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.

Chromium (VI) reduction in aqueous solutions by Fe3O4-stabilized Fe0 nanoparticles

“…The Fe 3 O 4 could act as an electron mediator which can facilitate the electron transport from Fe 0 to reducible species (chromium in this case). (3) As a result, Fe 3 O 4 -stabilized Fe 0 nanoparticles, acting much like catalysts, can undergo many successive reduction/oxidation cycles during the process [29]. Hence, the nanocomposites become much efficient nanostructures for the reduction of Cr(VI) in aqueous solutions: (i) Cr(VI) is being reduced by all reducing agents (Fe 0 , Fe II (s) , H 2(g) and Fe II (aq) ), and (ii) the resulting Cr(III) is being coagulated/co-precipitation by freshly formed Fe(III) agent.…”

Metallic iron for environmental remediation: Learning from electrocoagulation

“…The first step in the future is to investigate the well-established premise of reductive transformation of contaminants by Fe 0 or in Fe 0 /H 2 O systems. This premise was already shown to be inconsistent with many conceptual models for iron corrosion used in other branches of science including synthetic organic chemistry [65], hydrometallurgy [66,67] and iron corrosion produced in the petroleum [66]. Furthermore, newest mechanistic investigations with carbon tetrachloride (CT) using steady-state polarization curves and electrochemical impedance spectroscopy corroborate the fact iron corrosion is "helpful" for the reductive dechlorination of CT [10].…”

Biphasic reduction model for predicting the impacts of dye-bath constituents on the reduction of tris-azo dye Direct Green-1 by zero valent iron (Fe 0 )

“…This biphasic reaction pattern could result due to formation of iron oxide, the concentration of which increases with increasing reaction time. Accordingly, the heterogeneous reactions at the Fe 0 surface exhibits two phases: (1) an initial rapid and constant rate phase, followed by (2) a second phase, in which a steep decrease in rate is expected and was observed (Geetha and Surender, 1994;Noubactep, 2009b). The biphasic rate equation (Eq.…”

“…Besides, NH 4 + is reported to act as a buffer for hydroxyl ions (OH − ) and prevents excessively high local pH values, which might otherwise cause passivation of Fe 0 (Farrow et al, 1987;Noubactep, 2009b). Thus, NH 4 + might promote dye reduction.…”