Introduction

Dissolved species and pathogens are mostly quantitatively removed from aqueous solutions in the presence of metallic iron (Fe0) (Devonshire 1890; Baker 1934). This century-old knowledge has been intensively applied during the past three decades in solving the challenges for a clean environment and safe drinking water (Wakatsuki et al. 1993; Henderson and Demond 2007; Chen et al. 2014; Ghauch 2015; Guan et al. 2015; Singh et al. 2023; Lawrinenko et al. 2023a). Despite such a broad application, an understanding of the science of the Fe0 remediation technology has not yet been achieved (Cao et al. 2022; Lawrinenko et al. 2023b; Noubactep 2023). In particular, an understanding of the mechanisms of water decontamination in Fe0/H2O systems is useful for designing sustainable systems in practice (Comba et al. 2011; Fu et al 2014; Obiri-Nyarko et al 2014).

Mechanistic information regarding the operating mode of Fe0/H2O systems is conflicting. For example, Matheson and Tratnyek (1994) used slowly stirred batch experiments (≤ 15 rpm) to suggest that chlorinated carbons (RCl) are removed from aqueous solution mostly via reductive degradation mediated by electrons from Fe0. Weber (1996) reported to have confirmed that reductive transformation by Fe0 is a surface-mediated process while using 4-aminoazobenzene as a probe contaminant and a completely different experimental design. Weber (1996) also reported that the requirement that the contaminant contacts the Fe0 surface for electron transfer can be circumvented by the addition of soluble electron mediators. Roberts et al. (1996) and several other authors made similar conclusion which finally led to the “broad concensus” (O’Hannesin and Gillham 1998) that Fe0 oxidative dissolution is the anodic reaction occurring simultaneously to the cathodic reduction in the contaminant of interest (e.g. an electrochemical reaction). It should be pointed out that this reasoning is limited to reducible contaminants, while several other non-reducible species, including pathogens, have been quantitatively eliminated from the aqueous phase in the presence of Fe0 (You et al. 2005; Henderson and Demond 2007; Noubactep 2007). Thus, the removal of non-reducible contaminants and microbes cannot be explained by the concept of reductive transformation or degradation.

In contrast, Lipczynska-Kochany et al. (1994) questioned the validity of the reductive transformation concept in a context where iron corrosion increases the pH value of the system and the universal oxide scale on Fe0 impedes any (quantitative) electron transfer from the metal body. Similarly, Schreier and Reinhard (1994) found a lag time between the initiation of the reaction and the start of the degradation of chlorinated organic compounds in Fe0/H2O systems. Further results questioning the validity of the reductive transformation concept were presented by several authors including Cantrell et al. (1995), Warren et al. (1995), Fiedor et al. (1998), Qiu et al. (2000), Farrell et al. (2001), Lavine et al (2001), and Jiao et al. (2009).

While it is certain that the reductive transformation concept has never been univocally accepted/established, a cursory survey of the Fe0 literature reveals that it is favoured by the large majority of active researchers investigating Fe0/H2O systems (Chen et al. 2014; Guan et al. 2015; Hu et al. 2021; Lawrinenko et al. 2023a). In particular, Lawrinenko et al. (2023a) recently attempted to reiterate the validity of the named concept while characterizing divergent views as “isolated misconceptions”. The views reported by Lawrinenko et al. (2023a) contrast with recent developments that the observation that organic substances are reductively degraded in Fe0/H2O systems was not entirely new, rather, it has been known around 1990 as reported by Reynolds et al. (1990) (Hu et al. 2021; Konadu-Amoah et al. 2021). Moreover, the view that any dissolved species can exchange electrons with Fe0 disproves a century-old knowledge that protons (H+) and protons alone oxidize Fe0 under environmental conditions (pH > 4.5) (Whitney 1903). Actually, the pioneers of the Fe0 remediation technology have not considered Whitney (1903) and other ancient works like Gould (1982), Murphy (1988), Khudenko (1991), and Murphy (1991), while introducing their concept. Table 1 gives an overview of the extent to which six selected relevant pioneering scientific articles have been considered in the Fe0 remediation literature. Khudenko (1991) utilized Cu2+ cementation by Fe0 to generate FeII species (and H/H2 species) for the reductive destruction of organics. This approach was rooted in the knowledge that direct electron transfer from Fe0 is impossible (Whitney 1903; Khudenko 1985, 1987). In other words, it was established before 1994 that Fe0 is corroded by an electrochemical mechanism while contaminants are reduced by a chemical mechanism.

