The Suitability of Hybrid Fe-0/Aggregate Filtration Systems for Water Treatment

Metallic iron (Fe-0) corrosion under immersed conditions (Fe-0/H2O system) has been used for water treatment for the past 170 years. Fe-0 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 Fe-0 and reactive aggregates to build hybrid systems (e.g., Fe-0/FeS2, Fe-0/MnO2, Fe-0/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 Fe-0/H2O filtration systems. It is shown that any enhanced contaminant removal efficiency in Fe-0/aggregate/H2O systems relative to the Fe-0/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 Fe-0 is disproved by the evidence that Fe-0/sand systems are equally more efficient than pure Fe-0 systems. This demonstration corroborates the concept that aqueous contaminant removal in iron/water systems is not a process mediated by electrons from Fe-0. This communication reiterates that only hybrid Fe-0/H2O filtration systems are sustainable.