Metallic iron for environmental remediation: learning from the Becher process.

Abstract

Metallic iron (Fe(0)) is a moderately reducing agent that has been reported to be capable of reducing many environmental contaminants. Reduction by Fe(0) used for environmental remediation is a well-known process to organic chemists, corrosion scientists and hydrometallurgists. However, considering Fe(0) as a reducing agent for contaminants has faced considerable scepticism because of the universal role of oxide layers on Fe(0) in the process of electron transfer at the Fe(0)/oxide/water interface. This communication shows how progress achieved in developing the Becher process in hydrometallurgy could accelerate the comprehension of processes in Fe(0)/H(2)O systems for environmental remediation. The Becher process is an industrial process for the manufacture of synthetic rutile (TiO(2)) by selectively removing metallic iron (Fe(0)) from reduced ilmenite (RI). This process involves an aqueous oxygen leaching step at near neutral pH. Oxygen leaching suffers from serious limitations imposed by limited mass transport rates of dissolved oxygen across the matrix of iron oxides from initial Fe(0) oxidation. In a Fe(0)/H(2)O system pre-formed oxide layers similarly act as physical barrier limiting the transport of dissolved species (including contaminants and O(2)) to the Fe(0)/H(2)O interface. Instead of this universal role of oxide layers on Fe(0), improper conceptual models have been developed to rationalize electron transfer mechanisms at the Fe(0)/oxide/water interface.