Metallic iron (Fe⁰), also termed as zero-valent iron (ZVI) is widely considered as a cost-effective reducing agent for organic pollutants in groundwater (Henderson and Demond 2007; Guan et al., 2015; Cao et al., 2020; He et al., 2020). The Fe0-based permeable reactive barrier (PRB) technology for groundwater remediation is rooted on this premise (Gillham 2008; Chen et al., 2019; Xiao et al., 2020a; Xiao et al., 2020b; He et al., 2020; Wang et al., 2022). Fe⁰ has also been successfully used for the removal of various inorganic contaminants (e.g. As, NO₃₋) and pathogens (e.g., bacteria, viruses) from polluted waters (Richardson and Nicklow 2002; Henderson and Demond 2007; Gheju 2011; Guan et al., 2015; Cao et al., 2020; Kim et al., 2021; Noubactep, 2021). However, these applications are mainly perceived to be derived from the Fe⁰ PRB technology for organic pollutants (Obiri-Nyarko et al., 2014; Naseri et al., 2017). The concept that Fe⁰ is an electron donor under environmental conditions has never been experimentally established (Warren et al., 1995; Farrell et al., 2001; Lavine et al., 2001; Jiao et al., 2009; Naseri et al., 2017; Cao et al., 2020; Hu et al., 2020; Noubactep, 2022). For example, while investigating the reductive dechlorination of carbon tetrachloride (CT) in Fe⁰/H₂O systems, Jiao et al. (2009) clearly demonstrated that reducing electrons are not from Fe⁰, although iron corrosion was helpful for CT reductive dechlorination. Their conclusions read as: “The inherent relationship between the dechlorination of CT and the corrosion of iron is attributed to the fact that the adsorbed hydrogen atoms produced during the iron corrosion process are necessary for the dechlorination process of CT.” Table 1 summarizes some key arguments presented in the broad scientific literature prior to the advent of the recent Fe⁰ remediation technology, and disproving the reductive transformation concept.

The idea that organic pollutants are reductively transformed by Fe⁰ was introduced in the scientific literature by Reynolds et al. (1990). Scientists from the University of Waterloo (Canada) were investigating the potential for sampling bias caused by sorption of chlorinated organic species to materials commonly used in groundwater sampling (Lee et al., 2004; Gillham 2008; Cao et al., 2020). Their results revealed losses of chlorinated organic contaminants from water samples in contact with Fe⁰-based vessels. Hence, reductive dechlorination was proposed as the most likely reaction path (Reynolds et al., 1990; Gillham 2008). This observation coincided with a period when geochemists were looking for suitable materials for the realization of the concept of groundwater remediation using PRBs as introduced in the 1980s by McMurty and Elton (1985). In other words, Fe⁰ was considered a reducing agent (or an electron donor) for organic pollutants, because their reductive transformation was observed in its presence, in the Fe⁰/H₂O system. This coincidence was misinterpreted as a scientific fact and still prevails (Cao et al., 2020; Thakur et al., 2020; Noubactep, 2022). Investigations by Matheson and Tratnyek (1994) and Weber (1996) have been reported to confirm these observations. Moreover, it was claimed that the observation that organic pollutants can be reduced in Fe0/H2O systems was novel (Matheson and Tratnyek, 1994; Gillham 2008). Unfortunately, the then available seminal works of Khudenko (1985), Khudenko (1987), and Khudenko (1991) frontally contradict the claimed novelty (Cao et al., 2020). In particular, in the paper entitled, “Feasibility evaluation of a novel method for destruction of organics” (Khudenko, 1991), Boris Michael Khudenko demonstrated that Cu²⁺ cementation by Fe⁰ can be used to induce the reductive degradation of organic pollutants (Table 2). Clearly, Fe⁰ is oxidized by Cu²⁺ and reaction products (Fe^II and H/H₂ species) act as reducing agents for the (organic) contaminants of concern (COCs). In other words, Fe^II species resulting from iron corrosion are used for the “destruction of organics.” Factually, H₂ and H species also resulting from iron corrosion are reducing agents as demonstrated by Jiao et al. (2009).

