Materials for sustainable metallic iron-based water filters: a review

Water pollution is calling for a sustainable remediation method such as the use of metallic iron (Fe⁰) to reduce and filter some pollutants, yet the reactivity and hydraulic conductivity of iron filters decline over time under field conditions. Here we review iron filters with focus on metallic corrosion in porous media, flaws in designing iron filters, next-generation filters and perspectives such as safe drinking water supply, iron for anaemia control and coping with a reactive material. We argue that assumptions sustaining the design of current Fe⁰ filters are not valid because proposed solutions address the issues of declining iron reactivity and hydraulic conductivity separately. Alternatively, a recent approach suggest that each individual Fe⁰ atom corroding within a filter contributes to both reactivity and permeability loss. This approach applies well to alternative iron materials such as bimetallics, composites, hybrid aggregates, e.g. Fe⁰/sand, and nano-Fe⁰. Characterizing the intrinsic reactivity of individual Fe⁰ materials is a prerequisite to designing sustainable filters. Indeed, Fe⁰ ratio, Fe⁰ type, Fe⁰ shape, initial porosity, e.g. pore size and pore size distribution, and nature and size of admixing aggregates, e.g. pumice, pyrite and sand, are interrelated parameters which all influence the generation and accumulation of iron corrosion products. Fe⁰ should be characterized in long-term experiments, e.g. 12 months or longer, for Fe dissolution, H₂ generation and removal of contaminants in three media, i.e., tap water, spring water and saline water, to allow reactivity comparison and designing field-scale filters.