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.

Quiescent batch experiments were conducted to evaluate the influences of Cl^-, F^-, HCO₃^-, HPO₄^2-, and SO₄^2- on the reactivity of metallic iron (Fe⁰) for water remediation using the methylene blue (MB) method. Strong discoloration of MB indicates high availability of solid iron corrosion products (FeCPs). Tap water was used as an operational reference. Experiments were carried out in graduated test tubes (22 mL) for up to 45 d, using 0.1 g of Fe⁰ and 0.5 g of sand. Operational parameters investigated were (i) equilibration time (0-45 d), (ii) 4 different types of Fe⁰, (iii) anion concentration (10 values), and (iv) use of MB and Orange II (O-II). The degree of dye discoloration, the pH, and the iron concentration were monitored in each system. Relative to the reference system, HCO₃^- enhanced the extent of MB discoloration, while Cl^-, F^-, HPO₄^2-, and SO₄^2- inhibited it. A different behavior was observed for O-II discoloration: in particular, HCO₃^- inhibited O-II discoloration. The increased MB discoloration in the HCO₃^- system was justified by considering the availability of FeCPs as contaminant scavengers, pH increase, and contact time. The addition of any other anion initially delays the availability of FeCPs. Conflicting results in the literature can be attributed to the use of inappropriate experimental conditions. The results indicate that the application of Fe⁰-based systems for water remediation is a highly site-specific issue which has to include the anion chemistry of the water.

Granular metallic iron (gFe⁰) materials have been widely used for eliminating a wide range of pollutants from aqueous solutions over the past three decades. However, the intrinsic reactivity of gFe⁰ is rarely evaluated and existing methods for such evaluations have not been standardized. The aim of the present study was to develop a simple spectrophotometric method to characterize the intrinsic reactivity of gFe⁰ based on the extent of iron dissolution in an ascorbic acid (AA—0.002 M or 2 mM) solution. A modification of the ethylenediaminetetraacetic acid method (EDTA method) is suggested for this purpose. Being an excellent chelating agent for Fe^II and a reducing agent for Fe^III, AA sustains the oxidative dissolution of Fe⁰ and the reductive dissolution of Fe^III oxides from gFe⁰ specimens. In other words, Fe⁰ dissolution to Fe^II ions is promoted while the further oxidation to Fe^III ions is blocked. Thus, unlike the EDTA method that promotes Fe⁰ oxidation to Fe^III ions, the AA method promotes only the formation of Fe^II species, despite the presence of dissolved O₂. The AA test is more accurate than the EDTA test and is considerably less expensive. Eight selected gFe⁰ specimens (ZVI1 through ZVI8) with established diversity in intrinsic reactivity were tested in parallel batch experiments (for 6 days) and three of these specimens (ZVI1, ZVI3, ZVI5) were further tested for iron leaching in column experiments (for 150 days). Results confirmed the better suitability (e.g., accuracy in assessing Fe⁰ dissolution) of the AA test relative to the EDTA test as a powerful screening tool to select materials for various field applications. Thus, the AA test should be routinely used to characterize and rationalize the selection of gFe⁰ in individual studies.

Metallic iron (Fe⁰) is a reactive material for treating polluted water. The effect of water salinity on the efficiency of Fe⁰-based remediation systems is not yet established. This work aims to clarify the reasons why Cl⁻ ions are often reported to improve the efficiency of Fe⁰/H₂O remediation systems. Quiescent batch experiments were carried out to characterize the effect of chloride (Cl⁻) ions on the efficiency of methylene blue (MB) discoloration in the presence of Fe⁰. Cl⁻ was used in the form of NaCl at concentrations ranging from 0 to 40 g L⁻¹. The MB concentration was 10 mg L⁻¹, the Fe⁰ loading was 5 g L⁻¹, and the duration of the experiment varied from 2 to 46 days. Four different Fe⁰ materials were tested in parallel experiments. Tests with different NaCl levels were performed in parallel with three other organic dyes: Methyl orange (MO), orange II (OII), and reactive red 120 (RR 120). The results clearly show that the presence of Cl⁻ reduces the extent of dye discoloration in all systems investigated. The efficiency of the dyes increased in the order MB < MO < RR 120 < OII. In systems with varying NaCl concentrations, dye discoloration initially decreases with increasing NaCl and slightly increases for [NaCl] > 30 g L⁻¹. However, the extent of dye discoloration for [NaCl] = 40 g L⁻¹ remains much lower than for the system with [NaCl] = 0 g L⁻¹. The results clearly demonstrate that the presence of Cl⁻ fundamentally delays the process of contaminant removal in Fe⁰/H₂O systems, thus improving the understanding of the contaminant interactions in Fe⁰-based remediation systems. These results also suggest that the effects of other inorganic anions on the efficiency of Fe⁰/H₂O systems should be revisited for the design of field applications.

Metallic iron (Fe⁰) has been increasingly used to remove toxics from water over the past three decades. However, the idea that metallic iron (Fe⁰) is not an environmental reducing agent has been vigorously refuted. Researchers presenting their findings in a scientific journal have to accept the burden of proving that their argument has any validity. This 30-year-lasting discussion within the Fe⁰ remediation community is alien to electro-chemists, as it is a century-old knowledge. Nevertheless, the peer-reviewed literature on “remediation using Fe⁰” seems to be dominated by evaluators thinking that Fe⁰ is a reducing agent. This communication challenges the view that Fe⁰ donates any electron to any dissolved species. The sole goal is to reconcile a proven efficient technology with its scientific roots and enable the design of better Fe⁰ remediation systems.