Metallic Iron for Environmental Remediation: Starting an Overdue Progress in Knowledge

Abstract

A critical survey of the abundant literature on environmental remediation and water treatment using metallic iron (Fe0) as reactive agent raises two major concerns: (i) the peculiar properties of the used materials are not properly considered and characterized, and, (ii) the literature review in individual publications is very selective, thereby excluding some fundamental principles. Fe0 specimens for water treatment are typically small in size. Before the advent of this technology and its application for environmental remediation, such small Fe0 particles have never been allowed to freely corrode for the long-term spanning several years. As concerning the selective literature review, the root cause is that Fe0 was considered as a (strong) reducing agent under environmental conditions. Subsequent interpretation of research results was mainly directed at supporting this mistaken view. The net result is that, within three decades, the Fe0 research community has developed itself to a sort of modern knowledge system. This communication is a further attempt to bring Fe0 research back to the highway of mainstream corrosion science, where the fundamentals of Fe0 technology are rooted. The inherent errors of selected approaches, currently considered as countermeasures to address the inherent limitations of the Fe0 technology are demonstrated. The misuse of the terms “reactivity”, and “efficiency”, and adsorption kinetics and isotherm models for Fe0 systems is also elucidated. The immense importance of Fe0/H2O systems in solving the long-lasting issue of universal safe drinking water provision and wastewater treatment calls for a science-based system design.

Understanding the Operating Mode of Fe0/Fe-Sulfide/H2O Systems for Water Treatment

“…In other words, in Fe 0 /Fe-sulfide/H 2 O systems, contaminants are more or less removed by adsorption and co-precipitation, while some reducible contaminants are probably quantitatively reduced. However, two key issues should be considered: (i) observed reductive transformations are not the cathodic reactions coupled to iron dissolution (Equation (5)), and (ii) chemical reduction is not a relevant removal mechanism for many contaminants at the concentration ranges of natural waters [28,57,60,61].…”

“…For environmental remediation, it may suffice if the reaction products are biodegradable [10]. Clearly, one major problem of the research on using Fe 0 for water treatment has been to randomly interchange the terms "contaminant reduction" and "contaminant removal" as if reduced species are automatically removed [28,33,34]. It has been constantly pointed out that no satisfying mass balance of the species involved, including Fe 0, has been presented [63][64][65].…”

“…During the 1990s, Fe 0 has been rediscovered and successfully applied for environmental remediation, including in subsurface permeable reactive barriers (Fe 0 PRBs) [10][11][12][13][14][15][16][17][18]. New Fe 0 -based technologies for safe drinking water supply were then derived from the Fe 0 -based PRB technology [19][20][21][22][23][24][25][26][27][28].…”

“…The layer of (hydr)oxides or oxide scale acts as a physical and electronic barrier that "passivates" the material, thereby significantly reducing the efficiency of the Fe 0 -based system [10,18,[30][31][32]. In essence, "reactivity loss" is a misleading term, as reactivity is an intrinsic property of each material and cannot change with operational variables or under given operating conditions [28,[33][34][35][36]. That is the reason why it is restored by appropriate treatments (e.g., acid wash) [15,37] and/or sustained by other additive materials (e.g., using FeS 2 or MnO 2 ) [29,[38][39][40][41][42][43][44].…”

Steel Wool for Water Treatment: Intrinsic Reactivity and Defluoridation Efficiency

“…Required experiments should last for long times (at least one year) as there is no way to reproduce accelerated iron corrosion. The authors have been advocating for the past decade that designing such filters will universally provide safe drinking water to low-income communities worldwide [82][83][84].…”