Reduction of Hexavalent Chromium by Amorphous Iron Sulfide

Abstract

The reduction of Cr(VI) to Cr(III) decreases the toxicity and mobility of chromium contaminants in soils and water. In addition, the formation of a highly insoluble Cr(III) product would decrease the likelihood of future Cr(III) re-oxidation. Amorphous iron sulfide minerals like mackinawite (FeS1-x) have the potential to reduce large quantities of Cr(VI) and in the process form very stable [Cr, Fe](OH)3 solids. In this study, we examine the effectiveness of amorphous FeS as a reductant of Cr(VI) by identifying the solution and solid-phase products of the reaction between FeS suspensions and chromate. Iron sulfide suspensions at pH 5.0, 7.0, and 8.0 were reacted with a range of Cr(VI) solutions from 50 to 5000 μM in a N2 atmosphere glovebox for 3 d. Solutions were analyzed using ICP−AES, IC, and colorimetric methods; solids were analyzed using XRD, TEM, EDS, and XANES spectroscopy. Iron sulfide removed all of the added Cr(VI) from solution for the reaction conditions studied and reduced between 85% and 100% of the Cr(VI) to Cr(III). Chromate reduction occurred dominantly at the FeS surface and resulted in [Cr0.75,Fe0.25](OH)3; while less extensive, reduction of Cr(VI) by Fe(II) (aq) was noted and produced a solid with the opposite Cr:Fe ratio, [Cr0.25,Fe0.75](OH)3.