Researchers on metallic iron (Fe⁰) for environmental remediation and water treatment are walking in a valley of confusion for 25 years. This valley is characterized by the propagation of different beliefs that have resulted from a partial analysis of the Fe⁰/H₂O system as (1) a reductive chemical reaction was considered an electrochemical one and (2) the mass balance of iron has not been really addressed. The partial analysis in turn has been undermining the scientific method while discouraging any real critical argumentation. This communication re-establishes the complex nature of the Fe⁰/H₂O system while recalling that, finally, proper system analysis and chemical thermodynamics are the most confident ways to solve any conflicting situation in Fe⁰ environmental remediation.

This study characterizes the decrease of the hydraulic conductivity (permeability loss) of a metallic iron-based household water filter (Fe⁰ filter) for a duration of 12 months. A commercial steel wool (SW) is used as Fe⁰ source. The Fe⁰ unit containing 300 g of SW was sandwiched between two conventional biological sand filters (BSFs). The working solution was slightly turbid natural well water polluted with pathogens (total coliform = 1950 UFC mL⁻¹) and contaminated with nitrate ([NO₃⁻] = 24.0 mg L⁻¹). The system was monitored twice per month for pH value, removal of nitrate, coliforms, and turbidity, the iron concentration, as well as the permeability loss. Results revealed a quantitative removal of coliform (>99%), nitrate (>99%) and turbidity (>96%). The whole column effluent depicted drinking water quality. The permeability loss after one year of operation was about 40%, and the filter was still producing 200 L of drinking water per day at a flow velocity of 12.5 L h⁻¹. A progressive increase of the effluent pH value was also recorded from about 5.0 (influent) to 8.4 at the end of the experiment. The effluent iron concentration was constantly lower than 0.2 mg L⁻¹, which is within the drinking-water quality standards. This study presents an affordable design that can be one-to-one translated into the real world to accelerate the achievement of the UN Sustainable Development Goals for safe drinking water.

There are many factors to consider for the design of appropriate water treatment systems including: cost, the concentration and type of biological and/or chemical contamination, concentration limits at which contaminant(s) are required to be removed, required flow rate, level of local expertise for on-going maintenance, and social acceptance. An ideal technology should be effective at producing clean, potable water; however it must also be low-cost, low-energy (ideally energy-free) and require low-maintenance. The use of packed beds containing metallic iron (Fe⁰ filters) has the potential to become a cheap widespread technology for both safe drinking water provision and wastewater treatment. Fe⁰ filters have been intensively investigated over the past two decades, however, sound design criteria are still lacking. This article presents an overview of the design of Fe⁰ filters for decentralized water treatment particularly in the developing world. A design for safe drinking water to a community of 100 people is also discussed as starting module. It is suggested that Fe⁰ filters have the potential for significant worldwide applicability, but particularly in the developing world. The appropriate design of Fe⁰ filters, however, is site-specific and dependent upon the availability of local expertise/materials.

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.

Continuous gravity-fed column experiments using the methylene blue (MB) discoloration method were performed to characterize the suitability of a pozzolan (PZ) specimen as alternative admixing aggregate for metallic iron filters (Fe⁰-filters). Investigated systems were: (i) pure sand, (ii) pure PZ, (iii) pure Fe⁰, (iv) Fe⁰/sand, (v) Fe⁰/PZ, and (vi) Fe⁰/sand/PZ. The volumetric proportion of Fe⁰ was 25%. The volumetric proportions of the Fe⁰/sand/PZ system was 25/45/30. The initial MB concentration was 2.0 mg·L⁻¹, 6.0 g of Fe⁰ was used, and the experiments lasted for 46 days. The individual systems were fed with 3.9 to 8.4 L (7.80 to 16.69 mg of MB) and were characterized by the time-dependent changes of: (i) the pH value, (ii) the iron breakthrough, (iii) the MB breakthrough, and (iv) the hydraulic conductivity. Results showed that the Fe⁰/sand/PZ system was the most efficient. This ternary system was also the most permeable and therefore the most sustainable. The suitability of MB as a powerful operative indicator for the characterization of processes in the Fe⁰/H₂O system was confirmed. The tested PZ is recommended as an alternative material for efficient but sustainable Fe⁰ filters.

Metallic iron (Fe⁰) has been demonstrated as an excellent material for decentralized safe drinking water provision, wastewater treatment and environmental remediation. An open issue for all these applications is the rational material selection or quality assurance. Several methods for assessing Fe⁰ quality have been presented, but all of them are limited to characterizing its initial reactivity. The present study investigates H₂ evolution in an acidic solution (pH 2.0) as an alternative method, while comparing achieved results to those of uranium removal in quiescent batch experiments at neutral pH values. The unique feature of the H₂ evolution experiment is that quantitative H₂ production ceased when the pH reached a value of 3.1. A total of twelve Fe⁰ specimens were tested. The volume of molecular H₂ produced by 2.0 g of each Fe⁰ specimen in 560 mL H₂SO₄ (0.01 M) was monitored for 24 h. Additionally, the extent of U(VI) (0.084 mM) removal from an aqueous solution (20.0 mL) by 0.1 g of Fe⁰ was characterized. All U removal experiments were performed at room temperature (22 ± 2 °C) for 14 days. Results demonstrated the difficulty of comparing Fe⁰ specimens from different sources and confirmed that the elemental composition of Fe⁰ is not a stand-alone determining factor for reactivity. The time-dependent changes of H₂ evolution in H₂SO₄ confirmed that tests in the neutral pH range just address the initial reactivity of Fe⁰ materials. In particular, materials initially reacting very fast would experience a decrease in reactivity in the long-term, and this aspect must be incorporated in designing novel materials and sustainable remediation systems. An idea is proposed that could enable the manufacturing of intrinsically long-term efficient Fe⁰ materials for targeted operations as a function of the geochemistry.