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

The reaction of hexavalent chromium (Cr(VI)) with zerovalent iron (Fe-0) 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 Fe-0 particles (nano Fe-0) treated with Cr(III) and Cr(VI). X-ray diffraction and XPS analyses of oxidized nano Fe-0 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 Fe-0 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 Fe-0 were nearly identical to the Cr(VI)-nano Fe-0 reaction product. Detailed analysis of XPS O 1s line spectra revealed that both Cr(III)- and Cr(VI)-treated nano Fe-0 yielded a predominantly hydroxylated Cr(OH)(3) and/or a mixed phase CrxFe1-x(OH)(3) product. The structure of the Cr(III)- and Cr(VI)-treated nano Fe-0 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 Fe-0 is either a poorly ordered Cr(OH)(3) precipitate or possibly a mixed phase CrxFe1-x(OH)(3) product, both of which are highly insoluble under environmental conditions.