Water Treatment Using Metallic Iron: A Tutorial Review

Review Water Treatment Using Metallic Iron: A Tutorial Review Rui Hu 1, Willis Gwenzi 2, Viviane Raïssa Sipowo-Tala 3 and Chicgoua Noubactep 1,4,5,* School of Earth Science and Engineering, Hohai University, Fo Cheng Xi Road 8, Nanjing 211100, China; [email protected] 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; [email protected] 3 Faculty of Health Sciences, Campus of Banekane, Université des Montagnes, P.O. Box 208 Bangangté, Cameroon; [email protected] 4 Department of Applied Geology, University of Göttingen, Goldschmidtstraße 3, Göttingen D-37077, Germany; [email protected] 5 Department of Water and Environmental Science and Engineering, Nelson Mandela African Institution of Science and Technology, Arusha P.O. Box 447, Tanzania * Correspondence: [email protected] 1 Received: 15 August 2019; Accepted: 12 September 2019; Published: 14 September 2019 Abstract: Researchers and engineers using metallic iron (Fe0) for water treatment need a tutorial review on the operating mode of the Fe0/H2O system. There are few review articles attempting to present systematic information to guide proper material selection and application conditions. However, they are full of conflicting reports. This review seeks to: (i) Summarize the state-of-theart knowledge on the remediation Fe0/H2O system, (ii) discuss relevant contaminant removal mechanisms, and (iii) provide solutions for practical engineering application of Fe0-based systems for water treatment. Specifically, the following aspects are summarized and discussed in detail: (i) Fe0 intrinsic reactivity and material selection, (ii) main abiotic contaminant removal mechanisms, and (iii) relevance of biological and bio-chemical processes in the Fe0/H2O system. In addition, challenges for the design of the next generation Fe0/H2O systems are discussed. This paper serves as a handout to enable better practical engineering applications for environmental remediation using Fe0. Keywords: iron corrosion products; laboratory experiments; pilot tests; removal mechanisms; water treatment; zero-valent iron 1. Introduction The increased occurrence of micro-pollutants and pathogens in the hydrosphere is typically associated with population growth and increasing anthropogenic activities [1,2]. Historically, pollution of municipal water resources by human wastes was the starting point of industrial water treatment [1,3,4]. Public health and environmental concerns drive efforts to develop affordable, effective, and robust technologies for the removal of pollutants from water (water treatment). Related technologies are based on physical, chemical, electrical, thermal, and biological principles [5]. Filtration on fixed beds has been demonstrated as the most important one because of its wide range of applications [6,7] and ease of operation [8,9]. Adsorptive filtration is considered the most affordable water treatment technology due to the availability of a wide range of suitable adsorbents [5,6,8]. Adsorption also enables the removal of biological, chemical, and physical pollutants. However, adsorption has its limitations, such as finding suitable materials of high adsorption capacity [5,8,9]. For the past two or three decades, metallic iron (Fe0) has been discussed in the Processes 2019, 7, 622; doi:10.3390/pr7090622 www.mdpi.com/journal/processes Processes 2019, 7, 622 2 of 19 literature as an affordable reactive material for environmental remediation and decentralized safe drinking water provision [4]. However, the Fe0 remediation technology is an old one [10]. As early as 1856, a household water filter using metallic iron (Fe0) was patented [10]. Between 1881 and 1885, Fe0-based filters were successfully tested and used for the water supply of the city of Antwerp (Belgium) [10–13]. Afterwards, the city of Antwerp was supplied for some 30 years by water treated in a “revolving purifier”, a Fe0-based fluidized bed [13]. Thus, engineered Fe0-based systems for safe drinking water provision have a scientific history dating back to more than 160 years ago [14,15]. The long history of engineered Fe0-based systems for water treatment is not a continuous one [15]. Related systems have been abandoned and (partly) independently rediscovered several times [11,16–23]. In fact, after the first large scale applications in Antwerp and elsewhere [10–13], the Fe0 technology was abandoned after World War I and there was no trace of it in the Western peerreviewed scientific literature until 1951 [4,18]. On the other hand, the Harza Process (1986) [20] and all subsequent ones, including reactive barriers (1994), have not considered available knowledge from Western scientific journals [21–23]. Four examples will be given in a chronological order to illustrate the extent of confusion in the literature. (i) Example 1: The Emmons Process Lauderdale and Emmons [18] introduced the Emmons Process independently from past knowledge on using Fe0 (steel wool (SW) or Fe0 SW) for safe drinking water provision [15]. The Emmons Process is a compact unit designed at Oak Ridge National Laboratory (USA) to treat small volumes of radioactive polluted waters. This unit can be universally adapted for the following applications: (i) Emergency drinking water supply, and (ii) water supply in small communities. Lauderdale and Emmons [18] primarily added Fe0 SW to remove ruthenium, “for which it had been found to be very effective”. They hypothesized that Fe0 SW “serves both as a reducing agent and as a medium for the adsorption of radio-colloids”. The same authors also documented that radioactivity was not readily removed from the filter by washing it with water. Radioactive species were thus not removed by simple adsorption or “mechanical floc filtration” (i.e., pure size-exclusion). Another key observation by Lauderdale and Emmons [18] was that a band of rust appeared at the column inlet and progressed very slowly through the Fe0 bed. The columns clogged rapidly if: (i) Extremely fine grade of Fe0 SW was used, and/or (ii) fine iron filings were used. It was postulated that using a coarse grade of iron filings