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Opinion

Conceptualizing the Fe0/H2O System: A Call for Collaboration to Mark the 30th Anniversary of the Fe0-Based Permeable Reactive Barrier Technology

by
Viet Cao
1,
Omari Bakari
2,
Joseline Flore Kenmogne-Tchidjo
3,
Nadège Gatcha-Bandjun
4,
Arnaud Igor Ndé-Tchoupé
5,
Willis Gwenzi
6,
Karoli N. Njau
2 and
Chicgoua Noubactep
2,5,7,8,9,*
1
Faculty of Natural Sciences, Hung Vuong University, Nguyen Tat Thanh Street, Viet Tri 35120, Phu Tho, Vietnam
2
Department of Water and Environmental Science and Engineering, Nelson Mandela African Institution of Science and Technology, Arusha P.O. Box 447, Tanzania
3
Department of Chemistry, Faculty of Sciences, University of Yaoundé I, Yaoundé P.O. Box 812, Cameroon
4
Faculty of Science, Department of Chemistry, University of Maroua, Maroua P.O. Box 46, Cameroon
5
School of Earth Science and Engineering, Hohai University, Fo Cheng Xi Road 8, Nanjing 211100, China
6
Grassland Science and Renewable Plant Resources, Faculty of Organic Agricultural Science, University of Kassel, Steinstrasse 19, D-37213 Witzenhausen, Germany
7
Centre for Modern Indian Studies (CeMIS), Universität Göttingen, Waldweg 26, D-37073 Göttingen, Germany
8
Department of Applied Geology, University of Göttingen, Goldschmidtstraße 3, D-37077 Göttingen, Germany
9
Faculty of Science and Technology, Campus of Banekane, Université des Montagnes, Bangangté P.O. Box 208, Cameroon
*
Author to whom correspondence should be addressed.
Water 2022, 14(19), 3120; https://doi.org/10.3390/w14193120
Received: 4 September 2022 / Revised: 26 September 2022 / Accepted: 27 September 2022 / Published: 3 October 2022
(This article belongs to the Special Issue Sustainable Remediation Using Metallic Iron: Quo Vadis?)
Review Reports Versions Notes

Abstract

:
Science denial relates to rejecting well-established views that are no longer questioned by scientists within a given community. This expression is frequently connected with climate change and evolution. In such cases, prevailing views are built on historical facts and consensus. For water remediation using metallic iron (Fe0), also known as the remediation Fe0/H2O system, a consensus on electro-chemical contaminant reduction was established during the 1990s and still prevails. Arguments against the reductive transformation concept have been regarded for more than a decade as ‘science denial’. However, is it the prevailing concept that denies the science of aqueous iron corrosion? This article retraces the path taken by our research group to question the reductive transformation concept. It is shown that the validity of the following has been questioned: (i) analytical applications of the arsenazo III method for the determination of uranium, (ii) molecular diffusion as sole relevant mass-transport process in the vicinity of the Fe0 surface in filtration systems, and (iii) the volumetric expansive nature of iron corrosion at pH > 4.5. Item (i) questions the capability of Fe0 to serve as an electron donor for UVI reduction under environmental conditions. Items (ii) and (iii) are inter-related, as the Fe0 surface is permanently shielded by a non-conductive oxide scale acting as a diffusion barrier to dissolved species and a barrier to electrons from Fe0. The net result is that no electron transfer from Fe0 to contaminants is possible under environmental conditions. This conclusion refutes the validity of the reductive transformation concept and calls for alternative theories.

“In the sciences, people quickly come to regard as their own personal property that which they have learned and had passed on to them at the universities and academies. If someone else comes along with new ideas that contradict the Credo and in fact even threaten to overturn it, then all passions are raised against this threat and no method is left untried to suppress it. People resist it in every way possible: pretending not to have heard about it; speaking disparagingly of it, as if it were not even worth the effort of looking into the matter. And so a new truth can have a long wait before finally being accepted.” Johann Wolfgang von Goethe (28 August 1749–22 March 1832).

