Metallic iron for environmental remediation: Learning from the Becher process

Metallic iron (Fe0) is a moderately reducing agent that has been reported to be capable of reducing many environmental contaminants. Reduction by Fe0 used for environmental remediation is a well-known process to organic chemists, corrosion scientists and hydrometallurgists. However, considering Fe0 as a reducing agent for contaminants has faced considerable scepticism because of the universal role of oxide layers on Fe0 in the process of electron transfer at the Fe0/oxide/water interface. This communication shows how progress achieved in developing the Becher process in hydrometallurgy could accelerate the comprehension of processes in Fe0/H2O systems for environmental remediation. The Becher process is an industrial process for the manufacture of synthetic rutile (TiO2) by selectively removing metallic iron (Fe0) 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 Fe0 oxidation. In a Fe0/H2O system pre-formed oxide layers similarly act as physical barrier limiting the transport of dissolved species (including contaminants and O2) to the Fe0/H2O interface. Instead of this universal role of oxide layers on Fe0, improper conceptual models have been developed to rationalize electron transfer mechanisms at the Fe0/oxide/water interface.