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Review

Fe0/H2O Systems for Environmental Remediation: The Scientific History and Future Research Directions

by
Rui Hu
1,
Xuesong Cui
1,
Willis Gwenzi
2,
Shuanghong Wu
1 and
Chicgoua Noubactep
3,*
1
School of Earth Science and Engineering, Hohai University, Fo Cheng Xi Road 8, Nanjing 211100, China
2
Biosystems and Environmental Engineering Research Group, Department of Soil Science and Agricultural Engineering, Faculty of Agriculture, University of Zimbabwe, P.O. Box MP167, Mount Pleasant, Harare, Zimbabwe
3
Department of Applied Geology, Universität Göttingen, Goldschmidtstraße 3, D-37077 Göttingen, Germany
*
Author to whom correspondence should be addressed.
Water 2018, 10(12), 1739; https://doi.org/10.3390/w10121739
Submission received: 27 October 2018 / Revised: 23 November 2018 / Accepted: 23 November 2018 / Published: 27 November 2018
(This article belongs to the Special Issue Filters in Drinking Water Treatment)
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Abstract

:
Elemental iron (Fe0) has been widely used in groundwater/soil remediation, safe drinking water provision, and wastewater treatment. It is still mostly reported that a surface-mediated reductive transformation (direct reduction) is a dominant decontamination mechanism. Thus, the expressions “contaminant removal” and “contaminant reduction” are interchangeably used in the literature for reducible species (contaminants). This contribution reviews the scientific literature leading to the advent of the Fe0 technology and shows clearly that reductive transformations in Fe0/H2O systems are mostly driven by secondary (FeII, H/H2) and tertiary/quaternary (e.g., Fe3O4, green rust) reducing agents. The incidence of this original mistake on the Fe0 technology and some consequences for its further development are discussed. It is shown, in particular, that characterizing the intrinsic reactivity of Fe0 materials should be the main focus of future research.

Graphical Abstract

1. Introduction

Micrometre-size metallic iron (Fe0) is one of the most commonly used materials for permeable reactive barriers (PRBs) used in groundwater remediation [1,2,3,4,5,6,7,8,9,10]. Despite its large size and low surface area, it has been successfully used for groundwater treatment with more than 200 PRBs installed worldwide [10]. However, the contaminant removal mechanisms of Fe0 PRBs have not been elucidated.
Reductive degradation/precipitation using elemental iron (Fe0) as a reactive medium (electron donors) to treat contaminated soils and waters has been extensively investigated during the last 28 years [11,12,13,14,15,16]. Fe0 is a reducing agent (E0 = −0.44 V) with reaction products (FeII and FeIII species) which are (mostly) environmentally innocuous. Additionally, Fe0 is abundantly available, for instance in the form of scrap iron and steel wool [8,10,12,17,18,19,20,21,22].
When coupled with the chemical reduction (degradation) of an oxidized contaminant (Ox), the spontaneous reduction reaction yielding a more biodegradable and/or hopefully less toxic reduced form (Red) is given by Equation (1):
Fe0 + Ox ⇒ Fe2+ + Red
Fe0 permeable reactive barriers (Fe0 walls) have become an established technology for in-situ groundwater remediation [10,13,14,16,23]. There is an almost consistent agreement in the literature about direct reduction (electrons from Fe0) (Equation (1)) as the main mechanism by which Fe0 reduces aqueous contaminants [14,24,25,26,27,28]. Accordingly, Equation (1) alone is always given as the reduction scheme [28,29,30,31,32] and is implemented in modelling codes [13,33]. Therefore, other reductive mechanisms, including reduction by hydrogen (H/H2—Equation (2)), ferrous iron (FeII—Equation (3)) and solid corrosion products (e.g., green rust) are considered as side effects. Moreover, contaminant adsorption onto the Fe0 surface and oxide scale is acknowledged as an intermediate step towards reductive transformations [24,34,35,36,37,38].
H/H2 + Ox ⇒ H+ + Red
FeII + Ox ⇒ FeIII + Red
However, the validity of the concept of reductive degradation/precipitation has been severely challenged in the peer-reviewed literature since 2007 [28,39,40,41,42,43,44,45,46,47,48,49,50]. A new concept was introduced, that considers adsorption and co-precipitation of contaminants within the oxide scale on Fe0 as the primary removal mechanism [39,40,41,42]. The subsequent abiotic reduction is possible, but not necessarily a direct mechanism (Equation (1)). The new concept (adsorption/co-precipitation concept) is free of contradictions inherent to the reductive degradation/precipitation concept, and explains some seemingly controversial experimental results [31,32,39,40,51,52,53,54].
Noubactep [39,40] has demonstrated that the pioneers of environmental remediation using Fe0 materials have not properly considered information put forth by other branches of science using Fe0 materials, including; iron corrosion [55,56,57,58,59], organic synthetic chemistry [60,61], wastewater treatment [62,63], and the oil industry [64,65]. Furthermore, the experimental conditions used in the pioneering studies were not appropriate for traceable conclusions [39,40,41,66]. Despite some serious warnings [28,67,68,69,70,71,72,73], the inherent error of the pioneers has been perpetuated and propagated through the scientific literature [1,2,3,4,5,6,7,8,9,10,14,23,26,27,29,74,75].
The present contribution aims to demonstrate from a historical perspective that indirect reduction should have merited more attention in the Fe0 remediation research than other mechanisms. The demonstration is based on knowledge available to scientists before the advent of “Fe0 walls” [63,76]. The presentation is deliberately limited to the literature available before 1994 which corresponds to the first mechanistic investigations pertinent to the Fe0/H2O system [24]. It is further demonstrated that; (i) iron corrosion is volumetric expansive [77] making a pure Fe0 wall (100% Fe0) not sustainable [78,79] due to loss of porosity and hydraulic conductivity, and (ii) the Fe0/H2O system is ion-selective, making the Fe0 technology more suitable for the removal of negatively charged species than neutral and positively charged ones [80,81,82,83]. Based on the preceding, the future research directions on the Fe0/H2O system are highlighted.