or other metals in granular form would enable the design of more robust and sustainable filtration systems. A year later, Lacy [24] successfully tested aluminum (Al0), copper (Cu0), and zinc (Zn0) as alternative filter materials to Fe0. However, the relationship between Fe0 type and clogging was not further investigated. Nearly 30 years later, the Harza Process has experienced the same challenge. Moreover, even 50 years later, Westerhoff and James [25] have faced the same problem without going to the basic fundamentals and looking for the scientific origin of this key observation [26,27]. It is interesting to note that using mathematical modeling (ref. [26]) could establish that the 1:3 Fe0:sand ratio used in the water works in Antwerp is the optimal ratio concealing efficiency and permeability in the long term. (ii) Example 2: The Harza Process The Harza Engineering Company patented in 1986 an Fe0-based process known as the Harza Process for the removal of toxic metals from wastewaters [28]. The Harza Process was successfully pilot-tested for treating selenium (Se)-polluted agricultural drainage water. The Harza Process involved filtering Se-polluted water through beds packed with iron filings (100% Fe0) at controlled flow rates. Se removal was quantitative, but the filters clogged rapidly, thus the system was efficient but not sustainable. After three years of intensive research using several instrumental analytical tools, including Fourier transform infrared spectroscopy and Raman spectroscopy, the investigators realized that Se was removed by the action of in-situ generated iron oxyhydroxides, mostly at the inlet of the column. Adsorption and co-precipitation were the main removal mechanisms, even though some redox reactions were possible [19]. One important observation of Anderson [19] was that the clogging behavior was not dependent upon the type of water flowing through the Fe0 bed. Similar results were obtained whether Se was spiked to natural or to distilled waters. It is anticipated Processes 2019, 7, 622 3 of 19 here that the bed clogging is an intrinsic property of Fe0, as aqueous iron corrosion is accompanied by volumetric expansion [29]. Accordingly, properly considering the Fe0 mass balance in a porous system would have solved this problem [30,31]. (iii) Example 3: The SONO Arsenic Filter Intensive research on using Fe0 to treat water started around 1990 [32–34], when Fe0 was clearly used as a stand-alone contaminant-removing agent. However, Khan et al. [21] used Fe0 to increase the dissolved iron concentration and induce aqueous arsenic (As) removal by adsorption and coprecipitation. The resulting system, the SONO arsenic filter, was awarded the Grainger Challenge Gold Award [30]. While rationalizing the efficiency of SONO filter for As removal, Hussam and Munir [35] considered that inorganic AsIII species are oxidized to AsV species, which are strongly adsorbed onto hydrous ferric oxide. This explanation is acceptable when focusing the attention on As, but cannot explain why more than 20 other species are removed by SONO filters [36]. In fact, the efficiency of Fe0-based systems to remove pathogens and chemical contaminants from water was documented more than a century earlier [15,37]. In view of the diversity of contaminants that have been reported to be quantitatively removed in Fe0/H2O systems [38–40], it was stated that reduction is not likely to be a significant removal mechanism [41–44]. A decade after the first critical review severely questioning the reductive transformation concept [45], the view that Fe0 is an own reducing agent under environmental conditions is still prevailing [46–48]. It appears that within the Fe0 research community, there exists many individuals without adequate preparations to differentiate between chemical and electro-chemical reaction mechanisms. Yet, the viability and advancement of any scientific discipline largely depend on the quality of its investigators and the work they produce [49]. Considering its nature, this is a problem which cannot be resolved by some isolated research groups [43]. Therefore, the compilation of this tutorial review is a tool to make the problem better known to both the current and the future Fe0 research community. (iv) Example 4: Direct Versus Indirect Reduction Mechanisms Hu et al. [50] characterized the reductive process of nitrate in the Fe0/H2O system and concluded that “the indirect reduction of nitrate by hydrogen generated from the reaction between proton and metallic iron may be a major mechanism for the reduction of nitrate under the experimental conditions”. Although direct reduction by Fe0 is thermodynamically possible, and even more favorable (E0 = −0.44 V; [51]), indirect reduction by FeII species (E0 = 0.77 V) is also favorable as the electrode potential for the reduction of NO3− is higher (E0 > 0.80 V). Articles published after Hu et al. [50] have rarely tried to clarify the real mechanism of NO3− removal in Fe0/H2O systems, as reviewed by Vodyanitskii and Mineev [52]. They mostly just considered direct reduction, like the majority of available earlier works [46,47]. It is rather surprising that, nearly two decades after the work of Hu et al. [50], the indirect reduction mechanism is still being ignored by some in the Fe0 research community. Indeed, this behavior seems to be the rule in the Fe0 literature, and is a further motivation for the present tutorial review. Examples 1 to 4 have clearly shown that information regarding the applicability of Fe0 materials in the water treatment industry is conflicting and confusing. New information has been constantly added, independent from the available common database. As a consequence, some intrinsic impracticable designs have been published in the peer-reviewed literature [53,54]. Both references have not properly considered (i) the volumetric expansive nature of iron corrosion or (ii) the iron corrosion rate and its time-dependency. It is certain that many processes ridiculed by experts have been subsequently successfully applied [17]. However, the science of the system should be constantly considered. Systems (e.g., electrocoagulation) can work satisfactorily for decades before their operating mode is established [55,56]. The Fe0/H2O remediation system is no exception to this. The Fe0 literature is full of systems, whose functionality is rationalized by challenging the mainstream iron corrosion science [57,58]. Clearly, the present tutorial review aims at demonstrating that concepts deeply rooted in the Fe0 research community and generally taken for granted in daily research endeavors are not consistent with scientific principles. In such a context, aspects inherent to the established questionable concepts are investigated, rather than fundamentally questioning their benefit for science and engineering. Processes 2019, 7, 622 4 of 19 This work summarizes the science of aqueous iron corrosion and contrasts it with selected aspects of literature reports in order to improve the understanding of the interactions accounting for the removal of contaminants in Fe0/H2O systems. The bulk of published monitoring studies lack sufficient detail with respect to study design, thus making conclusive interpretation of results difficult [59–62]. For example, Wilkin et al. [61,62] investigated CrVI and trichloroethylene removal in subsurface Fe0 barriers without addressing the intrinsic characteristics (e.g., corrosion rate, extent of depletion) of the used materials. It is therefore imperative to routinely employ a ‘before’ and ‘after’ monitoring design to adequately assess potential impacts on selected operational parameters. Such a science-based understanding is crucial to design more efficient and sustainable Fe0-based remediation systems. 2. The Chemistry of the Fe0/H2O system A piece of metallic iron (Fe0) rusts when it is exposed to humid air. Here, Fe0 corrosion is caused by both water (H2O) and atmospheric oxygen (O2). In other words, under ambient conditions, Fe0 corrodes to form a reddish-brown hydrated metal oxide (Fe2O3•×H2O = rust) (Figure 1—top). Rust continually flakes off and exposes the Fe0 surface to abundant oxygen and trace amounts of water (humidity) [63]. Under atmospheric conditions, both air oxygen and water are required for rust to form. A piece of Fe0 immersed in water under ambient conditions rusts differently than that in air. Here, water is abundant and dissolved O2 is limited (8 mg L−1). Under quiescent conditions, the Fe0 surface is covered by layers of iron oxides in the sequence of increasing oxidation states: FeO–Fe3O4– Fe2O3 (oxide scale). This oxide scale is equally not protective because of it structural differences [63– 65]. The oxide scale also continually flakes off and exposes the Fe0 surface to abundant water, but not likely to dissolve oxygen which has to diffuse through the oxide scale to reach the Fe0 surface (Figure 1—bottom). If the immersing water is poor in O2 (anoxic conditions), Fe0 corrosion will occur at a lower kinetics and will be mostly made up of Fe3O4. However, some Fe2O3 (and other FeIII species) will be formed such that the oxide scale on Fe0 is never an electrically conductive one [47,66,67]. Under atmospheric conditions, Fe0 is oxidized to Fe2+ at an anodic site on the Fe0 surface (e.g., a lattice defect), while O2 is reduced to water at a different site on the Fe0 surface (the cathode). Electrons are transferred from the anode to the cathode through the electrically conductive metal (Fe0). Water is a solvent for the produced Fe2+, and also acts as a salt bridge (electrolyte). Rust is formed by the subsequent oxidation of Fe2+ by atmospheric O2 [68]. In other words, under atmospheric conditions, as long as the Fe0 surface is not completely covered by an oxide scale, O2 can be reduced to H2O in an electro-chemical reaction (Figure 1—top). Here, the perfect interplay between the four components (anode, cathode, conductive metal, electrolyte) of an electro-chemical cell are depicted. As a rule, under immersed conditions, dissolved O2 cannot quantitatively reach the Fe0 surface because the oxide scale acts as an O2 diffusion barrier [45,68]. Thus, Fe0 is corroded by water, while O2 is reduced by FeII species. Clearly, iron corrosion is still an electro-chemical reaction, but in this instance, O2 reduction is a chemical reaction (reduction by FeII species) (Figure 1—bottom). Processes 2019, 7, 622 5 of 19 Figure 1. Sketch of the electro-chemical process of iron corrosion as influenced by the abundance of water and dissolved O2 before (top) and after (bottom) the generation of a porous oxide scale. The porous oxide-film is a diffusion barrier to all dissolved species, including O2. To illustrate what has been said, the reactions that occur at the anode and the cathode in each case will be presented together with the corresponding overall cell reaction. In both cases, the anodic reaction is the oxidative dissolution of Fe0: Fe0  Fe2+ + 2 e−, (1) Under anoxic conditions, the cathodic reaction is certainly the reduction of water. Also, under immersed oxic conditions (presence of an oxide scale), the cathodic reaction may be the reduction of water: 2 H + + 2 e −  H 2, 2 H2O + 2 e−  H2 + 2OH−, (2) Processes 2019, 7, 622 6 of 19 It then follows that the overall cell reaction for immersed iron corrosion is: Fe0 + 2 H+  Fe2+ + H2, (3) Actually, when O2 or other oxidizing agents (e.g., reducible contaminants) are present, they are reduced by FeII species in a chemical reaction (Figure 1—bottom). Fe0 corrosion by water (Equation (3)) is accelerated because Fe2+ is consumed (Le Chatelier’s Principle). The electrode reaction for the reduction of oxygen reads as: O2 + 4 H+ + 4 e−  2 H2O, (4) The electrode reaction for the oxidation of Fe2+ reads as: Fe2+  Fe3+ + e−, (5) Under atmospheric, non-immersed conditions, electrochemical reduction of O2 by Fe0 is possible according to Equation (6): 2 Fe0 + O2 + 4 H+  2 Fe2+ + 2 H2O, (6) Under immersed conditions (oxide scale shields Fe0), O2 is reduced by Fe2+ according to Equation (7): 4 Fe2+ + O2 + 4 H+  4 Fe3+ + 2 H2O, (7) Similar to O2, all other reducible species must overcome the electrical non-conductive diffusive barrier (oxide scale) to reach the Fe0 surface (Figure 1—bottom). Due