Reduction and immobilization of chromium(VI) by iron(II)-treated faujasite

Removal of hexavalent chromium (Cr(VI)) from wastewater typically involves reduction of Cr(VI) to insoluble Cr(III) using zerovalent iron (Fe(0)) or ferrous iron (Fe(II)). This study investigates the effectiveness of Fe(II)-treated faujasite (zeolite Fe(II)-Y) for reduction of Cr(VI) and immobilization (adsorption/co-precipitation) of the Cr(III) reaction product. The Fe(II)-faujasite material effectively removed high concentrations of dissolved Cr(VI) from aqueous solution resulting in Cr solid loadings as high as 0.30 mmol Cr per gram Fe(II)-faujasite or approximately 1.5% Cr (w:w). Results of Cr K-edge X-ray absorption near edge spectroscopy (XANES) confirmed that the oxidation state of Cr in Cr(VI)-treated Fe(II)-faujasite was Cr(III). The local atomic structure of Cr was investigated by extended X-ray absorption fine structure (EXAFS) spectroscopy and the structure of Cr in the product was described by a Cr-O first shell of six O atoms at 1.98(+/-0.02)A plus a second atomic shell of metal (Cr, Fe) at 3.13(+/-0.02)A. The EXAFS results, combined with SEM imaging and X-ray diffraction analyses, suggested that the product of the reaction of Cr(VI) with Fe(II)-faujasite is primarily a poorly order Cr(x)Fe(1-x)(OH)(3) mixed phase similar to previous investigations of the reaction of Cr(VI) with Fe(0) and not solely Cr(III) bound directly to zeolite cation exchange sites.