Review: The fundamental mechanism of aqueous contaminant removal by metallic iron
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 Fe0/H2O systems at near neutral pH values. A primary reason for this is that the use of elemental iron (Fe0) in environmental remediation is based on the thermodynamic-founded premise that reducible contaminants are potentially reduced while Fe0 is oxidised. However, co-precipitation portends to be of fundamental importance for the process of contaminant removal in Fe0/H2O 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. CO2 corrosion, electrocoagulation using Fe0 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 Fe0/H2O 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.
Keywords: adsorption, co-precipitation, iron corrosion, iron oxhydroxides, zerovalent iron