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.

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.