Table 1 Bibliometric analysis of six important references pertinent to the understanding of the operating mode of Fe0/H2O systems using Google Scholar (https://scholar.google.de, Accessed 01.12.2022)
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Table 1 clearly shows that our research group (RGN) has largely considered the pioneering works with a citation rate varying from 6.1% for Gillham and O’Hannesin (1994) to 33.7% for Schreier and Reinhard (1994). Table 1 also reflects the fact that the reductive transformation concept was favoured (Sect. 1) as Gillham and O’Hannesin (1994) and Matheson and Tratnyek (1994) were roundly referenced 10 times more than Lipczynska-Kochany et al. (1994) and Schreier and Reinhard (1994). This selective citation is particularly evident considering that these papers were published in the same year (i.e., 1994). Another important feature reflected in Table 1 is that only a tiny fraction of published articles have referenced the original mechanistic works. In fact, recent bibliometric studies suggest that more than 10,000 scientific articles on “Fe0 for water remediation” have been published since 1994 (Chen et al. 2019; Antia 2020; Li et al. 2021). Actually, Google Scholar covers also graduation thesis and technical documents.

Given the crucial importance of the actual reaction mechanisms for the design of sustainable Fe0 remediation systems, the present work critically analyses the contaminant-free Fe0/H2O system to improve the mechanistic understanding of its operating mode. The presentation starts with a discussion of the chemistry of aqueous iron corrosion.

Aqueous iron corrosion: the Fe0/H2O system

Iron primarily exists in the environment in two oxidation states: (i) highly soluble Fe2+ (ferrous iron) and (ii) lowly soluble Fe3+ (ferric iron) (Langmuir 1997). Once immersed in water, reactive Fe0 undergoes oxidative dissolution (corrosion) owing to its instability. It was demonstrated 120 years ago that aqueous iron corrosion under environmental conditions is an electrochemical process, whereby Fe0 dissolves at an anode (Eq. 1) and proton (H+ from water dissociation) reduction takes place at a cathode (Eq. 2) (Whitney 1903). Clearly, the knowledge that water (H+) and only water (H+) accepts electrons from Fe0 (Eq. 3) is a century-old scientific fact.

$${\text{Fe}}^{0} \Leftrightarrow {\text{Fe}}^{{{2} + }} + {\text{ 2 e}}^{-}$$
(1)
$${\text{2 H}}^{ + } + {\text{ 2 e}}^{-} \Leftrightarrow {\text{H}}_{{2}}$$
(2)
$$\Leftrightarrow{{\text{Fe}}^{0} + \, 2{\text{ H}}^{ + } {\text{@Fe}}^{2 + } + {\text{ H}}_{2}}$$
(3)

It should be highlighted that Whitney (1903) explicitly excluded the possibility that dissolved oxygen accepts electrons from Fe0 (Eq. 1). This makes the following reaction (Eq. 4) impossible:

$$2{\text{Fe}}^{0} + \, 4{\text{H}}^{ + } + {\text{O}}_{2} @2{\text{Fe}}^{2 + } + 2{\text{H}}_{2} {\text{O}}$$
(4)

As shown in Table 1, Whitney (1903) has been overlooked in the Fe0 remediation literature until 2018. This means that the pioneers of the technology have performed incomplete literature research. This is unfortunate as the paper has been published twice: (i) in 1903 at the Journal of the American Chemical Society (original version) and (ii) in 1947 at CORROSION, The Journal of Science and Engineering. The textual reproduction was enriched with the mention: “Dr. W.R. Whitney’s classical paper on the electrochemical principles of corrosion is reproduced here by courtesy of the American Chemical Society from that Society’s 1903 journal. In recognition of Dr. Whitney’s work in establishing this theory of corrosion, the National Association of Corrosion Engineers this year founded the Willis Rodney Whitney Award in the Science of Corrosion, and designated Dr. Whitney the first recipient of the award, presentation of which was made during the NACE award meeting in Chicago, III, April 7–10, 1947.”

