REDUCTION OF HEXAVALENT CHROMIUM BY AMORPHOUS IRON SULFIDE

The reduction of Cr(VI) to Cr(III) decreases the toxicity and mobility of chromium contaminants in soils and water. In addition, the formati on 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 reducta nt of Gr(VI) by identifying the solution and solid-phase products of t he reaction between FeS suspensions and chromate. Iron sulfide suspens ions at pH 5.0, 7.0, and 8.0 were reacted with a range of Cr(VI) solut ions from 50 to 5000 mu M in a N-2 atmosphere glovebox for 3 d. Soluti ons were analyzed using ICP-AES, IC, and colorimetric methods; solids were analyzed using XRD, TEM, EDS, and XANES spectroscopy. Iron sulfid e removed all of the added Cr(VI) from solution for the reaction condi tions studied and reduced between 85% and 100% of the Cr(VI) to Cr(III ). Chromate reduction occurred dominantly at the FeS surface and resul ted in [Cr-0.75,Fe-0.25](OH)3; while less extensive, reduction of Cr(V I) by Fe(II) (ag) was noted and produced a solid with the opposite Cr: Fe ratio, [Cr-0.25,Fe-0.75](OH)(3).