2. The Interactions within Fe0/H2O Systems

The voluminous literature on the aqueous Fe0 reactivity under near-neutral pH conditions is characterized by a clear agreement on the formation of an oxide scale on the Fe0 surface (Figure 1) [17,55,56,58,59,60,84,85,86,87,88,89]. There is some controversy concerning the composition and structure of this oxide scale and the mechanism by which the film breaks down [35,86,89,90]. It is well-known that the formation of the oxide scale (and its further transformation) is influenced differently by major anions (Cl−, NO3−, PO43−, SO42−), but the mechanisms involved are unclear. In particular, there is a lack of agreement in the literature on the influence of Cl− on the oxide film. Cl− may; (i) adsorb on Fe0 and avoid formation and/or adherence of oxide film, (ii) cause a thinning of the oxide film and this can lead to pitting if the potential is above a critical value, and (iii) incorporate into the film lattice and induced a subsequent local breakdown [90]. The preceding recalls that iron is corroded by water (solvent—Equation (4)) and the extent of corrosion is more or less influenced by solutes [55,58,59,64].
Fe0 + 2 H+ ⇒ Fe2+ + H2
Furthermore, at neutral pH values, the presence of a universal oxide scale should be considered as a rule rather than an exception [65,87]. The exception of iron oxide film breakdown should be discussed on a case-by-case basis. For example, if water contains Cl−, it can be considered that the film formation is disturbed or delayed [92]. In this regard, Cohen [55] for instance, investigated the mechanism of oxide film formation on iron in aqueous solutions and the effects of oxygen (O2), oxidizing ions (chromate, molybdate, nitrite, tungstate), and non-oxidizing ions (acetate, carbonate, phosphate) on this process. In the recent Fe0 literature, chromate, molybdate, nitrite, tungstate, and phosphate have been investigated as contaminants in independent treatability studies, while O2, acetate, carbonate, and phosphate were mostly tested as site-specific environmental variables. For this reason, one may consider Cohen’s work as a seminal for an alternative and probably more accurate approach to investigate the process of contaminant removal in Fe0/H2O systems. Clearly, investigating contaminant removal in Fe0/H2O systems should entail characterizing the impact of individual contaminants on the process of oxide film formation (and transformation) under site-specific conditions (e.g., contaminant concentration, pH value, water velocity). This intuitive approach has not been considered, and the Fe0 remediation literature is full of studies investigating the same species (including NO3− and PO43−) as contaminants in one case and co-solutes in other cases [3,9,12,16,29]. Moreover, the bulk of such studies used artificial solutions under controlled laboratory conditions (e.g., constant temperature, agitation), while those using more complex contaminated aqueous systems under real-life conditions are rare. In particular, the use of technical tools (e.g., mixing, pumping, shaking) to accelerate mass transfer at laboratory scale has generated reproducible results with little practical meaning. The major limitation of accelerated mass transfer has been the delay of the formation of the oxide scale on Fe0 and, thus, minimizing the relevance of indirect reduction, while possibly favoring direct reduction at sites which would not be available under field conditions [32,44,46,67,93].

