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.

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