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

Abstract

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.

Fe0-Supported Anaerobic Digestion for Organics and Nutrients Removal from Domestic Sewage

“…Conversely, the removal of contaminants by solid iron corrosion products (FeCPs) and their hypothetical degradation by electrons from Fe 0 [81] are related to some stoichiometry [82][83][84][85]. Accordingly, higher Fe 0 levels in a system implies the in-situ generation of more FeCPs which are excellent scavengers of micro-pollutants and microorganisms, including pathogens [86,87]. As Fe 0 corrodes, FeCPs is continuously generated and acts as microorganism scavengers.…”

Realizing the potential of metallic iron for the mitigation of toxics: flee or adapt?

“…Assumptions for reductive transformation Counter-arguments against the reductive transformation concept RCl reduction is an electrochemical process (Matheson and Tratnyek 1994;Weber 1996) Oxide scale is a non-conductive barrier; thus, electron transfer is blocked RCl must diffuse across the oxide scale to the Fe 0 surface. This migration cannot be quantitative A fraction of electrons from Fe 0 is transferred to RCl (electron efficiency concept) (Liu et al 2013) No RCl can receive electrons from Fe 0 (Hu et al 2021b) RCl is reduced by an indirect mechanism (Jiao et al 2009) Shaking or stirring experimental vessels in batch studies accelerates mass transfer (Lee et al 2004) In field Fe 0 PRBs, diffusion controls all transport processes in the vicinity of Fe 0 (Hu et al 2021b) The Fe 0 amount for a PRB can be determined from the stoichiometry of the electrochemical reaction between Fe 0 and RCl (Sarr 2001) Non-reducible contaminants such as Zn 2+ (Njaramba et al 2021) and methylene blue (Konadu-Amoah et al 2022) have been quantitatively removed in Fe 0 /H 2 O systems Contaminants and O 2 are exclusively reduced by an indirect mechanism (Whitney 1903;Hu et al 2021b) The volumetric expansive nature of the corrosion process must be considered (Caré et al 2013;Domga et al 2015;Yang et al 2021;Tao et al 2022) 4. Because of the very low solubility of Fe II and Fe III species, iron oxides and hydroxides must precipitate close to the Fe 0 surface and cannot migrate far.…”

Metallic Iron for Water Remediation: Plenty of Room for Collaboration and Convergence to Advance the Science

“…In other words, admixing Fe 0 with non-expansive aggregates is even a prerequisite for sustainable Fe 0 filters [88,89]. Clearly, only hybrid Fe 0 /aggregate systems are sustainable (Figure 2) [90]. This evidence is still not clear to many active researchers, mainly because the Fe 0 PRB technology was demonstrated with a hybrid system (22% Fe 0 w/w) [91], but the majority of the first-generation Fe 0 PRBs were built with 100% Fe 0 .…”

“…Because the formation of oxide scale is spontaneous, it also occurs at defects such that there is always an Fe 0 /oxides interface. (iii) Improperly considering the kinetics of iron corrosion, which is non-constant and non-linearly varies with time [90,114,115].…”