Mass Transfer within Fe0/H2O Systems

A heterogeneous reaction involving the reduction of a contaminant in a Fe0/H2O system is considered to include the following eight steps [24,91,94,95]; (i) Transport of reactants to the water/oxide interface, (ii) adsorption of reactants onto the outer oxide surface, (iii) diffusion of reactants across the oxide scale, (iv) adsorption of reactants onto the Fe0 surface, (v) chemical reaction at the Fe0 surface; (vi) desorption of soluble reaction products from the Fe0 surface, (vii) diffusion of reaction products across the oxide scale, and (viii) transport of soluble reaction products into the bulk solution. Figure 1 presents some of these processes. Steps (i) and (viii) are controlled by the rate of mass transfer of the soluble species (reactants and products), and hence by the hydrodynamics of the system (mixing types and intensities, water flow velocities). Step (ii) is mainly controlled by electrostatic interactions between reactants and the oxide scale. Steps (iii), (iv), (v), (vi), and (vii) are chemically controlled processes (reaction and concentration gradient) [35,63,91,93,95].
The main purpose of a kinetic study is to identify the rate-limiting or slowest step in the overall process, and how its rate may be increased. However, for brevity, kinetic aspects, and the potential interactions of individual contaminants and other dissolved species with the oxide scale (Figure 1, Table 1) are not considered herein. In essence, the whole oxide scale is a reactive system in which complexation, dissolution, precipitation, and co-precipitation occur simultaneously (Table 1), implying some parallel chemical reactions, including contaminant reduction. Barring processes occurring on bulk Fe0 and in water, most of the processes occurring in the Fe0/H2O system have not been adequately addressed in the literature (Table 1).

3. The Importance of Indirect Reduction

The primary aim of using Fe0 in groundwater remediation under anoxic conditions is to exploit the negative potential of the FeII/Fe0 redox couple (E0 = −0.44 V) to degrade or immobilize redox amenable contaminants. However, dissolved ferrous iron from the FeIII/FeII redox couple (E0 = 0.77 V) can act as reducing agent for some contaminants (e.g., CrVI) [53,62,63]. Furthermore, it has been shown that adsorbed FeII (or structural FeII) (E0 = −0.34 to −65 V) [96] can be more powerful in reducing contaminants than Fe0 (for E < −0.44 V). Therefore, abiotic contaminant reduction in a Fe0/H2O system does not necessarily take place by reduction through electrons from Fe0 (direct reduction). Instead, indirect reduction (electrons from FeII or even H/H2) may occur and is most likely, for at least, two reasons; (i) for direct reduction to occur the contaminant should diffuse through the oxide film unless the film is conductible to cater for electron transfer, and (ii) oxide-film is a good adsorbent for both contaminant and Fe2+ ions (resulting in more reactive structural FeII) [17,93]. Yet indirect reduction is currently considered as a side effect in the ‘Fe0 remediation’ literature [10,13,14]. The next paragraph demonstrates that indirect reduction (electrons from FeII) of nitro aromatic compounds has been known for more than 160 years and observed at neutral pH solutions since 1927.

3.1. Aniline and the Fe0/H2O System

In 1854, Béchamp [97] discovered the quantitative reduction of nitro aromatic compounds to anilines by Fe0 materials in acetic acid (CH3COOH) [38]. The increased demand for aniline in the chemical industry led to the use of cost-effective hydrochloric acid (HCl) in place of CH3COOH (Equation (5)). Shortly afterwards, it was established that a minor fraction of the stoichiometric amount of HCl (1/20 to 1/60) was sufficient for quantitative reduction [85]. Muspratt [98] was the first to demonstrate this experimental observation (Equations (5)–(7)):
C6H5-NO2 + 6 HCl + 3 Fe0 ⇒ C6H5-NH2 + 3 FeCl2 + 2 H2O
2 C6H5-NH2 + FeCl2 + 2 H2O ⇒ 2 C6H5-NH2, HCl + Fe(OH)2
2 C6H5-NH2, HCl + Fe0 ⇒ 2 C6H5-NH2 + FeCl2 + H2
As evidenced in Equations (5)–(7), FeII species produced by Fe0 corrosion are reducing agents for nitro aromatic compounds (indirect reduction). An innovative extension of the work of Béchamp [97] was published by Lyons and Smith [85]. These authors achieved quantitative reduction of nitro aromatic compounds by Fe0 in the presence of NaCl and FeCl3 (neutral salts) instead of HCl and CH3COOH. In the same experiment, the efficiency of NaCl was only 84% of that of FeCl3. As stated above, Cl− disturbs the formation of oxide film. Thus, the reported efficiency difference between NaCl and FeCl3 primarily reflects the differential behavior of Na+ and Fe3+ in the Fe0/H2O system and will not be further discussed here.

