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 2O 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 2O 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 2O 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.
Highlights
Permeable reactive barriers of elemental iron (Fe0 walls or remediation Fe0/H2O systems) are a valuable technological application that has been shown to be both environmentally friendly and cost-effective in the removal of various substances from contaminated waters [1,2,3,4,5,6,7]
The results showed that the ammonium chloride plays three distinct and important roles in the selective leaching [21]: (i) NH4+ acts as a buffer for hydroxyl ions (OH-) and prevents excessively high local pH values, which might otherwise cause precipitation of iron (II) hydroxide before the iron (II) ions could diffuse from the rutile matrix; (ii) the ammonia (NH3) formed as a result of this buffering reaction complexes iron (II) ions until they have moved away from the regions of high pH, preventing the precipitation of iron (II) hydroxide in the pores of the rutile; (iii) the chloride ion helps to break down any passive films which might form during aeration
While the complexity of mass-transfer in remediation Fe0/H2O systems is yet to be properly assessed, the Becher process for the manufacture of synthetic rutile has already dealt with the prediction of solid-liquid mass transfer in systems in with particles of various densities and sizes are available [15,17,18,19]
Summary
Permeable reactive barriers of elemental iron (Fe0 walls or remediation Fe0/H2O systems) are a valuable technological application that has been shown to be both environmentally friendly and cost-effective in the removal of various substances from contaminated waters [1,2,3,4,5,6,7]. The large difference in the leaching rates for NaCl and NH4Cl could be explained using pH data, which indicate that most of the reactions in NH4Cl takes place at a pH of about 4, and most of the reactions in NaCl takes place at a pH between 9 and 10 These results show clearly that in investigating process relevant for remediation Fe0/H2O systems while avoiding iron oxide precipitation, it will be more advantageous to work with NH4Cl than with strong chelating agents like ethylenediaminetetraacetic acid [1,33] of larger molecular size and little buffering capacity. A further relevant result from the Becher process is obtained by Bruckard et al [22] on the optimisation of selective iron leaching using the anthraquinone-based redox catalysts while guaranteeing the formation of the preferred magnetite as the reaction product This result could be very important for enhancing long-term reactivity of Fe0/H2O systems. More research under conditions pertinent to remediation Fe0/H2O systems is needed to clarify this issue
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