The key role of contact time in elucidating the mechanisms of enhanced decontamination by Fe0/MnO2/sand systems

Metallic iron (Fe0)-based materials for aqueous phosphate removal: A critical review

The discharge of excessive phosphate from wastewater sources into the aquatic environment has been identified as a major environmental threat responsible for eutrophication. It has become essential to develop efficient but affordable techniques to remove excess phosphate from wastewater before discharging into freshwater bodies. The use of metallic iron (Fe⁰) as a reactive agent for aqueous phosphate removal has received a wide attention. Fe⁰ in-situ generates positively charged iron corrosion products (FeCPs) at pH > 4.5, with high binding affinity for anionic phosphate. This study critically reviews the literature that focuses on the utilization of Fe⁰-based materials for aqueous phosphate removal. The fundamental science of aqueous iron corrosion and historical background of the application of Fe⁰ for phosphate removal are elucidated. The main mechanisms for phosphate removal are identified and extensively discussed based on the chemistry of the Fe⁰/H₂O system. This critical evaluation confirms that the removal process is highly influenced by several operational factors including contact time, Fe⁰ type, influent geochemistry, initial phosphate concentration, mixing conditions, and pH value. The difficulty in comparing independent results owing to diverse experimental conditions is highlighted. Moreover, contemporary research in progress including Fe⁰/oxidant systems, nano-Fe⁰ application, Fe⁰ material selection, desorption studies, and proper design of Fe⁰-based systems for improved phosphate removal have been discussed. Finally, potential strategies to close the loop in Fe⁰-based phosphate remediation systems are discussed. This review presents a science-based guide to optimize the efficient design of Fe⁰-based systems for phosphate removal.

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.