3.2. MnO2 and the Fe0/H2O System

In 2003, Noubactep and colleagues presented the reductive dissolution of MnO2 as a tool to control the availability of iron corrosion products (FeCPs) in a Fe0/H2O system [99,100]. This tool was used to establish the mechanism of aqueous U(VI) removal in the presence of Fe0 [100,101,102]. The rationale for this application is that Fe0 is not the reducing agent for MnO2 reductive dissolution (Equation (8)). Contrary, Fe0 oxidative dissolution (Equation (4)) is accelerated by virtue of Fe2+ consumption by MnO2 (Equation (9)) (i.e., Le Chatelier’s Principle).
2 Fe0 + 3 MnO2 + 12 H+ ⇒ 2 Fe3+ + 3 Mn2+ + 6 H2O
2 Fe2+ + MnO2 + 4 H+ ⇒ 2 Fe3+ + Mn2+ + 2 H2O
Equation (9) implies that FeCPs are precipitated at the surface of MnO2 and not in the vicinity of Fe0. This fact delays ‘Fe0 passivation’ and supports the repeatedly reported increased efficiency of MnO2-amended Fe0/H2O systems [31,50,103,104,105,106,107]. Results of the investigation of the reductive dissolution of MnO2 using Fe0 (e.g., Fe0/MnO2/H2O system) in hydrometallurgy corroborate the validity of Equation (9) as the main reduction path [108]. Similar to the concept of Khudenko [67], MnO2 is efficient at sustaining iron corrosion and thus, the resulting parallel reactions, including indirect reduction.
Another important feature of MnO2 for Fe0-based filters is that its reductive dissolution is volumetric contraction [106], and Mn2+ is more labile than Fe2+ under subsurface conditions. This characteristic makes MnO2 an excellent admixing agent to sustain the permeability of Fe0-based filtration systems in the subsurface, in water treatment plants and/or in household water treatment filters.

4. Long-Term Permeability of Fe0 Walls

Following the reductive transformation paradigm, Fe0 filters have been designed based on the flux of contaminants and the stoichiometry of reductive transformations after Equation (1) (direct reduction) [9,10,13,32]. This paradigm implies that “the more Fe0 is available, the more efficient the filter”. This rule of thumb has not been validated [109,110,111]. For example, Bi et al. [110] observed that columns with less than 80% (w/w) Fe0 performed better than those with higher Fe0 ratios. The reason lies in the well-established knowledge that metals expand during oxidative transformation, including immersed corrosion [77,78,79]. In other words, the volume of each iron corrosion product (oxide) is larger than the volume of atomic iron in the metallic state (Voxide > Viron).
The proper consideration of this observation has led to the conclusion that pure Fe0 filters (100% Fe0) are not sustainable [78,79,112] due to loss of porosity and hydraulic conductivity over time. Proper consideration of the volumetric expansive nature of aqueous iron corrosion was possible, and even obvious in the 1990s when the first Fe0 wall was constructed [10,113]. A critical review of the scientific literature at that time would have been sufficient to reveal the expansive nature of iron corrosion. In fact, gravel and sand have been routinely mixed with Fe0 mostly to reduce iron costs rather than to minimize loss of porosity [110].
Independent efforts (not in the context of Fe0 walls) have used steel wool to improve the efficiency of sand filters for phosphate removal [17,114,115,116] and suggested up to 8% Fe0 to avoid clogging [115,116]. Steel wool has also been added to sand filters for better pathogen removal [117]. However, steel wool has a very low density (g/cm3) [6] such that deriving the volumetric proportion from the weight proportion is not straightforward like for granular iron. These results attest that a pragmatic approach can be successful, although a science-based design is still needed.

5. Ion-Selective Nature of Fe0/H2O Systems (Coulomb’s Law)

Evidence from the iron corrosion literature is that the Fe0 surface is permanently shielded by an oxide scale [55,58,59,65,118]. The oxide scale is made up of various iron hydroxides and oxides with positively charged surfaces at near-neutral pH values [119,120]. The fundamental Coulomb’s law of electrostatics states that, a positively charged surface attracts negatively charged solutes. However, this fundamental law is silent about the redox reactivity of the solute, meaning that even the most redox reactive solutes must be attracted by the oxide scale before quantitative reductive transformation occurs. The Fe0 remediation literature is currently full of examples of positively charged solutes (e.g., methylene blue) used to test the redox efficiency of Fe0/H2O systems, another avoidable mistake [121,122].

