A critical survey of the abundant literature on environmental remediation and water treatment using metallic iron (Fe⁰) as reactive agent raises two major concerns: (i) the peculiar properties of the used materials are not properly considered and characterized, and, (ii) the literature review in individual publications is very selective, thereby excluding some fundamental principles. Fe⁰ specimens for water treatment are typically small in size. Before the advent of this technology and its application for environmental remediation, such small Fe⁰ particles have never been allowed to freely corrode for the long-term spanning several years. As concerning the selective literature review, the root cause is that Fe⁰ was considered as a (strong) reducing agent under environmental conditions. Subsequent interpretation of research results was mainly directed at supporting this mistaken view. The net result is that, within three decades, the Fe⁰ research community has developed itself to a sort of modern knowledge system. This communication is a further attempt to bring Fe⁰ research back to the highway of mainstream corrosion science, where the fundamentals of Fe⁰ technology are rooted. The inherent errors of selected approaches, currently considered as countermeasures to address the inherent limitations of the Fe⁰ technology are demonstrated. The misuse of the terms “reactivity”, and “efficiency”, and adsorption kinetics and isotherm models for Fe⁰ systems is also elucidated. The immense importance of Fe⁰/H₂O systems in solving the long-lasting issue of universal safe drinking water provision and wastewater treatment calls for a science-based system design.

The general suitability of water treatment systems involving metallic iron (Fe⁰) is well-established. Various attempts have been made to improve the efficiency of conventional Fe⁰ systems. One promising approach combines granular Fe⁰ and an iron sulfide mineral to form Fe⁰/Fe-sulfide/H₂O systems. An improved understanding of the fundamental principles by which such systems operate is still needed. Through a systematic analysis of possible reactions and the probability of their occurrence, this study establishes that sulfide minerals primarily sustain iron corrosion by lowering the pH of the system. Thus, chemical reduction mediated by Fe^II species (indirect reduction) is a plausible explanation for the documented reductive transformations. Such a mechanism is consistent with the nature and distribution of reported reaction products. While considering the mass balance of iron, it appears that lowering the pH value increases Fe⁰ dissolution, and thus subsequent precipitation of hydroxides. This precipitation reaction is coupled with the occlusion of contaminants (co-precipitation or irreversible adsorption). The extent to which individual sulfides impact the efficiency of the tested systems depends on their intrinsic reactivities and the operational conditions (e.g., sulfide dosage, particle size, experimental duration). Future research directions, including the extension of Fe⁰/Fe-sulfide/H₂O systems to drinking water filters and (domestic) wastewater treatment using the multi-soil-layering method are highlighted.

Studies were undertaken to determine the reasons why published information regarding the efficiency of metallic iron (Fe⁰) for water treatment is conflicting and even confusing. The reactivity of eight Fe⁰ materials was characterized by Fe dissolution in a dilute solution of ethylenediaminetetraacetate (Na₂–EDTA; 2 mM). Both batch (4 days) and column (100 days) experiments were used. A total of 30 different systems were characterized for the extent of Fe release in EDTA. The effects of Fe⁰ type (granular iron, iron nails and steel wool) and pretreatment procedure (socking in acetone, EDTA, H₂O, HCl and NaCl for 17 h) were assessed. The results roughly show an increased iron dissolution with increasing reactive sites (decreasing particle size: wool > filings > nails), but there were large differences between materials from the same group. The main output of this work is that available results are hardly comparable as they were achieved under very different experimental conditions. A conceptual framework is presented for future research directed towards a more processed understanding.