1. Introduction

Water pollution has become a serious concern worldwide, as various discharges from agricultural, domestic, and industrial activities are sources and vectors of pollutants [1,2]. The quest for safe drinking water and a clean environment has motivated the use of metallic iron (Fe0) in water remediation [3,4]. Although Fe0 has been industrially used for water treatment for some 170 years [5,6,7,8,9,10,11], research on using Fe0 for water treatment boomed only after 1990, and following the advent of Fe0-based subsurface permeable reactive barriers (Fe0 PRBs) for groundwater remediation [12,13,14]. In particular, in 1990, Reynolds et al. [15] fortuitously found that Fe0-based sampling vessels eliminated trichloroethylene and other halogenated hydrocarbons from polluted groundwater [16,17,18]. This observation coincided with an active search for appropriate materials to realize the concept of subsurface reactive walls presented during the 1980s [18,19]. After some five years (1994–1998) of controversial discussion on the mechanisms of the reductive degradation of organics and reductive precipitation of inorganics in Fe0/H2O systems [13,20,21,22,23,24,25,26,27,28,29], a consensus was reached on the electro-chemical nature of these reductive transformations [14] (Table 1).
Table 1 is by no means a ‘pros and cons’ list to assist any decision-making process. A pros and cons list is conventionally used to help understand both sides of an argument. Pros are listed as arguments in favor of making a particular decision, while the cons are counter-arguments against the same decision. In Table 1, however, arguments are listed to recall the genesis of the reductive transformation concept, arguing that contaminant reduction is the cathodic reaction simultaneous to the oxidative dissolution of metallic iron (reductive transformation paradigm). In other words, the reductive transformation paradigm regards Fe0 as a relevant reducing agent under environmental conditions [13,18]. In this context, Fe0 is often regarded as ‘a fixed source of electrons’ for the reductive transformation of aqueous inorganic and organic contaminants [18]. Factually, this concept fails to justify the quantitative removal of microorganisms [30,31].
Sustained by the reductive transformation paradigm, the past 30 years have witnessed the application of Fe0-based systems for groundwater remediation [32,33,34,35,36,37]. Efficient systems for wastewater treatment [38,39,40,41] and safe drinking water supply [42,43,44,45] were also presented. Despite such an impressive record, with more than 5000 peer-reviewed articles [46,47], the Fe0 remediation technology is still an innovative one [37,48,49,50]. Nonetheless, the large majority of active researchers and practitioners regard Fe0 remediation as an established technology [33,34,51,52]. The main point of the discrepancy between the two groups is about the role of Fe0 in removing contaminants in Fe0/H2O systems [37,50]. Since 2007, our research group has strongly refuted the view that any electron from Fe0 can be transferred to dissolved contaminants under field or environmentally relevant conditions. Noubactep has been considered for more than a decade as a leading “science denier”: (i) questioning scientific milestones and spreading misinformation, and (ii) “contradicting decades of scientific endeavor” just like scientists denying climate change, pandemic issues, or the theory of evolution [53,54,55,56]. Our arguments have been mostly considered as “contrarian claims” [57] as opposed to sound scientific data supporting the view that contaminant reduction is the cathodic reaction simultaneous to Fe0 oxidation (Table 1). The following statement of a potential reviewer recently (2022) declining a manuscript submitted by Noubactep et al. at a “reputed” journal supports this negative view of the alternative concept: “You should not be sending these diatribes by Noubactep out for review. He submits variations on this paper all over the place. They mostly get rejected without review, but occasionally one slips through. They are an impenetrable mixture of about 1/3 creative critical reviewing and about 2/3 paranoid delusional nonsense. I stopped agreeing to review them almost 10 years ago” (Statement 1). This statement is just opposed to the opening quote by the German writer, pictorial artist, biologist, theoretical physicist, and polymath Johann Wolfgang von Goethe [58]. In particular, Statement 1 can be regarded as a dissuasive demonstration that it is “not even worth the effort of looking into” science denial by the submitting authors. Table 2 gives an overview of some of these ‘diatribes’ from 2007 to 2022. It is seen from the titles that the prevailing concept has been constantly challenged.
Statement 1 and the relatively low attention received by our published articles (Table 2) suggest that there is a communication problem. From our perspective, the contributions were well-conceived and timely. The often claimed ‘paucity of experimental support’ for our views is not acceptable (Section 3). Our efforts to promote the hypothesis that aqueous contaminant removal in the presence of Fe0 occurred primarily by adsorption and co-precipitation within the oxide scale even started earlier than 2007. Table 3 presents five articles written by the corresponding author in German on the same topic. The main feature from Table 3 is that the alternative concept was known to active German researchers on Fe0 for water remediation, even before 2007. Moreover, one of the papers at TerraTech has even been referenced by a research group from Bern, Switzerland [91], attesting that “German papers” were internationally known. It is also surprising that even German PhD candidates have not really considered the core of our work in their literature review. To the best of our knowledge, Burghardt [92] is the sole exception, explicitly basing some reasoning on Noubactep’s PhD and related articles. From the English language papers in Table 2, the most cited, with 396 counts, is a 15-year-old critical review. Two comparative review articles from the same year have received more attention as evidenced by their number of citations: (i) Cundy et al. [93] with 722 counts, and (ii) Thiruvenkatachari et al. [94] with 422 counts. The additional burden in getting some few articles published (statement 1) suggests that the research community is not willing to test any alternative view. This sentiment was reinforced while publishing Hu et al. [50] at Frontiers in Environmental Chemistry. Despite the innovative open peer-review process at Frontiers, the initial submission was rejected twice after evaluation; only the second re-submission was positively evaluated by all invited reviewers, who accepted to endorse the publication. The whole procedure lasted for some 18 months. Two recent articles [37,50] extensively present the state-of-the-art knowledge on the view that Fe0 is a generator of contaminant scavengers and secondary reducing agents (e.g., FeII species, FeII/FeIII species, H2). Interested readers are referred to these open-access papers. As part of a special issue, having been invited to encourage the scientific community to pay more attention to the adsorption/co-precipitation concept, the present article retraces the path of our research group through the past 15 years. The aim is to demonstrate the relevance of the opening quote by Johann Wolfgang von Goethe [58] for the science of aqueous iron corrosion. The presentation starts with an elucidation of the Fe0/H2O system and its aspects that have not been properly considered by pioneers of Fe0 remediation technology. Following this, the milestones of our research group are presented together with commentaries on how they were perceived, mostly by reviewers of journal articles and academic theses.