6. Discussion

Fe0 as a tool for remediation emerged in the early 1990s and rapidly went through the stages of laboratory testing [10,109,123], field demonstration [10,19,113,124], and commercial application [10,22,125,126,127]. In 2002, the US EPA designated the use of Fe0 in a permeable reactive barrier (PRB) as a standard remediation technology. During this time, Fe0 filtration was also established as a standard technology for municipal and/or industrial wastewater treatment [14,115,116]. However, the technology has not yet been universally accepted, partly due to the abundance of conflicting data in the literature [14,33,46,128,129]. In recent years, substantial efforts have been made to investigate such discrepancies and thereby address such knowledge gaps [14,30,31,120,124,125,126,127,128,129,130,131]. However, in some instances, such new data have added even more confusion. For example, Ruhl et al. [129] disproved the suitability of four granular inert aggregates (anthracite, gravel, pumice, and sand) to sustain the efficiency of Fe0 walls. This directly contradicts the results of Bi et al. [110] discussed earlier and would mean that pure Fe0 beds are sustainable. In another example, Firdous and Devlin [132] challenged the suitability of using methylene blue as a tracer to image processes occurring in dynamic Fe0/H2O systems as presented by Miyajima and Noubactep [133]. Firdous and Devlin [132] have simply not considered the fact that Miyajima and Noubactep [133] have not used methylene blue as a model contaminant, but as a tracer to detect the availability of iron oxides in the Fe0/H2O system.
This presentation has demonstrated that detailed processes occurring in the Fe0/H2O system are not currently being comprehensively considered within the Fe0 remediation scientific community [10,30,31,33,45,48,50]. The technology was fortuitously discovered in 1990 [134]. During the past 25 years, a myriad of tools has been applied to further constrain Fe0 corrosion processes [10,14,16,135,136]. Moreover, a myriad of sophisticated analytical tools has been used to characterize experimental observations [21,127,137,138,139,140]. However, because the kinetics of iron corrosion is neither constant nor linear [141,142,143], a typical analytical observation (e.g., an Eh measurement or XPS (X-ray photoelectron spectroscopy) analysis during or at the end of column studies) is simply a static snap-shot, hence does not describe the processes in a holistic way. As such, it is simply impossible to generate accurate models of the dynamic processes occurring within the Fe0/H2O system from such data, especially as these occur over an extremely variable range of time scales—from seconds to decades [142]. These difficulties are not unique to the Fe0/H2O system [144] but are also common to other remediation technologies such as biosorption [20,28,31,46].
Available laboratory data are mostly irrelevant for field situations, mainly because of the lack of any reference Fe0 material and the fact that experiments were performed under very different conditions, very far from those occurring under field conditions [31,46,71,139,142]. Additionally, it has been acknowledged that Fe0 materials currently used for water treatment and environmental remediation (e.g., Gotthart-Maier GmbH or iPutech—Rheinfelden, Germany, Connelly-GPM Inc.—Chicago, IL, USA, Peerless Metals Powder Inc.—Detroit, MI, USA) have not been specially produced for these applications [10,21,145,146]. For example, Gotthart-Maier GmbH and later iPutech widely used at many test sites in Germany is a mixture of scrap materials. This means that they do not represent tailored Fe0 reagents for environmental remediation. This suggests that the use of common materials (operational reference) in well-designed experiments is the first step towards improved and comparable data. It is certain, that such an approach, coupled with the consideration of the ion-selective nature of the Fe0/H2O system [80,81,82] and the volumetric expansive nature of iron corrosion [77,78,79] will accelerate knowledge acquisition and boost the wide acceptance and application of the Fe0 technology [143,147,148]. In these efforts, reduction through FeII (indirect reduction) should be considered as an independent and major pathway for abiotic contaminant removal [149]. However, reductive transformation alone is not universally sufficient for contaminant removal (at μg·L−1 level).
Given the preceding analysis, four arguments are highlighted to advance the Fe0 technology. First, adsorption onto and co-precipitation with iron oxyhydroxide and (hydr) oxide (corrosion products) enhance the removal process (Argument 1). Second, once the importance of reductive transformation for contaminant removal is accorded its rightful contribution, it is easy to accept Fe0 as a generator for removing agents (oxides and hydroxides) (Argument 2). Moreover, considering that the surface of Fe oxides/hydroxides being predominantly positively charged at neutral pH values, the Fe0/H2O system is necessarily ion-selective (Argument 3). Lastly, because iron oxides are larger in volume than the original Fe0, room for expansion should be factored into the design of Fe0 filters/walls (Argument 4). Arguments 1 through 4 should be considered simultaneously as they are not independent.

