Fe0/H2O Systems for Environmental Remediation: The Scientific History and Future Research Directions

Abstract

Elemental iron (Fe0) has been widely used in groundwater/soil remediation, safe drinking water provision, and wastewater treatment. It is still mostly reported that a surface-mediated reductive transformation (direct reduction) is a relevant decontamination mechanism. Thus, the expressions “contaminant removal” and “contaminant reduction” are interchangeably used in the literature for reducible species (contaminants). This contribution reviews the scientific literature leading to the advent of the Fe0 technology and shows clearly that reductive transformations in Fe0/H2O systems are mostly driven by secondary (FeII, H/H2) and tertiary/quaternary (e.g., Fe3O4, green rust) reducing agents. The incidence of this original mistake on the Fe0 technology and some consequences for its further development are discussed. It is shown, in particular, that characterizing the intrinsic reactivity of Fe0 materials should be the main focus of future research.

The Impact of Selected Pretreatment Procedures on Iron Dissolution from Metallic Iron Specimens Used in Water Treatment

“…Metallic iron (Fe 0 ) is a potential reducing agent for many reducible species, including water (H 2 O) and O 2 [15,26,56,57,114]. The redox potential of water is 0.00 V, making water a powerful oxidizing agent for Fe 0 (E 0 = −0.44 V).…”

“…This occurs because at pH > 4.5, the Fe 0 surface is constantly shielded by a universal oxide scale, acting as a diffusion barrier for any dissolved species [67][68][69][70][71][72]. Because the oxide scale is constantly present at the Fe 0 surface, the discussion of the mechanism of contaminant removal has been challenging [33,[55][56][57]72,111]. The results achieved herein contribute to clarify this dilemma because EDTA has no redox properties.…”

“…The long scientific history of using "Fe 0 for water treatment" demonstrates that the evidence that iron corrosion generates adsorbing species for decontamination was known before 1850 [2] and has been independently rediscovered several times thereafter [4,6,119,[121][122][123]. However, only in the context of subsurface reactive walls, Fe 0 was presented as its own reducing agent under environmental conditions, a possible mistake that is still difficult to address [55][56][57]109,111]. It is very interesting in this regard to note that, parallel to the discovery that Fe 0 may be a reducing agent [124], Khudenko [70,123] has presented cementation as a tool to induce the reductive transformation of organic contaminants.…”

“…Coagulation and flotation inducing coprecipitation and size-exclusion are less well understood [138,139] but are also well-established. However, it is clear from the published literature that there is a lack of a quantitative appreciation of the way in which these individual processes interact to provide efficient Fe 0 /H 2 O systems [9,26,28,33,47,[54][55][56][57][58][59][60]102,114,140] For the Fe 0 -based systems to become an accepted and dependable water treatment technology and play the expected wider role in decentralized water treatments, research is required that focuses neither on simply making a specific pollutant-centred application work (treatability studies) nor on any one of the named foundation processes but rather on the emphasis of explaining and quantifying the key interactions between electrochemistry, adsorption, coprecipitation and size-exclusion. The Fe 0 /MnO 2 /H 2 O system is a good candidate to start this systematic work.…”

“…Clearly, an understanding of the role of "passivation" in the process of decontamination using Fe 0 should be useful for enhancing the system's efficiency in practice [30,31,47].There is an agreement on the evidence that using Fe 0 for water treatment and environmental remediation is "putting corrosion to use" [51][52][53]. There is otherwise a contradiction on practically any other aspect concerning the Fe 0 /H 2 O system, including the reaction mechanisms and factors determining the long-term efficiency of such systems [9,28,47,[54][55][56][57][58]. A key reason for this is the large variability of experimental conditions used in testing Fe 0 materials [58][59][60][61][62].…”