to the non-conductive nature of the oxide scale, no quantitative electron transfer from the metal (Fe0) to the possibly thereon adsorbed species is possible. Clearly, contaminant reduction in an Fe0/H2O system is rarely (or even never) the cathodic reaction simultaneous to Fe0 oxidation. This fundamental knowledge preceded the mechanistic discussion within the Fe0 research community [69–71]. In fact, three years before Matheson and Tratnyek [69], Khudenko presented a concept for the cementation-induced oxidationreduction of organics [72–74]. The method makes use of the following: (i) Electronegative sacrificial metals (e.g., Fe0 or Al0), (ii) a salt of a sufficiently electropositive metal (e.g., CuSO4), and (iii) reagents for pH shift (e.g., pyrite or mineral acids) [72]. The redox transformation of target organics is induced as a parallel reaction to the cementation process. Clearly, aqueous Fe0 oxidation is accelerated by Cu2+ reduction (cementation), and redox transformation of organics is induced by parallel reactions. Again, Fe0 corrosion and redox transformation of organics are not simultaneous reactions [57,75,76]. Such reactions have been independently used over the years, the most recent example could be that of Xi et al. [48]. These authors used CuSO4 to accelerate the kinetics of arsenic removal in Fe0/H2O systems. It is important to recall that As is removed by adsorption and co-precipitation [77,78]. Accordingly, rationalizing the efficiency of Fe0/H2O systems for water decontamination by any electrochemical process involving the pollutants has been a huge mistake [69–71]. The electrochemistry-based reasoning implies the electrode potential of the couple FeII/Fe0 (E0 = –0.44 V), representing the anodic half-reaction in the electrochemical cell. This is thermodynamically possible (E0 values) but physically impossible because of the omnipresence of a non-conductive oxide scale. The next section recalls the fundamentals for an electrochemical cell and presents relevant influencing factors for their investigation. 3. Investigating the Electrochemical Corrosion in Fe0/H2O Systems For an electrochemical cell to be formed, the following four components must be available: (i) An anode where oxidation occurs, (ii) a cathode where reduction occurs, (iii) an external pathway to allow the flow of electrons, and (iv) a salt bridge (or a porous barrier) allowing ions to flow back and forth from the electrodes (anode and cathode) (Figure 1—top) [79–81]. In the context of remediation Fe0/H2O systems (immersed iron corrosion), anode and cathode are two different sites on the metal where Fe0 is dissolved to Fe2+ (Equation (1)) and H2O reduced to H2 (Equation (2)), respectively. Electron transfer is secured by the Fe0 body and contaminated water is the electrolyte (ionically conducting medium). A constant connection is required among the four components for an Processes 2019, 7, 622 7 of 19 electrochemical reaction to occur. As discussed in Section 2, the Fe0 is constantly shielded by an oxide scale which is electronically non-conductive. This is the reason why quantitative contaminant reduction is never the cathodic reaction coupled to Fe0 oxidative dissolution at the anode. Clearly, contaminant redox transformation in an Fe0/H2O system is not an electrochemical, but a chemical process [57,75,76,82]. Another key feature of the remediation Fe0/H2O system is a salt bridge (electrolyte), which is not always a ‘free’ aqueous solution, but a hydrated oxide scale, which is a porous barrier that allows the flow of anions and cations (Figure 1—bottom). The first consequence of this situation is that increasing ion mobility accelerates iron corrosion, and thus all related processes, including contaminant redox transformations. For this reason, fixing the experimental ionic strength while using high salt concentration is a huge conceptual mistake, as Fe0 corrosion is accelerated in a manner that will not be reproduced in natural waters [82,83]. The authors of the present communication have been avoiding this mistake for two decades by using natural or tap water as background electrolytes while investigating the Fe0/H2O system [84–86]. As Fe0/H2O systems are used in a variety of field conditions (static to fluidized bed), it is understood that operational conditions in laboratory experimentations should correspond to the mimicked real situations [41,45,87,88]. In particular, experiments pertaining to improved understanding of Fe0-based filtration systems should be performed under conditions enabling the formation of oxide scale at the Fe0 surface or in its vicinity [87]. On the contrary, the vast majority of experiments investigating the mechanism of contaminant removal in subsurface Fe0 barriers have been performed under agitated/stirred and controlled conditions, mostly for relatively short experimental durations [41,45]. Summarizing, the proper investigation of remediation Fe0/H2O systems implies that experiments are performed under conditions relevant to field situations, including the experimental duration. The importance of the experimental duration is reflected by the dynamic nature of iron corrosion, which implies Fe0 oxidation, Fe(OH)x formation, polymerization and precipitation, Fe oxide crystallization, and subsequent formation of the oxide scale. The dynamic nature of Fe0 corrosion is addressed in the next section. 