1. Introduction 

Metal corrosion is one of the most important problems in industry, transport and agriculture [1,2,3,4]. Understanding metal corrosion comprises its detection (e.g., analytical, visual), its monitoring (e.g., mass loss, H₂ evolution) and its long-term characterization under various field conditions [4,5]. The corrosion research aims at determining the durability of metallic structures under operational conditions (e.g., oil and gas pipelines, tanks) and revealing the mechanisms of corrosion process [5,6]. This mechanism can be chemical, electrochemical or mixed [1]. Various tools have been used to characterize the corrosion resistance of different metals under various application conditions [4]. The overall result is the availability of integrated approaches to assess and predict the corrosion processes and thus the longevity of metallic structures (e.g., buried pipes) [3,4]. However, the frequency and sudden nature of metallic pipe failures worldwide indicate the inadequacy of current knowledge related to the longevity of buried metallic pipes.

Metallic iron (Fe⁰) used as a reactive material in subsurface permeable reactive barriers is comparable to iron pipes with three major differences: (i) corrosion is welcome because it is a rather useful process [7,8,9], (ii) a reactive wall is ideally permanently water saturated, and (iii) the length of used particles (< 5 cm) is tiny compared to pipes which are up to 12 m in length. On the one hand, Fe⁰ specimens used in water treatment comprise steel wool with thickness varying between 25 and 90 μm [10,11]. On the other hand, the length of these particles is comparable to the wall thickness of iron pipes (2–4 mm). There has been no real system analysis for remediation Fe⁰ materials with the aim to outline the differences making their peculiar characteristics. In addition, traceably deriving the longevity of remediation Fe⁰ specimens from Fe⁰ pipes is impossible because of the differences highlighted.

The remediation Fe⁰ was termed as zero-valent iron (ZVI). This acronym is perhaps the first problem of this still innovative technology. A literature research with “zero-valent iron” as keyword would never reveal the ancient literature on Fe⁰ for water treatment [12,13,14,15,16]. Indeed, no research group until 2017 has referenced a single article from the ancient use of Fe⁰ in water treatment [17,18]. In chemistry, metallic elements are characterized by their oxidation state, the one of Fe⁰ is zero (0). Upon oxidation, Fe⁰ is transformed to Fe^II, Fe^III or Fe^IV species. Under environmental conditions, only Fe^II and Fe^III species are stable. The oxidation of Fe⁰ to Fe^II is a redox process characterized by an electrode potential whose value is –0.44 V [1]. According to the first principle of chemical thermodynamics, Fe⁰ can be oxidized by oxidizing agents from each redox couple having a higher electrode potential (E⁰ > –0.44 V). Water (H₂O or H⁺) is a relevant oxidizing agent for Fe⁰ under environmental conditions. The electrode potential for the redox couple H⁺/H₂ is 0.00 V. Fe⁰ immersed in (contaminated or polluted) water is corroded to form H/H₂ and Fe^II (and mixed Fe^II/Fe^III) species which are stand-alone reducing agents [19,20,21,22,23,24,25,26]. Clearly, it is not surprising that selected species undergo reductive transformations in an Fe⁰/H₂O system [19,26,27,28,29].

2. The Fe⁰/H₂O System

2.1. Overview of Fundamental Aspects

There is a transfer of electrons from the Fe⁰ body (solid state) to the Fe⁰/H₂O interface whenever a piece of a reactive Fe⁰ specimen is immersed in an aqueous solution (Fe⁰/H₂O system) [1,5,6,76]. This occurs because Fe⁰ is not stable under environmental conditions or because the redox couple H⁺/H₂ (E⁰ = 0.00) is higher than that of Fe^II/Fe⁰ (E⁰ = –0.44) in the electrochemical series [1,22,23,24,25,26].

2.2. Oxide Scale on Fe⁰ and the Decontamination Process

A look at the mechanism of oxide scale formation reveals that it cannot be electronically conductive. In fact, the initial scale is very porous and cannot transfer electrons because air and water are not electronically conductive (an aqueous solution can be ionic conductive—electrolyte). In subsequent stages, available pores are filled with nascent FeCPs, but they are never uniform and the oxide scale is a mixture of iron hydroxides and oxides [5]. An oxide scale made up of Fe₃O₄ alone would have been electronically conductive. However, such an Fe₃O₄ scale cannot exist under natural conditions (immersed Fe⁰). All other FeCPs are at best semi-conductors and cannot relay electrons from Fe⁰ under natural conditions. Clearly, reports justifying the reductive efficiency of Fe⁰/H₂O systems using the semi-conductive nature of FeCPs are mistaken [96]. This assertion encompasses the Fe⁰/pyrite/H₂O system, whose efficiency is mainly justified by the semi-conductive nature of FeS species [97,98,99].