2. The Fe0/H2O System

2.1. General Aspects

The treatment of polluted surface water and groundwater is a costly endeavor. The introduction of in-situ treatment technologies such as Fe0-based permeable reactive barriers (PRBs) has substantially reduced the costs of groundwater remediation [3]. Fe0 PRBs take advantage of the electro-chemical nature of aqueous iron corrosion to remove contaminants from the aqueous phase. In some cases, contaminants are transformed or degraded to less-toxic, non-toxic or immobilized chemical forms [12,13,29,100]. The current discrepancy in the Fe0 remediation literature stems from an insufficient analysis of the Fe0/H2O systems and their related dynamics [37,50]. This section presents the Fe0/H2O system and makes a holistic analysis of it.
Upon immersion in an aqueous environment, a piece of Fe0 is oxidized by water (H2O or H+) following an electro-chemical mechanism (Equation (1)).
Fe0 + 2 H+ ⇒ Fe2+ + H2
It is fundamental to state straight away that the reaction according to Equation (1) occurs both under anoxic (anaerobic) and oxic (aerobic) conditions. This phenomenon is published as an award-winning breakthrough which is a century old [101]. It was demonstrated by W.R. Whitney in 1903, and awarded the first Willis Rodney Whitney Award 44 years later in 1947. Even now, the Willis Rodney Whitney Award is given by the National Association of Corrosion Engineers (NACE—https://www.corrosionpedia.com (accessed on: 3 September 2022)) for “significant contributions to corrosion science, such as the development or improvement of a theory that provides a fundamental understanding of corrosion phenomena”. Clearly, the demonstration of W.R. Whitney advanced corrosion a century ago; it is thus strange that contrarian views have been introduced and supported for decades (Table 1) [37,50,59].

2.2. Fe0 Corrosion under Anoxic Conditions

Under anoxic conditions (no dissolved O2), Fe0 is corroded by H+ (Equation (1)) and traces of Fe3+ (Equation (2)).
Fe0 + 2 Fe3+ ⇒ 3 Fe2+
The net result is a system with various species of FeII (e.g., FeO, Fe(OH)2), FeII/FeIII (e.g., Fe3O4), FeIII (e.g., FeOOH) hydroxides/oxides, and H2. FeII species, FeII/FeIII species, and H2 are stand-alone reducing agents. There is a high density of reductive species but reduction of contaminants occurs according to a chemical mechanism, meaning that the reducing electrons are not from Fe0. This assertion refutes the discovery of Reynold et al. [15] and the whole consensus on which the reductive transformation concept is built (Table 1) [37,50]. Clearly, Fe0 corrosion by water is an electro-chemical process, but contaminant-reductive transformation in the presence of iron metal (Fe0) is a chemical reaction. This statement is indirectly supported by Burris et al. [20,24], who arbitrarily segregated the Fe0 surface into reactive and non-reactive sites. The same authors were the first to insist on the importance of adsorption processes for organics, beside reductive transformations. Their arguments were later supported by many other researchers, including Mantha et al. [102], Furukawa et al. [103], and Mielczarski et al. [104]. Our research group was the first to radically exclude Fe0 from the relevant reducing agents, while regarding solid iron corrosion products as the main contaminant scavengers [82,84,89,90].

2.3. Fe0 Corrosion under Oxic Conditions

Under oxic conditions (presence of O2), Fe0 is still corroded by H+ (Equation (1)), but generated Fe2+ is instantaneously used for O2 reduction (Equation (3)). In other words, corrosion is accelerated because Fe2+ (Equation (1)) is consumed for O2 reduction (Le Chatelier’s principle). Equation (3) considers the fact that, at circum-neutral pH, produced FeIII species hydrolyze and precipitate [105].
2 Fe2+ + 1/2 O2 + 5 H2O ⇒ 2 Fe(OH)3 + 4 H+
Despite the abundance of O2, the Fe0 surface is shielded by a non-conductive oxide scale (oxide film) acting as diffusion barrier to O2. The net result is that, in the vicinity of Fe0, the Fe0/oxide interface is still highly anoxic and on the exterior of the oxide scale, the oxide/water interface is highly oxic. In other words, under external oxic conditions, the oxide scale on Fe0 is highly layered. The outer layers are highly oxic, while the inner layers are highly anoxic comparable to the situation in Section 2.2. Under these conditions, redox transformations are possible, but electrons from Fe0 are not involved. Clearly, the oxide scale is the site for contaminant redox transformations and their scavenging as well [48,81,104,105,106].

2.4. Dynamics of the Fe0/H2O System

From the pure thermodynamic perspective, Fe0 corrosion results in Fe solid precipitates (FeCPs) that are contaminant scavengers. FeCPs remove contaminants from the aqueous phase by two main mechanisms: (i) enmeshment during their precipitation (co-precipitation), and (ii) adsorption onto their surface. Depending on the contaminant/FeCP molar ratio, even species without affinity to FeCPs can be quantitatively removed. Two particular examples are cationic methylene blue (MB) and Zn2+ [37,50]. Our research group has exploited these properties of MB and the simplicity of its analytical determination to develop a simple but efficient tool to characterize the reactivity of Fe0/H2O systems (the MB method). The MB method characterizes the extent of in-situ sand coating in a Fe0/sand system as iron corrodes [107,108]. In column experiments, the most reactive system is the one experiencing the earliest breakthrough [109].
The performance of a field Fe0 PRB depends on local hydrogeochemical conditions and barrier composition (e.g., Fe0 ratio). At the beginning of the implementation of Fe0 PRBs during the 1990s, there was an agreement on that field performance monitoring of contaminant level, Eh value, pH value, and permeability were needed to elucidate the operating mode and assess the potential limitations of the PRB technology. However, after two decades, published works on sustainable PRB systems still lack these before and after analyses [36,110,111]. Moreover, little or no effort was directed at characterizing the iron corrosion rate, that is, the rate at which a decrease in porosity in the reactive zone occurs, or the rate at which contaminant scavengers are generated. Clearly, the real problem of Fe0 PRBs resulting from the poor system analysis is that the rate at which FeCPs are produced is not known, and has even not been properly investigated [112,113,114]. Instead, the importance of foreign minerals (e.g., CaCO3, MgCO3) has been largely overemphasized, while the expansive nature of iron corrosion under aqueous conditions has been overlooked [112,114]. Even the Fe0 intrinsic reactivity has been poorly considered and experiments have rarely lasted for more than four months [115,116,117].

