Quantum chemical calculations of sulfate adsorption at the Al- and Fe-(hydr)oxide-H20 interface-estimation of gibbs free energies.
Quantum chemical calculations were performed to estimate relative Gibbs free energies of sulfate adsorption on variably charged Al- and Fe-(hydr)oxide clusters. Innersphere 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)). Atthe 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, chi. Predicted Gibbs free energies of bidentate bridging and monodentate sulfate adsorption on an Fe-(hydr)oxide cluster (charge = +1, chi = 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-').
Paul
Kristian W
KW
Department of Plant and Soil Sciences, 152 Townsend Hall, University of Delaware, Newark, Delaware 19716, USA. kpaul@udel.edu
Kubick
James D
JD
Sparks
Donald L
DL
D000327:Adsorption; D000536:Aluminum Hydroxide / Q000737:chemistry*; D005290:Ferric Compounds / Q000737:chemistry*; D008958:Models, Molecular; D011789:Quantum Theory; D013431:Sulfates / Q000737:chemistry*; D013816:Thermodynamics; D014867:Water / Q000737:chemistry*
D005290:Ferric Compounds; D013431:Sulfates; D014867:Water; C000499:ferric oxide; D000536:Aluminum Hydroxide