There is a burgeoning interest in reliably characterizing the intrinsic reactivity of metallic iron materials (Fe⁰) or zero-valent iron materials (ZVI) used in the water treatment industry. The present work is a contribution to a science-based selection of Fe⁰ for water treatment. A total of eight (8) granular ZVI materials (ZVI1 to ZVI8) were tested. Fe⁰ dissolution in ethylenediaminetetraacetic acid (EDTA test) and 1,10-Phenanthroline (Phen test) is characterized in parallel experiments for up to 250 h (10 days). 50 mL of each solution and 0.1 g of each Fe⁰ material are equilibrated in quiescent batch experiments using 2 mM EDTA or Phen. Results indicated a far higher extent of iron dissolution in EDTA than in Phen under the experimental conditions. The tested materials could be grouped into three reactivity classes: (i) low (ZVI4, ZVI6, ZVI7, and ZVI8), (ii) moderate (ZVI1 and ZVI5) and (iii) high (ZVI2 and ZVI3). The order of reactivity was the same for both tests: ZVI2 ≅ ZVI3 > ZVI1 ≅ ZVI5 > ZVI4 ≅ ZVI6 ≅ ZVI7 ≅ ZVI8. Phen results revealed for the first time the time-dependent variation of the kinetics of iron corrosion (corrosion rate) in short-term batch experiments. Overall, the results demonstrated the superiority of the Phen test for evaluating the initial stage of Fe⁰ dissolution. Long-term column experiments are recommended to deepen the acquired knowledge.

There is growing interest in using pyrite minerals (FeS₂) to enhance the efficiency of metallic iron (Fe⁰) for water treatment (Fe⁰/H₂O systems). This approach contradicts the thermodynamic predicting suppression of FeS₂ oxidation by Fe⁰ addition. Available results are rooted in time series correlations between aqueous and solid phases based on data collected under various operational conditions. Herein, the methylene blue method (MB method) is used to clarify the controversy. The MB method exploits the differential adsorptive affinity of MB onto sand and sand coated with iron corrosion products to assess the extent of Fe⁰ corrosion in Fe⁰/H₂O systems. The effects of the addition of various amounts of FeS₂ to a Fe⁰/sand mixture (FeS₂ method) on MB discoloration were characterized in parallel quiescent batch experiments for up to 71 d (pH₀ = 6.8). Pristine and aged FeS₂ specimens were used. Parallel experiments with methyl orange (MO) and reactive red 120 (RR120) enabled a better discussion of the achieved results. The results clearly showed that FeS₂ induces a pH shift and delays Fe precipitation and sand coating. Pristine FeS₂ induced a pH shift to values lower than 4.5, but no quantitative MB discoloration occurred after 45 d. Aged FeS₂ could not significantly shift the pH value (final pH ≥ 6.4) but improved the MB discoloration. The used systematic sequence of experiments demonstrated that adsorption and coprecipitation are the fundamental mechanisms of contaminant removal in Fe⁰/H₂O systems. This research has clarified the reason why a FeS₂ addition enhances the efficiency of Fe⁰ environmental remediation.

Metallic iron (Fe⁰) corrosion under immersed conditions (Fe⁰/H₂O system) has been used for water treatment for the past 170 years. Fe⁰ generates solid iron corrosion products (FeCPs) which are known to in situ coat the surface of aggregates, including granular activated carbon (GAC), gravel, lapillus, manganese oxide (MnO₂), pyrite (FeS₂), and sand. While admixing Fe⁰ and reactive aggregates to build hybrid systems (e.g., Fe⁰/FeS₂, Fe⁰/MnO₂, Fe⁰/sand) for water treatment, it has been largely overlooked that these materials would experience reactivity loss upon coating. This communication clarifies the relationships between aggregate addition and the sustainability of Fe⁰/H₂O filtration systems. It is shown that any enhanced contaminant removal efficiency in Fe⁰/aggregate/H₂O systems relative to the Fe⁰/H₂O system is related to the avoidance/delay of particle cementation by virtue of the non-expansive nature of the aggregates. The argument that aggregate addition sustains any reductive transformation of contaminants mediated by electrons from Fe⁰ is disproved by the evidence that Fe⁰/sand systems are equally more efficient than pure Fe⁰ systems. This demonstration corroborates the concept that aqueous contaminant removal in iron/water systems is not a process mediated by electrons from Fe⁰. This communication reiterates that only hybrid Fe⁰/H₂O filtration systems are sustainable.