The oxide scale on Fe⁰ is definitively a diffusion barrier for all dissolved species, including the pollutants. It is also the contaminant scavenger such that electrochemical corrosion of immersed Fe⁰ induces the generation of contaminant scavengers and other reducing agents. Thus the generation of solid FeCPs is a necessary process which has the (perceived negative) side effect of being expansive. Thus, designing an efficient and sustainable system requires answering the question: how long can FeCPs be generated to satisfactorily treat water while keeping a reasonable hydraulic conductivity (permeability)?

2.3. Chemical Aspects

Fe²⁺ from Equation 1 is transformed to ferrous hydroxide (Fe(OH)₂) and ferric hydroxide (Fe(OH)₃) which have a strong tendency to form colloids of particles that normally carry a positive charge [62,100,101]. These minerals are further transformed to other Fe^II/Fe^III minerals (e.g., Fe₂O₃, FeOOH, green rust) exhibiting different affinities to dissolved species. The nature of oxides in each individual system depends on the intrinsic reactivity of the used Fe⁰ material and the environmental conditions [5,79]. For example, two different Fe⁰ specimens corroding under the same environment will not necessarily produce the same iron oxides, because the composition of the oxide scale depends on the relative kinetics of Fe⁰ dissolution and Fe hydroxide precipitation, which in turn depends on the solution chemistry, including the pH value, dissolved ions and the salinity [5,6,102]. On the other hand, in-situ generated free Fe²⁺ are adsorbed to the surface of available minerals to form the so-called structural Fe^II with a reducing power far larger than that of the free Fe²⁺ (E⁰ < 0.77 V) and sometimes stronger than Fe⁰ (E⁰ < –0.44 V) [103]. The availability of several reducing agents in the Fe⁰/H₂O system, and especially from structural Fe^II, partly stronger than Fe⁰ implies that the electrochemical series of metal alone cannot predict the chemistry of the system.

2.4. Physical Aspects

The volumetric expansive nature of iron corrosion is the most important physical phenomenon occurring in Fe⁰/H₂O systems [104]. There is expansion because the parent metal (Fe⁰) produces in-situ both: (i) H₂, occupying a volume about 3100 times larger [26,105], and (ii) each solid oxide and hydroxide is at least twice larger in volume than Fe⁰ (V_oxide > V_iron) [106,107]. For example, the specific density of magnetite (Fe₃O₄) is about one half that of iron (Fe⁰). It implies that after corrosion a space twice larger than the initial space is occupied [108,109,110]. While it can be assumed that H₂ escapes from each open system, no free expansion of oxides in porous systems (e.g., water filters, reactive walls) can be assumed [26]. External or internal free expansion occurs in metallic pipes [111,112] and on the walls of steel canister for radioactive waste repositories [110,113]. On the contrary, free expansion cannot be expected in steel-reinforced concrete structures [108,109]. Accordingly, considering expansive iron corrosion, which culminates into permeability loss is an essential design parameter for porous Fe⁰/H₂O systems (Fe⁰ filters). For each Fe⁰ filter, the temporal production of both H₂ and oxide is decisive for the long-term efficiency and the permeability of the system [105,107,114,115].

2.5. Kinetic Aspects

Aqueous corrosion of Fe⁰ materials under environmental conditions (pH > 4.5) is an electrochemical process involving iron dissolution at the anode and H₂ evolution at the cathode (Equation 1). This electrochemical reaction is accompanied by the formation of an oxide scale on Fe⁰ which is not protective as a rule [6,116,117,118]. In general, oxide scale growth and its protectiveness depend primarily on the precipitation rate of iron hydroxides [5,102]. As the Fe⁰ surface corrodes under the initial scale, corrosion continuously undermines the scale. Voids are created and are progressively filled up by the ongoing hydroxide precipitation. The relative rate of (i) Fe⁰ oxidative dissolution and (ii) hydroxide precipitation in the Fe⁰ vicinity determine the protectiveness of the oxide scale. According to Nesic [5], when the rate of hydroxide precipitation exceeds the rate of Fe⁰ dissolution, a dense protective oxide scale is formed. Conversely, when Fe⁰ dissolution undermines the new oxide scale faster than hydroxide precipitation can fill in the voids, a porous and non-protective scale forms.

