Metallic iron for water treatment: Healing a research community

Despite two decades of intensive research on using metallic iron (Fe) for environmental remediation and water treatment, basic concerns about their efficiency still prevail. This communication presents the basic idea of the view that challenges the prevailing paradigm on the operating mode of Fe/H2O systems. The alternative paradigm is in tune with the mainstream science on aqueous iron corrosion. Its large scale adoption will enable a scientifically based system design and increase the acceptance of this already proven efficient technology.


Introduction
The quest for an affordable (low-cost), applicable (low-maintenance) and efficient technology for water treatment has culminated in the development of metallic iron technology ('Fe 0 technology'). Fe 0 is used both in the subsurface (reactive barriers) and above-ground treatment plants (Noubactep, 2013). Fe 0 has been demonstrated efficient at several sites for the remediation of biological and chemical contamination. However, the operating mode of Fe 0 is still not known (Crane and Scott, 2012;Noubactep, 2012;Noubactep et al., 2012;Togue-Kamga et al., 2012). This frustrating situation is currently endangering the spreading of this proven efficient technology (Mueller et al., 2011;Ruhl et al., 2012a). This communication argues that the major problem of the Fe 0 technology is that scientists are working on an incorrect basis.

The original mistake
The Fe 0 technology was born with the premise that contaminants are reduced as fortuitously observed by Reynolds et al. (1990). From 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). Moreover, any likely argument was suggested to justify how electron transfer occurs despite the presence of a (non conductive) diffusion layer (Noubactep, 2011). Any critical view was systematically ignored as presented in details elsewhere (Noubactep, 2011). 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)  Gheju and Balcu, 2011).

The true nature of metallic iron
It is frustrating to notice that equations similar to Eq. 1 are still written to rationalize contaminant reductive transformation.
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 Fe 0 is permanently covered by an oxide scale (Stratmann and Müller, 1994;Cole and Marney 2012;Wang et al. 2013 Disregarding the relative affinity of species of concern to iron oxides, the question arises why 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 Fe 0 filtration systems (Caré et al., 2013). 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), has not been properly considered in discussing the decrease of the hydraulic conductivity of Fe 0 /H 2 O systems (Henderson et al., 2011;Ruhl et al., 2012;Jeen et al., 2013). The ongoing discussion considers H 2 and foreign precipitates including CaCO 3 and FeCO 3 (Henderson et al., 2011;Henderson et al., 2013;Jeen et al., 2013) but not properly hydroxides and oxides. Overseeing the importance of volumetric expansion has let to various explanations of the fact that a pure Fe 0 system is not sustainable. The best illustration is perhaps a recent paper by Ruhl et al. (2012b)

Reactivity and efficiency of iron materials
Another important point is that the term 'reactivity' is confusing through the 'Fe 0 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 Fe 0 can be assessed by the extent of H 2 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 Fe 0 material is mixed with an inert sand, its reactivity is not changed but the extent of its dissolution (e.g. coupled to H 2 evolution) is modified as sand can not contribute to H 2 generation nor to porosity loss. In other words, mixing sand and Fe 0 is a tool to sustain the efficiency of the system (not the reactivity of Fe 0 ). Many reported discrepancies can be attributed to the randomly interchanged use of 'reactivity' and 'efficiency' (Miyajima, 2012). To clarify this semantic issue it could be stated that the efficiency is the expression of the reactivity as impacted by operational conditions.

Conclusion
The Fe 0 research community is aware on the instability of the concept that contaminants are removed in Fe 0 /H 2 O systems by a reductive transformation ) but is still not really willing to test new ideas. The situation is comparable to that of a person facing a blockade and being aware on it. Furthermore, the person can not recognize the way out of the blockade. Whenever this is the case, a professional assistance is needed: a modern psychologist or a traditional heeler? It seems that a heeler should intervene to redirect the Fe 0 research community on the highway of iron corrosion.