Table 2 summarizes the experimental conditions and results of Khudenko (1991) with regard to the Fe⁰/H₂O system. In Khudenko (1991)’s work, an aqueous solution of Direct Yellow 12 was acidified to an initial pH value of 4.5 using H2SO4. Two parallel experiments were performed differing in the addition or non-addition of 100 mg L-1 Cu²⁺ (CuSO₄). Used Fe⁰ was a 1 m long iron wire of 0.2 mm in diameter. The filtrate was stirred by a magnetic stirrer. Results demonstrated that with Cu²⁺ addition, complete discoloration was achieved within 2.5 min, while the system without Cu²⁺ could not be discolored at all after 60 min of stirring. More Fe⁰ was consumed in the absence of Cu²⁺ than with Cu²⁺. The second experiment was conducted with a wastewater from finishing operations of a textile mill. The wastewater had a rosy color, and an initial pH value of 8.0. Addition of Cu²⁺ alone changed the color to dark blue suggesting formation of Cu²⁺ complex. Cementation with Fe⁰ and Cu²⁺ resulted in 98.5% color removal, while the process without copper resulted in just 10% color removal (Table 2).

It is surprising that 3 years after the brilliant concept of Khudenko (1991) and its validation using both synthetic and real wastewater, Matheson and Tratnyek (1994) introduced the contradiction without proving Khudenko (1991) wrong. In fact, Matheson and Tratnyek (1994) did not even consider that earlier work by Khudenko (1991). Bigg and Judd, 2000 were the first scientists to cite Khudenko (1991), 2 years after the report that the concept that, Fe⁰ is a reducing agent was a “broad consensus” (O’Hannesin and Gillham, 1998). It is also surprising that the view of Matheson and Tratnyek (1994) was claimed to be experimentally validated (Roberts et al., 1996; Weber, 1996) and is still favored by the majority of active researchers on the remediation Fe⁰/H₂O system (Naseri et al., 2017; Xiao et al., 2020a; Xiao et al., 2020b; Cao et al., 2020; Hu et al., 2020; Hu et al., 2021; Noubactep, 2022). Accordingly, the whole mechanistic discussion is based on the idea that there is some electron transfer from the Fe⁰ bulk material to COCs, which are potentially transformed into non-toxic or less toxic species (He et al., 2020). The extent of this reaction was conventionally evaluated using the reaction rate constant k (kobs or kSA) (Johnson et al., 1996; McGeough et al., 2007). However, according to Liu et al. (2013) this is inappropriate because the extent of iron corrosion and the proportion of electrons used for the transformation of COCs should be considered to assess the economics of the system. In other words, the goal was to avoid superfluous Fe⁰ dosages which impede the economics of the designed systems (Wu et al., 2014; Shufen et al., 2018, He et al., 2020). During the past 8 years, an important number of papers has been published on the suitability of the electron efficiency concept (EE concept). He et al. (2020) give an excellent overview on the topic, and, interested readers are referred to this very recent review article.

To this point, the presentation has highlighted that the EE concept is intrinsically wrong, because Fe⁰ does not play any significant role in the process of contaminant reductive transformation in Fe⁰/H₂O systems (Whitney, 1903; Jiao et al., 2009; Noubactep, 2022). In fact, while Jiao et al. (2009) have proven Matheson and Tratnyek (1994) wrong, Whitney (1903) had already established that under environmental conditions, Fe⁰ is oxidized only by protons (H+), even in the presence of dissolved oxygen (O₂) and carbonic acid (H₂CO₃) (Table 1). The present communication aims at demonstrating the fallacy of the EE concept in order to avoid its further propagation. The presentation starts with chemistry of the Fe⁰/H₂O system, followed by a historical overview on water treatment using Fe⁰, and ends with a critical evaluation of the usefulness of the EE concept.

Aqueous iron corrosion is an electrochemical process which needs four compartments to occur: an anode, a cathode, a conductor and an electrolyte (Landolt, 2007; Groysman, 2010). In the remediation Fe⁰/H₂O system, the conductor is the metal body (Fe⁰), the electrolyte is the polluted water, the anode is an area of the Fe⁰ surface where oxidative dissolution occurs (releasing ferrous iron–Fe²⁺), and the cathode is an area of the Fe⁰ surface where electrons left behind by Fe²⁺ are transferred to a reducible species. A key feature of this process is that, the reactions at the anode and the cathode occur simultaneously, and the prerequisite is that the electrolyte must be in contact with both the anode and the cathode (Landolt, 2007; Noubactep, 2014; Noubactep, 2016).

