A critical review on the process of contaminant removal in Fe0-H2O systems

A central aspect of the contaminant removal by elemental iron materials (Fe-0 or Fe-0 materials) is that reduction reactions are mediated by the iron surface (direct reduction). This premise was introduced by the pioneers of the reactive wall technology and is widely accepted by the scientific community. In the meantime enough evidence has been provided to suggest that contaminant reduction through primary corrosion products (secondary reductants) does indeed occur (indirect reduction). It was shown for decades that iron corrosion in the pH range of natural waters (4-9) inevitably yields an obstructive oxide film of corrosion products at the metal surface (oxide film). Therefore, contaminant adsorption on to corrosion products and contaminant co-precipitation with corrosion products inevitably occurs. For adsorbed and co-precipitated contaminants to be directly reduced the oxide film should be electronic conductive. This study argues through a literature review a series of points which ultimately lead to the conclusion that, if any quantitative contaminant reduction occurs in the presence of Fe-0 materials, it takes place within the matrix of corrosion products and is not necessarily a direct reduction. It is concluded that Fe-0 materials act both as source of corrosion products for contaminant adsorption/coprecipitation and as a generator of Fe-II and H-2 (H) for possible catalytic contaminant reduction.