Metallic iron for environmental remediation: ending a myth

Iron corrosion is widely known as a destructive process. However, several useful applications of this process are known. Three examples are: consumption of humidity in food packaging, H2S removal from biogas and water treatment. Water treatment by metallic iron (Fe0) relies on producing iron corrosion products (hydroxides/oxides) for contaminant adsorption, co-precipitation and adsorptive size-exclusion. The same process occurred during electrocoagulation where iron hydroxides/oxides production is sustained by an electric power. A popular example for using Fe0 for water treatment is known from the aquaculture where steel wool has been long routinely used for phosphate removal. Corroding Fe0 for generating iron hydroxides/oxides is thus an established tool for water treatment as these phases readily remove several aqueous chemical and biological contaminants. A relative new branch of science exists regarding Fe0 as a reducing agent for contaminant degradation under environmental conditions. This seemingly scientific knowledge is already disseminated in lexica (including wikipedia) and textbooks. The fallacy of this view is demonstrated herein.

Fig. 1, Overview of a Fe0/H2O system labelling key features relevant for contaminant removal. Corrosion begins at a location where Fe2+ is generated (anode). Fe2+ goes into the aqueous solution and two electrons, left behind migrate to another location (cathode) where they are taken up by H+ from water dissociation (H2O  H+ + OH-). The resulting hydroxide ions (OH-) react with the Fe2+ to initially form hydrous iron oxides (Fe(OH)2) that precipitate. Depending from the environmental conditions Fe(OH)2 is oxidized and transformed to various FeII/FeIII oxides that form the oxide scale. The dynamic process of Fe(OH)2 formation and transformation continues ideally until Fe0 is depleted.

Fig. 1, Overview of a Fe0/H2O system labelling key features relevant for contaminant removal. Corrosion begins at a location where Fe2+ is generated (anode). Fe2+ goes into the aqueous solution and two electrons, left behind migrate to another location (cathode) where they are taken up by H+ from water dissociation (H2O <=> H+ + OH-). The resulting hydroxide ions (OH) react with the Fe2+ to initially form hydrous iron oxides (Fe(OH)2) that precipitate. Depending from the environmental conditions Fe(OH)2 is oxidized and transformed to various FeII/FeIII oxides that form the oxide scale. The dynamic process of Fe(OH)2 formation and transformation continues ideally until Fe0 is depleted.

The Fe0 filtration technology for environmental remediation was born with the observation that aqueous chlorinated hydrocarbons (RCl) disappeared from Fe0-based canisters. This experimental observation was characterized as a chemical reduction (degradation) and corresponding reaction products were identified. No particular attention was paid to the molar ratio Fe0:RCl in presence. During the past 25 years, intensive efforts were undertaken to optimize the efficiency of Fe0-based filtration systems (Fe0 filters). However, related efforts were mostly based on the premise that electrons from the Fe0 body are transferred to dissolved species (direct reduction) despite the presence of an oxide scale (Figure 1). The universal oxide scale has been widely described as a shield protecting the Fe0 surface from ‘aggressive’ dissolved electron acceptors.

Table 1. Key reactions relevant for the process of contaminant removal in Fe0/H2O systems. Cu2+, O2 and RCl are examples of reducible species (oxidizing agents). Fe2+ reacts with hydroxide ions from water dissociation to form the oxide scale. Because this scale is typically electronic non conductive, quantitative electrochemical reduction of dissolved species (red-marked) is not possible. The sole likely quantitative electrochemical process is iron corrosion by water (H+) (blue-marked). Contaminant are removed within the oxide scale. Chemical reduction (degradation) is not a removal mechanism at ug/L-level as it is not quantitative. On the other hand, reduced species must be removed from the aqueous phase as well.

Table 1. Key reactions relevant for the process of contaminant removal in Fe0/H2O systems. Cu2+, O2 and RCl are examples of reducible species (oxidizing agents). Fe2+ reacts with hydroxide ions from water dissociation to form the oxide scale. Because this scale is typically electronic non conductive, quantitative electrochemical reduction of dissolved species (red-marked) is not possible. The sole likely quantitative electrochemical process is iron corrosion by water (H+) (blue-marked). Contaminant are removed within the oxide scale. Chemical reduction (degradation) is not a removal mechanism at ug/L-level as it is not quantitative. On the other hand, reduced species must be removed from the aqueous phase as well.

Table 1 summarizes the chemical reactions relevant for the discussion of the process of contaminant reduction and removal in Fe0/H2O systems. The most probable location of their occurrence is also specified. The fundamental reaction is the Fe0 oxidative dissolution (Eq. 1). Eq. 1 describes the anodic process in the electrochemical corrosion of Fe0. It is known, that this anodic reaction is facilitated by the presence of suitable electron acceptors (oxidizing agents). Oxidizing agents in Tab. 1 are: protons (H+ – Eq. 2), dissolved O2 (Eq. 3), Cu2+ ions (Eq. 4), and RCl (Eq. 5). The electrochemical cells corresponding to the four electron acceptors are given in Eq. 6 through Eq. 9. Of these only Fe0 corrosion by water (H+) is certain because the oxide scale is typically non conductive. This makes the oxide scale a conductive barrier for electrons and a physical barrier for dissolved species. In other words, aqueous contaminants are reduced in a Fe0/H2O system by a pure chemical mechanism according to Eq. 10 through Eq. 12. This conclusion alone ends the myth of reductive transformations by Fe0 (electrons from Fe0).

The myth of reductive transformation by Fe0 has attempted to challenge mainstream iron corrosion science for 25 years. It was questioned at the introduction and during its whole lifetime. The wished contaminant degradation, achieved in the presence of Fe0, was constantly presented as proofs of concept. From 2006 on, the mistaken view has been systematically dismantled by a series of arguments which are supported since 2011 by sound experimental data. To date, some five research groups have realized the fallacy of the ‘reductive transformation concept’ worldwide. The alternative concept has already established (i) the ion-selective nature of Fe0 filters and (ii) some tools for the proper design of Fe0 filters.

 

Chicgoua Noubactep

Angewandte Geologie, Universität Göttingen, Germany.

 

Publication

Metallic iron for environmental remediation: A review of reviews.
Noubactep C
Water Res. 2015 Nov 15

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