Aqueous iron corrosion proceeds as follows: i) Fe⁰ is oxidatively dissolved at the anode to release Fe²⁺, ii) the generated Fe²⁺ ions migrate in the polluted water (electrolyte), and iii) then electrons left behind by Fe²⁺ are transferred through the metal body (conductor) to a reducible species at the cathode. It is crucial to underline that electrons are transported from the anode to the cathode by Fe⁰. Electron transfer to any adsorbed species only occurs if there is no conduction barrier at the Fe⁰ surface (Landolt, 2007; Nesic, 2007; Lazzari, 2008; Groysman, 2010). It is well-known that, at pH > 4.5, an oxide scale forms on the Fe⁰ surface and shields it from dissolved species, including dissolved O₂ (Stratmann and Mü ller, 1994; Lazzari, 2008). For the EE concept to be applicable, it means that the universal oxide scale on Fe⁰ should be electronically conductive, which is not the case in Fe⁰/H₂O systems (Noubactep 2007; Noubactep, 2008; Noubactep, 2014; Noubactep, 2016). Figure 1 summarized the pathways of contaminant removal and transformations in Fe⁰/H₂O systems.

The electrochemical reaction for aqueous iron corrosion is depicted in Eq. 1: F e 0 + 2 H + ⇒ F e 2+ + H 2 (1) F e 2+ + 2 O H - ⇒ Fe ( OH ) 2 (2)

Eq. 1 shows that iron corrosion consumes protons, thereby increasing the pH value. This means that adding protons (acidification) is a powerful tool to intensify iron corrosion where it is needed, for example for H₂ evolution (Ndé -Tchoup é et al., 2020). However, for environmental remediation, a pH shift to lower values is not typically envisaged such that a pH increase occurs as a rule (Lipczynska-Kochany et al., 1994; Matheson and Tratnyek, 1994; Schreier and Reinhard, 1994; Hu et al., 2021). This pH increase favours the formation of ferrous hydroxides [Fe(OH)₂], which polymerize and precipitate at the surface of Fe⁰ or in its vicinity (Eq. 2). When dissolved oxygen (O₂) is present, ferric hydroxides [Fe(OH)₃] are formed as well. In the real world, whether the conditions are anoxic or oxic, iron corrosion generates an oxide scale which permanently shields its surface and is made up of several oxides and hydroxides (Odziemkowski and Simpraga, 2004). The process of oxide scale formation and transformation is a dynamic one (Odziemkowski et al., 1998; Sikora and Macdonald, 2000; Nesic, 2007; Lazzari, 2008; Groysman, 2010). It is certain that no electronically conductive oxide scale can be formed at the Fe⁰ surface under environmental conditions (Nesic, 2007; Lazzari, 2008). In other words, the oxide scale, acting as diffusion barrier for contaminants and dissolved O₂, also represents a conductive barrier for electrons from Fe⁰. For this reason, electrons from the metal body cannot (quantitatively) reduce any initially dissolved species (contaminants and O₂) (Figure 1) (Stratmann and Mü ller, 1994; Noubactep 2007; Noubactep, 2008; Jiao et al., 2009; Noubactep, 2013; Noubactep, 2015; Noubactep, 2019; Hu et al., 2020). Clearly, the EE concept is built on a thinking mistake. Accordingly, scientists propagating this concept are justifying their own mistakes by citing past mistakes. The EE concept would have been valid to some extent, if the oxide scale was not present. That is under acidic conditions (pH < 4.5) which is not the pH range of environmental remediation (Gillham 2008; Ghauch, 2015). The barrier nature of the oxide film implies that all reductive transformations are mediated by corrosion products (e.g., Fe^II, H/H₂, Fe₃O₄, green rust). However, contaminant removal is mediated by adsorption onto and co-precipitation with solid iron corrosion products (FeCPs). In fixed beds, size-exclusion is the other relevant removal mechanism (Noubactep, 2007, Noubactep, 2008).