Dr. Whitney’s theory negates the popular view that Fe0 is corroded by water (Eq. 3) only under anoxic conditions (absence of O2) (Matheson and Tratnyek 1994; Zhang et al. 2022). It also reiterates that under oxic conditions (presence of O2), Fe0 corrosion is accelerated because O2 consumes Fe2+ for its reductive transformation (Eq. 5) (Le Chatelier Principle).

$${\text{4Fe}}^{{{2} + }} + {\text{4H}}^{ + } + {\text{O}}_{{2}} {\text{@4Fe}}^{{{3} + }} + {\text{2H}}_{{2}} {\text{O}}$$
(5)

In other words, the often-written reaction of Fe0 oxidation by O2 (under oxic conditions) (Eq. 4) should be regarded as impossible, even though it is still regarded as textbook knowledge. Once this misconception is fixed, scope exists for an agreement on the further transformations of ferrous and ferric ions to generate an oxide scale shielding the Fe0 surface and slowing the kinetics of Fe0 corrosion (Nesic 2007; Noubactep 2010). As evident from Eqs. 3 and 5, Fe0 corrosion consumes H+ and induces a significant increase in the pH value of the solution. A concomitant decline in solution redox potential (Eh) has been documented (Matheson and Tratnyek 1994; Singh et al. 2023). In other words, irrespective of the presence of any contaminant, the immersion of Fe0 in a system may lead to the development of highly reducing conditions.

Another key feature from Whitney (1903, 1947) is that depending on the salinity of natural waters, for pH > 4.5, Fe0 corrosion produces low-solubility hydroxides/oxides and more soluble salts (Antia 2020, 2022; Tao et al. 2023). The generation of iron hydroxides/oxides is certain as far as Fe0 is reactive. However, the amount generated is unknown and cannot be estimated because the corrosion rate is unknown (non-constant and nonlinear) (Angst 2019; Yang et al. 2021, 2022). In a porous system, the extent of Fe0 corrosion (spatially and temporally) depends on three key factors: (i) the relative solubility of hydroxides and salts, (ii) the concentration of dissolved O2, and (iii) the residual porosity and the pore size distribution. For the proper discussion of the mechanisms of water decontamination using Fe0/H2O systems, it is essential to consider the relative abundance of Fe0, O2 and salts. Assuming that Fe0 is available in stoichiometric abundance, for highly soluble salts (e.g. FeCl2, FeCl3), the extent of hydroxide precipitation depends on the time scale of Cl– exhaustion (Antia 2020; Tao et al. 2023).

This section has reiterated the complexity of the contaminant-free Fe0/H2O system. The challenge for scientists is to properly investigate its changes with the numerous influencing factors, including: (i) Fe0 intrinsic reactivity, (ii) water chemistry (electrolyte), (iii) water flow velocity (stagnant vs. convection), (iv) the presence of microorganisms, and (v) the presence of reactive phases (e.g. FeS2, MnO2) (Cao et al. 2022). In dynamic systems, the time-dependent properties of the porous medium (pore size and pore size distribution) must be considered as well (Wielinski et al. 2022). Clearly, it is about a holistic consideration of various interdependent dynamic processes such as Fe0 corrosion, contaminant transport, chemical reactions, and particle transport (Bedrikovetsky 2008; Yang et al. 2021).

