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Structure and specification of iron complexes in aqueous solutions determined by X-ray absorption spectroscopy
Authors:MJ Apted  GA Waychunas  GE Brown
Institution:Materials Department, Pacific Northwest Laboratory, Richland, Washington 99352 U.K.;Center for Materials Research, Stanford University, Stanford, California 94305 U.S.A.;Department of Geology, Stanford University, Stanford, California 94305 U.S.A.
Abstract:X-ray absorption spectroscopy, including extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) techniques, have been used to determine the structure and speciation of complexes for Fe2+ and Fe3+ chloride solutions at a variety of pH's, ionic strengths, and chloride/iron ratios.Low intensity K-edge transition features and analysis of modified pair correlation functions, derived from Fourier transformation of EXAFS spectra, show a regular octahedral coordination of Fe(II) by water molecules with a first-shell Fe2+-O bond distance, closely matching octahedral Fe2+-O bonds obtained from solid oxide model compounds. Solution Fe2+-O bond distances decrease with chloride/iron ratio, pH, and total FeCl2 concentration. A slight intensification of the 1s → 3d transition with increasing FeCl2 concentration suggests that chloride may begin to mix with water as a nearest-neighbor octahedral ligand. Fe3+ solutions show a pronounced increase in the 1s → 3d transition intensities between 1.0 M FeCl3/7.8 M Cl? to 1.0 M FeCl3/ 15 M Cl?, indicating a coordination change from octahedral to tetrahedral complexes. EXAFS analyses of these solutions show an increase in first-shell Fe3+-ligand distances despite this apparent reduction in coordination number. This can be best explained by a change from regular octahedral complexes of ferric iron (either Fe(H2O)63+ or trans-Fe(H2O)4Cl2 or both; Fe3+-O bond distances of 2.10 Å) to tetra-chloro complexes Fe3+-Cl bond distances of 2.25 Å].
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