3. Arguments against the Reductive Transformation Concept

The reductive transformation concept was adopted by a consensual approach as discussed in Section 1 (Table 1). This is simply not acceptable in natural sciences, and in the era of advanced instrumental analysis [118,119,120,121,122,123]. Moreover, the concept has ignored many important results from mainstream corrosion science, including the seminal work of Whitney [101]. Whitney [101] was a continuation of investigations on the electro-chemical nature of metal corrosion as started in 1819 by Michael Faraday or in 1830 by Auguste de la Rive (www.icorr.org/world-corrosion-science; accessed on: 3 September 2022). More recently, the results of Whitney [101] have been independently rediscovered by several researchers, including Michael Boris Khudenko [124], who used Cu2+ cementation by Fe0 to generate FeII and H+ for the degradation of organics in wastewater. In other words, the reductive transformation concept is a clear distortion of the science of aqueous iron corrosion [37,50]. Clearly, it is over to the followers of this concept to demonstrate its validity. This section will present three tangible arguments against the aforementioned concept.
In the second half of the 1990s, the mechanism of UVI removal by Fe0/H2O systems was discussed controversially [25,26,125]. Research groups with expertise on UVI interactions with iron oxides favored the view that UVI could not be quantitatively reduced under field conditions [25,125]. However, somehow, “Reductive precipitation of uranium (VI) by zero-valent iron” by Gu et al. [26] has been favored by subsequent investigators and is still considered as the paper that has demonstrated the mechanism of UVI removal in Fe0/H2O systems. This opinion has been challenged by three articles by Noubactep et al. [126,127,128] who have clearly demonstrated that there is no quantitative UVI removal under conditions where FeCP formation is hindered or delayed. A clear argument against the reductive precipitation concept comes from the arsenazo III method for U determination. In this method, UVI is reduced to UIV which forms stable complexes with arsenazo III. UVI reduction occurs around pH 2.0 upon addition of HCl (6M) using granular Bi0 or Zn0 as a reducing agent [129,130]. In the early phase of the arsenazo III method, granular Fe0 was used as a reducing agent, but was abandoned because the reaction was not really quantitative. The question arises, why a reaction that is not quantitative at pH 2.0 (free UO22+ in solution) should become quantitative at pH values where UO22+ (UVI) is not stable? At pH > 4.0, more than 90% of the initial UVI concentration used by Gu et al. [26] precipitates as schoepite (UO2(OH)2) [126,131,132].
The second argument against the reductive transformation concept is physical in nature. The presentation in Section 2 has recalled that the Fe0 surface is permanently covered with a non-conductive layered oxide scale. It is reasoned that the oxide scale is the location of the contaminant removal and should never be altered or removed during experiments [87,90]. In the early phase of Fe0 investigations, these prerequisites were largely observed, for example by Matheson and Tratnyek [13] and Burris et al. [20] who just stirred their experimental vessels at 15 and 8 rpm, respectively (Table 3). However, soon after, experimental vessels were typically stirred or shaken at speeds exceeding 200 rpm (Table 4). Surprisingly, such high homogenization speeds were explicitly intended to keep reactive particles in suspension to accelerate mass transfer. Results achieved under such conditions are inherently unrepresentative of practical or field environments. Thus, the large majority of data supporting the reductive transformation concept were obtained at mixing rates so high that no oxide scale could be generated in the vicinity of the Fe0 surface. The question arises, how can data obtained under undesirable conditions be used to support this concept.
The third argument against the reductive transformation concept is also physical in nature. It is about the suitability of hybrid Fe0/aggregate systems. The view that Fe0 is at least partly oxidized by contaminants has resulted in the consideration of the stoichiometry of reactions similar to Equation (4) for the design of Fe0 PRBs [33,51,140,141].
Fe0 + RCl + H+ ⇒ Fe2+ + RH + Cl−
Designing a PRB for the reductive transformation of RCl supposes that the more Fe0, the more efficient the system. Accordingly, a pure Fe0 filter (100% Fe0) should be more efficient than hybrid filters (e.g., Fe0/pyrite, Fe0/sand). Following this premise, hybrid pre-treatment zones have been tested, such that the pure filter works under perfect anoxic conditions [142,143,144]. However, pure Fe0 filters were proven efficient but not sustainable [109,145,146]. The reason for this is that columns packed with 100% Fe0 material left little room for solid phase expansion, because all particles are expansive. In fact, in-situ generated FeCPs that are useful for contaminant removal are equally undesirable, because they occupy the initial porosity, rendering the filter less and less permeable [112,113]. By establishing this, our research group has certainly reflected strong links between academic research and societal benefits. In particular, using this knowledge, we have presented the most efficient household water filters and ways to improve them while extending their use to small communities [45,147].

4. Arguments against the Adsorption/Co-Precipitation Concept

The overarching goal for water remediation is to (detect and) remove toxic substances from water, where possible affordably and robustly. The adsorption/co-precipitation concept demonstrates that this objective is achieved for all biological and chemical pollutants in well-designed, case-specific Fe0-based treatment systems. This is because Fe0 acts as source of FeCPs, which are excellent contaminant scavengers. All contaminants are removed regardless of their redox reactivity [89,90,148,149]. This knowledge is very old because Fe0 has been used for water treatment before the advent of coagulation/filtration [1,9,11].
It may be surprising to read that no argument against the adsorption/co-precipitation concept has been presented, only skepticism, as already expressed in 2009: “Noubactep questioned the premise that Fe0-induced contaminant removal is initiated by the direct electron transfer from Fe0 to substrates and added that “the premise was already questioned and/or proven inconsistent” while citing only his own papers [80,90]. This argument is hardly acceptable, since the role of the direct electron transfer in Fe0-mediated reactions is well-established and generally accepted among the research community” (Statement 2) Kang and Choi [150]. The authors further blamed Noubactep for referencing only two of his papers to support the statement. It would have been better to state what is wrong with the statement. Today, reviewers still claim that we are using “non-standard, too high self-citation” while recognizing that we have been walking almost alone for more than a decade (Statement 1).
The sole pseudo-scientific argument against the adsorption/co-precipitation concept has been the quest for proofs. However, as demonstrated in the previous sections, no proofs are needed as any relevant experimental result could at best falsify the theory [151]. Yet our publications have sufficiently falsified the reductive transformation concept. With the volume of skepticism, our research group has not received any funding since 2008, that is, for 14 good years equivalent to five generations of PhD students. The lack of funding later turned to a blessing. This is because we had to work with what we could afford, and that were the conditions for the development of the low-cost methylene blue method (MB method, Section 5) [107,108].

