Designing Metallic Iron Packed-Beds for Water Treatment: A Critical Review

Abstract

Filtration systems based on metallic iron (Fe-0 filters) have been successfully used for water treatment over the past two decades. Relevant Fe-0 filters expand from subsurface permeable reactive barriers (PRBs) to household filters. Fe-0 filters systems are shown efficient for the remediation of biological and chemical contamination. Properly designing a Fe-0 filter is finding a long-term balance between two major interdependent design parameters: (i) Fe-0 reactivity, and (ii) filter permeability. Other relevant design parameters include (i) aqueous flow velocity, (ii) bed thickness, and (iii) water chemistry. Water chemistry includes nature and extent of contamination. To date, attempts to design more sustainable Fe-0 filters have been mostly pragmatic as: (i) reactive Fe-0 has failed to be considered as in situ generator of contaminant collectors (and "secondary" reducing agents), and (ii) the volumetric expansive nature of iron corrosion has been overlooked. On the other hand, valuable design criteria were available in the hydrometallurgical literature (cementation using elemental metals) prior to the advent of Fe-0 filters. As a consequence the literature is full of seemingly controversial results which are easily conciliated by the physico-chemistry of the system. The present review is limited at identifying some misconceptions and demonstrating their proliferation. Tools for better analyses are recalled. Recent X-ray tomography data are used as illustration of how valuable data are insufficiently discussed. It is hoped that the present contribution will boost systematic research for the design of more sustainable Fe-0 filters.