Decontamination of hexavalent chromium-polluted waters: Significance of metallic iron technology

Abstract

Chromium (Cr) is an important metal used in a variety of industrial applications, which significantly contribute to pollution of air, soil, and waters. In natural environments, chromium can exist mainly in two of its most stable oxidation states, (+III) and (+VI). Among them, Cr(VI) is the most hazardous due to its high mobility in the environment and severe harmful effects exerted on all living matters. Therefore, it should be removed from all contaminated waters. During the last 25 years, there has been great interest in using metallic iron (Fe0) for the abatement of Cr(VI) pollution. The first mechanism, known as the reductiveprecipitation mechanism, was proposed at the beginning of the nineties, and attributed the efficiency of Fe0 in removing Cr(VI) mainly to the direct electron transfer from the Fe0 surface to Cr(VI), followed by precipitation of the resulted cations as simple hydroxides and/or mixed Fe(III)-Cr(III) (oxi)hydroxides. Recently, new perspectives were added to this early mechanism. A new concept, known as the adsorption-coprecipitation mechanism, suggests that direct reduction with Fe0, if applicable, is less important than had previously been assumed by the reductive-precipitation mechanism; accordingly, contaminants are quantitatively removed in Fe0/H2O systems principally by adsorption, coprecipitation, and size exclusion, while reduction, when possible, is mainly the result of indirect reducing agents produced by Fe0 corrosion. In spite of the substantial research work that has proven the capability of metallic iron as a reactive material to remove Cr(VI), there is no consensus at this time in what regards the mechanism of this process. Therefore, after providing an overview of chromium occurrence, chemistry, and toxicity, this work will critically review the existing knowledge on this subject, clearly demonstrating that mechanism of Cr(VI) removal with Fe0 is more complex than the simple reductive precipitation.