Investigating the remediation Fe0/H2O system

Section 2 recalls the major characteristics of the contaminant-free Fe0/H2O system which can be summarized in two points: (i) Fe0 corrodes, and an oxide scale is generated in its vicinity (not really at its surface), and (ii) the extent of Fe0 corrosion depends on various inter-related factors, including water chemistry (electrolyte’s nature) and the available volume of pores (e.g. pore size and pore size distribution). In other words, polluted water (to be remediated) is just an electrolyte containing elevated concentrations of selected contaminants, typically in trace amounts (µg/L or mg/L). However, the electrolyte is just one component of an electrochemical cell. The three other components are: (i) the anode where Fe0 is dissolved (to Fe2+) and electrons are released, (ii) the uncorroded Fe0 conducting the released electrons to the cathode, and (iii) the cathode where electrons from Fe0 are exchanged (with protons) and molecular H2 is generated (Groysman 2010; Noubactep 2014). At this stage of reasoning, four points need to be highlighted: (i) immersed Fe0 corrodes also in the absence of O2 and any contaminant (Fact 1), (ii) the oxide scale on Fe0 acts as a diffusion barrier for O2 and contaminants (Fact 2), (iii) the oxide scale on Fe0 is non-conductive and acts as conduction barriers for electrons from Fe0 (Fact 3), and (iv) Fe2+ and H2 are standalone reducing agents (Fact 4). Facts 1–4 have been presented by Noubactep (2007, 2008) to refute the validity of the theory that Fe0 donates electrons to any dissolved species. Attempts to falsify Noubactep’s view to date were collectively limited to supporting the view introduced in the 1990s. For example, Lawrinenko et al. (2023a, 2023b) supported their views while citing Cutler (1987), Gillham and O’Hannesin (1994), and Matheson and Tratnyek (1994). These three articles are claimed to have provided “evidence for direct electron transfer and the corrosion of Fe0 in contact with chlorinated hydrocarbons”. However, as shown in Table 1, the two papers from 1994 have not considered Whitney (1903), while Cutler (1987) is rather a review article on the interactions of Al0 with halogenated hydrocarbons. Clearly, since the rejection of the reductive transformation concept about 15 years ago, some colleagues have approved the alternative concept (Ghauch et al. 2011; Gheju 2011; Giles et al. 2011; Ghauch 2015; Sista et al. 2021; Gheju and Balcu 2019, 2023; Lan et al. 2023), while the large majority has overlooked it. Lawrinenko et al. (2023a) is the very first attempt to falsify the new concept.

Coming back to the contaminant-free Fe0/H2O system, and considering dissolved O2 as a non-polluting contaminant, its reduction by Fe2+ (Eq. 4) induces concentration changes in the vicinity of Fe0 and in the adjacent porous system. However, the dynamic relationships between these concentration changes and the extent of Fe0 corrosion are complex and depend on many factors, including: (i) chemical reactions (e.g. O2 reduction, oxide precipitation), (ii) local pore structure (e.g. size of Fe0 and sand particles), and (iii) ionic transport, diffusion within the oxide scale, and advection in moving water. The presence of contaminants and site-specific microorganisms are two other complicating parameters (Angst 2019; Yang et al. 2021; Tao et al. 2023). Thus, it is impossible to root the understanding of the Fe0/H2O system on simple engineering approaches, merely based on the nature of the contaminant and its redox reactivity relative to the redox couple Fe0/FeII (E0 = –0.44 V) (Gillham and O’Hannesin 1994; Matheson and Tratnyek 1994; Sarr 2001; Fu et al. 2014; Obiri-Nyarko et al. 2014; Lawrinenko et al. 2023a).

At this stage of reasoning, it should be highlighted that E0 = –0.44 V, the electrode potential of the FeII/Fe0 redox couple, is not the decisive factor to justify the use of Fe0 in water treatment. The first reason for this is that the oxide scale is positively charged and will selectively attract negatively charged pollutants (Sato 2001; Miyajima 2012; Phukan et al. 2015). On the other hand, Al0 (E0 = –1.66 V), Mg0 (E0 = –2.38 V), and Zn0 (E0 = –0.76 V) are more powerful reducing agents than Fe0 (Cutler 1987; Shreier and Reinhardt 1994; Warren et al. 1995). Proponents of the reductive transformation concept have not really considered this key aspect to explain why these metals are not used for the removal of contaminants in a similar way as Fe0. Rather, their reasoning was limited to toxicity and cost issues. However, particularly, Al0 and Zn0 are not suitable for long-term operations because they just form one hydroxide each (Al(OH)3 and Zn(OH)2) and one oxide (Al2O3 and ZnO). The unit cells in Al and Al2O3 on the one side and that of Zn and ZnO on the other side are very similar to one another. Thus, Al2O3 adheres tightly to Al0 beneath it, just as ZnO adheres tightly to Zn0. In both cases, the oxidized surface provides a protective layer that prevents O2 from getting to the underlying metal surface (Dickerson et al. 1979; Noubactep 2010). In contrast, the packing dimensions of Fe0 and Fe oxides are not particularly close; thus, there is no tendency for an iron oxide layer to adhere to Fe0. Accordingly, the oxide scale on Fe0 constantly flakes off and exposes fresh Fe0 surfaces for attack or corrosion (Dickerson et al. 1979). This is the reason why Fe0 is a long-term source of iron oxides for water remediation (James et al. 1992; Wakatsuki et al. 1993; Jia et al. 2007; Bradley et al. 2011), and not a long-term source of electrons for the reductive transformation of selected species (Lawrinenko et al. 2023a) (Fig. 1).

