Abstract
The suitability of remediation systems using metallic iron (Fe0) has been extensively discussed during the past 3 decades. It has been established that aqueous Fe0 oxidative dissolution is not caused by the presence of any contaminant. Instead, the reductive transformation of contaminants is a consequence of Fe0 oxidation. Yet researchers are still maintaining that electrons from the metal body are involved in the process of contaminant reduction. According to the electron efficiency concept, electrons from Fe0 should be redistributed to: i) contaminants of concern (COCs), ii) natural reducing agents (e.g., H2O, O2), and/or iii) reducible co-contaminants (e.g. NO3-). The electron efficiency is defined as the fraction of electrons from Fe0 oxidation which is utilized for the reductive transformations of COCs. This concept is in frontal contradiction with the view that Fe0 is not directly involved in the process of contaminant reduction. This communication recalls the universality of the concept that reductive processes observed in remediation Fe0/H2O systems are mediated by primary (e.g., FeII, H/H2) and secondary (e.g., Fe3O4, green rusts) products of aqueous iron corrosion. The critical evaluation of the electron efficiency concept suggests that it should be abandoned. Instead, research efforts should be directed towards tackling the real challenges for the design of sustainable Fe0-based water treatment systems based on fundamental mechanisms of iron corrosion.
Highlights
Metallic iron (Fe0), termed as zero-valent iron (ZVI) is widely considered as a cost-effective reducing agent for organic pollutants in groundwater (Henderson and Demond 2007; Guan et al, 2015; Cao et al, 2020; He et al, 2020)
Matheson and Tratnyek (1994) considered three possible pathways to justify the reductive transformation of chlorinated methanes (RCl) reduction in Fe0/H2O systems: i) reductive dehalogenation by Fe0 (Eq 3) which is equivalent to Fe0 oxidation by RCl (RCl oxidizing agent, electrochemical mechanism) (Pathway 1), ii) reductive dehalogenation by Fe2+ (Eq 4) which is equivalent to Fe2+ oxidation by RCl (Pathway 2), and iii) reductive dehalogenation by H+ (Eq 5) which is equivalent to H+ oxidation by RCl (Pathway 3)
Fe0/H2O systems have a long history of application in environmental remediation and water treating, and several new applications are emerging
Summary
Metallic iron (Fe0), termed as zero-valent iron (ZVI) is widely considered as a cost-effective reducing agent for organic pollutants in groundwater (Henderson and Demond 2007; Guan et al, 2015; Cao et al, 2020; He et al, 2020). He et al (2020) give an excellent overview on the topic, and, interested readers are referred to this very recent review article To this point, the presentation has highlighted that the EE concept is intrinsically wrong, because Fe0 does not play any significant role in the process of contaminant reductive transformation in Fe0/H2O systems (Whitney, 1903; Jiao et al, 2009; Noubactep, 2022). Aqueous iron corrosion proceeds as follows: i) Fe0 is oxidatively dissolved at the anode to release Fe2+, ii) the generated Fe2+ ions migrate in the polluted water (electrolyte), and iii) electrons left behind by Fe2+ are transferred through the metal body (conductor) to a reducible species at the cathode. “Citation” is the number of independent citations (excluding self-citations) according to Scopus (www.scopus.com: February 25, 2021)
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