Relevant reducing agents in remediation

Metallic iron (Fe) is often reported as a reducing agent for environmental remediation. There is still controversy as to whether Fe plays any significant direct role in the process of contaminant reductive transformation. The view that Fe is mostly a generator of reducing agents (e.g. H, H2 and Fe) and Fe oxyhydroxides has been either severely refuted or just tolerated. The tolerance is based on the simplification that, without Fe, no secondary reducing agents could be available. Accordingly, Fe serves as the original source of electron donors (including H, H2 and Fe). The objective of this communication is to refute the named simplification and establish that quantitative reduction results from secondary reducing agents. For this purpose, reports on aqueous contaminant removal by Al, Fe and Zn are comparatively discussed. Results indicated that reduction may be quantitative in aqueous systems containing Fe and Zn while no significant reduction is observed in Al/H2O systems. Given that Al is a stronger reducing agent than Fe and Zn, it is concluded that contaminant reduction in Fe/H2O systems results from synergic interactions between H/H2 and Fe within porous Fe oxyhydroxides. This conclusion corroborates the operating mode of Fe bimetallics as H/H2 producing systems for indirect contaminant reduction.