The Fe⁰ research community is reminded of a famous quote by Mahatma Gandhi “An error does not become truth by reason of multiplied propagation, nor does truth become error because nobody sees it.” Contextualizing this to the Fe⁰ literature, the highlighted mistake (error) has been propagated since the time the work of Khudenko (1991) was ignored. Despite several efforts by our group pointing out this conceptual mistake (Noubactep 2007, Noubactep, 2008; Noubactep, 2010a; Noubactep, 2010b; Noubactep, 2014; Noubactep, 2016), the truth has been ignored for 1 decade already (Xiao et al., 2020a; Xiao et al., 2020b; Cao et al., 2020; Cao et al., 2021a; Cao et al., 2021b; Cao et al., 2021c; Cao et al., 2021d; Hu et al., 2021; Noubactep, 2022). However, an accurate fundamental understanding of processes governing contaminant removal is critical in the design and operation of Fe⁰-based systems. This is particularly important given that Fe⁰ based remediation systems have wide practical applications. These applications have been discussed in earlier papers (Naseri et al., 2017; Antia, 2020, Huang et al., 2021a), thus, a detailed review is beyond the scope of the present paper. In summary, typical applications of Fe0-based remediation systems documented in literature include: i) decentralized safe drinking water provision in low-income settings (Huang et al., 2021b; Mueller et al., 2021), ii) industrial wastewater treatment systems (Li et al., 2019; Kulkarni et al., 2020), iii) recovery of heavy metals from industrial effluents (Vollprecht et al., 2018; Calabrò et al., 2021; Noubactep, 2021), iv) urban stormwater treatment (Rahman et al., 2013; Tian et al., 2019), v) treatment of drainage water from agroecosystems (Das et al., 2017; Lanet et al., 2021), vi) subsurface permeable reactive barriers (PRBs) for remediation of contaminated groundwater (Thakur et al., 2020; Njaramba et al., 2021, Wang et al., 2022), and vii) treatment of domestic wastewater (Wakatsuki et al., 1993; Latrach et al., 2018).

The next section gives some selected examples on how the past decade has ignored available knowledge.

Fe⁰ has been used in the following applications: i) H₂ production, ii) food packaging, iii) laboratory demonstration (e.g., practicals), iv) drinking water conservation, v) mining activities (e.g., copper cementation), and vi) safe drinking water provision for many decades/centuries (Davis, 1891; Bafghi et al., 2008; Noubactep, 2010a; Noubactep, 2013; Mwakabona et al., 2017; Antia, 2020; Cao et al., 2020; Noubactep, 2020; Rangan et al., 2020; Huang et al., 2021a). This section presents the various uses of the Fe⁰/H₂O system for water treatment in a historical perspective. Interested readers are referred to a recent overview summarizing 160 years of Fe⁰ technology based mainly on patent literature (Antia, 2020).

The present work intends to disprove the view that Fe⁰ is a reducing agent under environmental conditions. This section revisits Matheson and Tratnyek (1994) in the perspective of assessing whether or not their conclusions were supported by any methodical approach. The clear experimental observation is that there is reductive transformation of chlorinated methanes (RCl) in Fe⁰/H₂O systems (Reynolds et al., 1990; Gillham and O’ Hannesin, 1994; Matheson and Tratnyek, 1994). The question is whether Matheson and Tratnyek (1994) have given a molecular level understanding of this process or established the reaction mechanism. This information is very important for the design of sustainable Fe⁰/H₂O systems and was even the objective of their investigations (Matheson and Tratnyek, 1994). The history of science teaches that the most powerful advances in surface phenomena (including catalysis) are those that improve the ability to predict the efficiency of engineered systems (Buskirk and Baradaran, 2009). As recently recalled by Scott (2019), the fundamental understanding of the mechanisms of chemical reactions broadens the range of accurate predictions.

Any postulated reaction mechanism is a working hypothesis, whose predictions must be compared with experimental observations (Scott, 2019). Following this principle, Matheson and Tratnyek (1994) considered three possible pathways to justify the RCl reduction in Fe⁰/H₂O systems: i) reductive dehalogenation by Fe⁰ (Eq. 3) which is equivalent to Fe⁰ oxidation by RCl (RCl = oxidizing agent, electrochemical mechanism) (Pathway 1), ii) reductive dehalogenation by Fe²⁺ (Eq. 4) which is equivalent to Fe²⁺ oxidation by RCl (chemical mechanism) (Pathway 2), and iii) reductive dehalogenation by H⁺ (Eq. 5) which is equivalent to H⁺ oxidation by RCl (chemical mechanism) (Pathway 3). These three mechanisms are all consistent with the named experimental observation. However, the reaction stoichiometry and the corrosion rate are not known, making the discussion with the spectral signatures of reactants, intermediates and products highly speculative. A profound analysis by Lee et al. (2004) revealed that “no carbon balances between reactants and products have ever been successfully done for many chlorinated hydrocarbons, which indicates that reduction pathways of metal-mediated reactions are not fully understood yet.” The statement by Lee et al. (2004) is just another hint for the lack of concrete evidence on the reductive transformation concept. The lack of mass-balance disproves the concept in the sense that reduction is not necessarily quantitative as the missing fraction of carbon is rather enmeshed in the matrix of iron corrosion products (co-precipitation) (Eusterhues et al., 2011; Noubactep, 2011). However, in mechanistic discussions, co-precipitation is mostly considered relevant for metallic species (Henderdon and Demond 2007; Colabro et al., 2021). The lack of mass-balance collectively questions the validity of the three postulated mechanisms. 2 F e 2+ + R X + H + ⇒ RH+2F e 3+ + X - (4) H 2 + R X ⇒ RH + H + + X - (5)