Fig. 1
figure 1

Illustration of proposed hypotheses for contaminant (RCl) reduction in Fe0/H2O systems: (left) Fe0 is oxidized by RCl and water (H+) (Path 1), and (right) Fe0 is oxidized by water (H+) and RCl is reduced by FeII species and H2 (Path 2). Path 1 is not feasible because the oxide scale is a conduction barrier for electrons from Fe0. Path 2 is feasible and rooted in aqueous iron corrosion

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Figure 1 recalls that both concepts recognized the formation of the oxide scale on Fe0. On the one hand, the reductive transformation concept strives to explain why electron transfer from Fe0 occurs despite the presence of the oxide scale (Scherer et al. 2000; Lawrinenko et al. 2023a). On the other hand, the alternative concept supported by the seminal work of Whitney (1903) argues that such an electrochemical reaction is impossible (Cao et al. 2022; Noubactep 2023). It is very important to recall that for the concentration ranges of natural water, chemical reduction is not even a relevant remediation mechanism for many substances, particularly when it comes to safe drinking water (Noubactep 2011; Heimann et al. 2018). The reason is that even reaction products have to be removed from the aqueous phase. However, the maximum contamination limit (MCL) is generally lower than the solubility limit (Kalin et al. 2005; Ndé-Tchoupé et al. 2015). For this reason, adsorption, co-precipitation, and size-exclusion as occurring in all Fe0/H2O systems are fundamental (Noubactep 2007, 2008; Ghauch et al. 2011; Gheju 2011; Giles et al. 2011; Noubactep 2011; Gheju and Balcu 2019; Sista et al. 2021).

Designing sustainable Fe0 PRBs

The view that Fe0 is an electron donor under environmental conditions has led to an empirical design of the first generation of Fe0 PRBs. In fact, the stoichiometry of the reaction between Fe0 and the contaminants of concern were considered (Sarr 2001), and a “safety factor” was used to “increase assurance of successful remediation” (Lawrinenko et al. 2023a). In this approach, mixing Fe0 with non-expansive materials like pumice or sand was regarded as a “ Fe0 dilution” with cost benefits, but possible negative impacts on the efficiency of the resulting systems (Kenneke and McCutcheon 2003; Westerhoff and James 2003; Bi et al. 2009). Fe0/sand mixtures were also used as pre-treatment systems to free inflowing water from dissolved O2 such that the pure Fe0 main treatment system could be fully anoxic (O2 free) (Westerhoff and James 2003). Ironically, these efforts coexisted in the literature with the demonstration of the efficiency of Fe0 PRBs using a Fe0/sand mixture comprising 22% Fe0 (w/w) (O’Hannesin and Gillham 1998).