5. Development of a New Research Tool for Fe0/H2O Systems: The MB Method

The Fe0 PRB technology is definitively an innovative technology for groundwater remediation (Section 1) [152,153,154,155]. In innovation studies, it is crucial to properly derive the specific research methodology from the theory of the system [156,157]. In other words, it is crucial to assess how fit the methods used to answer the research question are. In our context, the common research question is: “What makes a Fe0-based remediation system efficient and sustainable?” The large majority of active researchers have coupled the answer to this question to the reductive potential of Fe0 for dissolved contaminants (e.g., chlorinated hydrocarbons, heavy metals). Accordingly, in the initial phase of the technology’s development, it was commonplace to compare the electrode potential of Fe0 (E0 = −0.44 V) to that of dissolved species [22,158,159]. However, we are in a context where it was established long ago that no dissolved species can oxidize Fe0 [101,124]. These ancient works were not considered when introducing the reductive transformation theory. Not knowing these works, Noubactep argued from 2006 onwards that because so many classes of contaminants, including reducible ones (e.g., AsV, CuII, CrVI, RCl, SeVI) are successfully treated in a Fe0/H2O system, reduction cannot be the fundamental removal mechanism [89,90]. Results on CrVI removal in Fe0/H2O systems have long falsified the reductive transformation concept [160].
In 2005, Song et al. [160] presented a mathematical model to explain why the presence of sand could enhance CrVI “reduction” by Fe0. The standard redox potential of the couple CrVI/CrIII is 1.52 V, making Fe0 (E0 = −0.44 V) a relevant reducing agent. However, reduction of CrVI by FeII is also possible (E0 = 0.77 V for the couple FeII/FeIII) and this is even well-documented [161,162]. In other words, CrVI reduction is possible by both Fe0 and FeII. However, the question remains open, why adding sand, which was then largely considered as “Fe0 dilution” [143,144,163], rather than enhancing the “reduction” efficiency. The answer is given when considering that, in the Fe0/sand system, the negatively charged sand surface is progressively coated with a positively charged oxide scale, which is good scavenger for negatively charged CrVI (chromate CrO42−) [164]. In other words, regardless of whether CrVI is reduced to CrIII or not, its quantitative removal in Fe0/sand systems is justified by a larger adsorptive surface made available by coated sand.
Our research group has focused on ‘contaminant removal’ and not on ‘contaminant reduction’. One additional reason was that contaminant reduction is rarely a removal mechanism in the range of concentration relevant to natural waters [10,48,82]. In other words, while having the same target (efficient and sustainable Fe0/H2O systems), the research community was not working on the same research question and that justifies the decade-old discrepancy [37,50,59]. Clearly, while using the same valid methodologies (e.g., batch tests, column experiments, structural analysis), the research question was not the same. Usually, the interested reader must assess whether the methodology used is suitable to answer the research question. The presentation herein demonstrates that the same research question was not asked, and even that it may not be worth insisting on results provided for the wrong research question (e.g., contaminant reduction by electrons from Fe0).
As a matter of fact, different methods provide different results, and these complementary methods are used to understand the Fe0/H2O system [118,120,164,165,166]. Moreover, there should be an openly explained reason for using each specific method [116,157,166]. What will each specific method reveal about the phenomenon that would otherwise remain hidden? Importantly, because the Fe0/H2O system is a dynamic one, it should be kept in mind that all observations from the laboratory and the field are just “static snapshots” and their measurements are inaccurate [50,151]. Therefore, innovative methodologies for characterizing the dynamics of the system are highly needed [156].
The conventional approach to investigating the Fe0/H2O system consists of characterizing the following at diverse timescales: (i) used Fe0 materials, (ii) in-situ generated FeCPs and other mineral phases, (iii) contaminant concentration, (iv) nature and concentration of daughter products, (v) contact time, (vi) pH value, and for column experiments, (vi) changes of the hydraulic conductivity (permeability). Because of the dynamic nature of the Fe0/H2O system, each recorded observation is a real static snapshot and its magnitude depends on variables like Fe0 intrinsic reactivity, proportion of Fe0 in the reactive zone, solution chemistry and temperature. Clearly, from the measurements and observations performed, a reconstruction of the phenomena (reverse modelling) is difficult and even impossible.
To improve system characterization, our research group introduced the MB method to follow the extent of Fe0 corrosion or the extent to which sand is coated in-situ by generated FeCPs. The MB method is rooted on the historical observation by Mitchell and colleagues that the efficiency of natural sand for MB discoloration depends on the extent to which its surface contains coated iron oxides [167]. Accordingly, if the same mass of two different Fe0 materials is added to a given amount of sand, the more reactive system is the one producing more FeCPs (e.g., for CrVI removal, see [160]), or the one depicting the lower extent of MB discoloration. Using this simple experimental tool, impressive results have been achieved, as recently summarized by Konadu-Amoah et al. [108]. For this article, it suffices to recall that the MB method has clarified the importance of hybrid systems (Fe0/aggregates) for sustainable Fe0 filters [109]. This challenging issue has been controversially discussed in the literature for at least a decade [31,71,142,163,168,169]. The key issue has been using a small amount of Fe0 (e.g., 100 g) and extending the contact time, for example, to more than 40 days in batch experiments and 4 months in column studies [109,170].

