Understanding the Operating Mode of Fe0/Fe-Sulfide/H2O Systems for Water Treatment

Investigating the Fe0/H2O systems using the methylene blue method: Validity, applications, and future directions

An innovative approach to characterize the reactivity of metallic iron (Fe⁰) for aqueous contaminant removal has been in use for a decade: The methylene blue method (MB method). The approach considers the differential adsorptive affinity of methylene blue (MB) for sand and iron oxides. The MB method characterizes MB discoloration by sand as it is progressively coated by in-situ generated iron corrosion products (FeCPs) to deduce the extent of iron corrosion. The MB method is a semi-quantitative tool that has successfully clarified some contradicting reports on the Fe⁰/H₂O system. Moreover, it has the potential to serve as a powerful tool for routine tests in the Fe⁰ remediation industry, including quality assurance and quality control (QA/QC). However, MB is widely used as a 'molecular probe' to characterize the Fe⁰/H₂O system, for instance for wastewater treatment. Thus, there is scope to avoid confusion created by the multiple uses of MB in Fe⁰/H₂O systems. The present communication aims at filling this gap by presenting the science of the MB method, and its application and limitations. It is concluded that the MB method is very suitable for Fe⁰ material screening and optimization of operational designs. However, the MB method only provides semi-quantitative information, but gives no data on the solid-phase characterization of solid Fe⁰ and its reaction products. In other words, further comprehensive investigations with microscopic and spectroscopic surface and solid-state analyses are needed to complement results from the MB method.

The Suitability of Hybrid Fe0/Aggregate Filtration Systems for Water Treatment

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.

Characterizing the impact of MnO2 addition on the efficiency of Fe0/H2O systems

The role of manganese dioxide (MnO₂) in the process of water treatment using metallic iron (Fe⁰/H₂O) was investigated in quiescent batch experiments for t ≤ 60 d. MnO₂ was used as an agent to control the availability of solid iron corrosion products (FeCPs) while methylene blue (MB) was an indicator of reactivity. The investigated systems were: (1) Fe⁰, (2) MnO₂, (3) sand, (4) Fe⁰/sand, (5) Fe⁰/MnO₂, and (6) Fe⁰/sand/MnO₂. The experiments were performed in test tubes each containing 22.0 mL of MB (10 mg L⁻¹) and the solid aggregates. The initial pH value was 8.2. Each system was characterized for the final concentration of H⁺, Fe, and MB. Results show no detectable level of dissolved iron after 47 days. Final pH values varied from 7.4 to 9.8. The MB discoloration efficiency varies from 40 to 80% as the MnO₂ loading increases from 2.3 to 45 g L⁻¹. MB discoloration is only quantitative when the operational fixation capacity of MnO₂ for Fe²⁺ was exhausted. This corresponds to the event where adsorption and co-precipitation with FeCPs is intensive. Adsorption and co-precipitation are thus the fundamental mechanisms of decontamination in Fe⁰/H₂O systems. Hybrid Fe⁰/MnO₂ systems are potential candidates for the design of more sustainable Fe⁰ filters.

Sulfidation of zerovalent iron for improving the selectivity toward Cr(VI) in oxic water: Involvements of FeSx

Recognition of the general roles of FeS_x in selectivity of zerovalent iron (ZVI) toward target contaminants is of great significance but challenging, especially in oxic water system. Herein, the ZVI amended with Na₂S₂O₃ (i.e., S-ZVI^Na2S2O3) and Na₂S₂O₄ (i.e., S-ZVI^Na2S2O4) were applied for the sequestration of Cr(VI) and corresponding FeS_x involvements were explored. Results revealed that the largest effect for S-ZVI^Na2S2O3 and S-ZVI^Na2S2O4 observed at S/Fe molar ratio of 0.05 were 7.9- and 11.6- folds increase in removal rate (k_obs) of Cr(VI), respectively. respectively. Correspondingly, the electron efficiency (EE) of S-ZVI for reducing Cr(VI) were mainly from 2.1- to 2.4- folds greater than that that of the ZVI^H2O. Further, this work suggested that the improved selectivity of ZVI toward Cr(VI) by sulfidation should be mainly ascribed to the involvements of FeS_x, which could tune the reactive sites and corrosion products of ZVI for synergistically improving the mass transfer of Cr(VI) and subsequent electron transfer from iron core to Cr(VI). Overall, this work offers a new platform for improving ZVI selectivity for water decontamination.

Validating the Efficiency of the FeS2 Method for Elucidating the Mechanisms of Contaminant Removal Using Fe0/H2O Systems

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.

Tracing the Scientific History of Fe0-Based Environmental Remediation Prior to the Advent of Permeable Reactive Barriers

The technology of using metallic iron (Fe0) for in situ generation of iron oxides for water treatment is a very old one. The Fe0 remediation technology has been re-discovered in the framework of groundwater remediation using permeable reactive barriers (PRBs). Despite its simplicity, the improvement of Fe0 PRBs is fraught with difficulties regarding their operating modes. The literature dealing with Fe0 remediation contains ambiguities regarding its invention and its development. The present paper examines the sequence of contributions prior to the advent of Fe0 PRBs in order to clarify the seemingly complex picture. To achieve this, the current paper addresses the following questions: (i) What were the motivations of various authors in developing their respective innovations over the years?, (ii) what are the ancient achievements which can accelerate progress in knowledge for the development of Fe0 PRBs?, and (iii) was Fe0 really used for the removal of organic species for the first time in the 1970s? A careful examination of ancient works reveals that: (i) The wrong questions were asked during the past three decades, as Fe0 was premised as a reducing agent, (ii) credit for using Fe0 for water treatment belongs to no individual scientist, and (iii) credit for the use of Fe0 in filtration systems for safe drinking water provision belongs to scientists from the 1850s, while credit for the use of Fe0 for the removal of aqueous organic species does not belong to the pioneers of the Fe0 PRB technology. However, it was these pioneers who exploited Fe0 for groundwater remediation, thereby extending its potential. Complementing recent achievements with the chemistry of the Fe0/H2O system would facilitate the design of more sustainable Fe0-remediation systems.

Designing the Next Generation of Fe0-Based Filters for Decentralized Safe Drinking Water Treatment: A Conceptual Framework

The ambitious United Nations Sustainable Development Goal for 2030 to “leave no one behind” concerning safe drinking water calls for the development of universally applicable and affordable decentralized treatment systems to provide safe drinking water. Published results suggest that well-designed biological sand filters (BSFs) amended with metallic iron (Fe0-BSFs) have the potential to achieve this goal. Fe0-BSFs quantitatively remove pathogens and a myriad of chemical pollutants. The available data were achieved under various operating conditions. A comparison of independent research results is almost impossible, especially because the used Fe0 materials are not characterized for their intrinsic reactivity. This communication summarizes the state-of-the-art knowledge on designing Fe0-BSFs for households and small communities. The results show that significant research progress has been made on Fe0-BSFs. However, well-designed laboratory and field experiments are required to improve the available knowledge in order to develop the next generation of adaptable and scalable designs of Fe0-BSFs in only two years. Tools to alleviate the permeability loss, the preferential flow, and the use of exhausted filters are presented.

Characterizing the Reactivity of Metallic Iron for Water Treatment: H2 Evolution in H2SO4 and Uranium Removal Efficiency

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.