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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}
}
  • No abstract prepared.
  • The authors have used Hartree-Fock theory and density functional theory to compute the enthalpy and entropy changes of dimerization for water, methanol, and the family of carboxylic acids. These results are used in a physical equation of state, the statistical associating fluid theory (SAFT), in order to model the phase behavior of these hydrogen-bonding compounds. A procedure has been developed to relate the calculated enthalpy and entropy changes to the association parameters in SAFT using only low-pressure data, as well as to relate molar volumes from molecular orbital calculations to the segment size and chain length parameters in SAFT. Bymore » doing so, the SAFT model is reduced to a three-parameter equation of state for associating fluids. The modified equation of state is shown to be as accurate as the original SAFT model for correlating pure-component vapor-liquid equilibrium data with fewer adjustable parameters.« less
  • In previous studies, the authors have used Hartree-Fock theory and density functional theory to compute the enthalpy and entropy changes of dimerization for methanol and a number of small carboxylic acids. They have shown that by using these results in a physical equation of state, the statistical associating fluid theory (SAFT), they are able to model the phase behavior of these pure hydrogen-bonding compounds with a reduction in the number of adjustable parameters; in this study, they use the pure-component parameters derived from the results of the molecular orbital calculations to describe the phase behavior of mixtures containing one associatingmore » and one nonassociating compound, again using the SAFT equation of state. They show that the use of the pure-component SAFT parameters derived from the quantum-mechanical calculations results in correlations of mixture VLE data with no loss of accuracy, and frequently with improved accuracy, compared to the original parameters reported for use with the SAFT model.« less
  • The thermal conductivity of 2,2,2-trifluoroethanol vapor was measured in a modified thick hot-wire cell between 338 and 385 K at pressures ranging from approximately 100 to 1300 Torr (13.3 to 173 kPa). Analysis of the data indicates that molecular association to form a dimeric species is the main source of the enhancement of the thermal conductivity of the vapor. The enthalpy of association of the trifluoroethanol dimer is -4753 cal/mol and the entropy of association is -18.73 cal/mol K. Ab initio calculations were carried out on several rotational isomers of trifluoroethanol and on four possible dimer structures. The most stablemore » dimer is a cyclic structure involving the gauche-staggered isomer of trifluoroethanol. An extra hydrogen bond due to the presence of the fluorines enhances the stability of the dimer compared to the methanol dimer.« less
  • The thermal conductivities of serveral methanol--water binary vapor mixtures were measured in a modified thick hot-wire cell between 352 and 375 K at pressures ranging from 100 to 1400 Torr. Analysis of the data indicates that molecular association to form a 2:1 methanol--water trimeric species is a major source of enhancement of the thermal conductivity. The enthalpy and entropy of association of this trimer are -10.48 kcal mol/sup -1/ and -38.24 cal mol/sup -1/ deg/sup -1/, respectively. Theoretical calculations, based on ab initio molecular orbital calculations, of the enthalpies and entropies of association for various open chain and cyclic methanol--watermore » trimers indicate that the 2:1 methanol--water trimeric species is more stable than the 1:2 trimeric species which is consistent with the results of the analysis of the thermal conductivity data. The theoretical calculations indicate that it may be an entropic effect which makes the 2:1 trimeric species more stable than the 1:2 species.« less