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.
The adsorption of saccharin on roughened nickel electrode has been studied with cyclic voltammetry ͑CV͒ and surface-enhanced Raman spectroscopy as well as complementary discrete Fourier transform calculations. CV measurements reveal a strong influence of both chloride concentration and pH of electrolyte on nickel surface and the inhibitory effect on nickel oxidation by the saccharin addition. Surface-enhanced Raman spectroscopy ͑SERS͒ measurements provide evidence for the strong interaction between the Ni surface and saccharin additives and dependence on the electrode potential. The overall behavior observed in SERS is explained based on the normal Raman spectra of saccharinate anion and its nickel-saccharinate complexes. Above the potential of zero charge ͑pzc͒ of the Ni surface, saccharinate-associated SERS peaks are dominant with O-bound nickel-saccharinate characteristics. Below the pzc, the SERS profile experiences a huge intensity enhancement and spectral change as well as substantial Raman frequency shifts. These shifts are interpreted in terms of a saccharinate-Ni complex, with both O and N bonding to the Ni electrode surface.
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