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.

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.

The typical time-dependent decrease of the iron corrosion rate is often difficult to consider while designing Fe⁰-based remediation systems. One of the most promising approaches is the amendment with manganese dioxide (Fe⁰/MnO₂ system). The resulting system is a very complex one where characterization is challenging. The present communication uses methylene blue discoloration (MB method) to characterize the Fe⁰/MnO₂ system. Shaken batch experiments (75 rpm) for 7 days were used. The initial MB concentration was 10 mg L⁻¹ with the following mass loading: [MnO₂] = 2.3 g L⁻¹, [sand] = 45 g L⁻¹, and 0 < [Fe⁰] (g L⁻¹) ≤ 45. The following systems where investigated: Fe⁰, MnO₂, sand, Fe⁰/MnO₂, Fe⁰/sand, and Fe⁰/MnO₂/sand. Results demonstrated that MB discoloration is influenced by the diffusive transport of MB from the solution to the aggregates at the bottom of the test-tubes. Results confirm the complexity of the Fe⁰/MnO₂/sand system, while establishing that both MnO₂ and sand improve the efficiency of Fe⁰/H₂O systems in the long-term. The mechanisms of water decontamination by amending Fe⁰-based systems with MnO₂ is demonstrated by the MB method.

This article critically evaluates the conventional Kanchan Arsenic Filter (KAF) in order to determine the main reasons for its reported poor performance. The KAF was introduced in 2004 in Nepal and makes use of non-galvanized nails as a Fe⁰ source for As removal. As early as 2009, the KAF was demonstrated to be ineffective for As removal in many cases. This was unambiguously attributed to the Fe⁰ layer which is placed on top of a sand filter instead of being incorporated into a sand matrix. Despite this conceptual mistake, the conventional KAF has been largely distributed in Asia, and recent articles have assessed its sustainability. This study reiterates that the suitability of the technology, rather than its sustainability, should be addressed. Evidence shows that the KAF has the following design limitations: (i) uses iron nails of unknown reactivity, and (ii) operates on the principle of a wet/dry cycle. The latter causes a decrease in the corrosion rate of the used nails, thereby limiting the availability of the iron corrosion products which act as contaminant scavengers. Taken together, these results confirm the unsuitability of the conventional KAF. Besides correcting the design mistakes, more attention should be paid to the intrinsic reactivity of the used iron nails, including using alternative Fe⁰ materials (e.g., iron filings, steel wool) for filters lasting for just 6 or 18 months. Specific design considerations to be addressed in the future are highlighted.

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