7. Fe0 for Environmental Remediation

During the first 2.5 decades of Fe0 technology, no concerted effort was directed at providing the appropriate Fe0 material for utility engineers [16,128,130,136,139]. Despite a remarkable success [10,15,21,22,116], the Fe0 research community should continue to seek better and new ways to make an already proven efficient and affordable technology more reliable [10,15,45,140].
The most important area of improved reliability is arguably the field of material selection. Characterizing the intrinsic reactivity of used materials should increasingly come to the forefront. Factors influencing the rate of iron corrosion and, thus, the remediation efficiency of Fe0/H2O systems including Fe0 type, water chemistry (e.g., local hydrogeology or admixing agents), and operation conditions (e.g., Fe0 mass, volumetric ratio of Fe0 in the mixture, temperature, thickness of the reactive layer (Fe0-based), water flow rate/hydraulic retention time) should be considered. Material selection is further complicated by the diversity in the production quality of available Fe0 types and the lack of linear correlation between the intrinsic reactivity and the elemental composition of Fe0 materials [21,138,139,140,150,151,152].
Available knowledge on aqueous corrosion science is wide and includes cementation, organic synthesis and results reported in utility pipes from the oil industry and water/wastewater reticulation systems [14,33,46,63]. For example, cementation studies and Béchamps-like reactions have been carried out for more than 150 years. Their fundamental understanding can be adapted to the Fe0 remediation industry, mainly characterized by lower concentration solutions and higher pH values. In particular, it is not acceptable, that 30 years of research on Fe0 remediation has not characterized used materials. In essence, the research performed by Fe0 remediation researchers is comparable to that used to develop hydrometallurgical methods for precipitating a target metal from a given source as economically as possible (on a case-to-case basis) [23,147,153,154]. However, a caveat to this approach is that this knowledge is not directly transferable to the Fe0 remediation industry because the size (e.g., diameter) of Fe0 materials used in environmental remediation is typically smaller than the thickness of the pipes used in the oil industry or for water pipes (Table 2) [142]. To ensure the reliability of Fe0 walls and household water filters, targeted replacement for depleted (corroded) materials is necessary. For a risk-based maintenance or replacement strategy of Fe0 walls in service, it is imperative to relate the Fe0 intrinsic reactivity to hydro-geochemical conditions as a standard practice for a geo-environmental engineer. Such standard practice or guidelines should be formulated for specific applications, e.g., ‘use a Fe0 with an average corrosion rate of xy for carbonate-rich waters’ or ‘use a Fe0 with an average corrosion rate of yz for chloride rich waters’. Based on such guidelines, process-based models for the design of appropriate materials can be developed and used by researchers, engineers and asset managers. Such tools for asset management of Fe0 walls would introduce a systematic approach in the design of new Fe0/H2O systems and the evaluation of those already in operation.

8. Lessons from the History and Future Directions

There are two important features from Table 2 for the future of Fe0 filters for water treatment and safe drinking water provision in particular: (i) The technology is old and has been successfully used both at household and large scale for water treatment [147,151,171,172,173,174,175,176], and (ii) the mode of operation of Fe0/H2O system was established in 1991 [67]. The ancient use of Fe0-based systems for safe drinking water provision is summarized by Mwakabona et al. [147] and will not be discussed herein. It is just recalled that these systems were very efficient to remove pathogens and organics (humic substances). Accordingly, properly designed Fe0/H2O systems will efficiently treat water as research drawn from the last two to three decades has demonstrated their suitability for all classes of chemical contamination [10,12,16].
In May 1991, Boris Mikhail Khudenko published a concept paper in ‘Water Science and Technology’ entitled ‘Feasibility evaluation of a novel method for destruction of organics’. Khudenko [67] where, the author presented cementation as a tool to accelerate Fe0 oxidative dissolution (Equation (4)) and produce H species for the reductive transformation of organic species. In other words, the oxidation–reduction of organics is induced as a parallel reaction to this cementation reaction. A SCOPUS search on 19 October 2018 indicated that Khudenko [67] has been only referenced eight (8) times (Table 3) [177,178,179,180,181] and has not been considered in some literature discussing the removal mechanisms of organics in Fe0/H2O systems [10,24,25,182]. The concept of Khudenko [67] was based on a profound understanding of the Fe0/H2O system [183,184,185] and corresponds to Noubactep’s concept [39,40] that has been difficult to accept [48,49]. It is hoped that this independent proof will end the mechanistic discussion and orient all energies to design the next generation of efficient Fe0/H2O systems for water treatment, including Fe0 filters.
To further validate the concepts presented here and improve the mechanistic understanding of Fe0/H2O remediation systems further research should address the following:
(1)
Understanding the role and mechanisms of interfering inorganic and organic species typically occurring in natural multi-component aqueous systems under relevant environmental conditions,
(2)
Elucidating the processes occurring on the various material phases (solid, liquid, and solid–liquid interface), and their effects on the formation and persistence of the iron oxide film. Recent advances in surface analytical techniques for solid-state characterization enable such detailed studies,
(3)
Long-term studies using typical multi-component contaminated aqueous media conducted in a quiescent mode are required to overcome some of the limitations associated with short-term studies based on artificial solutions and ideal experimental conditions (e.g., agitation, constant temperature),
(4)
Development and evaluation of tailor-made Fe0/H2O systems that accounts for the expansive nature of iron corrosion, ion-selectivity, and the role of co-solutes/agents. This will overcome the limitations associated with the use of materials not purposively developed for Fe0 remediation.