6. Concluding Remarks

This article is regarded as our contribution to the 30th anniversary of Fe0 PRB technology in 2022. It summarizes our perspective as requested by Special Issue invitation. It is certainly an expansion of some earlier ideas [37,50]. We hope to have delineated that the currently prevailing reductive transformation concept cannot enable the design of sustainable Fe0 filtration systems, because the issue of permeability loss cannot be solved by increasing the Fe0 ratio up to 100%. The way forward is to continue in the path of the adsorption/co-precipitation concept, a concept that has been around for 15 years, but has been largely ignored by active researchers. The adsorption/co-precipitation concept expresses a theory of how sustained permeability is achieved. It also advocates for a database of reactive Fe0 materials which, when complete, will simplify the design of non-site-specific Fe0/H2O systems.
According to the viewpoints discussed in this article, the following key conclusions and perspectives are put forward:
  • operational reference Fe0 materials are needed to enable at least a semi-quantitative comparison of results achieved under independent conditions.
  • experiments regarding the operating mode of filtration Fe0/H2O systems should be performed under diffusion-controlled conditions.
  • pure Fe0 filters (100% Fe0) are not sustainable. Thus, Fe0 should always be mixed with non-expansive aggregates like pumice or sand.
  • results based on the reductive transformation concept have been the cornerstone for the development of the adsorption/co-precipitation concept. Accordingly, observations and recommendations/suggestions anchored on the adsorption/co-precipitation concept should be acknowledged as valuable contributions in transferring scientific knowledge.
  • the entire environmental research community should question the validity of the view that Fe0 is a (strong) reducing agent under environmental conditions.
  • the MB method is a powerful tool for characterizing the dynamics of Fe0/H2O systems.
Finally, we wish to thank the active Fe0 research community for their skepticism and some few editors/reviewers for being patient listeners to our proposal. The aforementioned skepticism has been a driving force encouraging us to mine the literature for proofs. Among the findings, the evidence that using iron filings for floc generation (e.g., flocculation) [1] was perhaps as important as the “discovery” of Whitney [101] demonstrating that H+ and H+ alone are reducing agents for Fe0 under environmental conditions. These two papers definitively falsified the reductive transformation concept, and thus breaking the last trace of skepticism. It is our conviction that rooting future research on the adsorption/co-precipitation concept will help provide cost-effective, robust, and sustainable Fe0-based water treatment systems.

Author Contributions

Conceptualization: V.C., J.F.K.-T., N.G.-B., O.B., A.I.N.-T. and C.N.; methodology: O.B., K.N.N., A.I.N.-T. and W.G.; writing—original draft: V.C., J.F.K.-T., N.G.-B., O.B., A.I.N.-T., W.G. and C.N.; writing—review and editing: V.C., J.F.K.-T., N.G.-B., A.I.N.-T., W.G. and C.N.; supervision: C.N., K.N.N. and W.G. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

We would like to thank the peer reviewers for their valuable suggestions and comments that improved this paper. Viet Cao is supported by Hung Vuong University through the project “Synthesis of Fe3O4/graphene oxide nanocomposite for the treatment of organic contaminants. The Germany Research Foundation and the Open Access Publication Funds of the Göttingen University are acknowledged for funding the open access publication.

Conflicts of Interest

The authors declare no conflict of interest.