2.6. Investigating the Fe⁰/H₂O System

A Fe⁰/H₂O system is made up of two interfaces: (i) Fe⁰/oxide and (ii) oxide/H₂O. Because the oxide scale is never electronically conductive and is a diffusion barrier to many species, only water can quantitatively reach the Fe⁰ surface. The net result is that Fe⁰ oxidative dissolution is an electrochemical reaction (water is reduced) but all other observed/reported chemical reduction occur within the oxide film or at the oxide/H₂O interface [53,54,127]. For the Fe⁰/H₂O remediation system, it means that contaminants are not reduced by electrons from Fe⁰. This has important implications on the operating principles of Fe⁰/H₂O systems: first, using the electrochemical series to predict the reductive transformation of any species has been a mistake, and second, using the stoichiometry of any electrochemical reaction involving Fe⁰ is also a mistake. Accordingly, contaminants are ‘just’ dissolved species, capable of modifying: (i) the conductive properties of the electrolyte (H₂O), (ii) the ion conductivity of the oxide scale, and (iii) formation and the transformation of the oxide scale. Experiments pertaining to filtration systems must be performed under conditions enabling the formation of an oxide scale on the vicinity of Fe0 (e.g. quiescent batch experiments). For all other systems (e.g. fluidized systems), field dynamics (e.g. shaking intensity) must be reproduced during testing.

3. Selection and Characterization of Fe⁰ Materials

As discussed above, the scientific reason for using all Fe⁰ materials (e.g., granular iron and bimetallics, iron filings, iron nails, iron wire, nano-Fe⁰, scrap iron, steel wool) in water treatment is the electrode potential of the redox electrode Fe^II/Fe⁰: E⁰ = −0.44 V. Clearly, all reactive Fe⁰ materials have the same redox potential. The intrinsic reactivity of individual Fe⁰ specimens depends on a myriad of factors from which some are not readily accessible to the researcher. Relevant influencing factors include: alloying elements, Fe⁰ form, manufacturing processes, metallography, Fe⁰ grain size, surface area, and surface oxidation state. Accordingly, each Fe⁰ material has its own intrinsic reactivity which should be characterized in order to better understand how it is influenced by operational conditions to induce the intended remediation goal.

The long history of using Fe⁰ for technical chemical applications, including water treatment reveals that there has always been efforts to select appropriate materials for individual applications. For example, the porous “spongy iron” (sponge iron or direct reduced iron) was proven more suitable in filtration systems than dense materials [12,13,168]. Similarly, multi-metallic systems and nanoscale materials were recently developed to address (recalcitrant) contaminants that were less sensitive to treatment with granular materials [169]. For completeness, it should be stated that there is no Fe⁰ material for all situations such that one should rationally select appropriate materials for each specific application. For example, while treating water in fluidized systems (Anderson Process), dense materials were better than sponge iron [13,168]. Similarly, Hildebrant et al. [140] reported that Fe⁰ materials of low reactivity according to the EDTA test exhibited a better efficiency for fluoride removal. The question arises how to select the right Fe⁰ specimen for a given application? This calls for the development of standardized protocols for selection and characterization of Fe⁰ materials for various applications.

4. Conclution

The corrosion of iron in remediation Fe⁰/H₂O systems is an electrochemical process, coupling Fe⁰ oxidative dissolution to the reduction of water (protons) and to no other available oxidizing agent, including dissolved O₂. This is because the universal oxide scale on Fe⁰ acts as diffusion barrier to dissolved species and a conduction barrier to electrons from the metal body. In other words, water is the sole chemical which can remove electrons from the Fe⁰ surface. Fe⁰ oxidation and water reduction must not necessarily occur at the same locality. The spatial separation of oxidative (anodic) and reductive (cathodic) reactions is possible as the metal body allows the free flow of electrons from anodic to cathodic sites. The tendency of Fe⁰ to give off electrons (Equation 1) is the same for all Fe⁰-based materials (E⁰ = –0.44 V). This makes material selection and characterization critical in designing sustainable Fe⁰/H₂O systems.

This entry is adapted from the peer-reviewed paper 10.3390/w12030641