4. The Dynamic Nature of the Fe0/H2O System In aqueous solutions, iron corrosion is relentless, thus “rust does not rest”. This means that an immersed Fe0 specimen corrodes until it is completely depleted. A better understanding of the longterm corrosion process could hold clues for engineering improved Fe0-based remediation systems. A variety of Fe0 specimens have been tested and used for environmental remediation and water treatment [89–93]. However, these studies failed to pay particular attention to the iron corrosion rate [94,95]. The standard method for measuring the rate of corrosion entails immersing an Fe0 specimen in a salt solution (e.g., NaCl), and then periodically monitoring the mass loss. This approach has been proven time-consuming and has presented an operational barrier for the development of new Fe0 alloys, as test times of several months are necessary [96,97]. This evidence alone suggests that laboratory experiments pertinent to field Fe0 barriers would have to last for months or years. This has not been the case in existing studies, as even column experiments just lasted for some weeks or months [41,45,75,76]. The most important reason why column experiments should last for months and years is that iron corrosion is a volumetric expansive process [29,98]. It is well known that the volume of each iron corrosion product (Voxide) is larger than the volume of iron metal (Vmetal). In the context of iron corrosion in steel reinforcing bars, it has been established that 2.1  Voxide/Vmetal  6.4 [99,100]. Using this reasoning, Caré et al. [26] established that no Fe0 filtration system containing more than 53% Fe0 can be sustainable due to loss of porosity (and permeability) in the long term [27]. The reasoning assumed uniform spherical Fe0 particles having an initial size of 1.2 mm and filling the packed-bed with an initial porosity of 36%. To the best of the authors’ knowledge, no field application has used such spherical Fe0 particles, and measured initial porosity is often larger than 36%. However, it is evident that pure Fe0 (100% Fe0) beds are not sustainable. However, permeability loss and failure of Fe0 barriers have been attributed to all possible arguments, but not really the expansive corrosion of Processes 2019, 7, 622 8 of 19 Fe0 [59,101,102]. An evident merit of considering the volumetric expansive nature of iron corrosion has been to end the discussion as to whether mixing Fe0 and inert aggregates (e.g., gravel, MnO2, sand) is beneficial for packed beds. It was clearly established that mixing Fe0 with other nonexpansive aggregates is not a tool to reduce Fe0 cost [103,104], but a prerequisite for sustainable filtration systems [26,27,105]. Specifically, mixing Fe0 and non-expansive aggregates is meant to maintain porosity and permeability. A further key feature of the dynamic nature of the Fe0/H2O system is the differential reactivity of iron corrosion products (FeCPs) as they are produced and transformed in-situ [64,65,106]. Sikora and Macdonald [106] differentiated between aged and nascent FeCPs with respect to contaminant removal. Nascent FeCPs, or ‘living FeCPs’, are very reactive, while aged FeCPs are non-reactive and termed as ‘dead FeCPs’ [107,108]. It has already been discussed in detail that the dynamic nature of FeCP generation implies that contaminants are removed by adsorptive co-precipitation [41,43,45,109,110]. This corresponds to the view of Leffmann [17] that: “The most practical benefit of the application of electricity to water purification will come from the indirect methods in which the electrical energy used to produce an active disinfecting agent, and this is then applied to the water”. The presentation, until now, has clearly demonstrated that Fe0/H2O systems dynamically produce FeCPs for pollutant removal (water treatment). The dynamic nature of the system implies that their efficiency at any particular time depends on at least one key variable: The amount and the proportion of nascent FeCPs. For filtration systems, the extent of permeability loss should be considered as well [94,95]. 5. Investigating the Fe0/H2O System The dynamic nature of the Fe0/H2O system implies that the "bottle-point'' technique, traditionally used to characterize the contaminant removal efficiency of adsorbing agents in batch systems [111,112], should be profoundly revisited. The main reason being that Fe0 is a reactive material, producing adsorbing agents in-situ [20,21]. It is certain that discrepancies in published data are rationalized by the different experimental procedures employed by individual researchers [58,113,114]. Experimental procedures differ with respect to Fe0 size and type, Fe0 pre-treatment, Fe0 particle size, Fe0 dosage, volume of solution, shaking/stirring type and intensity, fraction of the bottles filled with solution, contaminant concentration, buffer application, and equilibration time [111,112,114]. In particular, the shaking/stirring type and intensity should not unnecessarily disturb the formation of an oxide scale on Fe0 [88]. There has been no systematic study of the effects of operational parameters on the decontamination process using Fe0/H2O systems [114]. Moreover, the nature of Fe0 as an in-situ generator of contaminant scavengers (FeCPs) is yet to be recognized, as many researchers are still regarding it as a reducing agent (under environmental conditions) [34,46]. This section paves the way for the much needed systematic studies which would enable the realization of the huge potential of Fe0 for environmental remediation and water treatment. Investigating an Fe0/H2O system implies that the four components (anode, cathode, conductive metal, electrolyte) of the electro-chemical iron corrosion are properly considered. The evidence that Fe0 is covered by an electrically non-conductive oxide scale implies that (quantitative) electron transfer from the anode to the cathode is only possible for water, the solvent. All other oxidizing species must migrate across the oxide scale to reach a cathodic site where reduction would occur. For this reason, reductive transformations of any species in Fe0/H2O systems should be regarded as a side effect (or a parallel reaction) of aqueous iron corrosion (Equation (3)) [19,41–45,72]. The presence of the oxide scale also implies that the electrolyte (contaminated water) is entrapped in a sort of porous barrier, where contaminant transport is an ion-selective process. It then follows that: (i) Quantitative contaminant transport depends on the relative surface charge of the oxide scale and one of the contaminant (at a given pH value), and (ii) contaminant transport, and thus iron corrosion, is favored by all factors sustaining ionic migration. The presentation until now suggests that there are three main groups of influencing factors for aqueous iron corrosion: (i) The nature of the electrodes (anode and cathode), (ii) the nature of the conductive metal (nature and proportion of alloying elements), and (iii) the nature of the electrolyte Processes 2019, 7, 622 9 of 19 (pH value, temperature, and solution chemistry). It is understood that environmental remediation using Fe0 is only possible in the pH ranges where the solubility of iron is low (pH > 4.5) [115-117]. The three groups of influencing factors are now discussed in some details. The impact of some common additives on the Fe0/H2O system are also discussed. 