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

Abstract

Filtration systems based on metallic iron (Fe0 filters) have been successfully used for water treatment over the past two decades. Relevant Fe0 filters expand from subsurface permeable reactive barriers (PRBs) to household filters. Fe0 filters systems are shown efficient for the remediation of biological and chemical contamination. Properly designing a Fe0 filter is finding a long‐term balance between two major interdependent design parameters: (i) Fe0 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 Fe0 filters have been mostly pragmatic as: (i) reactive Fe0 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 Fe0 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 Fe0 filters.

Testing Metallic Iron Filtration Systems for Decentralized Water Treatment at Pilot Scale

“…The two examples in Section 4.3 and the parameters in Tables 1 and 2 PRBs [83,141] have not been satisfactorily rationalized [122,123]. It is evident that the composition of contaminated waters and effluents vary to a large extent.…”

“…This clarification coupled to the consideration of the formation of voluminous Fe 0 corrosion products is the theoretical starting point for the design of next generation Fe 0 filters [16,17,25,98,100,[120][121][122][123]. No consideration of these key issues has led to controversial reports which render the assessment of progress in designing Fe [8,9,54,58,[88][89][90][124][125][126].…”

“…Furthermore, an empirical approach has been used to screen selected operational factors like grain sizes and grain size distributions [126], grain packing [129] or the mixture of Fe3O4 and external Fe II solutions [88][89][90]. This empirical approach is certainly costly but not necessarily effective [121][122][123]. An alternative approach is to develop the science of the system, which will serve as compass to evaluate experimental results [25,130,131].…”

“…A methodology to compare the intrinsic reactivity of Fe 0 materials was introduced [133], and was recently revisited [132,134], however it is yet to receive universal acceptance. On the other hand, a universal design rationale for the design of Fe 0 filters was presented by Noubactep and Caré [130,131] and progressively revisited [25,98,[121][122][123].…”

“…These authors used a 1:1 Fe 0 :sand (vol/vol) for methylene blue discoloration in column studies for four and three months respectively and observed that at the influent of the column, about 3 cm was brown colored (Figure 3) and hardly compacted, while the remaining Fe 0 /sand layer was less or not compacted and black colored. Recent works have recalled that the availability of dissolved O2 is the major factor causing particle cementation and permeability loss [25,100,122,123]. However, even without this "recent" knowledge, a pragmatic approach would have consisted of comparing the density of Fe 0 (7.8 g/cm 3 ) and Fe oxides (e.g., β-FeOOH; 3.6 g/cm 3 ) [137].…”