The most important feature from Matheson and Tratnyek (1994) is the approach they used to rule out reduction after Pathway 2 and Pathway 3: Uncatalyzed reduction by dissolved H₂ or Fe²⁺. They performed control experiments with H₂-saturated water and a 100 mg L⁻¹ FeCl₂ over 15 days in the absence of Fe⁰ and could not observe any dehalogenation. They acknowledged that it was “difficult to exclude the possibility that adsorbed Fe²⁺ or nascent hydrogen” from Eq. 1 “may be participating in the dehalogenation reaction.” Additionally, the amendment of Fe⁰/H₂O systems with external Fe²⁺ or H₂ did not impact RCl reduction. Finally, the addition of 0.5 mM ethylenediaminetetraacetic acid (EDTA) had no effect on the RCl dehalogenation rate. EDTA was supposed to fix Fe²⁺, avoid hydoxide precipitation and keep the Fe⁰ surface free for RCl electrochemical reduction. Matheson and Tratnyek (1994) concluded on the basis of the presented experiments, that “reductive dehalogenation directly coupled with oxidative dissolution of the metal” (Pathway 1) was the “dominant process.” It is very important to note that Matheson and Tratnyek (1994) have just initiated a discussion, but just 2 years later, Weber (1996) claimed to have confirmed the electrochemical nature of contaminant reduction in Fe⁰/H₂O systems. This section seeks to convince the reader that this concept was not established by any scientific approach.

In the absence of mass balance, Matheson and Tratnyek (1994) could not establish the mechanism of RCl reduction. However, there are two more intriguing facts: i) Fe⁰ oxidation by water is not discussed, and ii) water is just considered as a proton source for reaction after Eq. 3. The presentation of the authors textually reads, alkyl halides “can also be reduced by iron. In the presence of a proton donor like water, they typically undergo reductive dehalogenation.” This means that the seminal work of Whitney (1903) was ignored as well as thousands of works describing corrosion as resulting from the presence of water, including impurities in natural oil (Brondel et al., 1994) and atmospheric humidity (Stratmann and Müller, 1994). Moreover, the is no iron corrosion in dry (H₂O free) chlorinated solvents (Rhodes and Carty, 1925; Archer, 1979). Fe⁰ corrosion by H₂O (including moisture) and no Fe⁰ corrosion by dry RCl clearly indicates that more attention should have been paid to water as corroding agent in the Fe⁰ remediation literature (Ghauch, 2015; Cao et al., 2021b; Noubactep, 2022).

The extent to which it is possible to confirm a reaction mechanism is an issue that has preoccupied researchers in the chemical sciences for many decades (Brenner, 2010; Scott, 2019). In Geology, it is recommended to generate and test multiple working hypotheses in scientific inquiry to guard against drawing premature conclusions (Chamberlin, 1890). The science philosophers Karl Popper and Thomas Kuhn further asserted that scientific hypothesis must be falsifiable, or refutable (Scott, 2019). Following this approach, experiments should be designed to test the viability of multiple proposed reaction mechanisms. A mechanistic hypothesis can be falsified, resulting in its modification or even abandonment (Scott, 2019). Because the reaction mechanism of Matheson and Tratnyek (1994) was falsified by Jiao et al. (2009) it should be abandoned. Moreover, because their concept has not properly considered the redox reactivity of water (E⁰ = 0.00 V), the concept was false at the introduction.