5.1. The Nature of the Electrodes Thermodynamically, an immersed Fe0 specimen corrodes because there is a potential difference between two different sites at its surface. The site where Fe0 is oxidized (Fe2+ is produced) is the anode, while the site where water (H+) is reduced (H2 is produced) is the cathode. In other words, in aqueous iron corrosion, electrons are transferred from Fe0 to H2O (Equation (3)). The tendency of Fe0 to corrode in water is grounded in the difference in the electrode potential of the two involved redox couples: FeII/Fe0 (E0 = −0.44 V) and H+/H2 (E0 = −0.00 V). E0 = −0.44V is the electrode potential of Fe0 in all relevant reactive Fe0 alloys. Accordingly, any difference in reactivity between different Fe0 specimens is purely a kinetic issue, and depends mainly on the history of each individual specimen. Relevant parameters influencing the Fe0 intrinsic reactivity include: The manufacturing process, the surface area, the size and the form of the particles, the alloying elements and their proportions. While the details of manufacturing conditions are typically not accessible to the researcher, all other parameters can be analytically determined [93,118,119]. It is essential to recall that all relevant parameters are interdependent and none of them could be proven superior in determining the reactivity of Fe0 materials [89,92,120–122]. The evidence that E0 = −0.44 V is the driving force for all Fe0 materials implies that the nature of 0 Fe is a stand-alone variable in investigating the efficiency of Fe0 for environmental remediation. The recent literature on “remediation Fe0” reveals that this evidence has not been properly considered [91,93]. Moreover, there is still no standard protocol for characterizing the Fe0 intrinsic reactivity and there exists no reference material to which new materials should be compared [122]. On the other hand, available Fe0 materials are characterized for contaminant removal in parallel short-term experiments using pure adsorbents, including aged iron oxides [113,123,124]. For inert adsorbents (e.g., activated carbon, iron oxide, sand), an adsorption capacity is defined and gives the contaminant amount (e.g., mg) retained per unit adsorbent mass (g). The adsorption capacity (mg g−1) enables the prediction of packed-bed adsorbers [111]. The adsorption capacity has been translated to the Fe0 research, in a context where initial Fe0 is not depleted, the used material is not characterized for its intrinsic reactivity, and the kinetics of iron corrosion (corrosion rate) is not known [94,95]. Accordingly, the adsorption capacity is used without any knowledge on the available adsorbent amount and its intrinsic reactivity. This is not a good starting point for the comparison of independent results, even obtained under similar experimental conditions. Summarizing, this section clearly shows that various Fe0 materials, including nano-scale Fe0 and bimetallics, have been manufactured or selected and mostly reported to be successfully used for water treatment on a purely pragmatic basis. This is not a premise for progress in knowledge. Better systematic experimental work should be designed to rationalize the already documented success stories [30,31,35,61,62,125,126]. 5.2. The Nature of the Conductive Metal Conventional Fe0 materials used for reductive transformation of contaminants often produce reaction products which are sometimes more toxic than the parent compounds [127]. On the other hand, the transformation process is slowed down as the natural oxide scale develops at the Fe0 surface. It is in this context that bimetallic systems were introduced, wherein a second metal is combined with Fe0 [127,128]. The second metal primarily has three functions: (i) Acts as a hydrogenating catalyst, (ii) prevents the formation of the oxide film on the Fe0 surface, and (iii) induces Fe0 to release electrons due to the difference in reduction potentials [127,129,130]. However, the first function (hydrogenation catalyst) is questionable as H/H2 has to migrate through the oxide film to the site where the contaminant is adsorbed (Figure 1—bottom). The second metal certainly disturbs the formation of the oxide scale [63–65], but cannot really prevent it in the long term. The property of the second metal Processes 2019, 7, 622 10 of 19 to induce electron release from Fe0 is a fundamental aspect and likely the most accurate (Section 3) [79,128,131]. As demonstrated by Noubactep [128] in the pH range of natural waters, the second metal primarily induces Fe0 oxidative dissolution and accelerates—or at least sustains—the corrosion process, which in turn induces contaminant removal. Remember that Fe0 is a generator of contaminant scavengers. Another aspect to consider at this stage is that processes based on chemical reactions are never stand-alone ones for water decontamination at low concentration levels. In fact, the concentration corresponding to the solubility limit is often larger than the permissible maximum contamination level (MCL). For example, the MCL for fluoride is 1.5 mg L−1, while the concentration corresponding to the solubility limit of CaF2 is about 8.0 mg L−1 [86,132]. Moreover, even reaction products of reducible species must be removed from the aqueous phase. This means that adsorption, co-precipitation, and adsorptive size-filtration are the dominant removal mechanisms for contaminant removal in Fe0/H2O systems, including those using bimetallic systems [128,133]. In summary, this section recalls that alloying Fe0 to form bimetallic and multi-metallic systems are just a tool to sustain the reactivity of conventional Fe0/H2O systems. Contaminants are transformed and removed by the same mechanisms [133,134]. An affordable bimetallic system that has been successfully tested is one entailing the addition of sulfur [123,135,136]. 