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Table
Table 1. Outline of relevant arguments given to justify the process of contaminant removal using Fe0/H2O systems up to the “broad consensus” in 1998 [14].
Table 1. Outline of relevant arguments given to justify the process of contaminant removal using Fe0/H2O systems up to the “broad consensus” in 1998 [14].
ArticleObjectivesContaminants of ConcernConclusionImportant Remarks
Gillham and O’Hannesin [12]Assess the suitability of Fe0 for the dehalogenation of 14 chlorinated methanes, ethanes, and ethenes (RCl).CT; TCM; DCM; TBM; HCA; PCE; TCE; trans-DCE; cis-DCE; DCE; VC; 1,1,2,2-TECA; 1,1,1,2-TECA; and 1,1,1-TCADehalogenation of RCl by Fe0 occurs probably via direct reduction.Study focused on contaminant removal. Little attention was paid to the actual mechanisms.
Lipczynska-Kochany et al. [27]Investigate of the effect of acidification on the dehalogenation kinetics of carbon tetrachloride (RCl) by Fe0, and contribute to understanding the mechanisms of these processes.CCl4Acidification enhances RCl dehalogenation by Fe0.Study questioned the sustainability of direct reduction by increasing pH values.
Matheson and Tratnyek [13]Contribute to understanding the mechanism (and kinetics) of chlorinated methanes (RCl) transformations in the presence of granular Fe0.CCl4; CH3Cl; and TCEDehalogenation of RCl by Fe0 occurs mostly via direct reduction. Reductive transformation of RCl by Fe0 was favored.
Schreier and Reinhard [28]Investigate the ability of Fe and Mn powders to transform some chlorinated organic compounds (RCl) under anaerobic conditions. PCE; 1,1,1-TCA; 1,1-DCE; DCM; 1,1-DCA; and 1,4-DCBDehalogenation of RCl by Fe0 occurs via direct reduction.Study reported on a time lag prior to quantitative contaminant reduction.
Burris et al. [20]Determine the sorption and reduction kinetics of trichloroethylene and tetrachloroethylene (RCl) with Fe0 under anaerobic conditions.TCE and PCEReduction rates are first-order, thereby indicating that the bulk of sorption occurs on non-reactive sites.Study seeking to confirm the reductive transformation paradigm.
Cantrell et al. [29]Assess the suitability of Fe0 to remove some selected metals from groundwater, while characterizing the reaction kinetics and relating the findings to the thermodynamics of involved redox couples.UO22+; MoO42−; TcO4−; and CrO42− Metals removal by Fe0 occurs partly via direct reductive precipitation.Study conducted in analogy to Matheson and Tratnyek [13].
Warren et al. [21]Contribute to understanding the mechanism (and kinetics) of carbon tetrachloride (RCl) dehalogenation using Fe0. CCl4Reductive dehalogenation of RCl by Fe0 is mediated by hydrogen at the metal surface.The reductive transformation paradigm was questioned.
Roberts et al. [22]Assess whether β-elimination reactions of chlorinated ethylenes (RCl) occur in the presence of Fe0 and Zn0.DCE; trans-DCE; Cis-DCE; 1,1-DCE and VCDehalogenation of RCl by Fe0 occurs via direct reduction.Study conducted in analogy to Matheson and Tratnyek [13].
Weber [23]Study the reduction of 4-aminoazobenzene by Fe0 to determine whether the process is surface-mediated. 4-aminoazobenzene (4-AAB)Reductive transformation by Fe0 is a surface-mediated process with direct electron transfer from Fe0 to the substrate. Study seeking to confirm the reductive transformation paradigm.
Burris et al. [24]Examine the sorption of chlorinated ethenes (RCl) to cast iron (Fe0) surfaces to: (i) assess the generality of non-reactive sorption behavior for cast irons; (ii) determine the predominant non-reactive sorbent on the cast iron surface; (iii) determine whether sorption to cast iron adheres to Traube’s rule (sorption proportional to hydrophobicity); and (iv) evaluate rate-limited sorption/desorption for the non-reactive sites. TCE and PCE Significant mass transfer limitations to non-reactive sorption sites exist for PCE but not for TCE.Study seeking to confirm the reductive transformation paradigm.
Fiedor et al. [25]Investigate the removal mechanism of soluble uranium from groundwater by Fe0.UO22+ (i.e., U6+)Reduction of U6+ to U4+ by Fe0 is mediated by Fe2+ or H2, but the reaction is kinetically slow.The reductive transformation paradigm was questioned.
Gu et al. [26]Determine the effectiveness of Fe0 and several adsorbent materials in removing uranium (U) from contaminated groundwater, and to investigate the rates and mechanisms that are involved in the reactions.UO22+Uranium removal by Fe0 occurs via direct reductive precipitation.Study conducted in analogy to Matheson and Tratnyek [13].
O’Hannesin and Gillham [14]Long-term field investigation of the suitability of granular Fe0 for the in-situ degradation of dissolved chlorinated organic compounds (RCl). TCE and PCEDehalogenation of RCl by Fe0 is quantitative and occurs via direct reduction.Reductive transformation of RCl by Fe0 was favored and explicitly recognized as a “broad consensus”.
Table
Table 2. A selection of 34 articles of our research group refuting the view that Fe0 oxidative dissolution is the anodic half-reaction coupled to contaminant reduction in Fe0/H2O systems. Citations stands for the number of references according to Google Scholar (Accessed on 31 August 2022).
Table 2. A selection of 34 articles of our research group refuting the view that Fe0 oxidative dissolution is the anodic half-reaction coupled to contaminant reduction in Fe0/H2O systems. Citations stands for the number of references according to Google Scholar (Accessed on 31 August 2022).
YearTitleJournalCitationsReference
2022Metallic iron for water remediation: Plenty of room for collaboration and convergence to advance the scienceWater/MDPI2[37]
2022Should the term ‘metallic iron’ appear in the title of a research paper?Chemosphere9[59]
2021Metallic iron for environmental remediation: The fallacy of the electron efficiency conceptFront. Environ. Chem.10[50]
2021The mechanism of contaminant removal in Fe0/H2O systems: The burden of a poor literature reviewChemosphere9[60]
2020Tracing the scientific history of Fe0-based environmental remediation prior to the advent of permeable reactive barriersProcesses/MDPI17[61]
2020Metallic iron for environmental remediation: Starting an overdue progress in knowledge Water/MDPI23[62]
2019Redirecting research on Fe0 for environmental remediation: The search for synergyInt. J. Environ. Res. Public Health13[63]
