N. Khare, James D. Martin, Dean A. Hesterberg
Sep 15, 2007
Citations
6
Influential Citations
108
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Quality indicators
Journal
Geochimica et Cosmochimica Acta
Abstract
Abstract Sorption of phosphate by Fe(III)- and Al(III)-(hydr)oxide minerals regulates the mobility of this potential water pollutant in the environment. The objective of this research was to determine the molecular configuration of phosphate bound on ferrihydrite at pH 6 by interpreting P K-edge XANES spectra in terms of bonding mode. XANES and UV–visible absorption spectra for aqueous Fe(III)–PO 4 solutions (Fe/P molar ratio = 0–2.0) provided experimental trends for energies of P(3p)–O(2p) and Fe(3d)–O(2p) antibonding molecular orbitals. Molecular orbitals for Fe(III)–PO 4 or Al(III)–PO 4 complexes in idealized monodentate or bidentate bonding mode were generated by conceptual bonding arguments, and Extended-Huckel molecular orbital computations were used to understand and assign XANES spectral features to bound electronic states. The strong white line at the absorption edge in P K-edge XANES spectra for Fe–PO 4 or Al–PO 4 systems is attributable to an electronic transition from a P 1s atomic orbital into P(3p)–O(2p) or P(3p)–O(2p)–Al(3p) antibonding molecular orbitals, respectively. For Fe–PO 4 systems, a XANES peak at 2–5 eV below the edge was assigned to a P 1s electron transition into Fe(4p)–O(2p) antibonding molecular orbitals. Similarly, a shoulder on the low-energy side of the white line for variscite corresponds to a transition into Al(3p)–O(2p) orbitals. In monodentate-bonded phosphate, Fe–O bonding is optimized and P–O bonding is weakened, and the converse is true of bidentate-bonded phosphate. These differences explained an inverse correlation between energies of P(3p)–O(2p) and Fe(3d)–O(2p) antibonding molecular orbitals consistent with a monodentate-to-bidentate transition in aqueous Fe(III)–PO 4 solutions. The intensity of the XANES pre-edge feature in Fe(III)-bonded systems increased with increasing number of Fe(III)–O–P bonds. Based on the similarity of intensity and splitting of the pre-edge feature for phosphate sorbed on ferrihydrite at 750 mmol/kg at pH 6 and aqueous Fe–PO 4 solutions containing predominantly bidentate complexes, XANES results indicated that phosphate adsorbed on ferrihydrite was predominantly a bidentate–binuclear surface complex.