5.3. The Nature of the Electrolyte The importance of water composition (including pH value and temperature) as operational and environmental variables on the efficiency of Fe0/H2O systems is obvious. As a rule, the contaminants of concern are considered together with the pH value and the concentration of major ions, particularly anions (Cl−, HCO3−, NO3− and SO42−) [137–139]. Discrepancies among studies resulted mostly from the diversity of other operational variables like the Fe0 intrinsic reactivity, Fe0 dosage, the mixing rates, and experimental duration. It has already been stated that fixing the ionic strength with common salts is a conceptual mistake, as the ionic conductivity of the oxide scale is increased in a manner that will not be reproduced in nature. The pH values are the most influencing factor as contaminant removal is only quantitative at pH > 4.5. Fortunately, this corresponds to the pH range of natural waters. However, because aqueous iron corrosion consumes acidity (Equation (3)), even industrial wastewaters with lower pH values can be efficiently treated [114,140]. On the other hand, the pH value of natural waters can be artificially fixed, for example, to 4.0 using H2SO4, to optimize the efficiency of the Fe0/H2O system [132]. The pH value also determines the surface charge of adsorbents (including sand and iron oxides) and the speciation of dissolved contaminants, and thus the efficiency of the system (Figure 2) [82,141]. The question arises how to test systems at laboratory scale in a way that results would be transferable to field situations. The conventional approach is to vary individual or single background ions. Another approach is to use a single natural water or tap water as a background electrolyte. More reliable approaches exist [142,143]; the one introduced by Heffron et al. [142] is to use four synthetic waters representing the ionic composition of a wide range of natural waters. These synthetic test waters mimicked low and high ionic concentrations for both surface and groundwater (Table 1). Using these model waters and systematically varying all relevant operational parameters would accelerate knowledge acquisition for the design of more robust and efficient systems. Ideally, each tested system is monitored for contaminant removal efficiencies, residual Fe concentration, and pH value. Processes 2019, 7, 622 11 of 19 p K a2 p K a1 A 2- + 2 H - + AH + H + AH2 s o lid p h a s e s : p z c i , p z c j ... H 2 -c o rro s io n 0 2 O 2 -c o rro s io n 4 6 8 10 12 14 p H v a lu e Figure 2. pH dependence of (i) the iron corrosion mechanism (i.e., hydrogen evolution, oxygen corrosion) and (ii) the specification of dissolved substances (A2−, AH−, AH2). The solid phases existing in Fe0/H2O systems (mainly oxides) can be considered as poly-electrolytes. Each solid phase is characterized by an isoelectric point (pHpzc = pzc = "point of zero charge"). Above the pHpzc, the solid oxide surface is negatively charged. AH2 is a weak electrolyte with two acidity constants (pKa values). Similar to pHpzc values, pKa values are the limit of the predominance ranges (A 2−/AH− and AH−/AH2). Table 1. Composition of the four water matrices used by Heffron et al. [142]. Ion concentrations are in mmol L−1 (mM). The solutions were prepared using Milli-Q water and analytical grade reagents. The synthetic waters are SL (Surface Low), SH (Surface High), GL (Ground Low), and GH (Ground High). High and Low are related to the ionic strength (μ). SL SH GL [Ca2+] (mM) 0.399 0.898 2.300 [Mg2+] (mM) 0.181 0.333 1.400 [Na+] (mM) 1.200 1.950 5.560 [Cl−] (mM) 1.030 2.000 5.640 [HCO3−] (mM) 1.200 1.950 5.560 [SO42−] (mM) 0.067 0.229 0.874 μ (mM) 3.0 5.9 17.5 GH 2.920 1.780 22.800 18.900 9.090 2.100 39.0 Matrix 5.4. The Impact of Selected Additives Various aggregates have been added to granular Fe0 to modify the efficiency of the Fe0/H2O system for water treatment. Typical aggregates include activated carbon [40], anthracite [144], gravel [144], magnetite [145], MnO2 [58,146], pumice [95], sand [139,144], and zeolites [144]. While sand (inert) alone certainly increases the efficiency of Fe0/H2O systems, other more or less reactive aggregates have been introduced for the same purpose, but without systematic investigations demonstrating the corresponding concepts. For example, Huang et al. [145] reported that the copresence of Fe0, Fe3O4, and dissolved FeII creates a highly reactive system for molybdate removal. This is a tangible experimental observation. However, given that addition of both Fe3O4 and FeII enhances the Fe0 efficiency, it is difficult to assess the specificity of this ternary system. Moreover, the named system should have been compared to the Fe0/sand/FeII system and the affordability of using Fe3O4 discussed. For illustration, two examples are: (i) Song et al. [147], who found out that in batch experiments, Fe0/sand systems were more efficient in removing aqueous CrVI than pure Fe0 systems; Processes 2019, 7, 622 12 of 19 and (ii) Westerhoff and James [25], who reported that columns containing about 50% Fe0 (w/w) were more efficient at removing NO3− than pure Fe0 beds (100% Fe0). The rationale for this is that sand is in-situ coated with iron oxides that are better adsorbents for both negatively-charged HCrO4− and NO3− than the sand surface [85,148]. It is interesting to note that Noubactep et al. [82] initially used FeS2 and MnO2 as a tool to accumulate UVI in the vicinity of Fe0, and to enhance its reductive precipitation by Fe0 [149]. However, the experimental observation in both cases was a delay of UVI removal. In the Fe0/FeS2 system, pyrite dissolution induced a pH shift and quantitative contaminant removal was observed only in systems exhibiting a final pH > 4.5. In the Fe0/MnO2 system, MnO2 reductive dissolution consumed Fe2+ from iron corrosion (Equation (3)), and UVI removal was not quantitative until MnO2 was depleted. Both observations suggest that there is no quantitative UVI removal before iron hydroxides start to ‘freely’ precipitate [150]. It was clearly established that UVI removal is not a property of ‘reducing Fe0’, but a consequence of aqueous iron corrosion in the presence of dissolved UVI. This observation was generalized [41,45], and Fe0 was suggested as a suitable material for universal access to safe drinking water [107]. Overall, this section recalls that several efficient Fe0-based hybrid systems have been introduced and partly used over the years. The performance of these systems can be optimized based on the science of aqueous iron corrosion [113,151]. It should be noted that pre-washing Fe0 materials before use was also applied. This procedure solely frees the Fe0 surface from atmospheric corrosion products and thus accelerates ‘free’ precipitation of FeCPs [121,152]. 