Although differences in density between Fe0 (d = 7.6 g/cm3) and its corrosion products (d < 5.2 g/cm3) have been previously considered in justifying permeability loss in Fe0 PRBs (Liang et al. 2003, 2005), our research group led Dr. Noubactep was the first to properly consider the volumetric expansive nature of iron corrosion in designing Fe0 filters (Noubactep and Caré 2010). Accordingly, replacing a fraction of Fe0 in a filtration system with a non-expansive material (e.g. MnO2, pumice, sand) is a tool to enable complete exhaustion of Fe0 in the long-term (Moraci and Calabrò 2010; Noubactep et al. 2012; Caré et al. 2013; Bilardi et al. 2023). In 100% Fe0 filters (Fig. 2), clogging occurs before Fe0 exhaustion because of a lack of space for further dissolution. Figure 3 shows two systems, one with 100% Fe0 and the other with 50% Fe0 (vol/vol). Here, it is seen that the pure Fe0 system clogs when the 50% Fe0 has just lost 50% of its porosity. This reasoning implies that mixing Fe0 and a non-expansive material is a tool to enable better material efficiency (avoid Fe0 wastage), and a prerequisite for sustainability in terms of long-term permeability (Noubactep et al. 2012; Caré et al. 2013; Domga et al. 2015; Bilardi et al. 2023; Tao et al. 2023). Proponents of the reductive transformation concept have considered a decrease in the source of electrons (Fe0 ratio) as a cause of declined system performance for Fe0 filters (Kenneke and McCutcheon 2003; Bi et al. 2009). Yet data from both batch and column studies show that admixing Fe0 with non-expansive aggregates improves the performance of Fe0/H2O systems (Westerhoff and James 2003; Song et al. 2005; Moraci and Calabrò 2010; Miyajima 2012; Tepong-Tsindé et al. 2019; Gheju and Balcu 2023; Lan et al. 2023). Surprisingly, proponents of the reductive transformation concept have not yet revisited their theory in the light of this evidence (Wielinski et al. 2022; Lawrinenko et al. 2023a).

Fig. 2
figure 2

Illustration of changes in the residual porosity in a bed filled with spherical Fe0 particles: (left) at the start of the operation (t0 = 0), and (right) sometimes later (t > t0, but t < t∞) before Fe0 complete exhaustion at t∞. The time required for complete exhaustion (t∞) depends on a diversity of factors, including Fe0 intrinsic reactivity, initial porosity, water chemistry, and Fe0 ratio

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Fig. 3
figure 3

Illustration of changes in the residual porosity in a bed filled with spherical particles: a 100% Fe0, and b Fe0/sand with 50% Fe0 (vol/vol). The representation corresponds to a time (t) when the 100% Fe0 is completely clogged. It is considered that the Fe0/sand system has lost 50% of its initial porosity

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Another point of discrepancy between the two views is how to investigate spatio-temporal changes in Fe0/H2O systems. Proponents of the reductive transformation concept are still combining analytical tools to monitor changes in mineral phases in short-term experiments (some few weeks) (Wielinski et al. 2022), while our research group insists on the scarcity of long-term column studies (e.g. > 52 weeks, 1 year) (Tepong-Tsindé et al. 2019; Yang et al. 2022). In fact, using a pure thermodynamic approach (mass balance), the optimum Fe0 ratio to avoid material wastage was determined as 25% (vol/vol) in hybrid Fe0/sand systems (Miyajima 2012; Miyajima and Noubactep 2013). There is material wastage whenever Fe0 is not completely exhausted as the system is clogged (Caré et al. 2013; Domga et al 2015). This is when the time to material exhaustion (t∞) is lower than the time to clogging (tclog). A Fe0 volumetric ratio of 25% in a Fe0/sand system roughly corresponds to (i) 50% Fe0 (w/w) which has been used at several field installations (Henderson and Demond 2007; Guan et al. 2015) and (ii) the solution to the special case (t∞ = tclog, where clogging occurs promptly at Fe0 complete exhaustion). The temporal issue that is the extent of porosity loss at any time (or time to clogging) is difficult or even impossible to address because the corrosion kinetics is not known and has even not been really investigated in the Fe0 remediation context (Moraci et al. 2016; Noubactep 2016; Yang et al. 2022). Ironically, while Fe0 oxidation by water is mostly considered as a side reaction, modelling efforts, since Mayer et al. (2001), are mainly rooted in its corrosion rate as determined by Dr. Eric J. Reardon 33 years ago (Reardon 1995).

A last fundamental design aspect to consider is how the expected decrease of the permeability will influence the long-term efficiency of Fe0 PRBs. In fact, Fe0 PRBs are designed to be more permeable than the surrounding aquifer material (O’Hannesin and Gillham 1998; Zhang et al. 2022; Plessl et al. 2023). However, despite mixing Fe0 with non-expansive materials, the permeability of the reactive zone (Fe0 PRB) will still decrease. Therefore, it makes sense to forecast specific cases, for example, when the initial PRB‘s permeability will decrease by one-quarter, one-third, or one-half. Considering such scenarios in the design phase would be useful for better monitoring of the system’s performance (Yang et al. 2021, 2022).

