Friday, 14 July 2006

Quantum Chemical Calculations of Sulfate Adsorption at the Al- and Fe-(hydr)oxide-H2O Interface Estimation of Gibbs Free Energies.

Kristian W. Paul1, James D. Kubicki2, and Donald Sparks1. (1) University of Delaware, Department of Plant and Soil Science, 152 Townsend Hall, Newark, DE 19717, (2) The Pennsylvania State University, 308 Deike Building, University Park, PA 16802

Quantum chemical calculations were performed to estimate relative Gibbs free energies of sulfate adsorption on variably charged Al- and Fe-(hydr)oxides. Inner-sphere bidentate bridging and monodentate adsorption was predicted to be exothermic on positively charged Al- and Fe-(hydr)oxides (range -19 to -124 kJ mol-1) . However, inner-sphere and H-bonded adsorption on neutral Al- and Fe-(hydr)oxides was predicted to be endothermic (range +5 to +61 kJ mol-1). At highest positive surface charge, bidentate bridging adsorption was most thermodynamically favorable. At intermediate positive surface charge, bidentate bridging and monodentate adsorption were thermodynamically equivalent on Al-(hydr)oxides; monodentate adsorption was more thermodynamically favorable on Fe-(hydr)oxides compared with bidentate bridging adsorption. Predicted thermodynamic behavior of sulfate adsorption on Al- and Fe-(hydr)oxides qualitatively agreed with experimental pH-dependent surface charge adsorption measurements (i.e. adsorption edge).

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