b)²³ evaluating the suitability of admixing Fe⁰ with anthracite, gravel, pumice and sand in fixed bed filters for TCE removal. The authors concluded that none of the four dual systems was applicable for the remediation of tested groundwater. This conclusion disregards the historical work of O’Hannesin and Gillham (1998)¹⁹ at Borden, Ontario (Canada) which can be regarded as the cornerstone on which the ‘Fe⁰ technology’ is built. O’Hannesin and Gillham (1998)19 demonstrated the efficiency of a 22:78 Fe⁰:sand weight ratio for the removal of TCE (and PCE). The reactive wall in Borden (Canada) was the first full-scale Fe⁰ reactive barrier.

Another important point is that the term ‘reactivity’ is confusing through the ‘Fe⁰ technology’ literature. Reactivity is per definition an intrinsic, invariable characteristic, a trend that can not be strictly quantified but can be assessed by standard protocols (if available). For example, the intrinsic reactivity of Fe⁰ can be assessed by the extent of H₂ evolution under controlled conditions. It is essential to notice that the reactivity of a material does not depend on its amount or its proportion in a mixture. Accordingly, if a Fe⁰ material is mixed with an inert sand, its reactivity is not changed but the extent of its dissolution (e.g. coupled to H₂ evolution) is modified as sand can not contribute to H₂ generation nor to porosity loss. In other words, mixing sand and Fe⁰ is a tool to sustain the efficiency of the system (not the reactivity of Fe⁰). Many reported discrepancies can be attributed to the randomly interchanged use of ‘reactivity’ and ‘efficiency’ (Miyajima, 2012).¹³ To clarify this semantic issue it could be stated that the efficiency is the expression of the reactivity as impacted by operational conditions.

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