The authors have declared that no competing interests exist.
Despite two decades of intensive research on using metallic iron (Fe0) 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 Fe0/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.
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 (‘Fe0 technology‘). Fe0 is used both in the subsurface (reactive barriers) and above-ground treatment plants (Noubactep, 2013).
The Fe0 technology was born with the premise that contaminants are reduced as fortuitously observed by Reynolds et al. (1990).
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, 1994
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 FeII 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).
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).
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The Fe0 research community is aware on the instability of the concept that contaminants are removed in Fe0/H2O systems by a reductive transformation (Liu et al. 2013)