2019The operating mode of Fe0/H2O systems: Hidden truth or repeated nonsense? Fresenius Environ. Bull.12[64]
2019Metallic iron and the dialogue of the deafFresenius Environ. Bull.19[65]
2018Fe0/H2O systems for environmental remediation: The scientific history and future research directionsWater/MDPI16[66]
2018Iron corrosion: Scientific heritage in jeopardySustainability/MDPI6[67]
2018Metallic iron for environmental remediation: How experts maintain a comfortable status quoFresenius Environ. Bull.5[68]
2017Metallic iron for water treatment: Leaving the valley of confusion Appl. Water Sci.44[69]
2017Rescuing Fe0 remediation research from its systemic flawsRes. Rev. Insights23[70]
2016Predicting the hydraulic conductivity of metallic iron filters: Modeling gone astrayWater/MDPI36[71]
2016Research on metallic iron for environmental remediation: Stopping growing sloppy scienceChemosphere38[72]
2016No scientific debate in the zero-valent iron literatureFresenius Environ. Bull.17[73]
2015Metallic iron for environmental remediation: A review of reviewsWater Res.140[74]
2014Water remediation by metallic iron: Much ado about nothing—As profitless as water in a sieve?CLEAN-Soil, Air, Water8[75]
2014Flaws in the design of Fe0-based filtration systems?Chemosphere46[76]
2013Metallic iron for water treatment: Prevailing paradigm hinders progressFresenius Environ. Bull.12[77]
2013Metallic iron for environmental remediation: Missing the ‘valley of death’Fresenius Environ. Bull.10[78]
2013Metallic iron for environmental remediation: the long walk to evidenceCorros. Rev.19[79]
2012Metallic iron for environmental remediation: Back to textbooksFresenius Environ. Bull.19[80]
2011Metallic iron for water treatment: A knowledge system challenges mainstream scienceFresenius Environ. Bull.30[81]
2011Aqueous contaminant removal by metallic iron: Is the paradigm shifting?Water SA76[82]
2010On nanoscale metallic iron for groundwater remediationJ. Hazard. Mater.70[83]
2010The suitability of metallic iron for environmental remediationEnviron. Progr.78[84]
2009An analysis of the evolution of reactive species in Fe0/H2O systemsJ. Hazard. Mater.144[85]
2009Fe0-based alloys for environmental remediation: Thinking outside the boxJ. Hazard. Mater.34[86]
2009On the validity of specific rate constants (kSA) in Fe0/H2O systemsJ. Hazard. Mater.18[87]
2009On the operating mode of bimetallic systems for environmental remediationJ. Hazard. Mater.36[88]
2008A critical review on the mechanism of contaminant removal in Fe0–H2O systemsEnviron. Technol.396[89]
2007Processes of contaminant removal in “Fe0–H2O” systems revisited. The importance of co-precipitationOpen Environ. Sci.170[90]
Table
Table 3. The five articles in German (‘German papers’) refuting the reductive transformation concept and their citations according to Google Scholar (accessed on 31 August 2022). The original titles are given in the references.
Table 3. The five articles in German (‘German papers’) refuting the reductive transformation concept and their citations according to Google Scholar (accessed on 31 August 2022). The original titles are given in the references.
YearTitleJournalCitationsReference
2015New concepts for designing experiments regarding the process of water treatment in Fe0/H2O systemsAfrika and Wissenschaft0[95]
2007Response to the comments by colleagues Ebert and co-authors on my articles “The end of a myth” (TerraTech 11–12/2006) and “On the operating mode of reactive walls” (TerraTech 3–4/2007)TerraTech1[96]
2007On the operating mode of reactive walls: The emergence of the view that contaminant reduction occurs at the surface of elemental ironTerraTech1[97]
2006The end of a myth: Contaminant reduction by electrons from elemental iron contradicts three centuries of corrosion researchTerraTech4[98]
2003Investigations for the passive in-situ immobilization of U(VI) from waterWissenschaftliche Mitteilungen16[99]
Table
Table 4. Experimental conditions of some selected studies investigating the operating mode of the Fe0/H2O system in batch mode. It is seen that in the initial phase (here up to 1998) only low homogenization intensities were used; such conditions are representative of field situations.
Table 4. Experimental conditions of some selected studies investigating the operating mode of the Fe0/H2O system in batch mode. It is seen that in the initial phase (here up to 1998) only low homogenization intensities were used; such conditions are representative of field situations.
Fe0 MaterialContaminantVolume
(mL)		HomogenizationReference
Size
(mm)		Loading
(g/L)		 typespeed
0.151.7Chlorinated methanes60shaking15[13]
0.15250Halogenated alphatics40Shaking2[12]
0.4330Trichloroethylene and tetrachloroethylene15stirring8[19]
<0.15104-aminoazobenzene50shaking-[23]
0.012Trichloromethane and
trichloroethylene		2000stirring450 and 660[133]
1.6–2.520Uranium20quiescent0[126]
0.1533Nitrate60shaking60[134]
(5–8)
× 10−5		2Nitrate500stirring200[135]
0.31513Sulfate, chloride, nitrate, and bicarbonate186stirringvigorous[136]
0.6–0.42525Arsenite2000stirring55[137]
0.2–52.4Total organic carbon500stirring500 and 1000[138]
0.252.5Phosphorus100stirring165[139]
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Cao, V.; Bakari, O.; Kenmogne-Tchidjo, J.F.; Gatcha-Bandjun, N.; Ndé-Tchoupé, A.I.; Gwenzi, W.; Njau, K.N.; Noubactep, C. Conceptualizing the Fe0/H2O System: A Call for Collaboration to Mark the 30th Anniversary of the Fe0-Based Permeable Reactive Barrier Technology. Water 2022, 14, 3120. https://doi.org/10.3390/w14193120

AMA Style

Cao V, Bakari O, Kenmogne-Tchidjo JF, Gatcha-Bandjun N, Ndé-Tchoupé AI, Gwenzi W, Njau KN, Noubactep C. Conceptualizing the Fe0/H2O System: A Call for Collaboration to Mark the 30th Anniversary of the Fe0-Based Permeable Reactive Barrier Technology. Water. 2022; 14(19):3120. https://doi.org/10.3390/w14193120

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Cao, Viet, Omari Bakari, Joseline Flore Kenmogne-Tchidjo, Nadège Gatcha-Bandjun, Arnaud Igor Ndé-Tchoupé, Willis Gwenzi, Karoli N. Njau, and Chicgoua Noubactep. 2022. "Conceptualizing the Fe0/H2O System: A Call for Collaboration to Mark the 30th Anniversary of the Fe0-Based Permeable Reactive Barrier Technology" Water 14, no. 19: 3120. https://doi.org/10.3390/w14193120

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