This section calls authors to exercise vigilance to avoid making claims that a proof exists where it does not. To enable progress in the design of sustainable Fe⁰/H₂O systems, attempts should be made to discredit rather than prove available mechanistic concepts (Scott, 2019). Louis Pasteur once formulated the following advice: “When you believe you have found an important scientific fact, and are feverishly curious to publish it, constrain yourself for days, weeks, years sometimes, fight yourself, try and ruin your own experiments, and only proclaim your discovery after having exhausted all contrary hypotheses” (Scott, 2019).

Electron efficiency is defined as the fraction of total electrons from Fe⁰ that are used in the reduction of COCs (Eq. 3). The presentation until now has demonstrated that no single electron from Fe⁰ can be transferred to COCs because of the presence of the universal oxide scale which is never electronically conductive (Table 4). If electrons from Fe⁰ were transferred to any COC, there would have not been a lag time between the start of the experiment and the start of reductive transformation of COCs (Schreier and Reinhard, 1994; Huang et al., 1998; Hao et al., 2005; Cao et al., 2021d). Consequently, COCs, O₂ and co-contaminants are reduced by Fe^II, H₂, Fe₃O₄, green rust, and other reducing species generated in the Fe⁰/H₂O system (Figure 2). For simplification, it can be assumed that reductive transformations are mainly mediated by Fe²⁺ and H₂ from Eq. 1.

Eq. 1 implies that the oxidation of 1 mole of Fe⁰ produces 1 mole of Fe²⁺ and 1 mole of H₂. Fe²⁺ can donate one electron and H₂ two electrons. This means that the electrochemical oxidation of 1 mole of Fe⁰ by water (H⁺ or H₂O—the solvent), indirectly produces 3 moles of electrons for the reduction of O₂, NO₃ ⁻ and all other oxidizing agents. In other words, the EE concept has better considered how the 3 moles of electrons are distributed. In this laborious effort, the mass balance of all involved species must be performed, starting with that of iron, which implies the need for experiments entailing the controlled dissolution of FeCPs. To the best of the authors’ knowledge, such a work has not been published. It is very strange, that mechanistic discussions have been performed without complete mass balance for 3 decades (Lavine et al., 2001; Lee et al., 2004; Noubactep, 2011; Cao et al., 2021c). Yet mass balance analysis is critical for proving the validity of the EE concept.

Another important feature about the EE concept is the way it addresses the electrode redox potential. The electrode potential of the Fe^II/Fe⁰ redox couple (E⁰ = –0.44 V) is not relevant to explain the reduction potential of the system towards individual contaminants. Instead, the potential of the couples Fe^III/Fe^II and H⁺/H₂, both in adsorbed and dissolved states are considered. For Fe^II, it has been demonstrated that, while dissolved Fe^II is far less powerful than Fe⁰, adsorbed Fe^II, also referred to as structural Fe^II is sometimes more powerful than Fe⁰: −0.65 ≤ E⁰ (V) ≤ −0.34 (White and Peterson, 1996). This last argument demonstrates that reduction of COCs is favourable, but this reduction of COCs is a chemical process as demonstrated for example by Khudenko (1991) and Jiao et al. (2009).

The last important feature about the EE concept is its usefulness. Reactivity loss and permeability loss are the two main challenges of the Fe⁰ remediation technology (Henderson and Demond 2007; Ghauch, 2015; Guan et al., 2015; Cao et al., 2020). Avoiding material wastage is certainly a noble goal, but it should start when the intrinsic reactivity of each relevant material is documented, and its long-term changes characterized (Ndé -Tchoup é et al., 2020). This presents a challenge, because the current situation is that Fe⁰ materials are not characterized, and their long term behaviour has not been really investigated (Li et al., 2019; Lufingo et al., 2019; Ndé -Tchoup é et al., 2020). For example, steel wool (d ≤ 90 μm), iron wire (d = 200 μm) and iron nails (d > 200 μm) are just tested as “Fe⁰ materials” in independent researches (Tepong-Tsindé et al., 2019).

In summary, evidence shows that: i) the EE concept fails to account for the lag time widely reported in Fe⁰/H₂O systems, ii) the application of the EE concept is not based on any iron mass balance, iii) the EE concept disregards the decade old doubts on the reducing nature of Fe⁰, and iv) the EE concept does not contribute to solve the two major design issues for Fe⁰/H₂O systems (i.e., reactive loss and permeability loss).

