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

This paper describes the use of highly reactive magnetite (Fe3O4) nanoparticles-stabilized Fe-0 nanocomposites for the reduction and mitigation of hexavalent chromium Cr(VI) species in aqueous solutions. Higher proportions of Fe3O4 in the nanocomposites could increase the rate of Cr(VI) reduction. In the absence of magnetite, the Cr(VI) mitigation rate was just 51.4% after 60 min of reaction, while with an initial Fe3O4 mass loading of 3 g l(-1), the Cr(VI) mitigation rate was nearly 100% after 60 min. The optimal ratio of Fe3O4:Fe-0 for the mitigation of Cr(VI) was found to be 40:1. Otherwise, solution pHs significantly affected the rate of Cr(VI) reduction, with reactions occurring more rapidly under acidic or neutral than basic conditions. It is hypothesized that the high efficiency of the Fe3O4 nanoparticles-stabilized Fe-0 nanocomposites for Cr(VI) reduction was a direct result of the attachment of Fe-0 nanoparticles to the surface of magnetite, which prevents the aggregation of nano-Fe-0, moreover, the electron transfer during the reduction process most likely takes place via Fe-0 nanoparticles that are located at the magnetite octahedral sites, which are versatile redox centers as they can accommodate both Fe(III) and Fe(II), and this will promote the reduction of Cr(VI). Cr(VI) reduction is coupled with nano-Fe-0 oxidation. Nano-Fe-0 particles are located at the magnetite octahedral sites. Ions of Fe(II) and Fe(III) accommodated by magnetite octahedral sites are products of nano-Fe-0 oxidation. Therefore, Cr(VI) reduction is mediated either by nano-Fe-0 (direct reduction) or Fe(II) species (indirect reduction). Additionally, catalytic Cr(VI) reduction by molecular H-2 (or atomic H) is possible. (C) 2009 Elsevier B.V. All rights reserved.