Web Release Date: November 1,
Quantum Chemical Calculations of Sulfate Adsorption at the Al- and Fe-(Hydr)oxide-H2O Interface-Estimation of Gibbs Free Energies
and
Department of Plant and Soil Sciences, 152 Townsend Hall, University of Delaware, Newark, Delaware 19716, and Department of Geosciences and Earth and Environmental Systems Institute, The Pennsylvania State University, University Park, Pennsylvania 16802
Received for review May 12, 2006
Revised manuscript received August 4, 2006
Accepted August 15, 2006
Abstract:
Quantum chemical calculations were performed to
estimate relative Gibbs free energies of sulfate adsorption
on variably charged Al- and Fe-(hydr)oxide clusters. Inner-sphere bidentate bridging and monodentate adsorption were
predicted to be exergonic on positively charged Al- and Fe-(hydr)oxides (ranging from -19 to -124 kJ mol-1).
However, inner-sphere and H-bonded adsorption on
neutral Al- and Fe-(hydr)oxides was predicted to be
endergonic (ranging from +5 to +61 kJ mol-1). At the highest
positive surface charge, bidentate bridging adsorption
was most thermodynamically favorable. At intermediate
positive surface charge, bidentate bridging and monodentate
adsorption energies were equivalent on Al-(hydr)oxides;
monodentate adsorption was more thermodynamically
favorable on Fe-(hydr)oxides as compared with bidentate
bridging adsorption. The predicted thermodynamic favorability
of sulfate adsorption on Al- and Fe-(hydr)oxides was
directly related to positive surface charge and indirectly
related to the HO-/SO42- exchange stoichiometry, 
. Predicted
Gibbs free energies of bidentate bridging and monodentate
sulfate adsorption on an Fe-(hydr)oxide cluster (charge
= +1, = 1) agreed reasonably well with published
experimental estimates of sulfate adsorption on geothite
(predicted values -34 and -52 kJ mol-1, respectively, and
experimental range -36 to -30 kJ mol-1).
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