The two major problems of Fe⁰-based filters as documented from laboratory experiments, large-scale experiments and field implementations are reactivity loss and permeability loss (Guan et al., 2015; Cao et al., 2020). This corresponds to the state-of-the-art knowledge as summarized 14 years ago by Henderson and Demond (2007). Since then, the research community is divided in two schools. Some few researchers have realized that the dynamic nature of the Fe⁰/H₂O system implies a different approach of investigation (Ghauch, 2015) while the large majority is still considering that isolated instrumental characterizations would help to understand the system. Clearly, the still praised monitoring of physical changes of Fe⁰ features using sophisticated tools including X-ray microcomputer tomography have not mediated the expected understanding of the technology as a whole. In other words, changes of Fe⁰ surface, Fe⁰ particle size, nature of expanding FeCPs, the reducing porosity can be experimentally documented. However, all these observations are collectively only static snap-shots and their measurements are inaccurate. Therefore, they cannot enable the generation of non-trivial models of the dynamic processes within Fe⁰/H₂O systems (Santisukkasaem and Das, 2019), especially as these occur over an enormous range of time scales (from few seconds to some 2 decades) (Gillham 2008; Wilkin et al., 2014; Cao et al., 2020).

The core problem of the Fe⁰ remediation literature is that tools such as the first-order rate constants (k_obs) or the specific reaction rate first-order constants (k_SA) premised on the misconception that Fe⁰ is a reducing agent have had an adverse effect on its development. The EE concept is such a questionable tool. Rather than help the Fe⁰ research community to exploit the mainstream corrosion science, these inappropriate tools have isolated the community to a modern knowledge system, reasoning circularly on an avoidable mistake introduced by some few individuals nearly 3 decades ago. Yet despite several efforts highlighting the mistakes, the scientific community continues to propagate the same errors (Xiao et al., 2020a; Xiao et al., 2020b; Cao et al., 2020; Hu et al., 2020; Cao et al., 2021a; Cao et al., 2021b; Hu et al., 2021). Consequently, in terms of progress the Fe⁰ remediation technology has not progressed much in the last decade, a scenario referred to as the “lost decade” (Noubactep, 2019). The Fe⁰ research community cannot afford to further lose time by further perpetuating the errors of yesteryear, hence the need to root any further research on the correct mechanistic understanding of iron corrosion (Makota et al., 2017; Cao et al., 2020; Hu et al., 2021, Huang et al., 2021). Therefore, to design better systems, there is no other option besides the following: i) to come back to the historical work of Khudenko (1991) as a starting point, and ii) consider progress made by some few research groups during the last decades (Gheju, 2011; Ghauch, 2015; Noubactep, 2015). It is only on the basis of this consideration that research on designing efficient and sustainable Fe⁰ will progress in the next coming decades.

Notably, abandoning the view that Fe⁰ is an environmental reducing agent will not fix everything at once. It will take time to acquire relevant data pertaining to the specificity and long-term kinetics of iron corrosion as used in environmental remediation and water treatment. Further, it takes time for research funding institutions to adapt their funding practices to support a research in which pilot tests shall last for years exceeding the 2–3 years typical of most funded research project. It is particularly obvious that the common 2 or 3 years grants are not suitable for such research efforts. It is important to bear in mind that, Fe⁰ remediation research is going back to its roots: Corrosion Science (Whitney, 1903; Howard, 1910; Dickerson et al., 1979; Nesic, 2007).

The change in strategic direction suggested in this communication is seemingly controversial, because it refutes the consensus of the late 1990s (Gillham and O’ Hannesin, 1994). However, the consensual approach has not worked during the past 2 decades, resulting in unsatisfactory technical systems (Morrison et al., 2006; Ngai et al., 2007; Comba et al., 2011; Mueller et al., 2011; Kowalski and Sø gaard, 2014; Bretzler et al., 2020; Ogata et al., 2020; Huang et al., 2021b; Mueller et al., 2021). There is no alternative to abandoning the misleading concept for mere conveniences since the aspirations of younger scientists are yet to be met.

