skip to main content
OSTI.GOV title logo U.S. Department of Energy
Office of Scientific and Technical Information

Title: Phosphate Bonding Configuration on Ferrihydrite Based on Molecular Orbital Calculations and XANES Fingerprinting

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)-PO4 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)-PO4 or Al(III)-PO4 complexes in idealized monodentate or bidentate bonding mode were generated by conceptual bonding arguments, and Extended-Hueckel 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-PO4 or Al-PO4 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-PO4 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-Omore » 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)-PO4 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-PO4 solutions containing predominantly bidentate complexes, XANES results indicated that phosphate adsorbed on ferrihydrite was predominantly a bidentate-binuclear surface complex.« less

Authors:
; ;
Publication Date:
Research Org.:
Brookhaven National Laboratory (BNL) National Synchrotron Light Source
Sponsoring Org.:
Doe - Office Of Science
OSTI Identifier:
959676
Report Number(s):
BNL-82662-2009-JA
Journal ID: ISSN 0016-7037; GCACAK; TRN: US201016%%820
DOE Contract Number:  
DE-AC02-98CH10886
Resource Type:
Journal Article
Resource Relation:
Journal Name: Geochimica et Cosmochimica Acta; Journal Volume: 71
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; ABSORPTION; ABSORPTION SPECTRA; BONDING; CONFIGURATION; ELECTRONS; PHOSPHATES; POLLUTANTS; SORPTION; SPECTRA; WATER; national synchrotron light source

Citation Formats

Khare,N., Martin, J., and Hesterberg, D.. Phosphate Bonding Configuration on Ferrihydrite Based on Molecular Orbital Calculations and XANES Fingerprinting. United States: N. p., 2007. Web. doi:10.1016/j.gca.2007.07.008.
Khare,N., Martin, J., & Hesterberg, D.. Phosphate Bonding Configuration on Ferrihydrite Based on Molecular Orbital Calculations and XANES Fingerprinting. United States. doi:10.1016/j.gca.2007.07.008.
Khare,N., Martin, J., and Hesterberg, D.. Mon . "Phosphate Bonding Configuration on Ferrihydrite Based on Molecular Orbital Calculations and XANES Fingerprinting". United States. doi:10.1016/j.gca.2007.07.008.
@article{osti_959676,
title = {Phosphate Bonding Configuration on Ferrihydrite Based on Molecular Orbital Calculations and XANES Fingerprinting},
author = {Khare,N. and Martin, J. and Hesterberg, D.},
abstractNote = {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)-PO4 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)-PO4 or Al(III)-PO4 complexes in idealized monodentate or bidentate bonding mode were generated by conceptual bonding arguments, and Extended-Hueckel 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-PO4 or Al-PO4 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-PO4 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)-PO4 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-PO4 solutions containing predominantly bidentate complexes, XANES results indicated that phosphate adsorbed on ferrihydrite was predominantly a bidentate-binuclear surface complex.},
doi = {10.1016/j.gca.2007.07.008},
journal = {Geochimica et Cosmochimica Acta},
number = ,
volume = 71,
place = {United States},
year = {Mon Jan 01 00:00:00 EST 2007},
month = {Mon Jan 01 00:00:00 EST 2007}
}