Review: The fundamental mechanism of aqueous contaminant removal by metallic iron

Abstract

Contaminant co-precipitation with continuously generated and transformed iron corrosion products has received relatively little attention in comparison to other possible removal mechanisms (adsorption, oxidation, precipitation) in Fe 0 /H 2 O systems at near neutral pH values. A primary reason for this is that the use of elemental iron (Fe 0 ) in environmental remediation is based on the thermodynamic-founded premise that reducible contaminants are potentially reduced while Fe 0 is oxidised. However, co-precipitation portends to be of fundamental importance for the process of contaminant removal in Fe 0 /H 2 O systems, as the successful removal of bacteria, viruses and non reducible organic (e.g. methylene blue, triazoles) and inorganic (e.g. Zn) compounds has been reported. This later consideration has led to a search for the reasons why the importance of co-precipitation has almost been overlooked for more than a decade. Three major reasons have been identified: the improper consideration of the huge literature of iron corrosion by pioneer works, yielding to propagation of misconceptions in the iron technology literature; the improper consideration of available results from other branches of environmental science (e.g. CO 2 corrosion, electrocoagulation using Fe 0 electrodes, Fe or Mn geochemistry); and the use of inappropriate experimental procedures (in particular, mixing operations). The present paper demonstrates that contaminant co-precipitation with iron corrosion products is the fundamental mechanism of contaminant removal in Fe 0 /H 2 O systems. Therefore, the 'iron technology' as a whole is to be revisited as the 'know-why' of contaminant removal is yet to be properly addressed.

Impact of Fe0 amendment on methylene blue discoloration by sand columns

“…For Fe 0 -containing system, not such adsorption capacity can be defined because MB is not adsorbed onto a defined surface, rather MB is adsorbed or enmeshed in the matrix of iron corrosion products [46]. Additionally, the extent of Fe 0 depletion is difficult to access.…”

“…The fact that in situ dissolved iron mostly remains in the system is a hint, that MB should be more or less quantitatively removed by adsorption, co-precipitation and adsorptive size exclusion (straining) [46]. 4.…”

Impact of MnO2 on the efficiency of metallic iron for the removal of dissolved CrVI, CuII, MoVI, SbV, UVI and ZnII

“…suggesting that quantitative contaminant removal by adsorption and co-precipitation was likely to occur [39,40]. In fact, metal removal mostly occurs by adsorption and coprecipitation.…”

“…It should be kept in mind that iron oxides and hydroxides are in-situ generated and are in permanent transformation such that contaminant removal is not performed by a well-defined adsorbing agent [39,40].…”

Appropriate technology for domestic wastewater management in under-resourced regions of the world

“…The suitability of Fe 0 to treat a complex mixture of micro-pollutants arises from the fact that the primary removal mechanism is not a specific one. Concomitant occurrence of adsorption and co-precipitation in the removal of biological and chemical contaminants from aqua phase has been reported (Ghauch 2013;Noubactep 2010bNoubactep , 2013bNoubactep and 2014. Additionally, in column systems, in situ generation of volumetric expansive corrosion products enhances sizeexclusion (Ghauch 2013;Noubactep 2011).…”