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.

Spectroscopic investigation of Cr(III)- and Cr(VI)-treated nanoscale zerovalent iron.

The reaction of hexavalent chromium (Cr(VI)) with zerovalent iron (Fe0) during soil and groundwater remediation is an important environmental process. This study used several techniques including X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy to investigate nanometer scale Fe0 particles (nano Fe0) treated with Cr(III) and Cr(VI). X-ray diffraction and XPS analyses of oxidized nano Fe0 showed the crystalline Fe(III) phase is composed of lepidocrocite (gamma-FeOOH). Results of XPS Cr 2p data and Cr K-edge X-ray absorption near edge spectroscopy (XANES) provided evidence that Cr(VI) was entirely reduced to Cr(III) by nano Fe0 with no residual Cr(VI) after reaction. In addition, XPS and XANES results of Cr(III) precipitated as Cr(OH)3 in the presence of corroding nano Fe0 were nearly identical to the Cr(VI)-nano Fe0 reaction product. Detailed analysis of XPS O 1s line spectra revealed that both Cr(III)- and Cr(VI)-treated nano Fe0 yielded a predominantly hydroxylated Cr(OH)3 and/ or a mixed phase CrxFe(1 - x)(OH)3 product. The structure of the Cr(III)- and Cr(VI)-treated nano Fe0 determined using extended X-ray absorption fine structure spectroscopy (EXAFS) revealed octahedral Cr(III) with Cr-O interatomic distances between 1.97 and 1.98 A for both Cr(III) and Cr(VI) treatments and a pronounced Cr-Cr second interatomic shell at 3.01 A. Our results suggest that the reaction product of Cr(VI)-treated nano Fe0 is either a poorly ordered Cr(OH)3 precipitate or possibly a mixed phase CrxFe(1 - x)(OH)3 product, both of which are highly insoluble under environmental conditions.