Structure and specification of iron complexes in aqueous solutions determined by X-ray absorption spectroscopy

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 Fe 2+ and Fe 3+ 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 Fe 2+-O bond distance, closely matching octahedral Fe 2+-O bonds obtained from solid oxide model compounds. Solution Fe 2+-O bond distances decrease with chloride/iron ratio, pH, and total FeCl 2 concentration. A slight intensification of the 1s → 3d transition with increasing FeCl 2 concentration suggests that chloride may begin to mix with water as a nearest-neighbor octahedral ligand. Fe 3+ solutions show a pronounced increase in the 1s → 3d transition intensities between 1.0 M FeCl 3/7.8 M Cl − to 1.0 M FeCl 3/ 15 M Cl −, indicating a coordination change from octahedral to tetrahedral complexes. EXAFS analyses of these solutions show an increase in first-shell Fe 3+-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(H 2O) 6 3+ or trans-Fe(H 2O) 4Cl 2 or both; Fe 3+-O bond distances of 2.10 Å) to tetra-chloro complexes [Fe 3+-Cl bond distances of 2.25 Å].