9. Conclusions

The real challenge for active researchers on Fe0 for water treatment and environmental remediation is to properly use knowledge available in the mainstream corrosion science (Table 2) to design efficient Fe0/H2O systems. This task can be supported by available modern analytical tools (including surface-analytical techniques) to investigate well-known processes under relevant environmental conditions. However, the key lies in considering the system as a dynamic one comprising adsorbing, complexing, oxidizing and reducing species working in synergy for water decontamination (Arguments 1 through 4, Section 3). The three decades old work of Khudenko [67] recalls that scientific progress is at best achieved when the state-of-the-art knowledge is accurately given. Alternatively, a science-based approach can enable the rediscovery of the wheel. The fact that, one decade ago Noubactep [39] had independently achieved the conclusions of Khudenko [67] is regarded as the last proof for any researcher who might be impressed by the large volume of publications considering Fe0 as a reducing agent under environmental conditions. Despite three (1991–2018) or one (2007–2018) lost decade(s), future work should focus on directly investigating the highlighted arguments (1 to 4) to uncover the potential of the Fe0 technology and specify its limitations on a scientific basis. To this end, specific thematic areas warranting further research were highlighted.

Supplementary Materials

Supplementary File 1

Author Contributions

R.H., X.C., W.G., S.W. and C.N. contributed equally to manuscript compilation and revisions.

Funding

The work is supported by the Ministry of Education of the People’s Republic of China through the Program “Research on Mechanism of Groundwater Exploitation and Seawater Intrusion in Coastal Areas” (Project Code 20165037412) and by “the Fundamental Research Funds for the Central Universities” (“Research on the hydraulic tomographical method for aquifer characterization”, Project Code 2015B29314). It is also supported by Jiangsu Provincial Department of Education, Project Code 2016B1203503.

Acknowledgments

The author thanks Richard A. Crane (University of Cardiff, Cardiff, UK) and Mohammad A. Rahman (University of Hannover, Hannover, Germany) for helpful discussions, review of the manuscript, and the illustrative material. We acknowledge support by the German Research Foundation and the Open Access Publication Funds of the Göttingen University.

Conflicts of Interest

The authors declare no conflict of interest.