6. Conclusions The concept that contaminant removal in Fe0/H2O systems is caused by aqueous iron corrosion (Equation (3)) is consistent with many experimental observations, including successful technical applications. It appears to be a generalization of “the cementation-induced oxidation-reduction” of dissolved compounds, which was demonstrated to be technically feasible in the early 1990s [72]. It is somewhat surprising to note that Khudenko [72]: (i) Published the findings in Water Science and Technology (IWA Publishing), an English language journal, which is expected to be widely read; and (ii) focused on organic compounds, but was almost completely ignored for 28 years of intensive research on the remediation Fe0. This review clearly delineates the important role of system analysis in understanding the efficiency of Fe0/H2O systems for water treatment and environmental remediation. During the past three decades, the field of “remediation using Fe0” has been expanding at an amazing speed. This didactic review indicates that field applications of the named systems are mostly not based on their scientific understanding. The question then arises: What is next? Some trends should emerge on the horizon, and they are well-aligned with other remediation systems. First, paralleling the increased scope in treatability studies and field demonstrations, the quest to characterize the intrinsic reactivity of used materials has increased and even simpler protocols have been presented. As a consequence, it can be expected that both a standard protocol and a reference Fe0 material can be adopted in the coming years [108,122]. Second, the search for system design and operating principles has become popular [46,114,152– 154]. This search has been a central theme in ‘Fe0 remediation’ for a long time [155,156], but has been hampered by considering Fe0 as a reducing agent. It appears that the next phase on this path is to consider the dynamic nature of the process of iron corrosion and its volumetric expansive nature [26,27]. Despite the availability of a sound theoretical concept [26,157], it might be that additional concepts must be developed to grasp design and operating features spanning sustainable Fe0-based systems. Third, the community of remediation Fe0 is progressively recognizing the limitations of the reductive transformation concept [58,114]. Recognizing this deficiency implies that future developments should be based on long-term experiments (lab and pilot) to account for the long-term variability of the kinetics of iron corrosion and system clogging by iron corrosion products [94,95]. Processes 2019, 7, 622 13 of 19 Particular attention should be paid to the non-linear relationship between Fe0 size and corrosion rate [158]. Summarizing, thousands of papers are available on water treatment by Fe0-based systems using batch systems. Some few of them apply column systems at laboratory, pilot, and field scales, including commercial-scale applications. Unfortunately, the whole effort was based on a pragmatic, experience-based approach which cannot enable any reliable prediction of the long-term performance of any system under actual environments. Therefore, it is time to move towards longterm, well-designed experiments which could enable knowledge-based Fe0 selection for the design of sustainable systems. There is a great need to explore more granular Fe0 materials for developing commercial-scale decentralized water treatment systems. To the best of the authors’ knowledge and experience, the future of Fe0 in water treatment is bright. Collaborative efforts of research and industry are needed to materialize a dream of economical and feasible decentralized water treatment technology. Only by working together will it be possible to achieve universal safe drinking water provision and global clean environment. The present tutorial review has revealed that a major obstacle on this path is of educational nature. There is practically no formalized corrosion education of scientists and professionals working on Fe0 in water treatment. Thus, this article presents an opportunity for universities, educational institutions, and professional associations to play a lighthouse role in this field. Author Contributions: R.H., W.G., R.S., and C.N. contributed equally to the manuscript compilation and revisions. Funding: This work was supported by the Ministry of Science and Technology of China through the Program “Research on Mechanism of Groundwater Exploitation and Seawater Intrusion in Coastal Areas” (Project Code 20165037412) and by the Ministry of Education of China through “the Fundamental Research Funds for the Central Universities” (Project Code: 2015B29314). It was also supported by Jiangsu Provincial Department of Education (Project Code 2016B1203503). Acknowledgments: The manuscript was improved thanks to the insightful comments of anonymous reviewers from Processes. Conflicts of Interest: The authors declare no conflicts of interest. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. Howe, K.J.; Hand, D.W.; Crittenden, J.C.; Trussell, R.R.; Tchobanoglous, G.: Principles of Water Treatment. John Wiley & Sons, Inc.: Hoboken, NJ, USA, 2012; p. 674. 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