A further argument against the “reducing Fe⁰” is presented by Ito and colleagues (Satur et al., 2007; Tabelin et al., 2017a; Tabelin et al., 2017b; Seng et al., 2019; Parka et al., 2020). It took 1 decade to Ito and his collaborators to establish Fe⁰ addition as the most promising tool to suppress pyrite oxidation under environmental conditions (Seng et al., 2019). The idea of the so called carrier-microencapsulation (CME) technique, developed during the second half of the 2000s (Satur et al., 2007), is to coat pyrite with a layer of metal hydroxide or oxide, in order to stop mineral oxidative dissolution which causes acid mine drainage (AMD) (Tabelin et al., 2017a; Tabelin et al., 2017b). Several Al, Fe and Ti complexes with organics were tested over the years but were not really specific to FeS₂ in mine tailings and waste rocks. The bright idea, based on the electrochemistry of Fe⁰ (E⁰ = −0.44 V) and FeS₂ (E⁰ = 0.25 V), was that, in a Fe⁰/FeS₂ mixture, Fe⁰ is the anode and dissolved Fe²⁺ migrates to the cathode (FeS₂) which is passivated upon formation of an oxide scale. Seng et al. (2019) have verified this sound hypothesis using Al⁰ and Fe⁰. Interested readers are referred to the related publications. For the presentation herein, its suffices to recall that the semi-conductives properties of FeS₂ have been regarded as the reason for enhanced efficiency of Fe⁰/FeS₂ systems (Xiao et al., 2020a; Xiao et al., 2020b; Hu et al., 2021). It is obvious that thermodynamics is not all but the results of Sheng et al. (2019) are a further call to revisit the operating mode of remediation Fe⁰/H₂O systems.

The research group of the corresponding author and some few others have been working according to a new paradigm for a decade and have already achieved some good results than can be summarized as: i) Fe⁰ is not the source of electrons for the reduction of any contaminant. Tested species include organic (e.g., diclofenac, utriafol) and inorganic (e.g., Cr, Se) compounds (Gheju, 2011; Gheju, 2018; Gheju and Balcu, 2019), ii) the Fe⁰/H₂O system is ionic selective, negatively charged species are favoured (Phukan et al., 2015; Phukan et al., 2016), iii) only hybrid filtration systems (e.g., Fe⁰/pumice, Fe⁰/sand) are sustainable (Ghauch, 2015; Noubactep, 2016), iv) each Fe⁰ has a different corrosion rate which is never constant (Btatkeu et al., 2013; Lufingo et al., 2019; Hildebrant et al., 2020), and v) Fe0 filters are a special case of “corrosion in porous media” (Ndé -Tchoup é et al., 2018; Yang et al., 2021). Yang et al. (2021) have recently introduced the most really holistic attempt to predict the service life of Fe⁰ filters. They considered for the first time the Faraday’s law to tentatively predict the time to clogging. The limitation is that there are no reliable corrosion rates available, this limitation is common to all branches of science investigating corrosion in porous media, including corrosion of reinforcing steel in concrete (Caré et al., 2008; Zhao et al., 2011; Stefanoni et al., 2018; Stefanoni et al., 2019). Therefore, long-term data for iron corrosion are needed (Noubactep, 2022).

Fe⁰/H₂O systems have a long history of application in environmental remediation and water treating, and several new applications are emerging. However, the Fe⁰ technology suffers from a number of misconceptions and mistakes pertaining to the mechanisms of contaminant removal. In the current communication, evidence was presented highlighting the mistakes of the electron efficiency concept, based on the misunderstanding that contaminant removal occurs according to reaction stoichiometry involving Fe⁰ as electron donor (electrochemical mechanism). A number of lines of evidence were presented to demonstrate the invalidity of the EE concept, including: i) the lag time widely reported in Fe⁰/H₂O systems, ii) the universal formation of a non-conductive oxide layer that acts as a barrier for electron transfer, and iii) lack of mass balance data confirming its validity. Despite these limitations, and several studies demonstrating more plausible mechanisms based on iron corrosion, the bulk of the Fe⁰ research community continues to propagate the k_SA, k_obs and electron efficiency concepts. As a result, Fe⁰ research has been nearly stagnant for the past 2 decades, a scenario that has constrained the development of robust and efficient Fe⁰/H₂O systems. Hence, this communication highlighted the need for a paradigm shift from flawed historical concepts, to a future where the design of the next generation of Fe⁰ is rooted on sound fundamental principles based on the science of iron corrosion. The suggested approach would enable an improved exploitation of the capacity of Fe⁰ technology beyond it actual stand as summarized by Antia (2020). To achieve this, a critical evidence based on the reactivity and long-term kinetics of Fe⁰ is needed, and research to generate such evidence requires long-term funding commitments.