Practical significance

The analysis in this study shows that under given operational conditions, Fe0 can generate various amounts of FeCPs which are highly effective in removing many classes of contaminants from aqueous solutions (Bigg and Judd 2000; Henderson and Demond 2007; Cundy et al. 2008; Noubactep et al. 2009). Adsorption, co-precipitation, and size-exclusion (chromatographic effects) are the common underlying removal mechanisms, including for redox-active species. The robust design of sustainable Fe0 PRBs depends on the profound understanding of how site-specific characteristics (e.g. O2 availability, pH value, salinity) affect the long-term availability of FeCPs within the system (Naseri et al. 2017; Noubactep 2018; Tao et al. 2023). In other words, the analysis presented herein provides a new starting point for the conceptualization of future laboratory, pilot, and field-scale studies for the design of efficient and sustainable Fe0 PRBs for groundwater remediation as well as systems to treat wastewater and Fe0 filters for safe drinking water provision.

To the best of the authors’ knowledge, only the research group of Dr. Vollprecht (starting later) has designed their whole experiments correctly and interpreted the results according to the state-of-the-art knowledge (Vollprecht et al. 2018, 2020; Sedlazeck et al. 2019, 2020). In retrospect, Vollprecht’s articles on the “Ferrodecont Process” are built on Müller et al. (2014) entitled “Implementation of fluidized granulated iron reactors in a chromate remediation process”. Müller et al. (2014) is nothing new, but an independent rediscovery of the Anderson Process as implemented in the 1880s in the waterworks of Antwerpten (Belgium) (Devonshire 1890; van Craenenbroeck 1998; Mwakabona et al. 2017; Antia 2020). Both processes (Anderson and Ferrodecont) are rooted in the evidence that in filtration systems, clogging is a special (or geometrical) effect occurring because space lacks to store the in-situ generated corrosion products (contaminant scavengers) (Caré et al. 2013; Domga et al. 2015; Bilardi et al. 2023).

Conclusions and a look ahead

This article presents a critical assessment of the state-of-the-art knowledge on the operating mode of engineered Fe0-based systems for water remediation as used for safe drinking water supply, wastewater treatment, and environmental remediation. The following major conclusions are drawn:

Immersing a reactive Fe0 specimen in (polluted) water at a pH value higher than 4.5 results in its oxidative dissolution by H+ ions and the generation of solid iron corrosion products which act as contaminant scavengers. Fe0 dissolution also generates standalone reducing agents (e.g. FeII oxides, FeII/FeIII oxides, H2) which may reduce selected contaminants by a chemical reaction. In other words, although Fe0 corrosion in PRBs is an electrochemical process, oxidizing electrons are not supplied by any dissolved contaminant. This century-old knowledge has been overlooked for 28 years while discussing the mechanisms of contaminant removal in Fe0/H2O systems (Table 1).

Despite the discrepancy in the actual operating mode of engineered Fe0-based systems, a well-documented advantage of these systems is that their decontamination efficiency can be maintained for decades (Phillips et al. 2010; Wilkin et al. 2014, 2019). This is an additional motivation to root this technology in its scientific background (i.e. iron corrosion).

The relationship between Fe0 dissolution, porosity loss, and water decontamination is very complex. This is because of the numerous inter-related and partly dynamic influencing factors involved in the processes. Relevant influencing factors include: (i) the porous structure (e.g. size, distribution), (ii) the water chemistry (e.g. O2 availability, pH value, salinity), (iii) water flow velocity, and (iv) the action of microorganisms. Thus, engineering or pragmatic approaches for understanding the mechanisms of water treatment using Fe0/H2O systems have proven unreliable. The scientific challenge for the coming years is to properly consider advances in numerical modelling (e.g. reactive transport modelling in porous media) to devise models capable of considering: (i) the spatio-temporal changes within the Fe0/H2O system at the pore-scale level, and (ii) the interdependence between the involved processes (e.g. chemical reactions, Fe0 reactivity, Fe0 ratio, and transport processes).

A concerted effort is needed to pave the way out of the current “valley of confusion” characterized by a high level of empiricism. It is hoped that based on the clarifications presented herein, new engineering tools will be developed to increase the effectiveness and reliability of the next generation of Fe0 PRBs as an efficient and sustainable technology for groundwater remediation.