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Water 10 01739 g001 550
Figure 1. Schematic of the elemental iron (Fe0)/H2O system. Features relevant to contaminant removal are labelled. Modified after Taylor [91].
Figure 1. Schematic of the elemental iron (Fe0)/H2O system. Features relevant to contaminant removal are labelled. Modified after Taylor [91].
Water 10 01739 g001
Table
Table 1. Some relevant processes occurring in the elemental iron (Fe0)/H2O system and their spatial locations. The comments relate to the extent to which each process has been considered in the Fe0 literature, with “+ + +” denoting being regarded as satisfactorily, and “+” just acknowledged.
Table 1. Some relevant processes occurring in the elemental iron (Fe0)/H2O system and their spatial locations. The comments relate to the extent to which each process has been considered in the Fe0 literature, with “+ + +” denoting being regarded as satisfactorily, and “+” just acknowledged.
LocationProcessesComments
Bulk Fe0dealloying, electron transport+ + +
Fe0/H2O interfaceFe dissolution, complexation, precipitation+ +
Fe0/Oxidesoxide precipitation+
Oxide scalemigration of species, oxide recrystallization+
Oxides/H2Ooxide dissolution/precipitation, Fe complexation+
H2Omass transfer (advection and diffusion)+ + +
Table
Table 2. Chronology of selected historical observations showing that the science of the Fe0/H2O system was explored before the 1970s while cementation studies and Béchamps-like reactions have been carried out for more than 150 years.
Table 2. Chronology of selected historical observations showing that the science of the Fe0/H2O system was explored before the 1970s while cementation studies and Béchamps-like reactions have been carried out for more than 150 years.
YearEvent
<1850The cementation reaction is used for winning metals from ores [155,156,157]
<1850Iron shavings are used to treat drinking water [147,158,159]
1854Béchamp synthesized of aniline from nitrobenzene and Fe0 (iron and organic acid) [97]
1865Bekelov suggested that all cementation reactions are electrochemical in nature [155,157]
1873Bischoff established the spongy iron filter for household [147,159,160]
1881Spongy iron filters are tested at large scale in Antwerp (Belgium) [147,158,160]
1883Spongy iron filters secured water supply in Antwerp (Belgium) [147,160,161,162]
1885Revolving purifiers are installed in Antwerp (Belgium) [147,161,162]
1888Muspratt rationalized the successful use of HCl in the Béchamp reduction [98]
1914Holt used scrap iron instead of coarse scrap iron for the cementation of PbII [98,155,157]
1923Lueg showed that aniline and other substances inhibit iron corrosion [98,155,157]
1928Oldright and co-workers showed that only thin Fe0 beds are long-term sustainable [155]
1928Knowlton reported that the rate of iron corrosion is higher in NaCl solutions [60]
1928Knowlton reported that the used Fe0 type determines the extent of reduction [60]
1951Lauderdale and Emmons used steel wool to remove radioactive species from water [153]
1951–1961Werner published almost yearly review articles on “Amination by Reduction” [163,164,165,166,167,168,169]
1951–1961The Béchamp reduction is extended to other groups of compounds [163,164,165,166,167,168,169]
1969Case and Jones treated CrVI- and CuII-containing brass mill effluents with scrap iron [170]
1984Tseng et al. used steel wool to concentrate 60Co from nuclear effluent [154]
1986Harza Environmental Services patented Se(VI) removal in Fe0 beds [171]
1991Khudenko established the cementation based reductive degradation of organics [67]
1990Reynolds and co-authors observed dechlorination of RCl in Fe0-based vessels [11]
1994Fe0 is established as an efficient material for subsurface reactive walls [1,10,113]
Table
Table 3. SCOPUS chronological bibliometry of Khudenko [67] showing citation by eight (8) sources comprising four (4) review and four (4) research articles.
Table 3. SCOPUS chronological bibliometry of Khudenko [67] showing citation by eight (8) sources comprising four (4) review and four (4) research articles.
YearJournalTypeSectionReference
2015Water ResearchReviewIntroduction[16]
2014Environmental Science & Technology Letters ResearchDiscussion[75]
2010ACS Symposium SeriesReviewIntroduction[177]
2009Chemosphere ResearchDiscussion[178]
2007Environmental Science & TechnologyResearchIntroduction[179]
2004Environmental Science & TechnologyResearchDiscussion[145]
2000Environmental TechnologyReviewIntroduction/discussion[180]
1998ACS Symposium SeriesReviewDiscussion[181]

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Hu, R.; Cui, X.; Gwenzi, W.; Wu, S.; Noubactep, C. Fe0/H2O Systems for Environmental Remediation: The Scientific History and Future Research Directions. Water 2018, 10, 1739. https://doi.org/10.3390/w10121739

AMA Style

Hu R, Cui X, Gwenzi W, Wu S, Noubactep C. Fe0/H2O Systems for Environmental Remediation: The Scientific History and Future Research Directions. Water. 2018; 10(12):1739. https://doi.org/10.3390/w10121739

Chicago/Turabian Style

Hu, Rui, Xuesong Cui, Willis Gwenzi, Shuanghong Wu, and Chicgoua Noubactep. 2018. "Fe0/H2O Systems for Environmental Remediation: The Scientific History and Future Research Directions" Water 10, no. 12: 1739. https://doi.org/10.3390/w10121739

APA Style

Hu, R., Cui, X., Gwenzi, W., Wu, S., & Noubactep, C. (2018). Fe0/H2O Systems for Environmental Remediation: The Scientific History and Future Research Directions. Water, 10(12), 1739. https://doi.org/10.3390/w10121739

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